another day, another binary file format with a badly designed magic number not gonna call it out specifically but here are some RFC2119 MUSTs for magic number design: MUST be the very first N bytes in the file MUST be at least four bytes long, eight is better MUST include at least one byte with the high bit set MUST include a byte sequence that is invalid UTF-8 SHOULD include a zero byte, but you can usually get away with having that be part of the overall version number that immediately follows the magic number (did I mention that you really SHOULD put an overall version number right after the magic number, unless you know and have documented exactly why it's not necessary, e.g. PNG?) good examples: * PNG * ELF bad examples: * GIF * PE * PDF