gzip format

Offset   Length   Contents
  0      2 bytes  magic header  0x1f, 0x8b (\037 \213)  
  2      1 byte   compression method
                     0: store (copied)
                     1: compress
                     2: pack
                     3: lzh
                     4..7: reserved
                     8: deflate
  3      1 byte   flags
                     bit 0 set: file probably ascii text
                     bit 1 set: continuation of multi-part gzip file, part number present
                     bit 2 set: extra field present
                     bit 3 set: original file name present
                     bit 4 set: file comment present
                     bit 5 set: file is encrypted, encryption header present
                     bit 6,7:   reserved
  4      4 bytes  file modification time in Unix format
  8      1 byte   extra flags (depend on compression method)
  9      1 byte   OS type
[
         2 bytes  optional part number (second part=1)
]?
[
         2 bytes  optional extra field length (e)
        (e)bytes  optional extra field
]?
[
           bytes  optional original file name, zero terminated
]?
[
           bytes  optional file comment, zero terminated
]?
[
        12 bytes  optional encryption header
]?
           bytes  compressed data
         4 bytes  crc32
         4 bytes  uncompressed input size modulo 2^32