NewReaderAt creates a new CryptoReaderAt on the given io.ReaderAt, reads header and salt and verifies the stream follows the format specified in this library.

NewWriter creates a new CryptoWriteCloser, gets a random salt and writes the header to the underlying writer.

HeaderSize represents the metadata size prepended to the data stream and needs to be subtracted when passing the payload size by using the file size of the encrypted data.