draft-ietf-secsh-gsskeyex-10.txt   rfc4462.txt 
Secure Shell Working Group J. Hutzelman Network Working Group J. Hutzelman
Internet-Draft CMU Request for Comments: 4462 CMU
Expires: February 23, 2006 J. Salowey Category: Standards Track 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
August 22, 2005 May 2006
GSSAPI Authentication and Key Exchange for the Secure Shell Protocol
draft-ietf-secsh-gsskeyex-10
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at Generic Security Service Application Program Interface (GSS-API)
http://www.ietf.org/ietf/1id-abstracts.txt. Authentication and Key Exchange for the Secure Shell (SSH) Protocol
The list of Internet-Draft Shadow Directories can be accessed at Status of This Memo
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on February 23, 2006. This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
The Secure Shell protocol (SSH) is a protocol for secure remote login The Secure Shell protocol (SSH) is a protocol for secure remote 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)
provides security services to callers in a mechanism-independent 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 which uses a specified GSSAPI mechanism to authenticate a method that uses a specified GSS-API mechanism to authenticate a
user, and a family of SSH key exchange methods which use GSSAPI to user, and a family of SSH key exchange methods that use GSS-API to
authenticate a Diffie-Hellman key exchange. authenticate a Diffie-Hellman key exchange.
This memo also defines a new host public key algorithm which can be This memo also defines a new host public key algorithm that 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 that allows an authorization name to
be used in conjunction with any authentication which has already be used in conjunction with any authentication that has already
occurred as a side-effect of GSSAPI-based key exchange. occurred as a side-effect of GSS-API-based key exchange.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction ....................................................3
1.1. SSH terminology . . . . . . . . . . . . . . . . . . . . . 4 1.1. SSH Terminology ............................................3
1.2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Key Words ..................................................3
2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . . 5 2. GSS-API-Authenticated Diffie-Hellman Key Exchange ...............3
2.1. Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . 5 2.1. Generic GSS-API Key Exchange ...............................4
2.2. Group Exchange . . . . . . . . . . . . . . . . . . . . . . 11 2.2. Group Exchange ............................................10
2.3. gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . 13 2.3. gss-group1-sha1-* .........................................11
2.4. gss-group14-sha1-* . . . . . . . . . . . . . . . . . . . . 13 2.4. gss-group14-sha1-* ........................................12
2.5. gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . . . . 13 2.5. gss-gex-sha1-* ............................................12
2.6. Other GSSAPI key exchange methods . . . . . . . . . . . . 13 2.6. Other GSS-API Key Exchange Methods ........................12
3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . . 15 3. GSS-API User Authentication ....................................13
3.1. GSSAPI Authentication Overview . . . . . . . . . . . . . . 15 3.1. GSS-API Authentication Overview ...........................13
3.2. Initiating GSSAPI authentication . . . . . . . . . . . . . 15 3.2. Initiating GSS-API Authentication .........................13
3.3. Initial server response . . . . . . . . . . . . . . . . . 16 3.3. Initial Server Response ...................................14
3.4. GSSAPI session . . . . . . . . . . . . . . . . . . . . . . 16 3.4. GSS-API Session ...........................................15
3.5. Binding Encryption Keys . . . . . . . . . . . . . . . . . 17 3.5. Binding Encryption Keys ...................................16
3.6. Client acknowledgement . . . . . . . . . . . . . . . . . . 18 3.6. Client Acknowledgement ....................................16
3.7. Completion . . . . . . . . . . . . . . . . . . . . . . . . 19 3.7. Completion ................................................17
3.8. Error Status . . . . . . . . . . . . . . . . . . . . . . . 19 3.8. Error Status ..............................................17
3.9. Error Token . . . . . . . . . . . . . . . . . . . . . . . 20 3.9. Error Token ...............................................18
4. Authentication using GSSAPI Key Exchange . . . . . . . . . . . 21 4. Authentication Using GSS-API Key Exchange ......................19
5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . . 23 5. Null Host Key Algorithm ........................................20
6. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 24 6. Summary of Message Numbers .....................................21
7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . . 25 7. GSS-API Considerations .........................................22
7.1. Naming Conventions . . . . . . . . . . . . . . . . . . . . 25 7.1. Naming Conventions ........................................22
7.2. Channel Bindings . . . . . . . . . . . . . . . . . . . . . 25 7.2. Channel Bindings ..........................................22
7.3. SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.3. SPNEGO ....................................................23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 8. IANA Considerations ............................................24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28 9. Security Considerations ........................................24
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29 10. Acknowledgements ..............................................25
11. Changes the last version . . . . . . . . . . . . . . . . . . . 30 11. References ....................................................26
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 11.1. Normative References .....................................26
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31 11.2. Informative References ...................................27
12.2. Non-Normative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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 GSS-API.
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 GSS-API 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
describing the SSH protocol architecture [SSH-ARCH], transport layer describing the SSH protocol architecture [SSH-ARCH], transport layer
protocol [SSH-TRANSPORT], and user authentication protocol [SSH- protocol [SSH-TRANSPORT], and user authentication protocol
USERAUTH]. This document freely uses terminology and notation from [SSH-USERAUTH]. This document freely uses terminology and notation
the architecture document without reference or further explanation. from the architecture document 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 [SSH-ARCH]. It is particularly important to architecture document [SSH-ARCH]. It is particularly important to
note the definition of string allows binary content. note the definition of string allows binary content.
The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this service The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this service
name is an SSH service name, and has no relationship to GSSAPI name is an SSH service name and has no relationship to GSS-API
service names. Currently, the only defined service name is "ssh- service names. Currently, the only defined service name is
connection", which refers to the SSH connection protocol [SSH- "ssh-connection", which refers to the SSH connection protocol
CONNECT]. [SSH-CONNECT].
1.2. Keywords 1.2. Key Words
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 [KEYWORDS]. document are to be interpreted as described in [KEYWORDS].
2. GSSAPI Authenticated Diffie-Hellman Key Exchange 2. GSS-API-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 that combine the
the Diffie-Hellman key exchange from section 8 of [SSH-TRANSPORT] Diffie-Hellman key exchange from Section 8 of [SSH-TRANSPORT] with
with mutual authentication using GSSAPI. mutual authentication using GSS-API.
Since the GSSAPI key exchange methods described in this section do Since the GSS-API key exchange methods described in this section do
not require the use of public key signature or encryption algorithms, not require the use of public key signature or encryption algorithms,
they MAY be used with any host key algorithm, including the "null" they MAY be used with any host key algorithm, including the "null"
algorithm described in Section 5. algorithm described in Section 5.
2.1. Generic GSSAPI Key Exchange 2.1. Generic GSS-API 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 GF(p), o p is a large safe prime, g is a generator for a subgroup of 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, the received from S during this exchange, if any. For this call, the
client MUST set the mutual_req_flag to "true" to request that client MUST set mutual_req_flag to "true" to request that mutual
mutual authentication be performed. It also MUST set the authentication be performed. It also MUST set integ_req_flag to
integ_req_flag to "true" to request that per-message integrity "true" to request that per-message integrity protection be
protection be supported for this context. In addition, the supported for this context. In addition, deleg_req_flag MAY be
deleg_req_flag MAY be set to "true" to request access delegation, set to "true" to request access delegation, if requested by the
if requested by the user. Since the key exchange process user. Since the key exchange process authenticates only the
authenticates only the host, the setting of the anon_req_flag is host, the setting of anon_req_flag is immaterial to this process.
immaterial to this process. If the client does not support the If the client does not support the "gssapi-keyex" user
"gssapi-keyex" user authentication method described in Section 4, authentication method described in Section 4, or does not intend
or does not intend to use that method in conjunction with the to use that method in conjunction with the GSS-API context
GSSAPI context established during key exchange, then the established during key exchange, then anon_req_flag SHOULD be set
anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY to "true". Otherwise, this flag MAY be set to true if the client
be set to true if the client wishes to hide its identity. Since wishes to hide its identity. Since the key exchange process will
the key exchange process will involve the exchange of only a involve the exchange of only a single token once the context has
single token once the context has been established, it is not been established, it is not necessary that the GSS-API context
necessary that the GSSAPI context support detection of replayed support detection of replayed or out-of-sequence tokens. Thus,
or out-of-sequence tokens. Thus, the setting of the replay_det_req_flag and sequence_req_flag need not be set for
replay_det_req_flag and sequence_req_flag are not needed for this this process. These flags SHOULD be set to "false".
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 resulting the mutual_state and integ_avail flags are true, the 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 output_token is sent to S, which will reply with a new the output_token is sent to S, which will reply with a 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 non- * 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 key zero length to be sent to the server. In this case, the key
exchange MUST fail. exchange MUST fail.
3. S calls GSS_Accept_sec_context, using the token received from C. 3. S calls GSS_Accept_sec_context(), using the token received from
C.
* 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
skipping to change at page 7, line 7 skipping to change at page 5, line 44
* If the resulting major_status code is GSS_S_CONTINUE_NEEDED, * If the resulting major_status code is GSS_S_CONTINUE_NEEDED,
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 and obtain a GSS-API message integrity code for H. S then sends f
the MIC to C. and the message integrity code (MIC) to C.
5. This step is performed only if the server's final call to 5. This step is performed only (1) 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 (2) if 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
GSS_S_COMPLETE. Under these conditions, the client makes an of 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. If the MIC is not successfully that the MIC sent by S matches H. If the MIC is not successfully
verified, the key exchange MUST fail. verified, the key exchange MUST fail.
Either side MUST NOT send or accept e or f values that are not in the 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 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 returns If any call to GSS_Init_sec_context() or GSS_Accept_sec_context()
a major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or returns a major_status other than GSS_S_COMPLETE or
any other GSSAPI call returns a major_status other than GSS_S_CONTINUE_NEEDED, or any other GSS-API call returns a
GSS_S_COMPLETE, the key exchange fails. In this case, several major_status other than GSS_S_COMPLETE, the key exchange fails. In
mechanisms are available for communicating error information to the this case, several mechanisms are available for communicating error
peer before terminating the connection as required by [SSH- information to the peer before terminating the connection as required
TRANSPORT]: by [SSH-TRANSPORT]:
o If the key exchange fails due to any GSSAPI error on the server o If the key exchange fails due to any GSS-API error on the server
(including errors returned by GSS_Accept_sec_context), the server (including errors returned by GSS_Accept_sec_context()), the
MAY send a message informing the client of the details of the server MAY send a message informing the client of the details of
error. In this case, if an error token is also sent (see below), the error. In this case, if an error token is also sent (see
then this message MUST be sent before the error token. below), 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 GSS-API 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 GSS-API 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 policy As noted in Section 9, it may be desirable under site security policy
to obscure information about the precise nature of the error; thus, to obscure information about the precise nature of the error; thus,
it is RECOMMENDED that implementations provide a method to suppress it is RECOMMENDED that implementations provide a method to 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 is for computing the exchange hash is defined by the method name, and is
called HASH. The group used for Diffie-Hellman key exchange and the called HASH. The group used for Diffie-Hellman key exchange and the
underlying GSSAPI mechanism are also defined by the method name. underlying GSS-API 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 the used for any encryption operations, this message is OPTIONAL. If 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 public message MUST NOT be sent. If this message is sent, the server public
host key(s) and/or certificate(s) in this message are encoded as a host key(s) and/or certificate(s) in this message are encoded as a
single string, in the format specified by the public key type in use single string, in the format specified by the public key type in use
(see [SSH-TRANSPORT], section 6.6). (see [SSH-TRANSPORT], Section 6.6).
In traditional SSH deployments, host keys are normally expected to In traditional SSH deployments, host keys are normally expected to
change infrequently, and there is often no mechanism for validating change infrequently, and there is often no mechanism for validating
host keys not already known to the client. As a result, the use of a host keys not already known to the client. As a result, the use of a
new host key by an already-known host is usually considered an new host key by an already-known host is usually considered an
indication of a possible man-in-the-middle attack, and clients often indication of a possible man-in-the-middle attack, and clients often
present strong warnings and/or abort the connection in such cases. present strong warnings and/or abort the connection in such cases.
By contrast, when GSSAPI-based key exchange is used, host keys sent By contrast, when GSS-API-based key exchange is used, host keys sent
via the SSH_MSG_KEXGSS_HOSTKEY message are authenticated as part of via the SSH_MSG_KEXGSS_HOSTKEY message are authenticated as part of
the GSSAPI key exchange, even when previously unknown to the client. the GSS-API key exchange, even when previously unknown to the client.
Further, in environments in which GSSAPI-based key exchange is used Further, in environments in which GSS-API-based key exchange is used
heavily, it is possible and even likely that host keys will change heavily, it is possible and even likely that host keys will change
much more frequently and/or without advance warning. much more frequently and/or without advance warning.
Therefore, when a new key for an already-known host is received via Therefore, when a new key for an already-known host is received via
the SSH_MSG_KEXGSS_HOSTKEY message, clients SHOULD NOT issue strong the SSH_MSG_KEXGSS_HOSTKEY message, clients SHOULD NOT issue strong
warnings or abort the connection, provided the GSSAPI-based key warnings or abort the connection, provided the GSS-API-based key
exchange succeeds. exchange succeeds.
In order to facilitate key re-exchange after the user's GSSAPI In order to facilitate key re-exchange after the user's GSS-API
credentials have expired, client implementations SHOULD store host credentials have expired, client implementations SHOULD store host
keys received via SSH_MSG_KEXGSS_HOSTKEY for the duration of the keys received via SSH_MSG_KEXGSS_HOSTKEY for the duration of the
session, even when such keys are not stored for long-term use. session, even when such keys are not stored for long-term use.
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 as The server and client continue to trade these two messages as long as
the server's calls to GSS_Accept_sec_context result in major_status the server's calls to GSS_Accept_sec_context() result in major_status
codes of GSS_S_CONTINUE_NEEDED. When a call results in a codes of GSS_S_CONTINUE_NEEDED. When a call results in 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
major_status code of GSS_S_COMPLETE) returns a non-zero-length token a major_status code of GSS_S_COMPLETE) returns a non-zero-length
to be sent to the client, it sends the following: token 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
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.
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
major_status code of GSS_S_COMPLETE) returns a zero-length token or a major_status code of GSS_S_COMPLETE) returns a zero-length token or
no token at all, it sends the following: no token at all, 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 FALSE boolean FALSE
If the client receives this message when no call to If the client receives this message when no call to
GSS_Init_sec_context has yet resulted in a major_status code of GSS_Init_sec_context() has yet resulted in 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.
If either the client's call to GSS_Init_sec_context or the server's If either the client's call to GSS_Init_sec_context() or the server's
call to GSS_Accept_sec_context returns an error status and produces call to GSS_Accept_sec_context() returns an error status and produces
an output token (called an "error token"), then the following SHOULD an output token (called an "error token"), then the following SHOULD
be sent to convey the error information to the peer: be sent to convey the error information to the peer:
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 the In the event of a GSS-API error on the server, the server MAY send
following message before terminating the connection: the 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
[UTF8]. Language tags are those described in [LANGTAG]. Note that [UTF8]. Language tags are those described in [LANGTAG]. Note that
the message text may contain multiple lines separated by carriage the message text may contain multiple lines separated by carriage
skipping to change at page 11, line 27 skipping to change at page 10, line 15
This value is called the exchange hash, and it is used to This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept authenticate the key exchange. The exchange hash SHOULD be kept
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. Group Exchange 2.2. Group Exchange
This section describes a modification to the generic GSSAPI This section describes a modification to the generic GSS-API-
authenticated Diffie-Hellman key exchange to allow the negotiation of authenticated Diffie-Hellman key exchange to allow the negotiation of
the group to be used, using a method based on that described in the group to be used, using a method based on that described in
[GROUP-EXCHANGE]. [GROUP-EXCHANGE].
The server keeps a list of safe primes and corresponding generators The server keeps a list of safe primes and corresponding generators
that it can select from. These are chosen as described in section 3 that it can select from. These are chosen as described in Section 3
of [GROUP-EXCHANGE]. The client requests a modulus from the server, of [GROUP-EXCHANGE]. The client requests a modulus from the server,
indicating the minimum, maximum, and preferred sizes; the server indicating the minimum, maximum, and preferred sizes; the server
responds with a suitable modulus and generator. The exchange then responds with a suitable modulus and generator. The exchange then
proceeds as described in Section 2.1 above. proceeds as described in Section 2.1 above.
This description uses the following symbols, in addition to those This description uses the following symbols, in addition to those
defined above: defined above:
o n is the size of the modulus p in bits that the client would like o n is the size of the modulus p in bits that the client would like
to receive from the server to receive from the server
skipping to change at page 13, line 7 skipping to change at page 11, line 40
should send should send
uint32 max, maximal size in bits of an acceptable group uint32 max, maximal size in bits of an acceptable group
mpint p, safe prime mpint p, safe prime
mpint g, generator for subgroup in GF(p) mpint g, generator for subgroup in GF(p)
mpint e, exchange value sent by the client mpint e, exchange value sent by the client
mpint f, exchange value sent by the server mpint f, exchange value sent by the server
mpint K, the shared secret mpint K, the shared secret
2.3. gss-group1-sha1-* 2.3. gss-group1-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman Each of these methods specifies GSS-API-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 8.1 of [SSH-TRANSPORT]. The method name for group defined in Section 8.1 of [SSH-TRANSPORT]. The method name for
each method is the concatenation of the string "gss-group1-sha1-" each method is the concatenation of the string "gss-group1-sha1-"
with the Base64 encoding of the MD5 hash [MD5] of the ASN.1 DER with the Base64 encoding of the MD5 hash [MD5] of the ASN.1
encoding [ASN1] of the underlying GSSAPI mechanism's OID. Base64 Distinguished Encoding Rules (DER) encoding [ASN1] of the underlying
encoding is described in section 6.8 of [MIME]. GSS-API mechanism's Object Identifier (OID). Base64 encoding is
described in Section 6.8 of [MIME].
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 GSS-API mechanism.
2.4. gss-group14-sha1-* 2.4. gss-group14-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman Each of these methods specifies GSS-API 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 8.2 of [SSH-TRANSPORT]. The method name for group defined in Section 8.2 of [SSH-TRANSPORT]. The method name for
each method is the concatenation of the string "gss-group14-sha1-" each method is the concatenation of the string "gss-group14-sha1-"
with the Base64 encoding of the MD5 hash [MD5] of the ASN.1 DER with the Base64 encoding of the MD5 hash [MD5] of the ASN.1 DER
encoding [ASN1] of the underlying GSSAPI mechanism's OID. Base64 encoding [ASN1] of the underlying GSS-API mechanism's OID. Base64
encoding is described in section 6.8 of [MIME]. encoding is described in Section 6.8 of [MIME].
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 GSS-API mechanism.
2.5. gss-gex-sha1-* 2.5. gss-gex-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.2 with SHA-1 as HASH. The key exchange as described in Section 2.2 with SHA-1 as HASH. The
method name for each method is the concatenation of the string "gss- method name for each method is the concatenation of the string "gss-
gex-sha1-" with the Base64 encoding of the MD5 hash [MD5] of the gex-sha1-" with the Base64 encoding of the MD5 hash [MD5] of the
ASN.1 DER encoding [ASN1] of the underlying GSSAPI mechanism's OID. ASN.1 DER encoding [ASN1] of the underlying GSS-API mechanism's OID.
Base64 encoding is described in section 6.8 of [MIME]. Base64 encoding is described in Section 6.8 of [MIME].
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 GSS-API mechanism.
2.6. Other GSSAPI key exchange methods 2.6. Other GSS-API 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 that conform to this document; in particular, for
those methods which use the GSSAPI authenticated Diffie-Hellman key those methods that use the GSS-API-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 intent methods that use different groups and/or hash functions. The intent
is that the names for any such future methods methods be defined in a is that the names for any such future methods be defined in a similar
similar manner to that used in Section 2.3. manner to that used in Section 2.3.
3. GSSAPI User Authentication 3. GSS-API User Authentication
This section describes a general-purpose user authentication method This section describes a general-purpose user authentication method
based on [GSSAPI]. It is intended to be run over the SSH user based on [GSSAPI]. It is intended to be run over the SSH user
authentication protocol [SSH-USERAUTH]. authentication protocol [SSH-USERAUTH].
The authentication method name for this protocol is "gssapi-with- The authentication method name for this protocol is "gssapi-with-
mic". mic".
3.1. GSSAPI Authentication Overview 3.1. GSS-API Authentication Overview
GSSAPI authentication must maintain a context. Authentication begins GSS-API authentication must maintain a context. Authentication
when the client sends a SSH_MSG_USERAUTH_REQUEST, which specifies the begins when the client sends an SSH_MSG_USERAUTH_REQUEST, which
mechanism OIDs the client supports. specifies the 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 an 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 GSS-API context is completely
discarded and destroyed, and any further GSSAPI authentication MUST discarded and destroyed, and any further GSS-API authentication MUST
restart from the beginning. restart from the beginning.
3.2. Initiating GSSAPI authentication If the authentication succeeds and a non-empty user name is presented
by the client, the SSH server implementation verifies that the user
name is authorized based on the credentials exchanged in the GSS-API
exchange. If the user name is not authorized, then the
authentication MUST fail.
The GSSAPI authentication method is initiated when the client sends a 3.2. Initiating GSS-API Authentication
SSH_MSG_USERAUTH_REQUEST:
The GSS-API authentication method is initiated when the client sends
an 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-with-mic" (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 [ASN1] and in section 3.1 of Encoding Rules (DER), as described in [ASN1] and in Section 3.1 of
[GSSAPI]. The mechanism OIDs MUST be listed in order of preference, [GSSAPI]. The mechanism OIDs MUST be listed in order of preference,
and the server must choose the first mechanism OID on the list that and the server must choose the first mechanism OID on the list that
it supports. it supports.
The client SHOULD send GSSAPI mechanism OID's only for mechanisms The client SHOULD send GSS-API mechanism OIDs only for mechanisms
which are of the same priority, compared to non-GSSAPI authentication that are of the same priority, compared to non-GSS-API authentication
methods. Otherwise, authentication methods may be executed out of methods. Otherwise, authentication methods may be executed out of
order. Thus, the client could first send a SSH_MSG_USERAUTH_REQUEST order. Thus, the client could first send an SSH_MSG_USERAUTH_REQUEST
for one GSSAPI mechanism, then try public key authentication, and for one GSS-API mechanism, then try public key authentication, and
then try another GSSAPI mechanism. then try another GSS-API 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 an 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 empty, results of the GSS-API authentication. If the user name is not
and the requested user does not exist, the server MAY disconnect, or empty, and the requested user does not exist, the server MAY
MAY send a bogus list of acceptable authentications but never accept disconnect or MAY send a bogus list of acceptable authentications but
any. This makes it possible for the server to avoid disclosing never accept any. This makes it possible for the server to avoid
information about which accounts exist. In any case, if the user disclosing information about which accounts exist. In any case, if
does not exist, the authentication request MUST NOT be accepted. the user does not exist, the authentication request MUST NOT be
accepted.
Note that the 'user name' value is encoded in ISO-10646 UTF-8. It is
up to the server how it interprets the user name and determines
whether the client is authorized based on his GSS-API credentials.
In particular, the encoding used by the system for user names is a
matter for the ssh server implementation. However, if the client
reads the user name in some other encoding (e.g., ISO 8859-1 - ISO
Latin1), it MUST convert the user name to ISO-10646 UTF-8 before
transmitting, and the server MUST convert the user name to the
encoding used on that system for user names.
Any normalization or other preparation of names is done by the ssh
server based on the requirements of the system, and is outside the
scope of SSH. SSH implementations which maintain private user
databases SHOULD prepare user names as described by [SASLPREP].
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 new abandon the previous authentication attempt and continue with the 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 an
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 an 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
The mechanism OID must be one of the OIDs sent by the client in the The mechanism OID must be one of the OIDs sent by the client in the
SSH_MSG_USERAUTH_REQUEST packet. SSH_MSG_USERAUTH_REQUEST packet.
3.4. GSSAPI session 3.4. GSS-API Session
Once the mechanism OID has been selected, the client will then Once the mechanism OID has been selected, the client will then
initiate an exchange of one or more pairs of initiate an exchange of one or more pairs of
SSH_MSG_USERAUTH_GSSAPI_TOKEN packets. These packets contain the SSH_MSG_USERAUTH_GSSAPI_TOKEN packets. These packets contain the
tokens produced from the 'GSS_Init_sec_context()' and tokens produced from the 'GSS_Init_sec_context()' and
'GSS_Accept_sec_context()' calls. The actual number of packets 'GSS_Accept_sec_context()' calls. The actual number of packets
exchanged is determined by the underlying GSSAPI mechanism. exchanged is determined by the underlying GSS-API mechanism.
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 an 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.
When calling GSS_Init_sec_context(), the client MUST set the the When calling GSS_Init_sec_context(), the client MUST set
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,
deleg_req_flag MAY be set to "true" to request access delegation, if deleg_req_flag MAY be set to "true" to request access delegation, if
requested by the user. requested by the user.
Since the user authentication process by its nature authenticates Since the user authentication process by its nature authenticates
only the client, the setting of the mutual_req_flag is not needed for only the client, the setting of mutual_req_flag is not needed for
this process. This flag SHOULD be set to "false". this process. This flag SHOULD be set to "false".
Since the user authentication process will involve the exchange of Since the user authentication process will involve the exchange of
only a single token once the context has been established, it is not only a single token once the context has been established, it is not
necessary that the context support detection of replayed or out-of- necessary that the context support detection of replayed or out-of-
sequence tokens. Thus, the setting of the replay_det_req_flag and sequence tokens. Thus, the setting of replay_det_req_flag and
sequence_req_flag are not needed for this process. These flags sequence_req_flag are not needed for this process. These flags
SHOULD be set to "false". 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 non- only if the calls to the GSS-API routines produce send tokens of non-
zero length. 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. Binding Encryption Keys 3.5. Binding Encryption Keys
In some cases, it is possible to obtain improved security by allowing In some cases, it is possible to obtain improved security by allowing
access only if the client sends a valid message integrity code (MIC) access only if the client sends a valid message integrity code (MIC)
binding the GSSAPI context to the keys used for encryption and binding the GSS-API context to the keys used for encryption and
integrity protection of the SSH session. With this extra level of integrity protection of the SSH session. With this extra level of
protection, a "man-in-the-middle" attacker who has convinced a client protection, a "man-in-the-middle" attacker who has convinced a client
of his authenticity cannot then relay user authentication messages of his authenticity cannot then relay user authentication messages
between the real client and server, thus gaining access to the real between the real client and server, thus gaining access to the real
server. This additional protection is available when the negotiated server. This additional protection is available when the negotiated
GSSAPI context supports per-message integrity protection, as GSS-API context supports per-message integrity protection, as
indicated by the setting of the integ_avail flag on successful return indicated by the setting of the integ_avail flag on successful return
from GSS_Init_sec_context() or GSS_Accept_sec_context(). from GSS_Init_sec_context() or GSS_Accept_sec_context().
When the client's call to GSS_Init_sec_context() returns When the client's call to GSS_Init_sec_context() returns
GSS_S_COMPLETE with the integ_avail flag set, the client MUST GSS_S_COMPLETE with the integ_avail flag set, the client MUST
conclude the user authentication exchange by sending the following conclude the user authentication exchange by sending the following
message: message:
byte SSH_MSG_USERAUTH_GSSAPI_MIC byte SSH_MSG_USERAUTH_GSSAPI_MIC
string MIC string MIC
This message MUST be sent only if GSS_Init_sec_context() returned This message MUST be sent only if GSS_Init_sec_context() returned
GSS_S_COMPLETE. If a token is also 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.
The contents of the MIC field are obtained by calling GSS_GetMIC over The contents of the MIC field are obtained by calling GSS_GetMIC()
the following, using the GSSAPI context which was just established: over the following, using the GSS-API context that was just
established:
string session identifier string session identifier
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name string user name
string service string service
string "gssapi-with-mic" string "gssapi-with-mic"
If this message is received by the server before the GSSAPI context If this message is received by the server before the GSS-API context
is fully established, the server MUST fail the authentication. is fully established, the server MUST fail the authentication.
If this message is received by the server when the negotiated GSSAPI If this message is received by the server when the negotiated GSS-API
context does not support per-message integrity protection, the server context does not support per-message integrity protection, the server
MUST fail the authentication. MUST fail the authentication.
3.6. Client acknowledgement 3.6. Client Acknowledgement
Some servers may wish to permit user authentication to proceed even Some servers may wish to permit user authentication to proceed even
when the negotitated GSSAPI context does not support per-message when the negotiated GSS-API context does not support per-message
integrity protection. In such cases, it is possible for the server integrity protection. In such cases, it is possible for the server
to successfully complete the GSSAPI method, while the client's last to successfully complete the GSS-API method, while the client's last
call to GSS_Init_sec_context fails. If the server simply assumed call to GSS_Init_sec_context() fails. If the server simply assumed
success on the part of the client and completed the authentication success on the part of the client and completed the authentication
service, it is possible that the client would fail to complete the service, it is possible that the client would fail to complete the
authentication method, but not be able to retry other methods because authentication method, but not be able to retry other methods because
the server had already moved on. To protect against this, a final the server had already moved on. To protect against this, a final
message is sent by the client to indicate it has completed message is sent by the client to indicate it has completed
authentication. authentication.
When the client's call to GSS_Init_sec_context() returns When the client's call to GSS_Init_sec_context() returns
GSS_S_COMPLETE with the integ_avail flag not set, the client MUST GSS_S_COMPLETE with the integ_avail flag not set, the client MUST
conclude the user authentication exchange by sending the following conclude the user authentication exchange by sending the following
message: message:
byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE
This message MUST be sent only if GSS_Init_sec_context() returned This message MUST be sent only if GSS_Init_sec_context() returned
GSS_S_COMPLETE. If a token is also 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 this message is received by the server before the GSSAPI context If this message is received by the server before the GSS-API context
is fully established, the server MUST fail the authentication. is fully established, the server MUST fail the authentication.
If this message is received by the server when the negotiated GSSAPI If this message is received by the server when the negotiated GSS-API
context supports per-message integrity protection, the server MUST context supports per-message integrity protection, the server MUST
fail the authentication. fail the authentication.
It is a site policy descision for the server whether or not to permit It is a site policy decision for the server whether or not to permit
authentication using GSSAPI mechanisms and/or contexts which do not authentication using GSS-API mechanisms and/or contexts that do not
support per-message integrity protection. The server MAY fail the support per-message integrity protection. The server MAY fail the
otherwise valid gssapi-with-mic authentication if per-message otherwise valid gssapi-with-mic authentication if per-message
integrity protection is not supported. integrity protection is not supported.
3.7. Completion 3.7. Completion
As with all SSH authentication methods, successful completion is As with all SSH authentication methods, successful completion is
indicated by a SSH_MSG_USERAUTH_SUCCESS if no other authentication is indicated by an SSH_MSG_USERAUTH_SUCCESS if no other authentication
required, or a SSH_MSG_USERAUTH_FAILURE with the partial success flag is required, or an SSH_MSG_USERAUTH_FAILURE with the partial success
set if the server requires further authentication. This packet flag set if the server requires further authentication. This packet
SHOULD be sent immediately following receipt of the the SHOULD be sent immediately following receipt of the
SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet. SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet.
3.8. Error Status 3.8. Error Status
In the event a GSSAPI error occurs on the server during context In the event that a GSS-API 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 an
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
[UTF8]. Language tags are those described in [LANGTAG]. Note that [UTF8]. Language tags are those described in [LANGTAG]. Note that
skipping to change at page 20, line 8 skipping to change at page 18, line 24
return-line feed (CRLF) sequences. Application developers should return-line feed (CRLF) sequences. Application developers should
take this into account when displaying these messages. take 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 ignore report them to the user. Any server sending this message MUST ignore
any SSH_MSG_UNIMPLEMENTED sent by the client in response. any SSH_MSG_UNIMPLEMENTED sent by the client in response.
3.9. 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 is This message implies that the authentication is about to fail, and is
defined to allow the error token to be communicated without losing defined to allow the error token to be communicated without 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 an
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 this applying to the same authentication request. A client receiving 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 an 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 SSH_MSG_UNIMPLEMENTED Any server sending this message MUST ignore any SSH_MSG_UNIMPLEMENTED
sent by the client in response. If a server sends both this message sent by the client in response. If a server sends both this message
and an SSH_MSG_USERAUTH_GSSAPI_ERROR message, the and an SSH_MSG_USERAUTH_GSSAPI_ERROR message, the
SSH_MSG_USERAUTH_GSSAPI_ERROR message MUST be sent first, to allow SSH_MSG_USERAUTH_GSSAPI_ERROR message MUST be sent first, to allow
the client to store and/or display the error status before processing the client to store and/or display the error status before processing
the error token. the error token.
4. Authentication using GSSAPI Key Exchange 4. Authentication Using GSS-API 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 [SSH-USERAUTH]. This method performs user framework described in [SSH-USERAUTH]. This method performs user
authentication by making use of an existing GSSAPI context authentication by making use of an existing GSS-API context
established during key exchange. established during key exchange.
The authentication method name for this protocol is "gssapi-keyex". The authentication method name for this protocol is "gssapi-keyex".
This method may be used only if the initial key exchange was This method may be used only if the initial key exchange was
performed using a GSSAPI-based key exchange method defined in performed using a GSS-API-based key exchange method defined in
accordance with Section 2. The GSSAPI context used with this method accordance with Section 2. The GSS-API context used with this method
is always that established during an initial GSSAPI-based key is always that established during an initial GSS-API-based key
exchange. Any context established during key exchange for the exchange. Any context established during key exchange for the
purpose of rekeying MUST NOT be used with this method. purpose of rekeying MUST NOT be used with this method.
The server SHOULD include this user authentication method in the list The server SHOULD include this user authentication method in the list
of methods that can continue (in a SSH_MSG_USERAUTH_FAILURE) if the of methods that can continue (in an SSH_MSG_USERAUTH_FAILURE) if the
initial key exchange was performed using a GSSAPI-based key exchange initial key exchange was performed using a GSS-API-based key exchange
method and provides information about the user's identity which is method and provides information about the user's identity that is
useful to the server. It MUST NOT include this method if the initial useful to the server. It MUST NOT include this method if the initial
key exchange was not performed using a GSSAPI-based key exchange key exchange was not performed using a GSS-API-based key exchange
method defined in accordance with Section 2. method defined in accordance with Section 2.
The client SHOULD attempt to use this method if it is advertised by The client SHOULD attempt to use this method if it is advertised by
the server, initial key exchange was performed using a GSSAPI-based the server, initial key exchange was performed using a GSS-API-based
key exchange method, and this method has not already been tried. The key exchange method, and this method has not already been tried. The
client SHOULD NOT try this method more than once per session. It client SHOULD NOT try this method more than once per session. It
MUST NOT try this method if initial key exchange was not performed MUST NOT try this method if initial key exchange was not performed
using a GSSAPI-based key exchange method defined in accordance with using a GSS-API-based key exchange method defined in accordance with
Section 2. Section 2.
If a server receives a request for this method when initial key If a server receives a request for this method when initial key
exchange was not performed using a GSSAPI-based key exchange method exchange was not performed using a GSS-API-based key exchange method
defined in accordance with Section 2, it MUST return defined in accordance with Section 2, it MUST return
SSH_MSG_USERAUTH_FAILURE. SSH_MSG_USERAUTH_FAILURE.
This method is defined as a single message: This method is defined as a single message:
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name string user name
string service string service
string "gssapi-keyex" string "gssapi-keyex"
string MIC string MIC
The contents of the MIC field are obtained by calling GSS_GetMIC over The contents of the MIC field are obtained by calling GSS_GetMIC over
the following, using the GSSAPI context which was established during the following, using the GSS-API context that was established during
initial key exchange: initial key exchange:
string session identifier string session identifier
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name string user name
string service string service
string "gssapi-keyex" string "gssapi-keyex"
Upon receiving this message when initial key exchange was performed Upon receiving this message when initial key exchange was performed
using a GSSAPI-based key exchange method, the server uses using a GSS-API-based key exchange method, the server uses
GSS_VerifyMIC() to verify that the MIC received is valid. If the MIC GSS_VerifyMIC() to verify that the MIC received is valid. If the MIC
is not valid, the user authentication fails, and the server MUST is not valid, the user authentication fails, and the server MUST
return SSH_MSG_USERAUTH_FAILURE. return SSH_MSG_USERAUTH_FAILURE.
If the MIC is valid and the server is satisfied as to the user's If the MIC is valid and the server is satisfied as to the user's
credentials, it MAY return either SSH_MSG_USERAUTH_SUCCESS, or credentials, it MAY return either SSH_MSG_USERAUTH_SUCCESS or
SSH_MSG_USERAUTH_FAILURE with the partial success flag set, depending SSH_MSG_USERAUTH_FAILURE with the partial success flag set, depending
on whether additional authentications are needed. on whether additional authentications are needed.
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
and provides neither signature nor encryption algorithms. Thus, it provides neither signature nor encryption algorithms. Thus, it can
can be used only with key exchange methods that do not require any 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 2 are material. The key exchange methods described in Section 2 are
examples of such methods. examples of such methods.
This algorithm is used when, as a matter of configuration, the host This algorithm is used when, as a matter of configuration, the host
does not have or does not wish to use a public key. For example, it does not have or does not wish to use a public key. For example, it
can be used when the administrator has decided as a matter of policy can be used when the administrator has decided as a matter of policy
to require that all key exchanges be authenticated using Kerberos to require that all key exchanges be authenticated using Kerberos
[KRB5], and thus the only permitted key exchange method is the [KRB5], and thus the only permitted key exchange method is the
GSSAPI-authenticated Diffie-Hellman exchange described above, with GSS-API-authenticated Diffie-Hellman exchange described above, with
Kerberos V5 as the underlying GSSAPI mechanism. In such a Kerberos V5 as the underlying GSS-API mechanism. In such a
configuration, the server implementation supports the "ssh-dss" key configuration, the server implementation supports the "ssh-dss" key
algorithm (as required by [SSH-TRANSPORT]), but could be prohibited algorithm (as required by [SSH-TRANSPORT]), but could be prohibited
by configuration from using it. In this situation, the server needs by 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 that 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 key with implementations that do not support the GSS-API-authenticated
exchange and Kerberos. key exchange and Kerberos.
Any implementation supporting at least one key exchange method which Any implementation supporting at least one key exchange method that
conforms to Section 2 MUST also support the "null" host key conforms to Section 2 MUST also support the "null" host key
algorithm. Servers MUST NOT advertise the "null" host key algorithm algorithm. Servers MUST NOT advertise the "null" host key algorithm
unless it is the only algorithm advertised. 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 GSSAPI- The following message numbers have been defined for use with GSS-
based key exchange methods: API-based key exchange methods:
#define SSH_MSG_KEXGSS_INIT 30 #define SSH_MSG_KEXGSS_INIT 30
#define SSH_MSG_KEXGSS_CONTINUE 31 #define SSH_MSG_KEXGSS_CONTINUE 31
#define SSH_MSG_KEXGSS_COMPLETE 32 #define SSH_MSG_KEXGSS_COMPLETE 32
#define SSH_MSG_KEXGSS_HOSTKEY 33 #define SSH_MSG_KEXGSS_HOSTKEY 33
#define SSH_MSG_KEXGSS_ERROR 34 #define SSH_MSG_KEXGSS_ERROR 34
#define SSH_MSG_KEXGSS_GROUPREQ 40 #define SSH_MSG_KEXGSS_GROUPREQ 40
#define SSH_MSG_KEXGSS_GROUP 41 #define SSH_MSG_KEXGSS_GROUP 41
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
skipping to change at page 25, line 5 skipping to change at page 22, line 5
#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_ERRTOK 65
#define SSH_MSG_USERAUTH_GSSAPI_MIC 66 #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. GSS-API Considerations
7.1. Naming Conventions 7.1. Naming Conventions
In order to establish a GSSAPI security context, the SSH client needs In order to establish a GSS-API security context, the SSH client
to determine the appropriate targ_name to use in identifying the needs to determine the appropriate targ_name to use in identifying
server when calling GSS_Init_sec_context. For this purpose, the the server when calling GSS_Init_sec_context(). For this purpose,
GSSAPI mechanism-independent name form for host-based services is the GSS-API mechanism-independent name form for host-based services
used, as described in section 4.1 of [GSSAPI]. is used, as described in Section 4.1 of [GSSAPI].
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.
Because the GSS-API mechanism uses the targ_name to authenticate the
server's identity, it is important that it be determined in a secure
fashion. One common way to do this is to construct the targ_name
from the hostname as typed by the user; unfortunately, because some
GSS-API mechanisms do not canonicalize hostnames, it is likely that
this technique will fail if the user has not typed a fully-qualified,
canonical hostname. Thus, implementers may wish to use other
methods, but should take care to ensure they are secure. For
example, one should not rely on an unprotected DNS record to map a
host alias to the primary name of a server, or an IP address to a
hostname, since an attacker can modify the mapping and impersonate
the server.
Implementations of mechanisms conforming to this document MUST NOT
use the results of insecure DNS queries to construct the targ_name.
Clients MAY make use of a mapping provided by local configuration or
use other secure means to determine the targ_name to be used. If a
client system is unable to securely determine which targ_name to use,
then it SHOULD NOT use this mechanism.
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 and does not specify any standard data to be used as channel bindings,
the use of network addresses as channel bindings may break SSH in and the use of network addresses as channel bindings may break SSH 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
[SPNEGO] in conjunction with the authentication and key exchange [SPNEGO] in conjunction with the authentication and key exchange
methods described in this document is both unnecessary and methods described in this document is both unnecessary and
undesirable. As a result, mechanisms conforming to this document undesirable. As a result, mechanisms conforming to this document
MUST NOT use SPNEGO as the underlying GSSAPI mechanism. MUST NOT use SPNEGO as the underlying GSS-API 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 not exchange methods, the negotiation capability of SPNEGO alone does not
provide any added benefit. In fact, as described below, it has the provide any added benefit. In fact, as described below, it has 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 GSS-API
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, this checking that value at the client. In the case of key exchange, this
protection is not needed because the key exchange methods described protection is not needed because the key exchange methods described
here already perform an equivalent operation; namely, they generate a here already perform an equivalent operation; namely, they generate a
MIC of the SSH exchange hash, which is a hash of several items MIC of the SSH exchange hash, which is a hash of several items
including the lists of key exchange mechanisms supported by both including the lists of key exchange mechanisms supported by both
sides. In the case of user authentication, the protection is not sides. In the case of user authentication, the protection is not
needed because the negotiation occurs over a secure channel, and the needed because the negotiation occurs over a secure channel, and 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 SPNEGO The use of SPNEGO combined with GSS-API mechanisms used without
can lead to interoperability problems. For example, a client which SPNEGO can lead to interoperability problems. For example, a client
supports key exchange using the Kerberos V5 GSSAPI mechanism [KRB5- that supports key exchange using the Kerberos V5 GSS-API mechanism
GSS] only underneath SPNEGO will not interoperate with a server which [KRB5-GSS] only underneath SPNEGO will not interoperate with a server
supports key exchange only using the Kerberos V5 GSSAPI mechanism that supports key exchange only using the Kerberos V5 GSS-API
directly. As a result, allowing GSSAPI mechanisms to be used both mechanism directly. As a result, allowing GSS-API mechanisms to be
with and without SPNEGO is undesirable. used both 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 GSS-API-based key exchange
method X, then non-GSSAPI method Y, then GSSAPI-based method Z, and method X, then non-GSS-API method Y, then GSS-API-based method Z, and
if a server supports mechanisms Y and Z but not X, then an attempt to if a server supports mechanisms Y and Z but not X, then an attempt to
use SPNEGO to negotiate a GSSAPI mechanism might result in the use of use SPNEGO to negotiate a GSS-API mechanism might result in the use
method Z when method Y would have been preferable. As a result, the of method Z when method Y would have been preferable. As a result,
use of SPNEGO could result in the subversion of the negotiation the use of SPNEGO could result in the subversion of the negotiation
algorithm for key exchange methods as described in section 7.1 of algorithm for key exchange methods as described in Section 7.1 of
[SSH-TRANSPORT] and/or the negotiation algorithm for user [SSH-TRANSPORT] and/or the negotiation algorithm for user
authentication methods as described in [SSH-USERAUTH]. authentication methods as described in [SSH-USERAUTH].
8. IANA Considerations 8. IANA Considerations
Consistent with section 8 of [SSH-ARCH] and section 4.6 of [SSH- Consistent with Section 8 of [SSH-ARCH] and Section 4.6 of
NUMBERS], this document makes the following registrations: [SSH-NUMBERS], this document makes the following registrations:
The family of SSH key exchange method names beginning with "gss- The family of SSH key exchange method names beginning with "gss-
group1-sha1-" and not containing the at-sign ('@'), to name the group1-sha1-" and not containing the at-sign ('@'), to name the
key exchange methods defined in Section 2.3. key exchange methods defined in Section 2.3.
The family of SSH key exchange method names beginning with "gss- The family of SSH key exchange method names beginning with "gss-
gex-sha1-" and not containing the at-sign ('@'), to name the key gex-sha1-" and not containing the at-sign ('@'), to name the key
exchange methods defined in Section 2.5. exchange methods defined in Section 2.5.
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 2.6. noted in Section 2.6.
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-with-mic", to name The SSH user authentication method name "gssapi-with-mic", to name
the GSSAPI user authentication method defined in Section 3. the GSS-API user authentication method defined in Section 3.
The SSH user authentication method name "gssapi-keyex", to name The SSH user authentication method name "gssapi-keyex", to name
the GSSAPI user authentication method defined in Section 4. the GSS-API user authentication method defined in Section 4.
The SSH user authentication method name "gssapi" is to be The SSH user authentication method name "gssapi" is to be
reserved, in order to avoid conflicts with implementations reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification. supporting an earlier version of this specification.
The SSH user authentication method name "external-keyx" is to be The SSH user authentication method name "external-keyx" is to be
reserved, in order to avoid conflicts with implementations reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification. 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 such, security considerations are discussed throughout. As 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 GSS-API, any
underlying GSSAPI mechanisms which are used, and any protocols on underlying GSS-API mechanisms that are used, and any protocols on
which such mechanisms might depend. Each of these components plays a which such mechanisms might depend. Each of these components plays a
part in the security of the resulting connection, and each will have part in the security of the resulting connection, and each will have
its own security considerations. its own security considerations.
The key exchange method described in Section 2 depends on the The key exchange method described in Section 2 depends on the
underlying GSSAPI mechanism to provide both mutual authentication and underlying GSS-API mechanism to provide both mutual authentication
per-message integrity services. If either of these features is not and per-message integrity services. If either of these features is
supported by a particular GSSAPI mechanism, or by a particular not supported by a particular GSS-API mechanism, or by a particular
implementation of a GSSAPI mechanism, then the key exchange is not implementation of a GSS-API mechanism, then the key exchange is not
secure and MUST fail. 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 obtained used, it MUST have access to user authentication information obtained
as a side-effect of the key exchange. If this information is as a side-effect of the key exchange. If this information 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_KEXGSS_CONTINUE messages containing a GSSAPI error token. and SSH_MSG_KEXGSS_CONTINUE messages containing a GSS-API error
token.
10. Acknowledgements 10. Acknowledgements
The authors would like to thank the following individuals for their The authors would like to thank the following individuals for their
invaluable assistance and contributions to this document: invaluable assistance and contributions to this document:
o Sam Hartman o Sam Hartman
o Love Hornquist-Astrand o Love Hornquist-Astrand
o Joel N. Weber II o Joel N. Weber II
o Simon Wilkinson o Simon Wilkinson
o Nicolas Williams o Nicolas Williams
Much of the text describing DH group exchnage was borrowed from Much of the text describing DH group exchange was borrowed from
[GROUP-EXCHANGE], by Markus Friedl, Niels Provos, and William A. [GROUP-EXCHANGE], by Markus Friedl, Niels Provos, and William A.
Simpson. Simpson.
11. Changes the last version 11. References
This section lists important changes since the previous version of
this internet-draft. This section should be removed at the time of
publication of this document as an RFC.
o Fixed editorial issues raised in WGLC, AD review, and IETF last
call
o Fixed a SHOULD that should have been capitalized but wasn't (per
comments from David Leonard and confirmation from Joseph
Galbraith)
o Updated the reference to
draft-ietf-secsh-dh-group-exchange-05.txt, and corrected for
changes in the section numbering of that document.
12. References 11.1. Normative References
12.1. Normative References [ASN1] ISO/IEC, "ASN.1 Encoding Rules: Specification of
Basic Encoding Rules (BER), Canonical Encoding
Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690 (1997), ISO/
IEC 8825-1:1998, November 1998.
[ASN1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic [GROUP-EXCHANGE] Friedl, M., Provos, N., and W. Simpson, "Diffie-
Encoding Rules (BER), Canonical Encoding Rules (CER) and Hellman Group Exchange for the Secure Shell (SSH)
Distinguished Encoding Rules (DER)", ITU-T Transport Layer Protocol", RFC 4419, March 2006.
Recommendation X.690 (1997), ISO/IEC 8825-1:1998,
November 1998.
[GROUP-EXCHANGE] [GSSAPI] Linn, J., "Generic Security Service Application
Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman Program Interface Version 2, Update 1", RFC 2743,
Group Exchange for the SSH Transport Layer Protocol", January 2000.
draft-ietf-secsh-dh-group-exchange-05.txt (work in
progress), July 2005.
[GSSAPI] Linn, J., "Generic Security Service Application Program [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Interface Version 2, Update 1", RFC 2743, January 2000. Requirement Levels", BCP 14, RFC 2119, March 1997.
[KEYWORDS] [LANGTAG] Alvestrand, H., "Tags for the Identification of
Bradner, S., "Key words for use in RFCs to Indicate Languages", BCP 47, RFC 3066, January 2001.
Requirement Levels", RFC 2119, BCP 14, March 1997.
[LANGTAG] Alvestrand, H., "Tags for the Identification of [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC
Languages", RFC 3066, BCP 47, January 2001. 1321, April 1992.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet
April 1992. Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [SSH-ARCH] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Extensions (MIME) Part One: Format of Internet Message Protocol Architecture", RFC 4251, January 2006.
Bodies", RFC 2045, November 1996.
[SSH-ARCH] [SSH-CONNECT] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Ylonen, T. and C. Lonvick, "SSH Protocol Architecture", Connection Protocol", RFC 4254, January 2006.
draft-ietf-secsh-architecture-22.txt (work in progress),
March 2005.
[SSH-CONNECT] [SSH-NUMBERS] Lehtinen, S. and C. Lonvick, "The Secure Shell
Ylonen, T. and C. Lonvick, "SSH Connection Protocol", (SSH) Protocol Assigned Numbers", RFC 4250, January
draft-ietf-secsh-connect-25.txt (work in progress), 2006.
March 2005.
[SSH-NUMBERS] [SSH-TRANSPORT] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Lehtinen, S. and C. Lonvick, "SSH Connection Protocol", Transport Layer Protocol", RFC 4253, January 2006.
draft-ietf-secsh-assignednumbers-12.txt (work in
progress), March 2005.
[SSH-TRANSPORT] [SSH-USERAUTH] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Ylonen, T. and C. Lonvick, "SSH Transport Layer Protocol", Authentication Protocol", RFC 4252, January 2006.
draft-ietf-secsh-transport-24.txt (work in progress),
March 2005.
[SSH-USERAUTH] [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
Ylonen, T. and C. Lonvick, "SSH Authentication Protocol", 10646", STD 63, RFC 3629, November 2003.
draft-ietf-secsh-userauth-27.txt (work in progress),
March 2005.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 11.2. Informative References
10646", RFC 3629, STD 63, November 2003.
12.2. Non-Normative References [KRB5] Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
"The Kerberos Network Authentication Service (V5)",
RFC 4120, July 2005.
[KRB5] Kohl, J. and C. Neuman, "The Kerberos Network [KRB5-GSS] Zhu, L., Jaganathan, K., and S. Hartman, "The
Authentication Service (V5)", RFC 1510, September 1993. Kerberos Version 5 Generic Security Service
Application Program Interface (GSS-API) Mechanism:
Version 2", RFC 4121, July 2005.
[KRB5-GSS] [SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for
Linn, J., "The Kerberos Version 5 GSS-API Mechanism", User Names and Passwords", RFC 4013, February 2005.
RFC 1964, June 1996.
[SPNEGO] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API [SPNEGO] Zhu, L., Leach, P., Jaganathan, K., and W.
Negotiation Mechanism", RFC 2478, December 1998. Ingersoll, "The Simple and Protected Generic
Security Service Application Program Interface
(GSS-API) Negotiation Mechanism", RFC 4178, October
2005.
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
US US
Phone: +1 412 268 7225 Phone: +1 412 268 7225
Email: jhutz+@cmu.edu EMail: jhutz+@cmu.edu
URI: http://www.cs.cmu.edu/~jhutz/ URI: http://www.cs.cmu.edu/~jhutz/
Joseph Salowey Joseph Salowey
Cisco Systems Cisco Systems
2901 Third Avenue 2901 Third Avenue
Seattle, WA 98121 Seattle, WA 98121
US US
Phone: +1 206 256 3380 Phone: +1 206 256 3380
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
Intellectual Property Statement Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
skipping to change at page 34, line 29 skipping to change at page 29, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at this standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Disclaimer of Validity Acknowledgement
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
 End of changes. 170 change blocks. 
406 lines changed or deleted 413 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/