On 12/25/05, Nadav Har'El <

nyh@...> wrote:

> But how do the bad guys recognize credit card numbers? After all, a series

> of digits can easily be something else, like a phone number, a series of

> spreadsheet entries, and so on? Well, it's easy, thanks to the credit card

> companies! Credit card numbers have a check-digit (sifrat bikoret), which

> makes it trivial to recognize a niddle in haystack - a credit card number in

> a pile of other random digits...

This is not a security measure... it's called a check digit and it's

part of a well-known algorithm which your Israeli ID is also

verifiable with, called 'mod 10' or something. The algorithm is very

simple:

1. Take all the digits of the number

2. Multiply every odd digit by 2

3. Sum the digits (after multiplication)

If it's divisible by 10, the number is correct. Try it on your Israeli

ID, and if it doesn't work prepend a 0 to it and try again.

This was meant to prevent some of the most common KEYBOARD INPUT

mistakes, not to help security in any way. Originally credit cards

were imprinted on paper, and someone had to take all those paper slips

and punch the numbers into large IBM machines that did the actual

billing. Most common typing mistakes - omitting a digit, transposing

two adjacent digits - are detectable with this system.

-- Arik