| Internet-Draft | SAML Profile for the Metadata Query Prot | July 2024 |
| Young | Expires 9 January 2025 | [Page] |
This document profiles the Metadata Query Protocol for use with SAML metadata.¶
This document is a product of the Research and Education Federations (REFEDS) Working Group process.¶
Discussion of this draft takes place on the MDX mailing list (mdx@lists.iay.org.uk), which is accessed from [MDX.list].¶
XML versions, latest edits and the issues list for this document are available from [md-query].¶
The changes in this draft are summarized in Appendix A.22.¶
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Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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.¶
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.¶
This document makes use of the Augmented BNF metalanguage defined in [STD68].¶
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¶
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.¶
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.¶
Entities MAY also be associated with any number of additional responder-defined identifiers naming arbitrary groups of entities.¶
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.¶
A response which returns no EntityDescriptor elements
MUST be represented by an HTTP status code of 404 ("not found").¶
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¶
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¶
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.¶
MUST 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.¶
This profile mandates the availability of an 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.¶
The use of SHA-1 in section 3.6.4 of [SAML2Bind], and its resulting use in this protocol, would be vulnerable to an attack in which metadata was introduced into a system by an attacker capable of creating an entity identifier with the same SHA-1 hash as that of an existing entity's identifier.¶
Such an identifier is known as a second preimage of the original, and SHA-1's resistance to discovery of it is referred to as SHA-1's second-preimage resistance.¶
As demonstrated by the the [SHAttered] and [Shambles] attacks, the SHA-1 algorithm is known to have weak collision resistance. However, at the time of writing no attacks are known on SHA-1's second-preimage resistance; a result in this area would be required to provide the basis of an attack based on duplicating the SHA-1 hash of an existing identifier. As a result, the use of SHA-1 in SAML and in this protocol is not believed to introduce a security concern.¶
Implementations may guard against the possibility of a future
practical attack on the second-preimage resistance of SHA-1
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.¶
This document has no actions for IANA.¶
The editor would like to acknowledge the following individuals for their contributions to this document:¶
Initial version.¶
Added REFEDS RFC stream boilerplate.¶
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.¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Added an Editorial Note to help direct readers back to the discussion.¶
Fix reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Modernise normative language to include [RFC8174].¶
Improved references to RFCs.¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Replace citations in the abstract with straight textual mentions, as required by the ID-NITS checklist.¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
Strengthen Section 4.1 so that SHA-1 now MUST NOT be used in the context of digital signatures. This brings the section in line with current best practice recommendations, particularly in light of the [SHAttered] and [Shambles] attacks.¶
Revised Section 4.2 on the use of SHA-1 in transformed identifiers to:¶
Make clear that this is a SAML-level issue, not one introduced by the query protocol.¶
Reference the attacks demonstrating SHA-1's weak collision resistance.¶
Identify second-preimage resistance as the potential source of the attack we'd be concerned about for the query protocol.¶
Note that SHA-1's second-preimage resistance is at present uncompromised.¶
Bump reference to the Metadata Query Protocol [I-D.young-md-query].¶
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Formatting changes to allow rendering with xml2rfc version 3.¶
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