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/ccm/ccm_aead.c

@@ -343,7 +343,8 @@ METHOD(aead_t, destroy, void,
 /**
  * See header
  */
-ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo, size_t key_size)
+ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo,
+							size_t key_size, size_t salt_size)
 {
 	private_ccm_aead_t *this;
 	size_t icv_size;
@@ -360,6 +361,11 @@ ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo, size_t key_size)
 		default:
 			return NULL;
 	}
+	if (salt_size && salt_size != SALT_SIZE)
+	{
+		/* currently not supported */
+		return NULL;
+	}
 	switch (algo)
 	{
 		case ENCR_AES_CCM_ICV8:

src/libstrongswan/plugins/ccm/ccm_aead.h

@@ -44,8 +44,10 @@ struct ccm_aead_t {
  *
  * @param algo			algorithm to implement, a CCM mode
  * @param key_size		key size in bytes
+ * @param salt_size		size of implicit salt length
  * @return				aead, NULL if not supported
  */
-ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo, size_t key_size);
+ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo, size_t key_size,
+							size_t salt_size);
 
 #endif /** CCM_AEAD_H_ @}*/