**The pin number is the numerical code that is used as a verification system to enter certain systems, such as the SIM card of the mobile phone or the bank card.**

In other words, the pin number is a password, made up of digits, which allows the user’s identity to be verified. This, so that only this individual (or whom he or she authorizes) can use the service on her behalf.

The title pin or PIN comes from the acronym of its name in English: *Personal Identification Number*.

## Pin code features

Among the characteristics of the pin code we can highlight:

- Its purpose is to protect the user or client of the telephone or banking service so that no one can impersonate their identity, or that this is very difficult.
- The code is usually four digits. This means that there are 10,000 different possible codes that the user can assign.
- The above is possible to calculate if we take into account that we speak, in mathematical terms, of a permutation. This is an operation by which all or part of the different elements of a set are chosen and ordered. This, importing the arranged order and, in the case of the pin number, the element can be repeated. That is, if I have the numbers 1,2,3, to give a simple example, the possible permutations are: {1,2,3};{2,1,3};{1,3,2};{3 ,2,1};{3,1,2};{2,3,1};{1,1,1};{2,2,2};{3,3,3};{1,2 ,2};{2,2,1};{2,1,2};{1,2,1};{1,1,2;{2,1,1};{1,3,3} ;{3,3,1};{3,1,3};{1,3,1};{1,1,3};{3,1,1};{2,3,3};{ 3,3,2};{3,2,3};{2,3,2};{3,2,2};{2,2,3}. So, the same thing that we have done with these three digits, for the purpose of the pin number, we do it by considering ten digits, from 0 to 9, and choosing four of those digits in each permutation. For simplicity, the formula is n
^{x}, where n is the total of the elements, while x is the size of the subset. Thus, if there are ten digits in total and the pin number has four digits, the number of possible permutations is: 10^{4}=10,000. - Each user can receive a pin number already pre-established randomly (by their telephone service operator or by the banking company), which can be changed for another number that the person prefers.
- It is recommended to look for the highest level of security when creating the pin number. Thus, it is suggested to avoid, for example, easy-to-guess passwords such as the birthday date, or use a pin number where the same number is repeated four times.

## The pin number is not infallible or unique

The pin number is not infallible, as a hacker could, for example, make several attempts to find the password.

Also, it is important to note that this key is only one of the verification procedures that currently exist. In this sense, it is possible that the user can be asked not only for something that he knows (such as the pin number), but also something that he has (such as the token that is a physical device or available in a virtual way that every so often throws a random key) and/or something the person is (biometric data such as fingerprint).