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Tweak ristretto elligator to match ristretto.group description

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Henry de Valence 2018-11-01 15:19:13 -07:00
parent 4e2fc53e5b
commit 8f605dce6d
1 changed files with 14 additions and 19 deletions
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33
src/ristretto.rs
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@ -577,37 +577,32 @@ impl RistrettoPoint {
pub(crate) fn elligator_ristretto_flavor(r_0: &FieldElement) -> RistrettoPoint {
let (i, d) = (&constants::SQRT_M1, &constants::EDWARDS_D);
let one = FieldElement::one();
let one_minus_d_sq = &one - &d.square();
let d_minus_one_sq = (d - &one).square();
let r = i * &r_0.square();
// D = (dr -a)(ar-d) = -(dr+1)(r+d)
let D = -&( &(&(d * &r) + &one) * &(&r + d) );
// N = a(d-a)(d+a)(r+1) = -(r+1)(d^2 -1)
let d_sq = d.square();
let N = -&( &(&d_sq - &one) * &(&r + &one) );
let N_s = &(&r + &one) * &one_minus_d_sq;
let mut c = -&one;
let (N_over_D_is_square, mut s) = FieldElement::sqrt_ratio_i(&N, &D);
let D = &(&c - &(d * &r)) * &(&r + d);
let (Ns_D_is_sq, mut s) = FieldElement::sqrt_ratio_i(&N_s, &D);
let mut s_prime = &s * r_0;
let s_prime_is_pos = !s_prime.is_negative();
s_prime.conditional_negate(s_prime_is_pos);
s.conditional_assign(&s_prime, !N_over_D_is_square);
c.conditional_assign(&r, !N_over_D_is_square);
// T = (c * (r - one) * (d-one).square()) - D;
let T = &(&c * &(&(&r - &one) * &((d - &one).square()))) - &D;
s.conditional_assign(&s_prime, !Ns_D_is_sq);
c.conditional_assign(&r, !Ns_D_is_sq);
let N_t = &(&(&c * &(&r - &one)) * &d_minus_one_sq) - &D;
let s_sq = s.square();
let P = CompletedPoint{
// The conversion from W_i is exactly the conversion from P1xP1.
RistrettoPoint(CompletedPoint{
X: &(&s + &s) * &D,
Z: &T * &constants::SQRT_AD_MINUS_ONE,
Z: &N_t * &constants::SQRT_AD_MINUS_ONE,
Y: &FieldElement::one() - &s_sq,
T: &FieldElement::one() + &s_sq,
};
// Convert to extended and return.
RistrettoPoint(P.to_extended())
}.to_extended())
}
/// Return a `RistrettoPoint` chosen uniformly at random using a user-provided RNG.