Network Working Group J. Hutzelman
Internet-Draft CMU
Expires: August 31, 2001 J. Salowey
Cisco Systems
March 2, 2001
Using GSSAPI authentication for key exchange in Secure Shell
draft-ietf-secsh-gsskeyex-01
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This memo describes a method for using the Generic Security Service
Application Program Interface [2] for key exchange in the Secure
Shell protocol, by defining a class of SSH key exchange methods
which use GSSAPI to authenticate the Diffie-Hellman exchange
described in [10].
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 key exchange.
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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 [7].
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1. 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 [10] 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 2 of this document.
1.1 Generic method description
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 "external-keyx" user authentication method
described in section 3 of this document, or does not intend to
use that method, 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.
* 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
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protection is not available, and the key exchange MUST fail.
* If the resulting major_status code is GSS_S_COMPLETE and the
mutual_state flag is 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
mutual_state flag is 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
mod p. It computes K = e ^ y mod p, and H = hash(V_C || V_S ||
I_C || I_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
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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
|| e || f || K). It then calls GSS_VerifyMIC to verify that the
MIC sent by S matches H.
Either side MUST NOT send or accept e or f values that are not in
the range [1, p-1]. If this condition is violated, the key exchange
fails.
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.
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_GSSAPI_INIT
boolean TRUE
string output_token (from GSS_Init_sec_context)
mpint e
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_GSSAPI_CONTINUE
string output_token (from GSS_Accept_sec_context)
If the client receives this message appears 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_GSSAPI_INIT
boolean FALSE
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_contents (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_GSSAPI_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 appears 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_contents (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_GSSAPI_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.
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
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.
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The GSS_GetMIC call MUST be applied over H, not the original data.
1.2 gss-group1-sha1-*
Each of these methods specifies GSSAPI authenticated Diffie-Hellman
key exchange as described in section 1.1 of this document, with
SHA-1 as HASH, and the group defined in section 6.1 of [10]. The
method name for each method is the concatenation of the string
"gss-group1-sha1-" with the Base64 encoding of the MD5 hash [5] of
the ASN.1 DER encoding [1] of the underlying GSSAPI mechanism's OID.
Base64 encoding is described in section 6.8 of [6].
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.
1.3 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 1.1 of this document,
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 1.2
of this document.
1.4 SPNEGO
The use of the Simple and Protected GSS-API Negotiation Mechanism
[8] in conjunction with the key exchange methods described in this
document is both unnecessary and undesirable. As a result, key
exchange mechanisms conforming to this document MUST NOT use SPNEGO
as the underlying GSSAPI mechanism.
Since SSH performs its own negotiation of key exchange methods, and
there exists a separate method name corresponding to every possible
underlying GSSAPI mechanism, 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. The key exchange methods
described in this document already perform an equivalent operation;
namely, they generate a MIC of the SSH exchange hash, which is a
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hash of several items including the lists of key exchange mechanisms
supported by both sides. Thus, the extra level of protection
offered by SPNEGO is unnecessary in this case.
The use of SPNEGO combined with GSSAPI mechanisms used without
SPNEGO can lead to interoperability problems. For example, a client
which supports key exchange using the Kerberos V5 GSSAPI mechanism
[4] 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 [10].
1.5 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 [2].
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.
1.6 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.
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2. 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
[3], 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 [10], 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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3. External Key Exchange user authentication
This section describes a user authentication method building on the
framework described in [11]. This method relies upon the key
exchange to authenticate both the client and the server. If the key
exchange did not successfully perform these functions then the
server MUST always respond to this request with
SSH_MSG_USERAUTH_FAILURE with partial success set to false.
The new mechanism is defined as follows:
byte SSH_MSG_USERAUTH_REQUEST
string authorization-ID
string service
string "external-keyx"
If the user authenticated in the key-exchange is allowed to assume
the authorization identity, this method is successful, and the
server responds with SSH_MSG_USERAUTH_SUCCESS if no more
authentications are needed, or with SSH_MSG_USERAUTH_FAILURE with
partial success set to true if more authentications are needed.
If the user authenticated in the key-exchange is not allowed to
assume the authorization identity, then SSH_MSG_USERAUTH_FAILURE is
returned with partial success set to false.
Any implementation supporting at least one key exchange method which
conforms to section 1 of this document SHOULD also support the
"external-keyx" user authentication method, in order to allow user
authentication to be performed at the same time as key exchange,
thereby reducing the number of round trips needed for connection
setup.
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4. Summary of Message Numbers
The following message numbers have been defined in this document:
#define SSH_MSG_GSSAPI_INIT 30
#define SSH_MSG_GSSAPI_CONTINUE 31
#define SSH_MSG_GSSAPI_COMPLETE 32
The numbers 30-49 are key exchange specific and may be redefined by
other kex methods.
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5. Security Considerations
This document describes an authentication and key-exchange protocol.
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.
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6. Acknowledgements
The authors would like to thank Sam Hartman and Simon Wilkinson for
their invaluable assistance with this document.
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References
[1] ISO/IEC, "Specification of Abstract Syntax Notation One
(ASN.1)", ISO/IEC 8824, November 1998.
[2] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[3] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[4] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
1964, June 1996.
[5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[7] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[8] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
[9] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Protocol Architecture", November 2000.
[10] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Transport Layer Protocol", November 2000.
[11] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Authentication Protocol", November 2000.
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/
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Joseph Salowey
Cisco Systems
Bldg 20
725 Alder Drive
Milpitas, CA 95035
US
Phone: +1 408 525 6381
EMail: jsalowey@cisco.com
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