Content

A full node verifier feature that does not require checking signers' identities externally.

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Comments

The idea is to provide a core verifier (previously full-node verifier) that has

• A list of eligible parties,
• Total stake in the system,

and be able to verify the signatures that are generated without the registration information, i.e., avk.
In this case, the signer who generates this signature has all the fields of StmSigner except from closed_reg.
So, closed_reg field is updated as Option. If the signer wants to participate in Mithril certificate generation, then it must have the closed_reg and this signer is generated with the function StmInitializer::new_signer(). Otherwise, the signer can only generate signatures to be validated by a core verifier. We call this signer core_signer, and it is created with the function StmInitializer::new_core_signer(). Note that in this version the core signer is built upon the StmInitializer, but it does not need to be built upon the StmInitializer depending on the functionality it provides.

StmSigner now provides two signing functions, one for a core signature and one for a signature to participate in certificate generation.
A core signature is generated by StmSigner::core_sign() function without closed registration and can be verified by the core verifier. This function simply creates a signature for a given message. StmSigner::sign() function calls core_sign() with msgp = (Commitment || msg), and it produces a signature for the Mithril certificate.

Similarly, StmSig is updated to provide

• core_verify: Verifies sigma and checks the indices. This function verifies the StmSigs to be
  verified by full node verifier.
• verify: Runs core_verify with msgp. This function is responsible for verifying StmSigs for mithril functionality.

A new struct StmSigRegParty including StmSig and RegParty is created. Consequently, the field signatures of StmAggrSig is now of the form Vec<StmSigRegParty> instead of Vec<(StmSig, RegParty)>. This change is done to avoid duplicate codes since the core verifier and StmAggrSig have many common operations.

A struct CoreVerifier containing eligible_parties and total_stake is created. Core verifier implements the following:

• setup: Collects unique signers in eligible_parties, calculates the total_stake, and returns the CoreVerifier.
• dedup_sigs_for_indices: This function is adopted from StmClerk::dedup_sigs_for_indices, and the new function can be used by both StmClerk and CoreVerifier.
• preliminary_verify: Verify all signatures whether they all won the lottery, if the indices are unique, and if the quorum is achieved.
• collect_sigs_vks: Collect and return the signatures and verification keys for given lists of StmSigRegParty.
• verify: Maps the signatures with the eligible parties by map_sig_party. Runs dedup_sigs_for_indices to get unique signatures. After running preliminary_verify, it collects sigs and vks with collect_sigs_vks and verifies the aggregate.

The functions of CoreVerifier is designed to be used by StmAggrSig since they have similar operations.

StmAggrSig::preliminary_verify verifies all checks from signatures except for the signature verification itself.
Indices and quorum are checked by CoreVerifier::preliminary_verify with msgp.
It collects leaves from signatures and checks the batch proof. After the batch proof is checked, it collects and returns the signatures and verification keys to be used by aggregate verification.

Minor changes:

Error enums are updated for new functionality. to_bytes and from_bytes functions are implemented for StmSigRegParty, and they are used in StmAggrSig. Unused functions StmSigner::get_closed_reg() and StmSigner::compute_avk() are removed.

ToDo

• Implement module test for CoreVerifier.
• Remove StmClerk::dedup_sigs_for_indices() and its related tests.
• Implement proptest for CoreVerifier::dedup_sigs_for_indices().
• Implement full protocol test for CoreVerifier.
• Implement benches for CoreVerifier.
• Check the compatibility of the rest of the project with new features.
• Update Mithril change log.
• Update Mithril-stm README.
• Update PR description.

Issue(s)

Closes #839