Brute Force

The baseline of how strong a cipher, and what a cryptanalysist wishes to do better than, is an exhaustive search of the keyspace. In other words, any attack that is faster than simply enumerating all possible keys is considered a "cryptanalytic break" of the cipher. Whether such an attack is practical or not is a different story entirely.

PRESENT supports both 80-bit and 128-bits keys, which are then turned into thirty-two round keys using a key scheduling function. Right now, it's not too important how this function works, but more so the fact it exists. This means that despite the cipher consisting of thirty-two 64-bit round keys, its effective bit-security is only the size of the actual key. It should be noted that recovering the key is equivalent to recovering all round keys.

Despite this, it remains important to keep the way we generate our key important. To illustrate this point, we've encrypted a message with PRESENT. The key used was derived in the following way:

def random_roundkey(bits: int):
    t = randint(0, 2**bits)
    return int(sha256(str(t).encode()).hexdigest()[:16], 16)

cipher = Present(rounds=32, custom_roundkeys=[random_roundkey(20)]*32)

The ciphertext is TODO. Apply a brute force search to find a key, and decrypt the message.