Explain the threat in each of the following types of authentication and provide a solution for each scenario below:
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- Alice sends a password, and Bob compares it against a database of passwords.
- Alice sends a password, and Bob hashes it and compares it against a database of hashed passwords.
- Alice computes the hash of a password and uses it as secret key in challenge/response protocol.
- Alice computes the hash of a password and sends it to Bob, who hashes it and compares it against a database of doubly-hashed passwords.