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There are two ways to attack encryption that uses a derived key: You can attack the encryption algorithm. In the case of correctly used* 128-bit AES, that essentially amounts to a brute force attack on the 128-bit keyspace. This would succeed after on average $2^{127}$ tries (if it were practical). If you knew that two files had used the same password ...


Since there are only $16^6 \approx 16.8$ million keys, you can try them all and decrypt the message with each. In general you would have to know something about the plaintext to identify which of those decrypted candidates is the correct one. In this case it is known that the message is English ASCII, so the top bit of each plaintext byte will be 0. The ...


The difference in that answer is that brute force refers to an exhaustive guessing attack (having guaranteed success but requiring exponential time) whereas guessing (a polynomial number of) random values refers to exactly what it says (requiring polynomial time but having only a certain chance of success).

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