Implements KVAC from Improved Algebraic MACs and Practical Keyed-Verification Anonymous Credentials
An alternate implementation of proof of knowledge of MAC is added which is adapted from the protocol to prove knowledge of BBS+ signatures described in section 4.5 of the paper Anonymous Attestation Using the Strong Diffie Hellman Assumption Revisited
In addition, it supports generating proof of validity or invalidity of keyed-proofs, i.e. the proof verifying which requires the knowledge of secret key.