aead: Support custom AEAD salt sizes

The salt, or often called implicit nonce, varies between AEAD algorithms and
their use in protocols. For IKE and ESP, GCM uses 4 bytes, while CCM uses
3 bytes. With TLS, however, AEAD mode uses 4 bytes for both GCM and CCM.

Our GCM backends currently support 4 bytes and CCM 3 bytes only. This is fine
until we go for CCM mode support in TLS, which requires 4 byte nonces.

src/libstrongswan/plugins/openssl/openssl_gcm.c

@@ -202,7 +202,8 @@ METHOD(aead_t, destroy, void,
 /*
  * Described in header
  */
-aead_t *openssl_gcm_create(encryption_algorithm_t algo, size_t key_size)
+aead_t *openssl_gcm_create(encryption_algorithm_t algo,
+						   size_t key_size, size_t salt_size)
 {
 	private_aead_t *this;
 
@@ -236,6 +237,13 @@ aead_t *openssl_gcm_create(encryption_algorithm_t algo, size_t key_size)
 			return NULL;
 	}
 
+	if (salt_size && salt_size != SALT_LEN)
+	{
+		/* currently not supported */
+		free(this);
+		return NULL;
+	}
+
 	switch (algo)
 	{
 		case ENCR_AES_GCM_ICV8:

src/libstrongswan/plugins/openssl/openssl_gcm.h

@@ -30,8 +30,10 @@
  *
  * @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_gcm_create(encryption_algorithm_t algo, size_t key_size);
+aead_t *openssl_gcm_create(encryption_algorithm_t algo, size_t key_size,
+							size_t salt_size);
 
 #endif /** OPENSSL_GCM_H_ @}*/