f can never equal -m

In fact, before reaching this particular VERIFY_CHECK, we had already successfully passed through

    VERIFY_CHECK(secp256k1_modinv64_mul_cmp_62(&f, len, &modinfo->modulus, -1) > 0); /* f > -modulus */

ensuring that f is not -m.

Author: roconnor-blockstream

src/modinv32_impl.h

@@ -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);
@@ -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);

src/modinv64_impl.h

@@ -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);
@@ -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);