What are permutations in Cryptography and how do you compute the product of multiple ones?

Question

When there is a grouping of numbers how do you find the integer number that is equal to the product of two permutations? How do you solve and set it up?

Answer 1

Hi, I am not sure what example you might be looking at specifically in cryptography but I'll discuss one way permutations could be relevant in crypto and an example where you might multiply.

Lets talk about passwords. (note this could also be keys, etc)

If you're allowed to have passwords containing only digits 0-9, with no repeats. Then the number of 10 digit passwords would be 10!

Say you also needed a 4 digit PIN in addition to a password. Then the number of 4 digit pins ( no repeats ) is 10*9*8*7 = 5040

Now lets say in order to login to your banking app you need a PIN and a password. Then the number of combinations of pin and password pairs is 10! x 5040