draft-ietf-secsh-gsskeyex-06.txt   draft-ietf-secsh-gsskeyex-07.txt 
Network Working Group J. Hutzelman Secure Shell Working Group J. Hutzelman
Internet-Draft CMU Internet-Draft CMU
Expires: August 31, 2003 J. Salowey Expires: March 12, 2004 J. Salowey
Cisco Systems Cisco Systems
J. Galbraith J. Galbraith
Van Dyke Technologies, Inc. Van Dyke Technologies, Inc.
V. Welch V. Welch
U Chicago / ANL U Chicago / ANL
March 2, 2003 September 12, 2003
GSSAPI Authentication and Key Exchange for the Secure Shell Protocol GSSAPI Authentication and Key Exchange for the Secure Shell Protocol
draft-ietf-secsh-gsskeyex-06 draft-ietf-secsh-gsskeyex-07
Status of this Memo Status of this Memo
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all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
The Secure Shell protocol (SSH) is a protocol for secure remote The Secure Shell protocol (SSH) is a protocol for secure remote login
login and other secure network services over an insecure network. and other secure network services over an insecure network.
The Generic Security Service Application Program Interface (GSS-API) The Generic Security Service Application Program Interface (GSS-API)
[2] provides security services to callers in a mechanism-independent [2] provides security services to callers in a mechanism-independent
fashion. fashion.
This memo describes methods for using the GSS-API for authentication This memo describes methods for using the GSS-API for authentication
and key exchange in SSH. It defines an SSH user authentication and key exchange in SSH. It defines an SSH user authentication method
method which uses a specified GSSAPI mechanism to authenticate a which uses a specified GSSAPI mechanism to authenticate a user, and a
user, and a family of SSH key exchange methods which use GSSAPI to family of SSH key exchange methods which use GSSAPI to authenticate
authenticate the Diffie-Hellman exchange described in [8]. the Diffie-Hellman exchange described in [8].
This memo also defines a new host public key algorithm which can be This memo also defines a new host public key algorithm which can be
used when no operations are needed using a host's public key, and a used when no operations are needed using a host's public key, and a
new user authentication method which allows an authorization name to new user authentication method which allows an authorization name to
be used in conjunction with any authentication which has already be used in conjunction with any authentication which has already
occurred as a side-effect of key exchange. occurred as a side-effect of GSSAPI-based key exchange.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [5]. document are to be interpreted as described in [5].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 SSH terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . . 5
2.1 Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . . . 5
2.2 gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Other GSSAPI key exchange methods . . . . . . . . . . . . . . 11
3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . . 12
3.1 GSSAPI Authentication Overview . . . . . . . . . . . . . . . . 12
3.2 Initiating GSSAPI authentication . . . . . . . . . . . . . . . 12
3.3 Initial server response . . . . . . . . . . . . . . . . . . . 13
3.4 GSSAPI session . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5 Binding Encryption Keys . . . . . . . . . . . . . . . . . . . 14
3.6 Client acknowledgement . . . . . . . . . . . . . . . . . . . . 15
3.7 Completion . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.8 Error Status . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.9 Error Token . . . . . . . . . . . . . . . . . . . . . . . . . 17
4. Authentication using GSSAPI Key Exchange . . . . . . . . . . . 18
5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . . 20
6. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 21
7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . . 22
7.1 Naming Conventions . . . . . . . . . . . . . . . . . . . . . . 22
7.2 Channel Bindings . . . . . . . . . . . . . . . . . . . . . . . 22
7.3 SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 25
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
11. Changes the last version . . . . . . . . . . . . . . . . . . . 27
Normative References . . . . . . . . . . . . . . . . . . . . . 28
Normative References . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . 31
1. Introduction 1. Introduction
This document describes the methods used to perform key exchange and This document describes the methods used to perform key exchange and
user authentication in the Secure Shell protocol using the GSSAPI. user authentication in the Secure Shell protocol using the GSSAPI.
To do this, it defines a family of key exchange methods, two user To do this, it defines a family of key exchange methods, two user
authentication methods, and a new host key algorithm. These authentication methods, and a new host key algorithm. These
definitions allow any GSSAPI mechanism to be used with the Secure definitions allow any GSSAPI mechanism to be used with the Secure
Shell protocol. Shell protocol.
This document should be read only after reading the documents This document should be read only after reading the documents
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protocol [8], and user authentication protocol [9]. This document protocol [8], and user authentication protocol [9]. This document
freely uses terminology and notation from the architecture document freely uses terminology and notation from the architecture document
without reference or further explanation. without reference or further explanation.
1.1 SSH terminology 1.1 SSH terminology
The data types used in the packets are defined in the SSH The data types used in the packets are defined in the SSH
architecture document [6]. It is particularly important to note the architecture document [6]. It is particularly important to note the
definition of string allows binary content. definition of string allows binary content.
The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this service
service name is an SSH service name, and has no relationship to name is an SSH service name, and has no relationship to GSSAPI
GSSAPI service names. Currently, the only defined service name is service names. Currently, the only defined service name is
"ssh-connection", which refers to the SSH connection protocol [7]. "ssh-connection", which refers to the SSH connection protocol [7].
2. GSSAPI Authenticated Diffie-Hellman Key Exchange 2. GSSAPI Authenticated Diffie-Hellman Key Exchange
This section defines a class of key exchange methods which combine This section defines a class of key exchange methods which combine
the Diffie-Hellman key exchange from section 6 of [8] with mutual the Diffie-Hellman key exchange from section 6 of [8] with mutual
authentication using GSSAPI. authentication using GSSAPI.
Since the GSSAPI key exchange methods described in this section do Since the GSSAPI key exchange methods described in this section do
not require the use of public key signature or encryption not require the use of public key signature or encryption algorithms,
algorithms, they MAY be used with any host key algorithm, including they MAY be used with any host key algorithm, including the "null"
the "null" algorithm described in Section 5. algorithm described in Section 5.
2.1 Generic GSSAPI Key Exchange 2.1 Generic GSSAPI Key Exchange
The following symbols are used in this description: The following symbols are used in this description:
o C is the client, and S is the server o C is the client, and S is the server
o p is a large safe prime, g is a generator for a subgroup of o p is a large safe prime, g is a generator for a subgroup of GF(p),
GF(p), and q is the order of the subgroup and q is the order of the subgroup
o V_S is S's version string, and V_C is C's version string o V_S is S's version string, and V_C is C's version string
o I_C is C's KEXINIT message, and I_S is S's KEXINIT message o I_C is C's KEXINIT message, and I_S is S's KEXINIT message
1. C generates a random number x (1 < x < q) and computes e = g^x 1. C generates a random number x (1 < x < q) and computes e = g^x
mod p. mod p.
2. C calls GSS_Init_sec_context, using the most recent reply token 2. C calls GSS_Init_sec_context, using the most recent reply token
received from S during this exchange, if any. For this call, received from S during this exchange, if any. For this call, the
the client MUST set the mutual_req_flag to "true" to request client MUST set the mutual_req_flag to "true" to request that
that mutual authentication be performed. It also MUST set the mutual authentication be performed. It also MUST set the
integ_req_flag to "true" to request that per-message integrity integ_req_flag to "true" to request that per-message integrity
protection be supported for this context. In addition, the protection be supported for this context. In addition, the
deleg_req_flag MAY be set to "true" to request access deleg_req_flag MAY be set to "true" to request access delegation,
delegation, if requested by the user. Since the key exchange if requested by the user. Since the key exchange process
process authenticates only the host, the setting of the authenticates only the host, the setting of the anon_req_flag is
anon_req_flag is immaterial to this process. If the client does immaterial to this process. If the client does not support the
not support the "external-keyx" user authentication method "gssapi-keyex" user authentication method described in Section 4,
described in Section 4, or does not intend to use that method, or does not intend to use that method in conjunction with the
then the anon_req_flag SHOULD be set to "true". Otherwise, this GSSAPI context established during key exchange, then the
flag MAY be set to true if the client wishes to hide its anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY
identity. be set to true if the client wishes to hide its identity. Since
the key exchange process will involve the exchange of only a
single token once the context has been established, it is not
necessary that the GSSAPI context support detection of replayed
or out-of-sequence tokens. Thus, the setting of the
replay_det_req_flag and sequence_req_flag are not needed for this
process. These flags SHOULD be set to "false".
* If the resulting major_status code is GSS_S_COMPLETE and the * If the resulting major_status code is GSS_S_COMPLETE and the
mutual_state flag is not true, then mutual authentication has mutual_state flag is not true, then mutual authentication has
not been established, and the key exchange MUST fail. not been established, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and the * If the resulting major_status code is GSS_S_COMPLETE and the
integ_avail flag is not true, then per-message integrity integ_avail flag is not true, then per-message integrity
protection is not available, and the key exchange MUST fail. protection is not available, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and both * If the resulting major_status code is GSS_S_COMPLETE and both
the mutual_state and integ_avail flags are true, the the mutual_state and integ_avail flags are true, the resulting
resulting output token is sent to S. output token is sent to S.
* If the resulting major_status code is GSS_S_CONTINUE_NEEDED, * If the resulting major_status code is GSS_S_CONTINUE_NEEDED,
the the output_token is sent to S, which will reply with a the the output_token is sent to S, which will reply with a new
new token to be provided to GSS_Init_sec_context. token to be provided to GSS_Init_sec_context.
* The client MUST also include "e" with the first message it * The client MUST also include "e" with the first message it
sends to the server during this process; if the server sends to the server during this process; if the server
receives more than one "e" or none at all, the key exchange receives more than one "e" or none at all, the key exchange
fails. fails.
* It is an error if the call does not produce a token of * It is an error if the call does not produce a token of
non-zero length to be sent to the server. In this case, the non-zero length to be sent to the server. In this case, the
key exchange MUST fail. key exchange MUST fail.
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then the output token is sent to C, and processing continues then the output token is sent to C, and processing continues
with step 2. with step 2.
* If the resulting major_status code is GSS_S_COMPLETE, but a * If the resulting major_status code is GSS_S_COMPLETE, but a
non-zero-length reply token is returned, then that token is non-zero-length reply token is returned, then that token is
sent to the client. sent to the client.
4. S generates a random number y (0 < y < q) and computes f = g^y 4. S generates a random number y (0 < y < q) and computes f = g^y
mod p. It computes K = e ^ y mod p, and H = hash(V_C || V_S || mod p. It computes K = e ^ y mod p, and H = hash(V_C || V_S ||
I_C || I_S || K_S || e || f || K). It then calls GSS_GetMIC to I_C || I_S || K_S || e || f || K). It then calls GSS_GetMIC to
obtain a GSSAPI message integrity code for H. S then sends f obtain a GSSAPI message integrity code for H. S then sends f and
and the MIC to C. the MIC to C.
5. This step is performed only if the server's final call to 5. This step is performed only if the server's final call to
GSS_Accept_sec_context produced a non-zero-length final reply GSS_Accept_sec_context produced a non-zero-length final reply
token to be sent to the client _and_ no previous call by the token to be sent to the client _and_ no previous call by the
client to GSS_Init_sec_context has resulted in a major_status of client to GSS_Init_sec_context has resulted in a major_status of
GSS_S_COMPLETE. Under these conditions, the client makes an GSS_S_COMPLETE. Under these conditions, the client makes an
additional call to GSS_Init_sec_context to process the final additional call to GSS_Init_sec_context to process the final
reply token. This call is made exactly as described above. reply token. This call is made exactly as described above.
However, if the resulting major_status is anything other than However, if the resulting major_status is anything other than
GSS_S_COMPLETE, or a non-zero-length token is returned, it is an GSS_S_COMPLETE, or a non-zero-length token is returned, it is an
error and the key exchange MUST fail. error and the key exchange MUST fail.
6. C computes K = f^x mod p, and H = hash(V_C || V_S || I_C || I_S 6. C computes K = f^x mod p, and H = hash(V_C || V_S || I_C || I_S
|| K_S || e || f || K). It then calls GSS_VerifyMIC to verify || K_S || e || f || K). It then calls GSS_VerifyMIC to verify
that the MIC sent by S matches H. that the MIC sent by S matches H. If the MIC is not successfully
verified, the key exchange MUST fail.
Either side MUST NOT send or accept e or f values that are not in Either side MUST NOT send or accept e or f values that are not in the
the range [1, p-1]. If this condition is violated, the key exchange range [1, p-1]. If this condition is violated, the key exchange
fails. fails.
If any call to GSS_Init_sec_context or GSS_Accept_sec_context If any call to GSS_Init_sec_context or GSS_Accept_sec_context returns
returns a major_status other than GSS_S_COMPLETE or a major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or
GSS_S_CONTINUE_NEEDED, or any other GSSAPI call returns a any other GSSAPI call returns a major_status other than
major_status other than GSS_S_COMPLETE, the key exchange fails. In GSS_S_COMPLETE, the key exchange fails. In this case, several
this case, several mechanisms are available for communicating error mechanisms are available for communicating error information to the
information to the peer before terminating the connection as peer before terminating the connection as required by [8]:
required by [8]:
o If the key exchange fails due to any GSSAPI error on the server o If the key exchange fails due to any GSSAPI error on the server
(including errors returned by GSS_Accept_sec_context), the server (including errors returned by GSS_Accept_sec_context), the server
MAY send a message informing the client of the details of the MAY send a message informing the client of the details of the
error. In this case, if an error token is also sent (see below), error. In this case, if an error token is also sent (see below),
then this message MUST be sent before the error token. then this message MUST be sent before the error token.
o If the key exchange fails due to a GSSAPI error returned from the o If the key exchange fails due to a GSSAPI error returned from the
server's call to GSS_Accept_sec_context, and an "error token" is server's call to GSS_Accept_sec_context, and an "error token" is
also returned, then the server SHOULD send the error token to the also returned, then the server SHOULD send the error token to the
client to allow completion of the GSS security exchange. client to allow completion of the GSS security exchange.
o If the key exchange fails due to a GSSAPI error returned from the o If the key exchange fails due to a GSSAPI error returned from the
client's call to GSS_Init_sec_context, and an "error token" is client's call to GSS_Init_sec_context, and an "error token" is
also returned, then the client SHOULD send the error token to the also returned, then the client SHOULD send the error token to the
server to allow completion of the GSS security exchange. server to allow completion of the GSS security exchange.
As noted in Section 9, it may be desirable under site security As noted in Section 9, it may be desirable under site security policy
policy to obscure information about the precise nature of the error; to obscure information about the precise nature of the error; thus,
thus, it is RECOMMENDED that implementations provide a method to it is RECOMMENDED that implementations provide a method to suppress
suppress these messages as a matter of policy. these messages as a matter of policy.
This is implemented with the following messages. The hash algorithm This is implemented with the following messages. The hash algorithm
for computing the exchange hash is defined by the method name, and for computing the exchange hash is defined by the method name, and is
is called HASH. The group used for Diffie-Hellman key exchange and called HASH. The group used for Diffie-Hellman key exchange and the
the underlying GSSAPI mechanism are also defined by the method name. underlying GSSAPI mechanism are also defined by the method name.
After the client's first call to GSS_Init_sec_context, it sends the After the client's first call to GSS_Init_sec_context, it sends the
following: following:
byte SSH_MSG_KEXGSS_INIT byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context) string output_token (from GSS_Init_sec_context)
mpint e mpint e
Upon receiving the SSH_MSG_KEXGSS_INIT message, the server MAY send Upon receiving the SSH_MSG_KEXGSS_INIT message, the server MAY send
the following message, prior to any other messages, to inform the the following message, prior to any other messages, to inform the
client of its host key. client of its host key.
byte SSH_MSG_KEXGSS_HOSTKEY byte SSH_MSG_KEXGSS_HOSTKEY
string server public host key and certificates (K_S) string server public host key and certificates (K_S)
Since this key exchange method does not require the host key to be Since this key exchange method does not require the host key to be
used for any encryption operations, this message is OPTIONAL. If used for any encryption operations, this message is OPTIONAL. If the
the "null" host key algorithm described in Section 5 is used, this "null" host key algorithm described in Section 5 is used, this
message MUST NOT be sent. If this message is sent, the server message MUST NOT be sent. If this message is sent, the server public
public host key(s) and/or certificate(s) in this message are encoded host key(s) and/or certificate(s) in this message are encoded as a
as a single string, in the format specified by the public key type single string, in the format specified by the public key type in use
in use (see [8], section 4.6). (see [8], section 4.6).
Each time the server's call to GSS_Accept_sec_context returns a Each time the server's call to GSS_Accept_sec_context returns a
major_status code of GSS_S_CONTINUE_NEEDED, it sends the following major_status code of GSS_S_CONTINUE_NEEDED, it sends the following
reply to the client: reply to the client:
byte SSH_MSG_KEXGSS_CONTINUE byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context) string output_token (from GSS_Accept_sec_context)
If the client receives this message after a call to If the client receives this message after a call to
GSS_Init_sec_context has returned a major_status code of GSS_Init_sec_context has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail. MUST fail.
Each time the client receives the message described above, it makes Each time the client receives the message described above, it makes
another call to GSS_Init_sec_context. It then sends the following: another call to GSS_Init_sec_context. It then sends the following:
byte SSH_MSG_KEXGSS_CONTINUE byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Init_sec_context) string output_token (from GSS_Init_sec_context)
The server and client continue to trade these two messages as long The server and client continue to trade these two messages as long as
as the server's calls to GSS_Accept_sec_context result in the server's calls to GSS_Accept_sec_context result in major_status
major_status codes of GSS_S_CONTINUE_NEEDED. When a call results in codes of GSS_S_CONTINUE_NEEDED. When a call results in a
a major_status code of GSS_S_COMPLETE, it sends one of two final major_status code of GSS_S_COMPLETE, it sends one of two final
messages. messages.
If the server's final call to GSS_Accept_sec_context (resulting in a If the server's final call to GSS_Accept_sec_context (resulting in a
major_status code of GSS_S_COMPLETE) returns a non-zero-length token major_status code of GSS_S_COMPLETE) returns a non-zero-length token
to be sent to the client, it sends the following: to be sent to the client, it sends the following:
byte SSH_MSG_KEXGSS_COMPLETE byte SSH_MSG_KEXGSS_COMPLETE
mpint f mpint f
string per_msg_token (MIC of H) string per_msg_token (MIC of H)
boolean TRUE boolean TRUE
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byte SSH_MSG_KEXGSS_CONTINUE byte SSH_MSG_KEXGSS_CONTINUE
string error_token string error_token
If a server sends both this message and an SSH_MSG_KEXGSS_ERROR If a server sends both this message and an SSH_MSG_KEXGSS_ERROR
message, the SSH_MSG_KEXGSS_ERROR message MUST be sent first, to message, the SSH_MSG_KEXGSS_ERROR message MUST be sent first, to
allow clients to record and/or display the error information before allow clients to record and/or display the error information before
processing the error token. This is important because a client processing the error token. This is important because a client
processing an error token will likely disconnect without reading any processing an error token will likely disconnect without reading any
further messages. further messages.
In the event of a GSSAPI error on the server, the server MAY send In the event of a GSSAPI error on the server, the server MAY send the
the following message before terminating the connection: following message before terminating the connection:
byte SSH_MSG_KEXGSS_ERROR byte SSH_MSG_KEXGSS_ERROR
uint32 major_status uint32 major_status
uint32 minor_status uint32 minor_status
string message string message
string language tag string language tag
The message text MUST be encoded in the UTF-8 encoding described in The message text MUST be encoded in the UTF-8 encoding described in
[10]. Language tags are those described in [11]. Note that the [10]. Language tags are those described in [11]. Note that the
message text may contain multiple lines separated by carriage message text may contain multiple lines separated by carriage
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secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the
server or received by the client, then the empty string is used in server or received by the client, then the empty string is used in
place of K_S when computing the exchange hash. place of K_S when computing the exchange hash.
The GSS_GetMIC call MUST be applied over H, not the original data. The GSS_GetMIC call MUST be applied over H, not the original data.
2.2 gss-group1-sha1-* 2.2 gss-group1-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman Each of these methods specifies GSSAPI authenticated Diffie-Hellman
key exchange as described in Section 2.1 with SHA-1 as HASH, and the key exchange as described in Section 2.1 with SHA-1 as HASH, and the
group defined in section 6.1 of [8]. The method name for each group defined in section 6.1 of [8]. The method name for each method
method is the concatenation of the string "gss-group1-sha1-" with is the concatenation of the string "gss-group1-sha1-" with the Base64
the Base64 encoding of the MD5 hash [3] of the ASN.1 DER encoding encoding of the MD5 hash [3] of the ASN.1 DER encoding [1] of the
[1] of the underlying GSSAPI mechanism's OID. Base64 encoding is underlying GSSAPI mechanism's OID. Base64 encoding is described in
described in section 6.8 of [4]. section 6.8 of [4].
Each and every such key exchange method is implicitly registered by Each and every such key exchange method is implicitly registered by
this specification. The IESG is considered to be the owner of all this specification. The IESG is considered to be the owner of all
such key exchange methods; this does NOT imply that the IESG is such key exchange methods; this does NOT imply that the IESG is
considered to be the owner of the underlying GSSAPI mechanism. considered to be the owner of the underlying GSSAPI mechanism.
2.3 Other GSSAPI key exchange methods 2.3 Other GSSAPI key exchange methods
Key exchange method names starting with "gss-" are reserved for key Key exchange method names starting with "gss-" are reserved for key
exchange methods which conform to this document; in particular, for exchange methods which conform to this document; in particular, for
those methods which use the GSSAPI authenticated Diffie-Hellman key those methods which use the GSSAPI authenticated Diffie-Hellman key
exchange algorithm described in Section 2.1, including any future exchange algorithm described in Section 2.1, including any future
methods which use different groups and/or hash functions. The methods which use different groups and/or hash functions. The intent
intent is that the names for any such future methods methods be is that the names for any such future methods methods be defined in a
defined in a similar manner to that used in Section 2.2. similar manner to that used in Section 2.2.
3. GSSAPI User Authentication 3. GSSAPI User Authentication
This section describes a general-purpose user authentication method This section describes a general-purpose user authentication method
based on [2]. It is intended to be run over the SSH user based on [2]. It is intended to be run over the SSH user
authentication protocol [9]. authentication protocol [9].
The authentication method name for this protocol is "gssapi". The authentication method name for this protocol is
"gssapi-with-mic".
3.1 GSSAPI Authentication Overview 3.1 GSSAPI Authentication Overview
GSSAPI authentication must maintain a context. Authentication GSSAPI authentication must maintain a context. Authentication begins
begins when the client sends a SSH_MSG_USERAUTH_REQUEST, which when the client sends a SSH_MSG_USERAUTH_REQUEST, which specifies the
specifies the mechanism OIDs the client supports. mechanism OIDs the client supports.
If the server supports any of the requested mechanism OIDs, the If the server supports any of the requested mechanism OIDs, the
server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing
the mechanism OID. the mechanism OID.
After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the
client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets
until the authentication mechanism either succeeds or fails. until the authentication mechanism either succeeds or fails.
If at any time during the exchange, the client sends a new If at any time during the exchange, the client sends a new
SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely
discarded and destroyed, and any further GSSAPI authentication MUST discarded and destroyed, and any further GSSAPI authentication MUST
restart from the beginning. restart from the beginning.
3.2 Initiating GSSAPI authentication 3.2 Initiating GSSAPI authentication
The GSSAPI authentication method is initiated when the client sends The GSSAPI authentication method is initiated when the client sends a
a SSH_MSG_USERAUTH_REQUEST: SSH_MSG_USERAUTH_REQUEST:
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding) string user name (in ISO-10646 UTF-8 encoding)
string service name (in US-ASCII) string service name (in US-ASCII)
string "gssapi" (US-ASCII method name) string "gssapi-with-mic" (US-ASCII method name)
uint32 n, the number of mechanism OIDs client supports uint32 n, the number of mechanism OIDs client supports
string[n] mechanism OIDs string[n] mechanism OIDs
Mechanism OIDs are encoded according to the ASN.1 distinguished Mechanism OIDs are encoded according to the ASN.1 distinguished
encoding rules (DER), as described in [1] and in section 3.1 of [2]. encoding rules (DER), as described in [1] and in section 3.1 of [2].
The mechanism OIDs MUST be listed in order of preference, and the The mechanism OIDs MUST be listed in order of preference, and the
server must choose the first mechanism OID on the list that it server must choose the first mechanism OID on the list that it
supports. supports.
The client SHOULD NOT send more then one gssapi mechanism OID unless The client SHOULD send GSSAPI mechanism OID's only for mechanisms
there are no non-GSSAPI authentication methods between the GSSAPI which are of the same priority, compared to non-GSSAPI authentication
mechanisms in the order of preference, otherwise, authentication methods. Otherwise, authentication methods may be executed out of
methods may be executed out of order. order. Thus, the client could first send a SSH_MSG_USERAUTH_REQUEST
for one GSSAPI mechanism, then try public key authentication, and
then try another GSSAPI mechanism.
If the server does not support any of the specified OIDs, the server If the server does not support any of the specified OIDs, the server
MUST fail the request by sending a SSH_MSG_USERAUTH_FAILURE packet. MUST fail the request by sending a SSH_MSG_USERAUTH_FAILURE packet.
The user name may be an empty string if it can be deduced from the The user name may be an empty string if it can be deduced from the
results of the gssapi authentication. If the user name is not results of the GSSAPI authentication. If the user name is not empty,
empty, and the requested user does not exist, the server MAY and the requested user does not exist, the server MAY disconnect, or
disconnect, or MAY send a bogus list of acceptable authentications MAY send a bogus list of acceptable authentications but never accept
but never accept any. This makes it possible for the server to any. This makes it possible for the server to avoid disclosing
avoid disclosing information about which accounts exist. In any information about which accounts exist. In any case, if the user
case, if the user does not exist, the authentication request MUST does not exist, the authentication request MUST NOT be accepted.
NOT be accepted.
The client MAY at any time continue with a new The client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the abandon the previous authentication attempt and continue with the new
new one. one.
3.3 Initial server response 3.3 Initial server response
The server responds to the SSH_MSG_USERAUTH_REQUEST with either a The server responds to the SSH_MSG_USERAUTH_REQUEST with either a
SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or
with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows: with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows:
byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE
string selected mechanism OID string selected mechanism OID
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byte SSH_MSG_USERAUTH_GSSAPI_TOKEN byte SSH_MSG_USERAUTH_GSSAPI_TOKEN
string data returned from either GSS_Init_sec_context() string data returned from either GSS_Init_sec_context()
or GSS_Accept_sec_context() or GSS_Accept_sec_context()
If an error occurs during this exchange on server side, the server If an error occurs during this exchange on server side, the server
can terminate the method by sending a SSH_MSG_USERAUTH_FAILURE can terminate the method by sending a SSH_MSG_USERAUTH_FAILURE
packet. If an error occurs on client side, the client can terminate packet. If an error occurs on client side, the client can terminate
the method by sending a new SSH_MSG_USERAUTH_REQUEST packet. the method by sending a new SSH_MSG_USERAUTH_REQUEST packet.
The client MAY use the deleg_req_flag in calls to When calling GSS_Init_sec_context(), the client MUST set the the
GSS_Init_sec_context() to request credential delegation. integ_req_flag to "true" to request that per-message integrity
protection be supported for this context. In addition, the
deleg_req_flag MAY be set to "true" to request access delegation, if
requested by the user.
Since the user authentication process by its nature authenticates
only the client, the setting of the mutual_req_flag is not needed for
this process. This flag SHOULD be set to "false".
Since the user authentication process will involve the exchange of
only a single token once the context has been established, it is not
necessary that the context support detection of replayed or
out-of-sequence tokens. Thus, the setting of the replay_det_req_flag
and sequence_req_flag are not needed for this process. These flags
SHOULD be set to "false".
Additional SSH_MSG_USERAUTH_GSSAPI_TOKEN messages are sent if and Additional SSH_MSG_USERAUTH_GSSAPI_TOKEN messages are sent if and
only if the calls to the GSSAPI routines produce send tokens of only if the calls to the GSSAPI routines produce send tokens of
non-zero length. non-zero length.
Any major status code other than GSS_S_COMPLETE or Any major status code other than GSS_S_COMPLETE or
GSS_S_CONTINUE_NEEDED SHOULD be a failure. GSS_S_CONTINUE_NEEDED SHOULD be a failure.
3.5 Client acknowledgement 3.5 Binding Encryption Keys
It is possible for the server to successfully complete the GSSAPI In some cases, it is possible to obtain improved security by allowing
method and the client to fail. If the server simply assumed success access only if the client sends a valid message integrity code (MIC)
on the part of the client and completed the authentication service, binding the GSSAPI context to the keys used for encryption and
it is possible that the client would fail to complete the integrity protection of the SSH session. With this extra level of
authentication method, but not be able to retry other methods protection, a "man-in-the-middle" attacker who has convinced a client
because the server had already moved on. of his authenticity cannot then relay user authentication messages
between the real client and server, thus gaining access to the real
server. This additional protection is available when the negotiated
GSSAPI context supports per-message integrity protection, as
indicated by the setting of the integ_avail flag on successful return
from GSS_Init_sec_context() or GSS_Accept_sec_context().
Therefore, the client MUST send the following message when it has When the client's call to GSS_Init_sec_context() returns
successfully called GSS_Init_sec_context() and gotten GSS_S_COMPLETE: GSS_S_COMPLETE with the integ_avail flag set, the client MUST
conclude the user authentication exchange by sending the following
message:
byte SSH_MSG_USERAUTH_GSSAPI_MIC
string MIC
This message MUST be sent only if GSS_Init_sec_context() returned
GSS_S_COMPLETE. If a token is also returned then the
SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one.
The contents of the MIC field are obtained by calling GSS_GetMIC over
the following, using the GSSAPI context which was just established:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "gssapi-with-mic"
If this message is received by the server before the GSSAPI context
is fully established, the server MUST fail the authentication.
If this message is received by the server when the negotiated GSSAPI
context does not support per-message integrity protection, the server
MUST fail the authentication.
3.6 Client acknowledgement
Some servers may wish to permit user authentication to proceed even
when the negotitated GSSAPI context does not support per-message
integrity protection. In such cases, it is possible for the server
to successfully complete the GSSAPI method, while the client's last
call to GSS_Init_sec_context fails. If the server simply assumed
success on the part of the client and completed the authentication
service, it is possible that the client would fail to complete the
authentication method, but not be able to retry other methods because
the server had already moved on. To protect against this, a final
message is sent by the client to indicate it has completed
authentication.
When the client's call to GSS_Init_sec_context() returns
GSS_S_COMPLETE with the integ_avail flag not set, the client MUST
conclude the user authentication exchange by sending the following
message:
byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE
This message MUST be sent if and only if GSS_Init_sec_context() This message MUST be sent only if GSS_Init_sec_context() returned
returned GSS_S_COMPLETE. If a token is returned then the GSS_S_COMPLETE. If a token is also returned then the
SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one. SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one.
If GSS_Init_sec_context() failed, the client MUST terminate the If this message is received by the server before the GSSAPI context
method by sending a new SSH_MSG_USERAUTH_REQUEST. or by closing the is fully established, the server MUST fail the authentication.
connection
3.6 Completion If this message is received by the server when the negotiated GSSAPI
context supports per-message integrity protection, the server MUST
fail the authentication.
As with all SSH authentication methods, successful completion is It is a site policy descision for the server whether or not to permit
indicated by a SSH_MSG_USERAUTH_SUCCESS if no other authentication authentication using GSSAPI mechanisms and/or contexts which do not
is required, or a SSH_MSG_USERAUTH_FAILURE with the partial success support per-message integrity protection. The server MAY fail the
flag set if the server requires further authentication. otherwise valid gssapi-with-mic authentication if per-message
integrity protection is not supported.
This packet should be sent immediately following receipt of the the 3.7 Completion
As with all SSH authentication methods, successful completion is
indicated by a SSH_MSG_USERAUTH_SUCCESS if no other authentication is
required, or a SSH_MSG_USERAUTH_FAILURE with the partial success flag
set if the server requires further authentication. This packet
should be sent immediately following receipt of the the
SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet. SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet.
3.7 Error Status 3.8 Error Status
In the event a GSSAPI error occurs on the server during context In the event a GSSAPI error occurs on the server during context
establishment, the server MAY send the following message to inform establishment, the server MAY send the following message to inform
the client of the details of the error before sending a the client of the details of the error before sending a
SSH_MSG_USERAUTH_FAILURE message: SSH_MSG_USERAUTH_FAILURE message:
byte SSH_MSG_USERAUTH_GSSAPI_ERROR byte SSH_MSG_USERAUTH_GSSAPI_ERROR
uint32 major_status uint32 major_status
uint32 minor_status uint32 minor_status
string message string message
string language tag string language tag
The message text MUST be encoded in the UTF-8 encoding described in The message text MUST be encoded in the UTF-8 encoding described in
[10]. Language tags are those described in [11]. Note that the [10]. Language tags are those described in [11]. Note that the
message text may contain multiple lines separated by carriage message text may contain multiple lines separated by carriage
return-line feed (CRLF) sequences. Application developers should return-line feed (CRLF) sequences. Application developers should take
take this into account when displaying these messages. this into account when displaying these messages.
Clients receiving this message MAY log the error details and/or Clients receiving this message MAY log the error details and/or
report them to the user. Any server sending this message MUST report them to the user. Any server sending this message MUST ignore
ignore any SSH_MSG_UNIMPLEMENTED sent by the client in response. any SSH_MSG_UNIMPLEMENTED sent by the client in response.
3.8 Error Token 3.9 Error Token
In the event that, during context establishment, a client's call to In the event that, during context establishment, a client's call to
GSS_Init_sec_context or a server's call to GSS_Accept_sec_context GSS_Init_sec_context or a server's call to GSS_Accept_sec_context
returns a token along with an error status, the resulting "error returns a token along with an error status, the resulting "error
token" SHOULD be sent to the peer using the following message: token" SHOULD be sent to the peer using the following message:
byte SSH_MSG_USERAUTH_GSSAPI_ERRTOK byte SSH_MSG_USERAUTH_GSSAPI_ERRTOK
string error token string error token
This message implies that the authentication is about to fail, and This message implies that the authentication is about to fail, and is
is defined to allow the error token to be communicated without defined to allow the error token to be communicated without losing
losing synchronization. synchronization.
When a server sends this message, it MUST be followed by a When a server sends this message, it MUST be followed by a
SSH_MSG_USERAUTH_FAILURE message, which is to be interpreted as SSH_MSG_USERAUTH_FAILURE message, which is to be interpreted as
applying to the same authentication request. A client receiving applying to the same authentication request. A client receiving this
this message SHOULD wait for the following SSH_MSG_USERAUTH_FAILURE message SHOULD wait for the following SSH_MSG_USERAUTH_FAILURE
message before beginning another authentication attempt. message before beginning another authentication attempt.
When a client sends this message, it MUST be followed by a new When a client sends this message, it MUST be followed by a new
authentication request or by terminating the connection. A server authentication request or by terminating the connection. A server
receiving this message MUST NOT send a SSH_MSG_USERAUTH_FAILURE in receiving this message MUST NOT send a SSH_MSG_USERAUTH_FAILURE in
reply, since such a message might otherwise be interpreted by a reply, since such a message might otherwise be interpreted by a
client as a response to the following authentication sequence. client as a response to the following authentication sequence.
Any server sending this message MUST ignore any Any server sending this message MUST ignore any SSH_MSG_UNIMPLEMENTED
SSH_MSG_UNIMPLEMENTED sent by the client in response. If a server sent by the client in response. If a server sends both this message
sends both this message and an SSH_MSG_USERAUTH_GSSAPI_ERROR and an SSH_MSG_USERAUTH_GSSAPI_ERROR message, the
message, the SSH_MSG_USERAUTH_GSSAPI_ERROR message MUST be sent SSH_MSG_USERAUTH_GSSAPI_ERROR message MUST be sent first, to allow
first, to allow the client to store and/or display the error status the client to store and/or display the error status before processing
before processing the error token. the error token.
4. External Key Exchange User Authentication 4. Authentication using GSSAPI Key Exchange
This section describes a user authentication method building on the This section describes a user authentication method building on the
framework described in [9]. This method relies upon the key framework described in [9]. This method performs user authentication
exchange to authenticate both the client and the server. If the key by making use of an existing GSSAPI context established during key
exchange did not successfully perform these functions then the exchange.
server MUST always respond to this request with
SSH_MSG_USERAUTH_FAILURE with partial success set to false.
The new mechanism is defined as follows: The authentication method name for this protocol is "gssapi-keyex".
This method may be used only if the initial key exchange was
performed using a GSSAPI-based key exchange method defined in
accordance with Section 2. The GSSAPI context used with this method
is always that established during an initial GSSAPI-based key
exchange. Any context established during key exchange for the
purpose of rekeying MUST NOT be used with this method.
The server SHOULD include this user authentication method in the list
of methods that can continue (in a SSH_MSG_USERAUTH_FAILURE) if the
initial key exchange was performed using a GSSAPI-based key exchange
method and provides information about the user's identity which is
useful to the server. It MUST NOT include this method if the initial
key exchange was not performed using a GSSAPI-based key exchange
method defined in accordance with Section 2.
The client SHOULD attempt to use this method if it is advertised by
the server, initial key exchange was performed using a GSSAPI-based
key exchange method, and this method has already been tried. The
client SHOULD NOT try this method more than once per session. It
MUST NOT try this method if initial key exchange was not performed
using a GSSAPI-based key exchange method defined in accordance with
Section 2.
If a server receives a request for this method when initial key
exchange was not performed using a GSSAPI-based key exchange method
defined in accordance with Section 2, it MUST return
SSH_MSG_USERAUTH_FAILURE.
This method is defined as a single message:
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding) string user name
string service name (in US-ASCII) string service
string "external-keyx" (US-ASCII method name) string "gssapi-keyex"
string MIC
If the authentication performed as part of key exchange can be used The contents of the MIC field are obtained by calling GSS_GetMIC over
to authorize login as the requested user, this method is successful, the following, using the GSSAPI context which was established during
and the server responds with SSH_MSG_USERAUTH_SUCCESS if no more initial key exchange:
authentications are needed, or with SSH_MSG_USERAUTH_FAILURE with
partial success set to true if more authentications are needed.
If the authentication performed as part of key-exchange cannot be string session identifier
used to authorize login as the requested user, then byte SSH_MSG_USERAUTH_REQUEST
SSH_MSG_USERAUTH_FAILURE is returned with partial success set to string user name
false. string service
string "gssapi-keyex"
If the user name is not empty, and the requested user does not Upon receiving this message when initial key exchange was performed
exist, the server MAY disconnect, or MAY send a bogus list of using a GSSAPI-based key exchange method, the server uses
acceptable authentications but never accept any. This makes it GSS_VerifyMIC() to verify that the MIC received is valid. If the MIC
possible for the server to avoid disclosing information about which is not valid, the user authentication fails, and the server MUST
accounts exist. In any case, if the user does not exist, the return SSH_MSG_USERAUTH_FAILURE.
authentication request MUST NOT be accepted.
Any implementation supporting at least one key exchange method which If the MIC is valid and the server is satisfied as to the user's
conforms to section 1 of this document SHOULD also support the credentials, it MAY return either SSH_MSG_USERAUTH_SUCCESS, or
"external-keyx" user authentication method, in order to allow user SSH_MSG_USERAUTH_FAILURE with the partial success flag set, depending
authentication to be performed at the same time as key exchange, on whether additional authentications are needed.
thereby reducing the number of round trips needed for connection
setup.
5. Null Host Key Algorithm 5. Null Host Key Algorithm
The "null" host key algorithm has no associated host key material, The "null" host key algorithm has no associated host key material,
and provides neither signature nor encryption algorithms. Thus, it and provides neither signature nor encryption algorithms. Thus, it
can be used only with key exchange methods that do not require any can be used only with key exchange methods that do not require any
public-key operations and do not require the use of host public key public-key operations and do not require the use of host public key
material. The key exchange methods described in section 1 of this material. The key exchange methods described in section 1 of this
document are examples of such methods. document are examples of such methods.
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configuration, the server implementation supports the "ssh-dss" key configuration, the server implementation supports the "ssh-dss" key
algorithm (as required by [8]), but could be prohibited by algorithm (as required by [8]), but could be prohibited by
configuration from using it. In this situation, the server needs configuration from using it. In this situation, the server needs
some key exchange algorithm to advertise; the "null" algorithm fills some key exchange algorithm to advertise; the "null" algorithm fills
this purpose. this purpose.
Note that the use of the "null" algorithm in this way means that the Note that the use of the "null" algorithm in this way means that the
server will not be able to interoperate with clients which do not server will not be able to interoperate with clients which do not
support this algorithm. This is not a significant problem, since in support this algorithm. This is not a significant problem, since in
the configuration described, it will also be unable to interoperate the configuration described, it will also be unable to interoperate
with implementations that do not support the GSSAPI-authenticated with implementations that do not support the GSSAPI-authenticated key
key exchange and Kerberos. exchange and Kerberos.
Any implementation supporting at least one key exchange method which Any implementation supporting at least one key exchange method which
conforms to section 1 of this document MUST also support the "null" conforms to section 1 of this document MUST also support the "null"
host key algorithm. Servers MUST NOT advertise the "null" host key host key algorithm. Servers MUST NOT advertise the "null" host key
algorithm unless it is the only algorithm advertised. algorithm unless it is the only algorithm advertised.
6. Summary of Message Numbers 6. Summary of Message Numbers
The following message numbers have been defined for use with The following message numbers have been defined for use with
GSSAPI-based key exchange methods: GSSAPI-based key exchange methods:
skipping to change at page 18, line 26 skipping to change at page 21, line 26
The numbers 30-49 are specific to key exchange and may be redefined The numbers 30-49 are specific to key exchange and may be redefined
by other kex methods. by other kex methods.
The following message numbers have been defined for use with the The following message numbers have been defined for use with the
'gssapi' user authentication method: 'gssapi' user authentication method:
#define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60 #define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60
#define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61 #define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61
#define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63 #define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63
#define SSH_MSG_USERAUTH_GSSAPI_ERROR 64 #define SSH_MSG_USERAUTH_GSSAPI_ERROR 64
#define SSH_MSG_USERAUTH_GSSAPI_ERRTOK 65
#define SSH_MSG_USERAUTH_GSSAPI_MIC 66
The numbers 60-79 are specific to user authentication and may be The numbers 60-79 are specific to user authentication and may be
redefined by other user auth methods. Note that in the method redefined by other user auth methods. Note that in the method
described in this document, message number 62 is unused. described in this document, message number 62 is unused.
7. GSSAPI Considerations 7. GSSAPI Considerations
7.1 Naming Conventions 7.1 Naming Conventions
In order to establish a GSSAPI security context, the SSH client In order to establish a GSSAPI security context, the SSH client needs
needs to determine the appropriate targ_name to use in identifying to determine the appropriate targ_name to use in identifying the
the server when calling GSS_Init_sec_context. For this purpose, the server when calling GSS_Init_sec_context. For this purpose, the
GSSAPI mechanism-independent name form for host-based services is GSSAPI mechanism-independent name form for host-based services is
used, as described in section 4.1 of [2]. used, as described in section 4.1 of [2].
In particular, the targ_name to pass to GSS_Init_sec_context is In particular, the targ_name to pass to GSS_Init_sec_context is
obtained by calling GSS_Import_name with an input_name_type of obtained by calling GSS_Import_name with an input_name_type of
GSS_C_NT_HOSTBASED_SERVICE, and an input_name_string consisting of GSS_C_NT_HOSTBASED_SERVICE, and an input_name_string consisting of
the string "host@" concatenated with the hostname of the SSH server. the string "host@" concatenated with the hostname of the SSH server.
7.2 Channel Bindings 7.2 Channel Bindings
This document recommends that channel bindings SHOULD NOT be This document recommends that channel bindings SHOULD NOT be
specified in the calls during context establishment. This document specified in the calls during context establishment. This document
does not specify any standard data to be used as channel bindings does not specify any standard data to be used as channel bindings and
and the use of network addresses as channel bindings may break SSH the use of network addresses as channel bindings may break SSH in
in environments where it is most useful. environments where it is most useful.
7.3 SPNEGO 7.3 SPNEGO
The use of the Simple and Protected GSS-API Negotiation Mechanism The use of the Simple and Protected GSS-API Negotiation Mechanism
[14] in conjunction with the authentication and key exchange methods [14] in conjunction with the authentication and key exchange methods
described in this document is both unnecessary and undesirable. As described in this document is both unnecessary and undesirable. As a
a result, mechanisms conforming to this document MUST NOT use SPNEGO result, mechanisms conforming to this document MUST NOT use SPNEGO as
as the underlying GSSAPI mechanism. the underlying GSSAPI mechanism.
Since SSH performs its own negotiation of authentication and key Since SSH performs its own negotiation of authentication and key
exchange methods, the negotiation capability of SPNEGO alone does exchange methods, the negotiation capability of SPNEGO alone does not
not provide any added benefit. In fact, as described below, it has provide any added benefit. In fact, as described below, it has the
the potential to result in the use of a weaker method than desired. potential to result in the use of a weaker method than desired.
Normally, SPNEGO provides the added benefit of protecting the GSSAPI Normally, SPNEGO provides the added benefit of protecting the GSSAPI
mechanism negotiation. It does this by having the server compute a mechanism negotiation. It does this by having the server compute a
MIC of the list of mechanisms proposed by the client, and then MIC of the list of mechanisms proposed by the client, and then
checking that value at the client. In the case of key exchange, checking that value at the client. In the case of key exchange, this
this protection is not needed because the key exchange methods protection is not needed because the key exchange methods described
described here already perform an equivalent operation; namely, they here already perform an equivalent operation; namely, they generate a
generate a MIC of the SSH exchange hash, which is a hash of several MIC of the SSH exchange hash, which is a hash of several items
items including the lists of key exchange mechanisms supported by including the lists of key exchange mechanisms supported by both
both sides. In the case of user authentication, the protection is sides. In the case of user authentication, the protection is not
not needed because the negotiation occurs over a secure channel, and needed because the negotiation occurs over a secure channel, and the
the host's identity has already been proved to the user. host's identity has already been proved to the user.
The use of SPNEGO combined with GSSAPI mechanisms used without The use of SPNEGO combined with GSSAPI mechanisms used without SPNEGO
SPNEGO can lead to interoperability problems. For example, a client can lead to interoperability problems. For example, a client which
which supports key exchange using the Kerberos V5 GSSAPI mechanism supports key exchange using the Kerberos V5 GSSAPI mechanism [13]
[13] only underneath SPNEGO will not interoperate with a server only underneath SPNEGO will not interoperate with a server which
which supports key exchange only using the Kerberos V5 GSSAPI supports key exchange only using the Kerberos V5 GSSAPI mechanism
mechanism directly. As a result, allowing GSSAPI mechanisms to be directly. As a result, allowing GSSAPI mechanisms to be used both
used both with and without SPNEGO is undesirable. with and without SPNEGO is undesirable.
If a client's policy is to first prefer GSSAPI-based key exchange If a client's policy is to first prefer GSSAPI-based key exchange
method X, then non-GSSAPI method Y, then GSSAPI-based method Z, and method X, then non-GSSAPI method Y, then GSSAPI-based method Z, and
if a server supports mechanisms Y and Z but not X, then an attempt if a server supports mechanisms Y and Z but not X, then an attempt to
to use SPNEGO to negotiate a GSSAPI mechanism might result in the use SPNEGO to negotiate a GSSAPI mechanism might result in the use of
use of method Z when method Y would have been preferable. As a method Z when method Y would have been preferable. As a result, the
result, the use of SPNEGO could result in the subversion of the use of SPNEGO could result in the subversion of the negotiation
negotiation algorithm for key exchange methods as described in algorithm for key exchange methods as described in section 5.1 of [8]
section 5.1 of [8] and/or the negotiation algorithm for user and/or the negotiation algorithm for user authentication methods as
authentication methods as described in [9]. described in [9].
8. IANA Considerations 8. IANA Considerations
Consistent with section 7 of [6], the IANA is directed to make the Consistent with section 7 of [6], the IANA is directed to make the
following registrations: following registrations:
The family of SSH key exchange method names beginning with The family of SSH key exchange method names beginning with
"gss-group1-sha1-" and not containing the at-sign ('@'), to name "gss-group1-sha1-" and not containing the at-sign ('@'), to name
the key exchange methods defined in Section 2.2. the key exchange methods defined in Section 2.2.
All other SSH key exchange method names beginning with "gss-" and All other SSH key exchange method names beginning with "gss-" and
not containing the at-sign ('@'), to be reserved for future key not containing the at-sign ('@'), to be reserved for future key
exchange methods defined in conformance with this document, as exchange methods defined in conformance with this document, as
noted in Section 2.3. noted in Section 2.3.
The SSH host public key algorithm name "null", to name the NULL The SSH host public key algorithm name "null", to name the NULL
host key algorithm defined in Section 5. host key algorithm defined in Section 5.
The SSH user authentication method name "gssapi", to name the The SSH user authentication method name "gssapi-with-mic", to name
GSSAPI user authentication method defined in Section 3. the GSSAPI user authentication method defined in Section 3.
The SSH user authentication method name "external-keyx", to name The SSH user authentication method name "gssapi-keyex", to name
the "external key-exchange" user authentication method defined in the GSSAPI user authentication method defined in Section 4.
Section 4.
The SSH user authentication method name "gssapi" is to be
reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification.
The SSH user authentication method name "external-keyx" is to be
reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification.
This document creates no new registries. This document creates no new registries.
9. Security Considerations 9. Security Considerations
This document describes authentication and key-exchange protocols. This document describes authentication and key-exchange protocols. As
As such, security considerations are discussed throughout. such, security considerations are discussed throughout.
This protocol depends on the SSH protocol itself, the GSSAPI, any This protocol depends on the SSH protocol itself, the GSSAPI, any
underlying GSSAPI mechanisms which are used, and any protocols on underlying GSSAPI mechanisms which are used, and any protocols on
which such mechanisms might depend. Each of these components plays which such mechanisms might depend. Each of these components plays a
a part in the security of the resulting connection, and each will part in the security of the resulting connection, and each will have
have its own security considerations. its own security considerations.
The key exchange method described in section 1 of this document The key exchange method described in section 1 of this document
depends on the underlying GSSAPI mechanism to provide both mutual depends on the underlying GSSAPI mechanism to provide both mutual
authentication and per-message integrity services. If either of authentication and per-message integrity services. If either of
these features is not supported by a particular GSSAPI mechanism, or these features is not supported by a particular GSSAPI mechanism, or
by a particular implementation of a GSSAPI mechanism, then the key by a particular implementation of a GSSAPI mechanism, then the key
exchange is not secure and MUST fail. exchange is not secure and MUST fail.
In order for the "external-keyx" user authentication method to be In order for the "external-keyx" user authentication method to be
used, it MUST have access to user authentication information used, it MUST have access to user authentication information obtained
obtained as a side-effect of the key exchange. If this information as a side-effect of the key exchange. If this information is
is unavailable, the authentication MUST fail. unavailable, the authentication MUST fail.
Revealing information about the reason for an authentication failure Revealing information about the reason for an authentication failure
may be considered by some sites to be an unacceptable security risk may be considered by some sites to be an unacceptable security risk
for a production environment. However, having that information for a production environment. However, having that information
available can be invaluable for debugging purposes. Thus, it is available can be invaluable for debugging purposes. Thus, it is
RECOMMENDED that implementations provide a means for controlling, as RECOMMENDED that implementations provide a means for controlling, as
a matter of policy, whether to send SSH_MSG_USERAUTH_GSSAPI_ERROR, a matter of policy, whether to send SSH_MSG_USERAUTH_GSSAPI_ERROR,
SSH_MSG_USERAUTH_GSSAPI_ERRTOK, and SSH_MSG_KEXGSS_ERROR messages, SSH_MSG_USERAUTH_GSSAPI_ERRTOK, and SSH_MSG_KEXGSS_ERROR messages,
and SSH_MSG_KEXGEE_CONTINUE messages containing a GSSAPI error token. and SSH_MSG_KEXGEE_CONTINUE messages containing a GSSAPI error token.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank Sam Hartman, Simon Wilkinson, and The authors would like to thank the following individuals for their
Nicolas Williams for their invaluable assistance with this document. invaluable assistance and contributions to this document:
o Sam Hartman
o Love Hornquist-Astrand
o Joel N. Weber II
o Simon Wilkinson
o Nicolas Williams
11. Changes the last version 11. Changes the last version
This section lists important changes since the previous version of This section lists important changes since the previous version of
this internet-draft. This section should be removed at the time of this internet-draft. This section should be removed at the time of
publication of this document as an RFC. publication of this document as an RFC.
o Improved the description of error handling during key exchange. o Changed "gssapi" to "gssapi-with-mic", and added the description
and semantics of the SSH_MSG_USERAUTH_GSSAPI_MIC message.
o Specified that SSH_MSG_GSSKEX_CONTINUE SHOULD be used to send o Added information in user auth describing when integrity should be
error tokens in the event of a failure of GSS_Init_sec_context or requested.
GSS_Accept_sec_context during key exchange.
o Made SSH_MSG_GSSKEX_ERROR be OPTIONAL instead of RECOMMENDED. o Removed the definition of the "external-keyx" user authentication
method, and replaced it with the definition of the more secure
"gssapi-keyex" method.
o Specified a new SSH_MSG_USERAUTH_GSSAPI_ERRTOK message which o Added information in both key exchange and user auth describing
SHOULD be used to send error tokens in the event of a failure of why replay and out-of-sequence detection are not needed.
GSS_Init_sec_context or GSS_Accept_sec_context during user
authentication.
o Made SSH_MSG_USERAUTH_GSSAPI_ERROR be OPTIONAL instead of o Improved the description in user auth of when it is OK to list
RECOMMENDED. more than one mechanism OID in the same request,
o Added IANA considerations section. o Added the table of contents.
o Split normative and informative references.
o Added nemo and lha to the acknowledgements section.
Normative References Normative References
[1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic [1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic Encoding
Encoding Rules (BER), Canonical Encoding Rules (CER) and Rules (BER), Canonical Encoding Rules (CER) and Distinguished
Distinguished Encoding Rules (DER)", ITU-T Recommendation Encoding Rules (DER)", ITU-T Recommendation X.690 (1997), ISO/
X.690 (1997), ISO/IEC 8825-1:1998, November 1998. IEC 8825-1:1998, November 1998.
[2] Linn, J., "Generic Security Service Application Program [2] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
April 1992. 1992.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Extensions (MIME) Part One: Format of Internet Message Bodies",
Bodies", RFC 2045, November 1996. RFC 2045, November 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Requirement Levels", RFC 2119, BCP 14, March 1997. Levels", RFC 2119, BCP 14, March 1997.
[6] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. [6] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Protocol Architecture", Lehtinen, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-11.txt (work in progress), draft-ietf-secsh-architecture-11.txt (work in progress),
November 2001. November 2001.
[7] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. [7] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Connection Protocol", Lehtinen, "SSH Connection Protocol",
draft-ietf-secsh-connect-14.txt (work in progress), November draft-ietf-secsh-connect-14.txt (work in progress), November
2001. 2001.
skipping to change at page 25, line 45 skipping to change at page 28, line 45
[8] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. [8] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Transport Layer Protocol", Lehtinen, "SSH Transport Layer Protocol",
draft-ietf-secsh-transport-11.txt (work in progress), November draft-ietf-secsh-transport-11.txt (work in progress), November
2001. 2001.
[9] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. [9] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Authentication Protocol", Lehtinen, "SSH Authentication Protocol",
draft-ietf-secsh-userauth-13.txt (work in progress), November draft-ietf-secsh-userauth-13.txt (work in progress), November
2001. 2001.
[10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", [10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
RFC 2279, January 1998. 2279, January 1998.
[11] Alvestrand, H., "Tags for the Identification of Languages", [11] Alvestrand, H., "Tags for the Identification of Languages", RFC
RFC 1766, March 1995. 1766, March 1995.
Non-normative References Normative References
[12] Kohl, J. and C. Neuman, "The Kerberos Network Authentication [12] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993. Service (V5)", RFC 1510, September 1993.
[13] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC [13] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
1964, June 1996. June 1996.
[14] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API [14] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998. Negotiation Mechanism", RFC 2478, December 1998.
Authors' Addresses Authors' Addresses
Jeffrey Hutzelman Jeffrey Hutzelman
Carnegie Mellon University Carnegie Mellon University
5000 Forbes Ave 5000 Forbes Ave
Pittsburgh, PA 15213 Pittsburgh, PA 15213
skipping to change at page 26, line 45 skipping to change at page 30, line 4
EMail: jsalowey@cisco.com EMail: jsalowey@cisco.com
Joseph Galbraith Joseph Galbraith
Van Dyke Technologies, Inc. Van Dyke Technologies, Inc.
4848 Tramway Ridge Dr. NE 4848 Tramway Ridge Dr. NE
Suite 101 Suite 101
Albuquerque, NM 87111 Albuquerque, NM 87111
US US
EMail: galb@vandyke.com EMail: galb@vandyke.com
Von Welch Von Welch
University of Chicago & Argonne National Laboratory University of Chicago & Argonne National Laboratory
Distributed Systems Laboratory Distributed Systems Laboratory
701 E. Washington 701 E. Washington
Urbana, IL 61801 Urbana, IL 61801
US US
EMail: welch@mcs.anl.gov EMail: welch@mcs.anl.gov
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 End of changes. 

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