I have been selected as the General Area Review Team (Gen-ART)
reviewer for this draft (for background on Gen-ART, please see
Please resolve these comments along with any other Last Call comments
you may receive.
Reviewer: Spencer Dawkins
Review Date: 2009-11-30
IETF LC End Date: 2009-11-18 (oops!)
IESG Telechat date: 2009-12-03
Summary: This document is almost ready for publication as a Proposed
Standard. I did have one minor question about 13.3 (in my LATE review), but
it should not be difficult to resolve, if an AD agrees with my question.
I did tag a fair number of nits, but these aren't part of the Gen-ART
review, and are simply included as a convenience for anyone else who edits
The GS1 bridge failed to gain wide deployment for any GSS-API
mechanism other than The "Kerberos V5 GSS-API mechanism" [RFC1964]
Spencer (nit): s/The "Kerberos/"The Kerberos/
[RFC4121], and has a number of problems that lead us to desire a new
Spencer (nit): s/lead/led/
bridge. Specifically: a) GS1 was not round-trip optimized, b) GS1
did not support channel binding [RFC5056]. These problems and the
opportunity to create the next SASL password-based mechanism, SCRAM
Spencer (nit): please expand SCRAM on first use.
[I-D.ietf-sasl-scram], as a GSS-API mechanism used by SASL
applications via GS2, provide the motivation for GS2.
In particular, the current consensus of the SASL community appears to
be that SASL "security layers" (i.e., confidentiality and integrity
protection of application data after authentication) are too complex
and, since SASL applications tend to have an option to run over a
Transport Layer Security (TLS) [RFC5246] channel, redundant and best
replaced with channel binding.
Spencer (nit): it's a LONG way from "too complex" to "redundant" in this
sentence ;-) suggest moving "redundant" before the subclause, just for
The last step translate each decimal value using table 3 in Base32
Spencer (nit): s/translate/translates/?
[RFC4648]. Thus the SASL mechanism name for the SPKM-1 GSSAPI
mechanism is "GS2-DT4PIK22T6A".
8. GSS-API Parameters
The mutual_req_flag MUST be set. If channel binding is used then the
client MUST check that the corresponding ret_flag is set when the
context is fully establish, else authentication MUST fail.
Spencer (nit): s/establish/established/
Use or non-use of deleg_req_flag and anon_req_flag is an
implementation-specific detail. SASL and GS2 implementors are
encouraged to provide programming interfaces by which clients may
choose to delegate credentials and by which servers may receive them.
SASL and GS2 implementors are encouraged to provide programming
interfaces which provide a good mapping of GSS-API naming options.
11. GSS_Inquire_mech_for_SASLname call
To allow SASL clients to more efficiently identify which GSS-API
mechanism a particular SASL mechanism name refers to we specify a new
GSS-API utility function for this purpose.
Spencer (nit): whew! hard to parse. Suggest "We specify a new GSS-API
utility function to allow SASL clients to more efficiently identify the
GSS-API mechanism that a particular SASL mechanism name refers to", or
something like that?
13.3. Additional Recommendations
If the application requires security layers then it MUST prefer the
SASL "GSSAPI" mechanism over "GS2-KRB5" or "GS2-KRB5-PLUS".
Spencer (minor): If "prefer the mechanism" is the right way to describe
this, I apologize, but I don't know what the MUST means in practice - if
this needs to be at MUST strength, I'd expect text like "MUST use X and MUST
NOT use Y or Z", or "MUST use X unless the server doesn't support X".
14. GSS-API Mechanisms that negotiate other mechanisms
A GSS-API mechanism that negotiate other mechanisms interact badly
Spencer (nit): s/negotiate/negotiates/, and probably s/interact/will
with the SASL mechanism negotiation. There are two problems. The
first is an interoperability problem and the second is a security
concern. The problems are described and resolved below.
14.1. The interoperability problem
If a client implement GSS-API mechanism X, potentially negotiated
through a GSS-API mechanism Y, and the server also implement GSS-API
Spencer (nit): s/implement/implements/
mechanism X negotiated through a GSS-API mechanism Z, the
authentication negotiation will fail.
14.2. Security problem
If a client's policy is to first prefer GSSAPI mechanism X, then non-
GSSAPI mechanism Y, then GSSAPI mechanism Z, and if a server supports
mechanisms Y and Z but not X, then if the client attempts to
negotiate mechanism X by using a GSS-API mechanism that negotiate
Spencer (nit): s/negotiate/negotiates/
other mechanisms (such as SPNEGO), it may end up using mechanism Z
Spencer (nit): you provide a reference for SPNEGO in the next section, but
this is the first occurance...
when it ideally should have used mechanism Y. For this reason, the
use of GSS-API mechanisms that negotiate other mechanisms are
disallowed under GS2.
16. Security Considerations
GS2 does not directly use any cryptographic algorithms, therefore it
is automatically "algorithm agile", or, as agile as the GSS-API
mechanisms that are available for use in SASL applications via GS2.
The exception is the use of SHA-1 for deriving SASL mechanism names,
but no cryptographic properties are required. The required property
Spencer (nit): I would suggest "SHA-1 is used to derive SASL mechanism
names, but no cryptographic properties are required" - the current text says
"we don't use crypto, except when we do" :-)
is that the truncated output for distinct inputs are different for
practical input values.
Ietf mailing list