Make cert_validator_t.validate optional to implement

src/libstrongswan/credentials/cert_validator.h

@@ -43,6 +43,7 @@ struct cert_validator_t {
 	 * @param pathlen		the current length of the path bottom-up
 	 * @param anchor		is issuer trusted root anchor
 	 * @param auth			container for resulting authentication info
+	 * @return				TRUE if subject certificate valid
 	 */
 	bool (*validate)(cert_validator_t *this, certificate_t *subject,
 					 certificate_t *issuer, bool online, u_int pathlen,

src/libstrongswan/credentials/credential_manager.c

@@ -541,6 +541,10 @@ static bool check_certificate(private_credential_manager_t *this,
 	enumerator = this->validators->create_enumerator(this->validators);
 	while (enumerator->enumerate(enumerator, &validator))
 	{
+		if (!validator->validate)
+		{
+			continue;
+		}
 		if (!validator->validate(validator, subject, issuer,
 								 online, pathlen, trusted, auth))
 		{