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Use conditional_negate in Decaf

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Henry de Valence 2017-02-20 15:17:29 -08:00
parent c2bee88c0f
commit 9cb48c44ba
1 changed files with 10 additions and 8 deletions
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18
src/decaf.rs
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@ -24,6 +24,7 @@ use core::fmt::Debug;
use constants;
use field::FieldElement;
use subtle::CTAssignable;
use subtle::CTNegatable;
use core::ops::{Add, Sub, Neg};
@ -133,8 +134,7 @@ impl DecafPoint {
let iY = &Y * &constants::SQRT_M1;
X.conditional_assign(&iY, is_neg_mask);
Y.conditional_assign(&iX, is_neg_mask);
let minus_T = -&T;
T.conditional_assign(&minus_T, is_neg_mask);
T.conditional_negate(is_neg_mask);
// Step 1: Compute r = 1/sqrt((a-d)(Z+Y)(Z-Y))
let Z_plus_Y = &self.0.Z + &Y;
@ -149,17 +149,19 @@ impl DecafPoint {
// Step 3: Negate r if -2uZ is negative.
let uZ = &u * &self.0.Z;
let minus_r = -&r;
let m2uZ = -&(&uZ + &uZ);
let mask = m2uZ.is_negative_decaf();
r.conditional_assign(&minus_r, mask);
r.conditional_negate(m2uZ.is_negative_decaf());
// Step 4: Compute s = |u(r(aZX - dYT)+Y)/a|
// Step 4: Compute s = | u(r(aZX - dYT)+Y)/a|
// = |-u(r(aZX - dYT)+Y)|
let minus_ZX = -&(&self.0.Z * &X);
let dYT = &constants::d * &(&Y * &T);
// Compute s = u(r(aZX - dYT)+Y)
let mut s = &u * &(&(&r * &(&minus_ZX - &dYT)) + &Y);
s.negate();
CompressedDecaf(s.abs_decaf().to_bytes())
// Set s <- |-s|
let neg = s.is_nonnegative_decaf();
s.conditional_negate(neg);
CompressedDecaf(s.to_bytes())
}
/// Return the coset self + E[4], for debugging.