If the contestants were given both the plaintext and ciphertext, it's straightforward. Just bruteforce all 56-bit keys until you find one that maps the given plaintext to the given ciphertext.
But from what I understand, the contestants were given only the ciphertext and the initialization vector. I'm confused as to how they cracked the challenge without any plaintext.
What I imagine they did was:
- Ok, we know the plaintext is less than or equal to the size of the ciphertext.
- Calculate all possible plaintext from 1 bit up to the bit size of the ciphertext.
- Calculate all possible 56-bit keys.
- Run each plaintext through all 56-bit keys until they found a mapping.
But this doesn't make sense given the sheer size of the possibilities.
So since they weren't given any plaintext, how did they know they'd found the right plaintext/key combination?