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Whether the Shibboleth SP can consume a message signed with an algorithm other than SHA-1 depends on the underlying OpenSSL library. On RedHat Red Hat Enterprise Linux version 4 (a very old, unsupported version) the OpenSSL version is old enough version (0.9.7) is old enough that it cannot consume messages signed, for example, with any of the digest algorithms collectively known as SHA-2 (SHA-224, SHA-256, SHA-384 or SHA-512). SHA-2 support was introduced into OpenSSL with version 0.9.8 in July 2005, but this was too late for inclusion in RHEL 4. RHEL 5.x and 6.x do include OpenSSL version 0.9.8 or later.
A particularly difficult platform to assess is Solaris, and a lot of commercial vendors use it. Many different versions of OpenSSL may be in use depending on how open source software is managed in a particular Solaris environment. Heavy testing is recommended.
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