Network Working Group J. Hutzelman
Internet-Draft CMU
Expires: January 17, 2005 J. Salowey
Cisco Systems
J. Galbraith
Van Dyke Technologies, Inc.
V. Welch
U Chicago / ANL
July 19, 2004
GSSAPI Authentication and Key Exchange for the Secure Shell Protocol
draft-ietf-secsh-gsskeyex-08
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The Secure Shell protocol (SSH) is a protocol for secure remote
login and other secure network services over an insecure network.
The Generic Security Service Application Program Interface (GSS-API)
[GSSAPI] provides security services to callers in a
mechanism-independent fashion.
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This memo describes methods for using the GSS-API for authentication
and key exchange in SSH. It defines an SSH user authentication
method which uses a specified GSSAPI mechanism to authenticate a
user, and a family of SSH key exchange methods which use GSSAPI to
authenticate the Diffie-Hellman exchange described in
[SSH-TRANSPORT].
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
new user authentication method which allows an authorization name to
be used in conjunction with any authentication which has already
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].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 SSH terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. GSSAPI Authenticated Diffie-Hellman Key Exchange . . . . . . . 5
2.1 Generic GSSAPI Key Exchange . . . . . . . . . . . . . . . . . 5
2.2 Group Exchange . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 gss-group1-sha1-* . . . . . . . . . . . . . . . . . . . . . . 12
2.4 gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5 Other GSSAPI key exchange methods . . . . . . . . . . . . . . 13
3. GSSAPI User Authentication . . . . . . . . . . . . . . . . . . 14
3.1 GSSAPI Authentication Overview . . . . . . . . . . . . . . . . 14
3.2 Initiating GSSAPI authentication . . . . . . . . . . . . . . . 14
3.3 Initial server response . . . . . . . . . . . . . . . . . . . 15
3.4 GSSAPI session . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5 Binding Encryption Keys . . . . . . . . . . . . . . . . . . . 16
3.6 Client acknowledgement . . . . . . . . . . . . . . . . . . . . 17
3.7 Completion . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.8 Error Status . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.9 Error Token . . . . . . . . . . . . . . . . . . . . . . . . . 19
4. Authentication using GSSAPI Key Exchange . . . . . . . . . . . 20
5. Null Host Key Algorithm . . . . . . . . . . . . . . . . . . . 22
6. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 23
7. GSSAPI Considerations . . . . . . . . . . . . . . . . . . . . 24
7.1 Naming Conventions . . . . . . . . . . . . . . . . . . . . . . 24
7.2 Channel Bindings . . . . . . . . . . . . . . . . . . . . . . . 24
7.3 SPNEGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
11. Changes the last version . . . . . . . . . . . . . . . . . . . 29
References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
This document describes the methods used to perform key exchange and
user authentication in the Secure Shell protocol using the GSSAPI.
To do this, it defines a family of key exchange methods, two user
authentication methods, and a new host key algorithm. These
definitions allow any GSSAPI mechanism to be used with the Secure
Shell protocol.
This document should be read only after reading the documents
describing the SSH protocol architecture [SSH-ARCH], transport layer
protocol [SSH-TRANSPORT], and user authentication protocol
[SSH-USERAUTH]. This document freely uses terminology and notation
from the architecture document without reference or further
explanation.
1.1 SSH terminology
The data types used in the packets are defined in the SSH
architecture document [SSH-ARCH]. It is particularly important to
note the definition of string allows binary content.
The SSH_MSG_USERAUTH_REQUEST packet refers to a service; this
service name is an SSH service name, and has no relationship to
GSSAPI service names. Currently, the only defined service name is
"ssh-connection", which refers to the SSH connection protocol
[SSH-CONNECT].
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2. GSSAPI Authenticated Diffie-Hellman Key Exchange
This section defines a class of key exchange methods which combine
the Diffie-Hellman key exchange from section 6 of [SSH-TRANSPORT]
with mutual authentication using GSSAPI.
Since the GSSAPI key exchange methods described in this section do
not require the use of public key signature or encryption
algorithms, they MAY be used with any host key algorithm, including
the "null" algorithm described in Section 5.
2.1 Generic GSSAPI Key Exchange
The following symbols are used in this description:
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), 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 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
mod p.
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 client MUST set the mutual_req_flag to "true" to request
that mutual authentication be performed. It also MUST set the
integ_req_flag to "true" to request that per-message integrity
protection be supported for this context. In addition, the
deleg_req_flag MAY be set to "true" to request access
delegation, if requested by the user. Since the key exchange
process authenticates only the host, the setting of the
anon_req_flag is immaterial to this process. If the client does
not support the "gssapi-keyex" user authentication method
described in Section 4, or does not intend to use that method in
conjunction with the GSSAPI context established during key
exchange, then the anon_req_flag SHOULD be set to "true".
Otherwise, this flag MAY be set to true if the client wishes to
hide its identity. Since the key exchange process will involve
the exchange of only a single token once the context has been
established, it is not necessary that the GSSAPI context support
detection of replayed or out-of-sequence tokens. Thus, the
setting of the replay_det_req_flag and sequence_req_flag are not
needed for this process. These flags SHOULD be set to "false".
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* If the resulting major_status code is GSS_S_COMPLETE and the
mutual_state flag is not true, then mutual authentication has
not been established, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and the
integ_avail flag is not true, then per-message integrity
protection is not available, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and both
the mutual_state and integ_avail flags are true, the
resulting output token is sent to S.
* If the resulting major_status code is GSS_S_CONTINUE_NEEDED,
the the output_token is sent to S, which will reply with a
new token to be provided to GSS_Init_sec_context.
* The client MUST also include "e" with the first message it
sends to the server during this process; if the server
receives more than one "e" or none at all, the key exchange
fails.
* 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 exchange MUST fail.
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
mutual_state flag is not true, then mutual authentication has
not been established, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and the
integ_avail flag is not true, then per-message integrity
protection is not available, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and both
the mutual_state and integ_avail flags are true, then the
security context has been established, and processing
continues with step 4.
* If the resulting major_status code is GSS_S_CONTINUE_NEEDED,
then the output token is sent to C, and processing continues
with step 2.
* If the resulting major_status code is GSS_S_COMPLETE, but a
non-zero-length reply token is returned, then that token is
sent to the client.
4. S generates a random number y (0 < y < q) and computes f = g^y
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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
obtain a GSSAPI message integrity code for H. S then sends f
and the MIC to C.
5. This step is performed only if the server's final call to
GSS_Accept_sec_context produced a non-zero-length final reply
token to be sent to the client _and_ no previous call by the
client to GSS_Init_sec_context has resulted in a major_status of
GSS_S_COMPLETE. Under these conditions, the client makes an
additional call to GSS_Init_sec_context to process the final
reply token. This call is made exactly as described above.
However, if the resulting major_status is anything other than
GSS_S_COMPLETE, or a non-zero-length token is returned, it is an
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
|| 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 verified, the key exchange MUST fail.
Either side MUST NOT send or accept e or f values that are not in
the range [1, p-1]. If this condition is violated, the key exchange
fails.
If any call to GSS_Init_sec_context or GSS_Accept_sec_context
returns a major_status other than GSS_S_COMPLETE or
GSS_S_CONTINUE_NEEDED, or any other GSSAPI call returns a
major_status other than GSS_S_COMPLETE, the key exchange fails. In
this case, several mechanisms are available for communicating error
information to the peer before terminating the connection as
required by [SSH-TRANSPORT]:
o If the key exchange fails due to any GSSAPI error on the server
(including errors returned by GSS_Accept_sec_context), the server
MAY send a message informing the client of the details of the
error. In this case, if an error token is also sent (see below),
then this message MUST be sent before the error token.
o If the key exchange fails due to a GSSAPI error returned from the
server's call to GSS_Accept_sec_context, and an "error token" is
also returned, then the server SHOULD send the error token to the
client to allow completion of the GSS security exchange.
o If the key exchange fails due to a GSSAPI error returned from the
client's call to GSS_Init_sec_context, and an "error token" is
also returned, then the client SHOULD send the error token to the
server to allow completion of the GSS security exchange.
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As noted in Section 9, it may be desirable under site security
policy to obscure information about the precise nature of the error;
thus, it is RECOMMENDED that implementations provide a method to
suppress these messages as a matter of policy.
This is implemented with the following messages. The hash algorithm
for computing the exchange hash is defined by the method name, and
is called HASH. The group used for Diffie-Hellman key exchange and
the underlying GSSAPI mechanism are also defined by the method name.
After the client's first call to GSS_Init_sec_context, it sends the
following:
byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context)
mpint e
Upon receiving the SSH_MSG_KEXGSS_INIT message, the server MAY send
the following message, prior to any other messages, to inform the
client of its host key.
byte SSH_MSG_KEXGSS_HOSTKEY
string server public host key and certificates (K_S)
Since this key exchange method does not require the host key to be
used for any encryption operations, this message is OPTIONAL. If
the "null" host key algorithm described in Section 5 is used, this
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 single string, in the format specified by the public key type
in use (see [SSH-TRANSPORT], section 4.6).
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
reply to the client:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context)
If the client receives this message after a call to
GSS_Init_sec_context has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
Each time the client receives the message described above, it makes
another call to GSS_Init_sec_context. It then sends the following:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Init_sec_context)
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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 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
messages.
If the server's final call to GSS_Accept_sec_context (resulting in a
major_status code of GSS_S_COMPLETE) returns a non-zero-length token
to be sent to the client, it sends the following:
byte SSH_MSG_KEXGSS_COMPLETE
mpint f
string per_msg_token (MIC of H)
boolean TRUE
string output_token (from GSS_Accept_sec_context)
If the client receives this message after a call to
GSS_Init_sec_context has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
If the server's final call to GSS_Accept_sec_context (resulting in a
major_status code of GSS_S_COMPLETE) returns a zero-length token or
no token at all, it sends the following:
byte SSH_MSG_KEXGSS_COMPLETE
mpint f
string per_msg_token (MIC of H)
boolean FALSE
If the client receives this message when no call to
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
MUST fail.
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
an output token (called an "error token"), then the following SHOULD
be sent to convey the error information to the peer:
byte SSH_MSG_KEXGSS_CONTINUE
string error_token
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
allow clients to record and/or display the error information before
processing the error token. This is important because a client
processing an error token will likely disconnect without reading any
further messages.
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In the event of a GSSAPI error on the server, the server MAY send
the following message before terminating the connection:
byte SSH_MSG_KEXGSS_ERROR
uint32 major_status
uint32 minor_status
string message
string language tag
The message text MUST be encoded in the UTF-8 encoding described in
[UTF8]. Language tags are those described in [LANGTAG]. Note that
the message text may contain multiple lines separated by carriage
return-line feed (CRLF) sequences. Application developers should
take this into account when displaying these messages.
The hash H is computed as the HASH hash of the concatenation of the
following:
string V_C, the client's version string (CR and NL excluded)
string V_S, the server's version string (CR and NL excluded)
string I_C, the payload of the client's SSH_MSG_KEXINIT
string I_S, the payload of the server's SSH_MSG_KEXINIT
string K_S, the host key
mpint e, exchange value sent by the client
mpint f, exchange value sent by the server
mpint K, the shared secret
This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept
secret. 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
place of K_S when computing the exchange hash.
The GSS_GetMIC call MUST be applied over H, not the original data.
2.2 Group Exchange
This section describes a modification to the generic GSSAPI
authenticated Diffie-Hellman key exchange to allow the negotiation
of the group to be used, using a method based on that described in
[GROUP-EXCHANGE].
The server keeps a list of safe primes and corresponding generators
that it can select from. These are chosen as described in section 5
of [GROUP-EXCHANGE]. The client requests a modulus from the server,
indicating the minimum, maximum, and preferred sizes; the server
responds with a suitable modulus and generator. The exchange then
proceeds as described in Section 2.1 above.
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This description uses the following symbols, in addition to those
defined above:
o n is the size of the modulus p in bits that the client would like
to receive from the server
o min and max are the minimal and maximal sizes of p in bits that
are acceptable to the client
1. C sends "min || n || max" to S, indicating the minimal
acceptable group size, the preferred size of the group, and the
maximal group size in bits the client will accept.
2. S finds a group that best matches the client's request, and
sends "p || g" to C.
3. The exchange proceeds as described in Section 2.1 above,
beginning with step 1, except that the exchange hash is computed
as described below.
Servers and clients SHOULD support gorups with a modulus length of k
bits, where 1024 <= k <= 8192. The recommended values for min and
max are 1024 and 8192, respectively.
This is implemented using the following messages, in addition to
those described above:
First, the client sends:
byte SSH_MSG_KEXGSS_GROUPREQ
uint32 min, minimal size in bits of an acceptable group
uint32 n, preferred size in bits of the group the server should send
uint32 max, maximal size in bits of an acceptable group
The server responds with:
byte SSH_MSG_KEXGSS_GROUP
mpint p, safe prime
mpint g, generator for subgroup in GF(p)
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This is followed by the message exchange described above in Section
2.1, except that the exchange hash H is computed as the HASH hash of
the concatenation of the following:
string V_C, the client's version string (CR and NL excluded)
string V_S, the server's version string (CR and NL excluded)
string I_C, the payload of the client's SSH_MSG_KEXINIT
string I_S, the payload of the server's SSH_MSG_KEXINIT
string K_S, the host key
uint32 min, minimal size in bits of an acceptable group
uint32 n, preferred size in bits of the group the server should send
uint32 max, maximal size in bits of an acceptable group
mpint p, safe prime
mpint g, generator for subgroup in GF(p)
mpint e, exchange value sent by the client
mpint f, exchange value sent by the server
mpint K, the shared secret
2.3 gss-group1-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman
key exchange as described in Section 2.1 with SHA-1 as HASH, and the
group defined in section 6.1 of [SSH-TRANSPORT]. The method name
for 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 encoding [ASN1] of the underlying GSSAPI mechanism's
OID. Base64 encoding is described in section 6.8 of [MIME].
Each and every such key exchange method is implicitly registered by
this specification. The IESG is considered to be the owner of all
such key exchange methods; this does NOT imply that the IESG is
considered to be the owner of the underlying GSSAPI mechanism.
2.4 gss-gex-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman
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-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. Base64 encoding is described in section 6.8 of [MIME].
Each and every such key exchange method is implicitly registered by
this specification. The IESG is considered to be the owner of all
such key exchange methods; this does NOT imply that the IESG is
considered to be the owner of the underlying GSSAPI mechanism.
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2.5 Other GSSAPI key exchange methods
Key exchange method names starting with "gss-" are reserved for key
exchange methods which conform to this document; in particular, for
those methods which use the GSSAPI authenticated Diffie-Hellman key
exchange algorithm described in Section 2.1, including any future
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 similar manner to that used in Section 2.3.
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3. GSSAPI User Authentication
This section describes a general-purpose user authentication method
based on [GSSAPI]. It is intended to be run over the SSH user
authentication protocol [SSH-USERAUTH].
The authentication method name for this protocol is
"gssapi-with-mic".
3.1 GSSAPI Authentication Overview
GSSAPI authentication must maintain a context. Authentication
begins when the client sends a SSH_MSG_USERAUTH_REQUEST, which
specifies the mechanism OIDs the client supports.
If the server supports any of the requested mechanism OIDs, the
server sends a SSH_MSG_USERAUTH_GSSAPI_RESPONSE message containing
the mechanism OID.
After the client receives SSH_MSG_USERAUTH_GSSAPI_RESPONSE, the
client and server exchange SSH_MSG_USERAUTH_GSSAPI_TOKEN packets
until the authentication mechanism either succeeds or fails.
If at any time during the exchange, the client sends a new
SSH_MSG_USERAUTH_REQUEST packet, the GSSAPI context is completely
discarded and destroyed, and any further GSSAPI authentication MUST
restart from the beginning.
3.2 Initiating GSSAPI authentication
The GSSAPI authentication method is initiated when the client sends
a SSH_MSG_USERAUTH_REQUEST:
byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding)
string service name (in US-ASCII)
string "gssapi-with-mic" (US-ASCII method name)
uint32 n, the number of mechanism OIDs client supports
string[n] mechanism OIDs
Mechanism OIDs are encoded according to the ASN.1 distinguished
encoding rules (DER), as described in [ASN1] and in section 3.1 of
[GSSAPI]. The mechanism OIDs MUST be listed in order of preference,
and the server must choose the first mechanism OID on the list that
it supports.
The client SHOULD send GSSAPI mechanism OID's only for mechanisms
which are of the same priority, compared to non-GSSAPI
authentication methods. Otherwise, authentication methods may be
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executed out of order. Thus, the client could first send a
SSH_MSG_USERAUTH_REQUEST for one GSSAPI mechanism, then try public
key authentication, and then try another GSSAPI mechanism.
If the server does not support any of the specified OIDs, the server
MUST fail the request by sending a SSH_MSG_USERAUTH_FAILURE packet.
The user name may be an empty string if it can be deduced from the
results of the GSSAPI authentication. If the user name is not
empty, and the requested user does not exist, the server MAY
disconnect, or MAY send a bogus list of acceptable authentications
but never accept any. This makes it possible for the server to
avoid disclosing information about which accounts exist. In any
case, if the user does not exist, the authentication request MUST
NOT be accepted.
The client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the
new one.
3.3 Initial server response
The server responds to the SSH_MSG_USERAUTH_REQUEST with either a
SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported, or
with SSH_MSG_USERAUTH_GSSAPI_RESPONSE as follows:
byte SSH_MSG_USERAUTH_GSSAPI_RESPONSE
string selected mechanism OID
The mechanism OID must be one of the OIDs sent by the client in the
SSH_MSG_USERAUTH_REQUEST packet.
3.4 GSSAPI session
Once the mechanism OID has been selected, the client will then
initiate an exchange of one or more pairs of
SSH_MSG_USERAUTH_GSSAPI_TOKEN packets. These packets contain the
tokens produced from the 'GSS_Init_sec_context()' and
'GSS_Accept_sec_context()' calls. The actual number of packets
exchanged is determined by the underlying GSSAPI mechanism.
byte SSH_MSG_USERAUTH_GSSAPI_TOKEN
string data returned from either GSS_Init_sec_context()
or GSS_Accept_sec_context()
If an error occurs during this exchange on server side, the server
can terminate the method by sending a SSH_MSG_USERAUTH_FAILURE
packet. If an error occurs on client side, the client can terminate
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the method by sending a new SSH_MSG_USERAUTH_REQUEST packet.
When calling GSS_Init_sec_context(), the client MUST set the the
integ_req_flag to "true" to request that per-message integrity
protection be supported for this context. In addition, the
deleg_req_flag MAY be set to "true" to request access delegation, if
requested by the user.
Since the user authentication process by its nature authenticates
only the client, the setting of the mutual_req_flag is not needed
for this process. This flag SHOULD be set to "false".
Since the user authentication process will involve the exchange of
only a single token once the context has been established, it is not
necessary that the context support detection of replayed or
out-of-sequence tokens. Thus, the setting of the
replay_det_req_flag and sequence_req_flag are not needed for this
process. These flags SHOULD be set to "false".
Additional SSH_MSG_USERAUTH_GSSAPI_TOKEN messages are sent if and
only if the calls to the GSSAPI routines produce send tokens of
non-zero length.
Any major status code other than GSS_S_COMPLETE or
GSS_S_CONTINUE_NEEDED SHOULD be a failure.
3.5 Binding Encryption Keys
In some cases, it is possible to obtain improved security by
allowing access only if the client sends a valid message integrity
code (MIC) binding the GSSAPI context to the keys used for
encryption and integrity protection of the SSH session. With this
extra level of protection, a "man-in-the-middle" attacker who has
convinced a client of his authenticity cannot then relay user
authentication messages between the real client and server, thus
gaining access to the real server. This additional protection is
available when the negotiated GSSAPI context supports per-message
integrity protection, as indicated by the setting of the integ_avail
flag on successful return from GSS_Init_sec_context() or
GSS_Accept_sec_context().
When the client's call to GSS_Init_sec_context() returns
GSS_S_COMPLETE with the integ_avail flag set, the client MUST
conclude the user authentication exchange by sending the following
message:
byte SSH_MSG_USERAUTH_GSSAPI_MIC
string MIC
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This message MUST be sent only if GSS_Init_sec_context() returned
GSS_S_COMPLETE. If a token is also returned then the
SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one.
The contents of the MIC field are obtained by calling GSS_GetMIC
over the following, using the GSSAPI context which was just
established:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "gssapi-with-mic"
If this message is received by the server before the GSSAPI context
is fully established, the server MUST fail the authentication.
If this message is received by the server when the negotiated GSSAPI
context does not support per-message integrity protection, the
server MUST fail the authentication.
3.6 Client acknowledgement
Some servers may wish to permit user authentication to proceed even
when the negotitated GSSAPI context does not support per-message
integrity protection. In such cases, it is possible for the server
to successfully complete the GSSAPI method, while the client's last
call to GSS_Init_sec_context fails. If the server simply assumed
success on the part of the client and completed the authentication
service, it is possible that the client would fail to complete the
authentication method, but not be able to retry other methods
because the server had already moved on. To protect against this, a
final message is sent by the client to indicate it has completed
authentication.
When the client's call to GSS_Init_sec_context() returns
GSS_S_COMPLETE with the integ_avail flag not set, the client MUST
conclude the user authentication exchange by sending the following
message:
byte SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE
This message MUST be sent only if GSS_Init_sec_context() returned
GSS_S_COMPLETE. If a token is also returned then the
SSH_MSG_USERAUTH_GSSAPI_TOKEN message MUST be sent before this one.
If this message is received by the server before the GSSAPI context
is fully established, the server MUST fail the authentication.
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If this message is received by the server when the negotiated GSSAPI
context supports per-message integrity protection, the server MUST
fail the authentication.
It is a site policy descision for the server whether or not to
permit authentication using GSSAPI mechanisms and/or contexts which
do not support per-message integrity protection. The server MAY
fail the otherwise valid gssapi-with-mic authentication if
per-message integrity protection is not supported.
3.7 Completion
As with all SSH authentication methods, successful completion is
indicated by a SSH_MSG_USERAUTH_SUCCESS if no other authentication
is required, or a SSH_MSG_USERAUTH_FAILURE with the partial success
flag set if the server requires further authentication. This packet
should be sent immediately following receipt of the the
SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE packet.
3.8 Error Status
In the event a GSSAPI error occurs on the server during context
establishment, the server MAY send the following message to inform
the client of the details of the error before sending a
SSH_MSG_USERAUTH_FAILURE message:
byte SSH_MSG_USERAUTH_GSSAPI_ERROR
uint32 major_status
uint32 minor_status
string message
string language tag
The message text MUST be encoded in the UTF-8 encoding described in
[UTF8]. Language tags are those described in [LANGTAG]. Note that
the message text may contain multiple lines separated by carriage
return-line feed (CRLF) sequences. Application developers should
take this into account when displaying these messages.
Clients receiving this message MAY log the error details and/or
report them to the user. Any server sending this message MUST
ignore any SSH_MSG_UNIMPLEMENTED sent by the client in response.
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3.9 Error Token
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
returns a token along with an error status, the resulting "error
token" SHOULD be sent to the peer using the following message:
byte SSH_MSG_USERAUTH_GSSAPI_ERRTOK
string error token
This message implies that the authentication is about to fail, and
is defined to allow the error token to be communicated without
losing synchronization.
When a server sends this message, it MUST be followed by a
SSH_MSG_USERAUTH_FAILURE message, which is to be interpreted as
applying to the same authentication request. A client receiving
this message SHOULD wait for the following SSH_MSG_USERAUTH_FAILURE
message before beginning another authentication attempt.
When a client sends this message, it MUST be followed by a new
authentication request or by terminating the connection. A server
receiving this message MUST NOT send a SSH_MSG_USERAUTH_FAILURE in
reply, since such a message might otherwise be interpreted by a
client as a response to the following authentication sequence.
Any server sending this message MUST ignore any
SSH_MSG_UNIMPLEMENTED sent by the client in response. If a server
sends both this message and an SSH_MSG_USERAUTH_GSSAPI_ERROR
message, the 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 error token.
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4. Authentication using GSSAPI Key Exchange
This section describes a user authentication method building on the
framework described in [SSH-USERAUTH]. This method performs user
authentication by making use of an existing GSSAPI context
established during key exchange.
The authentication method name for this protocol is "gssapi-keyex".
This method may be used only if the initial key exchange was
performed using a GSSAPI-based key exchange method defined in
accordance with Section 2. The GSSAPI context used with this method
is always that established during an initial GSSAPI-based key
exchange. Any context established during key exchange for the
purpose of rekeying MUST NOT be used with this method.
The server SHOULD include this user authentication method in the
list of methods that can continue (in a SSH_MSG_USERAUTH_FAILURE) if
the initial key exchange was performed using a GSSAPI-based key
exchange method and provides information about the user's identity
which is useful to the server. It MUST NOT include this method if
the initial key exchange was not performed using a GSSAPI-based key
exchange method defined in accordance with Section 2.
The client SHOULD attempt to use this method if it is advertised by
the server, initial key exchange was performed using a GSSAPI-based
key exchange method, and this method has already been tried. The
client SHOULD NOT try this method more than once per session. It
MUST NOT try this method if initial key exchange was not performed
using a GSSAPI-based key exchange method defined in accordance with
Section 2.
If a server receives a request for this method when initial key
exchange was not performed using a GSSAPI-based key exchange method
defined in accordance with Section 2, it MUST return
SSH_MSG_USERAUTH_FAILURE.
This method is defined as a single message:
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "gssapi-keyex"
string MIC
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The contents of the MIC field are obtained by calling GSS_GetMIC
over the following, using the GSSAPI context which was established
during initial key exchange:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "gssapi-keyex"
Upon receiving this message when initial key exchange was performed
using a GSSAPI-based key exchange method, the server uses
GSS_VerifyMIC() to verify that the MIC received is valid. If the
MIC is not valid, the user authentication fails, and the server MUST
return SSH_MSG_USERAUTH_FAILURE.
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
SSH_MSG_USERAUTH_FAILURE with the partial success flag set,
depending on whether additional authentications are needed.
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5. Null Host Key Algorithm
The "null" host key algorithm has no associated host key material,
and provides neither signature nor encryption algorithms. Thus, it
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
material. The key exchange methods described in section 1 of this
document are examples of such methods.
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
can be used when the administrator has decided as a matter of policy
to require that all key exchanges be authenticated using Kerberos
[KRB5], and thus the only permitted key exchange method is the
GSSAPI-authenticated Diffie-Hellman exchange described above, with
Kerberos V5 as the underlying GSSAPI mechanism. In such a
configuration, the server implementation supports the "ssh-dss" key
algorithm (as required by [SSH-TRANSPORT]), but could be prohibited
by configuration from using it. In this situation, the server needs
some key exchange algorithm to advertise; the "null" algorithm fills
this purpose.
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
support this algorithm. This is not a significant problem, since in
the configuration described, it will also be unable to interoperate
with implementations that do not support the GSSAPI-authenticated
key exchange and Kerberos.
Any implementation supporting at least one key exchange method which
conforms to section 1 of this document MUST also support the "null"
host key algorithm. Servers MUST NOT advertise the "null" host key
algorithm unless it is the only algorithm advertised.
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6. Summary of Message Numbers
The following message numbers have been defined for use with
GSSAPI-based key exchange methods:
#define SSH_MSG_KEXGSS_INIT 30
#define SSH_MSG_KEXGSS_CONTINUE 31
#define SSH_MSG_KEXGSS_COMPLETE 32
#define SSH_MSG_KEXGSS_HOSTKEY 33
#define SSH_MSG_KEXGSS_ERROR 34
#define SSH_MSG_KEXGSS_GROUPREQ 40
#define SSH_MSG_KEXGSS_GROUP 41
The numbers 30-49 are specific to key exchange and may be redefined
by other kex methods.
The following message numbers have been defined for use with the
'gssapi' user authentication method:
#define SSH_MSG_USERAUTH_GSSAPI_RESPONSE 60
#define SSH_MSG_USERAUTH_GSSAPI_TOKEN 61
#define SSH_MSG_USERAUTH_GSSAPI_EXCHANGE_COMPLETE 63
#define SSH_MSG_USERAUTH_GSSAPI_ERROR 64
#define SSH_MSG_USERAUTH_GSSAPI_ERRTOK 65
#define SSH_MSG_USERAUTH_GSSAPI_MIC 66
The numbers 60-79 are specific to user authentication and may be
redefined by other user auth methods. Note that in the method
described in this document, message number 62 is unused.
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7. GSSAPI Considerations
7.1 Naming Conventions
In order to establish a GSSAPI security context, the SSH client
needs to determine the appropriate targ_name to use in identifying
the server when calling GSS_Init_sec_context. For this purpose, the
GSSAPI mechanism-independent name form for host-based services is
used, as described in section 4.1 of [GSSAPI].
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
GSS_C_NT_HOSTBASED_SERVICE, and an input_name_string consisting of
the string "host@" concatenated with the hostname of the SSH server.
7.2 Channel Bindings
This document recommends that channel bindings SHOULD NOT be
specified in the calls during context establishment. This document
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 environments where it is most useful.
7.3 SPNEGO
The use of the Simple and Protected GSS-API Negotiation Mechanism
[SPNEGO] in conjunction with the authentication and key exchange
methods described in this document is both unnecessary and
undesirable. As a result, mechanisms conforming to this document
MUST NOT use SPNEGO as the underlying GSSAPI mechanism.
Since SSH performs its own negotiation of authentication and key
exchange methods, the negotiation capability of SPNEGO alone does
not provide any added benefit. In fact, as described below, it has
the potential to result in the use of a weaker method than desired.
Normally, SPNEGO provides the added benefit of protecting the GSSAPI
mechanism negotiation. It does this by having the server compute a
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 protection is not needed because the key exchange methods
described here already perform an equivalent operation; namely, they
generate a MIC of the SSH exchange hash, which is a hash of several
items including the lists of key exchange mechanisms supported by
both sides. In the case of user authentication, the protection is
not needed because the negotiation occurs over a secure channel, and
the host's identity has already been proved to the user.
The use of SPNEGO combined with GSSAPI mechanisms used without
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SPNEGO can lead to interoperability problems. For example, a client
which supports key exchange using the Kerberos V5 GSSAPI mechanism
[KRB5-GSS] only underneath SPNEGO will not interoperate with a
server which supports key exchange only using the Kerberos V5 GSSAPI
mechanism directly. As a result, allowing GSSAPI mechanisms to be
used both with and without SPNEGO is undesirable.
If a client's policy is to first prefer GSSAPI-based key exchange
method X, then non-GSSAPI method Y, then GSSAPI-based method Z, and
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 method Z when method Y would have been preferable. As a
result, the use of SPNEGO could result in the subversion of the
negotiation algorithm for key exchange methods as described in
section 5.1 of [SSH-TRANSPORT] and/or the negotiation algorithm for
user authentication methods as described in [SSH-USERAUTH].
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8. IANA Considerations
Consistent with section 7 of [SSH-ARCH], this document makes the
following registrations:
The family of SSH key exchange method names beginning with
"gss-group1-sha1-" and not containing the at-sign ('@'), to name
the key exchange methods defined in Section 2.3.
The family of SSH key exchange method names beginning with
"gss-gex-sha1-" and not containing the at-sign ('@'), to name the
key exchange methods defined in Section 2.4.
All other SSH key exchange method names beginning with "gss-" and
not containing the at-sign ('@'), to be reserved for future key
exchange methods defined in conformance with this document, as
noted in Section 2.5.
The SSH host public key algorithm name "null", to name the NULL
host key algorithm defined in Section 5.
The SSH user authentication method name "gssapi-with-mic", to
name the GSSAPI user authentication method defined in Section 3.
The SSH user authentication method name "gssapi-keyex", to name
the GSSAPI user authentication method defined in Section 4.
The SSH user authentication method name "gssapi" is to be
reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification.
The SSH user authentication method name "external-keyx" is to be
reserved, in order to avoid conflicts with implementations
supporting an earlier version of this specification.
This document creates no new registries.
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9. Security Considerations
This document describes authentication and key-exchange protocols.
As such, security considerations are discussed throughout.
This protocol depends on the SSH protocol itself, the GSSAPI, any
underlying GSSAPI mechanisms which are used, and any protocols on
which such mechanisms might depend. Each of these components plays
a part in the security of the resulting connection, and each will
have its own security considerations.
The key exchange method described in section 1 of this document
depends on the underlying GSSAPI mechanism to provide both mutual
authentication and per-message integrity services. If either of
these features is not supported by a particular GSSAPI mechanism, or
by a particular implementation of a GSSAPI mechanism, then the key
exchange is not secure and MUST fail.
In order for the "external-keyx" user authentication method to be
used, it MUST have access to user authentication information
obtained as a side-effect of the key exchange. If this information
is unavailable, the authentication MUST fail.
Revealing information about the reason for an authentication failure
may be considered by some sites to be an unacceptable security risk
for a production environment. However, having that information
available can be invaluable for debugging purposes. Thus, it is
RECOMMENDED that implementations provide a means for controlling, as
a matter of policy, whether to send SSH_MSG_USERAUTH_GSSAPI_ERROR,
SSH_MSG_USERAUTH_GSSAPI_ERRTOK, and SSH_MSG_KEXGSS_ERROR messages,
and SSH_MSG_KEXGEE_CONTINUE messages containing a GSSAPI error token.
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10. Acknowledgements
The authors would like to thank the following individuals for their
invaluable assistance and contributions to this document:
o Sam Hartman
o Love Hornquist-Astrand
o Joel N. Weber II
o Simon Wilkinson
o Nicolas Williams
Much of the text describing DH group exchnage was borrowed from
[GROUP-EXCHANGE], by Markus Friedl, Niels Provos, and William A.
Simpson.
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11. Changes the last version
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 Added the description of the generic GSSAPI key exchange with
group exchange, and the definition of the gss-gex-sha1-* methods.
o Updated references to the ssh core documents.
o Converted from numbered to symbolic references.
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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.
[GROUP-EXCHANGE] Friedl, M., Provos, N. and W. A. Simpson,
"Diffie-Hellman Group Exchange for the SSH
Transport Layer Protocol",
draft-ietf-secsh-dh-group-exchange-04.txt (work in
progress), July 2003.
[GSSAPI] Linn, J., "Generic Security Service Application
Program Interface Version 2, Update 1", RFC 2743,
January 2000.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", RFC 2119, BCP 14,
March 1997.
[LANGTAG] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[MIME] Freed, N. and N. Borenstein, "Multipurpose
Internet Mail Extensions (MIME) Part One: Format
of Internet Message Bodies", RFC 2045, November
1996.
[SSH-ARCH] Ylonen, T. and D. Moffat, "SSH Protocol
Architecture",
draft-ietf-secsh-architecture-15.txt (work in
progress), October 2003.
[SSH-CONNECT] Ylonen, T. and D. Moffat, "SSH Connection
Protocol", draft-ietf-secsh-connect-18.txt (work
in progress), October 2003.
[SSH-TRANSPORT] Ylonen, T. and D. Moffat, "SSH Transport Layer
Protocol", draft-ietf-secsh-transport-17.txt (work
in progress), October 2003.
[SSH-USERAUTH] Ylonen, T. and D. Moffat, "SSH Authentication
Protocol", draft-ietf-secsh-userauth-18.txt (work
in progress), October 2003.
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[UTF8] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", RFC 2279, January 1998.
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Non-Normative References
[KRB5] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[KRB5-GSS] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[SPNEGO] Baize, E. and D. Pinkas, "The Simple and Protected
GSS-API Negotiation Mechanism", RFC 2478, December 1998.
Authors' Addresses
Jeffrey Hutzelman
Carnegie Mellon University
5000 Forbes Ave
Pittsburgh, PA 15213
US
Phone: +1 412 268 7225
EMail: jhutz+@cmu.edu
URI: http://www.cs.cmu.edu/~jhutz/
Joseph Salowey
Cisco Systems
2901 Third Avenue
Seattle, WA 98121
US
Phone: +1 206 256 3380
EMail: jsalowey@cisco.com
Joseph Galbraith
Van Dyke Technologies, Inc.
4848 Tramway Ridge Dr. NE
Suite 101
Albuquerque, NM 87111
US
EMail: galb@vandyke.com
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Von Welch
University of Chicago & Argonne National Laboratory
Distributed Systems Laboratory
701 E. Washington
Urbana, IL 61801
US
EMail: welch@mcs.anl.gov
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