Suppressing conf/integ flags in krb5 GSS tokens

[Greg Hudson]

MIT krb5 needs a way to suppress the confidentiality and integrity flags
in krb5 GSS context initiator tokens in order to interoperate with some
Microsoft servers.  The background is here:
http://krbdev.mit.edu/rt/Ticket/Display.html?id=6938

My plan had been to implement Heimdal's GSS_KRB5_CRED_NO_CI_FLAGS_X cred
option, with the same constant name and OID.  This option removes both
the conf and integ flags from the initiator token (and from ret_flags on
the initiator and acceptor), but the caller can still request them.

Unfortunately, those semantics don't play well with our SPNEGO mech,
which requests integrity from the underlying mechanism.  There would be
no way to actually suppress the integrity flag when going through
SPNEGO.  I don't think we want to change that about our SPNEGO mech; the
current behavior seems to match what RFC 4178 describes.

Taking that into account, I'm thinking we need to implement two cred
options to separately suppress the confidentiality and integrity flags,
even if the caller asks for them.  When used through SPNEGO, suppressing
the integrity flag would have the effect of disabling the SPNEGO MIC.
I'm a little concerned about that interaction, because Heimdal doesn't
seem to check whether the negotiated mech when deciding whether to
require a MIC.  But in general the other party will be a Microsoft
server (or maybe Samba) when these flags are used.

Comments?