The S/KEY One-Time Password System

Abstract

This document describes the S/KEY* One-Time Password system as released for public use by Bellcore and as described in reference [3].A reference implementation and documentation are available by anonymous ftp from ftp.bellcore.com in the directories pub/nmh/... Overview One form of attack on computing system connected to the Internet is eavesdropping on network connections to obtain login id's and passwords of legitimate users.The captured login id and password are, at a later time, used gain access to the system.The S/KEY One-Time Password system is designed to counter this type of attack, called a replay attack.With the S/KEY system, only a single use password ever crosses the network.The user's secret pass-phrase never crosses the network at any time, including during login or when executing other commands requiring authentication such as the UNIX commands passwd or su.Thus, it is not vulnerable to eavesdropping/replay attacks.Added security is provided by the property that no secret information need be stored on any system, including the host being protected.The S/KEY 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 "inside jobs" or against active attacks where the potential intruder as able to intercept and modify the packet stream.