I'm attempting to mimic the function used for creating CRC's in PNG files, I'm using the autodin II polynomial and the source code from: http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/libkern/crc32.c My tests have all been for the IHDR chunk, so my parameters have been: crc - 0xffffffff and 0 (both have been suggested) buff - the address of the IHDR Chunk's type. length - the IHDR Chunk's length + 4 (the length of the chunk's data + the length of the type) I printed the calculated CRC in binary, which I compared to the actual CRC of the chunk. I can see no similarities (little-big endian, reversed bits, XOR'd, etc). This is the data for the IHDR chunk (hexadecimal format): length(big endian): d0 00 00 00 (13) type: 49 48 44 52 data: 00 00 01 77 00 00 01 68 08 06 00 00 00 existing CRC: b0 bb 40 ac If anyone can tell me why my calculations are off, or give me a CRC32 function that will work I would greatly appreciate it. Thank-you!