RFC 1760 (rfc1760) - Page 1 of 12

The S/KEY One-Time Password System

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Network Working Group                                          N. Haller
Request for Comments: 1760                                      Bellcore
Category: Informational                                    February 1995


                   The S/KEY One-Time Password System

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

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.








Haller
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