POP3 AUTH: POP3 AUTHentication command
An optional POP3 command for choosing an authentication mechanism.
Myers, J., "POP3 AUTHentication command", RFC 1734, DOI 10.17487/RFC1734, December 1994, <https://www.rfc-editor.org/info/rfc1734>.
@misc{rfc1734,
series = {Request for Comments},
number = 1734,
howpublished = {RFC 1734},
publisher = {RFC Editor},
doi = {10.17487/RFC1734},
url = {https://www.rfc-editor.org/info/rfc1734},
author = {J. Myers},
title = {{POP3 AUTHentication command}},
pagetotal = 5,
year = 1994,
month = dec,
} RFC text
Contents
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
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
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.
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>
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
ISO Plain Language
On this page
§1Selecting an Authentication Method
The AUTH command names an authentication mechanism used by the POP3 client and server. The mechanism names and exchanges are based on those used by IMAP4, with pop used instead of imap in server authentication identities.
AUTH may be issued only in the AUTHORIZATION state, before the user has logged in. A server that does not support the requested mechanism should reject the command.
Before login, the client can ask the server to use a named authentication method. The server can reject a method it does not support.
§2The Challenge and Answer Exchange
If the server supports the method, it sends challenges and the client sends answers specific to that method. A challenge is a line beginning with a plus sign and a space, followed by a Base64-encoded string; the client's answer is also Base64 encoded.
The client can cancel by sending a line containing a single asterisk. If authentication succeeds, the server sends a positive response and the session enters the TRANSACTION state.
The server and client exchange encoded messages defined by the chosen method. The client can cancel the exchange, and successful authentication moves the POP3 session into its transaction state.
§2Optional Protection and Failed Attempts
An authentication method may also negotiate a protection mechanism that provides integrity and privacy for later traffic. If negotiated, it applies to all subsequent data on the connection and takes effect when the authentication exchange completes.
The RFC does not require a server to support a particular method or a method to provide protection. After a negative AUTH response, the session remains in AUTHORIZATION; the client may try another method or use USER/PASS or APOP.
Some methods can protect later messages from changes or disclosure. If an attempt fails, the client can try another method or another POP3 login command.