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Remove cross-group DLEq challenge bias as possible

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Luke Parker 2022-07-02 02:45:26 -04:00
parent 1c98f15d5b
commit 7e058f1c08
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GPG key ID: F9F1386DB1E119B6
1 changed files with 18 additions and 2 deletions
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20
crypto/dleq/src/cross_group/mod.rs
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@ -7,7 +7,7 @@ use transcript::Transcript;
use group::{ff::{Field, PrimeField, PrimeFieldBits}, prime::PrimeGroup};
use crate::{Generators, challenge};
use crate::Generators;
pub mod scalar;
use scalar::{scalar_normalize, scalar_convert};
@ -34,6 +34,11 @@ pub(crate) fn read_point<R: Read, G: PrimeGroup>(r: &mut R) -> std::io::Result<G
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Bit<G0: PrimeGroup, G1: PrimeGroup> {
commitments: (G0, G1),
// Merged challenges have a slight security reduction, yet one already applied to the scalar
// being proven for, and this saves ~8kb. Alternatively, challenges could be redefined as a seed,
// present here, which is then hashed for each of the two challenges, remaining unbiased/unique
// while maintaining the bandwidth savings, yet also while adding 252 hashes for
// Secp256k1/Ed25519
e: G0::Scalar,
s: [(G0::Scalar, G1::Scalar); 2]
}
@ -116,11 +121,22 @@ impl<G0: PrimeGroup, G1: PrimeGroup> DLEqProof<G0, G1>
blinding_key
}
fn mutual_scalar_from_bytes(bytes: &[u8]) -> (G0::Scalar, G1::Scalar) {
let capacity = usize::try_from(G0::Scalar::CAPACITY.min(G1::Scalar::CAPACITY)).unwrap();
debug_assert!((bytes.len() * 8) >= capacity);
let mut accum = G0::Scalar::zero();
for b in 0 .. capacity {
accum += G0::Scalar::from((bytes[b / 8] & (1 << (b % 8))).into());
}
(accum, scalar_convert(accum).unwrap())
}
#[allow(non_snake_case)]
fn nonces<T: Transcript>(mut transcript: T, nonces: (G0, G1)) -> (G0::Scalar, G1::Scalar) {
transcript.append_message(b"nonce_0", nonces.0.to_bytes().as_ref());
transcript.append_message(b"nonce_1", nonces.1.to_bytes().as_ref());
scalar_normalize(challenge(&mut transcript))
Self::mutual_scalar_from_bytes(transcript.challenge(b"challenge").as_ref())
}
#[allow(non_snake_case)]