One document matched: draft-ietf-sasl-gs2-13.xml
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<rfc category="std" ipr="pre5378Trust200902"
docName="draft-ietf-sasl-gs2-13">
<front>
<title abbrev="SASL GS2-*">
Using GSS-API Mechanisms in SASL:
The GS2 Mechanism Family
</title>
<author initials="S." surname="Josefsson" fullname="Simon Josefsson">
<organization>SJD AB</organization>
<address>
<postal>
<street>Hagagatan 24</street>
<city>Stockholm</city>
<code>113 47</code>
<country>SE</country>
</postal>
<email>simon@josefsson.org</email>
<uri>http://josefsson.org/</uri>
</address>
</author>
<author initials='N.' surname="Williams" fullname='Nicolas
Williams'>
<organization>Sun
Microsystems</organization>
<address>
<postal>
<street>5300
Riata
Trace
Ct</street>
<city>Austin</city>
<region>TX</region>
<code>78727</code>
<country>USA</country>
</postal>
<email>Nicolas.Williams@sun.com</email>
</address>
</author>
<date month="May" year="2009"/>
<abstract>
<t>This document describes how to use a Generic Security
Service Application Program Interface (GSS-API)
mechanism in the 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 are
supported.</t>
<t>See <http://josefsson.org/sasl-gs2-*/> for more
information.</t>
</abstract>
</front>
<middle>
<section anchor="intro"
title="Introduction">
<t>Generic Security Service Application Program Interface
(GSS-API) <xref target="RFC2743"/> is a framework that
provides security services to applications using a
variety of authentication "mechanisms". Simple
Authentication and Security Layer (SASL)
<xref target="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 <xref target='RFC2222'/>, now
<xref target='RFC4752'/>; we shall sometimes refer to
the original bridge as GS1 in this document.</t>
<t>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.</t>
<t>The GS1 bridge failed to gain wide deployment for any
GSS-API mechanism other than The "Kerberos V5 GSS-API
mechanism" <xref target="RFC1964"/> <xref
target="RFC4121"/>, and has a number of problems
that lead us to desire a new bridge. Specifically: a)
GS1 was not round-trip optimized, b) GS1 did not support
channel binding <xref target='RFC5056'/>. These
problems and the opportunity to create the next SASL
password-based mechanism,
<xref target='I-D.ietf-sasl-scram'>SCRAM</xref>, as a
GSS-API mechanism used by SASL applications via GS2,
provide the motivation for GS2.</t>
<t>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) <xref
target='RFC5246'/> channel, redundant and best
replaced with channel binding.</t>
<t>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 (2 bytes or 3 bytes plus the length of
the client requested SASL authorization ID (authzid))
to the initial context token and to the application
channel binding data, and it uses SASL mechanism
negotiation to implement channel binding negotiation.
All GS2 plaintext is protected via the use of GSS-API
channel binding. Additionally, to simplify the
implementation of GS2 mechanisms for implementors who
will not implement a GSS-API framework, we compress
the initial security context token header required by
<xref target='RFC2743'/> (see section 3.1).</t>
</section>
<section title="Conventions used in this document">
<t>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
<xref target="RFC2119" />.</t>
</section>
<section anchor="mechname"
title="Mechanism name">
<t>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.</t>
<t>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.</t>
<t>If the GSS_Inquire_SASLname_for_mech interface is not
used, the GS2 implementation need some other mechanism to
map mechanism OIDs to SASL name internally. In this case,
the implementation can only support the mechanisms for
which it knows the SASL name. If the
GSS_Inquire_SASLname_for_mech call fails, and the GS2
implementation cannot map the OID to a SASL mechanism name
using some other means, it cannot use the particular
GSS-API mechanism since it does not know its SASL
mechanism name.</t>
<section title="Generating SASL mechanism names from GSS-API OIDs">
<t>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 <xref
target="RFC4648">Base32 encoding</xref> (with an
upper case alphabet) of the first 55 bits of the
binary <xref target="FIPS.180-1.1995">SHA-1
hash</xref> string computed over the <xref
target="CCITT.X690.2002">ASN.1 DER
encoding</xref>, 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.</t>
</section>
<section title="Computing mechanism names manually">
<t>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.
It also obliterate 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
concern itself with that the mechanism is part of a
mechanism family.</t>
</section>
<section title="Examples">
<t>The OID for the <xref target="RFC2025">SPKM-1
mechanism</xref> 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:</t>
<figure>
<artwork>
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
base32 encoding:
D T 4 P I K 2 2 T 6 A
</artwork>
</figure>
<t>The last step translate each decimal value using table 3
in <xref target="RFC4648">Base32</xref>. Thus the SASL
mechanism name for the SPKM-1 GSSAPI mechanism is
"GS2-DT4PIK22T6A".</t>
<t>The OID for the <xref target="RFC1964">Kerberos V5
GSS-API mechanism</xref> 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:</t>
<figure>
<artwork>
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
</artwork>
</figure>
<t>The last step translate each decimal value using
table 3 in <xref target="RFC4648">Base32</xref>.
Thus the SASL mechanism name for the Kerberos V5
GSSAPI 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.</t>
</section>
<section title="Grandfathered mechanism names">
<t>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 <xref target='iana' />.</t>
</section>
</section>
<section anchor="messages"
title="SASL Authentication Exchange Message Format">
<section title="SASL Messages">
<t>During the SASL authentication exchange for GS2, a
number of messages following the following format is
sent between the client and server. 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 (or to fail to do
so).</t>
<t>Note that when using a GS2 mechanism the SASL client
is always a GSS-API initiator and the SASL server is
always a GSS-API acceptor. Thus the SASL client
calls GSS_Init_sec_context and the server calls
GSS_Accept_sec_context.</t>
<t>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.</t>
<t>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 indicate an error
condition, after sending the token, the sending
side should fail the authentication.</t>
<t>The initial security context token is modified as
follows:
<list style='symbols'>
<t>The <xref target='RFC2743'/> section 3.1
initial context token header 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.</t>
<t>A GS2 header MUST be prefixed to the
resulting initial context token. This
header has the form "gs2-header" given
below in ABNF <xref target='RFC5234'/>.</t>
</list>
</t>
<figure>
<artwork type='abnf'>
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
;; RFC2743 section 3.1 header was missing
gs2-cb-flag = "p" / "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.
</artwork>
</figure>
<t>When the "gs2-nonstd-flag" flag is present, the
client did not find/remove a
<xref target='RFC2743'/> section 3.1 token header
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.</t>
<t>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. A "n"
means that the client does not support channel
binding. A "y" means the client supports channel
binding, but believes the server does not, so it did
not use a channel binding. See the next section for
more details.</t>
<t>The "gs2-authzid" holds the SASL authorization
identity. It is encoded
using <xref target="RFC3629">UTF-8</xref> with three
exceptions:
<list style='symbols'>
<t>The NUL characters is forbidden as required
by section 3.4.1 of
<xref target="RFC4422"/>.</t>
<t>The server MUST replace any "," (comma) in
the string with "=2C".</t>
<t>The server MUST replace any "=" (equals) in
the string with "=3D".</t>
</list>
</t>
<t>If a server sends a string that does not conform to
this syntax, the client MUST reject
authentication.</t>
</section>
</section>
<section anchor="cb"
title="Channel Bindings">
<t>If the server supports channel binding then it MUST
list both forms of the SASL mechanism name for each
GSS-API mechanism supported via GS2 (i.e., GSS-API
mechanisms that support channel binding).</t>
<t>If the client supports channel binding and the server
does not (i.e., the server did not advertise the -PLUS
names) then the client MUST either fail authentication
or it MUST set the channel binding flag in the GS2
initial security context token to "y" and MUST NOT
include application channel binding data in the GSS-API
channel binding input to GSS_Init_sec_context.</t>
<t>If the client supports channel binding and the server
also does then the client MUST set the channel binding
flag in the GS2 initial security context token to "p"
and MUST include application channel binding data in
the GSS-API channel binding input to
GSS_Init_sec_context. This is done by pre-pending the
gs2-header to the application's channel binding data.
If the application did not provide channel binding
data then the GS2 header is used as though it were
application-provided channel binding data.</t>
<t>If the client does not support channel binding then it
MUST set the channel binding flag in the GS2 initial
security context token to "n" and MUST NOT include
application channel binding data in the GSS-API channel
binding input to GSS_Init_sec_context.</t>
<t>Upon receipt of the initial authentication message the
server checks the channel binding flag in the GS2 header
and constructs a channel binding data input for
GSS_Accept_sec_context accordingly. If the client
channel binding flag was "n" then the server MUST NOT
include application channel binding data in the GSS-API
channel binding input to GSS_Accept_sec_context. If
the client channel binding flag was "y" and the server
does support channel binding then the server MUST fail
authentication. If the client channel binding flag was
"p" the server MUST include application channel binding
data in the GSS-API channel binding input to
GSS_Accept_sec_context.</t>
<t>For more discussions of channel bindings, and the syntax
of the channel binding data for various security
protocols, see <xref target="RFC5056"/>.</t>
<section title="Channel Binding to TLS Channels">
<t>If an external TLS channel is to be bound into the
GS2 authentication, and if the channel was established
using a X.509 <xref target="RFC5280"/> server
certificate to authenticate the server, then the GS2
client and server MUST use the 'tls-server-end-point'
channel binding type. See the IANA Channel Binding
Types registry.</t>
<t>If an external TLS channel is to be bound into the
GS2 authentication, and if the channel was established
either without the use of any X.509 server certificate
to authenticate the server, or with a non X.509 server
certificate, then the GS2 client and server MUST use
the 'tls-unique' channel binding type.</t>
</section>
</section>
<section anchor="examples"
title="Examples">
<figure>
<preamble>Example #1: a one round-trip GSS-API context
token exchange, no channel binding, optional
authzid given.</preamble>
<artwork>
C: Request authentication exchange
S: Empty Challenge
C: na=someuser,<initial context token with standard
header removed>
S: Send reply context token as is
C: Empty message
S: Outcome of authentication exchange
</artwork>
</figure>
<figure>
<preamble>Example #2: a one and one half round-trip
GSS-API context token exchange.</preamble>
<artwork>
C: Request authentication exchange
S: Empty Challenge
C: na=someuser,<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
</artwork>
</figure>
<figure>
<preamble>Example #3: a two round-trip GSS-API context
token exchange, no standard token header.</preamble>
<artwork>
C: Request authentication exchange
S: Empty Challenge
C: Fna=someuser,<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
</artwork>
</figure>
<figure>
<preamble>Example #4: using channel binding</preamble>
<artwork>
C: Request authentication exchange
S: Empty Challenge
C: pa=someuser,<initial context token with standard
header removed>
S: Send reply context token as is
...
</artwork>
</figure>
<t>GSS-API authentication is always initiated by the client. The
SASL framework allows either the client and 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 <xref target="RFC4422"/>.</t>
</section>
<section anchor="authcond"
title="Authentication Conditions">
<t>Authentication MUST NOT succeed if any one of the following
conditions are true:</t>
<t>
<list style="symbols">
<t>GSS_Init/Accept_sec_context return anything
other than GSS_S_CONTINUE_NEEDED or
GSS_S_COMPLETE.</t>
<t>If the client's GS2 channel binding flag was "y"
and the server supports channel binding.</t>
<t>If the client requires use of channel binding and
the server did not advertise support for channel
binding.</t>
<t>Authorization of client principal (i.e., src_name
in GSS_Accept_sec_context) to requested
authzid failed.</t>
<t>If the client is not authorized to the requested
authzid or an authzid could not be derived from
the client's initiator principal name.</t>
</list>
</t>
</section>
<section anchor="gssapiparams"
title="GSS-API Parameters">
<t>GS2 does not use any GSS-API per-message tokens.
Therefore the setting of req_flags related to
per-message tokens is irrelevant.</t>
</section>
<section anchor="naming"
title="Naming">
<t>There's no requirement that any particular GSS-API
name-types be used. However, typically SASL servers
will have host-based acceptor principal names (see <xref
target='RFC2743'/> section 4.1) and clients will
typically have username initiator principal names (see
<xref target='RFC2743'/> section 4.2).</t>
</section>
<section anchor="GSS_Inquire_SASLname_for_mech call"
title="GSS_Inquire_SASLname_for_mech call">
<t>To allow SASL implementations to query for the SASL
mechanism name of a GSS-API mechanism, we specify a new
GSS-API function for this purpose.</t>
<figure>
<artwork>
Inputs:
o desired_mech OBJECT IDENTIFIER
Outputs:
o sasl_mech_name UTF-8 STRING -- SASL name for this mechanism
o mech_name UTF-8 STRING -- name of this mechanism, possibly
localized
o mech_description UTF-8 STRING -- possibly localized
description of this mechanism.
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.
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 a human readable 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 named computed from the OID as
described in section 3.1 of this document.
</artwork>
</figure>
<t><vspace blankLines="10000" /></t>
<section title="gss_inquire_saslname_for_mech">
<t>The C binding for the GSS_Inquire_SASLname_for_mech
call is as follows.</t>
<figure>
<artwork>
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
human readable form.
Parameters:
minor_status Integer, modify
Mechanism specific status code.
Function value: GSS status code
GSS_S_COMPLETE Successful completion
GSS_S_BAD_MECH The desired_mech OID is unsupported
</artwork>
</figure>
</section>
</section>
<section anchor="GSS_Inquire_mech_for_SASLname call"
title="GSS_Inquire_mech_for_SASLname call">
<t>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.</t>
<figure>
<artwork>
Inputs:
o sasl_mech_name UTF-8 STRING -- SASL name of mechanism
Outputs:
o mech_type OBJECT IDENTIFIER -- must be explicit mechanism,
and not "default" specifier
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.
The GSS_Inquire_mech_for_SASLname call is used to get the GSS-API
mechanism OID associated with a SASL mechanism name.
</artwork>
</figure>
<t><vspace blankLines="10000" /></t>
<section title="gss_inquire_mech_for_saslname">
<t>The C binding for the GSS_Inquire_mech_for_SASLname
call is as follows.</t>
<figure>
<artwork>
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.
Function value: GSS status code
GSS_S_COMPLETE Successful completion
GSS_S_BAD_MECH The desired_mech OID is unsupported
</artwork>
</figure>
</section>
</section>
<section anchor="seclayers"
title="Security Layers">
<t>GS2 does not currently support SASL security layers.
Applications that need integrity protection or
confidentiality and integrity protection MUST use either
channel binding to a secure external channel or a SASL
mechanism that does provide security layers.</t>
<t>NOTE WELL: the GS2 client's first authentication message
MUST always start with "F", "p", "n" or "y", otherwise
the server MUST fail authentication. This will allow us
to add support for security layers in the future if it
were to become necessary. Note that adding security
layer support to GS2 must not break existing SASL/GS2
applications, which can be accomplished by making
security layers optional.</t>
<t>[A sketch of how to add sec layer support... Add a way
for the client to: a) make an offer of sec layers and
max buffer, b) make an opportunistic selection of sec
layer and buffer size, both in the first client
authentication message, and starting with a character
other than "F", "n", "y" or "p". The server could
accept the opportunistic proposal (reply token prefixed
with a byte indicating acceptance) or reject it along
with an indication of the server's acceptable sec layers
and max buffer size. In the latter case the GSS-API
security context token exchange must be abandoned and
recommenced, although this would be a detail of the GS2
bridge not exposed to the SASL application. The
negotiation would be protected via GSS channel binding,
as with the rest of GS2.]</t>
</section>
<section anchor="gssapi"
title="Interoperability with the SASL GSSAPI mechanism">
<t>The <xref target="RFC1964">Kerberos V5 GSS-API</xref>
mechanism is currently used in SASL under the name
GSSAPI, see <xref target="RFC4752">GSSAPI
mechanism</xref>. The Kerberos V5 mechanism may also
be used with the GS2 family. This causes an
interoperability problem, which is discussed and
resolved below.</t>
<section title="The interoperability problem">
<t>The SASL "GSSAPI" mechanism is not wire-compatible
with the Kerberos V GSS-API mechanism used as a SASL
GS2 mechanism.</t>
<t>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.</t>
</section>
<section title="Resolving the problem">
<t>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.</t>
<t>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 <xref target='RFC5056'/>, <xref target='mitm'/>).</t>
</section>
<section title="Additional Recommendations">
<t>If the application requires security layers then it
MUST prefer the SASL "GSSAPI" mechanism over
"GS2-KRB5" or "GS2-KRB5-PLUS".</t>
<t>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.</t>
</section>
</section>
<section anchor='nego'
title="GSS-API Mechanisms that negotiate other mechanisms">
<t>A GSS-API mechanism that negotiate other mechanisms
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.</t>
<section title="The interoperability problem">
<t>If a client implement GSS-API mechanism X,
potentially negotiated through a GSS-API mechanism
Y, and the server also implement GSS-API mechanism
X negotiated through a GSS-API mechanism Z, the
authentication negotiation will fail.</t>
</section>
<section title="Security problem">
<t>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 other mechanisms (such as
SPNEGO), 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 are disallowed under GS2.</t>
</section>
<section title="Resolving the problems">
<t>GSS-API mechanisms that negotiate other mechanisms
MUST NOT be used with the GS2 SASL mechanism.
Specifically SPNEGO <xref target="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.
[What about SASL apps that don't do mechanism
negotiation? Probably none exist. But if any did
then presumably it would OK to use the SPNEGO
mechanism, no? -Nico]</t>
<t>The <xref
target="I-D.ietf-kitten-extended-mech-inquiry">
GSS_C_MA_MECH_NEGO attribute of
GSS_Inquire_attrs_for_mech</xref> can be used
to identify such mechanisms.</t>
</section>
</section>
<section anchor='iana'
title="IANA Considerations">
<t>The SASL names for the Kerberos V5 GSS-API mechanism <xref
target='RFC4121'/> <xref target='RFC1964'/> used via
GS2 SHALL be "GS2-KRB5" and "GS2-KRB5-PLUS".</t>
<t>The SASL names for the SPNEGO GSS-API mechanism used
via GS2 SHALL be "SPNEGO" and "SPNEGO-PLUS". As
described in <xref target='nego'/> the SASL "SPNEGO"
and "SPNEGO-PLUS" MUST NOT be used. These names are
provided as a convenience for SASL library
implementors.</t>
<t>The IANA is advised that SASL mechanism names starting
with "GS2-" are reserved for SASL mechanisms which
conform to this document. The IANA is directed to place
a statement to that effect in the sasl-mechanisms
registry.</t>
<t>The IANA is further advised that SASL mechanisms MUST NOT
end in "-PLUS" except as a version of another mechanism
name simply suffixed with "-PLUS".</t>
<figure>
<artwork>
Subject: Registration of SASL mechanism GS2-*
SASL mechanism prefix: GS2-
Security considerations: RFC [THIS-DOC]
Published specification: RFC [THIS-DOC]
Person & email address to contact for further information:
Simon Josefsson <simon@josefsson.org>
Intended usage: COMMON
Owner/Change controller: iesg@ietf.org
Note: Compare with the GSSAPI and GSS-SPNEGO mechanisms.
</artwork>
</figure>
</section>
<section anchor="security"
title="Security Considerations">
<t>Security issues are also discussed throughout this
memo.</t>
<t>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.</t>
<t>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 provided that the
GSS-API mechanism used is secure. Authentication
failure because of channel binding 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.</t>
<t>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.</t>
<t>The channel binding data MAY be sent (but 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
<xref target='RFC5056'/>.</t>
<t>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 <xref
target="RFC1034">Domain Name System</xref> without
<xref target="RFC4033">DNSSEC</xref>.</t>
<t>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.</t>
<t>The security considerations of SASL <xref
target='RFC4422'/>, the GSS-API <xref
target="RFC2743"/>, channel binding <xref
target="RFC5056"/>, any external channels (such as
TLS, <xref target="RFC5246"/>, channel binding types
(see the IANA channel binding type registry), and
GSS-API mechanisms (such as the Kerberos V5 mechanism
<xref target="RFC4121"/> <xref target="RFC1964"/>), may
also apply.</t>
</section>
<section anchor="ack"
title="Acknowledgements">
<t>The history of GS2 can be traced to the "GSSAPI"
mechanism originally specified by RFC2222. This
document was derived from draft-ietf-sasl-gssapi-02
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.</t>
<t>Contributions of many members of the SASL mailing list
are gratefully acknowledged. In particular, ideas and
feedback from Sam Hartman, Jeffrey Hutzelman, Alexey
Melnikov, and Tom Yu improved the document and the
protocol.</t>
</section>
</middle>
<back>
<references title="Normative References">
&fips180;
&rfc2119;
&rfc2743;
&rfc3629;
&rfc4422;
&rfc4648;
&rfc5056;
&rfc5234;
&x690;
</references>
<references title="Informative References">
&rfc1034;
&rfc1964;
&rfc2025;
&rfc2222;
&rfc4033;
&rfc4121;
&rfc4178;
&rfc4752;
&rfc5246;
&rfc5280;
&SCRAM;
&EMI;
<reference anchor="mitm">
<front>
<title>Man-in-the-Middle in Tunneled Authentication</title>
<author initials="N." surname="Asokan" fullname="N. Asokan"/>
<author initials="V." surname="Niemi" fullname="V. Niemi"/>
<author initials="K." surname="Nyberg" fullname="K. Nyberg"/>
</front>
<seriesInfo name="WWW"
value="http://www.saunalahti.fi/~asokan/research/mitm.html" />
</reference>
</references>
</back>
</rfc>
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