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Fixes intermittent failures in test_hqc192_roundtrip #321

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Fixes intermittent failures in test_hqc192_roundtrip #321

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2f4e40b
hqc-kem: fix constant-time nonzero test in compute_error_values
mikelodder7 Jun 1, 2026
d3a5fd4
fmt
mikelodder7 Jun 2, 2026
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8 changes: 4 additions & 4 deletions hqc-kem/src/reed_solomon.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -141,8 +141,8 @@ fn compute_error_values(error_values: &mut [u16], z: &[u16], error: &[u8; 256],
for j in 0..p.delta {
// Proper constant-time eq: both are u16
let diff = (j as u16) ^ delta_counter;
let zero_mask =
0u16.wrapping_sub(((diff as u32 | diff.wrapping_neg() as u32) >> 31) as u16);
let diff_u32 = diff as u32;
let zero_mask = 0u16.wrapping_sub(((diff_u32 | diff_u32.wrapping_neg()) >> 31) as u16);
let eq_mask2 = !zero_mask; // 0xFFFF if j == delta_counter

beta_j[j] ^= found_mask & eq_mask2 & GF_EXP[i];
Expand Down Expand Up @@ -184,8 +184,8 @@ fn compute_error_values(error_values: &mut [u16], z: &[u16], error: &[u8; 256],

for j in 0..p.delta {
let diff = (j as u16) ^ delta_counter;
let zero_mask =
0u16.wrapping_sub(((diff as u32 | diff.wrapping_neg() as u32) >> 31) as u16);
let diff_u32 = diff as u32;
let zero_mask = 0u16.wrapping_sub(((diff_u32 | diff_u32.wrapping_neg()) >> 31) as u16);
let eq_mask = !zero_mask;

error_values[i] ^= found_mask & eq_mask & e_j[j];
Expand Down