[SolarNet]

Well the brute forcing of it would be mighty suspicious: the number on the back has a 1,000 or 10,000 combinations. So that will be noticed, even if you got the first 12 numbers right on the first try. Also, theoretically, of the remaining 12 numbers, 6 should change with each new card, which is another 1,000,000 possibilities (and bigger banks may change more numbers than that)!

[mentat]

The number on the back is generated by an algorithm with secrets that are not very secret (though apparently "secret enough").