What is the X in X-mas?
in cryptography, an encryption/decryption value known only to the party or parties that exchange secret messages; also called secret key
Some systems use both a public and private key.
cryptography A piece of data used in private-key cryptography and public-key cryptography. In the former the private key is known by both sender and recipient whereas in the latter it is known only to the sender. (2008-02-07)