Explain the threat in each of the following types of authentication and provide a solution for each scenario below:

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.