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NTT: Simplify computation of zeta index #677

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NTT: Simplify computation of zeta index #677

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12 changes: 5 additions & 7 deletions mlkem/ntt.c
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Original file line number Diff line number Diff line change
Expand Up @@ -104,10 +104,8 @@ __contract__(
ensures(array_abs_bound(r, 0, MLKEM_N, (layer + 1) * MLKEM_Q)))
{
unsigned start, k;
/* `layer` is a ghost variable only needed in the CBMC specification */
((void)layer);
/* Twiddle factors for layer n start at index 2^(layer-1) */
k = MLKEM_N / (2 * len);
/* Twiddle factors for layer n are at indices 2^(n-1)..2^n-1. */
k = 1u << (layer - 1);
for (start = 0; start < MLKEM_N; start += 2 * len)
__loop__(
invariant(start < MLKEM_N + 2 * len)
Expand Down Expand Up @@ -172,9 +170,9 @@ __contract__(
ensures(array_abs_bound(r, 0, MLKEM_N, MLKEM_Q)))
{
unsigned start, k;
/* `layer` is a ghost variable used only in the specification */
((void)layer);
k = MLKEM_N / len - 1;
/* Twiddle factors for layer n are at indices 2^(n-1)..2^n-1.
* The inverse NTT uses them in backwards order. */
k = (1u << layer) - 1;
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for (start = 0; start < MLKEM_N; start += 2 * len)
__loop__(
invariant(array_abs_bound(r, 0, MLKEM_N, MLKEM_Q))
Expand Down
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