To prevent a key from being guessed, keys need to be generated truly randomly and contain sufficient entropy. The problem of how to safely generate truly random keys is difficult, and has been addressed in many ways by various cryptographic systems. There is a RFC on generating randomness (RFC 4086, Randomness Requirements for Security). Some operating systems include tools for "collecting" entropy from the timing of unpredictable operations such as disk drive head movements. For the production of small amounts of keying material, ordinary dice provide a good source of high quality randomness.
When a password (or passphrase) is used as an encryption key, well-designed cryptosystems first run it through a key derivation function which adds a salt and compresses or expands it to the key length desired, for example by compressing a long phrase into a 128-bit value suitable for use in a block cipher.
Read more about this topic: Key (cryptography)
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