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IMAP/POP AUTHorize Extension for Simple Challenge/Response (RFC2195) Disclosure Number: IPCOM000002753D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2019-Feb-15
Document File: 5 page(s) / 7K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

J. Klensin: AUTHOR [+2]

Related Documents

10.17487/RFC2195: DOI


This specification provides a simple challenge-response authentication protocol that is suitable for use with IMAP4. [STANDARDS-TRACK]

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Network Working Group J. Klensin Request for Comments: 2195 R. Catoe Category: Standards Track P. Krumviede Obsoletes: 2095 MCI September 1997

IMAP/POP AUTHorize Extension for Simple Challenge/Response

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.


While IMAP4 supports a number of strong authentication mechanisms as described in RFC 1731, it lacks any mechanism that neither passes cleartext, reusable passwords across the network nor requires either a significant security infrastructure or that the mail server update a mail-system-wide user authentication file on each mail access. This specification provides a simple challenge-response authentication protocol that is suitable for use with IMAP4. Since it utilizes Keyed-MD5 digests and does not require that the secret be stored in the clear on the server, it may also constitute an improvement on APOP for POP3 use as specified in RFC 1734.

1. Introduction

Existing Proposed Standards specify an AUTHENTICATE mechanism for the IMAP4 protocol [IMAP, IMAP-AUTH] and a parallel AUTH mechanism for the POP3 protocol [POP3-AUTH]. The AUTHENTICATE mechanism is intended to be extensible; the four methods specified in [IMAP-AUTH] are all fairly powerful and require some security infrastructure to support. The base POP3 specification [POP3] also contains a lightweight challenge-response mechanism called APOP. APOP is associated with most of the risks associated with such protocols: in particular, it requires that both the client and server machines have access to the shared secret in cleartext form. CRAM offers a method for avoiding such cleartext storage while retaining the algorithmic simplicity of APOP in using only MD5, though in a "keyed" method.

Klensin, Catoe & Krumviede Standards Track [Page 1]

RFC 2195 IMAP/POP AUTHorize Extension September 1997

At present, IMAP [IMAP] lacks any facility corresponding to APOP. The only alternative to the strong mechanisms identified in [IMAP- AUTH] is a presumably cleartext username and password, supported through the LOGIN command in [IMAP]. This document describes a simple challenge-response mechanism, similar to APOP and PPP CHAP [PPP], that can be used with IMAP (and, in principle, with POP3).

This mechanism also has the advantage over some possible alternatives of not requiring that the server maintain information about email "logins" on a per-login basis. While mechanisms that do require such per-login history records may offer enhanced security, protocols such as IMAP, which may have several connections between a given client and server open more or less simultaneous, may make their implementation particularly challenging.

2. Challenge-Response Aut...