Fixed carry propagation bug in m64 impl for P-256.

[BearSSL] / src / ec / ec_c25519_i31.c

diff --git a/src/ec/ec_c25519_i31.c b/src/ec/ec_c25519_i31.c
index aa88dd6..f8ffc2c 100644 (file)
--- a/src/ec/ec_c25519_i31.c
+++ b/src/ec/ec_c25519_i31.c
@@ -214,7 +214,7 @@ api_mul(unsigned char *G, size_t Glen,
         *    br_i31_decode_reduce(a, G, 32, C255_P);
         */
        br_i31_zero(b, 0x108);
-       b[9] = 0x0100;
+       b[9] = 0x0080;
        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)));
@@ -230,11 +230,14 @@ api_mul(unsigned char *G, size_t Glen,
        x2[1] = 0x13000000;
        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;
+       /*
+        * kb[] is in big-endian notation, but possibly shorter than k[].
+        */
+       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);
@@ -244,7 +247,7 @@ 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);