S/key

A security system developed by Bellecore. S/key is a one-time, challenge-response password scheme used to authenticate access to data. The purpose of S/key is to eliminate the need for the same password to be conveyed over a network each time a password is needed for access. A series of passwords are created at once, and each password is used only one time; therefore, someone obtaining the password cannot use the information because the password becomes invalid once it has been used the first time.