BabyEncryption — HackTheBox

Description

You are after an organised crime group which is responsible for the illegal weapon market in your country. As a secret agent, you have infiltrated the group enough to be included in meetings with clients. During the last negotiation, you found one of the confidential messages for the customer. It contains crucial information about the delivery. Do you think you can decrypt it?

Files:

• chall.py — encryption script
• msg.enc — encrypted message

Analysis

Encryption Source Code

import string
from secret import MSG

def encryption(msg):
    ct = []
    for char in msg:
        ct.append((123 * char + 18) % 256)
    return bytes(ct)

ct = encryption(MSG)
f = open('./msg.enc','w')
f.write(ct.hex())
f.close()

Vulnerability

The encryption uses an affine cipher — a linear transformation of each byte:

ct = (123 * pt + 18) % 256

Where:

• pt — plaintext byte
• ct — ciphertext byte
• 123 — multiplier (a)
• 18 — shift (b)
• 256 — modulus (byte size)

This is a reversible transformation if gcd(123, 256) = 1 (which is true since 123 is odd).

Solution

Decryption Math

To reverse the formula:

ct = (123 * pt + 18) % 256
ct - 18 = 123 * pt % 256
pt = (ct - 18) * inv(123) % 256

We need to find the modular inverse of 123 modulo 256.

Finding the Modular Inverse

Using the Extended Euclidean Algorithm:

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