A One-Time Password System

This document describes a one-time password authentication system (OTP).The system provides authentication for system access (login) and other applications requiring authentication that is secure against passive attacks based on replaying captured reusable passwords.OTP evolved from the S/KEY (S/KEY is a trademark of Bellcore) One-Time Password System that was released by Bellcore and is described in references [3] and [5]. OVERVIEWOne form of attack on networked computing systems is eavesdropping on network connections to obtain authentication information such as the login IDs and passwords of legitimate users.Once this information is captured, it can be used at a later time to gain access to the system.One-time password systems are designed to counter this type of attack, called a "replay attack" [4].The authentication system described in this document uses a secret pass-phrase to generate a sequence of one-time (single use) passwords.With this system, the user's secret pass-phrase never needs to cross the network at any time such as during authentication Haller Standards Track [Page 1] RFC 2289 A One-Time Password System February 1998or during pass-phrase changes.Thus, it is not vulnerable to replay attacks.Added security is provided by the property that no secret information need be stored on any system, including the server being protected.The OTP system protects against external passive attacks against the authentication subsystem.It does not prevent a network eavesdropper from gaining access to private information and does not provide protection against either "social engineering" or active attacks [9].