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RFC 1734

                      POP3 AUTHentication command

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



Myers                                                           [Page 1]

RFC 1734 POP3 AUTH December 1994 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 Myers [Page 2]
RFC 1734 POP3 AUTH December 1994 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. Myers [Page 3]
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> Myers [Page 4]
RFC 1734 POP3 AUTH December 1994 LF ::= <ASCII LF, line feed, 0x0A> SPACE ::= <ASCII SP, space, 0x20> TAB ::= <ASCII HT, tab, 0x09>



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