Does Salesforce support digital signatures on the outbound/inbound web service call to the endpoint? If so, where can this be configured?

I haven't had much luck getting SSO to work with my SAML assertion. Has anyone got this to work? If so, what does your saml response look like?  I signed my assertion and I believe everything is correct, yet the login history gives me "Failed: Assertion Invalid"

Any ideas?

Thank you.