File containing its own checksum

Question

Is it possible to create a file that will contain its own checksum (MD5, SHA1, whatever)? And to upset jokers I mean checksum in plain, not function calculating it.

Answer 1

There is a neat implementation of the Luhn Mod N algorithm in the python-stdnum library ( see luhn.py). The calc_check_digit function will calculate a digit or character which, when appended to the file (expressed as a string) will create a valid Luhn Mod N string. As noted in many answers above, this gives a sanity check on the validity of the file, but no significant security against tampering. The receiver will need to know what alphabet is being used to define Luhn mod N validity.