Abstract out verify logic for fe_set_b32

Author: sipa

src/field.h

@@ -85,6 +85,7 @@ static const secp256k1_fe secp256k1_const_beta = SECP256K1_FE_CONST(
 #  define secp256k1_fe_is_zero secp256k1_fe_impl_is_zero
 #  define secp256k1_fe_is_odd secp256k1_fe_impl_is_odd
 #  define secp256k1_fe_cmp_var secp256k1_fe_impl_cmp_var
+#  define secp256k1_fe_set_b32 secp256k1_fe_impl_set_b32
 #endif /* !defined(VERIFY) */
 
 /** Normalize a field element.
@@ -173,9 +174,15 @@ static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b);
  */
 static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const secp256k1_fe *b);
 
-/** Set a field element equal to 32-byte big endian value.
- *  Returns 1 if no overflow occurred, and then the output is normalized.
- *  Returns 0 if overflow occurred, and then the output is only weakly normalized. */
+/** Set a field element equal to a provided 32-byte big endian value.
+ *
+ * On input, r does not need to be initalized. a must be a pointer to an initialized 32-byte array.
+ * On output, r = a (mod p). It will have magnitude 1, and if (a < p), it will be normalized.
+ * If not, it will only be weakly normalized. Returns whether (a < p).
+ *
+ * Note that this function is unusual in that the normalization of the output depends on the
+ * run-time value of a.
+ */
 static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a);
 
 /** Convert a field element to a 32-byte big endian value. Requires the input to be normalized */

src/field_10x26_impl.h

@@ -298,8 +298,7 @@ static int secp256k1_fe_impl_cmp_var(const secp256k1_fe *a, const secp256k1_fe *
     return 0;
 }
 
-static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
-    int ret;
+static int secp256k1_fe_impl_set_b32(secp256k1_fe *r, const unsigned char *a) {
     r->n[0] = (uint32_t)a[31] | ((uint32_t)a[30] << 8) | ((uint32_t)a[29] << 16) | ((uint32_t)(a[28] & 0x3) << 24);
     r->n[1] = (uint32_t)((a[28] >> 2) & 0x3f) | ((uint32_t)a[27] << 6) | ((uint32_t)a[26] << 14) | ((uint32_t)(a[25] & 0xf) << 22);
     r->n[2] = (uint32_t)((a[25] >> 4) & 0xf) | ((uint32_t)a[24] << 4) | ((uint32_t)a[23] << 12) | ((uint32_t)(a[22] & 0x3f) << 20);
@@ -311,13 +310,7 @@ static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
     r->n[8] = (uint32_t)a[5] | ((uint32_t)a[4] << 8) | ((uint32_t)a[3] << 16) | ((uint32_t)(a[2] & 0x3) << 24);
     r->n[9] = (uint32_t)((a[2] >> 2) & 0x3f) | ((uint32_t)a[1] << 6) | ((uint32_t)a[0] << 14);
 
-    ret = !((r->n[9] == 0x3FFFFFUL) & ((r->n[8] & r->n[7] & r->n[6] & r->n[5] & r->n[4] & r->n[3] & r->n[2]) == 0x3FFFFFFUL) & ((r->n[1] + 0x40UL + ((r->n[0] + 0x3D1UL) >> 26)) > 0x3FFFFFFUL));
-#ifdef VERIFY
-    r->magnitude = 1;
-    r->normalized = ret;
-    secp256k1_fe_verify(r);
-#endif
-    return ret;
+    return !((r->n[9] == 0x3FFFFFUL) & ((r->n[8] & r->n[7] & r->n[6] & r->n[5] & r->n[4] & r->n[3] & r->n[2]) == 0x3FFFFFFUL) & ((r->n[1] + 0x40UL + ((r->n[0] + 0x3D1UL) >> 26)) > 0x3FFFFFFUL));
 }
 
 /** Convert a field element to a 32-byte big endian value. Requires the input to be normalized */

src/field_5x52_impl.h

@@ -244,8 +244,7 @@ static int secp256k1_fe_impl_cmp_var(const secp256k1_fe *a, const secp256k1_fe *
     return 0;
 }
 
-static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
-    int ret;
+static int secp256k1_fe_impl_set_b32(secp256k1_fe *r, const unsigned char *a) {
     r->n[0] = (uint64_t)a[31]
             | ((uint64_t)a[30] << 8)
             | ((uint64_t)a[29] << 16)
@@ -280,13 +279,7 @@ static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
             | ((uint64_t)a[2] << 24)
             | ((uint64_t)a[1] << 32)
             | ((uint64_t)a[0] << 40);
-    ret = !((r->n[4] == 0x0FFFFFFFFFFFFULL) & ((r->n[3] & r->n[2] & r->n[1]) == 0xFFFFFFFFFFFFFULL) & (r->n[0] >= 0xFFFFEFFFFFC2FULL));
-#ifdef VERIFY
-    r->magnitude = 1;
-    r->normalized = ret;
-    secp256k1_fe_verify(r);
-#endif
-    return ret;
+    return !((r->n[4] == 0x0FFFFFFFFFFFFULL) & ((r->n[3] & r->n[2] & r->n[1]) == 0xFFFFFFFFFFFFFULL) & (r->n[0] >= 0xFFFFEFFFFFC2FULL));
 }
 
 /** Convert a field element to a 32-byte big endian value. Requires the input to be normalized */

src/field_impl.h

@@ -237,6 +237,15 @@ SECP256K1_INLINE static int secp256k1_fe_cmp_var(const secp256k1_fe *a, const se
     VERIFY_CHECK(b->normalized);
     return secp256k1_fe_impl_cmp_var(a, b);
 }
+
+static int secp256k1_fe_impl_set_b32(secp256k1_fe *r, const unsigned char *a);
+SECP256K1_INLINE static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a) {
+    int ret = secp256k1_fe_impl_set_b32(r, a);
+    r->magnitude = 1;
+    r->normalized = ret;
+    secp256k1_fe_verify(r);
+    return ret;
+}
 #endif /* defined(VERIFY) */
 
 #endif /* SECP256K1_FIELD_IMPL_H */