You've got it backwards: it is the decompression that is predictable; compressors are free to transform the input in any way such that a compliant decompressor

You've got it backwards: it is the decompression that is predictable; compressors are free to transform the input in any way such that a compliant decompressor will extract the original file. The gzip format documentation is available here btw: http://www.gzip.org/zlib/rfc-gzip.html http://www.gzip.org/zlib/rfc-gzip.html