XOR is very simple and fast, which is a must for a graphics file translators/viewers that
must decrypt a file on the fly. A problem, however, is that most graphics files contain fixed size
headers which vary only slightly in content from file to file. If you knew the approximate
contents of the header of an encrypted file you could XOR a "decrypted" header with the
encrypted file and possibly produce the key used to encrypt the file. A short key might be very
easily discovered in this way.