# picoCTF 2022: Cryptography: basic-mod1

## Introduction
**Challenge:** [basic-mod1](https://play.picoctf.org/practice/challenge/253?originalEvent=70&page=1)

**Category:** Cryptography

**Description**:
We found this weird message being passed around on the servers, we think we have a working decryption scheme. Download the message [here](https://artifacts.picoctf.net/c/395/message.txt).

Take each number mod 37 and map it to the following character set: 0-25 is the alphabet (uppercase), 26-35 are the decimal digits, and 36 is an underscore.

Wrap your decrypted message in the picoCTF flag format (i.e. `picoCTF{decrypted_message}`)

## Solution
Here the problem is that we have a message that looks like this:
```txt
91 322 57 124 40 406 272 147 239 285 353 272 77 110 296 262 299 323 255 337 150 102 
```
Our goal is to decrypt. To do this we have to follow these steps:
1. **Take each number mod 37**: If you don't know what `mod 37` means. It means the reminder that we get after dividing the following number by 37. For example, `91 mod 37` will be `17`. And we need to do this  step for each number
2. **Map it with the character set**: After we have got the following number `mod 37`. We have to map it to the following character set like this: 0-25 is the alphabet (uppercase), 26-35 are the decimal digits, and 36 is an underscore. For example, 17 will be R starting from 0.
3. **Wrap it in picoCTF flag format**: Once we have decrypted our message. We have to wrap it in the picoCTF flag format like this: `picoCTF{decrypted_message}`

Doing this manually is not a good solution so let's write a Python and JavaScript script to do all these steps to decrypt this message.

**Python**:
```python
import string
characters = string.ascii_uppercase
characters += "0123456789_"

flag_enc = [91,322,57,124,40,406,272,147,239,285,353,272,77,110,296,262,299,323,255,337,150,102]
flag = ""
for char in flag_enc:
    mod = char % 37
    flag += characters[mod]

print('picoCTF{%s}' % flag)
```
**JavaScript**:
```javascript
const characterCodes = Array.from(Array(26)).map((e, i) => i + 65);
let characters = characterCodes.map((x) => String.fromCharCode(x));
characters = [...characters, ...Array.from(Array(10).keys()), "_"]

const flagEnc = [91,322,57,124,40,406,272,147,239,285,353,272,77,110,296,262,299,323,255,337,150,102]
let flag = ""
flagEnc.forEach(char => {
    const mod = char % 37
    flag += characters[mod]
})
console.log(`picoCTF{${flag}}`)
```
In both programs, the basic thing that is happening is we are creating a list of characters then mapping the `mod 37` and adding the letter to the `flag` string. After that just print it.

If you are on your browser reading this article. Just press `CTRL+Shift+J` to open the Browser Console and then just paste the above JavaScript code to decrypt this message.

## Conclusion
Again, This is a very simple message that we need to decrypt using a very simple decryption scheme.

**Flag:** `picoCTF{R0UND_N_R0UND_ADD17EC2}`