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Sapling Implementation Refactor #41

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ebfull opened this Issue Oct 15, 2018 · 0 comments

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ebfull commented Oct 15, 2018

Goals:

  1. Improve code quality and testing coverage to decrease the chance of critical bugs. This includes modifying code to match the specification in areas where the specification diverged. Also, clearly specifying code that is variable time.
  2. Separate components so that hardware wallets, light wallets etc. can use lightweight, portable implementations of the pieces that they need.
  3. Expose more engineers to the cryptographic underpinnings and algorithms, at the implementation level, to improve code maintenance.

Strategy:

Different components will be built and cleaned up independently and then brought together as git subtrees in this librustzcash repository as code matures.

Primitive crates (not Zcash specific, generally useful):

  1. zkcrypto/jubjub implements the Jubjub elliptic curve, the scalar field Fr, and the base field Fq, as defined in the Zcash protocol specification.
  2. zkcrypto/ff implements traits for finite fields. We will not be bringing in ff_derive or any other macros, to reduce dependency trees. jubjub will bring this in as a dependency.
  3. zkcrypto/group implements traits and generic group tools such as multi-exponentiation and FFTs. (TODO: multi-threaded variants of these algorithms; enabled by crate feature, or built outside this crate by exposing primitives?)
  4. zkcrypto/bellman implements circuit traits and primitive structures, as well as basic gadget implementations such as booleans, number abstractions, etc. Unlike now, bellman will be generic using ff and won't be pairing-specific.
  5. zkcrypto/pairing implements basic traits for pairing-friendly elliptic curve constructions.
  6. zkcrypto/bls12_381 implements BLS12-381, which brings in jubjub as a dependency (to use its Fq implementation).
  7. zkcrypto/groth16 implements the Groth16 proving system as used in Zcash. This brings in pairing and bellman as dependencies.

Zcash-specific crates:

  1. zcash_primitives contains implementations of crypto components in Zcash, such as keys and their derivations (ZIP32), addresses, notes, and any other structures or algorithms specific to Zcash, such as transactions.
  2. zcash_proofs contains an implementation of the prover and verifier for Sapling proofs. This brings in bls12-381, groth16 as dependencies and contains the implementation of the Spend/Output circuits.
  3. zcash_wallet contains tools for maintaining a Zcash wallet (something which can send and receive payments, maintain keys, etc.)
  4. librustzcash is a C FFI for consensus-rule verification of Sapling transactions, interacting with the prover from C++, etc.
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