Post Office Protocol Version 3 (POP3) Support for UTF-8

Internet Engineering Task Force (IETF)                        R. Gellens
Request for Comments: 6856                         QUALCOMM Incorporated
Obsoletes: 5721                                                C. Newman
Category: Standards Track                                         Oracle
ISSN: 2070-1721                                                   J. Yao
                                                                   CNNIC
                                                             K. Fujiwara
                                                                    JPRS
                                                              March 2013


        Post Office Protocol Version 3 (POP3) Support for UTF-8

Abstract

   This specification extends the Post Office Protocol version 3 (POP3)
   to support international strings encoded in UTF-8 in usernames,
   passwords, mail addresses, message headers, and protocol-level text
   strings.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6856.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  "UTF8" Capability  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The "UTF8" Command . . . . . . . . . . . . . . . . . . . .  5
     2.2.  USER Argument to "UTF8" Capability . . . . . . . . . . . .  6
   3.  "LANG" Capability  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Non-ASCII Character Maildrops  . . . . . . . . . . . . . . . .  9
   5.  "UTF8" Response Code . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Design Rationale  . . . . . . . . . . . . . . . . . . 13
   Appendix B.  Acknowledgments . . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   This document forms part of the Email Address Internationalization
   protocols described in the Email Address Internationalization
   Framework document [RFC6530].  As part of the overall Email Address
   Internationalization work, email messages can be transmitted and
   delivered containing a Unicode string encoded in UTF-8 in the header
   and/or body, and maildrops that are accessed using POP3 [RFC1939]
   might natively store Unicode characters.

   This specification extends POP3 using the POP3 extension mechanism
   [RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers and bodies
   (e.g., transferred using 8-bit content-transfer-encoding) as
   described in "Internationalized Email Headers" [RFC6532].  It also
   adds a mechanism to support login names and passwords containing a
   UTF-8 string (see Section 1.1 below), a mechanism to support UTF-8
   strings in protocol-level response strings, and the ability to
   negotiate a language for such response strings.

   This specification also adds a new response code to indicate that a
   message was not delivered because it required UTF-8 mode (as
   discussed in Section 2) and the server was unable or unwilling to
   create and deliver a surrogate form of the message as discussed in
   Section 7 of "IMAP Support for UTF-8" [RFC6855].

   This specification replaces an earlier, experimental, approach to the
   same problem [RFC5721].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC2119].

   The terms "UTF-8 string" or "UTF-8 character" are used to refer to
   Unicode characters, which may or may not be members of the ASCII
   repertoire, encoded in UTF-8 [RFC3629], a standard Unicode encoding
   form.  All other specialized terms used in this specification are
   defined in the Email Address Internationalization framework document.

   In examples, "C:" and "S:" indicate lines sent by the client and
   server, respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.

   Note that examples always use ASCII characters due to limitations of
   the RFC format; otherwise, some examples for the "LANG" command would
   have appeared incorrectly.

2.  "UTF8" Capability

   This specification adds a new POP3 Extension [RFC2449] capability
   response tag and command to specify support for header field
   information outside the ASCII repertoire.  The capability tag and new
   command and functionality are described below.

   CAPA tag:
      UTF8

   Arguments with CAPA tag:
      USER

   Added Commands:
      UTF8

   Standard commands affected:
      USER, PASS, APOP, LIST, TOP, RETR

   Announced states / possible differences:
      both / no

   Commands valid in states:
      AUTHORIZATION

   Specification reference:
      this document

   Discussion:

   This capability adds the "UTF8" command to POP3.  The "UTF8" command
   switches the session from the ASCII-only mode of POP3 [RFC1939] to
   UTF-8 mode.  The UTF-8 mode means that all messages transmitted
   between servers and clients are UTF-8 strings, and both servers and
   clients can send and accept UTF-8 strings.

2.1.  The "UTF8" Command

   The "UTF8" command enables UTF-8 mode.  The "UTF8" command has no
   parameters.

   UTF-8 mode has no effect on messages in an ASCII-only maildrop.
   Messages in native Unicode maildrops can be encoded in UTF-8 using
   internationalized headers [RFC6532], in 8bit
   content-transfer-encoding (see Section 2.8 of MIME [RFC2045]), in
   ASCII, or in any combination of these options.  In UTF-8 mode, if the
   character encoding format of maildrops is UTF-8 or ASCII, the
   messages are sent to the client as is; if the character encoding
   format of maildrops is a format other than UTF-8 or ASCII, the
   messages' encoding format SHOULD be converted to be UTF-8 before they
   are sent to the client.  When UTF-8 mode has not been enabled,
   character strings outside the ASCII repertoire MUST NOT be sent to
   the client as is.  If a client requests a UTF-8 message when UTF-8
   mode is not enabled, the server MUST either send the client a
   surrogate message that complies with unextended POP and Internet Mail
   Format without UTF-8 mode support, or fail the request with an -ERR
   response.  See Section 7 of "IMAP Support for UTF-8" [RFC6855] for
   information about creating a surrogate message and for a discussion
   of potential issues.  Section 5 of this document discusses "UTF8"
   response codes.  The server MAY respond to the "UTF8" command with an
   -ERR response.

   Note that even in UTF-8 mode, MIME binary content-transfer-encoding
   as defined in Section 6.2 of MIME [RFC2045] is still not permitted.
   MIME 8bit content-transfer-encoding (8BITMIME) [RFC6152] is obviously
   allowed.

   The octet count (size) of a message reported in a response to the
   "LIST" command SHOULD match the actual number of octets sent in a
   "RETR" response (not counting byte-stuffing).  Sizes reported
   elsewhere, such as in "STAT" responses and non-standardized,
   free-form text in positive status indicators (following "+OK") need
   not be accurate, but it is preferable if they are.

   Normal operation for maildrops that natively support non-ASCII
   characters will be for both servers and clients to support the
   extension discussed in this specification.  Upgrading both clients
   and servers is the only fully satisfactory way to support the
   capabilities offered by the "UTF8" extension and SMTPUTF8 mail more
   generally.  Servers must, however, anticipate the possibility of a
   client attempting to access a message that requires this extension
   without having issued the "UTF8" command.  There are no completely
   satisfactory responses for this case other than upgrading the client
   to support this specification.  One solution, unsatisfactory because
   the user may be confused by being able to access the message through
   some means and not others, is that a server MAY choose to reject the
   command to retrieve the message as discussed in Section 5.  Other
   alternatives, including the possibility of creating and delivering a
   surrogate form of the message, are discussed in Section 7 of "IMAP
   Support for UTF-8" [RFC6855].

   Clients MUST NOT issue the "STLS" command [RFC2595] after issuing
   UTF8; servers MAY (but are not required to) enforce this by rejecting
   with an -ERR response an "STLS" command issued subsequent to a
   successful "UTF8" command.  (Because this is a protocol error as
   opposed to a failure based on conditions, an extended response code
   [RFC2449] is not specified.)

2.2.  USER Argument to "UTF8" Capability

   If the USER argument is included with this capability, it indicates
   that the server accepts UTF-8 usernames and passwords.

   Servers that include the USER argument in the "UTF8" capability
   response SHOULD apply SASLprep [RFC4013] or one of its Standards
   Track successors to the arguments of the "USER" and "PASS" commands.

   A client or server that supports APOP and permits UTF-8 in usernames
   or passwords MUST apply SASLprep or one of its Standards Track
   successors to the username and password used to compute the APOP
   digest.

   When applying SASLprep, servers MUST reject UTF-8 usernames or
   passwords that contain a UTF-8 character listed in Section 2.3 of
   SASLprep.  When applying SASLprep to the USER argument, the PASS
   argument, or the APOP username argument, a compliant server or client
   MUST treat them as a query string [RFC3454].  When applying SASLprep
   to the APOP password argument, a compliant server or client MUST
   treat them as a stored string [RFC3454].

   If the server includes the USER argument in the UTF8 capability
   response, the client MAY use UTF-8 characters with a "USER", "PASS",
   or "APOP" command; the client MAY do so before issuing the "UTF8"
   command.  Clients MUST NOT use UTF-8 characters when authenticating
   if the server did not include the USER argument in the UTF8
   capability response.

   The server MUST reject UTF-8 usernames or passwords that fail to
   comply with the formal syntax in UTF-8 [RFC3629].

   Use of UTF-8 strings in the "AUTH" command is governed by the POP3
   SASL [RFC5034] mechanism.

3.  "LANG" Capability

   This document adds a new POP3 extension [RFC2449] capability response
   tag to indicate support for a new command: "LANG".

3.1.  Definition

   The capability tag and new command are described below.

   CAPA tag:
      LANG

   Arguments with CAPA tag:
      none

   Added Commands:
      LANG

   Standard commands affected:
      All

   Announced states / possible differences:
      both / no

   Commands valid in states:
      AUTHORIZATION, TRANSACTION

   Specification reference:
      this document

3.2.  Discussion

   POP3 allows most +OK and -ERR server responses to include human-
   readable text that, in some cases, might be presented to the user.
   But that text is limited to ASCII by the POP3 specification
   [RFC1939].  The "LANG" capability and command permit a POP3 client to
   negotiate which language the server uses when sending human-readable
   text.

   The "LANG" command requests that human-readable text included in all
   subsequent +OK and -ERR responses be localized to a language matching
   the language range argument (the "basic language range" as described
   by the "Matching of Language Tags" [RFC4647]).  If the command
   succeeds, the server returns a +OK response followed by a single
   space, the exact language tag selected, and another space.  Human-
   readable text in the appropriate language then appears in the rest of
   the line.  This, and subsequent protocol-level human-readable text,
   is encoded in the UTF-8 charset.

   If the command fails, the server returns an -ERR response and
   subsequent human-readable response text continues to use the language
   that was previously used.

   If the client issues a "LANG" command with the special "*" language
   range argument, it indicates a request to use a language designated
   as preferred by the server administrator.  The preferred language MAY
   vary based on the currently active user.

   If no argument is given and the POP3 server issues a positive
   response, that response will usually consist of multiple lines.
   After the initial +OK, for each language tag the server supports, the
   POP3 server responds with a line for that language.  This line is
   called a "language listing".

   In order to simplify parsing, all POP3 servers are required to use a
   certain format for language listings.  A language listing consists of
   the language tag [RFC5646] of the message, optionally followed by a
   single space and a human-readable description of the language in the
   language itself, using the UTF-8 charset.  There is no specific order
   to the listing of languages; the order may depend on configuration or
   implementation.

3.3.  Examples

   Examples for "LANG" capability usage are shown below.

      Note that some examples do not include the correct character
      accents due to limitations of the RFC format.

      C: USER karen
      S: +OK Hello, karen
      C: PASS password
      S: +OK karen's maildrop contains 2 messages (320 octets)

      Client requests deprecated MUL language [ISO639-2].  Server
      replies with -ERR response.

      C: LANG MUL
      S: -ERR invalid language MUL
      A LANG command with no parameters is a request for
      a language listing.

      C: LANG
      S: +OK Language listing follows:
      S: en English
      S: en-boont English Boontling dialect
      S: de Deutsch
      S: it Italiano
      S: es Espanol
      S: sv Svenska
      S: .

      A request for a language listing might fail.

      C: LANG
      S: -ERR Server is unable to list languages

      Once the client selects the language, all responses will be in
      that language, starting with the response to the "LANG" command.

      C: LANG es
      S: +OK es Idioma cambiado

      If a server returns an -ERR response to a "LANG" command
      that specifies a primary language, the current language
      for responses remains in effect.

      C: LANG uga
      S: -ERR es Idioma <<UGA>> no es conocido

      C: LANG sv
      S: +OK sv Kommandot "LANG" lyckades

      C: LANG *
      S: +OK es Idioma cambiado

4.  Non-ASCII Character Maildrops

   When a POP3 server uses a native non-ASCII character maildrop, it is
   the responsibility of the server to comply with the POP3 base
   specification [RFC1939] and Internet Message Format [RFC5322] when
   not in UTF-8 mode.  When the server is not in UTF-8 mode and the
   message requires that mode, requests to download the message MAY be
   rejected (as specified in the next section) or the various
   alternatives outlined in Section 2.1 above, including creation and
   delivery of surrogates for the original message, MAY be considered.

5.  "UTF8" Response Code

   Per "POP3 Extension Mechanism" [RFC2449], this document adds a new
   response code: UTF8, described below.

   Complete response code:
      UTF8

   Valid for responses:
      -ERR

   Valid for commands:
      LIST, TOP, RETR

   Response code meaning and expected client behavior:
      The "UTF8" response code indicates that a failure is due to a
      request for message content that contains a UTF-8 string when the
      client is not in UTF-8 mode.

      The client MAY reissue the command after entering UTF-8 mode.

6.  IANA Considerations

   Sections 2 and 3 of this specification update two capabilities
   ("UTF8" and "LANG") in the POP3 capability registry [RFC2449].

   Section 5 of this specification adds one new response code ("UTF8")
   to the POP3 response codes registry [RFC2449].

7.  Security Considerations

   The security considerations of UTF-8 [RFC3629], SASLprep [RFC4013],
   and the Unicode Format for Network Interchange [RFC5198] apply to
   this specification, particularly with respect to use of UTF-8 strings
   in usernames and passwords.

   The "LANG *" command might reveal the existence and preferred
   language of a user to an active attacker probing the system if the
   active language changes in response to the "USER", "PASS", or "APOP"
   commands prior to validating the user's credentials.  Servers are
   strongly advised to implement a configuration to prevent this
   exposure.

   It is possible for a man-in-the-middle attacker to insert a "LANG"
   command in the command stream, thus, making protocol-level diagnostic
   responses unintelligible to the user.  A mechanism to protect the
   integrity of the session can be used to defeat such attacks.  For
   example, a client can issue the "STLS" command [RFC2595] before
   issuing the "LANG" command.

   As with other internationalization upgrades, modifications to server
   authentication code (in this case, to support non-ASCII strings) need
   to be done with care to avoid introducing vulnerabilities (for
   example, in string parsing or matching).  This is particularly
   important if the native databases or mailstore of the operating
   system use some character set or encoding other than Unicode in
   UTF-8.

8.  References

8.1.  Normative References

   [RFC1939]   Myers, J. and M. Rose, "Post Office Protocol - Version
               3", STD 53, RFC 1939, May 1996.

   [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

   [RFC2047]   Moore, K., "MIME (Multipurpose Internet Mail Extensions)
               Part Three: Message Header Extensions for Non-ASCII
               Text", RFC 2047, November 1996.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2449]   Gellens, R., Newman, C., and L. Lundblade, "POP3
               Extension Mechanism", RFC 2449, November 1998.

   [RFC3454]   Hoffman, P. and M. Blanchet, "Preparation of
               Internationalized Strings ("stringprep")", RFC 3454,
               December 2002.

   [RFC3629]   Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, RFC 3629, November 2003.

   [RFC4013]   Zeilenga, K., "SASLprep: Stringprep Profile for User
               Names and Passwords", RFC 4013, February 2005.

   [RFC4647]   Phillips, A. and M. Davis, "Matching of Language Tags",
               BCP 47, RFC 4647, September 2006.

   [RFC5198]   Klensin, J. and M. Padlipsky, "Unicode Format for Network
               Interchange", RFC 5198, March 2008.

   [RFC5322]   Resnick, P., Ed., "Internet Message Format", RFC 5322,
               October 2008.

   [RFC5646]   Phillips, A. and M. Davis, "Tags for Identifying
               Languages", BCP 47, RFC 5646, September 2009.

   [RFC6152]   Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
               Service Extension for 8-bit MIME Transport", STD 71,
               RFC 6152, March 2011.

   [RFC6530]   Klensin, J. and Y. Ko, "Overview and Framework for
               Internationalized Email", RFC 6530, February 2012.

   [RFC6532]   Yang, A., Steele, S., and N. Freed, "Internationalized
               Email Headers", RFC 6532, February 2012.

   [RFC6855]   Resnick, P., Newman, C., and S. Shen, "IMAP Support for
               UTF-8", RFC 6855, March 2013.

8.2.  Informative References

   [ISO639-2]  International Organization for Standardization, "ISO
               639-2:1998.  Codes for the representation of names of
               languages -- Part 2: Alpha-3 code", October 1998.

   [RFC2231]   Freed, N. and K. Moore, "MIME Parameter Value and Encoded
               Word Extensions:
               Character Sets, Languages, and Continuations", RFC 2231,
               November 1997.

   [RFC2595]   Newman, C., "Using TLS with IMAP, POP3 and ACAP",
               RFC 2595, June 1999.

   [RFC5034]   Siemborski, R. and A. Menon-Sen, "The Post Office
               Protocol (POP3) Simple Authentication and Security Layer
               (SASL) Authentication Mechanism", RFC 5034, July 2007.

   [RFC5721]   Gellens, R. and C. Newman, "POP3 Support for UTF-8",
               RFC 5721, February 2010.

Appendix A.  Design Rationale

   This non-normative section discusses the reasons behind some of the
   design choices in this specification.

   Due to interoperability problems with the MIME Message Header
   Extensions [RFC2047] and limited deployment of the extended MIME
   parameter encodings [RFC2231], it is hoped these 7-bit encoding
   mechanisms can be deprecated in the future when UTF-8 header support
   becomes prevalent.

   The USER capability (Section 2.2) and hence the upgraded "USER"
   command and additional support for non-ASCII credentials, are
   optional because the implementation burden of SASLprep [RFC4013] is
   not well understood, and mandating such support in all cases could
   negatively impact deployment.

Appendix B.  Acknowledgments

   Thanks to John Klensin, Joseph Yee, Tony Hansen, Alexey Melnikov, and
   other Email Address Internationalization working group participants
   who provided helpful suggestions and interesting debate that improved
   this specification.

Authors' Addresses

   Randall Gellens
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92651
   USA

   EMail: rg+ietf@qualcomm.com


   Chris Newman
   Oracle
   800 Royal Oaks
   Monrovia, CA  91016-6347
   USA

   EMail: chris.newman@oracle.com


   Jiankang YAO
   CNNIC
   No.4 South 4th Street, Zhongguancun
   Beijing
   China

   Phone: +86 10 58813007
   EMail: yaojk@cnnic.cn


   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Tokyo
   Japan

   Phone: +81 3 5215 8451
   EMail: fujiwara@jprs.co.jp