>> Very vaguely similar real world example:        The last digit in 
>> your
>> VISA etc card number is a checksum which is derived in a complex 
>> but

> The LUHN check was intended to guard against manual entry errors.

But is sufficiently obscure in form that, while easily computed 
mentally, would never have it's algorithm be discovered by mistake or 
without determined analysis. Which points to it being meant to be a 
bit "hidden" as well as being for data checking. Otherwise a more 
direct modulo 10 sum would (I think, probably) be just as effective 
and error immune. (ie - not very error immune as it uses only a single 
digit with 10 choices.



        RM

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