Don't invert scalar in wnaf_fixed when it is even because a caller might
intentionally give a scalar with many leading zeros.
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@ -563,15 +563,11 @@ static size_t secp256k1_strauss_max_points(secp256k1_scratch *scratch) {
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* It has the following guarantees:
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* - each wnaf[i] is either 0 or an odd integer between -(1 << w) and (1 << w)
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* - the number of words set is always WNAF_SIZE(w)
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* - the returned skew is 0 without endomorphism, or 0 or 1 with endomorphism
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* - the returned skew is 0 or 1
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*/
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static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) {
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int sign = 0;
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int skew = 0;
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int pos = 1;
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#ifndef USE_ENDOMORPHISM
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secp256k1_scalar neg_s;
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#endif
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const secp256k1_scalar *work = s;
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if (secp256k1_scalar_is_zero(s)) {
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@ -583,16 +579,10 @@ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) {
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}
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if (secp256k1_scalar_is_even(s)) {
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#ifdef USE_ENDOMORPHISM
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skew = 1;
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#else
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secp256k1_scalar_negate(&neg_s, s);
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work = &neg_s;
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sign = -1;
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#endif
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}
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wnaf[0] = (secp256k1_scalar_get_bits_var(work, 0, w) + skew + sign) ^ sign;
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wnaf[0] = secp256k1_scalar_get_bits_var(work, 0, w) + skew;
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while (pos * w < WNAF_BITS) {
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int now = w;
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@ -602,10 +592,10 @@ static int secp256k1_wnaf_fixed(int *wnaf, const secp256k1_scalar *s, int w) {
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}
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val = secp256k1_scalar_get_bits_var(work, pos * w, now);
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if ((val & 1) == 0) {
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wnaf[pos - 1] -= ((1 << w) + sign) ^ sign;
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wnaf[pos] = (val + 1 + sign) ^ sign;
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wnaf[pos - 1] -= (1 << w);
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wnaf[pos] = (val + 1);
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} else {
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wnaf[pos] = (val + sign) ^ sign;
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wnaf[pos] = val;
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}
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/* Set a coefficient to zero if it is 1 or -1 and the proceeding digit
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* is strictly negative or strictly positive respectively. Only change
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@ -678,7 +668,6 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
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secp256k1_ge tmp;
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int idx;
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#ifdef USE_ENDOMORPHISM
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if (i == 0) {
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/* correct for wnaf skew */
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int skew = point_state.skew_na;
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@ -687,7 +676,6 @@ static int secp256k1_ecmult_pippenger_wnaf(secp256k1_gej *buckets, int bucket_wi
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secp256k1_gej_add_ge_var(&buckets[0], &buckets[0], &tmp, NULL);
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}
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}
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#endif
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if (n > 0) {
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idx = (n - 1)/2;
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secp256k1_gej_add_ge_var(&buckets[idx], &buckets[idx], &pt[point_state.input_pos], NULL);
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