Network Working Group T. Ylonen
Internet-Draft SSH Communications Security Corp
Expires: March 31, 2004 D. Moffat, Editor, Ed.
Sun Microsystems, Inc
Oct 2003
SSH Transport Layer Protocol
draft-ietf-secsh-transport-17.txt
Status of this Memo
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all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
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
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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. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . 3
4. Connection Setup . . . . . . . . . . . . . . . . . . . . . . 3
4.1 Use over TCP/IP . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Protocol Version Exchange . . . . . . . . . . . . . . . . . 4
4.3 Compatibility With Old SSH Versions . . . . . . . . . . . . 4
4.3.1 Old Client, New Server . . . . . . . . . . . . . . . . . . . 5
4.3.2 New Client, Old Server . . . . . . . . . . . . . . . . . . . 5
5. Binary Packet Protocol . . . . . . . . . . . . . . . . . . . 5
5.1 Maximum Packet Length . . . . . . . . . . . . . . . . . . . 6
5.2 Compression . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.4 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 9
5.5 Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 10
5.6 Public Key Algorithms . . . . . . . . . . . . . . . . . . . 11
6. Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1 Algorithm Negotiation . . . . . . . . . . . . . . . . . . . 13
6.2 Output from Key Exchange . . . . . . . . . . . . . . . . . . 16
6.3 Taking Keys Into Use . . . . . . . . . . . . . . . . . . . . 17
7. Diffie-Hellman Key Exchange . . . . . . . . . . . . . . . . 18
7.1 diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . . . 19
8. Key Re-Exchange . . . . . . . . . . . . . . . . . . . . . . 20
9. Service Request . . . . . . . . . . . . . . . . . . . . . . 21
10. Additional Messages . . . . . . . . . . . . . . . . . . . . 21
10.1 Disconnection Message . . . . . . . . . . . . . . . . . . . 22
10.2 Ignored Data Message . . . . . . . . . . . . . . . . . . . . 22
10.3 Debug Message . . . . . . . . . . . . . . . . . . . . . . . 23
10.4 Reserved Messages . . . . . . . . . . . . . . . . . . . . . 23
11. Summary of Message Numbers . . . . . . . . . . . . . . . . . 23
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 24
13. Security Considerations . . . . . . . . . . . . . . . . . . 24
14. Intellectual Property . . . . . . . . . . . . . . . . . . . 24
15. Additional Information . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26
Normative . . . . . . . . . . . . . . . . . . . . . . . . . 25
Informative . . . . . . . . . . . . . . . . . . . . . . . . 25
A. Contibutors . . . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . 28
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1. Contributors
The major original contributors of this document were: Tatu Ylonen,
Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH Communications
Security Corp), and Markku-Juhani O. Saarinen (University of
Jyvaskyla)
The document editor is: Darren.Moffat@Sun.COM. Comments on this
internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
2. 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, flexible, to allow
parameter negotiation, and to minimize the number of round-trips.
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.
3. Conventions Used in This Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119].
The used data types and terminology are specified in the architecture
document [SSH-ARCH].
The architecture document also discusses the algorithm naming
conventions that MUST be used with the SSH protocols.
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.
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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 of the form "SSH-protoversion-softwareversion
comments", followed by carriage return and newline characters (ASCII
13 and 10, respectively). Both sides MUST be able to process
identification strings without carriage return character. No null
character is sent. The maximum length of the string is 255
characters, including the carriage return and newline.
The part of the identification string preceding carriage return and
newline is used in the Diffie-Hellman key exchange (see Section
Section 7).
The server MAY send other lines of data before sending the version
string. Each line SHOULD be terminated by a carriage return and
newline. Such lines MUST NOT begin with "SSH-", and SHOULD be
encoded in ISO-10646 UTF-8 [RFC2279] (language is not specified).
Clients MUST be able to process such lines; they MAY be silently
ignored, or MAY be displayed to the client user; if they are
displayed, control character filtering 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.
Version strings MUST consist of printable US-ASCII characters, not
including whitespaces or a minus sign (-). The version string is
primarily used to trigger compatibility extensions and to indicate
the capabilities of an implementation. The comment string should
contain additional information that might be useful in solving user
problems.
The protocol version described in this document is 2.0.
Key exchange will begin immediately after sending this identifier.
All packets following the identification string SHALL use the binary
packet protocol, to be described below.
4.3 Compatibility With Old SSH Versions
During the transition period, it is important to be able to work in a
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way that is compatible with the installed SSH clients and servers
that use an older version of the protocol. Information in this
section is only relevant for implementations supporting compatibility
with SSH versions 1.x. There is no standards track or informational
draft available that defines the SSH 1.x protocol. The only known
documentation of the 1.x protocol is contained in README files that
are shipped along with the source code.
4.3.1 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 protocol version 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 version identification string.
In the compatibility mode the server SHOULD NOT send any further data
after its initialization 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 version string.
When compatibility with old clients is not needed, the server MAY
send its initial key exchange data immediately after the
identification string.
4.3.2 New Client, Old Server
Since the new client MAY immediately send additional data after its
identification string (before receiving server's identification), the
old protocol may already have been corrupted 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. 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); m = mac_length
packet_length
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The length of the packet (bytes), not including MAC or the
packet_length field itself.
padding_length
Length of 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 || 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 length of the concatenation of packet length, padding
length, payload, and 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.
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.
5.1 Maximum Packet Length
All implementations MUST be able to process packets with uncompressed
payload length of 32768 bytes or less and total packet size of 35000
bytes or less (including length, padding length, payload, padding,
and MAC.). The maximum of 35000 bytes is an arbitrary chosen value
larger than uncompressed size. Implementations SHOULD support longer
packets, where they might be needed, e.g. if an implementation wants
to send a very large number of certificates. Such packets MAY be
sent if the version string indicates that the other party is able to
process them. However, implementations SHOULD check that the packet
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length is reasonable for the implementation to avoid
denial-of-service and/or buffer overflow attacks.
5.2 Compression
If compression has been negotiated, the payload field (and only it)
will be compressed using the negotiated algorithm. The 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 independently choosing the algorithm for each direction.
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].
5.3 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,
and implementations MUST allow independently choosing the algorithm
for each direction (if multiple algorithms are allowed by local
policy).
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The following ciphers are currently defined:
3des-cbc REQUIRED three-key 3DES in CBC mode
blowfish-cbc OPTIONALi Blowfish in CBC mode
twofish256-cbc OPTIONAL Twofish in CBC mode,
with 256-bit key
twofish-cbc OPTIONAL alias for "twofish256-cbc" (this
is being retained for
historical reasons)
twofish192-cbc OPTIONAL Twofish with 192-bit key
twofish128-cbc OPTIONAL Twofish with 128-bit key
aes256-cbc OPTIONAL AES (Rijndael) in CBC mode,
with 256-bit key
aes192-cbc OPTIONAL AES with 192-bit key
aes128-cbc RECOMMENDED AES with 128-bit key
serpent256-cbc OPTIONAL Serpent in CBC mode, with
256-bit key
serpent192-cbc OPTIONAL Serpent with 192-bit key
serpent128-cbc OPTIONAL Serpent with 128-bit key
arcfour OPTIONAL the ARCFOUR stream cipher
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]
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. Same as above but with 192-bit key.
The "twofish128-cbc" cipher. Same as above but with 128-bit key.
The "aes256-cbc" cipher is AES (Advanced Encryption Standard)
[FIPS-197], formerly Rijndael, in CBC mode. This version uses 256-bit
key.
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The "aes192-cbc" cipher. Same as above but with 192-bit key.
The "aes128-cbc" cipher. Same as above but with 128-bit key.
The "serpent256-cbc" cipher in CBC mode, with 256-bit key as
described in the Serpent AES submission.
The "serpent192-cbc" cipher. Same as above but with 192-bit key.
The "serpent128-cbc" cipher. Same as above but with 128-bit key.
The "arcfour" is the Arcfour stream cipher with 128 bit keys. The
Arcfour cipher is believed to be compatible with the RC4 cipher
[SCHNEIER]. RC4 is a registered trademark of RSA Data Security Inc.
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
[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].
5.4 Data Integrity
Data integrity is protected by including with each packet a message
authentication code (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 selected MAC 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 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
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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.
The MAC bytes 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
Figure 1
The "hmac-*" algorithms are described in [RFC2104] The "*-n" MACs use
only the first n bits of the resulting value.
The hash algorithms are described in [SCHNEIER].
Additional methods may be defined as specified in [SSH-ARCH].
5.5 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.
Only one REQUIRED key exchange method has been defined:
diffie-hellman-group1-sha1 REQUIRED
This method is described later in this document.
Additional methods may be defined as specified in [SSH-ARCH].
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5.6 Public Key Algorithms
This protocol has been designed to be able 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 in e.g. 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
x509v3-sign-rsa OPTIONAL sign X.509 certificates (RSA key)
x509v3-sign-dss OPTIONAL sign X.509 certificates (DSS key)
spki-sign-rsa OPTIONAL sign SPKI certificates (RSA key)
spki-sign-dss OPTIONAL sign SPKI certificates (DSS 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].
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 have 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 / certifcate formats that do not explicitly specify a
signature format identifier MUST use the public key / certificate
format identifier as the signature identifier.
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Signatures are encoded as follows:
string signature format identifier (as specified by the
public key / cert 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] using the SHA-1 hash. A
description can also be found in [SCHNEIER].
The resulting signature is encoded as follows:
string "ssh-dss"
string dss_signature_blob
dss_signature_blob is encoded as a string containing r followed by s
(which are 160 bits long 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 done according to
[SCHNEIER] and [PKCS1] using the SHA-1 hash.
The resulting signature is encoded as follows:
string "ssh-rsa"
string rsa_signature_blob
rsa_signature_blob is encoded as a string containing s (which is an
integer, without lengths or padding, unsigned and in network byte
order).
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The "spki-sign-rsa" method indicates that the certificate blob
contains a sequence of SPKI certificates. The format of SPKI
certificates is described in [RFC2693]. This method indicates that
the key (or one of the keys in the certificate) is an RSA-key.
The "spki-sign-dss". As above, but indicates that the key (or one of
the keys in the certificate) is a DSS-key.
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". As above, but indicates that the key is a
DSS-key.
6. Key Exchange
Key exchange begins by each side sending 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.
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 Section 6.1).
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.
Server authentication in the key exchange MAY be implicit. After a
key exchange with implicit server authentication, the client MUST
wait for response to its service request message before sending any
further data.
6.1 Algorithm Negotiation
Key exchange begins by each side sending the following packet:
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byte SSH_MSG_KEXINIT
byte[16] cookie (random bytes)
string kex_algorithms
string server_host_key_algorithms
string encryption_algorithms_client_to_server
string encryption_algorithms_server_to_client
string mac_algorithms_client_to_server
string mac_algorithms_server_to_client
string compression_algorithms_client_to_server
string compression_algorithms_server_to_client
string languages_client_to_server
string languages_server_to_client
boolean first_kex_packet_follows
uint32 0 (reserved for future extension)
Each of the algorithm strings MUST be a comma-separated list of
algorithm names (see ''Algorithm Naming'' in [SSH-ARCH]). Each
supported (allowed) algorithm MUST be listed in order of preference.
The first algorithm in each list MUST be the preferred (guessed)
algorithm. Each string 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.
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server_host_key_algorithms
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 encryption capable host key.
It MUST be possible to determine this from the public key
algorithm name. The first algorithm on the client's 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
Lists the acceptable symmetric encryption algorithms in order
of preference. The chosen encryption algorithm to each
direction MUST be the first algorithm on the client's list
that is also on the server's 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
Section 5.3.
mac_algorithms
Lists the acceptable MAC algorithms in order of preference.
The chosen MAC algorithm MUST be the first algorithm on the
client's list that is also on the server's 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
Figure 1.
compression_algorithms
Lists the acceptable compression algorithms in order of
preference. The chosen compression algorithm MUST be the first
algorithm on the client's list that is also on the server's
list. If there is no such algorithm, both sides MUST
disconnect.
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Note that "none" must be explicitly listed if it is to be
acceptable. The compression algorithm names are listed in
Section Section 5.2.
languages
This is a comma-separated list of language tags in order of
preference [RFC3066]. Both parties MAY ignore this list. If
there are no language preferences, this list SHOULD be empty.
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 KEXINIT packet exchange, the key exchange algorithm is run.
It may involve several packet exchanges, as specified by the key
exchange method.
6.2 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:
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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. 128
bits (16 bytes) MUST be used for algorithms with variable-length
keys. The only variable key length algorithm defined in this document
is arcfour). For other algorithms, as many bytes as are 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.
6.3 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
taken into use for receiving.
This message is the only valid message after key exchange, in
addition to SSH_MSG_DEBUG, SSH_MSG_DISCONNECT and SSH_MSG_IGNORE
messages. The purpose of this message is to ensure that a party is
able to respond with a disconnect message that the other party can
understand if something goes wrong with the key exchange.
Implementations MUST NOT accept any other messages after key exchange
before receiving SSH_MSG_NEWKEYS.
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byte SSH_MSG_NEWKEYS
7. Diffie-Hellman Key Exchange
The Diffie-Hellman key exchange provides a shared secret that can not
be determined by either party alone. The key exchange is combined
with a signature with the host key to provide host authentication.
In the following description (C is the client, S is the server; p is
a large safe prime, g is a generator for a subgroup of GF(p), and q
is the order of the subgroup; V_S is S's version string; V_C is C's
version string; K_S is S's public host key; I_C is C's KEXINIT
message and I_S S's KEXINIT message which 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.
Either side MUST NOT send or accept e or f values that are not in the
range [1, p-1]. 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 KEXINIT messages.
First, the client sends the following:
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byte SSH_MSG_KEXDH_INIT
mpint e
The server 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 version string (CR and NL excluded)
string V_S, the server's version string (CR and NL 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. For example, "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.
7.1 diffie-hellman-group1-sha1
The "diffie-hellman-group1-sha1" method specifies Diffie-Hellman key
exchange with SHA-1 as HASH, and Oakley group 14 [RFC3526] (2048-bit
MODP Group). It is included below in hexadecimal and decimal.
The prime p is equal to 2^1024 - 2^960 - 1 + 2^64 * floor( 2^894 Pi +
129093 ). Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
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FFFFFFFF FFFFFFFF.
In decimal, this value is:
179769313486231590770839156793787453197860296048756011706444
423684197180216158519368947833795864925541502180565485980503
646440548199239100050792877003355816639229553136239076508735
759914822574862575007425302077447712589550957937778424442426
617334727629299387668709205606050270810842907692932019128194
467627007.
The generator used with this prime is g = 2. The group order q is (p
- 1) / 2.
8. 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 Section
6.1). 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 are 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.
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9. 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].
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
"servicename@domain" syntax.
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 Section 6 in [SSH-ARCH]). 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 response to its service request message
before sending any further data.
10. Additional Messages
Either party may send any of the following messages at any time.
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10.1 Disconnection Message
byte SSH_MSG_DISCONNECT
uint32 reason code
string description [RFC2279]
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 description field gives a more specific explanation in a
human-readable form. The error code gives the reason in a more
machine-readable format (suitable for localization), and can have the
following values:
#define SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT 1
#define SSH_DISCONNECT_PROTOCOL_ERROR 2
#define SSH_DISCONNECT_KEY_EXCHANGE_FAILED 3
#define SSH_DISCONNECT_RESERVED 4
#define SSH_DISCONNECT_MAC_ERROR 5
#define SSH_DISCONNECT_COMPRESSION_ERROR 6
#define SSH_DISCONNECT_SERVICE_NOT_AVAILABLE 7
#define SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED 8
#define SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE 9
#define SSH_DISCONNECT_CONNECTION_LOST 10
#define SSH_DISCONNECT_BY_APPLICATION 11
#define SSH_DISCONNECT_TOO_MANY_CONNECTIONS 12
#define SSH_DISCONNECT_AUTH_CANCELLED_BY_USER 13
#define SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE 14
#define SSH_DISCONNECT_ILLEGAL_USER_NAME 15
If the description string is displayed, control character filtering
discussed in [SSH-ARCH] should be used to avoid attacks by sending
terminal control characters.
10.2 Ignored Data Message
byte SSH_MSG_IGNORE
string data
All implementations MUST understand (and ignore) this message at any
time (after receiving the protocol version). No implementation is
required to send them. This message can be used as an additional
protection measure against advanced traffic analysis techniques.
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10.3 Debug Message
byte SSH_MSG_DEBUG
boolean always_display
string message [RFC2279]
string language tag [RFC3066]
All implementations MUST understand this message, but they are
allowed to ignore it. This message is used to pass the other side
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, terminal control character
filtering discussed in [SSH-ARCH] should be used to avoid attacks by
sending terminal control characters.
10.4 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
11. Summary of Message Numbers
The following message numbers have been defined in this protocol:
#define SSH_MSG_DISCONNECT 1
#define SSH_MSG_IGNORE 2
#define SSH_MSG_UNIMPLEMENTED 3
#define SSH_MSG_DEBUG 4
#define SSH_MSG_SERVICE_REQUEST 5
#define SSH_MSG_SERVICE_ACCEPT 6
#define SSH_MSG_KEXINIT 20
#define SSH_MSG_NEWKEYS 21
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/* Numbers 30-49 used for kex packets.
Different kex methods may reuse message numbers in
this range. */
#define SSH_MSG_KEXDH_INIT 30
#define SSH_MSG_KEXDH_REPLY 31
12. IANA Considerations
This document is part of a set, the IANA considerations for the SSH
protocol as defined in [SSH-ARCH], [SSH-TRANS], [SSH-USERAUTH],
[SSH-CONNECT] are detailed in [SSH-NUMBERS].
13. 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
Section 8 of [SSH-ARCH]
14. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
15. Additional Information
The current document editor is: Darren.Moffat@Sun.COM. Comments on
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this internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
Normative
[SSH-ARCH]
Ylonen, T., "SSH Protocol Architecture", I-D
draft-ietf-architecture-15.txt, Oct 2003.
[SSH-TRANS]
Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-17.txt, Oct 2003.
[SSH-USERAUTH]
Ylonen, T., "SSH Authentication Protocol", I-D
draft-ietf-userauth-18.txt, Oct 2003.
[SSH-CONNECT]
Ylonen, T., "SSH Connection Protocol", I-D
draft-ietf-connect-18.txt, Oct 2003.
[SSH-NUMBERS]
Lehtinen, S. and D. Moffat, "SSH Protocol Assigned
Numbers", I-D draft-ietf-secsh-assignednumbers-05.txt, Oct
2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative
[FIPS-186]
Federal Information Processing Standards Publication,
"FIPS PUB 186, Digital Signature Standard", May 1994.
[FIPS-197]
NIST, "FIPS PUB 197 Advanced Encryption Standard (AES)",
November 2001.
[FIPS-46-3]
U.S. Dept. of Commerce, "FIPS PUB 46-3, Data Encryption
Standard (DES)", October 1999.
[RFC2459] Housley, R., Ford, W., Polk, T. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January 1999.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
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Internet-Draft SSH Transport Layer Protocol Oct 2003
STD 13, RFC 1034, November 1987.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC1950] Deutsch, L. 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.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
May 1997.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[RFC2693] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
B. and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
September 1999.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[SCHNEIER]
Schneier, B., "Applied Cryptography Second Edition:
protocols algorithms and source in code in C", 1996.
[TWOFISH] Schneier, B., "The Twofish Encryptions Algorithm: A
128-Bit Block Cipher, 1st Edition", March 1999.
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Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo@ssh.com
Darren J. Moffat (editor)
Sun Microsystems, Inc
17 Network Circle
Menlo Park 95025
USA
EMail: Darren.Moffat@Sun.COM
Appendix A. Contibutors
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
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and distributed, in whole or in part, without restriction of any
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
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This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
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Ylonen & Moffat, Editor Expires March 31, 2004 [Page 29]
