1

All International Bank Account Numbers (IBANs) include two check digits that are the result of taking an intermediate value mod 97. Similarly, in France mod 97 is used to generate the National Check Digits for the national account number.

97 is the largest prime less than 100 but seems that it's primality is not used for generating and validating. What other reasons could there be for using 97 rather than say 100?

David Soroko
  • 121
  • Don't think so, my question is about the specific choice of 97 – David Soroko Oct 02 '22 at 11:32
  • Using $3$ digits is more "expensive", makes sense to use the largest two digit prime. $100$ would be an especially poor choice as it only picks up the last two digits of a base $10$ number. – lulu Oct 02 '22 at 11:34
  • Should have stressed, $10$ is a primitive root $\pmod {97}$ so the "weights" you get by looking at $10^n\pmod {97}$ are as varied as possible (given the size constraints). Remember, the goal is to trap common errors including single transpositions (and double transpositions if you can). – lulu Oct 02 '22 at 11:36
  • 3
    "So, the actual IBAN uses modulo 97, which is a nice prime number, so doesn't have the above problem and is large enough to efficiently use most of the 100 possible values of the checksum. Also because 97 is a prime swapping any two digits from any two places must get noticed as..." - see here. – Dietrich Burde Oct 02 '22 at 11:40

0 Answers0