Network Working Group                                           J. Myers
Request for Comments: 1734                               Carnegie Mellon
Category: Standards Track                                  December 1994


                      POP3 AUTHentication command

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.


1. Introduction

   This document describes the optional AUTH command, for indicating an
   authentication mechanism to the server, performing an authentication
   protocol exchange, and optionally negotiating a protection mechanism
   for subsequent protocol interactions.  The authentication and
   protection mechanisms used by the POP3 AUTH command are those used by
   IMAP4.


2. The AUTH command

   AUTH mechanism

         Arguments:
             a string identifying an IMAP4 authentication mechanism,
             such as defined by [IMAP4-AUTH].  Any use of the string
             "imap" used in a server authentication identity in the
             definition of an authentication mechanism is replaced with
             the string "pop".

         Restrictions:
             may only be given in the AUTHORIZATION state

         Discussion:
             The AUTH command indicates an authentication mechanism to
             the server.  If the server supports the requested
             authentication mechanism, it performs an authentication
             protocol exchange to authenticate and identify the user.
             Optionally, it also negotiates a protection mechanism for
             subsequent protocol interactions.  If the requested
             authentication mechanism is not supported, the server



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             should reject the AUTH command by sending a negative
             response.

             The authentication protocol exchange consists of a series
             of server challenges and client answers that are specific
             to the authentication mechanism.  A server challenge,
             otherwise known as a ready response, is a line consisting
             of a "+" character followed by a single space and a BASE64
             encoded string.  The client answer consists of a line
             containing a BASE64 encoded string.  If the client wishes
             to cancel an authentication exchange, it should issue a
             line with a single "*".  If the server receives such an
             answer, it must reject the AUTH command by sending a
             negative response.

             A protection mechanism provides integrity and privacy
             protection to the protocol session.  If a protection
             mechanism is negotiated, it is applied to all subsequent
             data sent over the connection.  The protection mechanism
             takes effect immediately following the CRLF that concludes
             the authentication exchange for the client, and the CRLF of
             the positive response for the server.  Once the protection
             mechanism is in effect, the stream of command and response
             octets is processed into buffers of ciphertext.  Each
             buffer is transferred over the connection as a stream of
             octets prepended with a four octet field in network byte
             order that represents the length of the following data.
             The maximum ciphertext buffer length is defined by the
             protection mechanism.

             The server is not required to support any particular
             authentication mechanism, nor are authentication mechanisms
             required to support any protection mechanisms.  If an AUTH
             command fails with a negative response, the session remains
             in the AUTHORIZATION state and client may try another
             authentication mechanism by issuing another AUTH command,
             or may attempt to authenticate by using the USER/PASS or
             APOP commands.  In other words, the client may request
             authentication types in decreasing order of preference,
             with the USER/PASS or APOP command as a last resort.

             Should the client successfully complete the authentication
             exchange, the POP3 server issues a positive response and
             the POP3 session enters the TRANSACTION state.

         Possible Responses:
             +OK maildrop locked and ready
             -ERR authentication exchange failed



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         Examples:
             S: +OK POP3 server ready
             C: AUTH KERBEROS_V4
             S: + AmFYig==
             C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
                +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
                WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
             S: + or//EoAADZI=
             C: DiAF5A4gA+oOIALuBkAAmw==
             S: +OK Kerberos V4 authentication successful
                ...
             C: AUTH FOOBAR
             S: -ERR Unrecognized authentication type

              Note: the line breaks in the first client answer  are
              for editorial clarity and are not in real authentica-
              tors.

































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RFC 1734                       POP3 AUTH                   December 1994


3. Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) notation as specified in RFC 822.

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of upper or lower case characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.

   ATOM_CHAR       ::= <any CHAR except atom_specials>

   atom_specials   ::= "(" / ")" / "{" / SPACE / CTLs / "%" / "*" /
                       <"> / "\"

   auth            ::= "AUTH" 1*(SPACE / TAB) auth_type *(CRLF base64)
                       CRLF

   auth_type       ::= 1*ATOM_CHAR

   base64          ::= *(4base64_CHAR) [base64_terminal]

   base64_char     ::= "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H" /
           "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P" /
                       "Q" / "R" / "S" / "T" / "U" / "V" / "W" / "X" /
                       "Y" / "Z" /
                       "a" / "b" / "c" / "d" / "e" / "f" / "g" / "h" /
                       "i" / "j" / "k" / "l" / "m" / "n" / "o" / "p" /
                       "q" / "r" / "s" / "t" / "u" / "v" / "w" / "x" /
                       "y" / "z" /
                       "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" /
                       "8" / "9" / "+" / "/"
                       ;; Case-sensitive

   base64_terminal ::= (2base64_char "==") / (3base64_char "=")

   CHAR            ::= <any 7-bit US-ASCII character except NUL,
                        0x01 - 0x7f>

   continue_req    ::= "+" SPACE base64 CRLF

   CR              ::= <ASCII CR, carriage return, 0x0C>

   CRLF            ::= CR LF

   CTL             ::= <any ASCII control character and DEL,
                        0x00 - 0x1f, 0x7f>




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RFC 1734                       POP3 AUTH                   December 1994


   LF              ::= <ASCII LF, line feed, 0x0A>

   SPACE           ::= <ASCII SP, space, 0x20>

   TAB             ::= <ASCII HT, tab, 0x09>



4. References

   [IMAP4-AUTH]  Myers, J., "IMAP4 Authentication Mechanisms", RFC 1731,
   Carnegie Mellon, December 1994.



5. Security Considerations

   Security issues are discussed throughout this memo.



6. Author's Address

   John G. Myers
   Carnegie-Mellon University
   5000 Forbes Ave
   Pittsburgh, PA 15213

   EMail: jgm+@cmu.edu






















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