Cracking block cipher algorithm refers to finding the key according to the information.
The decipherer here may only have ciphertext, there may be a certain number of plaintext-ciphertext pairs, or there is an encryption machine, and you can get ciphertext by inputting any plaintext. In short, the goal of deciphering is to "find the key".
The exhaustive method is to exhaust all possible 56-bit keys and try them one by one until the correct key is found.