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