+/*
+ * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "inner.h"
+
+/*
+ * Parameters for the field:
+ * - field modulus p = 2^255-19
+ * - R^2 mod p (R = 2^(15k) for the smallest k such that R >= p)
+ */
+
+static const uint16_t C255_P[] = {
+ 0x0110,
+ 0x7FED, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF,
+ 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF, 0x7FFF,
+ 0x7FFF
+};
+
+#define P0I 0x4A1B
+
+static const uint16_t C255_R2[] = {
+ 0x0110,
+ 0x0169, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000,
+ 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000,
+ 0x0000
+};
+
+static const uint16_t C255_A24[] = {
+ 0x0110,
+ 0x45D3, 0x0046, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000,
+ 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000,
+ 0x0000
+};
+
+static const unsigned char GEN[] = {
+ 0x09, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+};
+
+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
+};
+
+static const unsigned char *
+api_generator(int curve, size_t *len)
+{
+ (void)curve;
+ *len = 32;
+ return GEN;
+}
+
+static const unsigned char *
+api_order(int curve, size_t *len)
+{
+ (void)curve;
+ *len = 32;
+ return ORDER;
+}
+
+static void
+cswap(uint16_t *a, uint16_t *b, uint32_t ctl)
+{
+ int i;
+
+ ctl = -ctl;
+ for (i = 0; i < 18; i ++) {
+ uint32_t aw, bw, tw;
+
+ aw = a[i];
+ bw = b[i];
+ tw = ctl & (aw ^ bw);
+ a[i] = aw ^ tw;
+ b[i] = bw ^ tw;
+ }
+}
+
+static void
+c255_add(uint16_t *d, const uint16_t *a, const uint16_t *b)
+{
+ uint32_t ctl;
+ uint16_t t[18];
+
+ memcpy(t, a, sizeof t);
+ ctl = br_i15_add(t, b, 1);
+ ctl |= NOT(br_i15_sub(t, C255_P, 0));
+ br_i15_sub(t, C255_P, ctl);
+ memcpy(d, t, sizeof t);
+}
+
+static void
+c255_sub(uint16_t *d, const uint16_t *a, const uint16_t *b)
+{
+ uint16_t t[18];
+
+ memcpy(t, a, sizeof t);
+ br_i15_add(t, C255_P, br_i15_sub(t, b, 1));
+ memcpy(d, t, sizeof t);
+}
+
+static void
+c255_mul(uint16_t *d, const uint16_t *a, const uint16_t *b)
+{
+ uint16_t t[18];
+
+ br_i15_montymul(t, a, b, C255_P, P0I);
+ memcpy(d, t, sizeof t);
+}
+
+static void
+byteswap(unsigned char *G)
+{
+ int i;
+
+ for (i = 0; i < 16; i ++) {
+ unsigned char t;
+
+ t = G[i];
+ G[i] = G[31 - i];
+ G[31 - i] = t;
+ }
+}
+
+static uint32_t
+api_mul(unsigned char *G, size_t Glen,
+ const unsigned char *kb, size_t kblen, int curve)
+{
+ uint16_t x1[18], x2[18], x3[18], z2[18], z3[18];
+ uint16_t a[18], aa[18], b[18], bb[18];
+ uint16_t c[18], d[18], e[18], da[18], cb[18];
+ unsigned char k[32];
+ uint32_t swap;
+ int i;
+
+ (void)curve;
+
+ /*
+ * Points are encoded over exactly 32 bytes. Multipliers must fit
+ * in 32 bytes as well.
+ * RFC 7748 mandates that the high bit of the last point byte must
+ * be ignored/cleared.
+ */
+ if (Glen != 32 || kblen > 32) {
+ return 0;
+ }
+ G[31] &= 0x7F;
+
+ /*
+ * Byteswap the point encoding, because it uses little-endian, and
+ * the generic decoding routine uses big-endian.
+ */
+ byteswap(G);
+
+ /*
+ * 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);
+ br_i15_zero(z2, C255_P[0]);
+ memcpy(x2, z2, sizeof z2);
+ x2[1] = 19;
+ memcpy(z3, x2, sizeof x2);
+
+ memcpy(k, kb, kblen);
+ memset(k + kblen, 0, (sizeof k) - kblen);
+ k[0] &= 0xF8;
+ k[31] &= 0x7F;
+ k[31] |= 0x40;
+
+ swap = 0;
+ for (i = 254; i >= 0; i --) {
+ uint32_t kt;
+
+ kt = (k[i >> 3] >> (i & 7)) & 1;
+ swap ^= kt;
+ cswap(x2, x3, swap);
+ cswap(z2, z3, swap);
+ swap = kt;
+
+ c255_add(a, x2, z2);
+ c255_mul(aa, a, a);
+ c255_sub(b, x2, z2);
+ c255_mul(bb, b, b);
+ c255_sub(e, aa, bb);
+ c255_add(c, x3, z3);
+ c255_sub(d, x3, z3);
+ c255_mul(da, d, a);
+ c255_mul(cb, c, b);
+ c255_add(x3, da, cb);
+ c255_mul(x3, x3, x3);
+ c255_sub(z3, da, cb);
+ c255_mul(z3, z3, z3);
+ c255_mul(z3, z3, x1);
+ c255_mul(x2, aa, bb);
+ c255_mul(z2, C255_A24, e);
+ c255_add(z2, z2, aa);
+ c255_mul(z2, e, z2);
+ }
+ cswap(x2, x3, swap);
+ cswap(z2, z3, swap);
+
+ /*
+ * Inverse z2 with a modular exponentiation. This is a simple
+ * square-and-multiply algorithm; we mutualise most non-squarings
+ * since the exponent contains almost only ones.
+ */
+ memcpy(a, z2, sizeof z2);
+ for (i = 0; i < 15; i ++) {
+ c255_mul(a, a, a);
+ c255_mul(a, a, z2);
+ }
+ memcpy(b, a, sizeof a);
+ for (i = 0; i < 14; i ++) {
+ int j;
+
+ for (j = 0; j < 16; j ++) {
+ c255_mul(b, b, b);
+ }
+ c255_mul(b, b, a);
+ }
+ for (i = 14; i >= 0; i --) {
+ c255_mul(b, b, b);
+ if ((0xFFEB >> i) & 1) {
+ c255_mul(b, z2, b);
+ }
+ }
+ c255_mul(x2, x2, b);
+ br_i15_from_monty(x2, C255_P, P0I);
+ br_i15_encode(G, 32, x2);
+ byteswap(G);
+ return 1;
+}
+
+static size_t
+api_mulgen(unsigned char *R,
+ const unsigned char *x, size_t xlen, int curve)
+{
+ const unsigned char *G;
+ size_t Glen;
+
+ G = api_generator(curve, &Glen);
+ memcpy(R, G, Glen);
+ api_mul(R, Glen, x, xlen, curve);
+ return Glen;
+}
+
+static uint32_t
+api_muladd(unsigned char *A, const unsigned char *B, size_t len,
+ const unsigned char *x, size_t xlen,
+ const unsigned char *y, size_t ylen, int curve)
+{
+ /*
+ * We don't implement this method, since it is used for ECDSA
+ * only, and there is no ECDSA over Curve25519 (which instead
+ * uses EdDSA).
+ */
+ (void)A;
+ (void)B;
+ (void)len;
+ (void)x;
+ (void)xlen;
+ (void)y;
+ (void)ylen;
+ (void)curve;
+ return 0;
+}
+
+/* see bearssl_ec.h */
+const br_ec_impl br_ec_c25519_i15 = {
+ (uint32_t)0x20000000,
+ &api_generator,
+ &api_order,
+ &api_mul,
+ &api_mulgen,
+ &api_muladd
+};
projects / BearSSL / commitdiff