proof_system

The goal of this crate is to allow creating and combining zero knowledge proofs by executing several protocols as sub-protocols. The idea is to represent each relation to be proved as a Statement, and any relations between Statements as a MetaStatement. Both of these types contain public (known to both prover and verifier) information and are contained in a ProofSpec whose goal is to unambiguously define what needs to be proven. Some Statements are specific to either the prover or the verifier as those protocols require prover and verifier to use different public parameters. An example is Groth16 based SNARK protocols where the prover needs to have a proving key and the verifier needs to have a verifying key. Both the prover and verifier can know both the proving and verifying key but they don't need to thus for such protocols, there are different Statements for prover and verifier, like SaverProver and SaverVerifier are statements for prover and verifier respectively, executing SAVER protocol. Several Statements might need same public parameters like proving knowledge of several BBS+ from the same signer, or verifiable encryption of several messages for the same decryptor. Its not very efficient to pass the same parameters to each Statement especially when using this code's WASM bindings as the same values will be serialized and deserialized every time. To avoid this, caller can put all such public parameters as SetupParams in an array and then reference those by their index while creating an Statement. This array of SetupParams is then included in the ProofSpec and used by the prover and verifier during proof creation and verification respectively.

After creating the ProofSpec, the prover uses a Witness per Statement and creates a corresponding StatementProof. All StatementProofs are grouped together in a Proof. The verifier also creates its ProofSpec and uses it to verify the given proof. Currently it is assumed that there is one StatementProof per Statement and one Witness per Statement and StatementProofs appear in the same order in Proof as Statements do in ProofSpec.

Statement, Witness and StatementProof are enums whose variants will be entities from different protocols. Each of these protocols are variants of the enum SubProtocol. SubProtocols can internally call other SubProtocols, eg SaverProtocol invokes several SchnorrProtocols

Currently supports

• proof of knowledge of a BBS+ signature and signed messages
• proof of knowledge of multiple BBS+ signature and equality of certain messages
• proof of knowledge of accumulator membership and non-membership
• proof of knowledge of Pedersen commitment opening.
• proof of knowledge of a BBS+ signature and certain message satisfies given bounds (range proof)
• verifiable encryption of messages in a BBS+ signature

See following tests for examples:

• test pok_of_3_bbs_plus_sig_and_message_equality proves knowledge of 3 BBS+ signatures and also that certain messages are equal among them without revealing them.
• test pok_of_bbs_plus_sig_and_accumulator proves knowledge of a BBS+ signature and also that certain messages are present and absent in the 2 accumulators respectively.
• test pok_of_knowledge_in_pedersen_commitment_and_bbs_plus_sig proves knowledge of a BBS+ signature and opening of a Pedersen commitment.
• test requesting_partially_blind_bbs_plus_sig shows how to request a blind BBS+ signature by proving opening of a Pedersen commitment.
• test verifier_local_linkability shows how a verifier can link separate proofs from a prover (with prover's permission) and assign a unique identifier to the prover without learning any message from the BBS+ signature. Also this identifier cannot be linked across different verifiers (intentional by the prover).
• test pok_of_bbs_plus_sig_and_bounded_message shows proving knowledge of a BBS+ signature and that a specific message satisfies some upper and lower bounds i.e. min <= signed message <= max. This is a range proof.
• test pok_of_bbs_plus_sig_and_verifiable_encryption shows how to verifiably encrypt a message signed with BBS+ such that the verifier cannot decrypt it but still ensure that it is encrypted correctly for the specified decryptor.
• test pok_of_bbs_plus_sig_with_reusing_setup_params shows proving knowledge of several BBS+ signatures using SetupParamss. Here the same signers are used in multiple signatures thus their public params can be put as a variant of enum SetupParams. Similarly test pok_of_knowledge_in_pedersen_commitment_and_equality_with_commitment_key_reuse shows use of SetupParams when the same commitment key is reused in several commitments and test pok_of_bbs_plus_sig_and_verifiable_encryption_of_many_messages shows use of SetupParams when several messages are used in verifiable encryption for the same decryptor.

Note: This design is largely inspired from my work at Hyperledger Ursa.

Note: The design is tentative and will likely change as more protocols are integrated.

License: Apache-2.0