The Encoder – Reverse Engineering Challenge Writeup | KnightCTF 2022

The Encoder

Download the challenge file. Make that executabe: chmod +x the_encoder.out

Ran the encoder to see how it maps characters to numbers. I found this:

• A–Z → 1402–1427
• a–z → 1434–1459
• 0–9 → 1385–1394
• _ → 1432, space → 1369, { → 1460, } → 1462

Then I wrote a Python script to reverse the process using the offsets.

nums = [1412, 1404, 1421, 1407, 1460, 1452, 1386, 1414, 1449, 1445, 
        1388, 1432, 1388, 1415, 1436, 1385, 1405, 1388, 1451, 1432, 
        1386, 1388, 1388, 1392, 1462]

flag = ""
for n in nums:
    if 1402 <= n <= 1427:
        flag += chr((n - 1402) + ord('A'))
    elif 1434 <= n <= 1459:
        flag += chr((n - 1434) + ord('a'))
    elif 1385 <= n <= 1394:
        flag += chr((n - 1385) + ord('0'))
    elif n == 1369:
        flag += " "
    elif n == 1432:
        flag += "_"
    elif n == 1460:
        flag += "{"
    elif n == 1462:
        flag += "}"

print("Flag:", flag)

It gives the flag.