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Question:

Which of the following is the best attack vector for Joe and Bob to follow if they are both ethical hackers and have gained access to a folder when Joe has several encrypted files from the folder, and Bob has found one of them unencrypted?

A Known plain text
Explaination

The hacker has both plain-text and cipher-text communications in a known plain-text attack; the plain-text copies are scanned for repeating patterns, which are then compared to the cipher-text versions. This may be used to decipher the key over time and with effort.