Added API to share precomputations in EAX.

[BearSSL] / src / ec / ec_c25519_i31.c

diff --git a/src/ec/ec_c25519_i31.c b/src/ec/ec_c25519_i31.c
index 05ebe3c..aa88dd6 100644 (file)
--- a/src/ec/ec_c25519_i31.c
+++ b/src/ec/ec_c25519_i31.c
@@ -202,11 +202,27 @@ 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_i31_decode_reduce(). Instead, we use br_i31_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_i31_decode_mod() and not
+        * br_i31_decode(), because the ec_prime_i31 implementation uses
+        * the former but not the latter.
+        *    br_i31_decode_reduce(a, G, 32, C255_P);
+        */
+       br_i31_zero(b, 0x108);
+       b[9] = 0x0100;
+       br_i31_decode_mod(a, G, 32, b);
+       a[0] = 0x107;
+       br_i31_sub(a, C255_P, NOT(br_i31_sub(a, C255_P, 0)));
+
        /*
         * Initialise variables x1, x2, z2, x3 and z3. We set all of them
         * into Montgomery representation.
         */
-       br_i31_decode_reduce(a, G, 32, C255_P);
        br_i31_montymul(x1, a, C255_R2, C255_P, P0I);
        memcpy(x3, x1, sizeof x1);
        br_i31_zero(z2, C255_P[0]);
@@ -308,8 +324,18 @@ api_mul(unsigned char *G, size_t Glen,
                        c255_mul(b, z2, b);
                }
        }
-       c255_mul(x2, x2, b);
-       br_i31_from_monty(x2, C255_P, P0I);
+       c255_mul(b, x2, b);
+
+       /*
+        * To avoid a dependency on br_i31_from_monty(), we use
+        * a Montgomery multiplication with 1.
+        *    memcpy(x2, b, sizeof b);
+        *    br_i31_from_monty(x2, C255_P, P0I);
+        */
+       br_i31_zero(a, C255_P[0]);
+       a[1] = 1;
+       br_i31_montymul(x2, a, b, C255_P, P0I);
+
        br_i31_encode(G, 32, x2);
        byteswap(G);
        return 1;