The ZIP packer of DFIncBackup program uses the general purpose compression library ZLIB. ZLIB was written by Jean-loup Gailly (compression) and Mark Adler (decompression). This library contain general routines for ZIP files management.  ZLIB is designed to be a free, general-purpose, legally unencumbered -- that is, not covered by any patents -- lossless data-compression library for use on virtually any computer hardware and operating system.

You can find source and compiled versions of ZLIB library on internet site
http://www.zlib.net/.