As you guess the Peruvian DNI has a number, an 8-digit number that you have to remember for the rest of your life. But if you look carefully, following the 8-digit number there´s a number or a letter, the validation character.
I was googling for some time searching for the algorithm to calculate the validation character of the Peruvian DNI without success, so I decided to deduce it. My approach was this:
"Consider a 9 characters length string where the last of this characters is calculated from first eight ones by an unknown algorithm you have to deduce the algorithm from a set of examples."
The set of examples can be obtained from a public service here.
I treated this problem like an encryption challenge, my first guess was that the algorithm is a modified form of modulus 11, and I was right. Then, with a set of 40 strings was easy to deduce the coeficients and the variations to the original modulus 11 algorithm.
In my next post I'll show you the algoritm in C++.
