draft-ietf-secsh-gsskeyex-09.txt   draft-ietf-secsh-gsskeyex-10.txt 
Secure Shell Working Group J. Hutzelman Secure Shell Working Group J. Hutzelman
Internet-Draft CMU Internet-Draft CMU
Expires: November 7, 2005 J. Salowey Expires: February 23, 2006 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
May 6, 2005 August 22, 2005
GSSAPI Authentication and Key Exchange for the Secure Shell Protocol GSSAPI Authentication and Key Exchange for the Secure Shell Protocol
draft-ietf-secsh-gsskeyex-09 draft-ietf-secsh-gsskeyex-10
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
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have been or will be disclosed, and any of which he or she becomes 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. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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skipping to change at page 1, line 39 skipping to change at page 1, line 39
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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on November 7, 2005. This Internet-Draft will expire on February 23, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
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)
[GSSAPI] provides security services to callers in a mechanism- provides security services to callers in a mechanism-independent
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 which uses a specified GSSAPI 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 which use GSSAPI to
authenticate the Diffie-Hellman exchange described in [SSH- authenticate a Diffie-Hellman key exchange.
TRANSPORT].
This memo also defines a new host public key algorithm which can be This memo also defines a new host public key algorithm which can be
used when no operations are needed using a host's public key, and a used when no operations are needed using a host's public key, and a
new user authentication method which allows an authorization name to new user authentication method which allows an authorization name to
be used in conjunction with any authentication which has already be used in conjunction with any authentication which has already
occurred as a side-effect of GSSAPI-based key exchange. occurred as a side-effect of GSSAPI-based key exchange.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 SSH terminology . . . . . . . . . . . . . . . . . . . . . 4 1.1. SSH terminology . . . . . . . . . . . . . . . . . . . . . 4
2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . 5 1.2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . 5 2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . . 5
2.2 Group Exchange . . . . . . . . . . . . . . . . . . . . . . 11 2.1. Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . 5
2.3 gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . 13 2.2. Group Exchange . . . . . . . . . . . . . . . . . . . . . . 11
2.4 gss-group14-sha1-* . . . . . . . . . . . . . . . . . . . . 13 2.3. gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . 13
2.5 gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . . . . 13 2.4. gss-group14-sha1-* . . . . . . . . . . . . . . . . . . . . 13
2.6 Other GSSAPI key exchange methods . . . . . . . . . . . . 13 2.5. gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . . . . 13
3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . 15 2.6. Other GSSAPI key exchange methods . . . . . . . . . . . . 13
3.1 GSSAPI Authentication Overview . . . . . . . . . . . . . . 15 3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . . 15
3.2 Initiating GSSAPI authentication . . . . . . . . . . . . . 15 3.1. GSSAPI Authentication Overview . . . . . . . . . . . . . . 15
3.3 Initial server response . . . . . . . . . . . . . . . . . 16 3.2. Initiating GSSAPI authentication . . . . . . . . . . . . . 15
3.4 GSSAPI session . . . . . . . . . . . . . . . . . . . . . . 16 3.3. Initial server response . . . . . . . . . . . . . . . . . 16
3.5 Binding Encryption Keys . . . . . . . . . . . . . . . . . 17 3.4. GSSAPI session . . . . . . . . . . . . . . . . . . . . . . 16
3.6 Client acknowledgement . . . . . . . . . . . . . . . . . . 18 3.5. Binding Encryption Keys . . . . . . . . . . . . . . . . . 17
3.7 Completion . . . . . . . . . . . . . . . . . . . . . . . . 19 3.6. Client acknowledgement . . . . . . . . . . . . . . . . . . 18
3.8 Error Status . . . . . . . . . . . . . . . . . . . . . . . 19 3.7. Completion . . . . . . . . . . . . . . . . . . . . . . . . 19
3.9 Error Token . . . . . . . . . . . . . . . . . . . . . . . 20 3.8. Error Status . . . . . . . . . . . . . . . . . . . . . . . 19
4. Authentication using GSSAPI Key Exchange . . . . . . . . . . 21 3.9. Error Token . . . . . . . . . . . . . . . . . . . . . . . 20
5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . 23 4. Authentication using GSSAPI Key Exchange . . . . . . . . . . . 21
6. Summary of Message Numbers . . . . . . . . . . . . . . . . . 24 5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . . 23
7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . 25 6. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 24
7.1 Naming Conventions . . . . . . . . . . . . . . . . . . . . 25 7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . . 25
7.2 Channel Bindings . . . . . . . . . . . . . . . . . . . . . 25 7.1. Naming Conventions . . . . . . . . . . . . . . . . . . . . 25
7.3 SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . 25 7.2. Channel Bindings . . . . . . . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27 7.3. SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . 28 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11. Changes the last version . . . . . . . . . . . . . . . . . . 30 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 11. Changes the last version . . . . . . . . . . . . . . . . . . . 30
12.1 Normative References . . . . . . . . . . . . . . . . . . 31 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.2 Non-Normative References . . . . . . . . . . . . . . . . 32 12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 32 12.2. Non-Normative References . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . 34 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 GSSAPI.
To do this, it defines a family of key exchange methods, two user To do this, it defines a family of key exchange methods, two user
authentication methods, and a new host key algorithm. These authentication methods, and a new host key algorithm. These
definitions allow any GSSAPI mechanism to be used with the Secure definitions allow any GSSAPI mechanism to be used with the Secure
Shell protocol. Shell protocol.
This document should be read only after reading the documents This document should be read only after reading the documents
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 [SSH-
USERAUTH]. This document freely uses terminology and notation from USERAUTH]. This document freely uses terminology and notation from
the architecture document without reference or further explanation. 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 GSSAPI
service names. Currently, the only defined service name is "ssh- service names. Currently, the only defined service name is "ssh-
connection", which refers to the SSH connection protocol [SSH- connection", which refers to the SSH connection protocol [SSH-
CONNECT]. CONNECT].
1.2. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
2. GSSAPI Authenticated Diffie-Hellman Key Exchange 2. GSSAPI Authenticated Diffie-Hellman Key Exchange
This section defines a class of key exchange methods which combine This section defines a class of key exchange methods which combine
the Diffie-Hellman key exchange from section 8 of [SSH-TRANSPORT] the Diffie-Hellman key exchange from section 8 of [SSH-TRANSPORT]
with mutual authentication using GSSAPI. with mutual authentication using GSSAPI.
Since the GSSAPI key exchange methods described in this section do Since the GSSAPI key exchange methods described in this section do
not require the use of public key signature or encryption 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 GSSAPI Key Exchange
The following symbols are used in this description: The following symbols are used in this description:
o C is the client, and S is the server o C is the client, and S is the server
o p is a large safe prime, g is a generator for a subgroup of 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
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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 GSSAPI-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 GSSAPI key exchange, even when previously unknown to the client.
Further, in environements in which GSSAPI-based key exchange is used Further, in environments in which GSSAPI-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 GSSAPI-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 GSSAPI
credentials have expired, client implementations SHOULD store host credentials have expired, client implementations SHOULD store host
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mpint K, the shared secret mpint K, the shared secret
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 GSSAPI
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 5 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
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group size, the preferred size of the group, and the maximal group size, the preferred size of the group, and the maximal
group size in bits the client will accept. group size in bits the client will accept.
2. S finds a group that best matches the client's request, and sends 2. S finds a group that best matches the client's request, and sends
"p || g" to C. "p || g" to C.
3. The exchange proceeds as described in Section 2.1 above, 3. The exchange proceeds as described in Section 2.1 above,
beginning with step 1, except that the exchange hash is computed beginning with step 1, except that the exchange hash is computed
as described below. as described below.
Servers and clients SHOULD support gorups with a modulus length of k Servers and clients SHOULD support groups with a modulus length of k
bits, where 1024 <= k <= 8192. The recommended values for min and bits, where 1024 <= k <= 8192. The recommended values for min and
max are 1024 and 8192, respectively. max are 1024 and 8192, respectively.
This is implemented using the following messages, in addition to This is implemented using the following messages, in addition to
those described above: those described above:
First, the client sends: First, the client sends:
byte SSH_MSG_KEXGSS_GROUPREQ byte SSH_MSG_KEXGSS_GROUPREQ
uint32 min, minimal size in bits of an acceptable group uint32 min, minimal size in bits of an acceptable group
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uint32 min, minimal size in bits of an acceptable group uint32 min, minimal size in bits of an acceptable group
uint32 n, preferred size in bits of the group the server uint32 n, preferred size in bits of the group the server
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 GSSAPI authenticated Diffie-Hellman
key exchange as described in Section 2.1 with SHA-1 as HASH, and the key exchange as described in Section 2.1 with SHA-1 as HASH, and the
group defined in section 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 DER
encoding [ASN1] of the underlying GSSAPI mechanism's OID. Base64 encoding [ASN1] of the underlying GSSAPI 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 GSSAPI 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 GSSAPI authenticated Diffie-Hellman
key exchange as described in Section 2.1 with SHA-1 as HASH, and the key exchange as described in Section 2.1 with SHA-1 as HASH, and the
group defined in section 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 GSSAPI 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 GSSAPI 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 GSSAPI 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 GSSAPI 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 GSSAPI mechanism.
2.6 Other GSSAPI key exchange methods 2.6. Other GSSAPI key exchange methods
Key exchange method names starting with "gss-" are reserved for key Key exchange method names starting with "gss-" are reserved for key
exchange methods which conform to this document; in particular, for exchange methods which conform to this document; in particular, for
those methods which use the GSSAPI authenticated Diffie-Hellman key those methods which use the GSSAPI authenticated Diffie-Hellman key
exchange algorithm described in Section 2.1, including any future exchange algorithm described in Section 2.1, including any future
methods which use different groups and/or hash functions. The intent methods which 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 methods be defined in a
similar manner to that used in Section 2.3. similar manner to that used in Section 2.3.
3. GSSAPI User Authentication 3. GSSAPI User Authentication
This section describes a general-purpose user authentication method This section describes a general-purpose user authentication method
based on [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. GSSAPI Authentication Overview
GSSAPI authentication must maintain a context. Authentication begins GSSAPI authentication must maintain a context. Authentication begins
when the client sends a SSH_MSG_USERAUTH_REQUEST, which specifies the when the client sends a SSH_MSG_USERAUTH_REQUEST, which specifies the
mechanism OIDs the client supports. mechanism OIDs the client supports.
If the server supports any of the requested mechanism OIDs, the If the server supports any of the requested mechanism OIDs, the
server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing
the mechanism OID. the mechanism OID.
After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the
client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets
until the authentication mechanism either succeeds or fails. until the authentication mechanism either succeeds or fails.
If at any time during the exchange, the client sends a new If at any time during the exchange, the client sends a new
SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely
discarded and destroyed, and any further GSSAPI authentication MUST discarded and destroyed, and any further GSSAPI authentication MUST
restart from the beginning. restart from the beginning.
3.2 Initiating GSSAPI authentication 3.2. Initiating GSSAPI authentication
The GSSAPI authentication method is initiated when the client sends a The GSSAPI authentication method is initiated when the client sends a
SSH_MSG_USERAUTH_REQUEST: SSH_MSG_USERAUTH_REQUEST:
byte SSH_MSG_USERAUTH_REQUEST byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding) string user name (in ISO-10646 UTF-8 encoding)
string service name (in US-ASCII) string service name (in US-ASCII)
string "gssapi-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
skipping to change at page 16, line 25 skipping to change at page 16, line 25
MAY send a bogus list of acceptable authentications but never accept MAY send a bogus list of acceptable authentications but never accept
any. This makes it possible for the server to avoid disclosing any. This makes it possible for the server to avoid disclosing
information about which accounts exist. In any case, if the user information about which accounts exist. In any case, if the user
does not exist, the authentication request MUST NOT be accepted. does not exist, the authentication request MUST NOT be accepted.
The client MAY at any time continue with a new The client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the 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 a
SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or
with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows: with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows:
byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE
string selected mechanism OID string selected mechanism OID
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. GSSAPI 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 GSSAPI 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()
skipping to change at page 17, line 31 skipping to change at page 17, line 31
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 GSSAPI 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 GSSAPI 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 GSSAPI context supports per-message integrity protection, as
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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 GSSAPI 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 GSSAPI
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 negotitated GSSAPI 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 GSSAPI 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
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If this message is received by the server when the negotiated GSSAPI If this message is received by the server when the negotiated GSSAPI
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 descision for the server whether or not to permit
authentication using GSSAPI mechanisms and/or contexts which do not authentication using GSSAPI mechanisms and/or contexts which 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 a SSH_MSG_USERAUTH_SUCCESS if no other authentication is
required, or a SSH_MSG_USERAUTH_FAILURE with the partial success flag required, or a SSH_MSG_USERAUTH_FAILURE with the partial success flag
set if the server requires further authentication. This packet 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 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 a GSSAPI error occurs on the server during context
establishment, the server MAY send the following message to inform establishment, the server MAY send the following message to inform
the client of the details of the error before sending a the client of the details of the error before sending a
SSH_MSG_USERAUTH_FAILURE message: SSH_MSG_USERAUTH_FAILURE message:
byte SSH_MSG_USERAUTH_GSSAPI_ERROR byte SSH_MSG_USERAUTH_GSSAPI_ERROR
uint32 major_status uint32 major_status
uint32 minor_status uint32 minor_status
string message string message
skipping to change at page 20, line 5 skipping to change at page 20, line 5
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
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
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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 a SSH_MSG_USERAUTH_FAILURE) if the
initial key exchange was performed using a GSSAPI-based key exchange initial key exchange was performed using a GSSAPI-based key exchange
method and provides information about the user's identity which is method and provides information about the user's identity which 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 GSSAPI-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 GSSAPI-based
key exchange method, and this method has 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 GSSAPI-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 GSSAPI-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.
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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 provides neither signature nor encryption algorithms. Thus, it and provides neither signature nor encryption algorithms. Thus, it
can be used only with key exchange methods that do not require any can be used only with key exchange methods that do not require any
public-key operations and do not require the use of host public key public-key operations and do not require the use of host public key
material. The key exchange methods described in section 1 of this material. The key exchange methods described in Section 2 are
document 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 GSSAPI-authenticated Diffie-Hellman exchange described above, with
Kerberos V5 as the underlying GSSAPI mechanism. In such a Kerberos V5 as the underlying GSSAPI 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
skipping to change at page 23, line 35 skipping to change at page 23, line 35
this purpose. this purpose.
Note that the use of the "null" algorithm in this way means that the Note that the use of the "null" algorithm in this way means that the
server will not be able to interoperate with clients which do not server will not be able to interoperate with clients which do not
support this algorithm. This is not a significant problem, since in support this algorithm. This is not a significant problem, since in
the configuration described, it will also be unable to interoperate the configuration described, it will also be unable to interoperate
with implementations that do not support the GSSAPI-authenticated key with implementations that do not support the GSSAPI-authenticated key
exchange and Kerberos. exchange and Kerberos.
Any implementation supporting at least one key exchange method which Any implementation supporting at least one key exchange method which
conforms to section 1 of this document MUST also support the "null" conforms to Section 2 MUST also support the "null" host key
host key algorithm. Servers MUST NOT advertise the "null" host key algorithm. Servers MUST NOT advertise the "null" host key algorithm
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 GSSAPI-
based key exchange methods: 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
by other kex methods. by other kex methods.
The following message numbers have been defined for use with the The following message numbers have been defined for use with the
'gssapi' user authentication method: 'gssapi-with-mic' user authentication method:
#define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60 #define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60
#define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61 #define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61
#define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63 #define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63
#define SSH_MSG_USERAUTH_GSSAPI_ERROR 64 #define SSH_MSG_USERAUTH_GSSAPI_ERROR 64
#define SSH_MSG_USERAUTH_GSSAPI_ERRTOK 65 #define SSH_MSG_USERAUTH_GSSAPI_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. GSSAPI 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 GSSAPI security context, the SSH client needs
to determine the appropriate targ_name to use in identifying the to determine the appropriate targ_name to use in identifying the
server when calling GSS_Init_sec_context. For this purpose, the server when calling GSS_Init_sec_context. For this purpose, the
GSSAPI mechanism-independent name form for host-based services is GSSAPI mechanism-independent name form for host-based services is
used, as described in section 4.1 of [GSSAPI]. 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.
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 and
the use of network addresses as channel bindings may break SSH in 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 GSSAPI mechanism.
Since SSH performs its own negotiation of authentication and key Since SSH performs its own negotiation of authentication and key
exchange methods, the negotiation capability of SPNEGO alone does 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
skipping to change at page 28, line 16 skipping to change at page 28, line 16
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 GSSAPI, any
underlying GSSAPI mechanisms which are used, and any protocols on underlying GSSAPI mechanisms which are used, and any protocols on
which such mechanisms might depend. Each of these components plays 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 1 of this document The key exchange method described in Section 2 depends on the
depends on the underlying GSSAPI mechanism to provide both mutual underlying GSSAPI mechanism to provide both mutual authentication and
authentication and per-message integrity services. If either of per-message integrity services. If either of these features is not
these features is not supported by a particular GSSAPI mechanism, or supported by a particular GSSAPI mechanism, or by a particular
by a particular implementation of a GSSAPI mechanism, then the key implementation of a GSSAPI mechanism, then the key exchange is not
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_KEXGEE_CONTINUE messages containing a GSSAPI error token. and SSH_MSG_KEXGSS_CONTINUE messages containing a GSSAPI 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
skipping to change at page 30, line 11 skipping to change at page 30, line 11
Much of the text describing DH group exchnage was borrowed from Much of the text describing DH group exchnage 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. Changes the last version
This section lists important changes since the previous version of This section lists important changes since the previous version of
this internet-draft. This section should be removed at the time of this internet-draft. This section should be removed at the time of
publication of this document as an RFC. publication of this document as an RFC.
o Added text about not giving loud warnings when getting a new host o Fixed editorial issues raised in WGLC, AD review, and IETF last
key from an already-known host via GSSAPI. call
o Added gss-group14-sha1-*
o Updated references to the ssh core documents. o Fixed a SHOULD that should have been capitalized but wasn't (per
comments from David Leonard and confirmation from Joseph
Galbraith)
o Converted to RFC3978 boilerplate. 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 12. References
12.1 Normative References 12.1. Normative References
[ASN1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic [ASN1] ISO/IEC, "ASN.1 Encoding Rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Distinguished Encoding Rules (DER)", ITU-T
Recommendation X.690 (1997), ISO/IEC 8825-1:1998, Recommendation X.690 (1997), ISO/IEC 8825-1:1998,
November 1998. November 1998.
[GROUP-EXCHANGE] [GROUP-EXCHANGE]
Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
Group Exchange for the SSH Transport Layer Protocol", Group Exchange for the SSH Transport Layer Protocol",
draft-ietf-secsh-dh-group-exchange-04.txt (work in draft-ietf-secsh-dh-group-exchange-05.txt (work in
progress), July 2003. progress), July 2005.
[GSSAPI] Linn, J., "Generic Security Service Application Program [GSSAPI] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[KEYWORDS] [KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997. Requirement Levels", RFC 2119, BCP 14, March 1997.
[LANGTAG] Alvestrand, H., "Tags for the Identification of [LANGTAG] Alvestrand, H., "Tags for the Identification of
Languages", RFC 3066, BCP 47, January 2001. Languages", RFC 3066, BCP 47, January 2001.
skipping to change at page 32, line 18 skipping to change at page 32, line 18
March 2005. March 2005.
[SSH-USERAUTH] [SSH-USERAUTH]
Ylonen, T. and C. Lonvick, "SSH Authentication Protocol", Ylonen, T. and C. Lonvick, "SSH Authentication Protocol",
draft-ietf-secsh-userauth-27.txt (work in progress), draft-ietf-secsh-userauth-27.txt (work in progress),
March 2005. March 2005.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO [UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, STD 63, November 2003. 10646", RFC 3629, STD 63, November 2003.
12.2 Non-Normative References 12.2. Non-Normative References
[KRB5] Kohl, J. and C. Neuman, "The Kerberos Network [KRB5] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993. Authentication Service (V5)", RFC 1510, September 1993.
[KRB5-GSS] [KRB5-GSS]
Linn, J., "The Kerberos Version 5 GSS-API Mechanism", Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996. RFC 1964, June 1996.
[SPNEGO] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API [SPNEGO] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998. Negotiation Mechanism", RFC 2478, December 1998.
 End of changes. 

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