Network Working Group S. Josefsson
Internet-Draft February 13, 2006
Expires: August 17, 2006
Using GSS-API Mechanisms in SASL: The GS2 Mechanism Family
draft-ietf-sasl-gs2-00
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
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.
See <http://josefsson.org/sasl-gs2-*/> for more information.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . 3
3. Mechanism Name . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Generating SASL mechanism names from GSS-API OIDs . . . . 3
3.2. Computing mechanism names manually . . . . . . . . . . . . 4
3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Packet format . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SASL tokens . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Context token . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Wrap token . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3.1. Wrap token input for client requests . . . . . . . . . 7
4.3.2. Wrap token input for server responses . . . . . . . . 8
4.3.3. Wrap token input for server requests . . . . . . . . . 8
4.3.4. Wrap token input for client responses . . . . . . . . 9
5. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 9
6. Authentication Conditions . . . . . . . . . . . . . . . . . . 12
7. GSS-API parameters . . . . . . . . . . . . . . . . . . . . . . 12
8. Security layer bits . . . . . . . . . . . . . . . . . . . . . 13
9. Interoperability with the GSSAPI mechanism . . . . . . . . . . 13
9.1. Interoperability problem . . . . . . . . . . . . . . . . . 13
9.2. Resolving the problem . . . . . . . . . . . . . . . . . . 13
9.3. Additional recommendations . . . . . . . . . . . . . . . . 13
10. Mechanisms that negotiate other mechanisms . . . . . . . . . . 14
10.1. Interoperability problem . . . . . . . . . . . . . . . . . 14
10.2. Security problem . . . . . . . . . . . . . . . . . . . . . 14
10.3. Resolving the problems . . . . . . . . . . . . . . . . . . 14
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
12. Security Considerations . . . . . . . . . . . . . . . . . . . 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
14. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 16
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
15.1. Normative References . . . . . . . . . . . . . . . . . . . 16
15.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
Generic Security Service Application Program Interface (GSS-API) [3]
is a framework that provide security services to applications.
Simple Authentication and Security Layer (SASL) [2] is a framework to
provide authentication and security layers for connection based
protocols. This document describe how to use a GSS-API mechanism in
a connection-based protocol using the SASL framework.
All GSSAPI mechanisms are implicitly registered for use within SASL
by this specification. The SASL mechanism defined in this document
is known as the GS2 family.
The "Kerberos V5 GSS-API mechanism" [9] is also supported in SASL
through "SASL GSSAPI mechanisms" [11]. The difference between that
protocol and the one described here, is that this protocol offer more
features (i.e., channel bindings and round-trip optimizations) while
the other protocol is more widely deployed. There are
interoperability concerns by having the same GSS-API mechanism
available under more than one SASL mechanism name, see the section
"Interoperability with the GSSAPI mechanism" below.
There are interoperability and security concerns if this SASL
mechanism is used together with a GSS-API mechanism that negotiate
other GSS-API mechanisms (such as SPNEGO [10]), see the section
"Mechanisms that negotiate other mechanisms" below.
SASL mechanism names starting with "GS2-" are reserved for SASL
mechanisms which conform to this document.
The IESG is considered to be the owner of all SASL mechanisms which
conform to this document. This does not necessarily imply that the
IESG is considered to be the owner of the underlying GSSAPI
mechanism.
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 [1].
3. Mechanism Name
3.1. Generating SASL mechanism names from GSS-API OIDs
The SASL mechanism name for a GSS-API mechanism is the concatenation
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of the string "GS2-" and the Base32 encoding [5] (with an upper case
alphabet) of the first ten bytes of the binary SHA-1 hash [4] string
computed over the ASN.1 DER encoding [7] 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.
3.2. Computing mechanism names manually
The 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.
3.3. Example
For example, the OID for the SPKM-1 mechanism [12] is
1.3.6.1.5.5.1.1. The ASN.1 DER encoding of the OID is 06 07 2b 06 01
05 05 01 01. The SHA-1 hash of the ASN.1 DER encoding is
1cf8f42b5a9f80fae9f831226d5d9d56278661ad. The Base32 encoding of the
first ten bytes of this is "dt4pik22t6epv2py". Thus the SASL
mechanism name for the SPKM-1 GSSAPI mechanism is "GS2-
DT4PIK22T6EPV2PY".
4. Packet format
4.1. SASL tokens
All top-level SASL packets for the GS2 mechanism family follow the
following format:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Context length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ Context token /
/ --------------------/
/ ---------------------/ /
/--------------------/ /
/ [Wrap token] /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The "Context length" field is a 4 octet (32 bit) integer encoded in
network byte order, it indicate the length of the "Context token"
field.
The "Context token" field contain a GSS-API context establishment
token generated by GSS_Init_sec_context or GSS_Accept_sec_context.
The "Wrap token" field is optional, and if present will contain the
output generated by GSS_Wrap.
The length field does not include the length of the length field
itself. Whether the "Wrap token" field is included or not can be
infered from the length field; if the length field is shorter than
the entire packet size minus 4 octets, the "Wrap token" field is
present and begins after length+4 octets into the packet. The tokens
need not be aligned to 32-bit a boundary. There is no padding
between the tokens.
Packets shorter than 4 octets are invalid. If the length field is
longer than the entire packet size, minus 4 octets, the packet is
invalid.
4.2. Context token
The format of the "Context token" field inside the SASL token are
defined by the GSS-API specifications, see GSS_Init_sec_context and
GSS_Accept_sec_context.
The client calls GSS_Init_sec_context, passing in
input_context_handle of 0 (initially), mech_type of the GSSAPI
mechanism for which this SASL mechanism is registered, any
chan_binding if requested by the application (XXX?), and targ_name
equal to output_name from GSS_Import_Name called with input_name_type
of GSS_C_NT_HOSTBASED_SERVICE and input_name_string of
"service@hostname" where "service" is the service name specified in
the protocol's profile, and "hostname" is the fully qualified host
name of the server. If the client will be requesting a security
layer, it MUST also supply to the GSS_Init_sec_context a
mutual_req_flag of TRUE, a sequence_req_flag of TRUE, and an
integ_req_flag of TRUE. If the client will be requesting a security
layer providing confidentiality protection, it MUST also supply to
the GSS_Init_sec_context a conf_req_flag of TRUE. If
GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED, then the client
should expect the server to issue a token in a subsequent challenge
or as additional information to the outcome of the authentication.
The client must pass the context token to another call to
GSS_Init_sec_context, repeating the actions in this paragraph, until
GSS_S_COMPLETE is returned or authentication is aborted. If the
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server supply data beyond the context token, the context token should
be processed first, and then the overflow data should be passed to
GSS_Unwrap and the unwrapped data should be interpreted. During the
authentication exchange, the client will generate one Wrap token
using GSS_Wrap.
The server passes the first client response to GSS_Accept_sec_context
as input_token, setting input_context_handle to 0 (initially),
mech_type of the GSSAPI mechanism for which this SASL mechanism is
registered, any chan_binding if requested by the application (XXX?),
and acceptor_cred_handle equal to output_cred_handle from
GSS_Acquire_cred called with desired_name equal to output_name from
GSS_Import_name with input_name_type of GSS_C_NT_HOSTBASED_SERVICE
and input_name_string of "service@hostname" where "service" is the
service name specified in the protocol's profile, and "hostname" is
the fully qualified host name of the server. The server must pass
any resulting challenge from the client to another call to
GSS_Accept_sec_context, repeating the actions in this paragraph,
until GSS_S_COMPLETE is returned or authentication is aborted. If
the client supply data beyond the context token, the context token
should be processed first, and then the overflow data should be
passed to GSS_Unwrap and the unwrapped data should be interpreted.
During the authentication exchange, the server will generate one Wrap
token using GSS_Wrap.
4.3. Wrap token
The Wrap token MUST NOT be sent before the PROT_READY flag has been
set locally (by GSS_Init_sec_context or Gss_Accept_sec_context), or
if the PROT_READY flag is never set, before the context has been
fully established. The GSS_Wrap token does not have to be sent
directly when the PROT_READY flag is set. During any exchange,
exactly one GSS_Wrap token is sent in each direction. The input to
the GSS_Wrap function MUST follow the format described below. If not
exactly one GSS_Wrap token is received by the client and by the
server, the authentication MUST fail.
If PROT_READY is never set by GSS_Init_sec_context or
GSS_Accept_sec_context, the flag is implied by successful context
negotiation. This is for GSS-API v1 mechanisms that do not support
PROT_READY. This may result in a SASL token consisting of a context
token length of 0 and a Wrap token.
The entity that sends its first Wrap token will have to specify a
bitmap of supported and preferred quality of protection schemes. The
entity that reply to the Wrap tokens will pick a scheme, based on the
bitmask and local policy.
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Four input formats to the GSS_Wrap function are defined. The first
two input formats are used when the client sends the GSS_Wrap token
first and the server reponds. The other two input formats are used
when the server sends the GSS_Wrap token first and the client
responds.
The input formats below are passed to GSS_Wrap with conf_flag set to
FALSE, and the Wrap token output will be the generated
output_message.
Some fields in the input formats are optional, indicated by brackets
("[" and "]") and explained by the text below.
4.3.1. Wrap token input for client requests
The input to GSS_Wrap when the client sends the GSS_Wrap token first
is as follows.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client_qops | client_maxbuf |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| channel_binding_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|[client_cbqops]| [channel_binding_data] /
/ /
/ / [authzid] /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "client_qops" integer indicate the client's preferred quality of
protection if channel bindings are absent or the negotiation of the
channel binding fails.
The "client_maxbuf" field indicate the maximum protected buffer size
the client can receive.
The "channel_binding_length" is a network byte order integer that
indicate the length of the "channel_binding_data" field.
The optional field "client_cbqops" is present only if
"channel_binding_length" is non-zero, and indicate the client's
preferred quality of protection if channel binding negotiation
succeeds.
The optional field "channel_binding_data" is present only if
"channel_binding_length" is non-zero, and contain the actual channel
binding data.
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The optional field "authzid" contain the authorization identity. The
authorization identity is encoded using UTF-8 [6]. The authorization
identity is not terminated with the NUL (U+0000) character. Servers
MUST validate that the authorization identity is valid UTF-8.
4.3.2. Wrap token input for server responses
The data format for input to GSS_Wrap when the server responds to the
previous GSS_Wrap token data is as follows.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_qop | server_maxbuf |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "server_qop" field integer indicate the selected quality of
protection.
The "server_maxbuf" field indicate the maximum data buffer size the
server can receive. It MUST be 0 if the server doesn't advertise
support for any security layer, the client MUST verify this.
4.3.3. Wrap token input for server requests
The data format for input to GSS_Wrap when the server sends the
GSS_Wrap token first is as follows.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_qops | server_maxbuf |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| channel_binding_length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|[server_cbqops]| [channel_binding_data] /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "server_qops" field is an integer indicating the server's
preferred quality of protection if channel bindings are absent or the
negotiation of the channel binding fails.
The "server_maxbuf" is the same as above.
The "channel_binding_length" is a network byte order integer that
indicate the length of the "channel_binding_data" field.
The optional field "server_cbqops" is present only if
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"channel_binding_length" is non-zero, and indicate the server's
preferred quality of protection if channel binding negotiation
succeeds.
The optional field "channel_binding_data" is present only if
"channel_binding_length" is non-zero, and contain the actual channel
binding data.
4.3.4. Wrap token input for client responses
The data format for input to GSS_Wrap when the client responds to the
previous token is as follows.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| client_qop | client_maxbuf |
/ [authzid] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "client_qop" field is the selected quality of protection.
The "client_maxbuf" and "authzid" fields are as above.
5. Protocol overview
This section describe several high-level protocol exchanges. The
descriptions do not assume any properties of the actual GSS-API
mechanism. Protocol profiles, GSS-API mechanism specific behaviour,
and to some extent implementation and policy choices, will dictate
which packets are sent in what order. The protocol exchanges are
examples and other exchanges are permitted and will occur.
An authentication exchange using GS2 may look like:
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C: Request authentication exchange
S: Send [length=0] token
C: Send [length, GSS_Init_sec_context] token
...
S: After PROT_READY is set,
send [length, GSS_Accept_sec_context,
GSS_Wrap(server_qops, server_maxbuf,
channel_binding_length=0)]
...
C: After PROT_READY is set,
send [length, GSS_Init_sec_context,
GSS_Wrap (client_qop, client_maxbuf, authzid)]
S: Send [length, GSS_Accept_sec_context] token
C: Send [length, GSS_Init_sec_context] token
...
S: Outcome of authentication exchange
Because GSS-API authentication is initiated by the client, the length
field will be 0 in the initial token from the server to the client
when the protocol profile do not support additional information to be
sent together with the authentication request.
The next example illustrate when the client sends its Wrap token
first.
C: Request authentication exchange
S: Send [length=0] token
C: Send [length, GSS_Init_sec_context] token
...
C: After PROT_READY is set,
send [length, GSS_Init_sec_context,
GSS_Wrap(client_qops, client_maxbuf,
channel_binding_length=0, authzid)]
...
S: After PROT_READY is set,
send [length, GSS_Accept_sec_context,
GSS_Wrap (server_qop, server_maxbuf)]
C: Send [length, GSS_Init_sec_context] token
S: Send [length, GSS_Accept_sec_context] token
...
S: Outcome of authentication exchange
If the protocol profile support the optional initial client response,
the first empty message can be optimized away, and then the protocol
exchange will look like:
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C: Request authentication exchange and
send [length, GSS_Init_sec_context] token
S: Send [length, GSS_Accept_sec_context] token
...
S: Outcome of authentication exchange
If the protocol profile can also send additional information when
indicating the outcome of the authentication, then the protocol
exchange will look like:
C: Request authentication exchange and
send [length, GSS_Init_sec_context] token
S: Send [length, GSS_Accept_sec_context] token
...
C: Send [length, GSS_Init_sec_context] token
S: Indicate successful authentication and
send [length, GSS_Accept_sec_context] token
as additional information.
If the PROT_READY flag is never set by the GSS-API mechanism, the
GSS_Wrap message will be sent after the context has been established.
The protocol may look like:
C: Request authentication exchange
...
S: GSS_Accept_sec_context() returns GSS_S_COMPLETE and outputs
a token, send [length, context token,
GSS_Wrap(server_qops, server_maxbuf,
channel_binding_length=0)]
C: GSS_Init_sec_context() returns GSS_S_COMPLETE and does not
output a token, send
[length=0, context token,
GSS_Wrap(client_qop, client_maxbuf,
channel_binding_length=0, authzid)]
S: Outcome of authentication exchange
Alternatively, if the client finishes first, it may look like:
C: Request authentication exchange
...
C: GSS_Init_sec_context() returns GSS_S_COMPLETE and outputs a
token, send [length, context token,
GSS_Wrap(client_qops, client_maxbuf,
channel_binding_length=0, authzid)]
S: GSS_Accept_sec_context() returns GSS_S_COMPLETE and does not
output a token, send [length, context token,
GSS_Wrap(server_qop, server_maxbuf,
channel_binding_length=0)]
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S: Outcome of authentication exchange
If the protocol support initial data from the client, and the
PROT_READY flag is set in the client after the first call to
GSS_Init_sec_context, and the server can send additional data to the
client when indicating successful authentication, the following
protocol exchange will occur.
C: Request authentication exchange and
send [length, GSS_Init_sec_context,
GSS_Wrap (client_qops, client_maxbuf,
channel_binding_length=0, authzid)] token
S: Indicate successful authentication and
send [length, GSS_Accept_sec_context,
GSS_Wrap(server_qop, server_maxbuf)] token
as additional information.
The last example illustrate the optimal (round-trip wise)
authentication possible using this protocol.
6. Authentication Conditions
Authentication MUST NOT succeed if any one of the following
conditions are true:
o An invalid SASL token is received (i.e., length shorter than 4
octets).
o GSS_Init/Accept_sec_context() return anything other than
GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE.
o GSS_Wrap() returns anything other than GSS_S_COMPLETE.
o GSS_Unwrap() returns anything other than GSS_S_COMPLETE. (There
can't be supplementary status codes in GS2 at this point, so any
indications of out of order processing or replays should be
fatal.)
o The token returned from GSS_Unwrap fail to parse correctly (e.g.,
too short, invalid maximum buffer size) as the expected Wrap
token.
o Local policy reject the attempt. For example, client and server
can't agree on qop proposal.
o (server-side) Authorization of client principal (i.e., src_name in
GSS_Acecpt_sec_context) to requested authzid failed.
7. GSS-API parameters
The implementation MAY set any GSSAPI flags or arguments not
mentioned in this specification as is necessary for the
implementation to enforce its security policy.
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8. Security layer bits
The security layers and their corresponding bit-masks are as follows:
1 No security layer
2 Integrity protection.
Sender calls GSS_Wrap with conf_flag set to FALSE
4 Confidentiality protection.
Sender calls GSS_Wrap with conf_flag set to TRUE
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
Note that SASL negotiates the maximum size of the output_message to
send. Implementations can use the GSS_Wrap_size_limit call to
determine the corresponding maximum size input_message.
9. Interoperability with the GSSAPI mechanism
The GSSAPI mechanism [11] describe how the Kerberos V5 GSS-API
mechanism [9] is used in SASL under the mechanism name "GSSAPI". The
same mechanism may also be used with the GS2 family. This causes an
interopability problem, which is discussed and resolved below.
9.1. Interoperability problem
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 authentication negotiation will fail.
9.2. 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 Kerberos V5 "GSSAPI" SASL mechanism lack channel bindings, which
could enable certain tunnel attacks [16].
9.3. Additional recommendations
It is RECOMMENDED to negotiate Kerberos V5 through the GS2 mechanism
rather than through the "GSSAPI" mechanism, if both are available,
because of the additional features in the GS2 mechanism.
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10. Mechanisms that negotiate other mechanisms
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.
10.1. 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
mechanism X negotiated through a GSS-API mechanism Z, the
authentication negotiation will fail.
10.2. 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
other mechanisms (such as SPNEGO), it may end up using mechanism Z
when it should have used mechanism Y. For this reason, the use of
GSS-API mechanisms that negotiate other mechanisms are disallowed
under GS2.
10.3. Resolving the problems
GSS-API mechanisms that negotiate other mechanisms MUST NOT be used
with the GS2 SASL mechanism. This specifically exclude negotiating
SPNEGO [10] under GS2.
The GSS_C_MA_MECH_NEGO attribute of GSS_Inquire_attrs_for_mech() [15]
can be used to identify such mechanisms.
11. IANA Considerations
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.
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Subject: Registration of SASL mechanism GS2-*
Family of SASL mechanisms: YES
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.
12. Security Considerations
Security issues are discussed throughout this memo.
When a server or client supports multiple authentication mechanisms,
each of which has a different security strength, it is possible for
an active attacker to cause a party to use the least secure mechanism
supported. There are several ways to mitigate this problem:
1. Integrity protected transports can be used, e.g., TLS [13]. To
protect against certain tunnel attacks [16] with that solution, a
mechanism that support channel bindings that can bind the
security layer (e.g., the TLS session id) to the authentication
is required.
2. A client or server which supports mechanisms of different
strengths should have a configurable minimum strength that it
will use. It is not sufficient for this minimum strength check
to only be on the server, since an active attacker can change
which mechanisms the client sees as being supported, causing the
client to send authentication credentials for its weakest
supported mechanism.
Because the negotiation of a GSS-API mechanism may be done in the
clear, it is important for the GSS-API mechanisms to be designed such
that an active attacker cannot obtain an authentication with weaker
security properties by modifying the challenges and responses.
The integrity protection provided by the security layer is useless to
the client unless the client also requests mutual authentication.
Therefore, a client wishing to benefit from the integrity protection
of a security layer MUST pass to the GSS_Init_sec_context call a
mutual_req_flag of TRUE.
When constructing the input_name_string, the client should not
canonicalize the server's fully qualified domain name using an
insecure or untrusted directory service, e.g., the Domain Name System
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[8] without DNSSEC [14].
Additional security considerations are in the SASL and GSSAPI
specifications. Additional security considerations for the Kerberos
V5 GSSAPI mechanism can be found in [9]. We stress that service
names should not be canonicalized using an unsecured directory
service such as the DNS without DNSSEC.
13. Acknowledgements
This document is a revision of RFC 2222. This version was derived
from draft-ietf-sasl-gssapi-02 which was prepared by Alexey Melnikov
with significant contributions from John G. Myers, although this
document has been rewritten by the current author.
Contributions of many members of the SASL mailing list are gratefully
acknowledged. In particular, ideas from Sam Hartman, Jeffrey
Hutzelman, and Nicolas Williams influenced the design of this
protocol.
14. Copying conditions
Regarding the portion of this document that was written by Simon
Josefsson ("the author", for the remainder of this section), the
author makes no guarantees and is not responsible for any damage
resulting from its use. The author grants irrevocable permission to
anyone to use, modify, and distribute it in any way that does not
diminish the rights of anyone else to use, modify, and distribute it,
provided that redistributed derivative works do not contain
misleading author or version information. Derivative works need not
be licensed under similar terms. Contact the author to confirm which
sections are available under this license.
15. References
15.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
[3] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
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[4] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001.
[5] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
draft-josefsson-rfc3548bis-00 (work in progress), November 2005.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[7] "Information Processing Systems - Open Systems Interconnection -
Specification of Abstract Syntax Notation One (ASN.1)", ISO
Standard 8824.
15.2. Informative References
[8] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[9] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
June 1996.
[10] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.
[11] Melnikov, A., "SASL GSSAPI mechanisms",
draft-ietf-sasl-gssapi-03 (work in progress), September 2005.
[12] Adams, C., "The Simple Public-Key GSS-API Mechanism (SPKM)",
RFC 2025, October 1996.
[13] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[14] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[15] Williams, N., "Extended Generic Security Service Mechanism
Inquiry APIs", draft-ietf-kitten-extended-mech-inquiry-01 (work
in progress), October 2005.
[16] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in
Tunneled Authentication",
WWW http://www.saunalahti.fi/~asokan/research/mitm.html.
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Author's Address
Simon Josefsson
Email: simon@josefsson.org
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