Privacy-Preserving Authorization

Question 1

Anonymous authorization definition

Answer 1

• A cryptographic primitive that enables users to prove possession of an attribute
• Security property: the proving party cannot lie, and the verifying party cannot be convinced if not true
• Privacy property: the verifying party cannot learn anything, other than the veracity of the statement proven (and what one infers from the statement itself)
• Build on zero-knowledge proofs

Question 2

Properties of zero-knowledge proofs

Answer 2

• Completeness: If the statement is true, an honest prover can convince a honest verifier that the statement is true
• Soundness: If the statement is false, a cheating prover cannot convince an honest verifier with high probability
• Zero-knowledge: If the statement is true, no verifier learns anything other than the fact that the statement is true

Question 3

Properties of attribute-based credentials

Answer 3

• Unforgeability: only the issuer should be able to produce valid credentials
• Selective disclosure: the user can hide irrelevant attributes
• Issuer unlinkability: the issuer should not be able to recognize a credential that it previously issued
• Verifier unlinkability: the verifier should not be able to link two consecutive showings of the same credential