Iced Tea

3 minutes to read

We are given the Python source code that encrypts the flag:

import os
from secret import FLAG
from Crypto.Util.Padding import pad
from Crypto.Util.number import bytes_to_long as b2l, long_to_bytes as l2b
from enum import Enum

class Mode(Enum):
    ECB = 0x01
    CBC = 0x02

class Cipher:
    def __init__(self, key, iv=None):
        self.BLOCK_SIZE = 64
        self.KEY = [b2l(key[i:i+self.BLOCK_SIZE//16]) for i in range(0, len(key), self.BLOCK_SIZE//16)]  
        self.DELTA = 0x9e3779b9
        self.IV = iv
        if self.IV:
            self.mode = Mode.CBC
        else:
            self.mode = Mode.ECB
    
    def _xor(self, a, b):
        return b''.join(bytes([_a ^ _b]) for _a, _b in zip(a, b))

    def encrypt(self, msg):
        msg = pad(msg, self.BLOCK_SIZE//8)
        blocks = [msg[i:i+self.BLOCK_SIZE//8] for i in range(0, len(msg), self.BLOCK_SIZE//8)]
        
        ct = b''
        if self.mode == Mode.ECB:
            for pt in blocks:
                ct += self.encrypt_block(pt)
        elif self.mode == Mode.CBC:
            X = self.IV
            for pt in blocks:
                enc_block = self.encrypt_block(self._xor(X, pt))
                ct += enc_block
                X = enc_block
        return ct

    def encrypt_block(self, msg):
        m0 = b2l(msg[:4])
        m1 = b2l(msg[4:])
        K = self.KEY
        msk = (1 << (self.BLOCK_SIZE//2)) - 1

        s = 0
        for i in range(32):
            s += self.DELTA
            m0 += ((m1 << 4) + K[0]) ^ (m1 + s) ^ ((m1 >> 5) + K[1])
            m0 &= msk
            m1 += ((m0 << 4) + K[2]) ^ (m0 + s) ^ ((m0 >> 5) + K[3])
            m1 &= msk
        
        m = ((m0 << (self.BLOCK_SIZE//2)) + m1) & ((1 << self.BLOCK_SIZE) - 1) # m = m0 || m1

        return l2b(m)



if __name__ == '__main__':
    KEY = os.urandom(16)
    cipher = Cipher(KEY)
    ct = cipher.encrypt(FLAG)
    with open('output.txt', 'w') as f:
        f.write(f'Key : {KEY.hex()}\nCiphertext : {ct.hex()}')

We also have the output of the script:

Key : e25b3dd8f702879cccaec0280a29c66a
Ciphertext : 0ab76ebf55e0957ccaea74b440cb6f2ccef34969bdb22f87c73391d04dbe7fb272d7a6533a38a943449dda8cdce124be429e34e58847243904e8543ef33141d23da518b6bd1800f2

Source code analysis

The server uses a Cipher class to encrypt the flag. The server does not explicitly tell what encryption algorithm is using, but it is fairly easy to see that it is Tiny Encryption Algorithm (TEA), which is a symmetric block cipher.

Notice that we are given the encryption key and the ciphertext, so we only need to decrypt it. However, the Cipher class does not implement decrypt and decrypt_block functions, so we will need to implement them.

Decryption

The decrytion functions are easy to implement, we only need to do the inverse operations of encrypt and encrypt_block in reverse order. Otherwise, we can take a look at Wikipedia and translate the reference code in C to Python.

Anyways, these are the necessary functions:

    def decrypt(self, msg_ct):
        blocks = [msg_ct[i:i+self.BLOCK_SIZE//8] for i in range(0, len(msg_ct), self.BLOCK_SIZE//8)]  
        
        pt = b''
        if self.mode == Mode.ECB:
            for ct in blocks:
                pt += self.decrypt_block(ct)
        elif self.mode == Mode.CBC:
            X = self.IV
            for ct in blocks:
                dec_block = self._xor(X, self.decrypt_block(ct))
                pt += dec_block
                X = ct
        return unpad(pt, self.BLOCK_SIZE//8)

    def decrypt_block(self, m):
        m0 = b2l(m[:4])
        m1 = b2l(m[4:])
        K = self.KEY
        msk = 0xffffffff

        s = 32 * self.DELTA

        for _ in range(32):
            m1 -= (((m0 << 4) + K[2]) ^ (m0 + s) ^ ((m0 >> 5) + K[3]))
            m1 &= msk
            m0 -= (((m1 << 4) + K[0]) ^ (m1 + s) ^ ((m1 >> 5) + K[1]))
            m0 &= msk
            s -= self.DELTA

        msg = ((m0 << 32) + m1) & 0xffffffffffffffff # m = m0 || m1

        return l2b(msg)

At this point, we can take the encryption key and the ciphertext from output.txt and decrypt the flag:

KEY = bytes.fromhex('e25b3dd8f702879cccaec0280a29c66a')
ct = bytes.fromhex('0ab76ebf55e0957ccaea74b440cb6f2ccef34969bdb22f87c73391d04dbe7fb272d7a6533a38a943449dda8cdce124be429e34e58847243904e8543ef33141d23da518b6bd1800f2')  

print(Cipher(KEY).decrypt(ct).decode())

Flag

And here’s the flag:

$ python3 solve.py
HTB{th1s_ch4ll3ng3_1s_4n_1ntr0duct10n_t0_th3_T1ny_3ncrypt10n_41g0r1thm}