X-Git-Url: https://www.bearssl.org/gitweb//home/git/?p=BearSSL;a=blobdiff_plain;f=src%2Fec%2Fec_c25519_i15.c;h=8fadcf48039b5c9bca1f04aeab1c1c3ad4817fab;hp=6e4c4f8845766c0e3ed634dd9ac6c5d0e30782bb;hb=d5acc4f590a1b83b646dafa0232aa68912e00c07;hpb=693d2a0085f4c1d289cb6d79c02ddd23f18d9478;ds=sidebyside

diff --git a/src/ec/ec_c25519_i15.c b/src/ec/ec_c25519_i15.c
index 6e4c4f8..8fadcf4 100644
--- a/src/ec/ec_c25519_i15.c
+++ b/src/ec/ec_c25519_i15.c
@@ -82,10 +82,10 @@ static const unsigned char GEN[] = {
 };
 
 static const unsigned char ORDER[] = {
-	0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-	0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-	0x14, 0xDE, 0xF9, 0xDE, 0xA2, 0xF7, 0x9C, 0xD6,
-	0x58, 0x12, 0x63, 0x1A, 0x5C, 0xF5, 0xD3, 0xED
+	0x7F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
 };
 
 static const unsigned char *
@@ -104,6 +104,14 @@ api_order(int curve, size_t *len)
 	return ORDER;
 }
 
+static size_t
+api_xoff(int curve, size_t *len)
+{
+	(void)curve;
+	*len = 32;
+	return 0;
+}
+
 static void
 cswap(uint16_t *a, uint16_t *b, uint32_t ctl)
 {
@@ -171,8 +179,14 @@ static uint32_t
 api_mul(unsigned char *G, size_t Glen,
 	const unsigned char *kb, size_t kblen, int curve)
 {
+#define ILEN   (18 * sizeof(uint16_t))
+
+	/*
+	 * The a[] and b[] arrays have an extra word to allow for
+	 * decoding without using br_i15_decode_reduce().
+	 */
 	uint16_t x1[18], x2[18], x3[18], z2[18], z3[18];
-	uint16_t a[18], aa[18], b[18], bb[18];
+	uint16_t a[19], aa[18], b[19], bb[18];
 	uint16_t c[18], d[18], e[18], da[18], cb[18];
 	unsigned char k[32];
 	uint32_t swap;
@@ -197,23 +211,39 @@ api_mul(unsigned char *G, size_t Glen,
 	 */
 	byteswap(G);
 
+	/*
+	 * Decode the point ('u' coordinate). This should be reduced
+	 * modulo p, but we prefer to avoid the dependency on
+	 * br_i15_decode_reduce(). Instead, we use br_i15_decode_mod()
+	 * with a synthetic modulus of value 2^255 (this must work
+	 * since G was truncated to 255 bits), then use a conditional
+	 * subtraction. We use br_i15_decode_mod() and not
+	 * br_i15_decode(), because the ec_prime_i15 implementation uses
+	 * the former but not the latter.
+	 *    br_i15_decode_reduce(a, G, 32, C255_P);
+	 */
+	br_i15_zero(b, 0x111);
+	b[18] = 1;
+	br_i15_decode_mod(a, G, 32, b);
+	a[0] = 0x110;
+	br_i15_sub(a, C255_P, NOT(br_i15_sub(a, C255_P, 0)));
+
 	/*
 	 * Initialise variables x1, x2, z2, x3 and z3. We set all of them
 	 * into Montgomery representation.
 	 */
-	br_i15_decode_reduce(a, G, 32, C255_P);
 	br_i15_montymul(x1, a, C255_R2, C255_P, P0I);
-	memcpy(x3, x1, sizeof x1);
+	memcpy(x3, x1, ILEN);
 	br_i15_zero(z2, C255_P[0]);
-	memcpy(x2, z2, sizeof z2);
+	memcpy(x2, z2, ILEN);
 	x2[1] = 19;
-	memcpy(z3, x2, sizeof x2);
+	memcpy(z3, x2, ILEN);
 
-	memcpy(k, kb, kblen);
-	memset(k + kblen, 0, (sizeof k) - kblen);
-	k[0] &= 0xF8;
-	k[31] &= 0x7F;
-	k[31] |= 0x40;
+	memset(k, 0, (sizeof k) - kblen);
+	memcpy(k + (sizeof k) - kblen, kb, kblen);
+	k[31] &= 0xF8;
+	k[0] &= 0x7F;
+	k[0] |= 0x40;
 
 	/* obsolete
 	print_int_mont("x1", x1);
@@ -223,7 +253,7 @@ api_mul(unsigned char *G, size_t Glen,
 	for (i = 254; i >= 0; i --) {
 		uint32_t kt;
 
-		kt = (k[i >> 3] >> (i & 7)) & 1;
+		kt = (k[31 - (i >> 3)] >> (i & 7)) & 1;
 		swap ^= kt;
 		cswap(x2, x3, swap);
 		cswap(z2, z3, swap);
@@ -283,12 +313,12 @@ api_mul(unsigned char *G, size_t Glen,
 	 * square-and-multiply algorithm; we mutualise most non-squarings
 	 * since the exponent contains almost only ones.
 	 */
-	memcpy(a, z2, sizeof z2);
+	memcpy(a, z2, ILEN);
 	for (i = 0; i < 15; i ++) {
 		c255_mul(a, a, a);
 		c255_mul(a, a, z2);
 	}
-	memcpy(b, a, sizeof a);
+	memcpy(b, a, ILEN);
 	for (i = 0; i < 14; i ++) {
 		int j;
 
@@ -303,11 +333,23 @@ api_mul(unsigned char *G, size_t Glen,
 			c255_mul(b, z2, b);
 		}
 	}
-	c255_mul(x2, x2, b);
-	br_i15_from_monty(x2, C255_P, P0I);
+	c255_mul(b, x2, b);
+
+	/*
+	 * To avoid a dependency on br_i15_from_monty(), we use a
+	 * Montgomery multiplication with 1.
+	 *    memcpy(x2, b, ILEN);
+	 *    br_i15_from_monty(x2, C255_P, P0I);
+	 */
+	br_i15_zero(a, C255_P[0]);
+	a[1] = 1;
+	br_i15_montymul(x2, a, b, C255_P, P0I);
+
 	br_i15_encode(G, 32, x2);
 	byteswap(G);
 	return 1;
+
+#undef ILEN
 }
 
 static size_t
@@ -349,6 +391,7 @@ const br_ec_impl br_ec_c25519_i15 = {
 	(uint32_t)0x20000000,
 	&api_generator,
 	&api_order,
+	&api_xoff,
 	&api_mul,
 	&api_mulgen,
 	&api_muladd