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The `FPTX` batch threshold encryption scheme

crates/aptos-batch-encryption/README.md

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The FPTX batch threshold encryption scheme

Overview

This crate includes the batch threshold encryption scheme used in the Aptos encrypted mempool. There are several variants of the scheme:

  • FPTX is the original unweighted version of the scheme.
  • FPTXWeighted is a wrapper around FPTX where each master secret key share and decryption key share is weighted. This is in order to support the weighted setting of a proof-of-stake blockchain. FPTXWeighted is the scheme used in production.
  • FPTXSuccinct is a modified version of the scheme which saves a single group element per ciphertext. However, it requires a DKG which outputs $[\tau \cdot \mathsf{sk}]_2$, where $\mathsf{sk}$ is the per-epoch master secret key, which our DKG does not currently do. Because of this, it is not currently used in production.

Repo structure

  • src/traits.rs defines the interface which this crate exports to be used during consensus. Each of the schemes implement this trait.
  • The individual schemes are in src/schemes.
  • The elliptic curve group used in this crate is defined in src/group.rs.
  • Ciphertexts are defined in src/shared/ciphertext. Each scheme's ciphertext consists of two layers: an inner "batch IBE" ciphertext, defined in src/shared/ciphertext/{bibe.rs,bibe_succinct.rs}, and a wrapper, in src/shared/ciphertext/mod.rs, which generically turns a batch IBE ciphertext into a non-malleable (non-IBE) ciphertext. To do this, it uses various symmetric crypto primitives defined in src/shared/symmetric.rs.
    • Note: src/shared/ciphertext/bibe_succinct.rs is part of FPTXSuccinct, and is not used in production.
  • Block-specific decryption key reconstruction and verification are in src/shared/key_derivation.rs.
  • Types/code related to digest computation are in src/shared/digest.rs and src/shared/ids.rs.
  • Various required algebraic operations are implemented in src/shared/algebra. Most notably of these, two implementations of the generalized FK algorithm are in src/shared/algebra/fk_algorithm.rs. We currently only use the more naive of these two implementations, as it is faster for our batch size.