openssl: Generalize the GCM implementation a bit

This will allow us to use the implementation also for other algorithms.

src/libstrongswan/plugins/openssl/openssl_aead.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2013-2019 Tobias Brunner
+ * HSR Hochschule fuer Technik Rapperswil
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.  See <http://www.fsf.org/copyleft/gpl.txt>.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ */
+
+/**
+ * Implements the aead_t interface using OpenSSL.
+ *
+ * @defgroup openssl_aead openssl_aead
+ * @{ @ingroup openssl_p
+ */
+
+#ifndef OPENSSL_AEAD_H_
+#define OPENSSL_AEAD_H_
+
+#include <crypto/aead.h>
+
+/**
+ * Constructor to create aead_t implementation.
+ *
+ * @param algo			algorithm to implement
+ * @param key_size		key size in bytes
+ * @param salt_size		size of implicit salt length
+ * @return				aead_t object, NULL if not supported
+ */
+aead_t *openssl_aead_create(encryption_algorithm_t algo, size_t key_size,
+							size_t salt_size);
+
+#endif /** OPENSSL_AEAD_H_ @}*/