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As of January 1, 2014, NIST disallows the use of the SHA-1 digest algorithm in conjunction with digital signatures (see: NIST SP 800-57 Part 1, Revision 3, July 2012, Tables 3 and 4). This was a major driver behind the Phase 1 Recommendations of the Metadata Distribution Working Group, which gave rise to the Phase 1 Implementation Plan, a major effort to phase out the use of SHA-1 in InCommon metadata that began on December 18, 2013 and ended on June 30, 2014.
As of June 30, 2014, all metadata aggregates distributed by InCommon are signed using the SHA-256 digest algorithm. Because of this, we know that many of the SPs in the InCommon Federation support SHA-256.
Like metadata, SAML messages may be digitally signed for authenticity and integrity. Any SAML entity (IdP or SP) may sign a message but in practice it is the IdP that wields a signing key since SAML assertions issued by the IdP MUST be signed according to the SAML spec. The SP uses the IdP’s public signing key in metadata to verify the signature on the assertion just-in-time, that is, when the assertion is presented to the SP by the browser user.
It is believed that most IdPs in the InCommon Federation are signing assertions using the SHA-1 digest algorithm. This is because most IdP deployments use the Shibboleth IdP software, which is known to support SHA-1 only, at least out of the box.
To enable SHA-2 support in the Shibboleth IdP, you can use a 3rd-party extension to enable SHA-2 capability for Shibboleth IdP versions 2.3 and later. Note that the resulting Shibboleth configuration is all or none, either SHA-1 or SHA-256.
Sites running the Shibboleth IdP software should deploy a test IdP with SHA-2 support as described in the next section and then migrate to that test environment as appropriate.
In comparison, SimpleSAMLphp has good support for signing assertions using the SHA-256 digest algorithm. It can be configured to sign assertions on a per-SP basis, that is, a simpleSAMLphp IdP can sign assertions using the SHA-1 digest algorithm for some SPs and the SHA-256 digest algorithm for others.
Sites running the simpleSAMLphp IdP software can and should migrate to SHA-256 as soon as possible. A perfectly reasonable strategy would be to configure the use of SHA-256 by default but to fall back to SHA-1 for those few remaining SPs (perhaps external to InCommon) unable to handle SHA-256.
This section describes how to migrate a Shibboleth IdP from SHA-1 to SHA-256.
Deploy a test IdP. Configure this test IdP to be nearly identical to your production IdP (same entityID, same metadata sources, same attribute release policy, etc.).
There are at least two deployment options:
If you encounter a partner SP that is not compatible with SHA-256, with no hope of upgrading, you can work around the incompatibility by deploying an IdP Proxy. Initially, the SP component of the IdP Proxy is integrated with your test IdP and both are configured to sign assertions using the SHA-256 digest algorithm. The IdP component of the IdP Proxy is integrated with the partner SP and both are configured to sign assertions using the SHA-1 digest algorithm.
Note: If the partner SP does not support message-level encryption, the IdP Proxy can compensate for that shortcoming as well.