+/** \fn void aes256_dec(void* buffer, aes256_ctx_t* ctx)
+ * \brief decrypt with 256 bit key.
+ *
+ * This function decrypts one block with the AES algorithm under control of
+ * a keyschedule produced from a 256 bit key.
+ * \param buffer pointer to the block to decrypt
+ * \param ctx pointer to the key schedule
+ */