[keycloak-dev] Support for key rotation in SAML Redirect binding

Hi All, re KEYCLOAK-1881 [1]: Key rotation in SAML needs to distill key ID used for signing for every signature (assertion/document-level) to validate the signature with the correct key. In POST binding, dsig:KeyInfo element bears this information in dsig:KeyName both for signed assertions and for the whole document (a.k.a. protocol message). For REDIRECT binding in SAML, there is currently no place where KeyID can be inserted on document level. The document signature in REDIRECT binding has to be removed from the document (line 578 in [2]) and replaced by the Signature query parameter, so it is not advisable to put key ID there. To solve this, there are multiple options: 1) Modify SAML assertion by adding <Extensions> element that would include document signing key ID. For consistency reasons, this would also be present for POST binding SAML documents. This only works for SAML 2.0 but that does not matter as REDIRECT is new to SAML2.0 anyway. 2) Pass signing key ID in REDIRECT query parameter (next to Signature and SigAlg parameters) - seems not strictly against the spec but weird 3) Pass signing key ID in custom HTTP header - this might impact clustering setup and is even weirder than the previous item 4) Claim REDIRECT binding not supporting signatures signed with key rotation and only support them for POST bindings (for REDIRECT, it would only work for preset public keys). This is not a viable option for dynamic key retrieval, so including just for completeness. Seems like Option 1 or 2 is the one to go. Any thoughts/suggestions? Thanks --Hynek [1] https://issues.jboss.org/browse/KEYCLOAK-1881 [2] https://docs.oasis-open.org/security/saml/v2.0/saml-bindings-2.0-os.pdf