feat(crypto): introduce bn_reduce and bn_copy_lower

crypto/bignum.c

@@ -80,6 +80,15 @@
 #define BN_MAX_DECIMAL_DIGITS \
   79  // floor(log(2**(LIMBS * BITS_PER_LIMB), 10)) + 1
 
+// y = (bignum256) x
+// Assumes x is normalized and x < 2**261 == 2**(BITS_PER_LIMB * LIMBS)
+// Guarantees y is normalized
+void bn_copy_lower(const bignum512 *x, bignum256 *y) {
+  for (int i = 0; i < BN_LIMBS; i++) {
+    y->val[i] = x->val[i];
+  }
+}
+
 // out_number = (bignum256) in_number
 // Assumes in_number is a raw bigendian 256-bit number
 // Guarantees out_number is normalized
@@ -667,12 +676,11 @@ void bn_multiply_reduce_step(bignum512 *res, const bignum256 *prime,
   res->val[d + BN_LIMBS] = 0;
 }
 
-// Auxiliary function for bn_multiply
-// Partly reduces res and stores both in x and res
-// Assumes res in normalized and res < 2**519
+// Partly reduces x
+// Assumes x in normalized and res < 2**519
 // Guarantees x is normalized and partly reduced modulo prime
 // Assumes prime is normalized, 2**256 - 2**224 <= prime <= 2**256
-void bn_multiply_reduce(bignum256 *x, bignum512 *res, const bignum256 *prime) {
+void bn_reduce(bignum512 *x, const bignum256 *prime) {
   for (int i = BN_LIMBS - 1; i >= 0; i--) {
     // res < 2**(256 + 29*i + 31)
     // Proof:
@@ -683,11 +691,7 @@ void bn_multiply_reduce(bignum256 *x, bignum512 *res, const bignum256 *prime) {
     //   else:
     //     res < 2 * prime * 2**(29 * (i + 1))
     //       <= 2**256 * 2**(29*i + 29) < 2**(256 + 29*i + 31)
-    bn_multiply_reduce_step(res, prime, i);
-  }
-
-  for (int i = 0; i < BN_LIMBS; i++) {
-    x->val[i] = res->val[i];
+    bn_multiply_reduce_step(x, prime, i);
   }
 }
 
@@ -699,7 +703,8 @@ void bn_multiply(const bignum256 *k, bignum256 *x, const bignum256 *prime) {
   bignum512 res = {0};
 
   bn_multiply_long(k, x, &res);
-  bn_multiply_reduce(x, &res, prime);
+  bn_reduce(&res, prime);
+  bn_copy_lower(&res, x);
 
   memzero(&res, sizeof(res));
 }

crypto/bignum.h

@@ -72,6 +72,7 @@ static inline void write_le(uint8_t *data, uint32_t x) {
   data[0] = x;
 }
 
+void bn_copy_lower(const bignum512 *x, bignum256 *y);
 void bn_read_be(const uint8_t *in_number, bignum256 *out_number);
 void bn_write_be(const bignum256 *in_number, uint8_t *out_number);
 void bn_read_le(const uint8_t *in_number, bignum256 *out_number);
@@ -99,6 +100,7 @@ void bn_mult_half(bignum256 *x, const bignum256 *prime);
 void bn_mult_k(bignum256 *x, uint8_t k, const bignum256 *prime);
 void bn_mod(bignum256 *x, const bignum256 *prime);
 void bn_multiply(const bignum256 *k, bignum256 *x, const bignum256 *prime);
+void bn_reduce(bignum512 *x, const bignum256 *prime);
 void bn_fast_mod(bignum256 *x, const bignum256 *prime);
 void bn_power_mod(const bignum256 *x, const bignum256 *e,
                   const bignum256 *prime, bignum256 *res);

crypto/tests/test_bignum.py

@@ -197,6 +197,16 @@ class Random(random.Random):
         return self.rand_bignum(2 * limbs_number)
 
 
+def assert_bn_copy_lower(x):
+    x_number = int_to_bignum512(x)
+    y_number = bignum256()
+    lib.bn_copy_lower(x_number, y_number)
+    y = bignum256_to_int(y_number)
+
+    assert bignum_is_normalised(y_number)
+    assert y == x
+
+
 def assert_bn_read_be(in_number):
     raw_in_number = integer_to_raw_number256(in_number, "big")
     bn_out_number = bignum256()
@@ -458,6 +468,17 @@ def assert_bn_multiply(k, x_old, prime):
     assert x_new % prime == (k * x_old) % prime
 
 
+def assert_bn_reduce(x_old, prime):
+    bn_x = int_to_bignum512(x_old)
+    bn_prime = int_to_bignum256(prime)
+    lib.bn_reduce(bn_x, bn_prime)
+    x_new = bignum256_to_int(bn_x)
+
+    assert bignum_is_normalised(bn_x)
+    assert number_is_partly_reduced(x_new, prime)
+    assert x_new % prime == x_old % prime
+
+
 def assert_bn_fast_mod(x_old, prime):
     bn_x = int_to_bignum256(x_old)
     bn_prime = int_to_bignum256(prime)
@@ -731,6 +752,10 @@ def assert_bn_format(x, prefix, suffix, decimals, exponent, trailing, thousands)
     assert return_value == correct_return_value
 
 
+def test_bn_copy_lower(r):
+    assert_bn_copy_lower(r.rand_int_bitsize(261))
+
+
 def test_bn_read_be(r):
     assert_bn_read_be(r.rand_int_256())
 
@@ -916,6 +941,11 @@ def test_bn_multiply_reduce_step(r, prime):
     assert_bn_multiply_reduce_step(res, prime, k)
 
 
+def test_bn_reduce(r, prime):
+    x = r.rand_int_bitsize(519)
+    assert_bn_reduce(x, prime)
+
+
 def test_bn_multiply(r, prime):
     x = r.randrange(floor(sqrt(2**519)))
     k = r.randrange(floor(sqrt(2**519)))