Keyed-Verification Anonymous Credentials (KVAC)

Implements Keyed-Verification Anonymous Credentials (KVAC) schemes from the following papers. KVACs are supposed to be verified by the issuer only (or anyone who shares the issuer's key)

1. Improved Algebraic MACs and Practical Keyed-Verification Anonymous Credentials is implemented here
2. Fast Keyed-Verification Anonymous Credentials on Standard Smart Cards is specified here but is pending implementation.

Both implementations support additional verification methods that allow joint verification of proof of possession of credentials where one of the verifier is the issuer who knows the secret key and another verifier does not know secret key but learns the revealed attributes which are not shared with the issuer. This lets us build for a use-case where issuer wants to allow anytime its issued credential is used (eg. to get paid by the verifier) while still not harming the user's privacy as it doesn't learn any revealed attributes. The first verifier, i.e. the issuer can also provide a proof of validity or invalidity to the second verifier.