Network Working Group K. Wierenga
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track E. Lear
Expires: March 28, 2011 Cisco Systems GmbH
S. Josefsson
SJD AB
September 24, 2010
A SASL and GSS-API Mechanism for SAML
draft-ietf-kitten-sasl-saml-00.txt
Abstract
Security Assertion Markup Language (SAML) has found its usage on the
Internet for Web Single Sign-On. Simple Authentication and Security
Layer (SASL) and the Generic Security Service Application Program
Interface (GSS-API) are application frameworks to generalize
authentication. This memo specifies a SASL mechanism and a GSS-API
mechanism for SAML 2.0 that allows the integration of existing SAML
Identity Providers with applications using SASL and GSS-API.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 28, 2011.
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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability for non-HTTP Use Cases . . . . . . . . . . . . . 6
4. SAML SASL Mechanism Specification . . . . . . . . . . . . . . 9
4.1. Advertisement . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Initiation . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Server Redirect . . . . . . . . . . . . . . . . . . . . . 9
4.4. Client Empty Response and other . . . . . . . . . . . . . 9
4.5. Outcome and parameters . . . . . . . . . . . . . . . . . . 9
5. SAML GSS-API Mechanism Specification . . . . . . . . . . . . . 10
5.1. GSS-API Principal Name Types for SAML . . . . . . . . . . 10
6. Channel Binding . . . . . . . . . . . . . . . . . . . . . . . 11
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8.1. Binding SAML subject identifiers to Authorization
Identities . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2. User Privacy . . . . . . . . . . . . . . . . . . . . . . . 17
8.3. Collusion between RPs . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Normative References . . . . . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 20
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Security Assertion Markup Language (SAML) 2.0
[OASIS.saml-core-2.0-os] is a modular specification that provides
various means for a user to be identified to a relying party (RP)
through the exchange of (typically signed) assertions issued by an
identity provider (IdP). It includes a number of protocols, protocol
bindings [OASIS.saml-bindings-2.0-os], and interoperability profiles
[OASIS.saml-profiles-2.0-os] designed for different use cases.
Simple Authentication and Security Layer (SASL) [RFC4422] is a
generalized mechanism for identifying and authenticating a user and
for optionally negotiating a security layer for subsequent protocol
interactions. SASL is used by application protocols like IMAP, POP
and XMPP. The effect is to make modular authentication, so that
newer authentication mechanisms can be added as needed. This memo
specifies just such a mechanism.
The Generic Security Service Application Program Interface (GSS-API)
[RFC2743] provides a framework for applications to support multiple
authentication mechanisms through a unified programming interface.
This document defines a pure SASL mechanism for SAML, but it conforms
to the new bridge between SASL and the GSS-API called GS2 [RFC5801].
This means that this document defines both a SASL mechanism and a
GSS-API mechanism. We want to point out that the GSS-API interface
is optional for SASL implementers, and the GSS-API considerations can
be avoided in environments that uses SASL directly without GSS-API.
As currently envisioned, this mechanism is to allow the interworking
between SASL and SAML in order to assert identity and other
attributes to relying parties. As such, while servers (as relying
parties) will advertise SASL mechanisms (including SAML), clients
will select the SAML SASL mechanism as their SASL mechanism of
choice.
The SAML mechanism described in this memo aims to re-use the
available SAML deployment to a maximum extent and therefore does not
establish a separate authentication, integrity and confidentiality
mechanism. It is anticipated that existing security layers, such as
Transport Layer Security (TLS), will continued to be used.
Figure 1 describes the interworking between SAML and SASL: this
document requires enhancements to the Relying Party and to the Client
(as the two SASL communication end points) but no changes to the SAML
Identity Provider are necessary. To accomplish this goal some
indirect messaging is tunneled within SASL, and some use of external
methods is made.
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+-----------+
| |
>| Relying |
/ | Party |
// | |
// +-----------+
SAML/ // ^
HTTPs // +--|--+
// | S| |
/ S | A| |
// A | M| |
// S | L| |
// L | | |
// | | |
| Client |
| Provider | | |
+------------+ +----------+
Figure 1: Interworking Architecture
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2. Terminology
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 RFC 2119 [RFC2119].
The reader is assumed to be familiar with the terms used in the SAML
2.0 specification.
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3. Applicability for non-HTTP Use Cases
While SAML itself is merely a markup language, its common use case
these days is with HTTP. What follows is a typical flow:
1. The browser requests a resource of a Relying Party (RP) (via an
HTTP request).
2. The RP sends an HTTP redirect as described in Section 10.3 of
[RFC2616] to the browser to the Identity Provider (IdP) or an IdP
discovery service with an authentication request that contains
the name of resource being requested, some sort of a cookie and a
return URL,
3. The user authenticates to the IdP and perhaps authorizes the
authentication to the service provider.
4. In its authentication response, the IdP redirects the browser
back to the RP with an authentication assertion (stating that the
IdP vouches that the subject has successfully authenticated),
optionally along with some additional attributes.
5. RP now has sufficient identity information to approve access to
the resource or not, and acts accordingly. The authentication is
concluded.
When considering this flow in the context of SASL, we note that while
the RP and the client both must change their code to implement this
SASL mechanism, the IdP must remain untouched. The RP already has
some sort of session (probably a TCP connection) established with the
client. However, it may be necessary to redirect a SASL client to
another application or handler. This will be discussed below. The
steps are shown from below:
1. The Relying Party or SASL server advertises support for the SASL
SAML20 mechanism to the client
2. The client initiates a SASL authentication with SAML20 and sends
an IdP identity
3. The Relying Party transmits an authentication request encoded
using a Universal Resource Identifier (URI) as described in RFC
3986 [RFC3986] and a redirect to the IdP
4. The SASL client now sends an empty response, as authentication
continues via the normal SAML flow.
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5. At this point the SASL client MUST construct a URL containing the
content received in the previous message from the RP. This URL
is transmitted to the IdP either by the SASL client application
or an appropriate handler, such as a browser.
6. Next the client authenticates to the IdP. The manner in which
the end user is authenticated to the IdP and any policies
surrounding such authentication is out of scope for SAML and
hence for this draft. This step happens out of band from SASL.
7. The IdP will convey information about the success or failure of
the authentication back to the the RP in the form of an
Authentication Statement or failure, using a indirect response
via the client browser or the handler. This step happens out of
band from SASL.
8. The SASL Server sends an appropriate SASL response to the client,
along with an optional list of attributes
Please note: What is described here is the case in which the client
has not previously authenticated. If the client can handle SAML
internally it is possible that the client already holds a valid SAML
authentication token so that the user does not need to be involved in
the process anymore, but that would still be external to SASL.
With all of this in mind, the flow appears as follows:
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SASL Serv. Client IdP
|>-----(1)----->| | Advertisement
| | |
|<-----(2)-----<| | Initiation
| | |
|>-----(3)----->| | Authentication Request
| | |
|<-----(4)-----<| | Empty Response
| | |
| |< - - - - - ->| Client<>IDP
| | | Authentication
| | |
|<- - - - - - - - - - - - - - -| Authentication Statement
| | |
|>-----(6)----->| | SASL completion with
| | | status
| | |
----- = SASL
- - - = HTTP or HTTPs (external to SASL)
Figure 2: Authentication flow
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4. SAML SASL Mechanism Specification
Based on the previous figure, the following operations are performed
with the SAML SASL mechanism:
4.1. Advertisement
To advertise that a server supports SAML 2.0, during application
session initiation, it displays the name "SAML20" in the list of
supported SASL mechanisms.
4.2. Initiation
A client initiates a "SAML20" authentication with SASL by sending the
GS2 header followed by the authentication identifier. The GS2 header
carries the optional authorization identity.
initial-response = gs2-header Idp-Identifier
IdP-Identifier = Identifier ; IdP identifier
Identifier = URI ; IdP URI
The "gs2-header" is specified in [RFC5801], and it is used as
follows. The "gs2-nonstd-flag" MUST NOT be present. Regarding the
channel binding "gs2-cb-flag" field, see Section 5. The "gs2-
authzid" carries the optional authorization identity. URI is
specified in [RFC3986].
4.3. Server Redirect
The SASL Server transmits a redirect to the IdP that the user
provided, with a SAML authentication request in the form of a SAML
assertion as one of the parameters.
4.4. Client Empty Response and other
The SASL client hands the URI it received from the server in the
previous step to either a browser or other appropriate handler to
continue authentication externally while sending an empty response to
the SASL server. The URI is encoded according to Section 3.4 of the
SAML bindings 2.0 specification [OASIS.saml-bindings-2.0-os].
4.5. Outcome and parameters
The SAML authentication having completed externally, the SASL server
will transmit the outcome.
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5. SAML GSS-API Mechanism Specification
This section and its sub-sections and all normative references of it
not referenced elsewhere in this document are INFORMATIONAL for SASL
implementors, but they are NORMATIVE for GSS-API implementors.
The SAML SASL mechanism is actually also a GSS-API mechanism. The
messages are the same, but
a) the GS2 header on the client's first message and channel binding
data is excluded when SAML is used as a GSS-API mechanism, and
b) the RFC2743 section 3.1 initial context token header is prefixed
to the client's first authentication message (context token).
The GSS-API mechanism OID for SAML is 1.3.6.1.4.1.11591.4.8.
SAML security contexts always have the mutual_state flag
(GSS_C_MUTUAL_FLAG) set to TRUE. SAML does not support credential
delegation (FIXME), therefore SCRAM security contexts alway have the
deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.
The SAML mechanism does not support (FIXME) per-message tokens or
GSS_Pseudo_random.
5.1. GSS-API Principal Name Types for SAML
SAML supports standard generic name syntaxes for acceptors such as
GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4.1). SAML
supports only a single name type for initiators: GSS_C_NT_USER_NAME.
GSS_C_NT_USER_NAME is the default name type for SAML. The query,
display, and exported name syntaxes for SAML principal names are all
the same. There are no SAML-specific name syntaxes -- applications
should use generic GSS-API name types such as GSS_C_NT_USER_NAME and
GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4). The exported
name token does, of course, conform to [RFC2743], Section 3.2. GSS-
API name attributes may be defined in the future to hold the SAML
Subject Identifier.
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6. Channel Binding
The "gs2-cb-flag" MUST use "n" because channel binding data cannot be
integrity protected by the SAML negotiation.
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7. Example
Suppose the user has an identity at the SAML IdP saml.example.org and
a Jabber Identifier (JID) "somenode@example.com", and wishes to
authenticate his XMPP connection to xmpp.example.com. The
authentication on the wire would then look something like the
following:
Step 1: Client initiates stream to server:
Step 2: Server responds with a stream tag sent to client:
Step 3: Server informs client of available authentication mechanisms:
DIGEST-MD5
PLAIN
SAML20
Step 4: Client selects an authentication mechanism:
https://saml.example.org
Step 5: Server sends a BASE64 [RFC4648] encoded challenge to client
in the form of an HTTP Redirect to the SAML IdP with the SAML
Authentication Request as specified in the redirection url:
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SFRUUC8xLjEgMzAyIE9iamVjdCBNb3ZlZCBEYXRlOiAyMiBPY3QgMjAwOSAwNzowMDo0OS
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MlZ5ZEdsdg0KYmlJK0RRb2dJQ0FnSUdoMGRIQnpPaTh2ZUcxd2NDNWxlR0Z0Y0d4bExtTn
ZiUTBLSUR3dmMyRnRiRHBKYzNOMVpYSStEUW9nUEhOaA0KYld4d09rNWhiV1ZKUkZCdmJH
bGplU0I0Yld4dWN6cHpZVzFzY0QwaWRYSnVPbTloYzJsek9tNWhiV1Z6T25Sak9sTkJUVX
c2TWk0dw0KT25CeWIzUnZZMjlzSWcwS0lDQWdJQ0JHYjNKdFlYUTlJblZ5YmpwdllYTnBj
enB1WVcxbGN6cDBZenBUUVUxTU9qSXVNRHB1WVcxbA0KYVdRdFptOXliV0YwT25CbGNuTn
BjM1JsYm5RaURRb2dJQ0FnSUZOUVRtRnRaVkYxWVd4cFptbGxjajBpZUcxd2NDNWxlR0Z0
Y0d4bA0KTG1OdmJTSWdRV3hzYjNkRGNtVmhkR1U5SW5SeWRXVWlJQzgrRFFvZ1BITmhiV3
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I5cw0KSWlBTkNpQWdJQ0FnSUNBZ1EyOXRjR0Z5YVhOdmJqMGlaWGhoWTNRaVBnMEtJQ0E4
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c2NEcEJkWFJvYmxKbGNYVmxjM1Er
The decoded challenge is:
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HTTP/1.1 302 Object Moved Date: 22 Oct 2009 07:00:49 GMT Location:
https://saml.example.org/SAML/Browser?SAMLRequest=
PHNhbWxwOkF1dGhuUmVxdWVzdCB4bWxuczpzYW1scD0idXJuOm9hc2lzOm5hbWVzOnRjOl
NBTUw6Mi4wOnByb3RvY29sIg0KICAgIElEPSJfYmVjNDI0ZmE1MTAzNDI4OTA5YTMwZmYx
ZTMxMTY4MzI3Zjc5NDc0OTg0IiBWZXJzaW9uPSIyLjAiDQogICAgSXNzdWVJbnN0YW50PS
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pTQU1MOjIuMDpiaW5kaW5nczpIVFRQLVBPU1QiDQogICAgQXNzZXJ0aW9uQ29uc3VtZXJT
ZXJ2aWNlVVJMPQ0KICAgICAgICAiaHR0cHM6Ly94bXBwLmV4YW1wbGUuY29tL1NBTUwvQX
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Zm9ybWF0OnBlcnNpc3RlbnQiDQogICAgIFNQTmFtZVF1YWxpZmllcj0ieG1wcC5leGFtcG
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Q29udGV4dA0KICAgICB4bWxuczpzYW1scD0idXJuOm9hc2lzOm5hbWVzOnRjOlNBTUw6Mi
4wOnByb3RvY29sIiANCiAgICAgICAgQ29tcGFyaXNvbj0iZXhhY3QiPg0KICA8c2FtbDpB
dXRobkNvbnRleHRDbGFzc1JlZg0KICAgICAgeG1sbnM6c2FtbD0idXJuOm9hc2lzOm5hbW
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bnRleHQ+IA0KPC9zYW1scDpBdXRoblJlcXVlc3Q+
Where the decoded SAMLRequest looks like:
https://xmpp.example.com
urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
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Step 5 (alt): Server returns error to client:
Step 6: Client sends a BASE64 encoded empty response to the
challenge:
=
[ The client now sends the URL to a browser for processing. The
browser engages in a normal SAML authentication flow (external to
SASL), like redirection to the Identity Provider
(https://saml.example.org), the user logs into
https://saml.example.org, and agrees to authenticate to
xmpp.example.com. A redirect is passed back to the client browser
who sends the AuthN response to the server, containing the subject-
identifier as an attribute. If the AuthN response doesn't contain
the JID, the server maps the subject-identifier received from the IdP
to a JID]
Step 7: Server informs client of successful authentication:
Step 7 (alt): Server informs client of failed authentication:
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Step 8: Client initiates a new stream to server:
Step 9: Server responds by sending a stream header to client along
with any additional features (or an empty features element):
Step 10: Client binds a resource:
someresource
Step 11: Server informs client of successful resource binding:
somenode@example.com/someresource
Please note: line breaks were added to the base64 for clarity.
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8. Security Considerations
This section will address only security considerations associated
with the use of SAML with SASL applications. For considerations
relating to SAML in general, the reader is referred to the SAML
specification and to other literature. Similarly, for general SASL
Security Considerations, the reader is referred to that
specification.
8.1. Binding SAML subject identifiers to Authorization Identities
As specified in [RFC4422], the server is responsible for binding
credentials to a specific authorization identity. It is therefore
necessary that only specific trusted IdPs be allowed. This is
typical part of SAML trust establishment between RP's and IdP.
8.2. User Privacy
The IdP is aware of each RP that a user logs into. There is nothing
in the protocol to hide this information from the IdP. It is not a
requirement to track the visits, but there is nothing that prohibits
the collection of information. SASL servers should be aware that
SAML IdPs will track - to some extent - user access to their
services.
8.3. Collusion between RPs
It is possible for RPs to link data that they have collected on you.
By using the same identifier to log into every RP, collusion between
RPs is possible. In SAML, targeted identity was introduced.
Targeted identity allows the IdP to transform the identifier the user
typed in to an opaque identifier. This way the RP would never see
the actual user identifier, but a randomly generated identifier.
This is an option the user has to understand and decide to use if the
IdP is supporting it.
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9. IANA Considerations
The IANA is requested to register the following SASL profile:
SASL mechanism profile: SAML20
Security Considerations: See this document
Published Specification: See this document
For further information: Contact the authors of this document.
Owner/Change controller: the IETF
Note: None
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10. Normative References
[OASIS.saml-bindings-2.0-os]
Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E.
Maler, "Bindings for the OASIS Security Assertion Markup
Language (SAML) V2.0", OASIS
Standard saml-bindings-2.0-os, March 2005.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.
[OASIS.saml-profiles-2.0-os]
Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
P., Philpott, R., and E. Maler, "Profiles for the OASIS
Security Assertion Markup Language (SAML) V2.0", OASIS
Standard OASIS.saml-profiles-2.0-os, March 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[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.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
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Appendix A. Acknowledgments
The authors would like to thank Scott Cantor, Joe Hildebrand, Josh
Howlett, Leif Johansson, Diego Lopez, Hank Mauldin, RL 'Bob' Morgan,
Stefan Plug and Hannes Tschofenig for their review and contributions.
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Appendix B. Changes
This section to be removed prior to publication.
o 00 WG -00 draft. Updates GSS-API section, some fixes per Scott
Cantor
o 01 Added authorization identity, added GSS-API specifics, added
client supplied IdP
o 00 Initial Revision.
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Authors' Addresses
Klaas Wierenga
Cisco Systems, Inc.
Haarlerbergweg 13-19
Amsterdam, Noord-Holland 1101 CH
Netherlands
Phone: +31 20 357 1752
Email: klaas@cisco.com
Eliot Lear
Cisco Systems GmbH
Richtistrasse 7
Wallisellen, ZH CH-8304
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com
Simon Josefsson
SJD AB
Hagagatan 24
Stockholm 113 47
SE
Email: simon@josefsson.org
URI: http://josefsson.org/
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