Fixed carry propagation bug in P-256 'm62' implementation (found by Auke Zeilstra...

[BearSSL] / src / rsa / rsa_i15_priv.c

diff --git a/src/rsa/rsa_i15_priv.c b/src/rsa/rsa_i15_priv.c
index e692da4..177cc3a 100644 (file)
--- a/src/rsa/rsa_i15_priv.c
+++ b/src/rsa/rsa_i15_priv.c
@@ -27,24 +27,6 @@
 #define U      (2 + ((BR_MAX_RSA_FACTOR + 14) / 15))
 #define TLEN   (8 * U)
 
-/* obsolete
-static void
-print_int(const char *name, const uint16_t *x)
-{
-       extern int printf(const char *fmt, ...);
-       unsigned char tmp[1000];
-       size_t u, len;
-
-       len = (x[0] - (x[0] >> 4) + 7) >> 3;
-       br_i15_encode(tmp, len, x);
-       printf("%s = ", name);
-       for (u = 0; u < len; u ++) {
-               printf("%02X", tmp[u]);
-       }
-       printf("\n");
-}
-*/
-
 /* see bearssl_rsa.h */
 uint32_t
 br_rsa_i15_private(unsigned char *x, const br_rsa_private_key *sk)
@@ -53,7 +35,7 @@ br_rsa_i15_private(unsigned char *x, const br_rsa_private_key *sk)
        size_t plen, qlen;
        size_t fwlen;
        uint16_t p0i, q0i;
-       size_t xlen;
+       size_t xlen, u;
        uint16_t tmp[1 + TLEN];
        long z;
        uint16_t *mp, *mq, *s1, *s2, *t1, *t2, *t3;
@@ -117,19 +99,56 @@ br_rsa_i15_private(unsigned char *x, const br_rsa_private_key *sk)
        br_i15_decode(mq, q, qlen);
 
        /*
-        * Compute s2 = x^dq mod q.
+        * Decode p.
         */
-       q0i = br_i15_ninv15(mq[1]);
-       s2 = mq + fwlen;
-       br_i15_decode_reduce(s2, x, xlen, mq);
-       r = br_i15_modpow_opt(s2, sk->dq, sk->dqlen, mq, q0i,
-               mq + 2 * fwlen, TLEN - 2 * fwlen);
+       t1 = mq + fwlen;
+       br_i15_decode(t1, p, plen);
 
        /*
-        * Decode p.
+        * Compute the modulus (product of the two factors), to compare
+        * it with the source value. We use br_i15_mulacc(), since it's
+        * already used later on.
+        */
+       t2 = mq + 2 * fwlen;
+       br_i15_zero(t2, mq[0]);
+       br_i15_mulacc(t2, mq, t1);
+
+       /*
+        * We encode the modulus into bytes, to perform the comparison
+        * with bytes. We know that the product length, in bytes, is
+        * exactly xlen.
+        * The comparison actually computes the carry when subtracting
+        * the modulus from the source value; that carry must be 1 for
+        * a value in the correct range. We keep it in r, which is our
+        * accumulator for the error code.
+        */
+       t3 = mq + 4 * fwlen;
+       br_i15_encode(t3, xlen, t2);
+       u = xlen;
+       r = 0;
+       while (u > 0) {
+               uint32_t wn, wx;
+
+               u --;
+               wn = ((unsigned char *)t3)[u];
+               wx = x[u];
+               r = ((wx - (wn + r)) >> 8) & 1;
+       }
+
+       /*
+        * Move the decoded p to another temporary buffer.
         */
        mp = mq + 2 * fwlen;
-       br_i15_decode(mp, p, plen);
+       memmove(mp, t1, fwlen * sizeof *t1);
+
+       /*
+        * Compute s2 = x^dq mod q.
+        */
+       q0i = br_i15_ninv15(mq[1]);
+       s2 = mq + fwlen;
+       br_i15_decode_reduce(s2, x, xlen, mq);
+       r &= br_i15_modpow_opt(s2, sk->dq, sk->dqlen, mq, q0i,
+               mq + 3 * fwlen, TLEN - 3 * fwlen);
 
        /*
         * Compute s1 = x^dq mod q.