SAML Enhanced Client SASL and GSS-API Mechanisms
draft-ietf-kitten-sasl-saml-ec-20
| Document | Type | Active Internet-Draft (kitten WG) | |
|---|---|---|---|
| Authors | Scott Cantor , Margaret Cullen , Simon Josefsson | ||
| Last updated | 2021-05-10 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Robbie Harwood | ||
| Shepherd write-up | Show (last changed 2019-08-29) | ||
| IESG | IESG state | AD Evaluation::AD Followup | |
| Action Holders |
(None)
|
||
| Consensus Boilerplate | Yes | ||
| Telechat date | |||
| Responsible AD | Benjamin Kaduk | ||
| Send notices to | Robbie Harwood <rharwood@redhat.com> | ||
Network Working Group S. Cantor
Internet-Draft Shibboleth Consortium
Intended status: Standards Track M. Cullen
Expires: November 11, 2021 Painless Security
S. Josefsson
SJD AB
May 10, 2021
SAML Enhanced Client SASL and GSS-API Mechanisms
draft-ietf-kitten-sasl-saml-ec-20
Abstract
Security Assertion Markup Language (SAML) 2.0 is a generalized
framework for the exchange of security-related information between
asserting and relying parties. Simple Authentication and Security
Layer (SASL) and the Generic Security Service Application Program
Interface (GSS-API) are application frameworks that facilitate an
extensible authentication model, among other things. This document
specifies a SASL and GSS-API mechanism for SAML 2.0 that leverages
the capabilities of a SAML-aware "enhanced client" to address
significant barriers to federated authentication in a manner that
encourages reuse of existing SAML bindings and profiles designed for
non-browser scenarios.
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 https://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 November 11, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
Cantor, et al. Expires November 11, 2021 [Page 1]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability for Non-HTTP Use Cases . . . . . . . . . . . . 6
4. SAML Enhanced Client SASL Mechanism Specification . . . . . . 8
4.1. Advertisement . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Initiation . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Server Response . . . . . . . . . . . . . . . . . . . . . 9
4.4. User Authentication with Identity Provider . . . . . . . 10
4.5. Client Response . . . . . . . . . . . . . . . . . . . . . 10
4.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.7. Additional Notes . . . . . . . . . . . . . . . . . . . . 10
5. SAML EC GSS-API Mechanism Specification . . . . . . . . . . . 11
5.1. GSS-API Credential Delegation . . . . . . . . . . . . . . 12
5.2. GSS-API Channel Binding . . . . . . . . . . . . . . . . . 13
5.3. Session Key Derivation . . . . . . . . . . . . . . . . . 13
5.3.1. Generated by Identity Provider . . . . . . . . . . . 14
5.3.2. Alternate Key Derivation Mechanisms . . . . . . . . . 15
5.4. Per-Message Tokens . . . . . . . . . . . . . . . . . . . 15
5.5. Pseudo-Random Function (PRF) . . . . . . . . . . . . . . 15
5.6. GSS-API Principal Name Types for SAML EC . . . . . . . . 16
5.6.1. User Naming Considerations . . . . . . . . . . . . . 16
5.6.2. Service Naming Considerations . . . . . . . . . . . . 17
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7.1. Risks Left Unaddressed . . . . . . . . . . . . . . . . . 26
7.2. User Privacy . . . . . . . . . . . . . . . . . . . . . . 26
7.3. Collusion between RPs . . . . . . . . . . . . . . . . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
8.1. GSS-API and SASL Mechanism Registration . . . . . . . . . 27
8.2. XML Namespace Name for SAML-EC . . . . . . . . . . . . . 27
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.1. Normative References . . . . . . . . . . . . . . . . . . 28
9.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. XML Schema . . . . . . . . . . . . . . . . . . . . . 31
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 33
Appendix C. Changes . . . . . . . . . . . . . . . . . . . . . . 33
Cantor, et al. Expires November 11, 2021 [Page 2]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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).
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
[RFC3501], the Post Office Protocol(POP [RFC1939]) and XMPP
[RFC6120]. The effect of SASL is to make authentication modular, so
that newer authentication mechanisms can be added as needed.
There are related protocols, protocol bindings
[OASIS.saml-bindings-2.0-os], and interoperability profiles
[OASIS.saml-profiles-2.0-os] designed for different use cases.
Additional profiles and extensions are also routinely developed and
published.
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, as
well as additional optional cryptographic functionality. This
document defines a pure SASL mechanism for SAML, but it conforms to
the bridge between SASL and GSS-API called GS2 [RFC5801]. This means
that this document defines both a SASL mechanism and a GSS-API
mechanism. The GSS-API interface is optional for SASL implementers,
and the GSS-API considerations can be avoided in environments that
use SASL directly without GSS-API.
The mechanisms specified in this document allow a SASL- or GSS-API-
enabled server to act as a SAML relying party, or service provider
(SP), by advertising this mechanism as an option for SASL or GSS-API
clients that support the use of SAML to communicate identity and
attribute information. Clients supporting this mechanism are termed
"enhanced clients" in SAML terminology because they understand the
federated authentication model and have specific knowledge of the
IdP(s) associated with the user. This knowledge, and the ability to
act on it, addresses a significant problem with browser-based SAML
profiles known as the "discovery", or "where are you from?" (WAYF)
problem. In a "dumb" client such as a web browser, various intrusive
user interface techniques are used to determine the appropriate IdP
to use because the request to the IdP is generated as an HTTP
Cantor, et al. Expires November 11, 2021 [Page 3]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
redirect by the RP, which does not generally have prior knowledge of
the IdP to use. Obviating the need for the RP to interact with the
client to determine the right IdP (and its network location) is both
a user interface and security improvement.
The SAML mechanism described in this document is an adaptation of an
existing SAML profile, the Enhanced Client or Proxy (ECP) Profile
(V2.0) [SAMLECP20].
Figure 1 describes the interworking between SAML and SASL: this
document requires enhancements to the RP and to the client (as the
two SASL communication endpoints) but no changes to the SAML IdP are
assumed apart from its support for the applicable SAML profile. To
accomplish this, a SAML protocol exchange between the RP and the IdP,
brokered by the client, is tunneled within SASL. There is no assumed
communication between the RP and the IdP, but such communication may
occur in conjunction with additional SAML-related profiles not in
scope for this document.
+-----------+
| SAML |
| Relying |
| Party |
| |
+-----------+
^
+--|--+
| S| |
S | A| |
A | M| |
S | L| |
L | | |
| | |
+--|--+
+------------+ v
| | +----------+
| SAML | SAML SOAP | |
| Identity |<--------------->| Client |
| Provider | Binding | |
+------------+ +----------+
Figure 1: Interworking Architecture
Cantor, et al. Expires November 11, 2021 [Page 4]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
+-------+ +-------+ +-------+
| SAML | |Client | | SAML |
| IDP | | | | RP |
+---+---+ +---+---+ +---+---+
| | |
| |---------------------->|
| | Resource Request |
| | |
| | |
|<----------------------+-----------------------|
| | SAML Auth Request |
| | |
| | |
|<--------------------->| |
| User Authentication | |
| | |
| | |
|-----------------------+---------------------->|
| SAML Auth Response | |
| | |
| | |
| |<----------------------|
| | Requested Resource |
| | |
Figure 2: Communication Flow
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
The reader is also assumed to be familiar with the terms used in the
SAML 2.0 specification, and an understanding of the Enhanced Client
or Proxy (ECP) Profile (V2.0) [SAMLECP20] is necessary, as part of
this mechanism explicitly reuses and references it.
This document can be implemented without knowledge of GSS-API since
the normative aspects of the GS2 protocol syntax have been duplicated
in this document. The document may also be implemented to provide a
GSS-API mechanism, and then knowledge of GSS-API is essential.
Cantor, et al. Expires November 11, 2021 [Page 5]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
3. Applicability for Non-HTTP Use Cases
While SAML is designed to support a variety of application scenarios,
the profiles for authentication defined in the original standard are
designed around HTTP [RFC7230] applications. They are not, however,
limited to browsers, because browsers do not always meet the needs of
more security-sensitive applications. Specifically, the notion of an
"Enhanced Client" (or a proxy acting as one on behalf of a browser,
thus the term "ECP") was specified for a software component that acts
somewhat like a browser from an application perspective, but includes
limited, but sufficient, awareness of SAML to play a more conscious
role in the authentication exchange between the RP and the IdP. What
follows is an outline of the Enhanced Client or Proxy (ECP) Profile
(V2.0) [SAMLECP20], as applied to the web/HTTP service use case:
1. The Enhanced Client requests a resource of a Relying Party (RP)
(via an HTTP request). In doing so, it advertises its "enhanced"
capability using HTTP headers.
2. The RP, desiring SAML authentication and noting the client's
capabilities, responds not with an HTTP redirect or form, but
with a SOAP [W3C.soap11] envelope containing a SAML
<AuthnRequest> along with some supporting headers. This request
identifies the RP (and may be signed), and may provide hints to
the client as to what IdPs the RP finds acceptable, but the
choice of IdP is generally left to the client.
3. The client is then responsible for delivering the body of the
SOAP message to the IdP it is instructed to use (often via out-
of-band configuration). The user authenticates to the IdP ahead
of, during, or after the delivery of this message, and perhaps
explicitly authorizes the response to the RP.
4. Whether authentication succeeds or fails, the IdP responds with
its own SOAP envelope, generally containing a SAML <Response>
message for delivery to the RP. In a successful case, the
message will include one or more SAML <Assertion> elements
containing authentication, and possibly attribute, statements
about the subject. Either the response or each assertion is
signed, and the assertion(s) may be encrypted to a key negotiated
with or known to belong to the RP.
5. The client then delivers the SOAP envelope containing the
<Response> to the RP at a location the IdP directs (which acts as
an additional, though limited, defense against MITM attacks).
This completes the SAML exchange.
Cantor, et al. Expires November 11, 2021 [Page 6]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
6. The RP now has sufficient identity information to approve the
original HTTP request or not, and acts accordingly. Everything
between the original request and this response can be thought of
as an "interruption" of the original HTTP exchange.
When considering this flow in the context of an arbitrary application
protocol and SASL, the RP and the client both must change their code
to implement this SASL mechanism, but the IdP can remain unmodified.
The existing RP/client exchange that is tunneled through HTTP maps
well to the tunneling of that same exchange in SASL. In the parlance
of SASL [RFC4422], this mechanism is "client-first" for consistency
with GS2. The steps are shown below:
1. The server MAY advertise the SAML20EC and/or SAML20EC-PLUS
mechanisms.
2. The client initiates a SASL authentication with SAML20EC or
SAML20EC-PLUS.
3. The server sends the client a challenge consisting of a SOAP
envelope containing its SAML <AuthnRequest>.
4. The SASL client unpacks the SOAP message and communicates with
its chosen IdP to relay the SAML <AuthnRequest> to it. This
communication, and the authentication with the IdP, proceeds
separately from the SASL process.
5. Upon completion of the exchange with the IdP, the client responds
to the SASL server with a SOAP envelope containing the SAML
<Response> it obtained, or a SOAP fault, as warranted.
6. The SASL Server indicates success or failure.
Note: The details of the SAML processing, which are consistent with
the Enhanced Client or Proxy (ECP) Profile (V2.0) [SAMLECP20], are
such that the client MUST interact with the IdP in order to complete
any SASL exchange with the RP. The assertions issued by the IdP for
the purposes of the profile, and by extension this SASL mechanism,
are short lived, and therefore cannot be cached by the client for
later use.
Encompassed in step four is the client-driven selection of the IdP,
authentication to it, and the acquisition of a response to provide to
the SASL server. These processes are all external to SASL.
Note also that unlike an HTTP-based profile, the IdP cannot
participate in the selection of, or evaluation of, the location to
which the SASL Client Response will be delivered by the client. The
Cantor, et al. Expires November 11, 2021 [Page 7]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
use of GSS-API Channel Binding is an important mitigation of the risk
of a "Man in the Middle" attack between the client and RP, as is the
use of a negotiated or derived session key in whatever protocol is
secured by this mechanism.
With all of this in mind, the typical flow appears as follows:
SASL Serv. Client IdP
|------(1)----->| | Advertisement
| | |
|<-----(2)------| | Initiation
| | |
|------(3)----->| | SASL Server Response
| | |
| |<- - -(4)- - >| SOAP AuthnRequest + user authn
| | |
|<-----(5)------| | SASL Client Response
| | |
|------(6)----->| | Server sends Outcome
| | |
----- = SASL
- - - = SOAP over HTTPS (external to SASL)
Figure 3: Authentication flow
4. SAML Enhanced Client SASL Mechanism Specification
Based on the previous figures, the following operations are defined
by the SAML SASL mechanism:
4.1. Advertisement
To advertise that a server supports this mechanism, during
application session initiation, it displays the name "SAML20EC" and/
or "SAML20EC-PLUS" in the list of supported SASL mechanisms.
In accordance with [RFC5801] the "-PLUS" variant indicates that the
server supports channel binding and would be selected by a client
with that capability.
Cantor, et al. Expires November 11, 2021 [Page 8]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
4.2. Initiation
A client initiates "SAML20EC" or "SAML20EC-PLUS" authentication. If
supported by the application protocol, the client MAY include an
initial response, otherwise it waits until the server has issued an
empty challenge (because the mechanism is client-first).
The format of the initial client response ("initresp") is as follows:
hok = "urn:oasis:names:tc:SAML:2.0:cm:holder-of-key"
mut = "urn:oasis:names:tc:SAML:2.0:profiles:SSO:ecp:2.0:" \
"WantAuthnRequestsSigned"
del = "urn:oasis:names:tc:SAML:2.0:conditions:delegation"
initresp = gs2-cb-flag "," [gs2-authzid] "," [hok] "," [mut] "," [del]
The gs2-cb-flag flag MUST be set as defined in [RFC5801] to indicate
whether the client supports channel binding. This takes the place of
the PAOS HTTP header extension used in [SAMLECP20] to indicate
channel binding support.
The optional "gs2-authzid" field holds the authorization identity, as
requested by the client.
The optional "hok" field is a constant that signals the client's
support for stronger security by means of a locally held key. This
takes the place of the PAOS HTTP header extension used in [SAMLECP20]
to indicate "holder of key" support.
The optional "mut" field is a constant that signals the client's
desire for mutual authentication. If set, the SASL server MUST
digitally sign its SAML <AuthnRequest> message. The URN constant
above is a single string; the linefeed is shown for RFC formatting
reasons.
The optional "del" field is a constant that signals the client's
desire for the acceptor to request an assertion usable for delegation
of the client's identity to the acceptor.
4.3. Server Response
The SASL server responds with a SOAP envelope constructed in
accordance with section 2.3.2 of [SAMLECP20]. This includes adhering
to the SOAP header requirements of the SAML PAOS Binding
[OASIS.saml-bindings-2.0-os], for compatibility with the existing
Cantor, et al. Expires November 11, 2021 [Page 9]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
profile. Various SOAP headers are also consumed by the client in
exactly the same manner prescribed by that section.
4.4. User Authentication with Identity Provider
Upon receipt of the Server Response (Section 4.3), the steps
described in sections 2.3.3 through 2.3.6 of [SAMLECP20] are
performed between the client and the chosen IdP. The means by which
the client determines the IdP to use, and where it is located, are
out of scope of this mechanism.
The exact means of authentication to the IdP are also out of scope,
but clients supporting this mechanism MUST support HTTP Basic
Authentication as defined in [RFC7617] and TLS 1.3 client
authentication as defined in [RFC8446].
4.5. Client Response
Assuming a response is obtained from the IdP, the client responds to
the SASL server with a SOAP envelope constructed in accordance with
section 2.3.7 of [SAMLECP20]. This includes adhering to the SOAP
header requirements of the SAML PAOS Binding
[OASIS.saml-bindings-2.0-os], for compatibility with the existing
profile. If the client is unable to obtain a response from the IdP,
or must otherwise signal failure, it responds to the SASL server with
a SOAP envelope containing a SOAP fault.
4.6. Outcome
The SAML protocol exchange having completed, the SASL server will
transmit the outcome to the client depending on local validation of
the client responses (including the assertion conveyed from the
chosen IDP). This outcome is transmitted in accordance with the
application protocol in use.
4.7. Additional Notes
Because this mechanism is an adaptation of an HTTP-based profile,
there are a few requirements outlined in [SAMLECP20] that make
reference to a response URL that is normally used to regulate where
the client returns information to the RP. There are also security-
related checks built into the profile that involve this location.
For compatibility with existing IdP and profile behavior, and to
provide for mutual authentication, the SASL server MUST populate the
responseConsumerURL and AssertionConsumerServiceURL attributes with
its service name. As discussed in Section 5.6.2, most SASL profiles
rely on a service name format of "service@host", but regardless of
Cantor, et al. Expires November 11, 2021 [Page 10]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
the form, the service name is used directly rather than transformed
into an absolute URI if it is not already one, and MUST be percent-
encoded per [RFC3986].
The IdP MUST securely associate the service name with the SAML
entityID claimed by the SASL server, such as through the use of SAML
metadata [OASIS.saml-metadata-2.0-os]. If metadata is used, a SASL
service's <SPSSODescriptor> role MUST contain a corresponding
<AssertionConsumerService> whose Location attribute contains the
appropriate service name, as described above. The Binding attribute
MUST be one of "urn:ietf:params:xml:ns:samlec" (RECOMMENDED) or
"urn:oasis:names:tc:SAML:2.0:bindings:PAOS" (for compatibility with
older implementations of the ECP profile in existing IdP software).
Finally, note that the use of HTTP status signaling between the RP
and client mandated by [SAMLECP20] may not be applicable.
5. SAML EC 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 Enhanced Client SASL mechanism is also a GSS-API mechanism.
The messages are the same, but a) the GS2 [RFC5801] header on the
client's first authentication message is excluded when SAML EC is
used as a GSS-API mechanism, and b) the [RFC2743] section 3.1 initial
context token header is used for the client's first authentication
message (context token) instead, with the body of the message being
the same as for the SASL mechanism case.
The GSS-API mechanism OID for SAML EC is OID-TBD (IANA to assign: see
IANA considerations). The DER encoding of the OID is TBD.
The mutual_state request flag (GSS_C_MUTUAL_FLAG) MAY be set to TRUE,
resulting in the "mut" option set in the initial client response.
The security context mutual_state flag is set to TRUE only if the
server digitally signs its SAML <AuthnRequest> message and the
signature and signing credential are appropriately verified by the
IdP. The IdP signals this to the client in an
<ecp:RequestAuthenticated> SOAP header block.
The lifetime of a security context established with this mechanism
SHOULD be limited by the value of a SessionNotOnOrAfter attribute, if
any, in the <AuthnStatement> element(s) of the SAML assertion(s)
received by the RP. By convention, in the rare case that multiple
valid/confirmed assertions containing <AuthnStatement> elements are
Cantor, et al. Expires November 11, 2021 [Page 11]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
received, the most restrictive SessionNotOnOrAfter is generally
applied.
5.1. GSS-API Credential Delegation
This mechanism can support credential delegation through the issuance
of SAML assertions that an IdP will accept as proof of authentication
by a service on behalf of a subject. An initiator may request
delegation of its credentials by setting the "del" option field in
the initial client response to
"urn:oasis:names:tc:SAML:2.0:conditions:delegation".
An acceptor, upon receipt of this constant, requests a delegated
assertion by including in its <AuthnRequest> message a <Conditions>
element containing an <AudienceRestriction> identifying the IdP as a
desired audience for the assertion(s) to be issued. In the event
that the specific IdP to be used is unknown, the constant
"urn:oasis:names:tc:SAML:2.0:conditions:delegation" may be used as a
stand-in, per Section 2.3.2 of [SAMLECP20].
Upon receipt of an assertion satisfying this property, and containing
a <SubjectConfirmation> element that the acceptor can satisfy, the
security context will have its deleg_state flag (GSS_C_DELEG_FLAG)
set to TRUE.
The IdP, if it issues a delegated assertion to the acceptor, MUST
include in the SOAP response to the initiator a <samlec:Delegated>
SOAP header block, indicating that delegation was enabled. It has no
content, other than mandatory SOAP attributes (an example follows):
<samlec:Delegated xmlns:samlec="urn:ietf:params:xml:ns:samlec"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"
S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next" />
Upon receipt of such a header block, the initiator MUST fail the
establishment of the security context if it did not request
delegation in its initial client response to the acceptor. It SHOULD
signal this failure to the acceptor with a SOAP fault message in its
final client response.
As noted previously, the exact means of client authentication to the
IdP is formally out of scope of this mechanism. This extends to the
use of a delegation assertion as a means of authentication by an
acceptor acting as an initiator. In practice, some profile of
Cantor, et al. Expires November 11, 2021 [Page 12]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
[WSS-SAML] is used to attach the assertion and a confirmation proof
to the SOAP message from the client to the IdP.
5.2. GSS-API Channel Binding
GSS-API channel binding [RFC5554] is a protected facility for
exchanging a cryptographic identifier for an enclosing channel
between the initiator and acceptor. The initiator sends channel
binding data and the acceptor confirms that channel binding data has
been checked.
The acceptor SHOULD accept any channel binding provided by the
initiator if null channel bindings are passed into
gss_accept_sec_context. Protocols such as HTTP Negotiate [RFC4559]
depend on this behavior of some Kerberos implementations.
The exchange and verification of channel binding information is
described by [SAMLECP20].
5.3. Session Key Derivation
Some GSS-API features (discussed in the following sections) require a
session key be established as a result of security context
establishment. In the common case of a "bearer" assertion in SAML, a
mechanism is defined to communicate a key to both parties via the
IdP. In other cases such as assertions based on "holder of key"
confirmation bound to a client-controlled key, there may be
additional methods defined in the future, and extension points are
provided for this purpose.
Information defining or describing the session key, or a process for
deriving one, is communicated between the initiator and acceptor
using a <samlec:SessionKey> element, defined by the XML schema in
Appendix A. This element is a SOAP header block. The content of the
element further depends on the specific use in the mechanism. The
Algorithm XML attribute identifies a mechanism for key derivation.
It is omitted to identify the use of an IdP-generated key (see
following section) or will contain a URI value identifying a
derivation mechanism defined outside this specification. Each header
block's mustUnderstand and actor attributes MUST be set to "1" and
"http://schemas.xmlsoap.org/soap/actor/next" respectively.
In the acceptor's first response message containing its SAML request,
one or more <samlec:SessionKey> SOAP header blocks MUST be included.
The element MUST contain one or more <EncType> elements containing
the number of a supported encryption type defined in accordance with
[RFC3961]. Encryption types should be provided in order of
preference by the acceptor.
Cantor, et al. Expires November 11, 2021 [Page 13]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
In the final client response message, a single <samlec:SessionKey>
SOAP header block MUST be included. A single <EncType> element MUST
be included to identify the chosen encryption type used by the
initiator.
All parties MUST support the "aes128-cts-hmac-sha1-96" encryption
type, number 17, defined by [RFC3962].
Further details depend on the mechanism used, one of which is
described in the following section.
5.3.1. Generated by Identity Provider
The IdP, if issuing a bearer assertion for use with this mechanism,
SHOULD provide a generated key for use by the initiator and acceptor.
This key is used as pseudorandom input to the "random-to-key"
function for a specific encryption type defined in accordance with
[RFC3961]. The key is base64-encoded and placed inside a
<samlec:GeneratedKey> element. The IdP does not participate in the
selection of the encryption type and simply generates enough
pseudorandom bits to supply key material to the other parties.
The resulting <samlec:GeneratedKey> element is placed within the
<saml:Advice> element of the assertion issued. The identity provider
MUST encrypt the assertion (implying that it MUST have the means to
do so, typically knowledge of a key associated with the RP). If
multiple assertions are issued (allowed, but not typical), the
element need only be included in one of the assertions issued for use
by the relying party.
A copy of the element is also added as a SOAP header block in the
response from the IdP to the client (and then removed when
constructing the response to the acceptor).
If this mechanism is used by the initiator, then the
<samlec:SessionKey> SOAP header block attached to the final client
response message will identify this via the omission of the Algorithm
attribute and will identify the chosen encryption type using the
<samlec:EncType> element:
<samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"
S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next">
<samlec:EncType>17</samlec:EncType>
<samlec:SessionKey>
Cantor, et al. Expires November 11, 2021 [Page 14]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Both the initiator and acceptor MUST execute the chosen encryption
type's random-to-key function over the pseudorandom value provided by
the <samlec:GeneratedKey> element. The result of that function is
used as the protocol and session key. Support for subkeys from the
initiator or acceptor is not specified.
5.3.2. Alternate Key Derivation Mechanisms
In the event that a client is proving possession of a secret or
private key, a formal key agreement algorithm might be supported.
This specification does not define such a mechanism, but the
<samlec:SessionKey> element is extensible to allow for future work in
this space by means of the Algorithm attribute and an optional
<ds:KeyInfo> child element to carry extensible content related to key
establishment.
However a key is derived, the <samlec:EncType> element will identify
the chosen encrytion type, and both the initiator and acceptor MUST
execute the encryption type's random-to-key function over the result
of the key agreement or derivation process. The result of that
function is used as the protocol key.
5.4. Per-Message Tokens
The per-message tokens SHALL be the same as those for the Kerberos V5
GSS-API mechanism [RFC4121] (see Section 4.2 and sub-sections).
The replay_det_state (GSS_C_REPLAY_FLAG), sequence_state
(GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG) and integ_avail
(GSS_C_INTEG_FLAG) security context flags are always set to TRUE.
The "protocol key" SHALL be a key established in a manner described
in the previous section. "Specific keys" are then derived as usual
as described in Section 2 of [RFC4121], [RFC3961], and [RFC3962].
The terms "protocol key" and "specific key" are Kerberos V5 terms
[RFC3961].
SAML20EC is PROT_READY as soon as the SAML response message has been
seen.
5.5. Pseudo-Random Function (PRF)
The GSS-API has been extended with a Pseudo-Random Function (PRF)
interface in [RFC4401]. The purpose is to enable applications to
derive a cryptographic key from an established GSS-API security
context. This section defines a GSS_Pseudo_random that is applicable
for the SAML20EC GSS-API mechanism.
Cantor, et al. Expires November 11, 2021 [Page 15]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
The GSS_Pseudo_random() [RFC4401] SHALL be the same as for the
Kerberos V5 GSS-API mechanism [RFC7802]. There is no acceptor-
asserted sub-session key, thus GSS_C_PRF_KEY_FULL and
GSS_C_PRF_KEY_PARTIAL are equivalent. The protocol key to be used
for the GSS_Pseudo_random() SHALL be the same as the key defined in
the previous section.
5.6. GSS-API Principal Name Types for SAML EC
Services that act as SAML relying parties are typically identified by
means of a URI called an "entityID". Clients that are named in the
<Subject> element of a SAML assertion are typically identified by
means of a <NameID> element, which is an extensible XML structure
containing, at minimum, an element value that names the subject and a
Format attribute.
In practice, a GSS-API client and server are unlikely to know in
advance the name of the initiator as it will be expressed by the SAML
IdP upon completion of authentication. It is also generally
incorrect to assume that a particular acceptor name will directly map
into a particular RP entityID, because there is often a layer of
naming indirection between particular services on hosts and the
identity of a relying party in SAML terms.
To avoid complexity, and avoid unnecessary use of XML within the
naming layer, the SAML EC mechanism relies on the common/expected
name types used for acceptors and initiators,
GSS_C_NT_HOSTBASED_SERVICE and GSS_C_NT_USER_NAME. The mechanism
provides for validation of the host-based service name in conjunction
with the SAML exchange. It does not attempt to solve the problem of
mapping between an initiator "username", the user's identity while
authenticating to the IdP, and the information supplied by the IdP to
the acceptor. These relationships must be managed through local
policy at the initiator and acceptor.
SAML-based information associated with the initiator SHOULD be
expressed to the acceptor using GSS-API naming extensions [RFC6680],
in a similar manner to [RFC7056].
5.6.1. User Naming Considerations
The GSS_C_NT_USER_NAME form represents the name of an individual
user. Clients often rely on this value to determine the appropriate
credentials to use in authenticating to the IdP, and supply it to the
server for use by the acceptor.
Upon successful completion of this mechanism, the server MUST
construct the authenticated initiator name based on the <saml:NameID>
Cantor, et al. Expires November 11, 2021 [Page 16]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
element in the assertion it successfully validated. The name is
constructed as a UTF-8 string in the following form:
name = element-value "!" Format "!" NameQualifier
"!" SPNameQualifier "!" SPProvidedID
The "element-value" token refers to the content of the <saml:NameID>
element. The other tokens refer to the identically named XML
attributes defined for use with the element. If an attribute is not
present, which is common, it is omitted (i.e., replaced with the
empty string). The Format value is never omitted; if not present,
the SAML-equivalent value of "urn:oasis:names:tc:SAML:1.1:nameid-
format:unspecified" is used.
Not all SAML assertions contain a <saml:NameID> element. In the
event that no such element is present, including the exceptional
cases of a <saml:BaseID> element or a <saml:EncryptedID> element that
cannot be decrypted, the GSS_C_NT_ANONYMOUS name type MUST be used
for the initiator name.
As noted in the previous section, it is expected that most
applications able to rely on SAML authentication would make use of
naming extensions to obtain additional information about the user
based on the assertion. This is particularly true in the anonymous
case, or in cases in which the SAML name is pseudonymous or transient
in nature. The ability to express the SAML name in
GSS_C_NT_USER_NAME form is intended for compatibility with
applications that cannot make use of additional information.
5.6.2. Service Naming Considerations
The GSS_C_NT_HOSTBASED_SERVICE name form represents a service running
on a host; it is textually represented as "service@host". This name
form is required by most SASL profiles and is used by many existing
applications that use the Kerberos GSS-API mechanism. As described
in in the SASL mechanism's Section 4.7, such a name is used directly
by this mechanism as the effective AssertionConsumerService
"location" associated with the service and applied in IdP
verification of the request against the claimed SAML entityID.
6. 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 (and example.com
and example.org have established a SAML-capable trust relationship).
The authentication on the wire would then look something like the
following:
Cantor, et al. Expires November 11, 2021 [Page 17]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Step 1: Client initiates stream to server:
<stream:stream xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com' version='1.0'>
Step 2: Server responds with a stream tag sent to client:
<stream:stream
xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'
id='some_id' from='example.com' version='1.0'>
Step 3: Server informs client of available authentication mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
<mechanism>SAML20EC</mechanism>
</mechanisms>
</stream:features>
Step 4: Client selects an authentication mechanism and sends the
initial client response (it is base64 encoded as specified by the
XMPP SASL protocol profile):
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='SAML20EC'>
biwsLCw=
</auth>
The initial response is "n,,,," which signals that channel binding is
not used, there is no authorization identity, and the client does not
support key-based confirmation, or want mutual authentication or
delegation.
Step 5: Server sends a challenge to client in the form of a SOAP
envelope containing its SAML <AuthnRequest>:
Cantor, et al. Expires November 11, 2021 [Page 18]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>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</challenge>
The Base64 [RFC4648] decoded envelope:
Cantor, et al. Expires November 11, 2021 [Page 19]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<S:Envelope
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Header>
<paos:Request xmlns:paos="urn:liberty:paos:2003-08"
messageID="c3a4f8b9c2d" S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next"
responseConsumerURL="xmpp@xmpp.example.com"
service="urn:oasis:names:tc:SAML:2.0:profiles:SSO:ecp"/>
<ecp:Request
xmlns:ecp="urn:oasis:names:tc:SAML:2.0:profiles:SSO:ecp"
S:actor="http://schemas.xmlsoap.org/soap/actor/next"
S:mustUnderstand="1" ProviderName="Jabber at example.com">
<saml:Issuer>https://xmpp.example.com</saml:Issuer>
</ecp:Request>
<samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"
S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next">
<samlec:EncType>17</samlec:EncType>
<samlec:EncType>18</samlec:EncType>
<samlec:SessionKey>
</S:Header>
<S:Body>
<samlp:AuthnRequest
ID="c3a4f8b9c2d" Version="2.0" IssueInstant="2020-12-10T11:39:34Z"
AssertionConsumerServiceURL="xmpp@xmpp.example.com">
<saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
https://xmpp.example.com
</saml:Issuer>
<samlp:NameIDPolicy AllowCreate="true"
Format="urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"/>
<samlp:RequestedAuthnContext Comparison="exact">
<saml:AuthnContextClassRef>
urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
</saml:AuthnContextClassRef>
</samlp:RequestedAuthnContext>
</samlp:AuthnRequest>
</S:Body>
</S:Envelope>
Step 5 (alt): Server returns error to client:
Cantor, et al. Expires November 11, 2021 [Page 20]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
</stream:stream>
Step 6: Client relays the request to IdP in a SOAP message
transmitted over HTTP (over TLS). The HTTP portion is not shown, so
the use of Basic Authentication is assumed. The body of the SOAP
envelope is exactly the same as received in the previous step.
<S:Envelope
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Body>
<samlp:AuthnRequest>
<!-- same as above -->
</samlp:AuthnRequest>
</S:Body>
</S:Envelope>
Step 7: IdP responds to client with a SOAP response containing a SAML
<Response> containing a short-lived SSO assertion (shown as an
encrypted variant in the example). A generated key is included in
the assertion and in a header for the client.
Cantor, et al. Expires November 11, 2021 [Page 21]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<S:Envelope
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Header>
<ecp:Response S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next"
AssertionConsumerServiceURL="xmpp@xmpp.example.com"/>
<samlec:GeneratedKey xmlns:samlec="urn:ietf:params:xml:ns:samlec">
3w1wSBKUosRLsU69xGK7dg==
</samlec:GeneratedKey>
</S:Header>
<S:Body>
<samlp:Response ID="d43h94r389309r" Version="2.0"
IssueInstant="2020-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
Destination="xmpp@xmpp.example.com">
<saml:Issuer>https://saml.example.org</saml:Issuer>
<samlp:Status>
<samlp:StatusCode
Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
</samlp:Status>
<saml:EncryptedAssertion>
<!-- contents elided, copy of samlec:GeneratedKey in Advice -->
</saml:EncryptedAssertion>
</samlp:Response>
</S:Body>
</S:Envelope>
Step 8: Client sends SOAP envelope containing the SAML <Response> as
a response to the SASL server's challenge:
Cantor, et al. Expires November 11, 2021 [Page 22]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>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</response>
The Base64 [RFC4648] decoded envelope:
Cantor, et al. Expires November 11, 2021 [Page 23]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<S:Envelope
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/">
<S:Header>
<paos:Response xmlns:paos="urn:liberty:paos:2003-08"
S:actor="http://schemas.xmlsoap.org/soap/actor/next"
S:mustUnderstand="1" refToMessageID="6c3a4f8b9c2d"/>
<samlec:SessionKey xmlns:samlec="urn:ietf:params:xml:ns:samlec"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"
S:mustUnderstand="1"
S:actor="http://schemas.xmlsoap.org/soap/actor/next">
<samlec:EncType>17</samlec:EncType>
<samlec:SessionKey>
</S:Header>
<S:Body>
<samlp:Response ID="d43h94r389309r" Version="2.0"
IssueInstant="2020-12-10T11:42:34Z" InResponseTo="c3a4f8b9c2d"
Destination="xmpp@xmpp.example.com">
<saml:Issuer>https://saml.example.org</saml:Issuer>
<samlp:Status>
<samlp:StatusCode
Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
</samlp:Status>
<saml:EncryptedAssertion>
<!-- contents elided, copy of samlec:GeneratedKey in Advice -->
</saml:EncryptedAssertion>
</samlp:Response>
</S:Body>
</S:Envelope>
Step 9: Server informs client of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9 (alt): Server informs client of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 10: Client initiates a new stream to server:
Cantor, et al. Expires November 11, 2021 [Page 24]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<stream:stream xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com' version='1.0'>
Step 11: Server responds by sending a stream header to client along
with any additional features (or an empty features element):
<stream:stream xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_345' from='example.com' version='1.0'>
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
</stream:features>
Step 12: Client binds a resource:
<iq type='set' id='bind_1'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<resource>someresource</resource>
</bind>
</iq>
Step 13: Server informs client of successful resource binding:
<iq type='result' id='bind_1'>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>
<jid>somenode@example.com/someresource</jid>
</bind>
</iq>
Please note: line breaks were added to the base64 for clarity.
7. 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.
Cantor, et al. Expires November 11, 2021 [Page 25]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Version 2.0 of the Enhanced Client or Proxy Profile [SAMLECP20] adds
optional support for channel binding and use of "Holder of Key"
subject confirmation. The former is strongly recommended for use
with this mechanism to detect "Man in the Middle" attacks between the
client and the RP without relying on the commercial TLS
infrastructure that does not provide the level of assurance desired
by sensitive SAML applications. The latter may be impractical in
many cases, but is a valuable way of strengthening client
authentication, protecting against phishing, and improving the
overall mechanism.
7.1. Risks Left Unaddressed
The adaptation of a web-based profile that is largely designed around
security-oblivious clients and a bearer model for security token
validation results in a number of basic security exposures that
should be weighed against the compatibility and client simplification
benefits of this mechanism.
When channel binding is not used, protection against "Man in the
Middle" attacks is left solely to lower layer protocols such as TLS,
and the development of user interfaces able to implement that has not
been effectively demonstrated. Failure to detect a MITM can result
in phishing of the user's credentials if the attacker is between the
client and IdP, or the theft and misuse of a short-lived credential
(the SAML assertion) if the attacker is able to impersonate a RP.
SAML allows for source address checking as a minor mitigation to the
latter threat, but this is often impractical. IdPs can mitigate to
some extent the exposure of personal information to RP attackers by
encrypting assertions with authenticated keys.
7.2. 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 activity, but there is nothing technically
that prohibits the collection of this information. Servers should be
aware that SAML IdPs will track - to some extent - user access to
their services. This exposure extends to the use of session keys
generated by the IdP to secure messages between the parties, but note
that when bearer assertions are involved, the IdP can freely
impersonate the user to any relying party in any case.
It is also out of scope of the mechanism to determine under what
conditions an IdP will release particular information to a relying
party, and it is generally unclear in what fashion user consent could
be established in real time for the release of particular
Cantor, et al. Expires November 11, 2021 [Page 26]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
information. The SOAP exchange with the IdP does not preclude such
interaction, but neither does it define that interoperably.
7.3. Collusion between RPs
Depending on the information supplied by the IdP, it may be possible
for RPs to correlate data that they have collected. By using the
same identifier to log into every RP, collusion between RPs is
possible. SAML supports the notion of pairwise, or targeted/
directed, identity. This allows the IdP to manage opaque, pairwise
identifiers for each user that are specific to each RP. However,
correlation is often possible based on other attributes supplied, and
is generally a topic that is beyond the scope of this mechanism. It
is sufficient to say that this mechanism does not introduce new
correlation opportunities over and above the use of SAML in web-based
use cases.
8. IANA Considerations
8.1. GSS-API and SASL Mechanism Registration
The IANA is requested to assign a new entry for this GSS mechanism in
the sub-registry for SMI Security for Mechanism Codes, whose prefix
is iso.org.dod.internet.security.mechanisms (1.3.6.1.5.5) and to
reference this specification in the registry.
The IANA is requested to register the following SASL profile:
SASL mechanism profiles: SAML20EC and SAML20EC-PLUS
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
8.2. XML Namespace Name for SAML-EC
A URN sub-namespace for XML constructs introduced by this mechanism
is defined as follows:
URI: urn:ietf:params:xml:ns:samlec
Specification: See Appendix A of this document.
Cantor, et al. Expires November 11, 2021 [Page 27]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Description: This is the XML namespace name for XML constructs
introduced by the SAML Enhanced Client SASL and GSS-API Mechanisms.
Registrant Contact: the IESG
9. References
9.1. 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,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743,
DOI 10.17487/RFC2743, January 2000,
<https://www.rfc-editor.org/info/rfc2743>.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
2005, <https://www.rfc-editor.org/info/rfc3961>.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962,
DOI 10.17487/RFC3962, February 2005,
<https://www.rfc-editor.org/info/rfc3962>.
Cantor, et al. Expires November 11, 2021 [Page 28]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
DOI 10.17487/RFC4121, July 2005,
<https://www.rfc-editor.org/info/rfc4121>.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401,
DOI 10.17487/RFC4401, February 2006,
<https://www.rfc-editor.org/info/rfc4401>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5554] Williams, N., "Clarifications and Extensions to the
Generic Security Service Application Program Interface
(GSS-API) for the Use of Channel Bindings", RFC 5554,
DOI 10.17487/RFC5554, May 2009,
<https://www.rfc-editor.org/info/rfc5554>.
[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, DOI 10.17487/RFC5801,
July 2010, <https://www.rfc-editor.org/info/rfc5801>.
[RFC6680] Williams, N., Johansson, L., Hartman, S., and S.
Josefsson, "Generic Security Service Application
Programming Interface (GSS-API) Naming Extensions",
RFC 6680, DOI 10.17487/RFC6680, August 2012,
<https://www.rfc-editor.org/info/rfc6680>.
[RFC7056] Hartman, S. and J. Howlett, "Name Attributes for the GSS-
API Extensible Authentication Protocol (EAP) Mechanism",
RFC 7056, DOI 10.17487/RFC7056, December 2013,
<https://www.rfc-editor.org/info/rfc7056>.
Cantor, et al. Expires November 11, 2021 [Page 29]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
RFC 7617, DOI 10.17487/RFC7617, September 2015,
<https://www.rfc-editor.org/info/rfc7617>.
[RFC7802] Emery, S. and N. Williams, "A Pseudo-Random Function (PRF)
for the Kerberos V Generic Security Service Application
Program Interface (GSS-API) Mechanism", RFC 7802,
DOI 10.17487/RFC7802, March 2016,
<https://www.rfc-editor.org/info/rfc7802>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[SAMLECP20]
Cantor, S., "SAML V2.0 Enhanced Client or Proxy Profile
Version 2.0", OASIS Committee Specification OASIS.sstc-
saml-ecp-v2.0-cs01, August 2013.
[W3C.soap11]
Box, D., Ehnebuske, D., Kakivaya, G., Layman, A.,
Mendelsohn, N., Nielsen, H., Thatte, S., and D. Winer,
"Simple Object Access Protocol (SOAP) 1.1", W3C
Note soap11, May 2000, <http://www.w3.org/TR/SOAP/>.
9.2. Informative References
[OASIS.saml-metadata-2.0-os]
Cantor, S., Moreh, J., Philpott, R., and E. Maler,
"Metadata for the Security Assertion Markup Language
(SAML) V2.0", OASIS Standard saml-metadata-2.0-os, March
2005.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<https://www.rfc-editor.org/info/rfc1939>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
Cantor, et al. Expires November 11, 2021 [Page 30]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
<https://www.rfc-editor.org/info/rfc4559>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[W3C.REC-xmlschema-1]
Thompson, H., Beech, D., Maloney, M., and N. Mendelsohn,
"XML Schema Part 1: Structures", W3C REC-xmlschema-1, May
2001, <http://www.w3.org/TR/xmlschema-1/>.
[WSS-SAML]
Monzillo, R., "Web Services Security SAML Token Profile
Version 1.1.1", OASIS Standard OASIS.wss-SAMLTokenProfile,
May 2012.
Appendix A. XML Schema
The following schema formally defines the
"urn:ietf:params:xml:ns:samlec" namespace used in this document, in
conformance with [W3C.REC-xmlschema-1] While XML validation is
optional, the schema that follows is the normative definition of the
constructs it defines. Where the schema differs from any prose in
this specification, the schema takes precedence.
Cantor, et al. Expires November 11, 2021 [Page 31]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
<schema
targetNamespace="urn:ietf:params:xml:ns:samlec"
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:samlec="urn:ietf:params:xml:ns:samlec"
elementFormDefault="unqualified"
attributeFormDefault="unqualified"
blockDefault="substitution"
version="1.0">
<import namespace="http://www.w3.org/2000/09/xmldsig#"/>
<import namespace="http://schemas.xmlsoap.org/soap/envelope/"/>
<element name="SessionKey" type="samlec:SessionKeyType"/>
<complexType name="SessionKeyType">
<sequence>
<element ref="samlec:EncType" maxOccurs="unbounded"/>
<element ref="ds:KeyInfo" minOccurs="0"/>
</sequence>
<attribute ref="S:mustUnderstand" use="required"/>
<attribute ref="S:actor" use="required"/>
<attribute name="Algorithm"/>
</complexType>
<element name="EncType" type="integer"/>
<element name="GeneratedKey" type="samlec:GeneratedKeyType"/>
<complexType name="GeneratedKeyType">
<simpleContent>
<extension base="base64Binary">
<attribute ref="S:mustUnderstand"/>
<attribute ref="S:actor"/>
</extension>
</simpleContent>
</complexType>
<element name="Delegated" type="samlec:DelegatedType"/>
<complexType name="DelegatedType">
<sequence/>
<attribute ref="S:mustUnderstand" use="required"/>
<attribute ref="S:actor" use="required"/>
</complexType>
</schema>
Cantor, et al. Expires November 11, 2021 [Page 32]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
Appendix B. Acknowledgments
The authors would also like to thank Klaas Wierenga, Sam Hartman,
Nico Williams, Jim Basney, Venkat Yekkirala, and Ben Kaduk for their
contributions.
Appendix C. Changes
This section to be removed prior to publication.
o 20, address nits and easy fixes from Ben Kaduk's AD review
o 19, update obsoleted references
o 15,16,17,18 avoid expiration
o 14, address some minor comments
o 13, clarify SAML metadata usage, adding a recommended Binding
value alongside the backward-compatibility usage of PAOS
o 12, clarifying comments based on WG feedback, with a normative
change to use enctype numbers instead of names
o 11, update EAP Naming reference to RFC
o 10, update SAML ECP reference to final CS
o 09, align delegation signaling to updated ECP draft
o 08, more corrections, added a delegation signaling header
o 07, corrections, revised section on delegation
o 06, simplified session key schema, moved responsibility for
random-to-key to the endpoints, and defined advertisement of
session key algorithm and enctypes by acceptor
o 05, revised session key material, added requirement for random-to-
key, revised XML schema to capture enctype name, updated GSS
naming reference
o 04, stripped down the session key material to simplify it, and
define an IdP-brokered keying approach, moved session key XML
constructs from OASIS draft into this one
o 03, added TLS key export as a session key option, revised GSS
naming material based on list discussion
Cantor, et al. Expires November 11, 2021 [Page 33]
Internet-Draft SAML ECP SASL & GSS-API Mechanisms May 2021
o 02, major revision of GSS-API material and updated references
o 01, SSH language added, noted non-assumption of HTTP error
handling, added guidance on life of security context.
o 00, Initial Revision, first WG-adopted draft. Removed support for
unsolicited SAML responses.
Authors' Addresses
Scott Cantor
Shibboleth Consortium
1050 Carmack Rd
Columbus, Ohio 43210
United States
Phone: +1 614 247 6147
Email: cantor.2@osu.edu
Margaret Cullen
Painless Security
4 High St, Suite 134
North Andover, Massachusets 01845
United States
Phone: +1 781 405 7464
Email: mrcullen42@painless-security.com
Simon Josefsson
SJD AB
Hagagatan 24
Stockholm 113 47
SE
Email: simon@josefsson.org
URI: http://josefsson.org/
Cantor, et al. Expires November 11, 2021 [Page 34]