SAML Profile for the Metadata Query Protocol
draft-young-md-query-saml-02

Abstract

This document profiles the Metadata Query Protocol [I-D.young-md-query] for use with SAML metadata [SAML2Meta].

This document is a product of the Research and Education Federations (REFEDS) Working Group process.

Status of This Memo

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This Internet-Draft will expire on April 26, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Notation and Conventions
• 2. Request Profile
• 2.1. Content Type
• 2.2. Identifiers
• 2.2.1. Unique Identifier
• 2.2.2. Transformed Identifier
• 2.2.3. Additional Identifiers
• 3. Response Profile
• 3.1. Response Cardinality
• 3.1.1. No Entity Descriptors Returned
• 3.1.2. One Entity Descriptor Returned
• 3.1.3. More Than One Entity Descriptor Returned
• 4. Security Considerations
• 4.1. Integrity
• 4.2. Use of SHA-1 in Transformed Identifiers
• 5. IANA Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Change Log (to be removed by RFC Editor before publication)
• A.1. Since draft-young-md-query-saml-01
• A.2. Since draft-young-md-query-saml-00
• A.3. draft-young-md-query-saml-00

1. Introduction

This document profiles the Metadata Query Protocol [I-D.young-md-query] for use with SAML metadata [SAML2Meta].

The Research and Education Federations group ([REFEDS]) is the voice that articulates the mutual needs of research and education identity federations worldwide. It aims to represent the requirements of research and education in the ever-growing space of access and identity management.

From time to time REFEDS will wish to publish a document in the Internet RFC series. Such documents will be published as part of the RFC Independent Submission Stream [RFC4844]; however the REFEDS working group sign-off process will have been followed for these documents, as described in the REFEDS Participant's Agreement [REFEDS.agreement].

This document is a product of the REFEDS Working Group process.

1.1. Notation and Conventions

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 [BCP14].

This document makes use of the Augmented BNF metalanguage defined in [STD68].

2. Request Profile

2.1. Content Type

Requests compliant with this profile MUST include the following HTTP header to indicate that the metadata returned should be SAML metadata (see Appendix A of [SAML2Meta]):

Accept: application/samlmetadata+xml

2.2. Identifiers

2.2.1. Unique Identifier

Each entity known to the responder MUST be associated with the unique identifier of the entity, corresponding to the entityID attribute of the entity's EntityDescriptor element in SAML metadata.

2.2.2. Transformed Identifier

SAML 2.0 [SAML2Core] includes profiles based on the transfer of an "artifact" containing the unique identifier of a SAML entity transformed by means of the SHA-1 [RFC3174] hash algorithm (see [SAML2Bind] sections 3.6 and 3.6.4).

In order to support use cases in which clients may be in possession of only such a transformed representation of a SAML entity's unique identifier without any way to establish the original entity identifier, a responder compliant with this profile MUST associate each entity with an identifier matching the sha1id production in the following ABNF grammar, and treat such an identifier as equivalent to the corresponding untransformed identifier:

SHA1     = %x73 %x68 %x61 %x31 ; lower case "sha1"
DIGIT    = %x30-39
HEXDIGIT = DIGIT | %x61-66 ; lower case a-f
sha1id   = "{" SHA1 "}" sha1hex
sha1hex  = 40*HEXDIGIT

In the above, the sha1hex component encodes the 20-octet (160-bit) binary SHA-1 hash value as a sequence of 40 lower case hexadecimal digits.

For example, the identifier

http://example.org/service

transformed by means of SHA-1 hashing would become

{sha1}11d72e8cf351eb6c75c721e838f469677ab41bdb

Responder implementations MAY detect malformed SHA-1 transformed identifiers (for example where the string of characters following the "}" contains characters other than hexadecimal digits, or is other than exactly 40 characters in length) and return an HTTP status code of 400 ("bad request"). Alternatively, implementations MAY process these as normal identifiers and return an HTTP status code of 404 ("not found") if appropriate.

2.2.3. Additional Identifiers

Entities MAY also be associated with any number of additional responder-defined identifiers naming arbitrary groups of entities.

3. Response Profile

3.1. Response Cardinality

A request may return information for any number of entities, including none. Responses compliant with this profile MUST use the appropriate representation described below depending on the number of EntityDescriptor elements returned.

3.1.1. No Entity Descriptors Returned

A response which returns no EntityDescriptor elements MUST be represented by an HTTP status code of 404 ("not found").

3.1.2. One Entity Descriptor Returned

A response which returns a single EntityDescriptor element MUST use that element as its document element. The responder MUST NOT make use of a EntitiesDescriptor element in this situation (see [SAML2Meta] section 2.3).

Such a response MUST include the following HTTP header to indicate that the metadata returned is SAML metadata:

Content-Type: application/samlmetadata+xml

3.1.3. More Than One Entity Descriptor Returned

A response which returns more than one EntityDescriptor element MUST consist of a document element which is an EntitiesDescriptor element, containing the returned EntityDescriptor elements as children. Responses MUST NOT contain nested EntitiesDescriptor elements.

Such a response MUST include the following HTTP header to indicate that the metadata returned is SAML metadata:

Content-Type: application/samlmetadata+xml

4. Security Considerations

4.1. Integrity

As SAML metadata contains information necessary for the secure operation of interacting services it is strongly RECOMMENDED that a mechanism for integrity checking is provided to clients.

It is RECOMMENDED that the integrity checking mechanism provided by a responder is a digital signature embedded in the returned metadata document, as defined by [SAML2Meta] section 3.

Such digital signatures:

• SHOULD use an RSA keypair whose modulus is no less than 2048 bits in length.
• SHOULD NOT use the SHA-1 cryptographic hash algorithm as a digest algorithm.
• MUST NOT use the MD5 cryptographic hash algorithm as a digest algorithm.
• SHOULD otherwise follow current cryptographic best practices in algorithm selection.

4.2. Use of SHA-1 in Transformed Identifiers

This profile mandates the availability of a identifier synonym mechanism based on the SHA-1 cryptographic hash algorithm. Although SHA-1 is now regarded as weak enough to exclude it from use in new cryptographic systems, its use in this profile is necessary for full support of the SAML 2.0 standard.

Because the SHA-1 cryptographic hash is not being used within this profile in the context of a digital signature, it is not believed to introduce a security concern over and above that which already exists in SAML due to the possibility of a post-hash collision between entities whose entityID attributes hash to the same value.

Implementations may guard against this possibility by treating two entities whose entityID values have the same SHA-1 equivalent as an indicator of malicious intent on the part of the owner of one of the entities.

5. IANA Considerations

This document has no actions for IANA.

6. Acknowledgements

The editor would like to acknowledge the following individuals for their contributions to this document:

• Scott Cantor (The Ohio State University)
• Leif Johansson (SUNET)
• Joe St Sauver (University of Oregon)
• Tom Scavo (Internet2)

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Change Log (to be removed by RFC Editor before publication)

A.1. Since draft-young-md-query-saml-01

Bump reference to the Metadata Query Protocol [I-D.young-md-query].

Rework Section 2.2 to make the role of transformed identifiers clearer. This changes the semantics slightly (malformed transformed identifiers may now result in a 404 return rather than 400) but this gives implementers more latitude in the way that they handle the feature.

A.2. Since draft-young-md-query-saml-00

Added REFEDS RFC stream boilerplate.

A.3. draft-young-md-query-saml-00

Initial version.

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