uffer is destroyed. Returns the length of the decrypted plain-text, or olm_error() on failure.</p><p>On failure last_error will be set with an error code. The last_error will be: * OLM_OUTPUT_BUFFER_TOO_SMALL if the plain-text buffer is too small * OLM_INVALID_BASE64 if the message is not valid base-64 * OLM_BAD_MESSAGE_VERSION if the message was encrypted with an unsupported version of the protocol * OLM_BAD_MESSAGE_FORMAT if the message headers could not be decoded * OLM_BAD_MESSAGE_MAC if the message cou