Abstract out verify logic for fe_inv{,_var}

sipa · May 11, 2023 · d5aa2f0 · d5aa2f0
1 parent 3167646
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Showing 4 changed files with 43 additions and 51 deletions.
diff --git a/src/field.h b/src/field.h
@@ -95,6 +95,8 @@ static const secp256k1_fe secp256k1_const_beta = SECP256K1_FE_CONST(
 #  define secp256k1_fe_cmov secp256k1_fe_impl_cmov
 #  define secp256k1_fe_to_storage secp256k1_fe_impl_to_storage
 #  define secp256k1_fe_from_storage secp256k1_fe_impl_from_storage
+#  define secp256k1_fe_inv secp256k1_fe_impl_inv
+#  define secp256k1_fe_inv_var secp256k1_fe_impl_inv_var
 #endif /* !defined(VERIFY) */
 
 /** Normalize a field element.
@@ -258,11 +260,19 @@ static void secp256k1_fe_sqr(secp256k1_fe *r, const secp256k1_fe *a);
  */
 static int secp256k1_fe_sqrt(secp256k1_fe * SECP256K1_RESTRICT r, const secp256k1_fe * SECP256K1_RESTRICT a);
 
-/** Sets a field element to be the (modular) inverse of another. Requires the input's magnitude to be
- *  at most 8. The output magnitude is 1 (but not guaranteed to be normalized). */
+/** Compute the modular inverse of a field element.
+ *
+ * On input, a must be a valid field element; r need not be initialized.
+ * Performs {r = a**(p-2)} (which maps 0 to 0, and every other element to its
+ * inverse).
+ * On output, r will have magnitude (a.magnitude != 0) and be normalized.
+ */
 static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *a);
 
-/** Potentially faster version of secp256k1_fe_inv, without constant-time guarantee. */
+/** Compute the modular inverse of a field element, without constant-time guarantee.
+ *
+ * Behaves identically to secp256k1_fe_inv, but is not constant-time in a.
+ */
 static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *a);
 
 /** Convert a field element to secp256k1_fe_storage.

diff --git a/src/field_10x26_impl.h b/src/field_10x26_impl.h
@@ -1197,23 +1197,13 @@ static void secp256k1_fe_from_signed30(secp256k1_fe *r, const secp256k1_modinv32
     r->n[7] = (a6 >>  2           ) & M26;
     r->n[8] = (a6 >> 28 | a7 <<  2) & M26;
     r->n[9] = (a7 >> 24 | a8 <<  6);
-
-#ifdef VERIFY
-    r->magnitude = 1;
-    r->normalized = 1;
-    secp256k1_fe_verify(r);
-#endif
 }
 
 static void secp256k1_fe_to_signed30(secp256k1_modinv32_signed30 *r, const secp256k1_fe *a) {
     const uint32_t M30 = UINT32_MAX >> 2;
     const uint64_t a0 = a->n[0], a1 = a->n[1], a2 = a->n[2], a3 = a->n[3], a4 = a->n[4],
                    a5 = a->n[5], a6 = a->n[6], a7 = a->n[7], a8 = a->n[8], a9 = a->n[9];
 
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-#endif
-
     r->v[0] = (a0       | a1 << 26) & M30;
     r->v[1] = (a1 >>  4 | a2 << 22) & M30;
     r->v[2] = (a2 >>  8 | a3 << 18) & M30;
@@ -1231,34 +1221,24 @@ static const secp256k1_modinv32_modinfo secp256k1_const_modinfo_fe = {
     0x2DDACACFL
 };
 
-static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *x) {
-    secp256k1_fe tmp;
+static void secp256k1_fe_impl_inv(secp256k1_fe *r, const secp256k1_fe *x) {
+    secp256k1_fe tmp = *x;
     secp256k1_modinv32_signed30 s;
 
-    tmp = *x;
     secp256k1_fe_normalize(&tmp);
     secp256k1_fe_to_signed30(&s, &tmp);
     secp256k1_modinv32(&s, &secp256k1_const_modinfo_fe);
     secp256k1_fe_from_signed30(r, &s);
-
-#ifdef VERIFY
-    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == secp256k1_fe_normalizes_to_zero(&tmp));
-#endif
 }
 
-static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
-    secp256k1_fe tmp;
+static void secp256k1_fe_impl_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
+    secp256k1_fe tmp = *x;
     secp256k1_modinv32_signed30 s;
 
-    tmp = *x;
     secp256k1_fe_normalize_var(&tmp);
     secp256k1_fe_to_signed30(&s, &tmp);
     secp256k1_modinv32_var(&s, &secp256k1_const_modinfo_fe);
     secp256k1_fe_from_signed30(r, &s);
-
-#ifdef VERIFY
-    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == secp256k1_fe_normalizes_to_zero(&tmp));
-#endif
 }
 
 static int secp256k1_fe_is_square_var(const secp256k1_fe *x) {

diff --git a/src/field_5x52_impl.h b/src/field_5x52_impl.h
@@ -492,22 +492,12 @@ static void secp256k1_fe_from_signed62(secp256k1_fe *r, const secp256k1_modinv64
     r->n[2] = (a1 >> 42 | a2 << 20) & M52;
     r->n[3] = (a2 >> 32 | a3 << 30) & M52;
     r->n[4] = (a3 >> 22 | a4 << 40);
-
-#ifdef VERIFY
-    r->magnitude = 1;
-    r->normalized = 1;
-    secp256k1_fe_verify(r);
-#endif
 }
 
 static void secp256k1_fe_to_signed62(secp256k1_modinv64_signed62 *r, const secp256k1_fe *a) {
     const uint64_t M62 = UINT64_MAX >> 2;
     const uint64_t a0 = a->n[0], a1 = a->n[1], a2 = a->n[2], a3 = a->n[3], a4 = a->n[4];
 
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-#endif
-
     r->v[0] = (a0       | a1 << 52) & M62;
     r->v[1] = (a1 >> 10 | a2 << 42) & M62;
     r->v[2] = (a2 >> 20 | a3 << 32) & M62;
@@ -520,34 +510,24 @@ static const secp256k1_modinv64_modinfo secp256k1_const_modinfo_fe = {
     0x27C7F6E22DDACACFLL
 };
 
-static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *x) {
-    secp256k1_fe tmp;
+static void secp256k1_fe_impl_inv(secp256k1_fe *r, const secp256k1_fe *x) {
+    secp256k1_fe tmp = *x;
     secp256k1_modinv64_signed62 s;
 
-    tmp = *x;
     secp256k1_fe_normalize(&tmp);
     secp256k1_fe_to_signed62(&s, &tmp);
     secp256k1_modinv64(&s, &secp256k1_const_modinfo_fe);
     secp256k1_fe_from_signed62(r, &s);
-
-#ifdef VERIFY
-    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == secp256k1_fe_normalizes_to_zero(&tmp));
-#endif
 }
 
-static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
-    secp256k1_fe tmp;
+static void secp256k1_fe_impl_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
+    secp256k1_fe tmp = *x;
     secp256k1_modinv64_signed62 s;
 
-    tmp = *x;
     secp256k1_fe_normalize_var(&tmp);
     secp256k1_fe_to_signed62(&s, &tmp);
     secp256k1_modinv64_var(&s, &secp256k1_const_modinfo_fe);
     secp256k1_fe_from_signed62(r, &s);
-
-#ifdef VERIFY
-    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == secp256k1_fe_normalizes_to_zero(&tmp));
-#endif
 }
 
 static int secp256k1_fe_is_square_var(const secp256k1_fe *x) {

diff --git a/src/field_impl.h b/src/field_impl.h
@@ -351,6 +351,28 @@ SECP256K1_INLINE static void secp256k1_fe_from_storage(secp256k1_fe *r, const se
     r->normalized = 1;
     secp256k1_fe_verify(r);
 }
+
+static void secp256k1_fe_impl_inv(secp256k1_fe *r, const secp256k1_fe *x);
+SECP256K1_INLINE static void secp256k1_fe_inv(secp256k1_fe *r, const secp256k1_fe *x) {
+    int input_is_zero = secp256k1_fe_normalizes_to_zero(x);
+    secp256k1_fe_verify(x);
+    secp256k1_fe_impl_inv(r, x);
+    r->magnitude = x->magnitude > 0;
+    r->normalized = 1;
+    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == input_is_zero);
+    secp256k1_fe_verify(r);
+}
+
+static void secp256k1_fe_impl_inv_var(secp256k1_fe *r, const secp256k1_fe *x);
+SECP256K1_INLINE static void secp256k1_fe_inv_var(secp256k1_fe *r, const secp256k1_fe *x) {
+    int input_is_zero = secp256k1_fe_normalizes_to_zero(x);
+    secp256k1_fe_verify(x);
+    secp256k1_fe_impl_inv_var(r, x);
+    r->magnitude = x->magnitude > 0;
+    r->normalized = 1;
+    VERIFY_CHECK(secp256k1_fe_normalizes_to_zero(r) == input_is_zero);
+    secp256k1_fe_verify(r);
+}
 #endif /* defined(VERIFY) */
 
 #endif /* SECP256K1_FIELD_IMPL_H */