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Replace Ciphersuite::hash_to_F

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The prior-present `Ciphersuite::hash_to_F` was a sin. Implementations took a
DST, yet were not require to securely handle it. It was also biased towards the
requirements of `modular-frost` as `ciphersuite` was originally written all
those years ago, when `modular-frost` had needs exceeding what `ff`, `group`
satisfied.

Now, the hash is bound to produce an output which can be converted to a scalar
with `ff::FromUniformBytes`. A new `hash_to_F`, which accepts a single argument
of the value to hash (removing the potential to insecurely handle the DST by
removing the DST entirely). Due to `digest` yielding a `GenericArray`, yet
`FromUniformBytes` taking a `const usize`, the `ciphersuite` crate now defines
a `FromUniformBytes` trait taking an array (then implemented for all satisfiers
of `ff::FromUniformBytes`). In order to get the array type from the
`GenericArray`, the output of the hash, `digest` is updated to the `0.11`
release candidate which moves to `flexible-array` which solves that problem.

The existing, specific `hash_to_F` functions have been moved to `modular-frost`
as necessary.

`flexible-array` itself is patched to a fork due to
https://github.com/RustCrypto/hybrid-array/issues/131.
This commit is contained in:
Luke Parker
2025-08-29 05:04:03 -04:00
parent a4811c9a41
commit 90bc364f9f
37 changed files with 355 additions and 416 deletions
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20
crypto/frost/src/curve/mod.rs
View File

@@ -1,4 +1,4 @@
use core::ops::Deref;
use core::{ops::Deref, convert::AsRef};
use std::io::{self, Read};
use rand_core::{RngCore, CryptoRng};
@@ -6,9 +6,8 @@ use rand_core::{RngCore, CryptoRng};
use zeroize::{Zeroize, Zeroizing};
use subtle::ConstantTimeEq;
use digest::{Digest, Output};
pub use ciphersuite::{
digest::Digest,
group::{
ff::{Field, PrimeField},
Group,
@@ -46,24 +45,23 @@ pub trait Curve: Ciphersuite {
const CONTEXT: &'static [u8];
/// Hash the given dst and data to a byte vector. Used to instantiate H4 and H5.
fn hash(dst: &[u8], data: &[u8]) -> Output<Self::H> {
fn hash(dst: &[u8], data: &[u8]) -> impl AsRef<[u8]> {
Self::H::digest([Self::CONTEXT, dst, data].concat())
}
/// Field element from hash. Used during key gen and by other crates under Serai as a general
/// utility. Used to instantiate H1 and H3.
/// Field element from hash. Used to instantiate H1 and H3.
///
/// The `dst` MUST be prefixed by `Self::CONTEXT` by the implementor.
#[allow(non_snake_case)]
fn hash_to_F(dst: &[u8], msg: &[u8]) -> Self::F {
<Self as Ciphersuite>::hash_to_F(&[Self::CONTEXT, dst].concat(), msg)
}
fn hash_to_F(dst: &[u8], msg: &[u8]) -> Self::F;
/// Hash the message for the binding factor. H4 from the IETF draft.
fn hash_msg(msg: &[u8]) -> Output<Self::H> {
fn hash_msg(msg: &[u8]) -> impl AsRef<[u8]> {
Self::hash(b"msg", msg)
}
/// Hash the commitments for the binding factor. H5 from the IETF draft.
fn hash_commitments(commitments: &[u8]) -> Output<Self::H> {
fn hash_commitments(commitments: &[u8]) -> impl AsRef<[u8]> {
Self::hash(b"com", commitments)
}