Shared secret

In cryptography, a shared secret is a piece of data only known to the parties involved in a secure communication. The shared secret can be a password, a passphrase, a big number or an array of randomly chosen bytes.

The shared secret is either shared beforehand between the communicating parties, in which case it can also be called a pre-shared key. Or it is created at the start of the communication session by using a key-agreement protocol, for-instance using public-key cryptography such as Diffie-Hellman or using symmetric-key cryptography such as Kerberos.

The shared secret can be used for authentication (for instance when logging in to a remote system) using methods such as challenge-response or it can be fed to a key derivation function to produce one or more keys to use for encryption and/or MACing of messages.

To make unique session and message keys the shared secret is usually combined with an initialization vector (IV).