Pre-shared key

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A pre-shared key or PSK is a secret which was previously shared between the two parties using some secure channel before it needs to be used. Such systems almost always use symmetric key cryptographic algorithms. The characteristics of this secret or key are determined by the system which uses it; some system designs require that such keys be in a particular format. It can be a password like 'tom13i', a passphrase like 'idaho hung gear id gene', or a hexadecimal string like '65E4 E556 8622 EEE1'. The secret is used by all systems involved in the cryptographic processes used to secure the traffic between the systems (for example in WiFi encryption such as WEP or WPA).

Since one weakpoint of the crypto system is the encryption algorithm's key, the strength of the key is important, and since the strength of a key is in part dependent on its length, it is important to choose a key whose length is cryptographically secure (see password strength). There are several tools available to help one choose a strong key. Diceware is one example.

[edit] Examples

• A pre shared key is a password which is entered to access a secure wifi system using WEP or WPA. Both the wireless access point (AP) and the client share the same key.
• The book used by Alice and Bob when they use a book cipher.
• The passphrase entered to decrypt a file encrypted with PGP which did not use PKI.