Slight speed improvement for Curve25519 (m15 implementation on Cortex-M0+).

diff --git a/src/ec/ec_c25519_m15.c b/src/ec/ec_c25519_m15.c
index 3cb98d5..0373197 100644 (file)
--- a/src/ec/ec_c25519_m15.c
+++ b/src/ec/ec_c25519_m15.c
@@ -808,6 +808,7 @@ mul20(uint32_t *d, const uint32_t *a, const uint32_t *b)
        t[37] = MUL15(a[18], b[19])
                + MUL15(a[19], b[18]);
        t[38] = MUL15(a[19], b[19]);
+
        d[39] = norm13(d, t, 39);
 }
 
@@ -1026,6 +1027,7 @@ square20(uint32_t *d, const uint32_t *a)
                + ((MUL15(a[17], a[19])) << 1);
        t[37] = ((MUL15(a[18], a[19])) << 1);
        t[38] = MUL15(a[19], a[19]);
+
        d[39] = norm13(d, t, 39);
 }
 
@@ -1060,24 +1062,21 @@ reduce_final_f255(uint32_t *d)
        return cc;
 }
 
-/*
- * Perform a multiplication of two integers modulo 2^255-19.
- * Operands are arrays of 20 words, each containing 13 bits of data, in
- * little-endian order. Input value may be up to 2^256-1; on output, value
- * fits on 256 bits and is lower than twice the modulus.
- */
 static void
-f255_mul(uint32_t *d, const uint32_t *a, const uint32_t *b)
+f255_mulgen(uint32_t *d, const uint32_t *a, const uint32_t *b, int square)
 {
        uint32_t t[40], cc, w;
-       int i;
 
        /*
         * Compute raw multiplication. All result words fit in 13 bits
         * each; upper word (t[39]) must fit on 5 bits, since the product
         * of two 256-bit integers must fit on 512 bits.
         */
-       mul20(t, a, b);
+       if (square) {
+               square20(t, a);
+       } else {
+               mul20(t, a, b);
+       }
 
        /*
         * Modular reduction: each high word is added where necessary.
@@ -1088,61 +1087,80 @@ f255_mul(uint32_t *d, const uint32_t *a, const uint32_t *b)
         */
        cc = MUL15(t[19] >> 8, 19);
        t[19] &= 0xFF;
-       for (i = 0; i < 20; i ++) {
-               w = t[i] + cc + MUL15(t[i + 20], 608);
-               t[i] = w & 0x1FFF;
-               cc = w >> 13;
-       }
+
+#define MM1(x)   do { \
+               w = t[x] + cc + MUL15(t[(x) + 20], 608); \
+               t[x] = w & 0x1FFF; \
+               cc = w >> 13; \
+       } while (0)
+
+       MM1( 0);
+       MM1( 1);
+       MM1( 2);
+       MM1( 3);
+       MM1( 4);
+       MM1( 5);
+       MM1( 6);
+       MM1( 7);
+       MM1( 8);
+       MM1( 9);
+       MM1(10);
+       MM1(11);
+       MM1(12);
+       MM1(13);
+       MM1(14);
+       MM1(15);
+       MM1(16);
+       MM1(17);
+       MM1(18);
+       MM1(19);
+
+#undef MM1
+
        cc = MUL15(w >> 8, 19);
        t[19] &= 0xFF;
-       for (i = 0; i < 20; i ++) {
-               w = t[i] + cc;
-               d[i] = w & 0x1FFF;
-               cc = w >> 13;
-       }
+
+#define MM2(x)   do { \
+               w = t[x] + cc; \
+               d[x] = w & 0x1FFF; \
+               cc = w >> 13; \
+       } while (0)
+
+       MM2( 0);
+       MM2( 1);
+       MM2( 2);
+       MM2( 3);
+       MM2( 4);
+       MM2( 5);
+       MM2( 6);
+       MM2( 7);
+       MM2( 8);
+       MM2( 9);
+       MM2(10);
+       MM2(11);
+       MM2(12);
+       MM2(13);
+       MM2(14);
+       MM2(15);
+       MM2(16);
+       MM2(17);
+       MM2(18);
+       MM2(19);
+
+#undef MM2
 }
 
 /*
- * Square an integer modulo 2^255-19.
- * Operand is an array of 20 words, each containing 13 bits of data, in
+ * Perform a multiplication of two integers modulo 2^255-19.
+ * Operands are arrays of 20 words, each containing 13 bits of data, in
  * little-endian order. Input value may be up to 2^256-1; on output, value
  * fits on 256 bits and is lower than twice the modulus.
+ *
+ * f255_mul() is the general multiplication, f255_square() is specialised
+ * for squarings.
  */
-static void
-f255_square(uint32_t *d, const uint32_t *a)
-{
-       uint32_t t[40], cc, w;
-       int i;
-
-       /*
-        * Compute raw multiplication. All result words fit in 13 bits
-        * each; upper word (t[39]) must fit on 5 bits, since the product
-        * of two 256-bit integers must fit on 512 bits.
-        */
-       square20(t, a);
-
-       /*
-        * Modular reduction: each high word is added where necessary.
-        * Since the modulus is 2^255-19 and word 20 corresponds to
-        * offset 20*13 = 260, word 20+k must be added to word k with
-        * a factor of 19*2^5 = 608. The extra bits in word 19 are also
-        * added that way.
-        */
-       cc = MUL15(t[19] >> 8, 19);
-       t[19] &= 0xFF;
-       for (i = 0; i < 20; i ++) {
-               w = t[i] + cc + MUL15(t[i + 20], 608);
-               t[i] = w & 0x1FFF;
-               cc = w >> 13;
-       }
-       cc = MUL15(w >> 8, 19);
-       t[19] &= 0xFF;
-       for (i = 0; i < 20; i ++) {
-               w = t[i] + cc;
-               d[i] = w & 0x1FFF;
-               cc = w >> 13;
-       }
-}
+#define f255_mul(d, a, b)   f255_mulgen(d, a, b, 0)
+#define f255_square(d, a)   f255_mulgen(d, a, a, 1)
 
 /*
  * Add two values in F255. Partial reduction is performed (down to less