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 RFC822.

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", RFC1731,
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