Network Working Group Johansson
Internet-Draft SU
Intended status: Standards Track November 2, 2008
Expires: May 6, 2009
GSSAPI authentication for HTTP
draft-johansson-http-gss-04
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Abstract
This document specifies a template extension to the HTTP Negotiate
authentication mechanism defined in RFC4559 which supports mutual
authentication, fast session-based re-authentication and channel
bindings. An IANA registry for such GSS-API HTTP authentication
mechanisms is defined.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction and motivation . . . . . . . . . . . . . . . . . 4
3. HTTP GSS Authentication Mechanism . . . . . . . . . . . . . . 5
3.1. GSS Token Header Syntax . . . . . . . . . . . . . . . . . 5
3.2. Naming and Transport . . . . . . . . . . . . . . . . . . . 5
3.3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Intiating authentication . . . . . . . . . . . . . . . 6
3.3.2. The authentication phase . . . . . . . . . . . . . . . 7
3.3.3. The authorization phase . . . . . . . . . . . . . . . 8
3.3.4. Fast Renegotiation . . . . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Notes & TODO . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8.1. Registration Procedure . . . . . . . . . . . . . . . . . . 14
8.2. Change Control . . . . . . . . . . . . . . . . . . . . . . 15
9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2. 02 to 03 . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.3. 03 to 04 . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Terminology
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT"
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119]
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2. Introduction and motivation
[RFC4559] describes an authentication mechanism based on SPNEGO for
HTTP. This mechanism suffers from a couple of drawbacks, notably:
Only supports single-round-trip GSS-API mechanisms due to lack of
support for proxies.
Lack of channel bindings to the underlying HTTPS connection which
makes in unsuitable for deployment in situations where proxies
exists.
Lack of session-based re-authentication (compare with TLS).
This document is intended to solve these problems by introducing a
new authentication mechanism called 'GSS'. This mechanism is a
proper extension of Negotiate but since Negotiate is already widely
deployed this mechanism was given a separate name.
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3. HTTP GSS Authentication Mechanism
The GSS mechanism is an authentication mechanism for [RFC2616] based
on a multi-roundrip handshake using base64-encoded GSS-API [RFC2743]
tokens encoded in the WWW-Authenticate Response Header and the
Authorization Request Header. An important difference from [RFC4559]
is that multiple round trips are supported which means that the
server can be authenticated to the client (aka mutual
authentication). This document specifies a template authentication
mechanism with an associated IANA registry which provides input
parameters to the HTTP authentication mechanism describe below.
3.1. GSS Token Header Syntax
Both the Authorization and the WWW-Authenticate headers use the same
syntax throughout the handshake (cf below for details on the protocol
flow) specified by this Augmented BNF following [RFC2617] and
[RFC2616]:
challenge = auth-scheme-name 1*SP 1#auth-param
auth-scheme-name = token
auth-param = ( auth-data-value |
auth-param-type "=" auth-param-value )
auth-param-value = ( token | quoted-string )
auth-param-type = ( "auth-data" | "context-identifier" )
auth-data-value = 1*(UPALPHA|DIGIT) ;base64-encoded
context-identifier-value = 1*(UPALPHA|DIGIT) ;base64-encoded
The auth-param types defined by this specification (auth-data and
context-identifier) both have auth-param-value which contain base64
encoded data. Note that both the auth-data and context-identifier
auth-param may be absent. The semantics of these parameters will be
explained below. Each auth-param-type MUST NOT occur more than once
in a single challenge.
The auth-scheme-name token is the name of the mechanism and is
supplied in the IANA registry template described below.
For reasons of backwards compatibility with [RFC4559] two forms of
the auth-param are allowed - one version based on attribute-parameter
pairs and one where only GSS-API data is sent.
3.2. Naming and Transport
The GSS name of the server is "HTTP@<hostname>[:port]" where the
:port part is absent if the port == 80 or if the port == 443.
This mechanism SHOULD be used together with an HTTP transport
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providing session protection and encryption such as [RFC2817] or
[RFC2818] . Session protection is a requirement for fast re-
authentication described below.
Like [RFC4559] the mechanism described in this specification is based
on mapping the GSS-API protocol to HTTP requests and responses where
the GSS-API tokens are sent in the Authorization and WWW-
Authentication headers. Unlike [RFC4559] the entire handshake need
not take place using a single TCP connection or a single HTTP/1.1
session. Instead opaque identifiers in the GSS challenge option
field are optionally used together with channel bindings to provide a
way to share a security context over several HTTP connections. This
mechanism also serves as a way to let the client do a fast re-
authentication to the server.
3.3. Protocol Flow
3.3.1. Intiating authentication
Normally the server initiates an authentication handshake when the
client attempts to access a protected resource. The exception is
when the client knows that it is accessing a protected resource and
that the server supports the GSS mechanism, for instance when fast
re-authentication is attempted by the client (cf below). In both
cases the GSS-API negotiation is initiated by the client - i.e if the
server initiates the authentication it is only to inform the client
that authentication is required. The client SHOULD request mutual
authentication from the GSS-API layer.
Note that the first request by the client to a protected resource
will also serve to let the client and server establish channel
bindings in the sense of [RFC5056] using the 'tls-server-end-point'
CB type which means that this first request is not in general
"wasted" even in the case when the client has no prior knowledge
about the server or is attempting fast re-authentication.
If the client tries to access a protected resource the server may
return a code 401 response with an WWW-Authenticate header containing
a list of authentication challenges allowing the client to choose
among different authentication mechanisms supported by the server.
If the server supports the mechanism specified by the auth-scheme-
name the server returns a challenge with only the auth-scheme-name
part and no parameters along with any other challenges for mechanisms
supported by the server. This first request also allows the client
and server to establish channel-bindings.
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3.3.2. The authentication phase
In each case below when GSS-API tokens resulting from calls into the
GSS-API layer are sent from the server to the client or vice-versa,
the token is encoded using base64 and sent as the "auth-data"
parameter value of the Authorization and WWW-Authenticate headers
respectively.
A client initiates the authentication phase by sending the token
resulting from the first call to gss_init_security_context to the
server.
Upon receipt of token (i.e a request with an accompanying
Authenticate header with non-empty "auth-data" parameter value), the
server MUST return the token resulting from a call to
gss_accept_security_context in a code 401 response, unless the call
to gss_accept_security_context fails in which case a code 403
response is returned.
If the underlying transport provides session protection (eg HTTPS)
and if channel-bindings are in place (cf below) then the server MAY
include a unique identifier of the security context beeing negotiated
(or having been negotiated in the case of the last transaction) in
the "context-identifier" parameter value. The server MUST uniquely
associate this identifier with the client and the security context.
Upon receipt of a code 401 response from the server when the WWW-
Authenticate header contains a non-empty "auth-data" parameter value,
the client MUST return the token resulting from a call to
gss_initiate_security_context to the server in a new request to the
same resource. If the call fails the client MUST close the
connection. If a "context-identifier" parameter value is present in
the response from the server the client MUST include this in the
ensuing request as the "context-identifier" parameter value. If the
"context-identifier" parameter value is not present in the response
from the server the client MUST use the same HTTP/1.1 connection for
the entier handshake. If the client breaks the HTTP/1.1 connection
the server MUST invalidate the security context unless a context
identifier was sent to the client and returned to the server.
A client may close the connection both as the result of using the
context-identifier to spread the authentication over several
underlying connections or as the result of a failed call to
gss_initiate_security_context. This might at first seem like a
problem but the GSS-API layer combined with proper handling of the
context identifier will ensure that handling of these cases are
disambiguated at the server.
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The client and server continues the handshake until either an error
occurs (in which case a 403 is returned to the client or the client
closes the connection depending on where the error happens) or the
GSS-API layer has successfully completed the negotiation in which
case the server sends a normal response to the client. If the last
call to gss_accept_sec_context on the server resulted in a non-empty
token the server MUST include this in a WWW-Authenticate header in
the response to the client regardless of the return code which is
beeing sent to the client. If the underlying transport provides
session protection (eg HTTPS) and if channel-bindings are in place
(cf below) then the server MAY include a "context-identifier"
parameter value uniquely identifying the established security
context. The server MAY decide to limit the validity of the
established context and MAY choose not to consider references to the
context after a certain amount of time (cf below).
If the client receives a normal response with an non-empty "auth-
data" parameter value the client MUST call gss_initiate_sec_context
with this token as input to complete the authentication handshake.
If the final response contains a "context-identifier" parameter value
the client may cache it and use it to provide fast re-authentication
by including it in a Authorization header with auth-scheme-name and
empty "auth-data" parameter value.
3.3.3. The authorization phase
Authorization failures can occur even if the client is successfully
authenticated to the server. In this case the server will send a 403
response to the client even though the GSS-API handshake has
succeeded. It is important to let the client and server finish the
authentication handshake even if the client is not authorized to
access the resource. Therefore the client MUST call
gss_initiate_sec_context with any GSS-API token returned to the
client, even if the token was sent along with a 403 response.
During authorization the server MAY use the GSS-API name associated
with the established security context for authorization decisions and
should provide a string representation of the GSS-API name as the
REMOTE_USER meta-variable and the auth-scheme-name as the AUTH_TYPE
meta-variable if the Common Gateway Interface (CGI) is provided by
the server.
3.3.4. Fast Renegotiation
Upon receipt of a request containing an Authorization header with the
auth-scheme-name, an empty auth-data and the context-identifier
parameter value, the server MUST verify that the identifier
references a valid security context. If the security context is
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missing or invalid the server MUST return a 401 response prompting
the client to re-negotiate the security context. If the identifier
references a valid security context the server MUST process the
request as if the client had just completed the full authentication
handshake.
When this process is completed the client is authenticated to the
server and possibly (depending on the way the GSS-API layer was
called and which GSS-mechanism was used) the server is authenticated
to the client.
The use of fast regegotiation is optional and clients and servers
MUST NOT assume that this feature is supported.
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4. Examples
TODO
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5. Implementation Notes
The context-identifier could be produced by exporting the security
context through gss_export_sec_context which requires that the GSS-
API implementation supports exporting unfinished contexts.
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6. Security Considerations
Should channel-bindings be absent, the protocol is subject to a MITM
attack unless the authentication is between a client and a server
with no proxies in between and each request is sent over the same
HTTP/1.1 connection.
If fast re-authentication is used together with GSS-API credentials
delegation the server will need to associate forwarded credentials
with the negotiated security context. This presents a challenge for
server implementors since it must be guarateed that security states
and their associated credentials must be separated from each other.
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7. Notes & TODO
Examples
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8. IANA Considerations
IANA will create a new registry for HTTP authentication mechanisms
based on this document. The purpose of the registry is to bind the
HTTP authentication mechanism name (auth-scheme-name in the syntax
above) to the GSS-API mechanism OID. Such HTTP authentication
mechanisms will be called GSS-API HTTP authentication mechanisms.
Names for GSS-API HTTP authentication mechanisms must follow the
token syntax of section 2.2 of [RFC2616].
The procedure detailed in the section below is to be used for
registration of a value naming a specific GSS-API HTTP authentication
mechanism.
8.1. Registration Procedure
Registration of a new GSS-API HTTP authentication mechanism requires
expert review as defined in BCP 26 [RFC2434]. Registration of a GSS-
API HTTP authentication mechanism is requested by filling in the
following template:
Subject: Registration of GSS-API HTTP authentication mechanism X
GSS-API HTTP authentication mechanism name:
GSS-API mechanism OID:
Description or Published Specification:
State management: (one of INTERNAL or EXTERNAL)
Intended usage: (one of COMMON, LIMITED USE, OBSOLETE)
Person and email address to contact for further information:
Change manager name and email address:
Expert reviewer name and contact information: (leave blank)
Note: (Any other information deemed relevant)
and sending it via electronic mail to <gss-http@ietf.org> (a public
mailing list) and carbon copying (cc:) IANA at <iana@iana.org>.
After allowing new fewer than 2 weeks for community input on the
mailing list to be determined, an expert will determine the
appropriateness of the registration request and either approve or
disapprove the request with notice to the requester, the mailing list
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and IANA.
If the registration was approved the expert adds her name to the
submitted registration.
The expert is responsible for making sure that GSS-API authentication
scheme names are unique among all HTTP authentication mechanism names
and represent an appropriate name for the underlying GSS-API
mechanism.
Authors are encouraged to pursue community review by posting the
technical specification as an Internet-Draft and soliciting comment
by posting to appropriate IETF mailing lists.
8.2. Change Control
Once a GSS-API HTTP authentication mechanism has been published by
IANA, the author may request a change to its definition. The change
request follows the same procedure as the registration request. The
change manager is part of the registration template and controls who
may request changes to the registration. Passing control of a
registration is also accomplished by submitting a change request.
The IESG may also reassign control and responsibility for GSS-API
HTTP authentication mechanism registrations. This is expected to
happen when the author of a registration has died, has moved out of
contact, or is otherwise unable to make changes to the registered
mechanism(s)s. Furthermore the IESG is the owner of all GSS-API HTTP
authentication mechanisms that correspond to specifications on the
IETF standards track.
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9. Changes
9.1. 00 to 01
Changed from ABNF to Augmented BNF to align with [RFC2616].
9.2. 02 to 03
Added reference to rfc 5056.
Reference to tls-server-end-point channel binding mechanism.
9.3. 03 to 04
Generalized to IANA-controlled registry of authentication mechanisms.
Wrote IANA considerations section. Generalized the ABNF to cover old
Negotiate case which can now be turned into an IANA registration
covered by this specification.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
10.2. Informative References
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, June 2006.
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Author's Address
Leif Johansson
Stockholm university
Email: leifj@it.su.se
URI: http://www.su.se/
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