Network Working Group D. Eastlake 3rd
Request for Comments: 3275 Motorola
Obsoletes: 3075 J. Reagle
Category: Standards Track W3C
D. Solo
Citigroup
March 2002
(Extensible Markup Language) XML-Signature Syntax and Processing
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2002 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.
Abstract
This document specifies XML (Extensible Markup Language) digital
signature processing rules and syntax. XML Signatures provide
integrity, message authentication, and/or signer authentication
services for data of any type, whether located within the XML that
includes the signature or elsewhere.
Table of Contents
1. Introduction................................................... 3
1.1 Editorial and Conformance Conventions......................... 4
1.2 Design Philosophy............................................. 4
1.3 Versions, Namespaces and Identifiers.......................... 4
1.4 Acknowledgements.............................................. 6
1.5 W3C Status.................................................... 6
2. Signature Overview and Examples................................ 7
2.1 Simple Example (Signature, SignedInfo, Methods, and References) 8
2.1.1 More on Reference........................................... 9
2.2 Extended Example (Object and SignatureProperty)............... 10
2.3 Extended Example (Object and Manifest)........................ 12
3.0 Processing Rules.............................................. 13
3.1 Core Generation............................................... 13
3.1.1 Reference Generation........................................ 13
3.1.2 Signature Generation........................................ 13
3.2 Core Validation............................................... 14
3.2.1 Reference Validation........................................ 14
3.2.2 Signature Validation........................................ 15
4.0 Core Signature Syntax......................................... 15
4.0.1 The ds:CryptoBinary Simple Type............................. 17
4.1 The Signature element......................................... 17
4.2 The SignatureValue Element.................................... 18
4.3 The SignedInfo Element........................................ 18
4.3.1 The CanonicalizationMethod Element.......................... 19
4.3.2 The SignatureMethod Element................................. 21
4.3.3 The Reference Element....................................... 21
4.3.3.1 The URI Attribute......................................... 22
4.3.3.2 The Reference Processing Model............................ 23
4.3.3.3 Same-Document URI-References.............................. 25
4.3.3.4 The Transforms Element.................................... 26
4.3.3.5 The DigestMethod Element.................................. 28
4.3.3.6 The DigestValue Element................................... 28
4.4 The KeyInfo Element........................................... 29
4.4.1 The KeyName Element......................................... 31
4.4.2 The KeyValue Element........................................ 31
4.4.2.1 The DSAKeyValue Element................................... 32
4.4.2.2 The RSAKeyValue Element................................... 33
4.4.3 The RetrievalMethod Element................................. 34
4.4.4 The X509Data Element........................................ 35
4.4.5 The PGPData Element......................................... 38
4.4.6 The SPKIData Element........................................ 39
4.4.7 The MgmtData Element........................................ 40
4.5 The Object Element............................................ 40
5.0 Additional Signature Syntax................................... 42
5.1 The Manifest Element.......................................... 42
5.2 The SignatureProperties Element............................... 43
5.3 Processing Instructions in Signature Elements................. 44
5.4 Comments in Signature Elements................................ 44
6.0 Algorithms.................................................... 44
6.1 Algorithm Identifiers and Implementation Requirements......... 44
6.2 Message Digests............................................... 46
6.2.1 SHA-1....................................................... 46
6.3 Message Authentication Codes.................................. 46
6.3.1 HMAC........................................................ 46
6.4 Signature Algorithms.......................................... 47
6.4.1 DSA......................................................... 47
6.4.2 PKCS1 (RSA-SHA1)............................................ 48
6.5 Canonicalization Algorithms................................... 49
6.5.1 Canonical XML............................................... 49
6.6 Transform Algorithms.......................................... 50
6.6.1 Canonicalization............................................ 50
6.6.2 Base64...................................................... 50
6.6.3 XPath Filtering............................................. 51
6.6.4 Enveloped Signature Transform............................... 54
6.6.5 XSLT Transform.............................................. 54
7. XML Canonicalization and Syntax Constraint Considerations...... 55
7.1 XML 1.0, Syntax Constraints, and Canonicalization............. 56
7.2 DOM/SAX Processing and Canonicalization....................... 57
7.3 Namespace Context and Portable Signatures..................... 58
8.0 Security Considerations....................................... 59
8.1 Transforms.................................................... 59
8.1.1 Only What is Signed is Secure............................... 60
8.1.2 Only What is 'Seen' Should be Signed........................ 60
8.1.3 'See' What is Signed........................................ 61
8.2 Check the Security Model...................................... 62
8.3 Algorithms, Key Lengths, Certificates, Etc.................... 62
9. Schema, DTD, Data Model, and Valid Examples.................... 63
10. Definitions................................................... 63
Appendix: Changes from RFC 3075................................... 67
References........................................................ 67
Authors' Addresses................................................ 72
Full Copyright Statement.......................................... 73
1. Introduction
This document specifies XML syntax and processing rules for creating
and representing digital signatures. XML Signatures can be applied
to any digital content (data object), including XML. An XML
Signature may be applied to the content of one or more resources.
Enveloped or enveloping signatures are over data within the same XML
document as the signature; detached signatures are over data external
to the signature element. More specifically, this specification
defines an XML signature element type and an XML signature
application; conformance requirements for each are specified by way
of schema definitions and prose respectively. This specification
also includes other useful types that identify methods for
referencing collections of resources, algorithms, and keying and
management information.
The XML Signature is a method of associating a key with referenced
data (octets); it does not normatively specify how keys are
associated with persons or institutions, nor the meaning of the data
being referenced and signed. Consequently, while this specification
is an important component of secure XML applications, it itself is
not sufficient to address all application security/trust concerns,
particularly with respect to using signed XML (or other data formats)
as a basis of human-to-human communication and agreement. Such an
application must specify additional key, algorithm, processing and
rendering requirements. For further information, please see Security
Considerations (section 8).
1.1 Editorial and Conformance Conventions
For readability, brevity, and historic reasons this document uses the
term "signature" to generally refer to digital authentication values
of all types. Obviously, the term is also strictly used to refer to
authentication values that are based on public keys and that provide
signer authentication. When specifically discussing authentication
values based on symmetric secret key codes we use the terms
authenticators or authentication codes. (See Check the Security
Model, section 8.3.)
This specification provides an XML Schema [XML-schema] and DTD [XML].
The schema definition is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
specification are to be interpreted as described in RFC2119
[KEYWORDS]:
"they MUST only be used where it is actually required for
interoperation or to limit behavior which has potential for
causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized key words to unambiguously
specify requirements over protocol and application features and
behavior that affect the interoperability and security of
implementations. These key words are not used (capitalized) to
describe XML grammar; schema definitions unambiguously describe such
requirements and we wish to reserve the prominence of these terms for
the natural language descriptions of protocols and features. For
instance, an XML attribute might be described as being "optional."
Compliance with the Namespaces in XML specification [XML-ns] is
described as "REQUIRED."
1.2 Design Philosophy
The design philosophy and requirements of this specification are
addressed in the XML-Signature Requirements document [XML-Signature-
RD].
1.3 Versions, Namespaces and Identifiers
No provision is made for an explicit version number in this syntax.
If a future version is needed, it will use a different namespace.
The XML namespace [XML-ns] URI that MUST be used by implementations
of this (dated) specification is:
xmlns="http://www.w3.org/2000/09/xmldsig#"
This namespace is also used as the prefix for algorithm identifiers
used by this specification. While applications MUST support XML and
XML namespaces, the use of internal entities [XML] or our "dsig" XML
namespace prefix and defaulting/scoping conventions are OPTIONAL; we
use these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to
identify resources, algorithms, and semantics. The URI in the
namespace declaration above is also used as a prefix for URIs under
the control of this specification. For resources not under the
control of this specification, we use the designated Uniform Resource
Names [URN] or Uniform Resource Locators [URL] defined by its
normative external specification. If an external specification has
not allocated itself a Uniform Resource Identifier we allocate an
identifier under our own namespace. For instance:
SignatureProperties is identified and defined by this specification's
namespace:
http://www.w3.org/2000/09/xmldsig#SignatureProperties
XSLT is identified and defined by an external URI
http://www.w3.org/TR/1999/REC-xslt-19991116
SHA1 is identified via this specification's namespace and defined via
a normative reference
http://www.w3.org/2000/09/xmldsig#sha1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
Commerce/National Institute of Standards and Technology.
Finally, in order to provide for terse namespace declarations we
sometimes use XML internal entities [XML] within URIs. For instance:
<?xml version='1.0'?>
<!DOCTYPE Signature SYSTEM
"xmldsig-core-schema.dtd" [ <!ENTITY dsig
"http://www.w3.org/2000/09/xmldsig#"> ]>
<Signature xmlns="&dsig;" Id="MyFirstSignature">
<SignedInfo>
...
1.4 Acknowledgements
The contributions of the following Working Group members to this
specification are gratefully acknowledged:
* Mark Bartel, Accelio (Author)
* John Boyer, PureEdge (Author)
* Mariano P. Consens, University of Waterloo
* John Cowan, Reuters Health
* Donald Eastlake 3rd, Motorola (Chair, Author/Editor)
* Barb Fox, Microsoft (Author)
* Christian Geuer-Pollmann, University Siegen
* Tom Gindin, IBM
* Phillip Hallam-Baker, VeriSign Inc
* Richard Himes, US Courts
* Merlin Hughes, Baltimore
* Gregor Karlinger, IAIK TU Graz
* Brian LaMacchia, Microsoft (Author)
* Peter Lipp, IAIK TU Graz
* Joseph Reagle, W3C (Chair, Author/Editor)
* Ed Simon, XMLsec (Author)
* David Solo, Citigroup (Author/Editor)
* Petteri Stenius, DONE Information, Ltd
* Raghavan Srinivas, Sun
* Kent Tamura, IBM
* Winchel Todd Vincent III, GSU
* Carl Wallace, Corsec Security, Inc.
* Greg Whitehead, Signio Inc.
As are the Last Call comments from the following:
* Dan Connolly, W3C
* Paul Biron, Kaiser Permanente, on behalf of the XML Schema WG.
* Martin J. Duerst, W3C; and Masahiro Sekiguchi, Fujitsu; on
behalf of the Internationalization WG/IG.
* Jonathan Marsh, Microsoft, on behalf of the Extensible
Stylesheet Language WG.
1.5 W3C Status
The World Wide Web Consortium Recommendation corresponding to
this RFC is at:
http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
2. Signature Overview and Examples
This section provides an overview and examples of XML digital
signature syntax. The specific processing is given in Processing
Rules (section 3). The formal syntax is found in Core Signature
Syntax (section 4) and Additional Signature Syntax (section 5).
In this section, an informal representation and examples are used to
describe the structure of the XML signature syntax. This
representation and examples may omit attributes, details and
potential features that are fully explained later.
XML Signatures are applied to arbitrary digital content (data
objects) via an indirection. Data objects are digested, the
resulting value is placed in an element (with other information) and
that element is then digested and cryptographically signed. XML
digital signatures are represented by the Signature element which has
the following structure (where "?" denotes zero or one occurrence;
"+" denotes one or more occurrences; and "*" denotes zero or more
occurrences):
<Signature ID?>
<SignedInfo>
<CanonicalizationMethod/>
<SignatureMethod/>
(<Reference URI? >
(<Transforms>)?
<DigestMethod>
<DigestValue>
</Reference>)+
</SignedInfo>
<SignatureValue>
(<KeyInfo>)?
(<Object ID?>)*
</Signature>
Signatures are related to data objects via URIs [URI]. Within an XML
document, signatures are related to local data objects via fragment
identifiers. Such local data can be included within an enveloping
signature or can enclose an enveloped signature. Detached signatures
are over external network resources or local data objects that reside
within the same XML document as sibling elements; in this case, the
signature is neither enveloping (signature is parent) nor enveloped
attribute (signature is child). Since a Signature element (and its
Id value/name) may co-exist or be combined with other elements (and
their IDs) within a single XML document, care should be taken in
choosing names such that there are no subsequent collisions that
violate the ID uniqueness validity constraint [XML].
2.1 Simple Example (Signature, SignedInfo, Methods, and References)
The following example is a detached signature of the content of the
HTML4 in XML specification.
[s01] <Signature Id="MyFirstSignature"
xmlns="http://www.w3.org/2000/09/xmldsig#">
[s02] <SignedInfo>
[s03] <CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
[s04] <SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
[s05] <Reference
URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
[s06] <Transforms>
[s07] <Transform
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
[s08] </Transforms>
[s09] <DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
[s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[s11] </Reference>
[s12] </SignedInfo>
[s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue>
[s14] <KeyInfo>
[s15a] <KeyValue>
[s15b] <DSAKeyValue>
[s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y>
[s15d] </DSAKeyValue>
[s15e] </KeyValue>
[s16] </KeyInfo>
[s17] </Signature>
[s02-12] The required SignedInfo element is the information that is
actually signed. Core validation of SignedInfo consists of two
mandatory processes: validation of the signature over SignedInfo and
validation of each Reference digest within SignedInfo. Note that the
algorithms used in calculating the SignatureValue are also included
in the signed information while the SignatureValue element is outside
SignedInfo.
[s03] The CanonicalizationMethod is the algorithm that is used to
canonicalize the SignedInfo element before it is digested as part of
the signature operation. Note that this example, and all examples in
this specification, are not in canonical form.
[s04] The SignatureMethod is the algorithm that is used to convert
the canonicalized SignedInfo into the SignatureValue. It is a
combination of a digest algorithm and a key dependent algorithm and
possibly other algorithms such as padding, for example RSA-SHA1. The
algorithm names are signed to resist attacks based on substituting a
weaker algorithm. To promote application interoperability we specify
a set of signature algorithms that MUST be implemented, though their
use is at the discretion of the signature creator. We specify
additional algorithms as RECOMMENDED or OPTIONAL for implementation;
the design also permits arbitrary user specified algorithms.
[s05-11] Each Reference element includes the digest method and
resulting digest value calculated over the identified data object.
It may also include transformations that produced the input to the
digest operation. A data object is signed by computing its digest
value and a signature over that value. The signature is later
checked via reference and signature validation.
[s14-16] KeyInfo indicates the key to be used to validate the
signature. Possible forms for identification include certificates,
key names, and key agreement algorithms and information -- we define
only a few. KeyInfo is optional for two reasons. First, the signer
may not wish to reveal key information to all document processing
parties. Second, the information may be known within the
application's context and need not be represented explicitly. Since
KeyInfo is outside of SignedInfo, if the signer wishes to bind the
keying information to the signature, a Reference can easily identify
and include the KeyInfo as part of the signature.
2.1.1 More on Reference
[s05] <Reference
URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/">
[s06] <Transforms>
[s07] <Transform
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
[s08] </Transforms>
[s09] <DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
[s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[s11] </Reference>
[s05] The optional URI attribute of Reference identifies the data
object to be signed. This attribute may be omitted on at most one
Reference in a Signature. (This limitation is imposed in order to
ensure that references and objects may be matched unambiguously.)
[s05-08] This identification, along with the transforms, is a
description provided by the signer on how they obtained the signed
data object in the form it was digested (i.e., the digested content).
The verifier may obtain the digested content in another method so
long as the digest verifies. In particular, the verifier may obtain
the content from a different location such as a local store, as
opposed to that specified in the URI.
[s06-08] Transforms is an optional ordered list of processing steps
that were applied to the resource's content before it was digested.
Transforms can include operations such as canonicalization,
encoding/decoding (including compression/inflation), XSLT, XPath, XML
schema validation, or XInclude. XPath transforms permit the signer
to derive an XML document that omits portions of the source document.
Consequently those excluded portions can change without affecting
signature validity. For example, if the resource being signed
encloses the signature itself, such a transform must be used to
exclude the signature value from its own computation. If no
Transforms element is present, the resource's content is digested
directly. While the Working Group has specified mandatory (and
optional) canonicalization and decoding algorithms, user specified
transforms are permitted.
[s09-10] DigestMethod is the algorithm applied to the data after
Transforms is applied (if specified) to yield the DigestValue. The
signing of the DigestValue is what binds a resources content to the
signer's key.
2.2 Extended Example (Object and SignatureProperty)
This specification does not address mechanisms for making statements
or assertions. Instead, this document defines what it means for
something to be signed by an XML Signature (integrity, message
authentication, and/or signer authentication). Applications that
wish to represent other semantics must rely upon other technologies,
such as [XML, RDF]. For instance, an application might use a
foo:assuredby attribute within its own markup to reference a
Signature element. Consequently, it's the application that must
understand and know how to make trust decisions given the validity of
the signature and the meaning of assuredby syntax. We also define a
SignatureProperties element type for the inclusion of assertions
about the signature itself (e.g., signature semantics, the time of
signing or the serial number of hardware used in cryptographic
processes). Such assertions may be signed by including a Reference
for the SignatureProperties in SignedInfo. While the signing
application should be very careful about what it signs (it should
understand what is in the SignatureProperty) a receiving application
has no obligation to understand that semantic (though its parent
trust engine may wish to). Any content about the signature
generation may be located within the SignatureProperty element. The
mandatory Target attribute references the Signature element to which
the property applies.
Consider the preceding example with an additional reference to a
local Object that includes a SignatureProperty element. (Such a
signature would not only be detached [p02] but enveloping [p03].)
[ ] <Signature Id="MySecondSignature" ...>
[p01] <SignedInfo>
[ ] ...
[p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/">
[ ] ...
[p03] <Reference URI="#AMadeUpTimeStamp"
[p04]
Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties">
[p05] <DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
[p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[p07] </Reference>
[p08] </SignedInfo>
[p09] ...
[p10] <Object>
[p11] <SignatureProperties>
[p12] <SignatureProperty Id="AMadeUpTimeStamp"
Target="#MySecondSignature">
[p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt">
[p14] <date>19990908</date>
[p15] <time>14:34:34:34</time>
[p16] </timestamp>
[p17] </SignatureProperty>
[p18] </SignatureProperties>
[p19] </Object>
[p20]</Signature>
[p04] The optional Type attribute of Reference provides information
about the resource identified by the URI. In particular, it can
indicate that it is an Object, SignatureProperty, or Manifest
element. This can be used by applications to initiate special
processing of some Reference elements. References to an XML data
element within an Object element SHOULD identify the actual element
pointed to. Where the element content is not XML (perhaps it is
binary or encoded data) the reference should identify the Object and
the Reference Type, if given, SHOULD indicate Object. Note that Type
is advisory and no action based on it or checking of its correctness
is required by core behavior.
[p10] Object is an optional element for including data objects within
the signature element or elsewhere. The Object can be optionally
typed and/or encoded.
[p11-18] Signature properties, such as time of signing, can be
optionally signed by identifying them from within a Reference.
(These properties are traditionally called signature "attributes"
although that term has no relationship to the XML term "attribute".)
2.3 Extended Example (Object and Manifest)
The Manifest element is provided to meet additional requirements not
directly addressed by the mandatory parts of this specification. Two
requirements and the way the Manifest satisfies them follow.
First, applications frequently need to efficiently sign multiple data
objects even where the signature operation itself is an expensive
public key signature. This requirement can be met by including
multiple Reference elements within SignedInfo since the inclusion of
each digest secures the data digested. However, some applications
may not want the core validation behavior associated with this
approach because it requires every Reference within SignedInfo to
undergo reference validation -- the DigestValue elements are checked.
These applications may wish to reserve reference validation decision
logic to themselves. For example, an application might receive a
signature valid SignedInfo element that includes three Reference
elements. If a single Reference fails (the identified data object
when digested does not yield the specified DigestValue) the signature
would fail core validation. However, the application may wish to
treat the signature over the two valid Reference elements as valid or
take different actions depending on which fails. To accomplish this,
SignedInfo would reference a Manifest element that contains one or
more Reference elements (with the same structure as those in
SignedInfo). Then, reference validation of the Manifest is under
application control.
Second, consider an application where many signatures (using
different keys) are applied to a large number of documents. An
inefficient solution is to have a separate signature (per key)
repeatedly applied to a large SignedInfo element (with many
References); this is wasteful and redundant. A more efficient
solution is to include many references in a single Manifest that is
then referenced from multiple Signature elements.
The example below includes a Reference that signs a Manifest found
within the Object element.
[ ] ...
[m01] <Reference URI="#MyFirstManifest"
[m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest">
[m03] <DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
[m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
[m05] </Reference>
[ ] ...
[m06] <Object>
[m07] <Manifest Id="MyFirstManifest">
[m08] <Reference>
[m09] ...
[m10] </Reference>
[m11] <Reference>
[m12] ...
[m13] </Reference>
[m14] </Manifest>
[m15] </Object>
3.0 Processing Rules
The sections below describe the operations to be performed as part of
signature generation and validation.
3.1 Core Generation
The REQUIRED steps include the generation of Reference elements and
the SignatureValue over SignedInfo.
3.1.1 Reference Generation
For each data object being signed:
1. Apply the Transforms, as determined by the application, to the
data object.
2. Calculate the digest value over the resulting data object.
3. Create a Reference element, including the (optional)
identification of the data object, any (optional) transform
elements, the digest algorithm and the DigestValue. (Note, it is
the canonical form of these references that are signed in 3.1.2
and validated in 3.2.1.)
3.1.2 Signature Generation
1. Create SignedInfo element with SignatureMethod,
CanonicalizationMethod and Reference(s).
2. Canonicalize and then calculate the SignatureValue over SignedInfo
based on algorithms specified in SignedInfo.
3. Construct the Signature element that includes SignedInfo,
Object(s) (if desired, encoding may be different than that used
for signing), KeyInfo (if required), and SignatureValue.
Note, if the Signature includes same-document references, [XML] or
[XML-schema] validation of the document might introduce changes that
break the signature. Consequently, applications should be careful to
consistently process the document or refrain from using external
contributions (e.g., defaults and entities).
3.2 Core Validation
The REQUIRED steps of core validation include (1) reference
validation, the verification of the digest contained in each
Reference in SignedInfo, and (2) the cryptographic signature
validation of the signature calculated over SignedInfo.
Note, there may be valid signatures that some signature applications
are unable to validate. Reasons for this include failure to
implement optional parts of this specification, inability or
unwillingness to execute specified algorithms, or inability or
unwillingness to dereference specified URIs (some URI schemes may
cause undesirable side effects), etc.
Comparison of values in reference and signature validation are over
the numeric (e.g., integer) or decoded octet sequence of the value.
Different implementations may produce different encoded digest and
signature values when processing the same resources because of
variances in their encoding, such as accidental white space. But if
one uses numeric or octet comparison (choose one) on both the stated
and computed values these problems are eliminated.
3.2.1 Reference Validation
1. Canonicalize the SignedInfo element based on the
CanonicalizationMethod in SignedInfo.
2. For each Reference in SignedInfo:
2.1 Obtain the data object to be digested. (For example, the
signature application may dereference the URI and execute
Transforms provided by the signer in the Reference element, or
it may obtain the content through other means such as a local
cache.)
2.2 Digest the resulting data object using the DigestMethod
specified in its Reference specification.
2.3 Compare the generated digest value against DigestValue in the
SignedInfo Reference; if there is any mismatch, validation
fails.
Note, SignedInfo is canonicalized in step 1. The application must
ensure that the CanonicalizationMethod has no dangerous side affects,
such as rewriting URIs, (see CanonicalizationMethod (section 4.3))
and that it Sees What is Signed, which is the canonical form.
3.2.2 Signature Validation
1. Obtain the keying information from KeyInfo or from an external
source.
2. Obtain the canonical form of the SignatureMethod using the
CanonicalizationMethod and use the result (and previously obtained
KeyInfo) to confirm the SignatureValue over the SignedInfo
element.
Note, KeyInfo (or some transformed version thereof) may be signed via
a Reference element. Transformation and validation of this reference
(3.2.1) is orthogonal to Signature Validation which uses the KeyInfo
as parsed.
Additionally, the SignatureMethod URI may have been altered by the
canonicalization of SignedInfo (e.g., absolutization of relative
URIs) and it is the canonical form that MUST be used. However, the
required canonicalization [XML-C14N] of this specification does not
change URIs.
4.0 Core Signature Syntax
The general structure of an XML signature is described in Signature
Overview (section 2). This section provides detailed syntax of the
core signature features. Features described in this section are
mandatory to implement unless otherwise indicated. The syntax is
defined via DTDs and [XML-Schema] with the following XML preamble,
declaration, and internal entity.
Schema Definition:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE schema
PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
"http://www.w3.org/2001/XMLSchema.dtd"
[
<!ATTLIST schema
xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#">
<!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'>
<!ENTITY % p ''>
<!ENTITY % s ''>
]>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
targetNamespace="http://www.w3.org/2000/09/xmldsig#"
version="0.1" elementFormDefault="qualified">
DTD:
<!--
The following entity declarations enable external/flexible content
in the Signature content model.
#PCDATA emulates schema:string; when combined with element types
it emulates schema mixed="true".
%foo.ANY permits the user to include their own element types from
other namespaces, for example:
<!ENTITY % KeyValue.ANY '| ecds:ECDSAKeyValue'>
...
<!ELEMENT ecds:ECDSAKeyValue (#PCDATA) >
-->
<!ENTITY % Object.ANY ''>
<!ENTITY % Method.ANY ''>
<!ENTITY % Transform.ANY ''>
<!ENTITY % SignatureProperty.ANY ''>
<!ENTITY % KeyInfo.ANY ''>
<!ENTITY % KeyValue.ANY ''>
<!ENTITY % PGPData.ANY ''>
<!ENTITY % X509Data.ANY ''>
<!ENTITY % SPKIData.ANY ''>
4.0.1 The ds:CryptoBinary Simple Type
This specification defines the ds:CryptoBinary simple type for
representing arbitrary-length integers (e.g., "bignums") in XML as
octet strings. The integer value is first converted to a "big
endian" bitstring. The bitstring is then padded with leading zero
bits so that the total number of bits == 0 mod 8 (so that there are
an integral number of octets). If the bitstring contains entire
leading octets that are zero, these are removed (so the high-order
octet is always non-zero). This octet string is then base64 [MIME]
encoded. (The conversion from integer to octet string is equivalent
to IEEE 1363's I2OSP [1363] with minimal length).
This type is used by "bignum" values such as RSAKeyValue and
DSAKeyValue. If a value can be of type base64Binary or
ds:CryptoBinary they are defined as base64Binary. For example, if
the signature algorithm is RSA or DSA then SignatureValue represents
a bignum and could be ds:CryptoBinary. However, if HMAC-SHA1 is the
signature algorithm then SignatureValue could have leading zero
octets that must be preserved. Thus SignatureValue is generically
defined as of type base64Binary.
Schema Definition:
<simpleType name="CryptoBinary">
<restriction base="base64Binary">
</restriction>
</simpleType>
4.1 The Signature element
The Signature element is the root element of an XML Signature.
Implementation MUST generate laxly schema valid [XML-schema]
Signature elements as specified by the following schema:
Schema Definition:
<element name="Signature" type="ds:SignatureType"/>
<complexType name="SignatureType">
<sequence>
<element ref="ds:SignedInfo"/>
<element ref="ds:SignatureValue"/>
<element ref="ds:KeyInfo" minOccurs="0"/>
<element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
DTD:
<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?,
Object*) >
<!ATTLIST Signature
xmlns CDATA #FIXED 'http://www.w3.org/2000/09/xmldsig#'
Id ID #IMPLIED >
4.2 The SignatureValue Element
The SignatureValue element contains the actual value of the digital
signature; it is always encoded using base64 [MIME]. While we
identify two SignatureMethod algorithms, one mandatory and one
optional to implement, user specified algorithms may be used as well.
Schema Definition:
<element name="SignatureValue" type="ds:SignatureValueType"/>
<complexType name="SignatureValueType">
<simpleContent>
<extension base="base64Binary">
<attribute name="Id" type="ID" use="optional"/>
</extension>
</simpleContent>
</complexType>
DTD:
<!ELEMENT SignatureValue (#PCDATA) >
<!ATTLIST SignatureValue
Id ID #IMPLIED>
4.3 The SignedInfo Element
The structure of SignedInfo includes the canonicalization algorithm,
a signature algorithm, and one or more references. The SignedInfo
element may contain an optional ID attribute that will allow it to be
referenced by other signatures and objects.
SignedInfo does not include explicit signature or digest properties
(such as calculation time, cryptographic device serial number, etc.).
If an application needs to associate properties with the signature or
digest, it may include such information in a SignatureProperties
element within an Object element.
Schema Definition:
<element name="SignedInfo" type="ds:SignedInfoType"/>
<complexType name="SignedInfoType">
<sequence>
<element ref="ds:CanonicalizationMethod"/>
<element ref="ds:SignatureMethod"/>
<element ref="ds:Reference" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
DTD:
<!ELEMENT SignedInfo (CanonicalizationMethod,
SignatureMethod, Reference+) >
<!ATTLIST SignedInfo
Id ID #IMPLIED
4.3.1 The CanonicalizationMethod Element
CanonicalizationMethod is a required element that specifies the
canonicalization algorithm applied to the SignedInfo element prior to
performing signature calculations. This element uses the general
structure for algorithms described in Algorithm Identifiers and
Implementation Requirements (section 6.1). Implementations MUST
support the REQUIRED canonicalization algorithms.
Alternatives to the REQUIRED canonicalization algorithms (section
6.5), such as Canonical XML with Comments (section 6.5.1) or a
minimal canonicalization (such as CRLF and charset normalization),
may be explicitly specified but are NOT REQUIRED. Consequently,
their use may not interoperate with other applications that do not
support the specified algorithm (see XML Canonicalization and Syntax
Constraint Considerations, section 7). Security issues may also
arise in the treatment of entity processing and comments if non-XML
aware canonicalization algorithms are not properly constrained (see
section 8.2: Only What is "Seen" Should be Signed).
The way in which the SignedInfo element is presented to the
canonicalization method is dependent on that method. The following
applies to algorithms which process XML as nodes or characters:
* XML based canonicalization implementations MUST be provided
with a [XPath] node-set originally formed from the document
containing the SignedInfo and currently indicating the
SignedInfo, its descendants, and the attribute and namespace
nodes of SignedInfo and its descendant elements.
* Text based canonicalization algorithms (such as CRLF and
charset normalization) should be provided with the UTF-8 octets
that represent the well-formed SignedInfo element, from the
first character to the last character of the XML
representation, inclusive. This includes the entire text of
the start and end tags of the SignedInfo element as well as all
descendant markup and character data (i.e., the text) between
those tags. Use of text based canonicalization of SignedInfo
is NOT RECOMMENDED.
We recommend applications that implement a text-based instead of
XML-based canonicalization -- such as resource constrained apps --
generate canonicalized XML as their output serialization so as to
mitigate interoperability and security concerns. For instance, such
an implementation SHOULD (at least) generate standalone XML instances
[XML].
NOTE: The signature application must exercise great care in accepting
and executing an arbitrary CanonicalizationMethod. For example, the
canonicalization method could rewrite the URIs of the References
being validated. Or, the method could massively transform SignedInfo
so that validation would always succeed (i.e., converting it to a
trivial signature with a known key over trivial data). Since
CanonicalizationMethod is inside SignedInfo, in the resulting
canonical form it could erase itself from SignedInfo or modify the
SignedInfo element so that it appears that a different
canonicalization function was used! Thus a Signature which appears to
authenticate the desired data with the desired key, DigestMethod, and
SignatureMethod, can be meaningless if a capricious
CanonicalizationMethod is used.
Schema Definition:
<element name="CanonicalizationMethod"
type="ds:CanonicalizationMethodType"/>
<complexType name="CanonicalizationMethodType" mixed="true">
<sequence>
<any namespace="##any" minOccurs="0" maxOccurs="unbounded"/>
<!-- (0,unbounded) elements from (1,1) namespace -->
</sequence>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT CanonicalizationMethod (#PCDATA %Method.ANY;)* >
<!ATTLIST CanonicalizationMethod
Algorithm CDATA #REQUIRED >
4.3.2 The SignatureMethod Element
SignatureMethod is a required element that specifies the algorithm
used for signature generation and validation. This algorithm
identifies all cryptographic functions involved in the signature
operation (e.g., hashing, public key algorithms, MACs, padding,
etc.). This element uses the general structure here for algorithms
described in section 6.1: Algorithm Identifiers and Implementation
Requirements. While there is a single identifier, that identifier
may specify a format containing multiple distinct signature values.
Schema Definition:
<element name="SignatureMethod" type="ds:SignatureMethodType"/>
<complexType name="SignatureMethodType" mixed="true">
<sequence>
<element name="HMACOutputLength" minOccurs="0"
type="ds:HMACOutputLengthType"/>
<any namespace="##other" minOccurs="0" maxOccurs="unbounded"/>
<!-- (0,unbounded) elements from (1,1) external namespace -->
</sequence>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT SignatureMethod
(#PCDATA|HMACOutputLength %Method.ANY;)* >
<!ATTLIST SignatureMethod
Algorithm CDATA #REQUIRED >
4.3.3 The Reference Element
Reference is an element that may occur one or more times. It
specifies a digest algorithm and digest value, and optionally an
identifier of the object being signed, the type of the object, and/or
a list of transforms to be applied prior to digesting. The
identification (URI) and transforms describe how the digested content
(i.e., the input to the digest method) was created. The Type
attribute facilitates the processing of referenced data. For
example, while this specification makes no requirements over external
data, an application may wish to signal that the referent is a
Manifest. An optional ID attribute permits a Reference to be
referenced from elsewhere.
Schema Definition:
<element name="Reference" type="ds:ReferenceType"/>
<complexType name="ReferenceType">
<sequence>
<element ref="ds:Transforms" minOccurs="0"/>
<element ref="ds:DigestMethod"/>
<element ref="ds:DigestValue"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
<attribute name="URI" type="anyURI" use="optional"/>
<attribute name="Type" type="anyURI" use="optional"/>
</complexType>
DTD:
<!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) >
<!ATTLIST Reference
Id ID #IMPLIED
URI CDATA #IMPLIED
Type CDATA #IMPLIED>
4.3.3.1 The URI Attribute
The URI attribute identifies a data object using a URI-Reference, as
specified by RFC2396 [URI]. The set of allowed characters for URI
attributes is the same as for XML, namely [Unicode]. However, some
Unicode characters are disallowed from URI references including all
non-ASCII characters and the excluded characters listed in RFC2396
[URI, section 2.4]. However, the number sign (#), percent sign (%),
and square bracket characters re-allowed in RFC 2732 [URI-Literal]
are permitted. Disallowed characters must be escaped as follows:
1. Each disallowed character is converted to [UTF-8] as one or more
octets.
2. Any octets corresponding to a disallowed character are escaped
with the URI escaping mechanism (that is, converted to %HH, where
HH is the hexadecimal notation of the octet value).
3. The original character is replaced by the resulting character
sequence.
XML signature applications MUST be able to parse URI syntax. We
RECOMMEND they be able to dereference URIs in the HTTP scheme.
Dereferencing a URI in the HTTP scheme MUST comply with the Status
Code Definitions of [HTTP] (e.g., 302, 305 and 307 redirects are
followed to obtain the entity-body of a 200 status code response).
Applications should also be cognizant of the fact that protocol
parameter and state information, (such as HTTP cookies, HTML device
profiles or content negotiation), may affect the content yielded by
dereferencing a URI.
If a resource is identified by more than one URI, the most specific
should be used (e.g., http://www.w3.org/2000/06/interop-
pressrelease.html.en instead of http://www.w3.org/2000/06/interop-
pressrelease). (See the Reference Validation (section 3.2.1) for a
further information on reference processing.)
If the URI attribute is omitted altogether, the receiving application
is expected to know the identity of the object. For example, a
lightweight data protocol might omit this attribute given the
identity of the object is part of the application context. This
attribute may be omitted from at most one Reference in any particular
SignedInfo, or Manifest.
The optional Type attribute contains information about the type of
object being signed. This is represented as a URI. For example:
Type="http://www.w3.org/2000/09/xmldsig#Object"
Type="http://www.w3.org/2000/09/xmldsig#Manifest"
The Type attribute applies to the item being pointed at, not its
contents. For example, a reference that identifies an Object element
containing a SignatureProperties element is still of type #Object.
The type attribute is advisory. No validation of the type
information is required by this specification.
4.3.3.2 The Reference Processing Model
Note: XPath is RECOMMENDED. Signature applications need not conform
to [XPath] specification in order to conform to this specification.
However, the XPath data model, definitions (e.g., node-sets) and
syntax is used within this document in order to describe
functionality for those that want to process XML-as-XML (instead of
octets) as part of signature generation. For those that want to use
these features, a conformant [XPath] implementation is one way to
implement these features, but it is not required. Such applications
could use a sufficiently functional replacement to a node-set and
implement only those XPath expression behaviors REQUIRED by this
specification. However, for simplicity we generally will use XPath
terminology without including this qualification on every point.
Requirements over "XPath node-sets" can include a node-set functional
equivalent. Requirements over XPath processing can include
application behaviors that are equivalent to the corresponding XPath
behavior.
The data-type of the result of URI dereferencing or subsequent
Transforms is either an octet stream or an XPath node-set.
The Transforms specified in this document are defined with respect to
the input they require. The following is the default signature
application behavior:
* If the data object is an octet stream and the next transform
requires a node-set, the signature application MUST attempt to
parse the octets yielding the required node-set via [XML]
well-formed processing.
* If the data object is a node-set and the next transform
requires octets, the signature application MUST attempt to
convert the node-set to an octet stream using Canonical XML
[XML-C14N].
Users may specify alternative transforms that override these defaults
in transitions between transforms that expect different inputs. The
final octet stream contains the data octets being secured. The
digest algorithm specified by DigestMethod is then applied to these
data octets, resulting in the DigestValue.
Unless the URI-Reference is a 'same-document' reference as defined in
[URI, Section 4.2], the result of dereferencing the URI-Reference
MUST be an octet stream. In particular, an XML document identified
by URI is not parsed by the signature application unless the URI is a
same-document reference or unless a transform that requires XML
parsing is applied. (See Transforms (section 4.3.3.1).)
When a fragment is preceded by an absolute or relative URI in the
URI-Reference, the meaning of the fragment is defined by the
resource's MIME type. Even for XML documents, URI dereferencing
(including the fragment processing) might be done for the signature
application by a proxy. Therefore, reference validation might fail
if fragment processing is not performed in a standard way (as defined
in the following section for same-document references).
Consequently, we RECOMMEND that the URI attribute not include
fragment identifiers and that such processing be specified as an
additional XPath Transform.
When a fragment is not preceded by a URI in the URI-Reference, XML
signature applications MUST support the null URI and barename
XPointer. We RECOMMEND support for the same-document XPointers
'#xpointer(/)' and '#xpointer(id('ID'))' if the application also
intends to support any canonicalization that preserves comments.
(Otherwise URI="#foo" will automatically remove comments before the
canonicalization can even be invoked.) All other support for
XPointers is OPTIONAL, especially all support for barename and other
XPointers in external resources since the application may not have
control over how the fragment is generated (leading to
interoperability problems and validation failures).
The following examples demonstrate what the URI attribute identifies
and how it is dereferenced:
URI="http://example.com/bar.xml"
Identifies the octets that represent the external resource
'http://example.com/bar.xml', that is probably an XML document
given its file extension.
URI="http://example.com/bar.xml#chapter1"
Identifies the element with ID attribute value 'chapter1' of the
external XML resource 'http://example.com/bar.xml', provided as
an octet stream. Again, for the sake of interoperability, the
element identified as 'chapter1' should be obtained using an
XPath transform rather than a URI fragment (barename XPointer
resolution in external resources is not REQUIRED in this
specification).
URI=""
Identifies the node-set (minus any comment nodes) of the XML
resource containing the signature
URI="#chapter1"
Identifies a node-set containing the element with ID attribute
value 'chapter1' of the XML resource containing the signature.
XML Signature (and its applications) modify this node-set to
include the element plus all descendents including namespaces and
attributes -- but not comments.
4.3.3.3 Same-Document URI-References
Dereferencing a same-document reference MUST result in an XPath
node-set suitable for use by Canonical XML [XML-C14N]. Specifically,
dereferencing a null URI (URI="") MUST result in an XPath node-set
that includes every non-comment node of the XML document containing
the URI attribute. In a fragment URI, the characters after the
number sign ('#') character conform to the XPointer syntax [Xptr].
When processing an XPointer, the application MUST behave as if the
root node of the XML document containing the URI attribute were used
to initialize the XPointer evaluation context. The application MUST
behave as if the result of XPointer processing were a node-set
derived from the resultant location-set as follows:
1. discard point nodes
2. replace each range node with all XPath nodes having full or
partial content within the range
3. replace the root node with its children (if it is in the node-set)
4. replace any element node E with E plus all descendants of E (text,
comment, PI, element) and all namespace and attribute nodes of E
and its descendant elements.
5. if the URI is not a full XPointer, then delete all comment nodes
The second to last replacement is necessary because XPointer
typically indicates a subtree of an XML document's parse tree using
just the element node at the root of the subtree, whereas Canonical
XML treats a node-set as a set of nodes in which absence of
descendant nodes results in absence of their representative text from
the canonical form.
The last step is performed for null URIs, barename XPointers and
child sequence XPointers. It's necessary because when [XML-C14N] is
passed a node-set, it processes the node-set as is: with or without
comments. Only when it's called with an octet stream does it invoke
its own XPath expressions (default or without comments). Therefore
to retain the default behavior of stripping comments when passed a
node-set, they are removed in the last step if the URI is not a full
XPointer. To retain comments while selecting an element by an
identifier ID, use the following full XPointer:
URI='#xpointer(id('ID'))'. To retain comments while selecting the
entire document, use the following full XPointer: URI='#xpointer(/)'.
This XPointer contains a simple XPath expression that includes the
root node, which the second to last step above replaces with all
nodes of the parse tree (all descendants, plus all attributes, plus
all namespaces nodes).
4.3.3.4 The Transforms Element
The optional Transforms element contains an ordered list of Transform
elements; these describe how the signer obtained the data object that
was digested. The output of each Transform serves as input to the
next Transform. The input to the first Transform is the result of
dereferencing the URI attribute of the Reference element. The output
from the last Transform is the input for the DigestMethod algorithm.
When transforms are applied the signer is not signing the native
(original) document but the resulting (transformed) document. (See
Only What is Signed is Secure (section 8.1).)
Each Transform consists of an Algorithm attribute and content
parameters, if any, appropriate for the given algorithm. The
Algorithm attribute value specifies the name of the algorithm to be
performed, and the Transform content provides additional data to
govern the algorithm's processing of the transform input. (See
Algorithm Identifiers and Implementation Requirements (section 6).)
As described in The Reference Processing Model (section 4.3.3.2),
some transforms take an XPath node-set as input, while others require
an octet stream. If the actual input matches the input needs of the
transform, then the transform operates on the unaltered input. If
the transform input requirement differs from the format of the actual
input, then the input must be converted.
Some Transforms may require explicit MIME type, charset (IANA
registered "character set"), or other such information concerning the
data they are receiving from an earlier Transform or the source data,
although no Transform algorithm specified in this document needs such
explicit information. Such data characteristics are provided as
parameters to the Transform algorithm and should be described in the
specification for the algorithm.
Examples of transforms include but are not limited to base64 decoding
[MIME], canonicalization [XML-C14N], XPath filtering [XPath], and
XSLT [XSLT]. The generic definition of the Transform element also
allows application-specific transform algorithms. For example, the
transform could be a decompression routine given by a Java class
appearing as a base64 encoded parameter to a Java Transform
algorithm. However, applications should refrain from using
application-specific transforms if they wish their signatures to be
verifiable outside of their application domain. Transform Algorithms
(section 6.6) define the list of standard transformations.
Schema Definition:
<element name="Transforms" type="ds:TransformsType"/>
<complexType name="TransformsType">
<sequence>
<element ref="ds:Transform" maxOccurs="unbounded"/>
</sequence>
</complexType>
<element name="Transform" type="ds:TransformType"/>
<complexType name="TransformType" mixed="true">
<choice minOccurs="0" maxOccurs="unbounded">
<any namespace="##other" processContents="lax"/>
<!-- (1,1) elements from (0,unbounded) namespaces -->
<element name="XPath" type="string"/>
</choice>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT Transforms (Transform+)>
<!ELEMENT Transform (#PCDATA|XPath %Transform.ANY;)* >
<!ATTLIST Transform
Algorithm CDATA #REQUIRED >
<!ELEMENT XPath (#PCDATA) >
4.3.3.5 The DigestMethod Element
DigestMethod is a required element that identifies the digest
algorithm to be applied to the signed object. This element uses the
general structure here for algorithms specified in Algorithm
Identifiers and Implementation Requirements (section 6.1).
If the result of the URI dereference and application of Transforms is
an XPath node-set (or sufficiently functional replacement implemented
by the application) then it must be converted as described in the
Reference Processing Model (section 4.3.3.2). If the result of URI
dereference and application of transforms is an octet stream, then no
conversion occurs (comments might be present if the Canonical XML
with Comments was specified in the Transforms). The digest algorithm
is applied to the data octets of the resulting octet stream.
Schema Definition:
<element name="DigestMethod" type="ds:DigestMethodType"/>
<complexType name="DigestMethodType" mixed="true">
<sequence>
<any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</sequence>
<attribute name="Algorithm" type="anyURI" use="required"/>
</complexType>
DTD:
<!ELEMENT DigestMethod (#PCDATA %Method.ANY;)* >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
4.3.3.6 The DigestValue Element
DigestValue is an element that contains the encoded value of the
digest. The digest is always encoded using base64 [MIME].
Schema Definition:
<element name="DigestValue" type="ds:DigestValueType"/>
<simpleType name="DigestValueType">
<restriction base="base64Binary"/>
</simpleType>
DTD:
<!ELEMENT DigestValue (#PCDATA) >
<!-- base64 encoded digest value -->
4.4 The KeyInfo Element
KeyInfo is an optional element that enables the recipient(s) to
obtain the key needed to validate the signature. KeyInfo may contain
keys, names, certificates and other public key management
information, such as in-band key distribution or key agreement data.
This specification defines a few simple types but applications may
extend those types or all together replace them with their own key
identification and exchange semantics using the XML namespace
facility. [XML-ns] However, questions of trust of such key
information (e.g., its authenticity or strength) are out of scope of
this specification and left to the application.
If KeyInfo is omitted, the recipient is expected to be able to
identify the key based on application context. Multiple declarations
within KeyInfo refer to the same key. While applications may define
and use any mechanism they choose through inclusion of elements from
a different namespace, compliant versions MUST implement KeyValue
(section 4.4.2) and SHOULD implement RetrievalMethod (section 4.4.3).
The schema/DTD specifications of many of KeyInfo's children (e.g.,
PGPData, SPKIData, X509Data) permit their content to be
extended/complemented with elements from another namespace. This may
be done only if it is safe to ignore these extension elements while
claiming support for the types defined in this specification.
Otherwise, external elements, including alternative structures to
those defined by this specification, MUST be a child of KeyInfo. For
example, should a complete XML-PGP standard be defined, its root
element MUST be a child of KeyInfo. (Of course, new structures from
external namespaces can incorporate elements from the &dsig;
namespace via features of the type definition language. For
instance, they can create a DTD that mixes their own and dsig
qualified elements, or a schema that permits, includes, imports, or
derives new types based on &dsig; elements.)
The following list summarizes the KeyInfo types that are allocated to
an identifier in the &dsig; namespace; these can be used within the
RetrievalMethod Type attribute to describe a remote KeyInfo
structure.
* http://www.w3.org/2000/09/xmldsig#DSAKeyValue
* http://www.w3.org/2000/09/xmldsig#RSAKeyValue
* http://www.w3.org/2000/09/xmldsig#X509Data
* http://www.w3.org/2000/09/xmldsig#PGPData
* http://www.w3.org/2000/09/xmldsig#SPKIData
* http://www.w3.org/2000/09/xmldsig#MgmtData
In addition to the types above for which we define an XML structure,
we specify one additional type to indicate a binary (ASN.1 DER) X.509
Certificate.
* http://www.w3.org/2000/09/xmldsig#rawX509Certificate
Schema Definition:
<element name="KeyInfo" type="ds:KeyInfoType"/>
<complexType name="KeyInfoType" mixed="true">
<choice maxOccurs="unbounded">
<element ref="ds:KeyName"/>
<element ref="ds:KeyValue"/>
<element ref="ds:RetrievalMethod"/>
<element ref="ds:X509Data"/>
<element ref="ds:PGPData"/>
<element ref="ds:SPKIData"/>
<element ref="ds:MgmtData"/>
<any processContents="lax" namespace="##other"/>
<!-- (1,1) elements from (0,unbounded) namespaces -->
</choice>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
DTD:
<!ELEMENT KeyInfo (#PCDATA|KeyName|KeyValue|RetrievalMethod|
X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* >
<!ATTLIST KeyInfo
Id ID #IMPLIED >
4.4.1 The KeyName Element
The KeyName element contains a string value (in which white space is
significant) which may be used by the signer to communicate a key
identifier to the recipient. Typically, KeyName contains an
identifier related to the key pair used to sign the message, but it
may contain other protocol-related information that indirectly
identifies a key pair. (Common uses of KeyName include simple string
names for keys, a key index, a distinguished name (DN), an email
address, etc.)
Schema Definition:
<element name="KeyName" type="string"/>
DTD:
<!ELEMENT KeyName (#PCDATA) >
4.4.2 The KeyValue Element
The KeyValue element contains a single public key that may be useful
in validating the signature. Structured formats for defining DSA
(REQUIRED) and RSA (RECOMMENDED) public keys are defined in Signature
Algorithms (section 6.4). The KeyValue element may include
externally defined public key values represented as PCDATA or element
types from an external namespace.
Schema Definition:
<element name="KeyValue" type="ds:KeyValueType"/>
<complexType name="KeyValueType" mixed="true">
<choice>
<element ref="ds:DSAKeyValue"/>
<element ref="ds:RSAKeyValue"/>
<any namespace="##other" processContents="lax"/>
</choice>
</complexType>
DTD:
<!ELEMENT KeyValue (#PCDATA|DSAKeyValue|RSAKeyValue
%KeyValue.ANY;)* >
4.4.2.1 The DSAKeyValue Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#DSAKeyValue" (this can be
used within a RetrievalMethod or Reference element to identify the
referent's type)
DSA keys and the DSA signature algorithm are specified in [DSS]. DSA
public key values can have the following fields:
P
a prime modulus meeting the [DSS] requirements
Q
an integer in the range 2**159 < Q < 2**160 which is a prime
divisor of P-1
G
an integer with certain properties with respect to P and Q
Y
G**X mod P (where X is part of the private key and not made
public)
J
(P - 1) / Q
seed
a DSA prime generation seed
pgenCounter
a DSA prime generation counter
Parameter J is available for inclusion solely for efficiency as it is
calculatable from P and Q. Parameters seed and pgenCounter are used
in the DSA prime number generation algorithm specified in [DSS]. As
such, they are optional, but must either both be present or both be
absent. This prime generation algorithm is designed to provide
assurance that a weak prime is not being used and it yields a P and Q
value. Parameters P, Q, and G can be public and common to a group of
users. They might be known from application context. As such, they
are optional but P and Q must either both appear or both be absent.
If all of P, Q, seed, and pgenCounter are present, implementations
are not required to check if they are consistent and are free to use
either P and Q or seed and pgenCounter. All parameters are encoded
as base64 [MIME] values.
Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema Definition:
<element name="DSAKeyValue" type="ds:DSAKeyValueType"/>
<complexType name="DSAKeyValueType">
<sequence>
<sequence minOccurs="0">
<element name="P" type="ds:CryptoBinary"/>
<element name="Q" type="ds:CryptoBinary"/>
</sequence>
<element name="G" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Y" type="ds:CryptoBinary"/>
<element name="J" type="ds:CryptoBinary" minOccurs="0"/>
<sequence minOccurs="0">
<element name="Seed" type="ds:CryptoBinary"/>
<element name="PgenCounter" type="ds:CryptoBinary"/>
</sequence>
</sequence>
</complexType>
DTD Definition:
<!ELEMENT DSAKeyValue ((P, Q)?, G?, Y, J?, (Seed, PgenCounter)?) >
<!ELEMENT P (#PCDATA) >
<!ELEMENT Q (#PCDATA) >
<!ELEMENT G (#PCDATA) >
<!ELEMENT Y (#PCDATA) >
<!ELEMENT J (#PCDATA) >
<!ELEMENT Seed (#PCDATA) >
<!ELEMENT PgenCounter (#PCDATA) >
4.4.2.2 The RSAKeyValue Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#RSAKeyValue" (this can be
used within a RetrievalMethod or Reference element to identify the
referent's type)
RSA key values have two fields: Modulus and Exponent.
<RSAKeyValue>
<Modulus>
xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W
jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRg
BUwUlV5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U=
</Modulus>
<Exponent>AQAB</Exponent>
</RSAKeyValue>
Arbitrary-length integers (e.g., "bignums" such as RSA moduli) are
represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema Definition:
<element name="RSAKeyValue" type="ds:RSAKeyValueType"/>
<complexType name="RSAKeyValueType">
<sequence>
<element name="Modulus" type="ds:CryptoBinary"/>
<element name="Exponent" type="ds:CryptoBinary"/>
</sequence>
</complexType>
DTD Definition:
<!ELEMENT RSAKeyValue (Modulus, Exponent) >
<!ELEMENT Modulus (#PCDATA) >
<!ELEMENT Exponent (#PCDATA) >
4.4.3 The RetrievalMethod Element
A RetrievalMethod element within KeyInfo is used to convey a
reference to KeyInfo information that is stored at another location.
For example, several signatures in a document might use a key
verified by an X.509v3 certificate chain appearing once in the
document or remotely outside the document; each signature's KeyInfo
can reference this chain using a single RetrievalMethod element
instead of including the entire chain with a sequence of
X509Certificate elements.
RetrievalMethod uses the same syntax and dereferencing behavior as
Reference's URI (section 4.3.3.1) and the Reference Processing Model
(section 4.3.3.2) except that there is no DigestMethod or DigestValue
child elements and presence of the URI is mandatory.
Type is an optional identifier for the type of data to be retrieved.
The result of dereferencing a RetrievalMethod Reference for all
KeyInfo types defined by this specification (section 4.4) with a
corresponding XML structure is an XML element or document with that
element as the root. The rawX509Certificate KeyInfo (for which there
is no XML structure) returns a binary X509 certificate.
Schema Definition:
<element name="RetrievalMethod" type="ds:RetrievalMethodType"/>
<complexType name="RetrievalMethodType">
<sequence>
<element ref="ds:Transforms" minOccurs="0"/>
</sequence>
<attribute name="URI" type="anyURI"/>
<attribute name="Type" type="anyURI" use="optional"/>
</complexType>
DTD:
<!ELEMENT RetrievalMethod (Transforms?) >
<!ATTLIST RetrievalMethod
URI CDATA #REQUIRED
Type CDATA #IMPLIED >
4.4.4 The X509Data Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#X509Data" (this can be
used within a RetrievalMethod or Reference element to identify the
referent's type)
An X509Data element within KeyInfo contains one or more identifiers
of keys or X509 certificates (or certificates' identifiers or a
revocation list). The content of X509Data is:
1. At least one element, from the following set of element types; any
of these may appear together or more than once if (if and only if)
each instance describes or is related to the same certificate:
2.
o The X509IssuerSerial element, which contains an X.509 issuer
distinguished name/serial number pair that SHOULD be compliant
with RFC 2253 [LDAP-DN],
o The X509SubjectName element, which contains an X.509 subject
distinguished name that SHOULD be compliant with RFC 2253
[LDAP-DN],
o The X509SKI element, which contains the base64 encoded plain
(i.e., non-DER-encoded) value of a X509 V.3
SubjectKeyIdentifier extension.
o The X509Certificate element, which contains a base64-encoded
[X509v3] certificate, and
o Elements from an external namespace which
accompanies/complements any of the elements above.
o The X509CRL element, which contains a base64-encoded
certificate revocation list (CRL) [X509v3].
Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
appear MUST refer to the certificate or certificates containing the
validation key. All such elements that refer to a particular
individual certificate MUST be grouped inside a single X509Data
element and if the certificate to which they refer appears, it MUST
also be in that X509Data element.
Any X509IssuerSerial, X509SKI, and X509SubjectName elements that
relate to the same key but different certificates MUST be grouped
within a single KeyInfo but MAY occur in multiple X509Data elements.
All certificates appearing in an X509Data element MUST relate to the
validation key by either containing it or being part of a
certification chain that terminates in a certificate containing the
validation key.
No ordering is implied by the above constraints. The comments in the
following instance demonstrate these constraints:
<KeyInfo>
<X509Data> <!-- two pointers to certificate-A -->
<X509IssuerSerial>
<X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data><!-- single pointer to certificate-B -->
<X509SubjectName>Subject of Certificate B</X509SubjectName>
</X509Data>
<X509Data> <!-- certificate chain -->
<!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
<X509Certificate>MIICXTCCA..</X509Certificate>
<!-- Intermediate cert subject CN=arbolCA,OU=FVT,O=IBM,C=US
issuer CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICPzCCA...</X509Certificate>
<!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICSTCCA...</X509Certificate>
</X509Data>
</KeyInfo>
Note, there is no direct provision for a PKCS#7 encoded "bag" of
certificates or CRLs. However, a set of certificates and CRLs can
occur within an X509Data element and multiple X509Data elements can
occur in a KeyInfo. Whenever multiple certificates occur in an
X509Data element, at least one such certificate must contain the
public key which verifies the signature.
Also, strings in DNames (X509IssuerSerial,X509SubjectName, and
KeyNameif appropriate) should be encoded as follows:
* Consider the string as consisting of Unicode characters.
* Escape occurrences of the following special characters by
prefixing it with the "\" character: a "#" character occurring
at the beginning of the string or one of the characters ",",
"+", """, "\", "<", ">" or ";"
* Escape all occurrences of ASCII control characters (Unicode
range \x00 - \x 1f) by replacing them with "\" followed by a
two digit hex number showing its Unicode number.
* Escape any trailing white space by replacing "\ " with "\20".
* Since a XML document logically consists of characters, not
octets, the resulting Unicode string is finally encoded
according to the character encoding used for producing the
physical representation of the XML document.
Schema Definition:
<element name="X509Data" type="ds:X509DataType"/>
<complexType name="X509DataType">
<sequence maxOccurs="unbounded">
<choice>
<element name="X509IssuerSerial"
type="ds:X509IssuerSerialType"/>
<element name="X509SKI" type="base64Binary"/>
<element name="X509SubjectName" type="string"/>
<element name="X509Certificate" type="base64Binary"/>
<element name="X509CRL" type="base64Binary"/>
<any namespace="##other" processContents="lax"/>
</choice>
</sequence>
</complexType>
<complexType name="X509IssuerSerialType">
<sequence>
<element name="X509IssuerName" type="string"/>
<element name="X509SerialNumber" type="integer"/>
</sequence>
</complexType>
DTD:
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName
| X509Certificate | X509CRL)+ %X509.ANY;)>
<!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) >
<!ELEMENT X509IssuerName (#PCDATA) >
<!ELEMENT X509SubjectName (#PCDATA) >
<!ELEMENT X509SerialNumber (#PCDATA) >
<!ELEMENT X509SKI (#PCDATA) >
<!ELEMENT X509Certificate (#PCDATA) >
<!ELEMENT X509CRL (#PCDATA) >
<!-- Note, this DTD and schema permit X509Data to be empty; this is
precluded by the text in KeyInfo Element (section 4.4) which states
that at least one element from the dsig namespace should be present
in the PGP, SPKI, and X509 structures. This is easily expressed for
the other key types, but not for X509Data because of its rich
structure. -->
4.4.5 The PGPData Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#PGPData" (this can be used
within a RetrievalMethod or Reference element to identify the
referent's type)
The PGPData element within KeyInfo is used to convey information
related to PGP public key pairs and signatures on such keys. The
PGPKeyID's value is a base64Binary sequence containing a standard PGP
public key identifier as defined in [PGP, section 11.2]. The
PGPKeyPacket contains a base64-encoded Key Material Packet as defined
in [PGP, section 5.5]. These children element types can be
complemented/extended by siblings from an external namespace within
PGPData, or PGPData can be replaced all together with an alternative
PGP XML structure as a child of KeyInfo. PGPData must contain one
PGPKeyID and/or one PGPKeyPacket and 0 or more elements from an
external namespace.
Schema Definition:
<element name="PGPData" type="ds:PGPDataType"/>
<complexType name="PGPDataType">
<choice>
<sequence>
<element name="PGPKeyID" type="base64Binary"/>
<element name="PGPKeyPacket" type="base64Binary"
minOccurs="0"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
<sequence>
<element name="PGPKeyPacket" type="base64Binary"/>
<any namespace="##other" processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</sequence>
</choice>
</complexType>
DTD:
<!ELEMENT PGPData ((PGPKeyID, PGPKeyPacket?) | (PGPKeyPacket)
%PGPData.ANY;) >
<!ELEMENT PGPKeyPacket (#PCDATA) >
<!ELEMENT PGPKeyID (#PCDATA) >
4.4.6 The SPKIData Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#SPKIData" (this can be
used within a RetrievalMethod or Reference element to identify the
referent's type)
The SPKIData element within KeyInfo is used to convey information
related to SPKI public key pairs, certificates and other SPKI data.
SPKISexp is the base64 encoding of a SPKI canonical S-expression.
SPKIData must have at least one SPKISexp; SPKISexp can be
complemented/extended by siblings from an external namespace within
SPKIData, or SPKIData can be entirely replaced with an alternative
SPKI XML structure as a child of KeyInfo.
Schema Definition:
<element name="SPKIData" type="ds:SPKIDataType"/>
<complexType name="SPKIDataType">
<sequence maxOccurs="unbounded">
<element name="SPKISexp" type="base64Binary"/>
<any namespace="##other" processContents="lax" minOccurs="0"/>
</sequence>
</complexType>
DTD:
<!ELEMENT SPKIData (SPKISexp %SPKIData.ANY;) >
<!ELEMENT SPKISexp (#PCDATA) >
4.4.7 The MgmtData Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#MgmtData" (this can be
used within a RetrievalMethod or Reference element to identify the
referent's type)
The MgmtData element within KeyInfo is a string value used to convey
in-band key distribution or agreement data. For example, DH key
exchange, RSA key encryption, etc. Use of this element is NOT
RECOMMENDED. It provides a syntactic hook where in-band key
distribution or agreement data can be placed. However, superior
interoperable child elements of KeyInfo for the transmission of
encrypted keys and for key agreement are being specified by the W3C
XML Encryption Working Group and they should be used instead of
MgmtData.
Schema Definition:
<element name="MgmtData" type="string"/>
DTD:
<!ELEMENT MgmtData (#PCDATA)>
4.5 The Object Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#Object" (this can be used
within a Reference element to identify the referent's type)
Object is an optional element that may occur one or more times. When
present, this element may contain any data. The Object element may
include optional MIME type, ID, and encoding attributes.
The Object's Encoding attributed may be used to provide a URI that
identifies the method by which the object is encoded (e.g., a binary
file).
The MimeType attribute is an optional attribute which describes the
data within the Object (independent of its encoding). This is a
string with values defined by [MIME]. For example, if the Object
contains base64 encoded PNG, the Encoding may be specified as
'base64' and the MimeType as 'image/png'. This attribute is purely
advisory; no validation of the MimeType information is required by
this specification. Applications which require normative type and
encoding information for signature validation should specify
Transforms with well defined resulting types and/or encodings.
The Object's Id is commonly referenced from a Reference in
SignedInfo, or Manifest. This element is typically used for
enveloping signatures where the object being signed is to be included
in the signature element. The digest is calculated over the entire
Object element including start and end tags.
Note, if the application wishes to exclude the <Object> tags from the
digest calculation, the Reference must identify the actual data
object (easy for XML documents) or a transform must be used to remove
the Object tags (likely where the data object is non-XML). Exclusion
of the object tags may be desired for cases where one wants the
signature to remain valid if the data object is moved from inside a
signature to outside the signature (or vice versa), or where the
content of the Object is an encoding of an original binary document
and it is desired to extract and decode so as to sign the original
bitwise representation.
Schema Definition:
<element name="Object" type="ds:ObjectType"/>
<complexType name="ObjectType" mixed="true">
<sequence minOccurs="0" maxOccurs="unbounded">
<any namespace="##any" processContents="lax"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
<attribute name="MimeType" type="string" use="optional"/>
<attribute name="Encoding" type="anyURI" use="optional"/>
</complexType>
DTD:
<!ELEMENT Object (#PCDATA|Signature|SignatureProperties|Manifest
%Object.ANY;)* >
<!ATTLIST Object
Id ID #IMPLIED
MimeType CDATA #IMPLIED
Encoding CDATA #IMPLIED >
5.0 Additional Signature Syntax
This section describes the optional to implement Manifest and
SignatureProperties elements and describes the handling of XML
processing instructions and comments. With respect to the elements
Manifest and SignatureProperties, this section specifies syntax and
little behavior -- it is left to the application. These elements can
appear anywhere the parent's content model permits; the Signature
content model only permits them within Object.
5.1 The Manifest Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#Manifest" (this can be
used within a Reference element to identify the referent's type)
The Manifest element provides a list of References. The difference
from the list in SignedInfo is that it is application defined which,
if any, of the digests are actually checked against the objects
referenced and what to do if the object is inaccessible or the digest
compare fails. If a Manifest is pointed to from SignedInfo, the
digest over the Manifest itself will be checked by the core signature
validation behavior. The digests within such a Manifest are checked
at the application's discretion. If a Manifest is referenced from
another Manifest, even the overall digest of this two level deep
Manifest might not be checked.
Schema Definition:
<element name="Manifest" type="ds:ManifestType"/>
<complexType name="ManifestType">
<sequence>
<element ref="ds:Reference" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
DTD:
<!ELEMENT Manifest (Reference+) >
<!ATTLIST Manifest
Id ID #IMPLIED >
5.2 The SignatureProperties Element
Identifier
Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties" (this
can be used within a Reference element to identify the referent's
type)
Additional information items concerning the generation of the
signature(s) can be placed in a SignatureProperty element (i.e.,
date/time stamp or the serial number of cryptographic hardware used
in signature generation).
Schema Definition:
<element name="SignatureProperties"
type="ds:SignaturePropertiesType"/>
<complexType name="SignaturePropertiesType">
<sequence>
<element ref="ds:SignatureProperty" maxOccurs="unbounded"/>
</sequence>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
<element name="SignatureProperty"
type="ds:SignaturePropertyType"/>
<complexType name="SignaturePropertyType" mixed="true">
<choice maxOccurs="unbounded">
<any namespace="##other" processContents="lax"/>
<!-- (1,1) elements from (1,unbounded) namespaces -->
</choice>
<attribute name="Target" type="anyURI" use="required"/>
<attribute name="Id" type="ID" use="optional"/>
</complexType>
DTD:
<!ELEMENT SignatureProperties (SignatureProperty+) >
<!ATTLIST SignatureProperties
Id ID #IMPLIED >
<!ELEMENT SignatureProperty (#PCDATA %SignatureProperty.ANY;)* >
<!ATTLIST SignatureProperty
Target CDATA #REQUIRED
Id ID #IMPLIED >
5.3 Processing Instructions in Signature Elements
No XML processing instructions (PIs) are used by this specification.
Note that PIs placed inside SignedInfo by an application will be
signed unless the CanonicalizationMethod algorithm discards them.
(This is true for any signed XML content.) All of the
CanonicalizationMethods identified within this specification retain
PIs. When a PI is part of content that is signed (e.g., within
SignedInfo or referenced XML documents) any change to the PI will
obviously result in a signature failure.
5.4 Comments in Signature Elements
XML comments are not used by this specification.
Note that unless CanonicalizationMethod removes comments within
SignedInfo or any other referenced XML (which [XML-C14N] does), they
will be signed. Consequently, if they are retained, a change to the
comment will cause a signature failure. Similarly, the XML signature
over any XML data will be sensitive to comment changes unless a
comment-ignoring canonicalization/transform method, such as the
Canonical XML [XML-C14N], is specified.
6.0 Algorithms
This section identifies algorithms used with the XML digital
signature specification. Entries contain the identifier to be used
in Signature elements, a reference to the formal specification, and
definitions, where applicable, for the representation of keys and the
results of cryptographic operations.
6.1 Algorithm Identifiers and Implementation Requirements
Algorithms are identified by URIs that appear as an attribute to the
element that identifies the algorithms' role (DigestMethod,
Transform, SignatureMethod, or CanonicalizationMethod). All
algorithms used herein take parameters but in many cases the
parameters are implicit. For example, a SignatureMethod is
implicitly given two parameters: the keying info and the output of
CanonicalizationMethod. Explicit additional parameters to an
algorithm appear as content elements within the algorithm role
element. Such parameter elements have a descriptive element name,
which is frequently algorithm specific, and MUST be in the XML
Signature namespace or an algorithm specific namespace.
This specification defines a set of algorithms, their URIs, and
requirements for implementation. Requirements are specified over
implementation, not over requirements for signature use.
Furthermore, the mechanism is extensible; alternative algorithms may
be used by signature applications.
Digest
1. Required SHA1
http://www.w3.org/2000/09/xmldsig#sha1
Encoding
1. Required base64
http://www.w3.org/2000/09/xmldsig#base64
MAC
1. Required HMAC-SHA1
http://www.w3.org/2000/09/xmldsig#hmac-sha1
Signature
1. Required DSAwithSHA1 (DSS)
http://www.w3.org/2000/09/xmldsig#dsa-sha1
2. Recommended RSAwithSHA1
http://www.w3.org/2000/09/xmldsig#rsa-sha1
Canonicalization
1. Required Canonical XML (omits comments)
http://www.w3.org/TR/2001/REC-xml-c14n-20010315
2. Recommended Canonical XML with Comments
http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments
Transform
1. Optional XSLT
http://www.w3.org/TR/1999/REC-xslt-19991116
2. Recommended XPath
http://www.w3.org/TR/1999/REC-xpath-19991116
3. Required Enveloped Signature*
http://www.w3.org/2000/09/xmldsig#enveloped-signature
* The Enveloped Signature transform removes the Signature element
from the calculation of the signature when the signature is within
the content that it is being signed. This MAY be implemented via the
RECOMMENDED XPath specification specified in 6.6.4: Enveloped
Signature Transform; it MUST have the same effect as that specified
by the XPath Transform.
6.2 Message Digests
Only one digest algorithm is defined herein. However, it is expected
that one or more additional strong digest algorithms will be
developed in connection with the US Advanced Encryption Standard
effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances
in cryptanalysis have cast doubt on its strength.
6.2.1 SHA-1
Identifier:
http://www.w3.org/2000/09/xmldsig#sha1
The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example
of an SHA-1 DigestAlg element is:
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
A SHA-1 digest is a 160-bit string. The content of the DigestValue
element shall be the base64 encoding of this bit string viewed as a
20-octet octet stream. For example, the DigestValue element for the
message digest:
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be:
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
6.3 Message Authentication Codes
MAC algorithms take two implicit parameters, their keying material
determined from KeyInfo and the octet stream output by
CanonicalizationMethod. MACs and signature algorithms are
syntactically identical but a MAC implies a shared secret key.
6.3.1 HMAC
Identifier:
http://www.w3.org/2000/09/xmldsig#hmac-sha1
The HMAC algorithm (RFC2104 [HMAC]) takes the truncation length in
bits as a parameter; if the parameter is not specified then all the
bits of the hash are output. An example of an HMAC SignatureMethod
element:
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1">
<HMACOutputLength>128</HMACOutputLength>
</SignatureMethod>
The output of the HMAC algorithm is ultimately the output (possibly
truncated) of the chosen digest algorithm. This value shall be
base64 encoded in the same straightforward fashion as the output of
the digest algorithms. Example: the SignatureValue element for the
HMAC-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [HMAC] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
Schema Definition:
<simpleType name="HMACOutputLengthType">
<restriction base="integer"/>
</simpleType>
DTD:
<!ELEMENT HMACOutputLength (#PCDATA)>
6.4 Signature Algorithms
Signature algorithms take two implicit parameters, their keying
material determined from KeyInfo and the octet stream output by
CanonicalizationMethod. Signature and MAC algorithms are
syntactically identical but a signature implies public key
cryptography.
6.4.1 DSA
Identifier:
http://www.w3.org/2000/09/xmldsig#dsa-sha1
The DSA algorithm [DSS] takes no explicit parameters. An example of
a DSA SignatureMethod element is:
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
The output of the DSA algorithm consists of a pair of integers
usually referred by the pair (r, s). The signature value consists of
the base64 encoding of the concatenation of two octet-streams that
respectively result from the octet-encoding of the values r and s in
that order. Integer to octet-stream conversion must be done
according to the I2OSP operation defined in the RFC 2437 [PKCS1]
specification with a l parameter equal to 20. For example, the
SignatureValue element for a DSA signature (r, s) with values
specified in hexadecimal:
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
<SignatureValue>
i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==
</SignatureValue>
6.4.2 PKCS1 (RSA-SHA1)
Identifier:
http://www.w3.org/2000/09/xmldsig#rsa-sha1
The expression "RSA algorithm" as used in this document refers to the
RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. The RSA
algorithm takes no explicit parameters. An example of an RSA
SignatureMethod element is:
<SignatureMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
The SignatureValue content for an RSA signature is the base64 [MIME]
encoding of the octet string computed as per RFC 2437 [PKCS1, section
8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature
scheme]. As specified in the EMSA-PKCS1-V1_5-ENCODE function RFC
2437 [PKCS1, section 9.2.1], the value input to the signature
function MUST contain a pre-pended algorithm object identifier for
the hash function, but the availability of an ASN.1 parser and
recognition of OIDs are not required of a signature verifier. The
PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
01 | FF* | 00 | prefix | hash
where "|" is concatenation, "01", "FF", and "00" are fixed octets of
the corresponding hexadecimal value, "hash" is the SHA1 digest of the
data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix
required in PKCS1 [RFC 2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard
cryptographic libraries. The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being CRYPTed is
one octet shorter than the RSA modulus.
The resulting base64 [MIME] string is the value of the child text
node of the SignatureValue element, e.g.,
<SignatureValue>
IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639
In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw=
</SignatureValue>
6.5 Canonicalization Algorithms
If canonicalization is performed over octets, the canonicalization
algorithms take two implicit parameters: the content and its charset.
The charset is derived according to the rules of the transport
protocols and media types (e.g., RFC2376 [XML-MT] defines the media
types for XML). This information is necessary to correctly sign and
verify documents and often requires careful server side
configuration.
Various canonicalization algorithms require conversion to [UTF-8].
The two algorithms below understand at least [UTF-8] and [UTF-16] as
input encodings. We RECOMMEND that externally specified algorithms
do the same. Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode
encoding to Unicode. The two algorithms below perform text
normalization during transcoding [NFC, NFC-Corrigendum]. We
RECOMMEND that externally specified canonicalization algorithms do
the same. (Note, there can be ambiguities in converting existing
charsets to Unicode, for an example see the XML Japanese Profile
[XML-Japanese] Note.)
6.5.1 Canonical XML
Identifier for REQUIRED Canonical XML (omits comments):
http://www.w3.org/TR/2001/REC-xml-c14n-20010315
Identifier for Canonical XML with Comments:
http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments
An example of an XML canonicalization element is:
<CanonicalizationMethod
Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
The normative specification of Canonical XML is [XML-C14N]. The
algorithm is capable of taking as input either an octet stream or an
XPath node-set (or sufficiently functional alternative). The
algorithm produces an octet stream as output. Canonical XML is
easily parameterized (via an additional URI) to omit or retain
comments.
6.6 Transform Algorithms
A Transform algorithm has a single implicit parameter: an octet
stream from the Reference or the output of an earlier Transform.
Application developers are strongly encouraged to support all
transforms listed in this section as RECOMMENDED unless the
application environment has resource constraints that would make such
support impractical. Compliance with this recommendation will
maximize application interoperability and libraries should be
available to enable support of these transforms in applications
without extensive development.
6.6.1 Canonicalization
Any canonicalization algorithm that can be used for
CanonicalizationMethod (such as those in Canonicalization Algorithms
(section 6.5)) can be used as a Transform.
6.6.2 Base64
Identifiers:
http://www.w3.org/2000/09/xmldsig#base64
The normative specification for base64 decoding transforms is [MIME].
The base64 Transform element has no content. The input is decoded by
the algorithms. This transform is useful if an application needs to
sign the raw data associated with the encoded content of an element.
This transform requires an octet stream for input. If an XPath
node-set (or sufficiently functional alternative) is given as input,
then it is converted to an octet stream by performing operations
logically equivalent to 1) applying an XPath transform with
expression self::text(), then 2) taking the string-value of the
node-set. Thus, if an XML element is identified by a barename
XPointer in the Reference URI, and its content consists solely of
base64 encoded character data, then this transform automatically
strips away the start and end tags of the identified element and any
of its descendant elements as well as any descendant comments and
processing instructions. The output of this transform is an octet
stream.
6.6.3 XPath Filtering
Identifier:
http://www.w3.org/TR/1999/REC-xpath-19991116
The normative specification for XPath expression evaluation is
[XPath]. The XPath expression to be evaluated appears as the
character content of a transform parameter child element named XPath.
The input required by this transform is an XPath node-set. Note that
if the actual input is an XPath node-set resulting from a null URI or
barename XPointer dereference, then comment nodes will have been
omitted. If the actual input is an octet stream, then the
application MUST convert the octet stream to an XPath node-set
suitable for use by Canonical XML with Comments. (A subsequent
application of the REQUIRED Canonical XML algorithm would strip away
these comments.) In other words, the input node-set should be
equivalent to the one that would be created by the following process:
1. Initialize an XPath evaluation context by setting the initial node
equal to the input XML document's root node, and set the context
position and size to 1.
2. Evaluate the XPath expression (//. | //@* | //namespace::*)
The evaluation of this expression includes all of the document's
nodes (including comments) in the node-set representing the octet
stream.
The transform output is also an XPath node-set. The XPath expression
appearing in the XPath parameter is evaluated once for each node in
the input node-set. The result is converted to a boolean. If the
boolean is true, then the node is included in the output node-set.
If the boolean is false, then the node is omitted from the output
node-set.
Note: Even if the input node-set has had comments removed, the
comment nodes still exist in the underlying parse tree and can
separate text nodes. For example, the markup <e>Hello, <!-- comment
-->world!</e> contains two text nodes. Therefore, the expression
self::text()[string()="Hello, world!"] would fail. Should this
problem arise in the application, it can be solved by either
canonicalizing the document before the XPath transform to physically
remove the comments or by matching the node based on the parent
element's string value (e.g., by using the expression
self::text()[string(parent::e)="Hello, world!"]).
The primary purpose of this transform is to ensure that only
specifically defined changes to the input XML document are permitted
after the signature is affixed. This is done by omitting precisely
those nodes that are allowed to change once the signature is affixed,
and including all other input nodes in the output. It is the
responsibility of the XPath expression author to include all nodes
whose change could affect the interpretation of the transform output
in the application context.
An important scenario would be a document requiring two enveloped
signatures. Each signature must omit itself from its own digest
calculations, but it is also necessary to exclude the second
signature element from the digest calculations of the first signature
so that adding the second signature does not break the first
signature.
The XPath transform establishes the following evaluation context for
each node of the input node-set:
* A context node equal to a node of the input node-set.
* A context position, initialized to 1.
* A context size, initialized to 1.
* A library of functions equal to the function set defined in
[XPath] plus a function named here.
* A set of variable bindings. No means for initializing these is
defined. Thus, the set of variable bindings used when
evaluating the XPath expression is empty, and use of a variable
reference in the XPath expression results in an error.
* The set of namespace declarations in scope for the XPath
expression.
As a result of the context node setting, the XPath expressions
appearing in this transform will be quite similar to those used in
[XSLT], except that the size and position are always 1 to reflect the
fact that the transform is automatically visiting every node (in
XSLT, one recursively calls the command apply-templates to visit the
nodes of the input tree).
The function here() is defined as follows:
Function: node-set here()
The here function returns a node-set containing the attribute or
processing instruction node or the parent element of the text node
that directly bears the XPath expression. This expression results in
an error if the containing XPath expression does not appear in the
same XML document against which the XPath expression is being
evaluated.
As an example, consider creating an enveloped signature (a Signature
element that is a descendant of an element being signed). Although
the signed content should not be changed after signing, the elements
within the Signature element are changing (e.g., the digest value
must be put inside the DigestValue and the SignatureValue must be
subsequently calculated). One way to prevent these changes from
invalidating the digest value in DigestValue is to add an XPath
Transform that omits all Signature elements and their descendants.
For example,
<Document>
...
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
...
<Reference URI="">
<Transforms>
<Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
<XPath xmlns:dsig="&dsig;">
not(ancestor-or-self::dsig:Signature)
</XPath>
</Transform>
</Transforms>
<DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<DigestValue></DigestValue>
</Reference>
</SignedInfo>
<SignatureValue></SignatureValue>
</Signature>
...
</Document>
Due to the null Reference URI in this example, the XPath transform
input node-set contains all nodes in the entire parse tree starting
at the root node (except the comment nodes). For each node in this
node-set, the node is included in the output node-set except if the
node or one of its ancestors, has a tag of Signature that is in the
namespace given by the replacement text for the entity &dsig;.
A more elegant solution uses the here function to omit only the
Signature containing the XPath Transform, thus allowing enveloped
signatures to sign other signatures. In the example above, use the
XPath element:
<XPath xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
Since the XPath equality operator converts node sets to string values
before comparison, we must instead use the XPath union operator (|).
For each node of the document, the predicate expression is true if
and only if the node-set containing the node and its Signature
element ancestors does not include the enveloped Signature element
containing the XPath expression (the union does not produce a larger
set if the enveloped Signature element is in the node-set given by
ancestor-or-self::Signature).
6.6.4 Enveloped Signature Transform
Identifier:
http://www.w3.org/2000/09/xmldsig#enveloped-signature
An enveloped signature transform T removes the whole Signature
element containing T from the digest calculation of the Reference
element containing T. The entire string of characters used by an XML
processor to match the Signature with the XML production element is
removed. The output of the transform is equivalent to the output
that would result from replacing T with an XPath transform containing
the following XPath parameter element:
<XPath xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to
those of the XPath transform, but may only be applied to a node-set
from its parent XML document. Note that it is not necessary to use
an XPath expression evaluator to create this transform. However,
this transform MUST produce output in exactly the same manner as the
XPath transform parameterized by the XPath expression above.
6.6.5 XSLT Transform
Identifier:
http://www.w3.org/TR/1999/REC-xslt-19991116
The normative specification for XSL Transformations is [XSLT].
Specification of a namespace-qualified stylesheet element, which MUST
be the sole child of the Transform element, indicates that the
specified style sheet should be used. Whether this instantiates in-
line processing of local XSLT declaration within the resource is
determined by the XSLT processing model; the ordered application of
multiple stylesheet may require multiple Transforms. No special
provision is made for the identification of a remote stylesheet at a
given URI because it can be communicated via an xsl:include or
xsl:import within the stylesheet child of the Transform.
This transform requires an octet stream as input. If the actual
input is an XPath node-set, then the signature application should
attempt to convert it to octets (apply Canonical XML]) as described
in the Reference Processing Model (section 4.3.3.2).
The output of this transform is an octet stream. The processing
rules for the XSL style sheet or transform element are stated in the
XSLT specification [XSLT]. We RECOMMEND that XSLT transform authors
use an output method of xml for XML and HTML. As XSLT
implementations do not produce consistent serializations of their
output, we further RECOMMEND inserting a transform after the XSLT
transform to canonicalize the output. These steps will help to
ensure interoperability of the resulting signatures among
applications that support the XSLT transform. Note that if the
output is actually HTML, then the result of these steps is logically
equivalent [XHTML].
7. XML Canonicalization and Syntax Constraint Considerations
Digital signatures only work if the verification calculations are
performed on exactly the same bits as the signing calculations. If
the surface representation of the signed data can change between
signing and verification, then some way to standardize the changeable
aspect must be used before signing and verification. For example,
even for simple ASCII text there are at least three widely used line
ending sequences. If it is possible for signed text to be modified
from one line ending convention to another between the time of
signing and signature verification, then the line endings need to be
canonicalized to a standard form before signing and verification or
the signatures will break.
XML is subject to surface representation changes and to processing
which discards some surface information. For this reason, XML
digital signatures have a provision for indicating canonicalization
methods in the signature so that a verifier can use the same
canonicalization as the signer.
Throughout this specification we distinguish between the
canonicalization of a Signature element and other signed XML data
objects. It is possible for an isolated XML document to be treated
as if it were binary data so that no changes can occur. In that
case, the digest of the document will not change and it need not be
canonicalized if it is signed and verified as such. However, XML
that is read and processed using standard XML parsing and processing
techniques is frequently changed such that some of its surface
representation information is lost or modified. In particular, this
will occur in many cases for the Signature and enclosed SignedInfo
elements since they, and possibly an encompassing XML document, will
be processed as XML.
Similarly, these considerations apply to Manifest, Object, and
SignatureProperties elements if those elements have been digested,
their DigestValue is to be checked, and they are being processed as
XML.
The kinds of changes in XML that may need to be canonicalized can be
divided into four categories. There are those related to the basic
[XML], as described in 7.1 below. There are those related to [DOM],
[SAX], or similar processing as described in 7.2 below. Third, there
is the possibility of coded character set conversion, such as between
UTF-8 and UTF-16, both of which all [XML] compliant processors are
required to support, which is described in the paragraph immediately
below. And, fourth, there are changes that related to namespace
declaration and XML namespace attribute context as described in 7.3
below.
Any canonicalization algorithm should yield output in a specific
fixed coded character set. All canonicalization algorithms
identified in this document use UTF-8 (without a byte order mark
(BOM)) and do not provide character normalization. We RECOMMEND that
signature applications create XML content (Signature elements and
their descendents/content) in Normalization Form C [NFC, NFC-
Corrigendum] and check that any XML being consumed is in that form as
well; (if not, signatures may consequently fail to validate).
Additionally, none of these algorithms provide data type
normalization. Applications that normalize data types in varying
formats (e.g., (true, false) or (1,0)) may not be able to validate
each other's signatures.
7.1 XML 1.0, Syntax Constraints, and Canonicalization
XML 1.0 [XML] defines an interface where a conformant application
reading XML is given certain information from that XML and not other
information. In particular,
1. line endings are normalized to the single character #xA by
dropping #xD characters if they are immediately followed by a #xA
and replacing them with #xA in all other cases,
2. missing attributes declared to have default values are provided to
the application as if present with the default value,
3. character references are replaced with the corresponding
character,
4. entity references are replaced with the corresponding declared
entity,
5. attribute values are normalized by
5.1 replacing character and entity references as above,
5.2 replacing occurrences of #x9, #xA, and #xD with #x20 (space)
except that the sequence #xD#xA is replaced by a single space,
and
5.3 if the attribute is not declared to be CDATA, stripping all
leading and trailing spaces and replacing all interior runs of
spaces with a single space.
Note that items (2), (4), and (5.3) depend on the presence of a
schema, DTD or similar declarations. The Signature element type is
laxly schema valid [XML-schema], consequently external XML or even
XML within the same document as the signature may be (only) well-
formed or from another namespace (where permitted by the signature
schema); the noted items may not be present. Thus, a signature with
such content will only be verifiable by other signature applications
if the following syntax constraints are observed when generating any
signed material including the SignedInfo element:
1. attributes having default values be explicitly present,
2. all entity references (except "amp", "lt", "gt", "apos", "quot",
and other character entities not representable in the encoding
chosen) be expanded,
3. attribute value white space be normalized
7.2 DOM/SAX Processing and Canonicalization
In addition to the canonicalization and syntax constraints discussed
above, many XML applications use the Document Object Model [DOM] or
the Simple API for XML [SAX]. DOM maps XML into a tree structure of
nodes and typically assumes it will be used on an entire document
with subsequent processing being done on this tree. SAX converts XML
into a series of events such as a start tag, content, etc. In either
case, many surface characteristics such as the ordering of attributes
and insignificant white space within start/end tags is lost. In
addition, namespace declarations are mapped over the nodes to which
they apply, losing the namespace prefixes in the source text and, in
most cases, losing where namespace declarations appeared in the
original instance.
If an XML Signature is to be produced or verified on a system using
DOM or SAX processing, a canonical method is needed to serialize the
relevant part of a DOM tree or sequence of SAX events. XML
canonicalization specifications, such as [XML-C14N], are based only
on information which is preserved by DOM and SAX. For an XML
Signature to be verifiable by an implementation using DOM or SAX, not
only must the XML 1.0 syntax constraints given in the previous
section be followed, but an appropriate XML canonicalization MUST be
specified so that the verifier can re-serialize DOM/SAX mediated
input into the same octet stream that was signed.
7.3 Namespace Context and Portable Signatures
In [XPath] and consequently the Canonical XML data model an element
has namespace nodes that correspond to those declarations within the
element and its ancestors:
"Note: An element E has namespace nodes that represent its
namespace declarations as well as any namespace declarations made
by its ancestors that have not been overridden in E's
declarations, the default namespace if it is non-empty, and the
declaration of the prefix xml." [XML-C14N]
When serializing a Signature element or signed XML data that's the
child of other elements using these data models, that Signature
element and its children, may contain namespace declarations from its
ancestor context. In addition, the Canonical XML and Canonical XML
with Comments algorithms import all xml namespace attributes (such as
xml:lang) from the nearest ancestor in which they are declared to the
apex node of canonicalized XML unless they are already declared at
that node. This may frustrate the intent of the signer to create a
signature in one context which remains valid in another. For
example, given a signature which is a child of B and a grandchild of
A:
<A xmlns:n1="&foo;">
<B xmlns:n2="&bar;">
<Signature xmlns="&dsig;"> ...
<Reference URI="#signme"/> ...
</Signature>
<C ID="signme" xmlns="&baz;"/>
</B>
</A>
when either the element B or the signed element C is moved into a
[SOAP] envelope for transport:
<SOAP:Envelope
xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/">
...
<SOAP:Body>
<B xmlns:n2="&bar;">
<Signature xmlns="&dsig;">
...
</Signature>
<C ID="signme" xmlns="&baz;"/>
</B>
</SOAP:Body>
</SOAP:Envelope>
The canonical form of the signature in this context will contain new
namespace declarations from the SOAP:Envelope context, invalidating
the signature. Also, the canonical form will lack namespace
declarations it may have originally had from element A's context,
also invalidating the signature. To avoid these problems, the
application may:
1. Rely upon the enveloping application to properly divorce its body
(the signature payload) from the context (the envelope) before the
signature is validated. Or,
2. Use a canonicalization method that "repels/excludes" instead of
"attracts" ancestor context. [XML-C14N] purposefully attracts
such context.
8.0 Security Considerations
The XML Signature specification provides a very flexible digital
signature mechanism. Implementors must give consideration to their
application threat models and to the following factors.
8.1 Transforms
A requirement of this specification is to permit signatures to "apply
to a part or totality of a XML document." (See [XML-Signature-RD,
section 3.1.3].) The Transforms mechanism meets this requirement by
permitting one to sign data derived from processing the content of
the identified resource. For instance, applications that wish to
sign a form, but permit users to enter a limited field data without
invalidating a previous signature on the form might use [XPath] to
exclude those portions the user needs to change. Transforms may be
arbitrarily specified and may include encoding transforms,
canonicalization instructions or even XSLT transformations. Three
cautions are raised with respect to this feature in the following
sections.
Note, core validation behavior does not confirm that the signed data
was obtained by applying each step of the indicated transforms.
(Though it does check that the digest of the resulting content
matches that specified in the signature.) For example, some
applications may be satisfied with verifying an XML signature over a
cached copy of already transformed data. Other applications might
require that content be freshly dereferenced and transformed.
8.1.1 Only What is Signed is Secure
First, obviously, signatures over a transformed document do not
secure any information discarded by transforms: only what is signed
is secure.
Note that the use of Canonical XML [XML-C14N] ensures that all
internal entities and XML namespaces are expanded within the content
being signed. All entities are replaced with their definitions and
the canonical form explicitly represents the namespace that an
element would otherwise inherit. Applications that do not
canonicalize XML content (especially the SignedInfo element) SHOULD
NOT use internal entities and SHOULD represent the namespace
explicitly within the content being signed since they cannot rely
upon canonicalization to do this for them. Also, users concerned
with the integrity of the element type definitions associated with
the XML instance being signed may wish to sign those definitions as
well (i.e., the schema, DTD, or natural language description
associated with the namespace/identifier).
Second, an envelope containing signed information is not secured by
the signature. For instance, when an encrypted envelope contains a
signature, the signature does not protect the authenticity or
integrity of unsigned envelope headers nor its ciphertext form, it
only secures the plaintext actually signed.
8.1.2 Only What is 'Seen' Should be Signed
Additionally, the signature secures any information introduced by the
transform: only what is "seen" (that which is represented to the user
via visual, auditory or other media) should be signed. If signing is
intended to convey the judgment or consent of a user (an automated
mechanism or person), then it is normally necessary to secure as
exactly as practical the information that was presented to that user.
Note that this can be accomplished by literally signing what was
presented, such as the screen images shown a user. However, this may
result in data which is difficult for subsequent software to
manipulate. Instead, one can sign the data along with whatever
filters, style sheets, client profile or other information that
affects its presentation.
8.1.3 'See' What is Signed
Just as a user should only sign what he or she "sees," persons and
automated mechanism that trust the validity of a transformed document
on the basis of a valid signature should operate over the data that
was transformed (including canonicalization) and signed, not the
original pre-transformed data. This recommendation applies to
transforms specified within the signature as well as those included
as part of the document itself. For instance, if an XML document
includes an embedded style sheet [XSLT] it is the transformed
document that should be represented to the user and signed. To meet
this recommendation where a document references an external style
sheet, the content of that external resource should also be signed
via a signature Reference, otherwise the content of that external
content might change which alters the resulting document without
invalidating the signature.
Some applications might operate over the original or intermediary
data but should be extremely careful about potential weaknesses
introduced between the original and transformed data. This is a
trust decision about the character and meaning of the transforms that
an application needs to make with caution. Consider a
canonicalization algorithm that normalizes character case (lower to
upper) or character composition ('e and accent' to 'accented-e'). An
adversary could introduce changes that are normalized and
consequently inconsequential to signature validity but material to a
DOM processor. For instance, by changing the case of a character one
might influence the result of an XPath selection. A serious risk is
introduced if that change is normalized for signature validation but
the processor operates over the original data and returns a different
result than intended.
As a result:
* All documents operated upon and generated by signature
applications MUST be in [NFC, NFC-Corrigendum] (otherwise
intermediate processors might unintentionally break the
signature)
* Encoding normalizations SHOULD NOT be done as part of a
signature transform, or (to state it another way) if
normalization does occur, the application SHOULD always "see"
(operate over) the normalized form.
8.2 Check the Security Model
This specification uses public key signatures and keyed hash
authentication codes. These have substantially different security
models. Furthermore, it permits user specified algorithms which may
have other models.
With public key signatures, any number of parties can hold the public
key and verify signatures while only the parties with the private key
can create signatures. The number of holders of the private key
should be minimized and preferably be one. Confidence by verifiers
in the public key they are using and its binding to the entity or
capabilities represented by the corresponding private key is an
important issue, usually addressed by certificate or online authority
systems.
Keyed hash authentication codes, based on secret keys, are typically
much more efficient in terms of the computational effort required but
have the characteristic that all verifiers need to have possession of
the same key as the signer. Thus any verifier can forge signatures.
This specification permits user provided signature algorithms and
keying information designators. Such user provided algorithms may
have different security models. For example, methods involving
biometrics usually depend on a physical characteristic of the
authorized user that can not be changed the way public or secret keys
can be and may have other security model differences.
8.3 Algorithms, Key Lengths, Certificates, Etc.
The strength of a particular signature depends on all links in the
security chain. This includes the signature and digest algorithms
used, the strength of the key generation [RANDOM] and the size of the
key, the security of key and certificate authentication and
distribution mechanisms, certificate chain validation policy,
protection of cryptographic processing from hostile observation and
tampering, etc.
Care must be exercised by applications in executing the various
algorithms that may be specified in an XML signature and in the
processing of any "executable content" that might be provided to such
algorithms as parameters, such as XSLT transforms. The algorithms
specified in this document will usually be implemented via a trusted
library, but even there perverse parameters might cause unacceptable
processing or memory demand. Even more care may be warranted with
application defined algorithms.
The security of an overall system will also depend on the security
and integrity of its operating procedures, its personnel, and on the
administrative enforcement of those procedures. All the factors
listed in this section are important to the overall security of a
system; however, most are beyond the scope of this specification.
9. Schema, DTD, Data Model, and Valid Examples
XML Signature Schema Instance
http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-core-
schema.xsd
Valid XML schema instance based on the 20001024 Schema/DTD
[XML-Schema].
XML Signature DTD
http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-core-
schema.dtd
RDF Data Model
http://www.w3.org/Signature/Drafts/xmldsig-core/xmldsig-datamodel-
20000112.gif
XML Signature Object Example
http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example.xml
A cryptographical fabricated XML example that includes foreign
content and validates under the schema, it also uses schemaLocation
to aid automated schema fetching and validation.
RSA XML Signature Example
http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example-
rsa.xml
An XML Signature example with generated cryptographic values by
Merlin Hughes and validated by Gregor Karlinger.
DSA XML Signature Example
http://www.w3.org/Signature/Drafts/xmldsig-core/signature-example-
dsa.xml
Similar to above but uses DSA.
10. Definitions
Authentication Code (Protected Checksum)
A value generated from the application of a shared key to a
message via a cryptographic algorithm such that it has the
properties of message authentication (and integrity) but not
signer authentication. Equivalent to protected checksum, "A
checksum that is computed for a data object by means that protect
against active attacks that would attempt to change the checksum
to make it match changes made to the data object." [SEC]
Authentication, Message
The property, given an authentication code/protected checksum,
that tampering with both the data and checksum, so as to introduce
changes while seemingly preserving integrity, are still detected.
"A signature should identify what is signed, making it
impracticable to falsify or alter either the signed matter or the
signature without detection." [Digital Signature Guidelines, ABA].
Authentication, Signer
The property of the identity of the signer is as claimed. "A
signature should indicate who signed a document, message or
record, and should be difficult for another person to produce
without authorization." [Digital Signature Guidelines, ABA] Note,
signer authentication is an application decision (e.g., does the
signing key actually correspond to a specific identity) that is
supported by, but out of the scope of, this specification.
Checksum
"A value that (a) is computed by a function that is dependent on
the contents of a data object and (b) is stored or transmitted
together with the object, for the purpose of detecting changes in
the data." [SEC]
Core
The syntax and processing defined by this specification, including
core validation. We use this term to distinguish other markup,
processing, and applications semantics from our own.
Data Object (Content/Document)
The actual binary/octet data being operated on (transformed,
digested, or signed) by an application -- frequently an HTTP
entity [HTTP]. Note that the proper noun Object designates a
specific XML element. Occasionally we refer to a data object as a
document or as a resource's content. The term element content is
used to describe the data between XML start and end tags [XML].
The term XML document is used to describe data objects which
conform to the XML specification [XML].
Integrity
"The property that data has not been changed, destroyed, or lost
in an unauthorized or accidental manner." [SEC] A simple checksum
can provide integrity from incidental changes in the data; message
authentication is similar but also protects against an active
attack to alter the data whereby a change in the checksum is
introduced so as to match the change in the data.
Object
An XML Signature element wherein arbitrary (non-core) data may be
placed. An Object element is merely one type of digital data (or
document) that can be signed via a Reference.
Resource
"A resource can be anything that has identity. Familiar examples
include an electronic document, an image, a service (e.g.,
'today's weather report for Los Angeles'), and a collection of
other resources.... The resource is the conceptual mapping to an
entity or set of entities, not necessarily the entity which
corresponds to that mapping at any particular instance in time.
Thus, a resource can remain constant even when its content---the
entities to which it currently corresponds---changes over time,
provided that the conceptual mapping is not changed in the
process." [URI] In order to avoid a collision of the term entity
within the URI and XML specifications, we use the term data
object, content or document to refer to the actual bits/octets
being operated upon.
Signature
Formally speaking, a value generated from the application of a
private key to a message via a cryptographic algorithm such that
it has the properties of integrity, message authentication and/or
signer authentication. (However, we sometimes use the term
signature generically such that it encompasses Authentication Code
values as well, but we are careful to make the distinction when
the property of signer authentication is relevant to the
exposition.) A signature may be (non-exclusively) described as
detached, enveloping, or enveloped.
Signature, Application
An application that implements the MANDATORY (REQUIRED/MUST)
portions of this specification; these conformance requirements are
over application behavior, the structure of the Signature element
type and its children (including SignatureValue) and the specified
algorithms.
Signature, Detached
The signature is over content external to the Signature element,
and can be identified via a URI or transform. Consequently, the
signature is "detached" from the content it signs. This
definition typically applies to separate data objects, but it also
includes the instance where the Signature and data object reside
within the same XML document but are sibling elements.
Signature, Enveloping
The signature is over content found within an Object element of
the signature itself. The Object (or its content) is identified
via a Reference (via a URI fragment identifier or transform).
Signature, Enveloped
The signature is over the XML content that contains the signature
as an element. The content provides the root XML document
element. Obviously, enveloped signatures must take care not to
include their own value in the calculation of the SignatureValue.
Transform
The processing of a data from its source to its derived form.
Typical transforms include XML Canonicalization, XPath, and XSLT.
Validation, Core
The core processing requirements of this specification requiring
signature validation and SignedInfo reference validation.
Validation, Reference
The hash value of the identified and transformed content,
specified by Reference, matches its specified DigestValue.
Validation, Signature
The SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified in
Core Validation (section 3.2).
Validation, Trust/Application
The application determines that the semantics associated with a
signature are valid. For example, an application may validate the
time stamps or the integrity of the signer key -- though this
behavior is external to this core specification.
Appendix: Changes from RFC 3075
Numerous minor editorial changes were made. In addition, the
following substantive changes have occurred based on interoperation
experience or other considerations:
1. Minor but incompatible changes in the representation of DSA keys.
In particular, the optionality of several fields was changed and
two fields were re-ordered.
2. Minor change in the X509Data KeyInfo structure to allow multiple
CRLs to be grouped with certificates and other X509 information.
Previously CRLs had to occur singly and each in a separate
X509Data structure.
3. Incompatible change in the type of PGPKeyID, which had previously
been string, to the more correct base64Binary since it is actually
a binary quantity.
4. Several warnings have been added. Of particular note, because it
reflects a problem actually encountered in use and is the only
warning added that has its own little section, is the warning of
canonicalization problems when the namespace context of signed
material changes.
References
[ABA] Digital Signature Guidelines.
http://www.abanet.org/scitech/ec/isc/dsgfree.html
[DOM] Document Object Model (DOM) Level 1 Specification.
W3C Recommendation. V. Apparao, S. Byrne, M.
Champion, S. Isaacs, I. Jacobs, A. Le Hors, G.
Nicol, J. Robie, R. Sutor, C. Wilson, L. Wood.
October 1998.
http://www.w3.org/TR/1998/REC-DOM-Level-1-
19981001/
[DSS] FIPS PUB 186-2 . Digital Signature Standard (DSS).
U.S. Department of Commerce/National Institute of
Standards and Technology.
http://csrc.nist.gov/publications/fips/fips186-
2/fips186-2.pdf
[HMAC] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC
2104, February 1997.
[HTTP] Fielding, R. Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC
2616, June 1999.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[LDAP-DN] Wahl, M., Kille, S. and T. Howes, "Lightweight
Directory Access Protocol (v3): UTF-8 String
Representation of Distinguished Names", RFC 2253,
December 1997.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[MIME] Freed, N. and N. Borenstein, "Multipurpose
Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", RFC 2045, November
1996.
[NFC] TR15, Unicode Normalization Forms. M. Davis, M.
Drst. Revision 18: November 1999.
http://www.unicode.org/unicode/reports/tr15/tr15-
18.html. NFC-Corrigendum Normalization
Corrigendum. The Unicode Consortium.
http://www.unicode.org/unicode/uni2errata/
Normalization_Corrigendum.html.
[PGP] Callas, J., Donnerhacke, L., Finney, H. and R.
Thayer, "OpenPGP Message Format", RFC 2440,
November 1998.
[RANDOM] Eastlake, 3rd, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
[RDF] Resource Description Framework (RDF) Schema
Specification 1.0. W3C Candidate Recommendation.
D. Brickley, R.V. Guha. March 2000.
http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
Resource Description Framework (RDF) Model and
Syntax Specification. W3C Recommendation. O.
Lassila, R. Swick. February 1999.
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/
[1363] IEEE 1363: Standard Specifications for Public Key
Cryptography. August 2000.
[PKCS1] Kaliski, B. and J. Staddon, "PKCS #1: RSA
Cryptography Specifications Version 2.0", RFC
2437, October 1998.
[SAX] SAX: The Simple API for XML. D. Megginson, et al.
May 1998. http://www.megginson.com/SAX/index.html
(THIS PAGE OUT OF DATE; GO TO www.saxproject.org)
[SEC] Shirey, R., "Internet Security Glossary", FYI 36,
RFC 2828, May 2000.
[SHA-1] FIPS PUB 180-1. Secure Hash Standard. U.S.
Department of Commerce/National Institute of
Standards and Technology.
http://csrc.nist.gov/publications/fips/fips180-
1/fip180-1.txt
[SOAP] Simple Object Access Protocol (SOAP) Version 1.1.
W3C Note. D. Box, D. Ehnebuske, G. Kakivaya, A.
Layman, N. Mendelsohn, H. Frystyk Nielsen, S.
Thatte, D. Winer. May 2001.
http://www.w3.org/TR/2000/NOTE-SOAP-20000508/
[Unicode] The Unicode Consortium. The Unicode Standard.
http://www.unicode.org/unicode/standard/
standard.html
[UTF-16] Hoffman, P. and F. Yergeau, "UTF-16, an encoding
of ISO 10646", RFC 2781, February 2000.
[UTF-8] Yergeau, R., "UTF-8, a transformation format of
ISO 10646", RFC 2279, January 1998.
[URI] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
[URI-Literal] Hinden, R., Carpenter, B. and L. Masinter, "Format
for Literal IPv6 Addresses in URL's", RFC 2732,
December 1999.
[URL] Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738,
December 1994.
[URN] Moats, R., "URN Syntax", RFC 2141, May 1997.
[X509v3] ITU-T Recommendation X.509 version 3 (1997).
"Information Technology - Open Systems
Interconnection - The Directory Authentication
Framework" ISO/IEC 9594-8:1997.
[XHTML 1.0] XHTML(tm) 1.0: The Extensible Hypertext Markup
Language. W3C Recommendation. S. Pemberton, D.
Raggett, et al. January 2000.
http://www.w3.org/TR/2000/REC-xhtml1-20000126/
[XLink] XML Linking Language. W3C Recommendation. S.
DeRose, E. Maler, D. Orchard. June 2001.
http://www.w3.org/TR/2000/REC-xlink-20010627/
[XML] Extensible Markup Language (XML) 1.0 (Second
Edition). W3C Recommendation. T. Bray, E. Maler,
J. Paoli, C. M. Sperberg-McQueen. October 2000.
http://www.w3.org/TR/2000/REC-xml-20001006
[XML-C14N] Boyer, J., "Canonical XML Version 1.0", RFC 3076,
March 2001.
[XML-Japanese] XML Japanese Profile. W3C Note. M. Murata April
2000 http://www.w3.org/TR/2000/NOTE-japanese-xml-
20000414/
[XML-MT] Whitehead, E. and M. Murata, "XML Media Types",
RFC 2376, July 1998.
[XML-ns] Namespaces in XML. W3C Recommendation. T. Bray, D.
Hollander, A. Layman. January 1999.
http://www.w3.org/TR/1999/REC-xml-names-19990114
[XML-schema] XML Schema Part 1: Structures. W3C Recommendation.
D. Beech, M. Maloney, N. Mendelsohn, H. Thompson.
May 2001. http://www.w3.org/TR/2001/REC-
xmlschema-1-20010502/ XML Schema Part 2: Datatypes
W3C Recommendation. P. Biron, A. Malhotra. May
2001. http://www.w3.org/TR/2001/REC-xmlschema-2-
20010502/
[XML-Signature-RD] Reagle, J., "XML Signature Requirements", RFC
2807, July 2000.
[XPath] XML Path Language (XPath) Version 1.0. W3C
Recommendation. J. Clark, S. DeRose. October 1999.
http://www.w3.org/TR/1999/REC-xpath-19991116
[XPointer] XML Pointer Language (XPointer). W3C Working
Draft. S. DeRose, R. Daniel, E. Maler. January
2001. http://www.w3.org/TR/2001/WD-xptr-20010108
[XSL] Extensible Stylesheet Language (XSL). W3C Proposed
Recommendation. S. Adler, A. Berglund, J. Caruso,
S. Deach, P. Grosso, E. Gutentag, A. Milowski, S.
Parnell, J. Richman, S. Zilles. August 2001.
http://www.w3.org/TR/2001/PR-xsl-20010828/
[XSLT] XSL Transforms (XSLT) Version 1.0. W3C
Recommendation. J. Clark. November 1999.
http://www.w3.org/TR/1999/REC-xslt-19991116.html
Authors' Addresses
Donald E. Eastlake 3rd
Motorola, 20 Forbes Boulevard
Mansfield, MA 02048 USA
Phone: 1-508-851-8280
EMail: Donald.Eastlake@motorola.com
Joseph M. Reagle Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: +1.617.258.7621
EMail: reagle@w3.org
David Solo
Citigroup
909 Third Ave, 16th Floor
NY, NY 10043 USA
Phone +1-212-559-2900
EMail: dsolo@alum.mit.edu
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