Network Working Group T. Ylonen
Request for Comments: 4253 SSH Communications Security Corp
Category: Standards Track C. Lonvick, Ed.
Cisco Systems, Inc.
January 2006
The Secure Shell (SSH) Transport Layer Protocol
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) The Internet Society (2006).
Abstract
The Secure Shell (SSH) is a protocol for secure remote login and
other secure network services over an insecure network.
This document describes the SSH transport layer protocol, which
typically runs on top of TCP/IP. The protocol can be used as a basis
for a number of secure network services. It provides strong
encryption, server authentication, and integrity protection. It may
also provide compression.
Key exchange method, public key algorithm, symmetric encryption
algorithm, message authentication algorithm, and hash algorithm are
all negotiated.
This document also describes the Diffie-Hellman key exchange method
and the minimal set of algorithms that are needed to implement the
SSH transport layer protocol.
Table of Contents
1. Introduction ....................................................3
2. Contributors ....................................................3
3. Conventions Used in This Document ...............................3
4. Connection Setup ................................................4
4.1. Use over TCP/IP ............................................4
4.2. Protocol Version Exchange ..................................4
5. Compatibility With Old SSH Versions .............................5
5.1. Old Client, New Server .....................................6
5.2. New Client, Old Server .....................................6
5.3. Packet Size and Overhead ...................................6
6. Binary Packet Protocol ..........................................7
6.1. Maximum Packet Length ......................................8
6.2. Compression ................................................8
6.3. Encryption .................................................9
6.4. Data Integrity ............................................12
6.5. Key Exchange Methods ......................................13
6.6. Public Key Algorithms .....................................13
7. Key Exchange ...................................................15
7.1. Algorithm Negotiation .....................................17
7.2. Output from Key Exchange ..................................20
7.3. Taking Keys Into Use ......................................21
8. Diffie-Hellman Key Exchange ....................................21
8.1. diffie-hellman-group1-sha1 ................................23
8.2. diffie-hellman-group14-sha1 ...............................23
9. Key Re-Exchange ................................................23
10. Service Request ...............................................24
11. Additional Messages ...........................................25
11.1. Disconnection Message ....................................25
11.2. Ignored Data Message .....................................26
11.3. Debug Message ............................................26
11.4. Reserved Messages ........................................27
12. Summary of Message Numbers ....................................27
13. IANA Considerations ...........................................27
14. Security Considerations .......................................28
15. References ....................................................29
15.1. Normative References .....................................29
15.2. Informative References ...................................30
Authors' Addresses ................................................31
Trademark Notice ..................................................31
1. Introduction
The SSH transport layer is a secure, low level transport protocol.
It provides strong encryption, cryptographic host authentication, and
integrity protection.
Authentication in this protocol level is host-based; this protocol
does not perform user authentication. A higher level protocol for
user authentication can be designed on top of this protocol.
The protocol has been designed to be simple and flexible to allow
parameter negotiation, and to minimize the number of round-trips.
The key exchange method, public key algorithm, symmetric encryption
algorithm, message authentication algorithm, and hash algorithm are
all negotiated. It is expected that in most environments, only 2
round-trips will be needed for full key exchange, server
authentication, service request, and acceptance notification of
service request. The worst case is 3 round-trips.
2. Contributors
The major original contributors of this set of documents have been:
Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
Communications Security Corp), and Markku-Juhani O. Saarinen
(University of Jyvaskyla). Darren Moffat was the original editor of
this set of documents and also made very substantial contributions.
Many people contributed to the development of this document over the
years. People who should be acknowledged include Mats Andersson, Ben
Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
Tadayoshi Kohno. Listing their names here does not mean that they
endorse this document, but that they have contributed to it.
3. Conventions Used in This Document
All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
requirements. These keywords are to be interpreted as described in
[RFC2119].
The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
this document when used to describe namespace allocation are to be
interpreted as described in [RFC2434].
Protocol fields and possible values to fill them are defined in this
set of documents. Protocol fields will be defined in the message
definitions. As an example, SSH_MSG_CHANNEL_DATA is defined as
follows.
byte SSH_MSG_CHANNEL_DATA
uint32 recipient channel
string data
Throughout these documents, when the fields are referenced, they will
appear within single quotes. When values to fill those fields are
referenced, they will appear within double quotes. Using the above
example, possible values for 'data' are "foo" and "bar".
4. Connection Setup
SSH works over any 8-bit clean, binary-transparent transport. The
underlying transport SHOULD protect against transmission errors, as
such errors cause the SSH connection to terminate.
The client initiates the connection.
4.1. Use over TCP/IP
When used over TCP/IP, the server normally listens for connections on
port 22. This port number has been registered with the IANA, and has
been officially assigned for SSH.
4.2. Protocol Version Exchange
When the connection has been established, both sides MUST send an
identification string. This identification string MUST be
SSH-protoversion-softwareversion SP comments CR LF
Since the protocol being defined in this set of documents is version
2.0, the 'protoversion' MUST be "2.0". The 'comments' string is
OPTIONAL. If the 'comments' string is included, a 'space' character
(denoted above as SP, ASCII 32) MUST separate the 'softwareversion'
and 'comments' strings. The identification MUST be terminated by a
single Carriage Return (CR) and a single Line Feed (LF) character
(ASCII 13 and 10, respectively). Implementers who wish to maintain
compatibility with older, undocumented versions of this protocol may
want to process the identification string without expecting the
presence of the carriage return character for reasons described in
Section 5 of this document. The null character MUST NOT be sent.
The maximum length of the string is 255 characters, including the
Carriage Return and Line Feed.
The part of the identification string preceding the Carriage Return
and Line Feed is used in the Diffie-Hellman key exchange (see Section
8).
The server MAY send other lines of data before sending the version
string. Each line SHOULD be terminated by a Carriage Return and Line
Feed. Such lines MUST NOT begin with "SSH-", and SHOULD be encoded
in ISO-10646 UTF-8 [RFC3629] (language is not specified). Clients
MUST be able to process such lines. Such lines MAY be silently
ignored, or MAY be displayed to the client user. If they are
displayed, control character filtering, as discussed in [SSH-ARCH],
SHOULD be used. The primary use of this feature is to allow TCP-
wrappers to display an error message before disconnecting.
Both the 'protoversion' and 'softwareversion' strings MUST consist of
printable US-ASCII characters, with the exception of whitespace
characters and the minus sign (-). The 'softwareversion' string is
primarily used to trigger compatibility extensions and to indicate
the capabilities of an implementation. The 'comments' string SHOULD
contain additional information that might be useful in solving user
problems. As such, an example of a valid identification string is
SSH-2.0-billsSSH_3.6.3q3<CR><LF>
This identification string does not contain the optional 'comments'
string and is thus terminated by a CR and LF immediately after the
'softwareversion' string.
Key exchange will begin immediately after sending this identifier.
All packets following the identification string SHALL use the binary
packet protocol, which is described in Section 6.
5. Compatibility With Old SSH Versions
As stated earlier, the 'protoversion' specified for this protocol is
"2.0". Earlier versions of this protocol have not been formally
documented, but it is widely known that they use 'protoversion' of
"1.x" (e.g., "1.5" or "1.3"). At the time of this writing, many
implementations of SSH are utilizing protocol version 2.0, but it is
known that there are still devices using the previous versions.
During the transition period, it is important to be able to work in a
way that is compatible with the installed SSH clients and servers
that use the older version of the protocol. Information in this
section is only relevant for implementations supporting compatibility
with SSH versions 1.x. For those interested, the only known
documentation of the 1.x protocol is contained in README files that
are shipped along with the source code [ssh-1.2.30].
5.1. Old Client, New Server
Server implementations MAY support a configurable compatibility flag
that enables compatibility with old versions. When this flag is on,
the server SHOULD identify its 'protoversion' as "1.99". Clients
using protocol 2.0 MUST be able to identify this as identical to
"2.0". In this mode, the server SHOULD NOT send the Carriage Return
character (ASCII 13) after the identification string.
In the compatibility mode, the server SHOULD NOT send any further
data after sending its identification string until it has received an
identification string from the client. The server can then determine
whether the client is using an old protocol, and can revert to the
old protocol if required. In the compatibility mode, the server MUST
NOT send additional data before the identification string.
When compatibility with old clients is not needed, the server MAY
send its initial key exchange data immediately after the
identification string.
5.2. New Client, Old Server
Since the new client MAY immediately send additional data after its
identification string (before receiving the server's identification
string), the old protocol may already be corrupt when the client
learns that the server is old. When this happens, the client SHOULD
close the connection to the server, and reconnect using the old
protocol.
5.3. Packet Size and Overhead
Some readers will worry about the increase in packet size due to new
headers, padding, and the Message Authentication Code (MAC). The
minimum packet size is in the order of 28 bytes (depending on
negotiated algorithms). The increase is negligible for large
packets, but very significant for one-byte packets (telnet-type
sessions). There are, however, several factors that make this a
non-issue in almost all cases:
o The minimum size of a TCP/IP header is 32 bytes. Thus, the
increase is actually from 33 to 51 bytes (roughly).
o The minimum size of the data field of an Ethernet packet is 46
bytes [RFC0894]. Thus, the increase is no more than 5 bytes.
When Ethernet headers are considered, the increase is less than 10
percent.
o The total fraction of telnet-type data in the Internet is
negligible, even with increased packet sizes.
The only environment where the packet size increase is likely to have
a significant effect is PPP [RFC1661] over slow modem lines (PPP
compresses the TCP/IP headers, emphasizing the increase in packet
size). However, with modern modems, the time needed to transfer is
in the order of 2 milliseconds, which is a lot faster than people can
type.
There are also issues related to the maximum packet size. To
minimize delays in screen updates, one does not want excessively
large packets for interactive sessions. The maximum packet size is
negotiated separately for each channel.
6. Binary Packet Protocol
Each packet is in the following format:
uint32 packet_length
byte padding_length
byte[n1] payload; n1 = packet_length - padding_length - 1
byte[n2] random padding; n2 = padding_length
byte[m] mac (Message Authentication Code - MAC); m = mac_length
packet_length
The length of the packet in bytes, not including 'mac' or the
'packet_length' field itself.
padding_length
Length of 'random padding' (bytes).
payload
The useful contents of the packet. If compression has been
negotiated, this field is compressed. Initially, compression
MUST be "none".
random padding
Arbitrary-length padding, such that the total length of
(packet_length || padding_length || payload || random padding)
is a multiple of the cipher block size or 8, whichever is
larger. There MUST be at least four bytes of padding. The
padding SHOULD consist of random bytes. The maximum amount of
padding is 255 bytes.
mac
Message Authentication Code. If message authentication has
been negotiated, this field contains the MAC bytes. Initially,
the MAC algorithm MUST be "none".
Note that the length of the concatenation of 'packet_length',
'padding_length', 'payload', and 'random padding' MUST be a multiple
of the cipher block size or 8, whichever is larger. This constraint
MUST be enforced, even when using stream ciphers. Note that the
'packet_length' field is also encrypted, and processing it requires
special care when sending or receiving packets. Also note that the
insertion of variable amounts of 'random padding' may help thwart
traffic analysis.
The minimum size of a packet is 16 (or the cipher block size,
whichever is larger) bytes (plus 'mac'). Implementations SHOULD
decrypt the length after receiving the first 8 (or cipher block size,
whichever is larger) bytes of a packet.
6.1. Maximum Packet Length
All implementations MUST be able to process packets with an
uncompressed payload length of 32768 bytes or less and a total packet
size of 35000 bytes or less (including 'packet_length',
'padding_length', 'payload', 'random padding', and 'mac'). The
maximum of 35000 bytes is an arbitrarily chosen value that is larger
than the uncompressed length noted above. Implementations SHOULD
support longer packets, where they might be needed. For example, if
an implementation wants to send a very large number of certificates,
the larger packets MAY be sent if the identification string indicates
that the other party is able to process them. However,
implementations SHOULD check that the packet length is reasonable in
order for the implementation to avoid denial of service and/or buffer
overflow attacks.
6.2. Compression
If compression has been negotiated, the 'payload' field (and only it)
will be compressed using the negotiated algorithm. The
'packet_length' field and 'mac' will be computed from the compressed
payload. Encryption will be done after compression.
Compression MAY be stateful, depending on the method. Compression
MUST be independent for each direction, and implementations MUST
allow independent choosing of the algorithm for each direction. In
practice however, it is RECOMMENDED that the compression method be
the same in both directions.
The following compression methods are currently defined:
none REQUIRED no compression
zlib OPTIONAL ZLIB (LZ77) compression
The "zlib" compression is described in [RFC1950] and in [RFC1951].
The compression context is initialized after each key exchange, and
is passed from one packet to the next, with only a partial flush
being performed at the end of each packet. A partial flush means
that the current compressed block is ended and all data will be
output. If the current block is not a stored block, one or more
empty blocks are added after the current block to ensure that there
are at least 8 bits, counting from the start of the end-of-block code
of the current block to the end of the packet payload.
Additional methods may be defined as specified in [SSH-ARCH] and
[SSH-NUMBERS].
6.3. Encryption
An encryption algorithm and a key will be negotiated during the key
exchange. When encryption is in effect, the packet length, padding
length, payload, and padding fields of each packet MUST be encrypted
with the given algorithm.
The encrypted data in all packets sent in one direction SHOULD be
considered a single data stream. For example, initialization vectors
SHOULD be passed from the end of one packet to the beginning of the
next packet. All ciphers SHOULD use keys with an effective key
length of 128 bits or more.
The ciphers in each direction MUST run independently of each other.
Implementations MUST allow the algorithm for each direction to be
independently selected, if multiple algorithms are allowed by local
policy. In practice however, it is RECOMMENDED that the same
algorithm be used in both directions.
The following ciphers are currently defined:
3des-cbc REQUIRED three-key 3DES in CBC mode
blowfish-cbc OPTIONAL Blowfish in CBC mode
twofish256-cbc OPTIONAL Twofish in CBC mode,
with a 256-bit key
twofish-cbc OPTIONAL alias for "twofish256-cbc"
(this is being retained
for historical reasons)
twofish192-cbc OPTIONAL Twofish with a 192-bit key
twofish128-cbc OPTIONAL Twofish with a 128-bit key
aes256-cbc OPTIONAL AES in CBC mode,
with a 256-bit key
aes192-cbc OPTIONAL AES with a 192-bit key
aes128-cbc RECOMMENDED AES with a 128-bit key
serpent256-cbc OPTIONAL Serpent in CBC mode, with
a 256-bit key
serpent192-cbc OPTIONAL Serpent with a 192-bit key
serpent128-cbc OPTIONAL Serpent with a 128-bit key
arcfour OPTIONAL the ARCFOUR stream cipher
with a 128-bit key
idea-cbc OPTIONAL IDEA in CBC mode
cast128-cbc OPTIONAL CAST-128 in CBC mode
none OPTIONAL no encryption; NOT RECOMMENDED
The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-
encrypt), where the first 8 bytes of the key are used for the first
encryption, the next 8 bytes for the decryption, and the following 8
bytes for the final encryption. This requires 24 bytes of key data
(of which 168 bits are actually used). To implement CBC mode, outer
chaining MUST be used (i.e., there is only one initialization
vector). This is a block cipher with 8-byte blocks. This algorithm
is defined in [FIPS-46-3]. Note that since this algorithm only has
an effective key length of 112 bits ([SCHNEIER]), it does not meet
the specifications that SSH encryption algorithms should use keys of
128 bits or more. However, this algorithm is still REQUIRED for
historical reasons; essentially, all known implementations at the
time of this writing support this algorithm, and it is commonly used
because it is the fundamental interoperable algorithm. At some
future time, it is expected that another algorithm, one with better
strength, will become so prevalent and ubiquitous that the use of
"3des-cbc" will be deprecated by another STANDARDS ACTION.
The "blowfish-cbc" cipher is Blowfish in CBC mode, with 128-bit keys
[SCHNEIER]. This is a block cipher with 8-byte blocks.
The "twofish-cbc" or "twofish256-cbc" cipher is Twofish in CBC mode,
with 256-bit keys as described [TWOFISH]. This is a block cipher
with 16-byte blocks.
The "twofish192-cbc" cipher is the same as above, but with a 192-bit
key.
The "twofish128-cbc" cipher is the same as above, but with a 128-bit
key.
The "aes256-cbc" cipher is AES (Advanced Encryption Standard)
[FIPS-197], in CBC mode. This version uses a 256-bit key.
The "aes192-cbc" cipher is the same as above, but with a 192-bit key.
The "aes128-cbc" cipher is the same as above, but with a 128-bit key.
The "serpent256-cbc" cipher in CBC mode, with a 256-bit key as
described in the Serpent AES submission.
The "serpent192-cbc" cipher is the same as above, but with a 192-bit
key.
The "serpent128-cbc" cipher is the same as above, but with a 128-bit
key.
The "arcfour" cipher is the Arcfour stream cipher with 128-bit keys.
The Arcfour cipher is believed to be compatible with the RC4 cipher
[SCHNEIER]. Arcfour (and RC4) has problems with weak keys, and
should be used with caution.
The "idea-cbc" cipher is the IDEA cipher in CBC mode [SCHNEIER].
The "cast128-cbc" cipher is the CAST-128 cipher in CBC mode with a
128-bit key [RFC2144].
The "none" algorithm specifies that no encryption is to be done.
Note that this method provides no confidentiality protection, and it
is NOT RECOMMENDED. Some functionality (e.g., password
authentication) may be disabled for security reasons if this cipher
is chosen.
Additional methods may be defined as specified in [SSH-ARCH] and in
[SSH-NUMBERS].
6.4. Data Integrity
Data integrity is protected by including with each packet a MAC that
is computed from a shared secret, packet sequence number, and the
contents of the packet.
The message authentication algorithm and key are negotiated during
key exchange. Initially, no MAC will be in effect, and its length
MUST be zero. After key exchange, the 'mac' for the selected MAC
algorithm will be computed before encryption from the concatenation
of packet data:
mac = MAC(key, sequence_number || unencrypted_packet)
where unencrypted_packet is the entire packet without 'mac' (the
length fields, 'payload' and 'random padding'), and sequence_number
is an implicit packet sequence number represented as uint32. The
sequence_number is initialized to zero for the first packet, and is
incremented after every packet (regardless of whether encryption or
MAC is in use). It is never reset, even if keys/algorithms are
renegotiated later. It wraps around to zero after every 2^32
packets. The packet sequence_number itself is not included in the
packet sent over the wire.
The MAC algorithms for each direction MUST run independently, and
implementations MUST allow choosing the algorithm independently for
both directions. In practice however, it is RECOMMENDED that the
same algorithm be used in both directions.
The value of 'mac' resulting from the MAC algorithm MUST be
transmitted without encryption as the last part of the packet. The
number of 'mac' bytes depends on the algorithm chosen.
The following MAC algorithms are currently defined:
hmac-sha1 REQUIRED HMAC-SHA1 (digest length = key
length = 20)
hmac-sha1-96 RECOMMENDED first 96 bits of HMAC-SHA1 (digest
length = 12, key length = 20)
hmac-md5 OPTIONAL HMAC-MD5 (digest length = key
length = 16)
hmac-md5-96 OPTIONAL first 96 bits of HMAC-MD5 (digest
length = 12, key length = 16)
none OPTIONAL no MAC; NOT RECOMMENDED
The "hmac-*" algorithms are described in [RFC2104]. The "*-n" MACs
use only the first n bits of the resulting value.
SHA-1 is described in [FIPS-180-2] and MD5 is described in [RFC1321].
Additional methods may be defined, as specified in [SSH-ARCH] and in
[SSH-NUMBERS].
6.5. Key Exchange Methods
The key exchange method specifies how one-time session keys are
generated for encryption and for authentication, and how the server
authentication is done.
Two REQUIRED key exchange methods have been defined:
diffie-hellman-group1-sha1 REQUIRED
diffie-hellman-group14-sha1 REQUIRED
These methods are described in Section 8.
Additional methods may be defined as specified in [SSH-NUMBERS]. The
name "diffie-hellman-group1-sha1" is used for a key exchange method
using an Oakley group, as defined in [RFC2409]. SSH maintains its
own group identifier space that is logically distinct from Oakley
[RFC2412] and IKE; however, for one additional group, the Working
Group adopted the number assigned by [RFC3526], using diffie-
hellman-group14-sha1 for the name of the second defined group.
Implementations should treat these names as opaque identifiers and
should not assume any relationship between the groups used by SSH and
the groups defined for IKE.
6.6. Public Key Algorithms
This protocol has been designed to operate with almost any public key
format, encoding, and algorithm (signature and/or encryption).
There are several aspects that define a public key type:
o Key format: how is the key encoded and how are certificates
represented. The key blobs in this protocol MAY contain
certificates in addition to keys.
o Signature and/or encryption algorithms. Some key types may not
support both signing and encryption. Key usage may also be
restricted by policy statements (e.g., in certificates). In this
case, different key types SHOULD be defined for the different
policy alternatives.
o Encoding of signatures and/or encrypted data. This includes but
is not limited to padding, byte order, and data formats.
The following public key and/or certificate formats are currently
defined:
ssh-dss REQUIRED sign Raw DSS Key
ssh-rsa RECOMMENDED sign Raw RSA Key
pgp-sign-rsa OPTIONAL sign OpenPGP certificates (RSA key)
pgp-sign-dss OPTIONAL sign OpenPGP certificates (DSS key)
Additional key types may be defined, as specified in [SSH-ARCH] and
in [SSH-NUMBERS].
The key type MUST always be explicitly known (from algorithm
negotiation or some other source). It is not normally included in
the key blob.
Certificates and public keys are encoded as follows:
string certificate or public key format identifier
byte[n] key/certificate data
The certificate part may be a zero length string, but a public key is
required. This is the public key that will be used for
authentication. The certificate sequence contained in the
certificate blob can be used to provide authorization.
Public key/certificate formats that do not explicitly specify a
signature format identifier MUST use the public key/certificate
format identifier as the signature identifier.
Signatures are encoded as follows:
string signature format identifier (as specified by the
public key/certificate format)
byte[n] signature blob in format specific encoding.
The "ssh-dss" key format has the following specific encoding:
string "ssh-dss"
mpint p
mpint q
mpint g
mpint y
Here, the 'p', 'q', 'g', and 'y' parameters form the signature key
blob.
Signing and verifying using this key format is done according to the
Digital Signature Standard [FIPS-186-2] using the SHA-1 hash
[FIPS-180-2].
The resulting signature is encoded as follows:
string "ssh-dss"
string dss_signature_blob
The value for 'dss_signature_blob' is encoded as a string containing
r, followed by s (which are 160-bit integers, without lengths or
padding, unsigned, and in network byte order).
The "ssh-rsa" key format has the following specific encoding:
string "ssh-rsa"
mpint e
mpint n
Here the 'e' and 'n' parameters form the signature key blob.
Signing and verifying using this key format is performed according to
the RSASSA-PKCS1-v1_5 scheme in [RFC3447] using the SHA-1 hash.
The resulting signature is encoded as follows:
string "ssh-rsa"
string rsa_signature_blob
The value for 'rsa_signature_blob' is encoded as a string containing
s (which is an integer, without lengths or padding, unsigned, and in
network byte order).
The "pgp-sign-rsa" method indicates the certificates, the public key,
and the signature are in OpenPGP compatible binary format
([RFC2440]). This method indicates that the key is an RSA-key.
The "pgp-sign-dss" is as above, but indicates that the key is a
DSS-key.
7. Key Exchange
Key exchange (kex) begins by each side sending name-lists of
supported algorithms. Each side has a preferred algorithm in each
category, and it is assumed that most implementations, at any given
time, will use the same preferred algorithm. Each side MAY guess
which algorithm the other side is using, and MAY send an initial key
exchange packet according to the algorithm, if appropriate for the
preferred method.
The guess is considered wrong if:
o the kex algorithm and/or the host key algorithm is guessed wrong
(server and client have different preferred algorithm), or
o if any of the other algorithms cannot be agreed upon (the
procedure is defined below in Section 7.1).
Otherwise, the guess is considered to be right, and the
optimistically sent packet MUST be handled as the first key exchange
packet.
However, if the guess was wrong, and a packet was optimistically sent
by one or both parties, such packets MUST be ignored (even if the
error in the guess would not affect the contents of the initial
packet(s)), and the appropriate side MUST send the correct initial
packet.
A key exchange method uses explicit server authentication if the key
exchange messages include a signature or other proof of the server's
authenticity. A key exchange method uses implicit server
authentication if, in order to prove its authenticity, the server
also has to prove that it knows the shared secret, K, by sending a
message and a corresponding MAC that the client can verify.
The key exchange method defined by this document uses explicit server
authentication. However, key exchange methods with implicit server
authentication MAY be used with this protocol. After a key exchange
with implicit server authentication, the client MUST wait for a
response to its service request message before sending any further
data.
7.1. Algorithm Negotiation
Key exchange begins by each side sending the following packet:
byte SSH_MSG_KEXINIT
byte[16] cookie (random bytes)
name-list kex_algorithms
name-list server_host_key_algorithms
name-list encryption_algorithms_client_to_server
name-list encryption_algorithms_server_to_client
name-list mac_algorithms_client_to_server
name-list mac_algorithms_server_to_client
name-list compression_algorithms_client_to_server
name-list compression_algorithms_server_to_client
name-list languages_client_to_server
name-list languages_server_to_client
boolean first_kex_packet_follows
uint32 0 (reserved for future extension)
Each of the algorithm name-lists MUST be a comma-separated list of
algorithm names (see Algorithm Naming in [SSH-ARCH] and additional
information in [SSH-NUMBERS]). Each supported (allowed) algorithm
MUST be listed in order of preference, from most to least.
The first algorithm in each name-list MUST be the preferred (guessed)
algorithm. Each name-list MUST contain at least one algorithm name.
cookie
The 'cookie' MUST be a random value generated by the sender.
Its purpose is to make it impossible for either side to fully
determine the keys and the session identifier.
kex_algorithms
Key exchange algorithms were defined above. The first
algorithm MUST be the preferred (and guessed) algorithm. If
both sides make the same guess, that algorithm MUST be used.
Otherwise, the following algorithm MUST be used to choose a key
exchange method: Iterate over client's kex algorithms, one at a
time. Choose the first algorithm that satisfies the following
conditions:
+ the server also supports the algorithm,
+ if the algorithm requires an encryption-capable host key,
there is an encryption-capable algorithm on the server's
server_host_key_algorithms that is also supported by the
client, and
+ if the algorithm requires a signature-capable host key,
there is a signature-capable algorithm on the server's
server_host_key_algorithms that is also supported by the
client.
If no algorithm satisfying all these conditions can be found, the
connection fails, and both sides MUST disconnect.
server_host_key_algorithms
A name-list of the algorithms supported for the server host
key. The server lists the algorithms for which it has host
keys; the client lists the algorithms that it is willing to
accept. There MAY be multiple host keys for a host, possibly
with different algorithms.
Some host keys may not support both signatures and encryption
(this can be determined from the algorithm), and thus not all
host keys are valid for all key exchange methods.
Algorithm selection depends on whether the chosen key exchange
algorithm requires a signature or an encryption-capable host
key. It MUST be possible to determine this from the public key
algorithm name. The first algorithm on the client's name-list
that satisfies the requirements and is also supported by the
server MUST be chosen. If there is no such algorithm, both
sides MUST disconnect.
encryption_algorithms
A name-list of acceptable symmetric encryption algorithms (also
known as ciphers) in order of preference. The chosen
encryption algorithm to each direction MUST be the first
algorithm on the client's name-list that is also on the
server's name-list. If there is no such algorithm, both sides
MUST disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The defined algorithm names are listed in Section
6.3.
mac_algorithms
A name-list of acceptable MAC algorithms in order of
preference. The chosen MAC algorithm MUST be the first
algorithm on the client's name-list that is also on the
server's name-list. If there is no such algorithm, both sides
MUST disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The MAC algorithm names are listed in Section 6.4.
compression_algorithms
A name-list of acceptable compression algorithms in order of
preference. The chosen compression algorithm MUST be the first
algorithm on the client's name-list that is also on the
server's name-list. If there is no such algorithm, both sides
MUST disconnect.
Note that "none" must be explicitly listed if it is to be
acceptable. The compression algorithm names are listed in
Section 6.2.
languages
This is a name-list of language tags in order of preference
[RFC3066]. Both parties MAY ignore this name-list. If there
are no language preferences, this name-list SHOULD be empty as
defined in Section 5 of [SSH-ARCH]. Language tags SHOULD NOT
be present unless they are known to be needed by the sending
party.
first_kex_packet_follows
Indicates whether a guessed key exchange packet follows. If a
guessed packet will be sent, this MUST be TRUE. If no guessed
packet will be sent, this MUST be FALSE.
After receiving the SSH_MSG_KEXINIT packet from the other side,
each party will know whether their guess was right. If the
other party's guess was wrong, and this field was TRUE, the
next packet MUST be silently ignored, and both sides MUST then
act as determined by the negotiated key exchange method. If
the guess was right, key exchange MUST continue using the
guessed packet.
After the SSH_MSG_KEXINIT message exchange, the key exchange
algorithm is run. It may involve several packet exchanges, as
specified by the key exchange method.
Once a party has sent a SSH_MSG_KEXINIT message for key exchange or
re-exchange, until it has sent a SSH_MSG_NEWKEYS message (Section
7.3), it MUST NOT send any messages other than:
o Transport layer generic messages (1 to 19) (but
SSH_MSG_SERVICE_REQUEST and SSH_MSG_SERVICE_ACCEPT MUST NOT be
sent);
o Algorithm negotiation messages (20 to 29) (but further
SSH_MSG_KEXINIT messages MUST NOT be sent);
o Specific key exchange method messages (30 to 49).
The provisions of Section 11 apply to unrecognized messages.
Note, however, that during a key re-exchange, after sending a
SSH_MSG_KEXINIT message, each party MUST be prepared to process an
arbitrary number of messages that may be in-flight before receiving a
SSH_MSG_KEXINIT message from the other party.
7.2. Output from Key Exchange
The key exchange produces two values: a shared secret K, and an
exchange hash H. Encryption and authentication keys are derived from
these. The exchange hash H from the first key exchange is
additionally used as the session identifier, which is a unique
identifier for this connection. It is used by authentication methods
as a part of the data that is signed as a proof of possession of a
private key. Once computed, the session identifier is not changed,
even if keys are later re-exchanged.
Each key exchange method specifies a hash function that is used in
the key exchange. The same hash algorithm MUST be used in key
derivation. Here, we'll call it HASH.
Encryption keys MUST be computed as HASH, of a known value and K, as
follows:
o Initial IV client to server: HASH(K || H || "A" || session_id)
(Here K is encoded as mpint and "A" as byte and session_id as raw
data. "A" means the single character A, ASCII 65).
o Initial IV server to client: HASH(K || H || "B" || session_id)
o Encryption key client to server: HASH(K || H || "C" || session_id)
o Encryption key server to client: HASH(K || H || "D" || session_id)
o Integrity key client to server: HASH(K || H || "E" || session_id)
o Integrity key server to client: HASH(K || H || "F" || session_id)
Key data MUST be taken from the beginning of the hash output. As
many bytes as needed are taken from the beginning of the hash value.
If the key length needed is longer than the output of the HASH, the
key is extended by computing HASH of the concatenation of K and H and
the entire key so far, and appending the resulting bytes (as many as
HASH generates) to the key. This process is repeated until enough
key material is available; the key is taken from the beginning of
this value. In other words:
K1 = HASH(K || H || X || session_id) (X is e.g., "A")
K2 = HASH(K || H || K1)
K3 = HASH(K || H || K1 || K2)
...
key = K1 || K2 || K3 || ...
This process will lose entropy if the amount of entropy in K is
larger than the internal state size of HASH.
7.3. Taking Keys Into Use
Key exchange ends by each side sending an SSH_MSG_NEWKEYS message.
This message is sent with the old keys and algorithms. All messages
sent after this message MUST use the new keys and algorithms.
When this message is received, the new keys and algorithms MUST be
used for receiving.
The purpose of this message is to ensure that a party is able to
respond with an SSH_MSG_DISCONNECT message that the other party can
understand if something goes wrong with the key exchange.
byte SSH_MSG_NEWKEYS
8. Diffie-Hellman Key Exchange
The Diffie-Hellman (DH) key exchange provides a shared secret that
cannot be determined by either party alone. The key exchange is
combined with a signature with the host key to provide host
authentication. This key exchange method provides explicit server
authentication as defined in Section 7.
The following steps are used to exchange a key. In this, C is the
client; S is the server; p is a large safe prime; g is a generator
for a subgroup of GF(p); q is the order of the subgroup; V_S is S's
identification string; V_C is C's identification string; K_S is S's
public host key; I_C is C's SSH_MSG_KEXINIT message and I_S is S's
SSH_MSG_KEXINIT message that have been exchanged before this part
begins.
1. C generates a random number x (1 < x < q) and computes
e = g^x mod p. C sends e to S.
2. S generates a random number y (0 < y < q) and computes
f = g^y mod p. S receives e. It computes K = e^y mod p,
H = hash(V_C || V_S || I_C || I_S || K_S || e || f || K)
(these elements are encoded according to their types; see below),
and signature s on H with its private host key. S sends
(K_S || f || s) to C. The signing operation may involve a
second hashing operation.
3. C verifies that K_S really is the host key for S (e.g., using
certificates or a local database). C is also allowed to accept
the key without verification; however, doing so will render the
protocol insecure against active attacks (but may be desirable for
practical reasons in the short term in many environments). C then
computes K = f^x mod p, H = hash(V_C || V_S || I_C || I_S || K_S
|| e || f || K), and verifies the signature s on H.
Values of 'e' or 'f' that are not in the range [1, p-1] MUST NOT be
sent or accepted by either side. If this condition is violated, the
key exchange fails.
This is implemented with the following messages. The hash algorithm
for computing the exchange hash is defined by the method name, and is
called HASH. The public key algorithm for signing is negotiated with
the SSH_MSG_KEXINIT messages.
First, the client sends the following:
byte SSH_MSG_KEXDH_INIT
mpint e
The server then responds with the following:
byte SSH_MSG_KEXDH_REPLY
string server public host key and certificates (K_S)
mpint f
string signature of H
The hash H is computed as the HASH hash of the concatenation of the
following:
string V_C, the client's identification string (CR and LF
excluded)
string V_S, the server's identification string (CR and LF
excluded)
string I_C, the payload of the client's SSH_MSG_KEXINIT
string I_S, the payload of the server's SSH_MSG_KEXINIT
string K_S, the host key
mpint e, exchange value sent by the client
mpint f, exchange value sent by the server
mpint K, the shared secret
This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept
secret.
The signature algorithm MUST be applied over H, not the original
data. Most signature algorithms include hashing and additional
padding (e.g., "ssh-dss" specifies SHA-1 hashing). In that case, the
data is first hashed with HASH to compute H, and H is then hashed
with SHA-1 as part of the signing operation.
8.1. diffie-hellman-group1-sha1
The "diffie-hellman-group1-sha1" method specifies the Diffie-Hellman
key exchange with SHA-1 as HASH, and Oakley Group 2 [RFC2409] (1024-
bit MODP Group). This method MUST be supported for interoperability
as all of the known implementations currently support it. Note that
this method is named using the phrase "group1", even though it
specifies the use of Oakley Group 2.
8.2. diffie-hellman-group14-sha1
The "diffie-hellman-group14-sha1" method specifies a Diffie-Hellman
key exchange with SHA-1 as HASH and Oakley Group 14 [RFC3526] (2048-
bit MODP Group), and it MUST also be supported.
9. Key Re-Exchange
Key re-exchange is started by sending an SSH_MSG_KEXINIT packet when
not already doing a key exchange (as described in Section 7.1). When
this message is received, a party MUST respond with its own
SSH_MSG_KEXINIT message, except when the received SSH_MSG_KEXINIT
already was a reply. Either party MAY initiate the re-exchange, but
roles MUST NOT be changed (i.e., the server remains the server, and
the client remains the client).
Key re-exchange is performed using whatever encryption was in effect
when the exchange was started. Encryption, compression, and MAC
methods are not changed before a new SSH_MSG_NEWKEYS is sent after
the key exchange (as in the initial key exchange). Re-exchange is
processed identically to the initial key exchange, except for the
session identifier that will remain unchanged. It is permissible to
change some or all of the algorithms during the re-exchange. Host
keys can also change. All keys and initialization vectors are
recomputed after the exchange. Compression and encryption contexts
are reset.
It is RECOMMENDED that the keys be changed after each gigabyte of
transmitted data or after each hour of connection time, whichever
comes sooner. However, since the re-exchange is a public key
operation, it requires a fair amount of processing power and should
not be performed too often.
More application data may be sent after the SSH_MSG_NEWKEYS packet
has been sent; key exchange does not affect the protocols that lie
above the SSH transport layer.
10. Service Request
After the key exchange, the client requests a service. The service
is identified by a name. The format of names and procedures for
defining new names are defined in [SSH-ARCH] and [SSH-NUMBERS].
Currently, the following names have been reserved:
ssh-userauth
ssh-connection
Similar local naming policy is applied to the service names, as is
applied to the algorithm names. A local service should use the
PRIVATE USE syntax of "servicename@domain".
byte SSH_MSG_SERVICE_REQUEST
string service name
If the server rejects the service request, it SHOULD send an
appropriate SSH_MSG_DISCONNECT message and MUST disconnect.
When the service starts, it may have access to the session identifier
generated during the key exchange.
If the server supports the service (and permits the client to use
it), it MUST respond with the following:
byte SSH_MSG_SERVICE_ACCEPT
string service name
Message numbers used by services should be in the area reserved for
them (see [SSH-ARCH] and [SSH-NUMBERS]). The transport level will
continue to process its own messages.
Note that after a key exchange with implicit server authentication,
the client MUST wait for a response to its service request message
before sending any further data.
11. Additional Messages
Either party may send any of the following messages at any time.
11.1. Disconnection Message
byte SSH_MSG_DISCONNECT
uint32 reason code
string description in ISO-10646 UTF-8 encoding [RFC3629]
string language tag [RFC3066]
This message causes immediate termination of the connection. All
implementations MUST be able to process this message; they SHOULD be
able to send this message.
The sender MUST NOT send or receive any data after this message, and
the recipient MUST NOT accept any data after receiving this message.
The Disconnection Message 'description' string gives a more specific
explanation in a human-readable form. The Disconnection Message
'reason code' gives the reason in a more machine-readable format
(suitable for localization), and can have the values as displayed in
the table below. Note that the decimal representation is displayed
in this table for readability, but the values are actually uint32
values.
Symbolic name reason code
------------- -----------
SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT 1
SSH_DISCONNECT_PROTOCOL_ERROR 2
SSH_DISCONNECT_KEY_EXCHANGE_FAILED 3
SSH_DISCONNECT_RESERVED 4
SSH_DISCONNECT_MAC_ERROR 5
SSH_DISCONNECT_COMPRESSION_ERROR 6
SSH_DISCONNECT_SERVICE_NOT_AVAILABLE 7
SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED 8
SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE 9
SSH_DISCONNECT_CONNECTION_LOST 10
SSH_DISCONNECT_BY_APPLICATION 11
SSH_DISCONNECT_TOO_MANY_CONNECTIONS 12
SSH_DISCONNECT_AUTH_CANCELLED_BY_USER 13
SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE 14
SSH_DISCONNECT_ILLEGAL_USER_NAME 15
If the 'description' string is displayed, the control character
filtering discussed in [SSH-ARCH] should be used to avoid attacks by
sending terminal control characters.
Requests for assignments of new Disconnection Message 'reason code'
values (and associated 'description' text) in the range of 0x00000010
to 0xFDFFFFFF MUST be done through the IETF CONSENSUS method, as
described in [RFC2434]. The Disconnection Message 'reason code'
values in the range of 0xFE000000 through 0xFFFFFFFF are reserved for
PRIVATE USE. As noted, the actual instructions to the IANA are in
[SSH-NUMBERS].
11.2. Ignored Data Message
byte SSH_MSG_IGNORE
string data
All implementations MUST understand (and ignore) this message at any
time (after receiving the identification string). No implementation
is required to send them. This message can be used as an additional
protection measure against advanced traffic analysis techniques.
11.3. Debug Message
byte SSH_MSG_DEBUG
boolean always_display
string message in ISO-10646 UTF-8 encoding [RFC3629]
string language tag [RFC3066]
All implementations MUST understand this message, but they are
allowed to ignore it. This message is used to transmit information
that may help debugging. If 'always_display' is TRUE, the message
SHOULD be displayed. Otherwise, it SHOULD NOT be displayed unless
debugging information has been explicitly requested by the user.
The 'message' doesn't need to contain a newline. It is, however,
allowed to consist of multiple lines separated by CRLF (Carriage
Return - Line Feed) pairs.
If the 'message' string is displayed, the terminal control character
filtering discussed in [SSH-ARCH] should be used to avoid attacks by
sending terminal control characters.
11.4. Reserved Messages
An implementation MUST respond to all unrecognized messages with an
SSH_MSG_UNIMPLEMENTED message in the order in which the messages were
received. Such messages MUST be otherwise ignored. Later protocol
versions may define other meanings for these message types.
byte SSH_MSG_UNIMPLEMENTED
uint32 packet sequence number of rejected message
12. Summary of Message Numbers
The following is a summary of messages and their associated message
number.
SSH_MSG_DISCONNECT 1
SSH_MSG_IGNORE 2
SSH_MSG_UNIMPLEMENTED 3
SSH_MSG_DEBUG 4
SSH_MSG_SERVICE_REQUEST 5
SSH_MSG_SERVICE_ACCEPT 6
SSH_MSG_KEXINIT 20
SSH_MSG_NEWKEYS 21
Note that numbers 30-49 are used for kex packets. Different kex
methods may reuse message numbers in this range.
13. IANA Considerations
This document is part of a set. The IANA considerations for the SSH
protocol as defined in [SSH-ARCH], [SSH-USERAUTH], [SSH-CONNECT], and
this document, are detailed in [SSH-NUMBERS].
14. Security Considerations
This protocol provides a secure encrypted channel over an insecure
network. It performs server host authentication, key exchange,
encryption, and integrity protection. It also derives a unique
session ID that may be used by higher-level protocols.
Full security considerations for this protocol are provided in
[SSH-ARCH].
15. References
15.1. Normative References
[SSH-ARCH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Protocol Architecture", RFC 4251, January 2006.
[SSH-USERAUTH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Authentication Protocol", RFC 4252, January
2006.
[SSH-CONNECT] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Connection Protocol", RFC 4254, January 2006.
[SSH-NUMBERS] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell
(SSH) Protocol Assigned Numbers", RFC 4250, January
2006.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
1321, April 1992.
[RFC1950] Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
Format Specification version 3.3", RFC 1950, May 1996.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC 1951, May 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
2144, May 1997.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R.
Thayer, "OpenPGP Message Format", RFC 2440, November
1998.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential
(MODP) Diffie-Hellman groups for Internet Key Exchange
(IKE)", RFC 3526, May 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[FIPS-180-2] US National Institute of Standards and Technology,
"Secure Hash Standard (SHS)", Federal Information
Processing Standards Publication 180-2, August 2002.
[FIPS-186-2] US National Institute of Standards and Technology,
"Digital Signature Standard (DSS)", Federal
Information Processing Standards Publication 186-2,
January 2000.
[FIPS-197] US National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", Federal
Information Processing Standards Publication 197,
November 2001.
[FIPS-46-3] US National Institute of Standards and Technology,
"Data Encryption Standard (DES)", Federal Information
Processing Standards Publication 46-3, October 1999.
[SCHNEIER] Schneier, B., "Applied Cryptography Second Edition:
protocols algorithms and source in code in C", John
Wiley and Sons, New York, NY, 1996.
[TWOFISH] Schneier, B., "The Twofish Encryptions Algorithm: A
128-Bit Block Cipher, 1st Edition", March 1999.
15.2. Informative References
[RFC0894] Hornig, C., "Standard for the transmission of IP
datagrams over Ethernet networks", STD 41, RFC 894,
April 1984.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol",
RFC 2412, November 1998.
[ssh-1.2.30] Ylonen, T., "ssh-1.2.30/RFC", File within compressed
tarball ftp://ftp.funet.fi/pub/unix/security/
login/ssh/ssh-1.2.30.tar.gz, November 1995.
Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Valimotie 17
00380 Helsinki
Finland
EMail: ylo@ssh.com
Chris Lonvick (editor)
Cisco Systems, Inc.
12515 Research Blvd.
Austin 78759
USA
EMail: clonvick@cisco.com
Trademark Notice
"ssh" is a registered trademark in the United States and/or other
countries.
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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