- DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, &k, dsa->p, ctx,
- (BN_MONT_CTX *)dsa->method_mont_p);
+
+ if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0)
+ {
+ if (!BN_copy(&kq, &k)) goto err;
+
+ /* We do not want timing information to leak the length of k,
+ * so we compute g^k using an equivalent exponent of fixed length.
+ *
+ * (This is a kludge that we need because the BN_mod_exp_mont()
+ * does not let us specify the desired timing behaviour.) */
+
+ if (!BN_add(&kq, &kq, dsa->q)) goto err;
+ if (BN_num_bits(&kq) <= BN_num_bits(dsa->q))
+ {
+ if (!BN_add(&kq, &kq, dsa->q)) goto err;
+ }
+
+ K = &kq;
+ }
+ else
+ {
+ K = &k;
+ }
+ DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, K, dsa->p, ctx,
+ dsa->method_mont_p);