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/* ======================================================================== * Copyright 1988-2006 University of Washington * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * * ======================================================================== */ MD5 Based Authentication Mark Crispin 1 November 1999 The IMAP toolkit makes available two MD5 based authentication mechanisms, CRAM-MD5 and APOP. CRAM-MD5 is described in RFC 2195, and is a SASL (RFC 2222) authentication mechanism. APOP is described in RFC 1939, the standard document for the POP3 protocol. These mechanisms use the same general idea. The server issues a challenge; the client responds with an MD5 checksum of the challenge plus the password; the server in compares the client's response with its own calculated value of the checksum. If the client's response matches the server's calulated value, the client is authenticated. Unlike plaintext passwords, this form of authentication is believed to be secure against the session being monitored; "sniffing" the session will not disclose the password nor will it provide usable information to authenticate in another session without knowing the password. The key disadvantage with this form of authentication is that the server must know a plaintext form of the password. In traditional UNIX authentication, the server only knows an encrypted form of the password. Consequently, the authentication database for this form of authentication must be kept strictly confidential; a bad guy who acquires access to this database can access any account in the database. CRAM-MD5 client support is implemented unconditionally; any client application built with the IMAP toolkit will use CRAM-MD5 with any server which advertises CRAM-MD5 SASL support. CRAM-MD5 and APOP server support is implemented if, and only if, the CRAM-MD5 authentication database exists. By default, the CRAM-MD5 authentication database is in a UNIX file called /etc/cram-md5.pwd It is recommended that this file be protected 0400. NOTE: FAILURE TO PROTECT THIS FILE AGAINST UNAUTHORIZED ACCESS WILL COMPROMSE CRAM-MD5 AND APOP AUTHENTICATION FOR ALL USERS LISTED IN THIS DATABASE. If the CRAM-MD5 authentication database exists, then plaintext password authentication (e.g. the LOGIN command) will also use the CRAM-MD5 passwords instead of UNIX passwords. Alternatively, it is possible to build the IMAP toolkit so that plaintext password authentication is disabled entirely, by using PASSWDTYPE=nul, e.g. make aix PASSWDTYPE=nul The CRAM-MD5 authentication database file consists of a series of text lines, consisting of a UNIX user name, a single tab, and the password. A line starting with a "#" character is ignored, as are any lines which are not in valid format. For example: ------------------------------Sample------------------------------ # CRAM-MD5 authentication database # Entries are in form <user><tab><password> # Lines starting with "#" are comments bill hubba-hubba hillary nysenator monica beret tripp wired kenstarr inquisitor reno waco jessie thebody billgates ruleworld ------------------------------Sample------------------------------ Every entry in the CRAM-MD5 authentication database must have a corresponding entry in the /etc/passwd file. It is STRONGLY RECOMMENDED that the CRAM-MD5 password NOT be the same as the /etc/passwd password. It is permitted for the /etc/passwd password to be disabled; /etc/passwd is just used to get the UID, GID, and home directory information.