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f can never equal -m #1603

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Oct 8, 2024
Merged

f can never equal -m #1603

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10 changes: 4 additions & 6 deletions src/modinv32_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -565,13 +565,12 @@ static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_m

/* g == 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, 9, &SECP256K1_SIGNED30_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
/* |f| == 1, or (x == 0 and d == 0 and f == modulus) */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, 9, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, 9, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, -1) == 0)));
secp256k1_modinv32_mul_cmp_30(&f, 9, &modinfo->modulus, 1) == 0));

/* Optionally negate d, normalize to [0,modulus), and return it. */
secp256k1_modinv32_normalize_30(&d, f.v[8], modinfo);
Expand Down Expand Up @@ -643,13 +642,12 @@ static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256

/* g == 0 */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&g, len, &SECP256K1_SIGNED30_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
/* |f| == 1, or (x == 0 and d == 0 and f == modulus) */
VERIFY_CHECK(secp256k1_modinv32_mul_cmp_30(&f, len, &SECP256K1_SIGNED30_ONE, -1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, len, &SECP256K1_SIGNED30_ONE, 1) == 0 ||
(secp256k1_modinv32_mul_cmp_30(x, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
secp256k1_modinv32_mul_cmp_30(&d, 9, &SECP256K1_SIGNED30_ONE, 0) == 0 &&
(secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, -1) == 0)));
secp256k1_modinv32_mul_cmp_30(&f, len, &modinfo->modulus, 1) == 0));

/* Optionally negate d, normalize to [0,modulus), and return it. */
secp256k1_modinv32_normalize_30(&d, f.v[len - 1], modinfo);
Expand Down
10 changes: 4 additions & 6 deletions src/modinv64_impl.h
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Expand Up @@ -621,13 +621,12 @@ static void secp256k1_modinv64(secp256k1_modinv64_signed62 *x, const secp256k1_m

/* g == 0 */
VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, 5, &SECP256K1_SIGNED62_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
/* |f| == 1, or (x == 0 and d == 0 and f == modulus) */
VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, 5, &SECP256K1_SIGNED62_ONE, -1) == 0 ||
secp256k1_modinv64_mul_cmp_62(&f, 5, &SECP256K1_SIGNED62_ONE, 1) == 0 ||
(secp256k1_modinv64_mul_cmp_62(x, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
secp256k1_modinv64_mul_cmp_62(&d, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
(secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, -1) == 0)));
secp256k1_modinv64_mul_cmp_62(&f, 5, &modinfo->modulus, 1) == 0));

/* Optionally negate d, normalize to [0,modulus), and return it. */
secp256k1_modinv64_normalize_62(&d, f.v[4], modinfo);
Expand Down Expand Up @@ -698,13 +697,12 @@ static void secp256k1_modinv64_var(secp256k1_modinv64_signed62 *x, const secp256

/* g == 0 */
VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&g, len, &SECP256K1_SIGNED62_ONE, 0) == 0);
/* |f| == 1, or (x == 0 and d == 0 and |f|=modulus) */
/* |f| == 1, or (x == 0 and d == 0 and f == modulus) */
VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &SECP256K1_SIGNED62_ONE, -1) == 0 ||
secp256k1_modinv64_mul_cmp_62(&f, len, &SECP256K1_SIGNED62_ONE, 1) == 0 ||
(secp256k1_modinv64_mul_cmp_62(x, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
secp256k1_modinv64_mul_cmp_62(&d, 5, &SECP256K1_SIGNED62_ONE, 0) == 0 &&
(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) == 0 ||
secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) == 0)));
secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, 1) == 0));

/* Optionally negate d, normalize to [0,modulus), and return it. */
secp256k1_modinv64_normalize_62(&d, f.v[len - 1], modinfo);
Expand Down
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