It's a secret (18)

Computers are particularly good at one thing - number-crunching.  That is doing maths with large numbers.  And this ability has led to the ultimate in cryptography. If I ask you to multiply two numbers together and give me the answer, that's not much of a problem - even if they're big numbers, and you only have pencil and paper, it shouldn't cause you too much trouble.  But if I give you the answer and ask you to find the two numbers which were multiplied together to get that answer (this is called factorising) that's a whole different ball-game.  This is because there is no real formula for doing this.  You are thrown back on what mathematicians are pleased to call the "brute force" method - in other words trying all the various possibilities until you hit on the right one.  And clearly the bigger the number you are trying to factorise, the more possibilities there will be, and the longer it will take you.  And this forms the basis of what has become known as public-private-key cryptography (PPK). Of which, more to come.