Password-based cryptography generally refers to two distinct classes of methods:
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Single party methods
Some systems attempt to derive a cryptographic key directly from a password. However, such practice is generally ill-advised when there is a threat of brute-force attack. Techniques to mitigate such attack include passphrases and iterated (deliberately slow) password-based key derivation functions such as PBKDF2 (RFC 2898).
Multi-party methods
Password-authenticated key agreement systems allow two or more parties that agree on a password (or password-related data) to derive shared keys without exposing the password or keys to network attack. Earlier generations of challenge-response authentication systems have also been used with passwords, but these have generally been subject to eavesdropping and/or brute-force attacks on the password.