Re: [TLS] Review of draft-santesson-tls-gssapi-03

Larry Zhu wrote:
> 
> > 5) Does the wire protocol differentiate between GSS_S_CONTINUE_NEEDED
> > and GSS_S_COMPLETE?  I'm thinking if the initial GSS_Init_sec_context
> > call returns GSS_S_COMPLETE.  The server will call
> > GSS_Accept_sec_context which shouldn't return any data.  The server
> > sends back a 0b string.  How can the client differentiate this from when
> > GSS_Accept_sec_context returned GSS_S_CONTINUE_NEEDED?  Perhaps the
> > answer is that the client simply disconnect in this situation, but maybe
> > that has to be noted.
> 
> A note is added as follows:
> 
>    As implied by GSS-API, a GSS-API token SHOULD NOT be received after
>    the context is established (GSS_S_COMPLETE is returned), the receiver
>    MUST tear down the connection in this case.

Only nitpicking: In the GSS-API original design, it is possible to
receive a context-level token after having seen GSS_S_COMPLETE from
the local context establishment function.

The obvious and non-controversial case has been deprecated
(the context deletion token).  The specified behaviour was too
pass this token to gss_process_context_token().

Whether or not an peer is allowed to create an context-level error
token (not a delete token) that the peer does not expect is fairly
unclear, however.  Implementations of the Kerberos 5 gssapi
mechanisms send such error tokens (containing a KRB_ERROR message)
in some situations.  I don't know whether these implementations refrain
from sending the error token if the peer does not expect it
(e.g. when the client does not request mutual authentication
 and uses an incorrect target name on a Microsoft Windows platform).

I don't know what other GSS-API mechanisms (e.g. based on SPKM)
have been doing in this area.

The one thing that is clear from the GSS-API spec is that an application
caller MUST NOT call a context establishment call (init_sec_context,
accept_sec_context) on a context for which a previous call to
the context establishment functions returned GSS_S_COMPLETE.
If at all, such a token must be consumed through gss_process_context_token().
and in the two scenarios described in the GSS-API standard, the
context deletion token and the context error token, this will result
in the security context to become invalid/unusable...

-Martin

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