A password authentication enhancement, the one?time password allows you to log on only once with
a password, after which that password is no longer valid. The way this works is that instead of having
a normal password you would memorize, you are given a list of passwords. You use each password
sequentially. You might use a hardware device that maintains the list for you. Each time you login,
you ask the hardware device for the next password. Key point: The true secret (such as the password
used to encrypt the passwords) is never sent across the wire. A hacker could certainly sniff the
password from the wire, but it is now useless. Example: The original OTP system was named
“S/Key”; a term trademarked by Bellcore. The idea was to create a password authentication system
that integrated seamlessly to existing UNIX systems. Other approaches require replacing existing
protocols/software with secure password exchanges (like challenge?responses or public?key crypto).
However, it should be noted that the S/Key protocol is still vulnerable to man?in?the?middle attacks.

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