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Expose a Ristretto::from_uniform_bytes function.

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Why expose this instead of `from_hash`? Because it allows constructing
arbitrary-length chains of orthogonal generators from a XOF.
This commit is contained in:
Henry de Valence 2018-07-26 19:04:56 -07:00
parent 920bdb3279
commit 259e2cd188
1 changed files with 22 additions and 14 deletions
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36
src/ristretto.rs
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@ -576,19 +576,10 @@ impl RistrettoPoint {
/// point should be unknown. The map is applied twice and the
/// results are added, to ensure a uniform distribution.
pub fn random<T: Rng + CryptoRng>(rng: &mut T) -> Self {
let mut field_bytes = [0u8; 32];
let mut uniform_bytes = [0u8; 64];
rng.fill(&mut uniform_bytes);
rng.fill(&mut field_bytes);
let r_1 = FieldElement::from_bytes(&field_bytes);
let R_1 = RistrettoPoint::elligator_ristretto_flavor(&r_1);
rng.fill(&mut field_bytes);
let r_2 = FieldElement::from_bytes(&field_bytes);
let R_2 = RistrettoPoint::elligator_ristretto_flavor(&r_2);
// Applying Elligator twice and adding the results ensures a
// uniform distribution.
&R_1 + &R_2
RistrettoPoint::from_uniform_bytes(&uniform_bytes)
}
/// Hash a slice of bytes into a `RistrettoPoint`.
@ -639,14 +630,31 @@ impl RistrettoPoint {
{
// dealing with generic arrays is clumsy, until const generics land
let output = hash.result();
let mut output_bytes = [0u8; 64];
output_bytes.copy_from_slice(&output.as_slice());
RistrettoPoint::from_uniform_bytes(&output_bytes)
}
/// Construct a `RistrettoPoint` from 64 bytes of data.
///
/// If the input bytes are uniformly distributed, the resulting
/// point will be uniformly distributed over the group, and its
/// discrete log with respect to other points should be unknown.
///
/// # Implementation
///
/// This function splits the input array into two 32-byte halves,
/// takes the low 255 bits of each half mod p, applies the
/// Ristretto-flavored Elligator map to each, and adds the results.
pub fn from_uniform_bytes(bytes: &[u8; 64]) -> RistrettoPoint {
let mut r_1_bytes = [0u8; 32];
r_1_bytes.copy_from_slice(&output.as_slice()[0..32]);
r_1_bytes.copy_from_slice(&bytes[0..32]);
let r_1 = FieldElement::from_bytes(&r_1_bytes);
let R_1 = RistrettoPoint::elligator_ristretto_flavor(&r_1);
let mut r_2_bytes = [0u8; 32];
r_2_bytes.copy_from_slice(&output.as_slice()[32..64]);
r_2_bytes.copy_from_slice(&bytes[32..64]);
let r_2 = FieldElement::from_bytes(&r_2_bytes);
let R_2 = RistrettoPoint::elligator_ristretto_flavor(&r_2);