A One-Time Password System (RFC1938)
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
N. Haller: AUTHOR [+1]
This document describes a one-time password authentication system (OTP). [STANDARDS-TRACK]
Network Working Group N. Haller Request for Comments: 1938 Bellcore Category: Standards Track C. Metz Kaman Sciences Corporation May 1996
A One-Time Password System
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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  and .
One 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" .
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 or 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
Haller & Metz Standards Track [Page 1]
RFC 1938 A One-Time Password System May 1996
protection against either "social engineering" or active attacks .
There are two entities in the operation of the OTP one-time password system. The generator must produce the appropriate one-time password from the user’s secret pass-phrase and from information provided in the challenge from the server. The server must send a challenge that includes the appropriate generation parameters to the generator, must verify the one-time password received, must store the last valid one-time password it received, and must store the corresponding one- time password sequence number. The server must also facilitate the changing of the user’s secret pass-phrase in a secure manner.
The OTP system generator passes the user’s secret pass-phrase, along with a seed received from the server as part of the challenge, through multiple iterations of a secure hash func...