Forgotten Cipher

VishwaCTF{VIC_Decoded_113510}

This one was little bit tricky. Encrypted text and some key evoluation formula was given. We have to decrypt the text. description was pointing towards the modified VIC cipher.

Encrypted Message :- 0d4ac648a2f0bee7bccf0231c35e13ba7bc93a2d8f7d9498885e3f4998

Key Evolution Formula :- K(n) = [ K(n−1) × 3 + index ] mod 256

With the help of Wikipedia & ChatGPT, we understood the VIC cipher first. Getting the key was easy as basekey is single digit in VIC cipher (0 - 9).

After little bit of fidling we created the following script to decrypt the message. Tries all basekeys (10)

def rotate_left(val, r_bits, max_bits=8):
    return ((val << r_bits) & (2**max_bits - 1)) | (val >> (max_bits - r_bits))

def rotate_right(val, r_bits, max_bits=8):
    return (val >> r_bits) | ((val << (max_bits - r_bits)) & (2**max_bits - 1))

def encrypt(plaintext, base_key):
    encrypted_bytes = []
    key = base_key
    for i, char in enumerate(plaintext):
        # K(n) = (K(n-1) * 3 + index) mod 256
        key = (key * 3 + i) % 256
        xored = ord(char) ^ key
        if i % 2 == 0:
            shifted = rotate_left(xored, 2)
        else:
            shifted = rotate_right(xored, 2)
        encrypted_bytes.append(shifted)
    return ''.join(f'{b:02x}' for b in encrypted_bytes)

def decrypt(ciphertext, base_key):
    encrypted_bytes = [int(ciphertext[i:i+2], 16) for i in range(0, len(ciphertext), 2)]
    decrypted_chars = []
    key = base_key
    for i, b in enumerate(encrypted_bytes):
        if i % 2 == 0:
            unshifted = rotate_right(b, 2)
        else:
            unshifted = rotate_left(b, 2)
        # Evolve key in the same way as during encryption
        key = (key * 3 + i) % 256
        original_val = unshifted ^ key
        decrypted_chars.append(chr(original_val))
    return ''.join(decrypted_chars)

if __name__ == "__main__":
    enc_text = "0d4ac648a2f0bee7bccf0231c35e13ba7bc93a2d8f7d9498885e3f4998"
    for i in range(0, 10):
        v = decrypt(enc_text, i)
        if 'Vishwa' in v:
            print(v)
            break