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|| The TCP/IP Guide|
9 TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
9 TCP/IP Key Applications and Application Protocols
9 TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
9 TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
9 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
9 TCP/IP Post Office Protocol (POP/POP3)
POP3 Authorization State: User Authentication Process and Commands
(Page 3 of 3)
Alternative Authentication Using APOP
Since user/password authorization
is considered by many people to be insufficient for the security needs
of modern internetworks, the POP3 standard also defines an alternative
authentication method, using the APOP command. This is a more
sophisticated technique based on the MD5 message digest
If the server supports this technique,
in its opening greeting it provides a string indicating a timestamp
that is unique for each POP3 session. The client then performs an MD5
calculation using this timestamp value and a shared secret
known by the server and client. The result of this calculation is included
in the client's APOP command. If it matches the server's calculation,
authentication is successful; otherwise the session remains in the Authorization
The Post Office Protocol was also
designed to allow it to be extended through the addition of other authentication
mechanisms. This process is based on the use of the optional AUTH
command, as described in RFC 1734.
Key Concept: A POP3 session begins in the Authorization state, where the client device is expected to authenticate with the server. By default, POP3 uses only a simple username/password authentication method. Optional authentication methods are also defined for applications requiring more security.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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