Implement CollisionResistance for XOFs#694
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## Changes to `ExpandMsg` - Move generic parameter `K` from `ExpandMsg` implementers to `trait ExpandMsg` itself. This was necessary to be able to enforce the correct `K` instead of letting users insert an arbitrary one. E.g. `GroupDigest::hash_from_bytes()` can now `where X: ExpandMsg<Self::K>` instead of users calling `hash_from_bytes::<ExpandMsgXmd<Sha256, U0>>()`. However, this made calling `ExpandMsg` implementers directly more difficult. E.g. instead of `ExpandMsgXmd::<Sha256, U32>::expand_msg(...)` users now have to write `<ExpandMsgXmd<Sha256> as ExpandMsg<U32>>::expand_message()`. If we want to address this, I propose adding `RawExpandMsgXmd`. - Add `CollisionResistance` requirement to `ExpandMsgXof`. - Move the lifetime on `ExpandMsg` to the associated `type Expander<'dst>`. - Fix `dst` not actually being checked to be empty, but instead checked for number of slices. - Move `GroupDigest`s `ProjectivePoint: CofactorGroup` bound to super trait bounds. This makes it less "poisoning" so downstream users don't have to write `where ProjectivePoint: CofactorGroup` every time they use `GroupDigest`. - Move `GroupDigest::hash_to_scalar()`s `Scalar: FromOkm` bounds to `GroupDigest`. I believe this was a historical leftover when `FromOkm` wasn't implemented for `Scalar`s yet. - Improved some documentation around hash2curve and updated all mentions of the draft to RFC9380. - Rename parameter names `msgs` and `dsts` to singular `msg` and `dst`. This is to avoid confusion: even though the type is `&[&[u8]]`, it doesn't represent multiple messages or DSTs, but single concated ones. - Remove non-functioning examples. While I don't think the examples are necessary, I'm happy to re-add them if desired, but I would have to add `GroupDigest` to the `Dev` curve. ## Changes to VOPRF While I was at it, I also adjusted a couple of things around `VoprfParameters` (but I'm happy to split this into its own PR): - Renamed all mentions of VOPRF to OPRF. VOPRF was the old name of the draft when it was just a single "mode", the RFC is split into three modes: OPRF, VOPRF and POPRF. The RFC itself is called ["Oblivious Pseudorandom Functions (OPRFs)"](https://www.rfc-editor.org/rfc/rfc9497.html). - Changed associated `const ID` from `&str` to `&[u8]`. - Changed associated `type Hash` from requiring `Digest` to `Default + FixedOutput + Update`. - Updated all mentions of the VOPRF draft to RFC9497. --- Related: RustCrypto/hashes#694. Companion PR: RustCrypto/elliptic-curves#1203.
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This PR implements
CollisionResistancefor all XOFs. I started with those to add support forExpandMsgXofinelliptic-curveand will do follow-up PRs for at least SHA2 and SHA3 fixed output hashes.Companion PR: RustCrypto/traits#1862.
See RustCrypto/traits#1816 for previous discussions.