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A key derivation function, or KDF, derives one or many secret keys from a secret value. If you've ever needed to store a password in a database or create a private key from a password, you may have used a KDF. KDFs allow child keys to be created from a master key. They can be used in applications like Bitcoin where child keys can control sections of a wallet. The number of sites that use plain-text or weak-hash password storage is frightening.