MIME Header Encoding: Representation of Non-ASCII Text in Internet Message Headers

Network Working Group                                           K. Moore
Request for Comments: 1342                       University of Tennessee
                                                               June 1992


      Representation of Non-ASCII Text in Internet Message Headers

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo describes an extension to the message format defined in [1]
   (known to the IETF Mail Extensions Working Group as "RFC 1341"), to
   allow the representation of character sets other than ASCII in RFC
   822 message headers.  The extensions described were designed to be
   highly compatible with existing Internet mail handling software, and
   to be easily implemented in mail readers that support RFC 1341.

Introduction

   RFC 1341 describes a mechanism for denoting textual body parts which
   are coded in various character sets, as well as methods for encoding
   such body parts as sequences of printable ASCII characters.  This
   memo describes similar techniques to allow the encoding of non-ASCII
   text in various portions of a RFC 822 [2] message header, in a manner
   which is unlikely to confuse existing message handling software.

   Like the encoding techniques described in RFC 1341, the techniques
   outlined here were designed to allow the use of non-ASCII characters
   in message headers in a way which is unlikely to be disturbed by the
   quirks of existing Internet mail handling programs.  In particular,
   some mail relaying programs are known to (a) delete some message
   header fields while retaining others, (b) rearrange the order of
   addresses in To or Cc fields, (c) rearrange the (vertical) order of
   header fields, and/or (d) "wrap" message headers at different places
   than those in the original message.  In addition, some mail reading
   programs are known to have difficulty correctly parsing message
   headers which, while legal according to RFC 822, make use of
   backslash-quoting to "hide" special characters such as "<", ",", or
   or which exploit other infrequently-used features of that
   specification.

   While it is unfortunate that these programs do not correctly
   interpret RFC 822 headers, to "break" these programs would cause
   severe operational problems for the Internet mail system.  The
   extensions described in this memo therefore do not rely on little-
   used features of RFC 822.  Instead, certain sequences of "ordinary"
   printable ASCII characters (which are assumed to be unlikely to
   otherwise appear in message headers) are reserved for use as encoded
   data.  The characters used in these encodings are restricted to those
   which do not have special meanings in the context in which the
   encoded text appears.

Encodings

   An "encoded-word" is a sequence of printable ASCII characters that
   begins with "=?", ends with "?=", and has two "?"s in between.  It
   specifies a character set and an encoding method, and also includes
   the original text encoded as ASCII characters, according to the rules
   for that encoding method.

   A mail composer that implements this specification will provide a
   means of inputing non-ASCII text in header fields, but will translate
   these fields (or appropriate portions of these fields) into encoded-
   words before inserting them into the message header.

   A mail reader that implements this specification will recognize
   encoded-words when they appear in certain portions of the message
   header.  Instead of displaying the encoded-word "as is", it will
   reverse the encoding and display the original text in the designated
   character set.

   An "encoded-word" is more precisely defined by the following EBNF
   grammar, using the notation of RFC 822:

   encoded-word = "=" "?" charset "?" encoding "?" encoded-text "?" "="

   charset = token    ; legal charsets defined by RFC 1341

   encoding = token   ; Either "B" or "Q"

   token = 1*<Any CHAR except SPACE, CTLs, and tspecials>

   tspecials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
               <"> / "/" / "[" / "]" / "?" / "." / "="

   encoded-text = 1*<Any printable ASCII character other than "?" or
                  ; SPACE> (but see "Use of encoded-words in message
                  ; headers", below)

   An encoded-word may not be more than 75 characters long, including
   charset, encoding, encoded-text, and delimiters.  If it is desirable
   to encode more text than will fit in an encoded-word of 75
   characters, multiple encoded-words (separated by SPACE or newline)
   may be used.  Message header lines that contain one or more encoded-
   words should be no more than 76 characters long.  NOTE: These
   restrictions are included not only to ease interoperbility through
   internetwork mail gateways, but also to impose a limit on the amount
   of lookahead a header parser must employ (while looking for a final
   ?= delimiter) before it can decide whether a token is an encoded-word
   or something else.

   Initially, the legal values for "encoding" are "Q" and "B".  These
   encodings are described below.  The "Q" encoding is recommended for
   use with Latin character sets, and the "B" encoding for all others.
   Nevertheless, a mail reader which claims to recognize encoded-words
   MUST be able to accept either encoding for any character set which it
   supports.

   Only a subset of the printable ASCII characters may be used in
   encoded-text.  The SPACE character is not allowed, so that the
   beginning and end of an encoded-word are obvious.  The "?" character
   is used within an encoded-word to separate the various portions of
   the encoded-word from one another, and thus cannot appear in the
   encoded-text portion.  Other characters are also illegal in certain
   contexts.  For example, an encoded-word in a "phrase" preceeding an
   address in a From header field may not contain any of the "specials"
   defined in RFC 822.  Finally, certain other characters are disallowed
   in some contexts, to ensure reliability for messages that pass
   through internetwork mail gateways.

   The "B" encoding automatically meets these requirements.  The "Q"
   encoding allows a wide range of printable characters to be used in
   non-critical locations in the message header (e.g., Subject), with
   fewer characters available for use in other locations.

The "B" encoding

   The "B" encoding is identical to the "BASE64" encoding defined by RFC
   1341.

The "Q" encoding

   The "Q" encoding is similar to the "Quoted-Printable" content-
   transfer-encoding defined in RFC 1341.  It is designed to allow text
   containing mostly ASCII characters to be decipherable on an ASCII
   terminal without decoding.

   1.  Any 8-bit value may be represented by a "=" followed by two
       hexadecimal digits.  For example, if the character set in use
       were ISO-8859-1, the "=" character would thus be encoded as
       "=3D", and a SPACE by "=20".

   2.  The 8-bit hexadecimal value 20 (e.g., IS0-8859-1 SPACE) may be
       represented as "_" (underscore, ASCII 95.).  (This character may
       not pass through some internetwork mail gateways, but its use
       will greatly enhance readability of "Q" encoded data with mail
       readers that do not support this encoding.)  Note that the "_"
       always represents hexadecimal 20, even if the SPACE character
       occupies a different code position in the character set in use.

   3.  8-bit values which correspond to printable ASCII characters other
       than "=", "?", "_" (underscore), and SPACE may be represented as
       those characters.  (But see "Use of encoded-words in message
       headers", below).

Character sets

   In an encoded-word, the character set associated with the unencoded
   text is specified by a charset.  A charset can be any of the
   character set names allowed in an RFC 1341 "charset" parameter of a
   "text/plain" body part.  (See section 7.1.1 of RFC 1341 for a list of
   valid charset parameters).

   When there is a possibility of using more than one character set to
   represent the text in an encoded-word, and in the absence of private
   agreements between sender and recipients of a message, it is
   recommended that members of the ISO-8859-* series be used in
   preference to other character sets.  Among the various ISO-8859-*
   character sets, the lowest-numbered set which contains all of the
   required characters should be used.

Use of encoded-words in message headers

   A sequence of one or more encoded-words is used to represent non-
   ASCII textual data within a header field.  An encoded-word must be
   separated from an adjacent encoded-word, "word", "text", "ctext", or
   "special" by a linear white-space character or a newline.  When
   displaying a particular header field" (in the RFC 822 sense)
   containing one or more encoded-words, an unencoded SPACE character
   that immediately follows the encoded-word is not displayed.  A
   newline that immediately follows an encoded-word is not displayed
   unless the encoded-word is the last token in that "field".  (This is
   to allow the use of multiple encoded-words to represent long strings
   of unencoded text, without having to separate encoded-words where
   spaces occur in the unencoded text.)

   An encoded-word may appear in a message header or body part header
   according to the following rules:

- An encoded-word may replace a "text" token (as defined by RFC 822) in:
  (1) a Subject or Comments header field, (2) any extension message
  header field, (3) any user-defined message header field, or (4) any
  RFC 1341 body part header field (such as Content-Description) for
  which the field body contains only "text"s.

- An encoded-word may appear within a comment delimited by "(" and ")",
  i.e., wherever a "ctext" is allowed.  More precisely, the RFC 822 EBNF
  definition for "comment" is amended as follows:

  comment = "(" *(ctext / quoted-pair / comment / encoded-word) ")"

  A "Q"-encoded encoded-word which appears in a comment MUST NOT contain
  the characters "(", ")" or "\".

- As a replacement for a "word" entity within a "phrase", for example,
  one that precedes an address in a From, To, or Cc header.  The EBNF
  definition for phrase from RFC 822 thus becomes:

  phrase = 1*(encoded-word / word)

  In this case the set of characters that may be used in a "Q"-encoded
  encoded-word is restricted to: <upper and lower case ASCII letters,
  decimal digits, "!", "*", "+", "-", "/", "=", and "_" (underscore,
  ASCII 95.)>.

  These are the ONLY locations where an encoded-word may appear.  In
  particular, an encoded-word MUST NOT appear in any portion of an
  "address".  In addition, an encoded-word MUST NOT be used in a
  Received header field.

  Whenever such words appear in a header being displayed, an enlightened
  mail reader will decode the text and render it appropriately.

  Only textual data (printable and white space characters) should be
  encoded using this scheme.  However, since these encoding schemes
  allow the encoding of arbitrary 8-bit values, mail readers that
  implement this decoding should also ensure that display of the
  decoded data on the recipient's terminal will not cause unwanted
  side-effects.

  Use of these methods to encode non-textual data (e.g., pictures or
  sounds) is not defined by this memo.  Use of encoded-words to
  represent strings of purely ASCII characters is allowed, but
  discouraged.

Recognition of encoded-words in message headers.

   An encoded-word may be distinguished from an ordinary "word", "text",
   or "ctext", as follows: An encoded-word begins with "=?", ends with
   "?=", contains exactly four "?" characters including the delimiters,
   and is followed by a SPACE or newline.  If the "word", "text", or
   "ctext" does not meet the above tests, it should be displayed as it
   appears in the message header.

   If the mail reader does not support the character set used, it may
   either display the encoded-word as ordinary text (i.e., as it appears
   in the header), or it may substitute an appropriate message
   indicating that the decoded text could not be displayed.

Conformance

   A mail composing program claiming compliance with this specification
   MUST ensure that any string of printable ASCII characters in a
   message header that begins with "=?" and ends with "?=" be a valid
   encoded-word.

   A mail reading program claiming compliance with this specification
   must be able to distinguish encoded-words from "text", "ctext", or
   "word"s anytime they appear in appropriate places in message headers.
   The program must be able to display unencoded text if the character
   set is "US-ASCII".  For the ISO-8859-* character sets, the mail
   reading program must at least be able to display the characters which
   are also in the ASCII set.

Examples

   From: =?US-ASCII?Q?Keith_Moore?= <moore@cs.utk.edu>
   To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <keld@dkuug.dk>
   CC: =?ISO-8859-1?Q?Andr=E9_?= Pirard <PIRARD@vm1.ulg.ac.be>
   Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=
    =?ISO-8859-2?B?dSB1bmRlcnN0YW5kIHRoZSBleGFtcGxlLg==?=

   From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <ojarnef@admin.kth.se>
   To: ietf-822@dimacs.rutgers.edu, ojarnef@admin.kth.se
   Subject: Time for ISO 10646?

   To: Dave Crocker <dcrocker@mordor.stanford.edu>
   Cc: ietf-822@dimacs.rutgers.edu, paf@comsol.se
   From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <paf@nada.kth.se>
   Subject: Re: RFC-HDR care and feeding
   From: Nathaniel Borenstein <nsb@thumper.bellcore.com>
           (=?iso-8859-8?b?7eXs+SDv4SDp7Oj08A==?=)
   To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed
   <ned@innosoft.com>,
           Keith Moore <moore@cs.utk.edu>
   Subject: Test of new header generator
   MIME-Version: 1.0
   Content-type: text/plain; charset=ISO-8859-1

References

   [1] Borenstein N., and N. Freed, "MIME (Multipurpose Internet Mail
       Extensions):  Mechanisms for Specifying and Describing the Format
       of Internet Message Bodies", RFC 1341, Bellcore, Innosoft,
       June 1992.

   [2] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", RFC 822, UDEL, August 1982.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   Keith Moore
   University of Tennessee
   107 Ayres Hall
   Knoxville TN 37996-1301

   EMail: moore@cs.utk.edu