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patch: remove ConditionallyNegatable for Aeneas/Charon transpilation

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Upstream: anza-xyz/cryptography
Required for: formal verification via Aeneas bf13c42e + Charon 9dd7f23c
This commit is contained in:
mrwulf 2026-06-30 17:30:34 +02:00
parent 0a54ccaf04
commit e5f4598766
1 changed files with 9 additions and 2 deletions
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11
curve25519/solana-ed25519/src/field.rs
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@ -26,7 +26,6 @@
#![allow(unused_qualifications)] #![allow(unused_qualifications)]
use subtle::Choice; use subtle::Choice;
use subtle::ConditionallyNegatable;
use subtle::ConditionallySelectable; use subtle::ConditionallySelectable;
use subtle::ConstantTimeEq; use subtle::ConstantTimeEq;
@ -355,8 +354,15 @@ impl FieldElement {
r.conditional_assign(&r_prime, flipped_sign_sqrt | flipped_sign_sqrt_i); r.conditional_assign(&r_prime, flipped_sign_sqrt | flipped_sign_sqrt_i);
// Choose the nonnegative square root. // Choose the nonnegative square root.
//
// NOTE: written as negate-then-conditional-assign rather than
// `r.conditional_negate(r_is_negative)`: semantically identical and
// still constant-time, but it avoids subtle's `ConditionallyNegatable`
// blanket impl (whose `for<'a> &'a T: Neg` bound breaks the Aeneas
// verification toolchain; see verification/field/README.md).
let r_is_negative = r.is_negative(); let r_is_negative = r.is_negative();
r.conditional_negate(r_is_negative); let r_neg = -&r;
r.conditional_assign(&r_neg, r_is_negative);
let was_nonzero_square = correct_sign_sqrt | flipped_sign_sqrt; let was_nonzero_square = correct_sign_sqrt | flipped_sign_sqrt;
@ -516,6 +522,7 @@ where
#[cfg(test)] #[cfg(test)]
mod test { mod test {
use crate::field::*; use crate::field::*;
use subtle::ConditionallyNegatable;
/// Random element a of GF(2^255-19), from Sage /// Random element a of GF(2^255-19), from Sage
/// a = 1070314506888354081329385823235218444233221\ /// a = 1070314506888354081329385823235218444233221\