---
title: peaCTF 2019 (Round1) - [Crypto] Broken Keyboard (50pts)
author: Maltemo
tags: CTF, peaCTF, Crypto, ASCII
---
peaCTF 2019 (Round1) - [Crypto] Broken Keyboard (50pts)
===
Written by [Maltemo](https://twitter.com/Maltemo), member of team [SinHack](https://sinhack.blog/).

[TOC]

## Statement of the challenge

### Description
Help! My keyboard only types numbers!

### Encrypted file :lock:
The file attached was `enc.txt` and contained :

112 101 97 67 84 70 123 52 115 99 49 49 105 115 99 48 48 108 125

## Analysis :mag:

Thanks to the challenge's description, we get some informations that those numbers come from a keyboard.

We can expect that those numbers corresponds to letters.
And what makes numbers correspond to letter in IT ?

:arrow_forward: ASCII Table :arrow_backward:

Here is a quick explanation of what is the ASCII Table :

:::info
>ASCII stands for American Standard Code for Information Interchange. Computers can only understand numbers, so an ASCII code is the numerical representation of a character such as 'a' or '@' or an action of some sort.

Source : https://www.asciitable.com/
:::

## Solution :unlock:

At this point, many methods can solve the challenge. We could do it only manualy with the ASCII Table and the message, but I will do it with javascript language.

Why ? Because it is fast, and I :heartbeat: javascript !

First step consists in starting node in the console :

node

:::warning
If you don't have nodejs installed on your laptop, you can still execute this code from the console of your own browser. Or you can install it on the [official website](https://nodejs.org/en/download/).
:::

Then we can write down some javascript, and everytime we will hit the `Enter` button, it will execute the written code.

First step will be to store the encoded string in a variable :

let stringEnc = "112 101 97 67 84 70 123 52 115 99 49 49 105 115 99 48 48 108 125";

Then, we want to get an array of those numbers :

let arrayEnc = stringEnc.split(" ")

`.split(" ")` will cut the string in little pieces every time he finds the given string in parameter (here a space)

Now, we have an array of strings of encoded characters.

console.log(arrayEnc);

//[ '112','101','97','67','84','70','123','52','115',
//'99','49','49','105','115','99','48','48','108','125' ]

But we want to have number to decode them back to characters.
So we are going to apply a function on every element of the array, with the `map()` function.

We convert the element from `String` to `Number` with `Number()`
And then we convert this number to the corresponding character in the ASCII table with `String.fromCharCode()`.

let arrayDec = arrayEnc.map(function(element){
    return String.fromCharCode(Number(element));
});

Finaly, we are going to get back a single string from the array that now contains the letters of the flag.
`.join("")` function will do the trick.
It will create a string by concatenating every element and spliting them with the given string (here an empty one).

arrayDec.join("");

If we are in node console, it will display the code. In the browser too normaly.

But if not, you can still display the output with `console.log()` :

console.log(arrayDec.join(""));

### TL;DR

The numbers where numbers of the ASCII Table, that corresponds to letters.

:::success
One line solution in javascript :

"112 101 97 67 84 70 123 52 115 99 49 49 105 115 99 48 48 108 125".split(" ").map(elt => String.fromCharCode(Number(elt))).join("");

:::

### Flag :white_check_mark:
The flag is : **peaCTF{4sc11isc00l}**

___
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