# Problem 79

Solution to Problem 79. Solved using pencil/paper.

Problem: A common security method used for online banking is to ask the user for three random characters from a passcode. For example, if the passcode was 531278, they may ask for the 2nd, 3rd, and 5th characters; the expected reply would be: 317.

The text file, keylog.txt, contains fifty successful login attempts.

Given that the three characters are always asked for in order, analyse the file so as to determine the shortest possible secret passcode of unknown length.

The solution to this problem is actually quite simple. Think of all the numbers in keylog.txt as rules that need to be satisifed.

We start with writing down the first and second rule next to each other. You should have `319680`

written down. We can see the third rule, `180`

, already has the digits `1`

, `8`

, and `0`

in order in the number `319680`

. The fourth rule, `690`

, is where things start to change. In order to satisfy this rule, we need to swap the `6`

and `9`

to arrive at `316980`

.

The next rule, `129`

, introduces a new digit that we haven’t seen before, but it hasn’t changed the way we solve the problem. `1`

and `9`

appear to be satisfied, but in order to solve for this rule, we need to put the `2`

somewhere. Putting it right after the `1`

is fine; you should now have `3126980`

written down. Keep doing the same thing all the way down the list, and you’ll arrive at your answer.