Check ECC-CDH is compliant with SP800-56A-r3
Added comments and cleared an intermediate result.
KAT tests already exist in evppkey.txt (Search for "KAS_ECC_CDH_PrimitiveTest")
@@ -31,9 +31,14 @@ int ossl_ecdh_compute_key(unsigned char **psec, size_t *pseclen,
}
/*-
- * This implementation is based on the following primitives in the IEEE 1363 standard:
+ * This implementation is based on the following primitives in the
+ * IEEE 1363 standard:
* - ECKAS-DH1
* - ECSVDP-DH
+ *
+ * It also conforms to SP800-56A r3
+ * See Section 5.7.1.2 "Elliptic Curve Cryptography Cofactor Diffie-Hellman
+ * (ECC CDH) Primitive:". The steps listed below refer to SP800-56A.
*/
int ecdh_simple_compute_key(unsigned char **pout, size_t *poutlen,
const EC_POINT *pub_key, const EC_KEY *ecdh)
@@ -64,6 +69,10 @@ int ecdh_simple_compute_key(unsigned char **pout, size_t *poutlen,
group = EC_KEY_get0_group(ecdh);
+ /*
+ * Step(1) - Compute the point tmp = cofactor * owners_private_key
+ * * peer_public_key.
+ */
if (EC_KEY_get_flags(ecdh) & EC_FLAG_COFACTOR_ECDH) {
if (!EC_GROUP_get_cofactor(group, x, NULL) ||
!BN_mul(x, x, priv_key, ctx)) {
@@ -83,11 +92,20 @@ int ecdh_simple_compute_key(unsigned char **pout, size_t *poutlen,
goto err;
}
+ /*
+ * Step(2) : If point tmp is at infinity then clear intermediate values and
+ * exit. Note: getting affine coordinates returns 0 if point is at infinity.
+ * Step(3a) : Get x-coordinate of point x = tmp.x
+ */
if (!EC_POINT_get_affine_coordinates(group, tmp, x, NULL, ctx)) {
ECerr(EC_F_ECDH_SIMPLE_COMPUTE_KEY, EC_R_POINT_ARITHMETIC_FAILURE);
goto err;
}
+ /*
+ * Step(3b) : convert x to a byte string, using the field-element-to-byte
+ * string conversion routine defined in Appendix C.2
+ */
buflen = (EC_GROUP_get_degree(group) + 7) / 8;
len = BN_num_bytes(x);
if (len > buflen) {
@@ -112,6 +130,8 @@ int ecdh_simple_compute_key(unsigned char **pout, size_t *poutlen,
ret = 1;
err:
+ /* Step(4) : Destroy all intermediate calculations */
+ BN_clear(x);
EC_POINT_clear_free(tmp);
BN_CTX_end(ctx);
BN_CTX_free(ctx);
