add cofactor ECDH support from fips branch

crypto/ecdh/ecdh.h

@@ -85,6 +85,8 @@
 extern "C" {
 #endif
 
+#define EC_FLAG_COFACTOR_ECDH	0x1000
+
 const ECDH_METHOD *ECDH_OpenSSL(void);
 
 void	  ECDH_set_default_method(const ECDH_METHOD *);

crypto/ecdh/ech_ossl.c

@@ -146,6 +146,18 @@ static int ecdh_compute_key(void *out, size_t outlen, const EC_POINT *pub_key,
 		}
 
 	group = EC_KEY_get0_group(ecdh);
+
+	if (EC_KEY_get_flags(ecdh) & EC_FLAG_COFACTOR_ECDH)
+		{
+		if (!EC_GROUP_get_cofactor(group, x, ctx) ||
+			!BN_mul(x, x, priv_key, ctx))
+			{
+			ECDHerr(ECDH_F_ECDH_COMPUTE_KEY, ERR_R_MALLOC_FAILURE);
+			goto err;
+			}
+		priv_key = x;
+		}
+
 	if ((tmp=EC_POINT_new(group)) == NULL)
 		{
 		ECDHerr(ECDH_F_ECDH_COMPUTE_KEY,ERR_R_MALLOC_FAILURE);