Internet Engineering Task Force (IETF) S. Josefsson

Internet Engineering Task Force (IETF) S. Josefsson
Request for Comments: 5801 SJD AB
Category: Standards Track N. Williams
ISSN: 2070-1721 Oracle
July 2010

Using Generic Security Service Application Program Interface (GSS-API)
Mechanisms in Simple Authentication and Security Layer (SASL):
The GS2 Mechanism Family

Abstract

This document describes how to use a Generic Security Service
Application Program Interface (GSS-API) mechanism in the Simple
Authentication and Security Layer (SASL) framework. This is done by
defining a new SASL mechanism family, called GS2. This mechanism
family offers a number of improvements over the previous "SASL/
GSSAPI" mechanism: it is more general, uses fewer messages for the
authentication phase in some cases, and supports negotiable use of
channel binding. Only GSS-API mechanisms that support channel
binding and mutual authentication are supported.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5801.

Josefsson & Williams Standards Track [Page 1]

RFC 5801 SASL GS2-* July 2010

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Josefsson & Williams Standards Track [Page 2]

RFC 5801 SASL GS2-* July 2010

Table of Contents

1. Introduction ....................................................4
2. Conventions Used in This Document ...............................5
3. Mechanism Name ..................................................5
3.1. Generating SASL Mechanism Names from GSS-API OIDs ..........5
3.2. Computing Mechanism Names Manually .........................6
3.3. Examples ...................................................6
3.4. Grandfathered Mechanism Names ..............................7
4. SASL Authentication Exchange Message Format .....................8
5. Channel Bindings ...............................................10
5.1. Content of GSS-CHANNEL-BINDINGS Structure .................11
5.2. Default Channel Binding ...................................12
6. Examples .......................................................12
7. Authentication Conditions ......................................14
8. GSS-API Parameters .............................................15
9. Naming .........................................................15
10. GSS_Inquire_SASLname_for_mech Call ............................15
10.1. gss_inquire_saslname_for_mech ............................16
11. GSS_Inquire_mech_for_SASLname Call ............................18
11.1. gss_inquire_mech_for_saslname ............................19
12. Security Layers ...............................................20
13. Interoperability with the SASL GSSAPI Mechanism ...............20
13.1. The Interoperability Problem .............................20
13.2. Resolving the Problem ....................................20
13.3. Additional Recommendations ...............................20
14. GSS-API Mechanisms That Negotiate Other Mechanisms ............21
14.1. The Interoperability Problem .............................21
14.2. Security Problem .........................................21
14.3. Resolving the Problems ...................................21
15. IANA Considerations ...........................................22
16. Security Considerations .......................................22
17. Acknowledgements ..............................................24
18. References ....................................................24
18.1. Normative References .....................................24
18.2. Informative References ...................................25

Josefsson & Williams Standards Track [Page 3]

RFC 5801 SASL GS2-* July 2010

1. Introduction

Generic Security Service Application Program Interface (GSS-API)
[RFC2743] is a framework that provides security services to
applications using a variety of authentication mechanisms. Simple
Authentication and Security Layer (SASL) [RFC4422] is a framework to
provide authentication and security layers for connection-based
protocols, also using a variety of mechanisms. This document
describes how to use a GSS-API mechanism as though it were a SASL
mechanism. This facility is called GS2 -- a moniker that indicates
that this is the second GSS-API->SASL mechanism bridge. The original
GSS-API->SASL mechanism bridge was specified by [RFC2222], now
[RFC4752]; we shall sometimes refer to the original bridge as GS1 in
this document.

All GSS-API mechanisms are implicitly registered for use within SASL
by this specification. The SASL mechanisms defined in this document
are known as the GS2 family of mechanisms.

The GS1 bridge failed to gain wide deployment for any GSS-API
mechanism other than "The Kerberos Version 5 GSS-API Mechanism"
[RFC1964] [RFC4121], and has a number of problems that led us to
desire a new bridge. Specifically, a) GS1 was not round-trip
optimized and b) GS1 did not support channel binding [RFC5056].
These problems and the opportunity to create the next SASL password-
based mechanism, "Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms" [RFC5802], 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 redundant because SASL applications tend to have an option to run
over a Transport Layer Security (TLS) [RFC5246] channel. Use of SASL
security layers is best replaced with channel binding to a TLS
channel.

GS2 is designed to be as simple as possible. It adds to GSS-API
security context token exchanges only the bare minimum to support
SASL semantics and negotiation of use of channel binding.
Specifically, GS2 adds a small header (a few bytes plus the length of
the client-requested SASL authorization identity) to the initial GSS-
API context token and to the application channel binding data. GS2
uses SASL mechanism negotiation to implement channel binding
negotiation. Security-relevant GS2 plaintext is protected via the
use of GSS-API channel binding. Additionally, to simplify the

Josefsson & Williams Standards Track [Page 4]

RFC 5801 SASL GS2-* July 2010

implementation of GS2 mechanisms for implementors who will not
implement a GSS-API framework, we compress the initial security
context token header required by [RFC2743], Section 3.1.

GS2 does not protect any plaintext exchanged outside GS2, such as
SASL mechanism negotiation plaintext, or application messages
following authentication. But using channel binding to a secure
channel over which all SASL and application plaintext is sent will
cause all that plaintext to be authenticated.

2. Conventions Used in This Document

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 [RFC2119].

The document uses many terms and function names defined in [RFC2743],
as updated by [RFC5554].

3. Mechanism Name

There are two SASL mechanism names for any GSS-API mechanism used
through this facility. One denotes that the server supports channel
binding. The other denotes that it does not.

The SASL mechanism name for a GSS-API mechanism is that which is
provided by that mechanism when it was specified, if one was
specified. This name denotes that the server does not support
channel binding. Add the suffix "-PLUS" and the resulting name
denotes that the server does support channel binding. SASL
implementations can use the GSS_Inquire_SASLname_for_mech call (see
below) to query for the SASL mechanism name of a GSS-API mechanism.

If the GSS_Inquire_SASLname_for_mech interface is not used, the GS2
implementation needs some other mechanism to map mechanism Object
Identifiers (OIDs) to SASL names internally. In this case, the
implementation can only support the mechanisms for which it knows the
SASL name. If GSS_Inquire_SASLname_for_mech() fails and the GS2
implementation cannot map the OID to a SASL mechanism name via some
other means, then the GS2 implementation MUST NOT use the given GSS-
API mechanism.

3.1. Generating SASL Mechanism Names from GSS-API OIDs

For GSS-API mechanisms whose SASL names are not defined together with
the GSS-API mechanism or in this document, the SASL mechanism name is
concatenation of the string "GS2-" and the Base32 encoding [RFC4648]
(with an uppercase alphabet) of the first 55 bits of the binary SHA-1

Josefsson & Williams Standards Track [Page 5]

RFC 5801 SASL GS2-* July 2010

hash [FIPS.180-1.1995] string computed over the ASN.1 DER encoding
[CCITT.X690.2002], including the tag and length octets, of the GSS-
API mechanism's Object Identifier. The Base32 rules on padding
characters and characters outside of the Base32 alphabet are not
relevant to this use of Base32. If any padding or non-alphabet
characters are encountered, the name is not a GS2 family mechanism
name. This name denotes that the server does not support channel
binding. Add the suffix "-PLUS" and the resulting name denotes that
the server does support channel binding.

A GS2 mechanism that has a non-OID-derived SASL mechanism name is
said to have a "user-friendly SASL mechanism name".

3.2. Computing Mechanism Names Manually

The hash-derived GS2 SASL mechanism name may be computed manually.
This is useful when the set of supported GSS-API mechanisms is known
in advance. This eliminates the need to implement Base32, SHA-1, and
DER in the SASL mechanism. The computed mechanism name can be used
directly in the implementation, and the implementation need not be
concerned if the mechanism is part of a mechanism family.

3.3. Examples

The OID for the Simple Public-Key GSS-API Mechanism (SPKM-1)
[RFC2025] is 1.3.6.1.5.5.1.1. The ASN.1 DER encoding of the OID,
including the tag and length, is (in hex) 06 07 2b 06 01 05 05 01 01.
The SHA-1 hash of the ASN.1 DER encoding is (in hex) 1c f8 f4 2b 5a
9f 80 fa e9 f8 31 22 6d 5d 9d 56 27 86 61 ad. Convert the first 7
octets to binary, drop the last bit, and re-group them in groups of
5, and convert them back to decimal, which results in these
computations:

hex:
1c f8 f4 2b 5a 9f 80

binary:
00011100 11111000 11110100 00101011 01011010
10011111 1000000

binary in groups of 5:
00011 10011 11100 01111 01000 01010 11010 11010
10011 11110 00000

decimal of each group:
3 19 28 15 8 10 26 26 19 30 0

Josefsson & Williams Standards Track [Page 6]

RFC 5801 SASL GS2-* July 2010

base32 encoding:
D T 4 P I K 2 2 T 6 A

The last step translates each decimal value using table 3 in Base32
[RFC4648]. Thus, the SASL mechanism name for the SPKM-1 GSSAPI
mechanism is "GS2-DT4PIK22T6A".

The OID for the Kerberos V5 GSS-API mechanism [RFC1964] is
1.2.840.113554.1.2.2 and its DER encoding is (in hex) 06 09 2A 86 48
86 F7 12 01 02 02. The SHA-1 hash is 82 d2 73 25 76 6b d6 c8 45 aa
93 25 51 6a fc ff 04 b0 43 60. Convert the 7 octets to binary, drop
the last bit, and re-group them in groups of 5, and convert them back
to decimal, which results in these computations:

hex:
82 d2 73 25 76 6b d6

binary:
10000010 11010010 01110011 00100101 01110110
01101011 1101011

binary in groups of 5:
10000 01011 01001 00111 00110 01001 01011 10110
01101 01111 01011

decimal of each group:
16 11 9 7 6 9 11 22 13 15 11

base32 encoding:
Q L J H G J L W N P L

The last step translates each decimal value using table 3 in Base32
[RFC4648]. Thus, the SASL mechanism name for the Kerberos V5 GSS-API
mechanism would be "GS2-QLJHGJLWNPL" and (because this mechanism
supports channel binding) "GS2-QLJHGJLWNPL-PLUS". Instead, the next
section assigns the Kerberos V5 mechanism a non-hash-derived
mechanism name.

3.4. Grandfathered Mechanism Names

Some older GSS-API mechanisms were not specified with a SASL GS2
mechanism name. Using a shorter name can be useful, nonetheless. We
specify the names "GS2-KRB5" and "GS2-KRB5-PLUS" for the Kerberos V5
mechanism, to be used as if the original specification documented it,
see Section 15.

Josefsson & Williams Standards Track [Page 7]

RFC 5801 SASL GS2-* July 2010

4. SASL Authentication Exchange Message Format

During the SASL authentication exchange for GS2, a number of messages
following the following format are sent between the client and
server. On success, this number is the same as the number of context
tokens that the GSS-API mechanism would normally require in order to
establish a security context. On failures, the exchange can be
terminated early by any party.

When using a GS2 mechanism the SASL client is always a GSS-API
initiator and the SASL server is always a GSS-API acceptor. The
client calls GSS_Init_sec_context and the server calls
GSS_Accept_sec_context.

All the SASL authentication messages exchanged are exactly the same
as the security context tokens of the GSS-API mechanism, except for
the initial security context token.

The client and server MAY send GSS-API error tokens (tokens output by
GSS_Init_sec_context() or GSS_Accept_sec_context() when the major
status code is other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED).
As this indicates an error condition, after sending the token, the
sending side should fail the authentication.

The initial security context token is modified as follows:

o The initial context token header (see Section 3.1 of [RFC2743])
MUST be removed if present. If the header is not present, the
client MUST send a "gs2-nonstd-flag" flag (see below). On the
server side, this header MUST be recomputed and restored prior to
passing the token to GSS_Accept_sec_context, except when the "gs2-
nonstd-flag" is sent.

o A GS2 header MUST be prefixed to the resulting initial context
token. This header has the form "gs2-header" given below in ABNF
[RFC5234].

The figure below describes the permissible attributes, their use, and
the format of their values. All attribute names are single US-ASCII
letters and are case sensitive.

Josefsson & Williams Standards Track [Page 8]

RFC 5801 SASL GS2-* July 2010

UTF8-1-safe = %x01-2B / %x2D-3C / %x3E-7F
;; As UTF8-1 in RFC 3629 except
;; NUL, "=", and ",".
UTF8-2 = <as defined in RFC 3629 (STD 63)>
UTF8-3 = <as defined in RFC 3629 (STD 63)>
UTF8-4 = <as defined in RFC 3629 (STD 63)>
UTF8-char-safe = UTF8-1-safe / UTF8-2 / UTF8-3 / UTF8-4

saslname = 1*(UTF8-char-safe / "=2C" / "=3D")
gs2-authzid = "a=" saslname
;; GS2 has to transport an authzid since
;; the GSS-API has no equivalent
gs2-nonstd-flag = "F"
;; "F" means the mechanism is not a
;; standard GSS-API mechanism in that the
;; RFC 2743, Section 3.1 header was missing
cb-name = 1*(ALPHA / DIGIT / "." / "-")
;; See RFC 5056, Section 7.
gs2-cb-flag = ("p=" cb-name) / "n" / "y"
;; GS2 channel binding (CB) flag
;; "p" -> client supports and used CB
;; "n" -> client does not support CB
;; "y" -> client supports CB, thinks the server
;; does not
gs2-header = [gs2-nonstd-flag ","] gs2-cb-flag "," [gs2-authzid] ","
;; The GS2 header is gs2-header.

When the "gs2-nonstd-flag" flag is present, the client did not find/
remove a token header ([RFC2743], Section 3.1) from the initial token
returned by GSS_Init_sec_context. This signals to the server that it
MUST NOT re-add the data that is normally removed by the client.

The "gs2-cb-flag" signals the channel binding mode. One of "p", "n",
or "y" is used. A "p" means the client supports and used a channel
binding, and the name of the channel binding type is indicated. An
"n" means that the client does not support channel binding. A "y"
means the client supports channel binding, but believes the server
does not support it, so it did not use a channel binding. See the
next section for more details.

The "gs2-authzid" holds the SASL authorization identity. It is
encoded using UTF-8 [RFC3629] with three exceptions:

o The NUL character is forbidden as required by section 3.4.1 of
[RFC4422].

o The server MUST replace any "," (comma) in the string with "=2C".

Josefsson & Williams Standards Track [Page 9]

RFC 5801 SASL GS2-* July 2010

o The server MUST replace any "=" (equals) in the string with "=3D".

Upon receipt of this value, the server verifies its correctness
according to the used SASL protocol profile. Failed verification
results in a failed authentication exchange.

5. Channel Bindings

GS2 supports channel binding to external secure channels, such as
TLS. Clients and servers may or may not support channel binding;
therefore, the use of channel binding is negotiable. However, GS2
does not provide security layers; therefore, it is imperative that
GS2 provide integrity protection for the negotiation of channel
binding.

Use of channel binding is negotiated as follows:

o Servers that support the use of channel binding SHOULD advertise
both the non-PLUS and PLUS-variant of each GS2 mechanism name. If
the server cannot support channel binding, it SHOULD advertise
only the non-PLUS-variant. If the server would never succeed in
the authentication of the non-PLUS-variant due to policy reasons,
it MUST advertise only the PLUS-variant.

o If the client supports channel binding and the server does not
appear to (i.e., the client did not see the -PLUS name advertised
by the server), then the client MUST NOT use an "n" gs2-cb-flag.

o Clients that support mechanism negotiation and channel binding
MUST use a "p" gs2-cb-flag when the server offers the PLUS-variant
of the desired GS2 mechanism.

o If the client does not support channel binding, then it MUST use
an "n" gs2-cb-flag. Conversely, if the client requires the use of
channel binding then it MUST use a "p" gs2-cb-flag. Clients that
do not support mechanism negotiation never use a "y" gs2-cb-flag,
they use either "p" or "n" according to whether they require and
support the use of channel binding or whether they do not,
respectively.

o The client generates the chan_bindings input parameter for
GSS_Init_sec_context as described below.

o Upon receipt of the initial authentication message, the server
checks the gs2-cb-flag in the GS2 header and constructs a
chan_bindings parameter for GSS_Accept_sec_context as described
below. If the client channel binding flag was "y" and the server
did advertise support for channel bindings (by advertising the

Josefsson & Williams Standards Track [Page 10]

RFC 5801 SASL GS2-* July 2010

PLUS-variant of the mechanism chosen by the client), then the
server MUST fail authentication. If the client channel binding
flag was "p" and the server does not support the indicated channel
binding type, then the server MUST fail authentication.

o If the client used an "n" gs2-cb-flag and the server requires the
use of channel binding, then the server MUST fail authentication.

FLAG CLIENT CB SUPPORT SERVER CB SUPPORT DISPOSITION
---- ----------------- ----------------- -----------

n no support N/A If server disallows
non-channel-bound
authentication, then
fail

y Yes, not required No Authentication may
succeed; CB not used

y Yes, not required Yes Authentication must fail

p Yes Yes Authentication may
succeed, with CB used

p Yes No Authentication will fail

N/A Yes, required No Client does not even try

For more discussion of channel bindings, and the syntax of the
channel binding data for various security protocols, see [RFC5056].

5.1. Content of GSS-CHANNEL-BINDINGS Structure

The calls to GSS_Init_sec_context and GSS_Accept_sec_context take a
chan_bindings parameter. The value is a GSS-CHANNEL-BINDINGS
structure [RFC5554].

The initiator-address-type and acceptor-address-type fields of the
GSS-CHANNEL-BINDINGS structure MUST be set to 0. The initiator-
address and acceptor-address fields MUST be the empty string.

The application-data field MUST be set to the gs2-header, excluding
the initial [gs2-nonstd-flag ","] part, concatenated with, when a
gs2-cb-flag of "p" is used, the application's channel binding data.

Josefsson & Williams Standards Track [Page 11]

RFC 5801 SASL GS2-* July 2010

5.2. Default Channel Binding

A default channel binding type agreement process for all SASL
application protocols that do not provide their own channel binding
type agreement is provided as follows.

'tls-unique' is the default channel binding type for any application
that doesn't specify one.

Servers MUST implement the "tls-unique" [RFC5929] channel binding
type, if they implement any channel binding. Clients SHOULD
implement the "tls-unique" channel binding type, if they implement
any channel binding. Clients and servers SHOULD choose the highest-
layer/innermost end-to-end TLS channel as the channel to which to
bind.

Servers MUST choose the channel binding type indicated by the client,
or fail authentication if they don't support it.

6. Examples

Example #1: a one round-trip GSS-API context token exchange, no
channel binding, optional authzid given.

C: Request authentication exchange
S: Empty Challenge
C: n,a=someuser,<initial context token with standard
header removed>
S: Send reply context token as is
C: Empty message
S: Outcome of authentication exchange

Example #2: a one and one half round-trip GSS-API context token
exchange, no channel binding.

C: Request authentication exchange
S: Empty Challenge
C: n,,<initial context token with standard
header removed>
S: Send reply context token as is
C: Send reply context token as is
S: Outcome of authentication exchange

Josefsson & Williams Standards Track [Page 12]

RFC 5801 SASL GS2-* July 2010

Example #3: a two round-trip GSS-API context token exchange, no
channel binding, no standard token header.

C: Request authentication exchange
S: Empty Challenge
C: F,n,,<initial context token without
standard header>
S: Send reply context token as is
C: Send reply context token as is
S: Send reply context token as is
C: Empty message
S: Outcome of authentication exchange

Example #4: using channel binding, optional authzid given.

C: Request authentication exchange
S: Empty Challenge
C: p=tls-unique,a=someuser,<initial context token with standard
header removed>
S: Send reply context token as is
...

Example #5: using channel binding.

C: Request authentication exchange
S: Empty Challenge
C: p=tls-unique,,<initial context token with standard
header removed>
S: Send reply context token as is
...

Example #6: using non-standard channel binding (requires out-of-band
negotiation).

C: Request authentication exchange
S: Empty Challenge
C: p=tls-server-end-point,,<initial context token with standard
header removed>
S: Send reply context token as is
...

Josefsson & Williams Standards Track [Page 13]

RFC 5801 SASL GS2-* July 2010

Example #7: client supports channel bindings but server does not,
optional authzid given.

C: Request authentication exchange
S: Empty Challenge
C: y,a=someuser,<initial
context token with standard header removed>
S: Send reply context token as is
...

GSS-API authentication is always initiated by the client. The SASL
framework allows either the client or the server to initiate
authentication. In GS2, the server will send an initial empty
challenge (zero-byte string) if it has not yet received a token from
the client. See Section 3 of [RFC4422].

7. Authentication Conditions

Authentication MUST NOT succeed if any one of the following
conditions are true:

o If GSS_Init/Accept_sec_context returns anything other than
GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE.

o If the client's initial GS2 header does not match the ABNF.

o In particular, if the initial character of the client message is
anything except "F", "p", "n", or "y".

o If the client's GS2 channel binding flag was "y" and the server
supports channel bindings.

o If the client's GS2 channel binding flag was "p" and the server
does not support the indicated channel binding.

o If the client requires use of channel binding and the server did
not advertise support for channel binding.

o If authorization of client principal (i.e., src_name in
GSS_Accept_sec_context) to requested authzid failed.

o If the client is not authorized to the requested authzid or an
authzid could not be derived from the client's initiator principal
name.

Josefsson & Williams Standards Track [Page 14]

RFC 5801 SASL GS2-* July 2010

8. GSS-API Parameters

GS2 does not use any GSS-API per-message tokens. Therefore, the per-
message token ret_flags from GSS_Init_sec_context() and
GSS_Accept_sec_context() are irrelevant; implementations SHOULD NOT
set the per-message req_flags.

The mutual_req_flag MUST be set. Clients MUST check that the
corresponding ret_flag is set when the context is fully established,
else authentication MUST fail.

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 that provide a good mapping of GSS-API naming options.

9. Naming

There is no requirement that any particular GSS-API name-types be
used. However, typically, SASL servers will have host-based acceptor
principal names (see [RFC2743], Section 4.1) and clients will
typically have username initiator principal names (see [RFC2743],
Section 4.2). When a host-based acceptor principal name is used
("service@hostname"), "service" is the service name specified in the
protocol's profile and "hostname" is the fully qualified host name of
the server.

10. GSS_Inquire_SASLname_for_mech Call

We specify a new GSS-API utility function to allow SASL
implementations to more efficiently identify the GSS-API mechanism to
which a particular SASL mechanism name refers.

Inputs:

o desired_mech OBJECT IDENTIFIER

Outputs:

o major_status INTEGER

o minor_status INTEGER

o sasl_mech_name UTF-8 STRING -- SASL name for this
mechanism; caller must release with
GSS_Release_buffer()

Josefsson & Williams Standards Track [Page 15]

RFC 5801 SASL GS2-* July 2010

o mech_name UTF-8 STRING -- name of this mechanism, possibly
localized; caller must release with GSS_Release_buffer()

o mech_description UTF-8 STRING -- possibly localized
description of this mechanism; caller must release with
GSS_Release_buffer()

Return major_status codes:

o GSS_S_COMPLETE indicates successful completion, and that
output parameters holds correct information.

o GSS_S_BAD_MECH indicates that a desired_mech was unsupported
by the GSS-API implementation.

o GSS_S_FAILURE indicates that the operation failed for reasons
unspecified at the GSS-API level.

The GSS_Inquire_SASLname_for_mech call is used to get the SASL
mechanism name for a GSS-API mechanism. It also returns a name
and description of the mechanism in user-friendly form.

The output variable sasl_mech_name will hold the IANA registered
mechanism name for the GSS-API mechanism, or if none is
registered, a mechanism name computed from the OID as described
in Section 3.1 of this document.

10.1. gss_inquire_saslname_for_mech

The C binding for the GSS_Inquire_SASLname_for_mech call is as
follows.

As mentioned in [RFC2744], routines may return GSS_S_FAILURE,
indicating an implementation-specific or mechanism-specific error
condition, further details of which are reported via the minor_status
parameter.

Josefsson & Williams Standards Track [Page 16]

RFC 5801 SASL GS2-* July 2010

OM_uint32 gss_inquire_saslname_for_mech(
OM_uint32 *minor_status,
const gss_OID desired_mech,
gss_buffer_t sasl_mech_name,
gss_buffer_t mech_name,
gss_buffer_t mech_description
);

Purpose:

Output the SASL mechanism name of a GSS-API mechanism.
It also returns a name and description of the mechanism in a
user-friendly form.

Parameters:

minor_status Integer, modify
Mechanism-specific status code.

desired_mech OID, read
Identifies the GSS-API mechanism to query.

sasl_mech_name buffer, character-string, modify, optional
Buffer to receive SASL mechanism name.
The application must free storage associated
with this name after use with a call to
gss_release_buffer().

mech_name buffer, character-string, modify, optional
Buffer to receive human-readable mechanism name.
The application must free storage associated
with this name after use with a call to
gss_release_buffer().

mech_description buffer, character-string, modify, optional
Buffer to receive description of mechanism.
The application must free storage associated
with this name after use with a call to
gss_release_buffer().

Function value: GSS status code:

GSS_S_COMPLETE Successful completion.

GSS_S_BAD_MECH The desired_mech OID is unsupported.

Josefsson & Williams Standards Track [Page 17]

RFC 5801 SASL GS2-* July 2010

11. GSS_Inquire_mech_for_SASLname Call

To allow SASL clients to more efficiently identify to which GSS-API
mechanism a particular SASL mechanism name refers, we specify a new
GSS-API utility function for this purpose.

Inputs:

o sasl_mech_name UTF-8 STRING -- SASL name of mechanism.

Outputs:

o major_status INTEGER

o minor_status INTEGER

o mech_type OBJECT IDENTIFIER -- must be explicit mechanism,
and not "default" specifier. Caller should treat as read-only
and should not attempt to release.

Return major_status codes:

o GSS_S_COMPLETE indicates successful completion, and that output
parameters holds correct information.

o GSS_S_BAD_MECH indicates that no supported GSS-API mechanism
had the indicated sasl_mech_name.

o GSS_S_FAILURE indicates that the operation failed for reasons
unspecified at the GSS-API level.

The GSS_Inquire_mech_for_SASLname call is used to get the GSS-API
mechanism OID associated with a SASL mechanism name.

Josefsson & Williams Standards Track [Page 18]

RFC 5801 SASL GS2-* July 2010

11.1. gss_inquire_mech_for_saslname

The C binding for the GSS_Inquire_mech_for_SASLname call is as
follows.

As mentioned in [RFC2744], routines may return GSS_S_FAILURE,
indicating an implementation-specific or mechanism-specific error
condition, further details of which are reported via the minor_status
parameter.

OM_uint32 gss_inquire_mech_for_saslname(
OM_uint32 *minor_status,
const gss_buffer_t sasl_mech_name,
gss_OID *mech_type
);

Purpose:

Output GSS-API mechanism OID of mechanism associated with given
sasl_mech_name.

Parameters:

minor_status Integer, modify
Mechanism-specific status code.

sasl_mech_name buffer, character-string, read
Buffer with SASL mechanism name.

mech_type OID, modify, optional
Actual mechanism used. The OID returned via
this parameter will be a pointer to static
storage that should be treated as read-only.
In particular, the application should not attempt
to free it. Specify NULL if not required.

Function value: GSS status code:

GSS_S_COMPLETE Successful completion.

GSS_S_BAD_MECH There is no GSS-API mechanism known
as sasl_mech_name.

Josefsson & Williams Standards Track [Page 19]

RFC 5801 SASL GS2-* July 2010

12. Security Layers

GS2 does not support SASL security layers. Applications that need
integrity or confidentiality protection can use either channel
binding to a secure external channel or another SASL mechanism that
does provide security layers.

13. Interoperability with the SASL GSSAPI Mechanism

The Kerberos V5 GSS-API [RFC1964] mechanism is currently used in SASL
under the name GSSAPI, see [RFC4752]. The Kerberos V5 mechanism may
also be used with the GS2 family. This causes an interoperability
problem, which is discussed and resolved below.

13.1. The Interoperability Problem

The SASL "GSSAPI" mechanism is not wire compatible with the Kerberos
V GSS-API mechanism used as a SASL GS2 mechanism.

If a client (or server) only support Kerberos V5 under the "GSSAPI"
name, and the server (or client) only support Kerberos V5 under the
GS2 family, the mechanism negotiation will fail.

13.2. Resolving the Problem

If the Kerberos V5 mechanism is supported under GS2 in a server, the
server SHOULD also support Kerberos V5 through the "GSSAPI"
mechanism, to avoid interoperability problems with older clients.

Reasons for violating this recommendation may include security
considerations regarding the absent features in the GS2 mechanism.
The SASL "GSSAPI" mechanism lacks support for channel bindings, which
means that using an external secure channel may not be sufficient
protection against active attackers (see [RFC5056] and [MITM]).

13.3. Additional Recommendations

If the application requires SASL security layers, then it MUST use
the SASL "GSSAPI" mechanism [RFC4752] instead of "GS2-KRB5" or "GS2-
KRB5-PLUS".

If the application can use channel binding to an external channel,
then it is RECOMMENDED that it select Kerberos V5 through the GS2
mechanism rather than the "GSSAPI" mechanism.

Josefsson & Williams Standards Track [Page 20]

RFC 5801 SASL GS2-* July 2010

14. GSS-API Mechanisms That Negotiate Other Mechanisms

A GSS-API mechanism that negotiates other mechanisms will interact
badly 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 implements GSS-API mechanism X, potentially negotiated
through a GSS-API mechanism Y, and the server also implements GSS-API
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 negotiates
other mechanisms (such as Simple and Protected GSS-API Negotiation
(SPNEGO) [RFC4178]), it may end up using mechanism Z when it ideally
should have used mechanism Y. For this reason, the use of GSS-API
mechanisms that negotiate other mechanisms is disallowed under GS2.

14.3. Resolving the Problems

GSS-API mechanisms that negotiate other mechanisms MUST NOT be used
with the GS2 SASL mechanism. Specifically, SPNEGO [RFC4178] MUST NOT
be used as a GS2 mechanism. To make this easier for SASL
implementations, we assign a symbolic SASL mechanism name to the
SPNEGO GSS-API mechanism, "SPNEGO". SASL client implementations MUST
NOT choose the SPNEGO mechanism under any circumstances.

The GSS_C_MA_MECH_NEGO attribute of GSS_Inquire_attrs_for_mech
[RFC5587] can be used to identify such mechanisms.

Josefsson & Williams Standards Track [Page 21]

RFC 5801 SASL GS2-* July 2010

15. IANA Considerations

The IANA has registered a SASL mechanism family as per [RFC4422]
using the following information.

Subject: Registration of SASL mechanism family GS2-*
SASL mechanism prefix: GS2-
Security considerations: RFC 5801
Published specification: RFC 5801
Person & email address to contact for further information:
Simon Josefsson <[email protected]>
Intended usage: COMMON
Owner/Change controller: [email protected]
Note: Compare with the GSSAPI and GSS-SPNEGO mechanisms.

The IANA is advised that SASL mechanism names starting with "GS2-"
are reserved for SASL mechanisms that conform to this document. The
IANA has placed a statement to that effect in the SASL Mechanisms
registry.

The IANA is further advised that GS2 SASL mechanism names MUST NOT
end in "-PLUS" except as a version of another mechanism name simply
suffixed with "-PLUS".

The SASL names for the Kerberos V5 GSS-API mechanism [RFC4121]
[RFC1964] used via GS2 SHALL be "GS2-KRB5" and "GS2-KRB5-PLUS".

The SASL names for the SPNEGO GSS-API mechanism used via GS2 SHALL be
"SPNEGO" and "SPNEGO-PLUS". As described in Section 14, the SASL
"SPNEGO" and "SPNEGO-PLUS" MUST NOT be used. These names are
provided as a convenience for SASL library implementors.

16. Security Considerations

Security issues are also discussed throughout this memo.

The security provided by a GS2 mechanism depends on the security of
the GSS-API mechanism. The GS2 mechanism family depends on channel
binding support, so GSS-API mechanisms that do not support channel
binding cannot be successfully used as SASL mechanisms via the GS2
bridge.

Because GS2 does not support security layers, it is strongly
RECOMMENDED that channel binding to a secure external channel be
used. Successful channel binding eliminates the possibility of man-
in-the-middle (MITM) attacks, provided that the external channel and
its channel binding data are secure and that the GSS-API mechanism
used is secure. Authentication failure because of channel binding

Josefsson & Williams Standards Track [Page 22]

RFC 5801 SASL GS2-* July 2010

failure may indicate that an MITM attack was attempted, but note that
a real MITM attacker would likely attempt to close the connection to
the client or simulate network partition; thus, MITM attack detection
is heuristic.

Use of channel binding will also protect the SASL mechanism
negotiation -- if there is no MITM, then the external secure channel
will have protected the SASL mechanism negotiation.

The channel binding data MAY be sent (by the actual GSS-API mechanism
used) without confidentiality protection and knowledge of it is
assumed to provide no advantage to an MITM (who can, in any case,
compute the channel binding data independently). If the external
channel does not provide confidentiality protection and the GSS-API
mechanism does not provide confidentiality protection for the channel
binding data, then passive attackers (eavesdroppers) can recover the
channel binding data, see [RFC5056].

When constructing the input_name_string for GSS_Import_name with the
GSS_C_NT_HOSTBASED_SERVICE name type, the client SHOULD NOT
canonicalize the server's fully qualified domain name using an
insecure or untrusted directory service, such as the Domain Name
System [RFC1034] without DNS Security (DNSSEC) [RFC4033].

SHA-1 is used to derive SASL mechanism names, but no traditional
cryptographic properties are required -- the required property is
that the truncated output for distinct inputs are different for
practical input values. GS2 does not use any other cryptographic
algorithm. Therefore, GS2 is "algorithm agile", or, as agile as the
GSS-API mechanisms that are available for use in SASL applications
via GS2.

GS2 does not protect against downgrade attacks of channel binding
types. Negotiation of channel binding type was intentionally left
out of scope for this document.

The security considerations of SASL [RFC4422], the GSS-API [RFC2743],
channel binding [RFC5056], any external channels (such as TLS,
[RFC5246], channel binding types (see the IANA channel binding type
registry), and GSS-API mechanisms (such as the Kerberos V5 mechanism
[RFC4121] [RFC1964]), also apply.

Josefsson & Williams Standards Track [Page 23]

RFC 5801 SASL GS2-* July 2010

17. Acknowledgements

The history of GS2 can be traced to the "GSSAPI" mechanism originally
specified by RFC 2222. This document was derived from [SASL-GSSAPI],
which was prepared by Alexey Melnikov with significant contributions
from John G. Myers, although the majority of this document has been
rewritten by the current authors.

Contributions of many members of the SASL mailing list are gratefully
acknowledged. In particular, ideas and feedback from Pasi Eronen,
Sam Hartman, Jeffrey Hutzelman, Alexey Melnikov, and Tom Yu improved
the document and the protocol. Other suggestions to the documents
were made by Spencer Dawkins, Ralph Droms, Adrian Farrel, Robert
Sparks, and Glen Zorn.

18. References

18.1. Normative References

[FIPS.180-1.1995]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-1, April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.

[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

Josefsson & Williams Standards Track [Page 24]

RFC 5801 SASL GS2-* July 2010

[RFC5554] Williams, N., "Clarifications and Extensions to the
Generic Security Service Application Program Interface
(GSS-API) for the Use of Channel Bindings", RFC 5554,
May 2009.

[CCITT.X690.2002]
International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.

[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.

18.2. Informative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.

[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.

[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism
(SPKM)", RFC 2025, October 1996.

[RFC2222] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.

[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.

[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.

[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
Simple and Protected Generic Security Service Application
Program Interface (GSS-API) Negotiation Mechanism",
RFC 4178, October 2005.

Josefsson & Williams Standards Track [Page 25]

RFC 5801 SASL GS2-* July 2010

[RFC4752] Melnikov, A., "The Kerberos V5 ("GSSAPI") Simple
Authentication and Security Layer (SASL) Mechanism",
RFC 4752, November 2006.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5587] Williams, N., "Extended Generic Security Service Mechanism
Inquiry APIs", RFC 5587, July 2009.

[RFC5802] Menon-Sen, A., Melnikov, A., Newman, C., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

[MITM] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunnelled Authentication", in 11th Security
Protocols Workshop, 2002.

[SASL-GSSAPI]
Melnikov, A., "The Kerberos V5 ("GSSAPI") SASL mechanism",
Work in Progress, March 2005.

Authors' Addresses

Simon Josefsson
SJD AB
Hagagatan 24
Stockholm 113 47
SE

EMail: [email protected]
URI: http://josefsson.org/

Nicolas Williams
Oracle
5300 Riata Trace Ct
Austin, TX 78727
USA

EMail: [email protected]

Josefsson & Williams Standards Track [Page 26]