# Lossless Data Compression

Some lossless data compression algorithms are available in botan, currently all via third party libraries - these include zlib (including deflate and gzip formats), bzip2, and lzma. Support for these must be enabled at build time; you can check for them using the macros `BOTAN_HAS_ZLIB`, `BOTAN_HAS_BZIP2`, and `BOTAN_HAS_LZMA`.

Note

You should always compress before you encrypt, because encryption seeks to hide the redundancy that compression is supposed to try to find and remove.

Compression is done through the `Compression_Algorithm` and `Decompression_Algorithm` classes, both defined in compression.h

Compression and decompression both work in three stages: starting a message (`start`), continuing to process it (`update`), and then finally completing processing the stream (`finish`).

class `Compression_Algorithm`
  void `start`(size_t level)

    Initialize the compression engine. This must be done before calling `update` or `finish`. The meaning of the level parameter varies by the algorithm but generally takes a value between 1 and 9, with higher values implying typically better compression from and more memory and/or CPU time consumed by the compression process. The decompressor can always handle input from any compressor.

  void `update`(secure_vector<uint8_t> &buf, size_t offset = 0, bool flush = false)

    > Compress the material in the in/out parameter `buf`. The leading `offset` bytes of `buf` are ignored and remain untouched; this can be useful for ignoring packet headers. If `flush` is true, the compression state is flushed, allowing the decompressor to recover the entire message up to this point without having the see the rest of the compressed stream.

class `Decompression_Algorithm`
  void `start`()

    Initialize the decompression engine. This must be done before calling `update` or `finish`. No level is provided here; the decompressor can accept input generated by any compression parameters.

  void `update`(secure_vector<uint8_t> &buf, size_t offset = 0)

    > Decompress the material in the in/out parameter `buf`. The leading `offset` bytes of `buf` are ignored and remain untouched; this can be useful for ignoring packet headers.

    > This function may throw if the data seems to be invalid.

The easiest way to get a compressor is via the functions

Compression_Algorithm *`make_compressor`(std::string type)

Decompression_Algorithm *`make_decompressor`(std::string type)

Supported values for type include zlib (raw zlib with no checksum), deflate (zlib’s deflate format), gzip, bz2, and lzma. A null pointer will be returned if the algorithm is unavailable.

To use a compression algorithm in a Pipe use the adapter types Compression_Filter and Decompression_Filter from comp_filter.h. The constructors of both filters take a std::string argument (passed to make_compressor or make_decompressor), the compression filter also takes a level parameter. Finally both constructors have a parameter buf_sz which specifies the size of the internal buffer that will be used - inputs will be broken into blocks of this size. The default is 4096.