This document defines a <saml:Condition> type for expressing a chain of intermediaries acting on behalf of the subject of an assertion, requring relying parties to distinguish between direct and indirect access.

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1 Introduction 5

1.1 Notation 5

1.2 Normative References 6

1.3 Non-Normative References 6

2 SAML V2.0 Condition for Delegation Restriction 7

2.1 Required Information 7

2.2 Overview 7

2.2.1 Terminology and Motivation 7

2.3 Element <Delegate> 9

2.4 Complex Type DelegationRestrictionType 10

2.5 Use of Identifiers Within <saml:SubjectConfirmation> 10

2.6 Security Considerations 11

3 Conformance 12

3.1 SAML V2.0 Condition for Delegation Restriction 12

Appendix A. Acknowledgements 13

Appendix B. Revision History 14

1 Introduction

Some advanced SAML use cases involve a single logical transaction that spans one or more intermediate clients or servers. A common example includes a SAML-enabled web site acting on behalf of a logged-in user while accessing additional SAML-enabled web services. Generalizing this example, a number of intermediaries might be transited before the final point of access. If a SAML assertion is used as a security token to authenticate and authorize such access, it is important that the identity and order of intermediaries, if any, be expressed within the token in some fashion.

Existing mechanisms designed for this purpose, such as the <saml:SubjectConfirmation> element definition in the SAML V2.0 core specification [SAML2Core], or the extended syntax found in the Liberty ID-WSF Security Mechanisms specification [LibSecMech20], suffer from the drawback that they have advisory semantics for a relying party and are likey to be ignored by delegation-unaware SAML processing. While backward compatibility can be an advantage, ignoring security-relevant details that might impact upon a relying party's policy is unacceptable in some scenarios.

This specification provides for the expression of delegation information with normative SAML processing semantics through the use of a <saml:Condition> extension type.

1.1 Notation

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119]:

…they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)…

These keywords are thus capitalized when used to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. When these words are not capitalized, they are meant in their natural-language sense.

Conventional XML namespace prefixes are used throughout the listings in this specification to stand for their respective namespaces as follows, whether or not a namespace declaration is present in the example:

Prefix	XML Namespace	Comments
saml:	urn:oasis:names:tc:SAML:2.0:assertion	This is the SAML V2.0 assertion namespace defined in the SAML V2.0 core specification [SAML2Core].
del:	urn:oasis:names:tc:SAML:2.0:conditions:delegation	This is the namespace defined by this specification.
xsd:	http://www.w3.org/2001/XMLSchema	This namespace is defined in the W3C XML Schema specification [Schema1]. In schema listings, this is the default namespace and no prefix is shown.
xsi:	http://www.w3.org/2001/XMLSchema-instance	This is the XML Schema namespace for schema-related markup that appears in XML instances [Schema1].

This specification uses the following typographical conventions in text: <SAMLElement>, <ns:ForeignElement>, Attribute, Datatype, OtherCode.

1.2 Normative References

[RFC2119] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. IETF RFC 2119, March 1997. http://www.ietf.org/rfc/rfc2119.txt

1.3 Non-Normative References

2 SAML V2.0 Condition for Delegation Restriction

2.1 Required Information

2.2 Overview

The SAML V2.0 core specification [SAML2Core] defines the saml:ConditionAbstractType complex type as a basis for extensions with mandatory processing semantics for relying parties. This specification defines such an extension as a supplement for the presence of an identifier within the <saml:SubjectConfirmation> element.

Rather than an advisory mechanism for identifying a single delegate, the extension provides for a normative mechanism that identifies an ordered sequence of delegates, along with optional detail about the acts of delegation.

2.2.1 Terminology and Motivation

Delegation can be complex and is frequently conflated, combined, or confused with a number of related approaches. Without attempting to address all the myriad ways of describing such interactions, for the purposes of this profile the following is an attempt to capture some of the alternatives encountered and how the notion of delegation is meant for the purposes of this profile. In most of the cases presented, the flows involved are simplified for illustration. These are not meant as normative scenarios.

As described by section 3.4.1.5 of the SAML V2.0 core specification [SAML2Core], proxying occurs when an intermediate identity provider issues an assertion to a relying party on the basis of an assertion issued to it. Proxying is a gateway-like function in which the subect of the assertion is presumed to directly interact with each party directly.

In Figure 1: Proxying, the sequence proceeds from left to right, with the user agent authenticating to the left-most system to obtain an assertion for the proxy, which issues an assertion for the user agent to deliver to the service.

An impersonation model is one in which an entity acting on behalf of an assertion subject is able to obtain and use an assertion indistinguishable from an assertion that would be issued directly to the subject. A typical example of such a scenario might include a portal that holds credentials for users and is able to authenticate directly to obtain assertions about them.

In Figure 2: Impersonation, the user agent supplies its credentials (e.g., a password) to the portal, which relays them to the identity provider on the left, obtaining an assertion that it supplies to the service. As far as the service knows, the portal is a direct user agent.

Forwarding is a form of impersonation in which an assertion is directly reused by an intermediary to impersonate a subject from whom an assertion was obtained. It is distinguished from delegation in that assertions are passed along unchanged, and because (as an impersonation model) the assertion is not modified in a fashion that would identify the intermediary.

In Figure 3: Forwarding, the user agent authenticates directly to the identity provider, obtaining an assertion that it delivers to the service in the middle. The service directly forwards the assertion to the second service on the right, acting as the subject. As far as the second service knows, the first service is a direct user agent (as with impersonation).

Finally, delegation moves beyond the forwarding scenario by adding information to the assertion that explicitly identifies all the parties through which a transaction flows. While it is ultimately a matter for the identity provider as to how and on what basis this information is collected, it is often assured by routing requests back through the identity provider at each hop to cryptographically guarantee that each party has been authenticated and appropriate policy enforced. This fine-grained and real-time enforcement capability is a key advantage over pure forwarding or impersonation.

In Figure 4: Delegation, the forwarding model is extended by adding a request back to the identity provider by the intermediary service in the middle. This allows a new assertion to be issued to it that, while it may identify the same subject as the original, also identifies the service as a delegate of the subject. This identification can be performed advisedly, in a manner defined by [SAML2Core], or with a normative semantic as defined by this profile.

So, in summary, this profile is intended to address scenarios in which assertions are materially altered to reflect the path of a transaction through one or more intermediaries that act on behalf of the subject of the assertion. These intermediaries are termed delegates, and an assertion carrying the condition type defined in this profile is termed a delegate assertion. The act of producing such an assertion is then termed delegation and we can say that the credentials from which the assertion is derived have been delegated. Such credentials are therefore delegatable. In the context of SAML, an assertion might be used as a delegatable credential, so it's possible in multiple-tier scenarios for a delegate assertion to itself be delegatable.

There are no normative requirements associated with the use of these terms, and they do not materially affect the semantics of the profile, but using terms consistently across implementations and scenarios is likely to aid understanding and deployment.

2.3 Element <Delegate>

The <Delegate> element is a container for a single intermediary/delegate represented by the assertion. It contains the following elements and attributes:

A timestamp indicating the approximate time at which the act of delegation occurred, if known.

Identifies the subject confirmation method used, if the delegate presented a SAML assertion to authenticate itself to the issuing authority.

The delegate is identified by a required child element in the usual SAML fashion. The optional attributes, if present, supply additional information about the act of delegation.

The following schema fragment defines the <Delegate> element and its DelegateType complex type:

2.4 Complex Type DelegationRestrictionType

The DelegationRestrictionType complex type defines a subtype of saml:ConditionType representing one or more acts of delegation that are represented by the containing assertion. It contains the following elements:

An element identifying a delegate of the subject of the containing assertion. The delegates MUST be ordered from least to most recent; thus the earliest element is the farthest removed from the immediate use of the assertion.

A relying party MUST evaluate the list of delegates, and SHOULD NOT accept the assertion unless it wishes to permit each delegate to act on behalf of the subject of the containing assertion.

A SAML authority MUST NOT include more than one <saml:Condition> element of this type within a <saml:Conditions> element of an assertion.

For the purposes of determining the validity of the <saml:Conditions> element, this condition type is always considered to be valid. That is, this condition type does not affect assertion validity, but is a condition on use.

The following schema fragment defines the DelegationRestrictionType complex type:

2.5 Use of Identifiers Within <saml:SubjectConfirmation>

For consistency with the existing SAML-defined syntax, it is RECOMMENDED that the identifier of the most recent delegate (within the last element in the condition, per section 2.4) be duplicated within the relevant <saml:SubjectConfirmation> elements in the containing assertion.

2.6 Security Considerations

The content of this condition type is directly impacted by the security semantics of the flow of activity that leads to the issuance of the containing assertion. This specification does not define the exchanges that must take place, and relies on composition with other profiles that logically represent acts of delegation that require representation in an assertion.

Relying parties are not required to apply any particular policies with regard to the information represented by this condition type. Rather, it is expected that such information will naturally be significant in the enforcement of existing policies, and that the presence of delegation is significant enough to warrant the disruption of existing services designed to consume SAML assertions until those policies reflect a willingness to accept more indirect forms of access.

3 Conformance

3.1 SAML V2.0 Condition for Delegation Restriction

An assertion issuer conforms to this specification if it can generate assertions containing a <saml:Condition> of type DelegationRestrictionType, per section 2.

A relying party conforms to this specification if it can successfully process assertions containing a <saml:Condition> of type DelegationRestrictionType, per section 2.

The editors would like to acknowledge the contributions of the OASIS Security Services Technical Committee, whose voting members at the time of publication were: