Files
serai/crypto/transcript
Luke Parker a141deaf36 Smash the singular Ciphersuite trait into multiple
This helps identify where the various functionalities are used, or rather, not
used. The `Ciphersuite` trait present in `patches/ciphersuite`, facilitating
the entire FCMP++ tree, only requires the markers _and_ canonical point
decoding. I've opened a PR to upstream such a trait into `group`
(https://github.com/zkcrypto/group/pull/68).

`WrappedGroup` is still justified for as long as `Group::generator` exists.
Moving `::generator()` to its own trait, on an independent structure (upstream)
would be massively appreciated. @tarcieri also wanted to update from
`fn generator()` to `const GENERATOR`, which would encourage further discussion
on https://github.com/zkcrypto/group/issues/32 and
https://github.com/zkcrypto/group/issues/45, which have been stagnant.

The `Id` trait is occasionally used yet really should be first off the chopping
block.

Finally, `WithPreferredHash` is only actually used around a third of the time,
which more than justifies it being a separate trait.

---

Updates `dalek_ff_group::Scalar` to directly re-export
`curve25519_dalek::Scalar`, as without issue. `dalek_ff_group::RistrettoPoint`
also could be replaced with an export of `curve25519_dalek::RistrettoPoint`,
yet the coordinator relies on how we implemented `Hash` on it for the hell of
it so it isn't worth it at this time. `dalek_ff_group::EdwardsPoint` can't be
replaced for an re-export of `curve25519_dalek::SubgroupPoint` as it doesn't
implement `zeroize`, `subtle` traits within a released, non-yanked version.
Relevance to https://github.com/serai-dex/serai/issues/201 and
https://github.com/dalek-cryptography/curve25519-dalek/issues/811#issuecomment-3247732746.

Also updates the `Ristretto` ciphersuite to prefer `Blake2b-512` over
`SHA2-512`. In order to maintain compliance with FROST's IETF standard,
`modular-frost` defines its own ciphersuite for Ristretto which still uses
`SHA2-512`.
2025-09-03 13:50:20 -04:00
..
2025-08-25 10:06:35 -04:00
2024-06-01 21:46:49 -04:00

Flexible Transcript

Flexible Transcript is a crate offering:

  • Transcript, a trait offering functions transcripts should implement.
  • DigestTranscript, a competent transcript format instantiated against a provided hash function.
  • MerlinTranscript, a wrapper of merlin into the trait (available via the merlin feature).
  • RecommendedTranscript, a transcript recommended for usage in applications. Currently, this is DigestTranscript<Blake2b512> (available via the recommended feature).

The trait was created while working on an IETF draft which defined an incredibly simple transcript format. Extensions of the protocol would quickly require a more competent format, yet implementing the one specified was mandatory to meet the specification. Accordingly, the library implementing the draft defined an IetfTranscript, dropping labels and not allowing successive challenges, yet thanks to the trait, allowed protocols building on top to provide their own transcript format as needed.

DigestTranscript takes in any hash function implementing Digest, offering a secure transcript format around it. All items are prefixed by a flag, denoting their type, and their length.

MerlinTranscript was used to justify the API, and if any issues existed with DigestTranscript, enable a fallback. It was also meant as a way to be compatible with existing Rust projects using merlin.

This library was audited by Cypher Stack in March 2023, culminating in commit 669d2dbffc1dafb82a09d9419ea182667115df06. Any subsequent changes have not undergone auditing.

This library is usable under no_std.