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Network Working Group K. Moore Request for Comments: 1522 University of Tennessee Obsoletes: 1342 September 1993 Category: Standards Track

       MIME (Multipurpose Internet Mail Extensions) Part Two:
            Message Header Extensions for Non-ASCII Text

Status of this Memo

 This RFC 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" for the standardization state and status
 of this protocol.  Distribution of this memo is unlimited.


 This memo describes an extension to the message format defined in RFC
 1521 [1], to allow the representation of character sets other than
 ASCII in RFC 822 (STD 11) 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 1521.

1. Introduction

 RFC 1521 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 1521, 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

Moore [Page 1] RFC 1522 MIME Part Two September 1993

 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
 (known as "encoded-words") are reserved for use as encoded data.  The
 syntax of encoded-words is such that they are unlikely to
 "accidentally" appear as normal text in message headers.
 Furthermore, the characters used in encoded-words are restricted to
 those which do not have special meanings in the context in which the
 encoded-word appears.
 Generally, 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 graphic ASCII characters,
 according to the rules for that encoding method.
 A mail composer that implements this specification will provide a
 means of inputting 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.
    This memo relies heavily on notation and terms defined STD 11, RFC
    822 and RFC 1521.  In particular, the syntax for the ABNF used in
    this memo is defined in STD 11, RFC 822, as well as many of the
    terms used in the grammar for the header extensions defined here.
    Successful implementation of this protocol extension requires
    careful attention to the details of both STD 11, RFC 822 and RFC
    When the term "ASCII" appears in this memo, it refers to the "7-
    Bit American Standard Code for Information Interchange", ANSI
    X3.4-1986.  The MIME charset name for this character set is "US-
    ASCII".  When not specifically referring to the MIME charset name,
    this document uses the term "ASCII", both for brevity and for

Moore [Page 2] RFC 1522 MIME Part Two September 1993

    consistency with STD 11, RFC 822.  However, implementors are
    warned that the character set name must be spelled "US-ASCII" in
    MIME message and body part headers.

2. Syntax of encoded-words

 An "encoded-word" is defined by the following ABNF grammar.  The
 notation of RFC 822 is used, with the exception that white space
 characters MAY NOT appear between components of an encoded-word.
 encoded-word = "=?" charset "?" encoding "?" encoded-text "?="
 charset = token    ; see section 3
 encoding = token   ; see section 4
 token = 1*<Any CHAR except SPACE, CTLs, and especials>
 especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "
             <"> / "/" / "[" / "]" / "?" / "." / "="
 encoded-text = 1*<Any printable ASCII character other
                   than "?" or SPACE>
                   ; (but see "Use of encoded-words in message
                   ; headers", section 5)
 Both "encoding" and "charset" names are case-independent.  Thus the
 charset name "ISO-8859-1" is equivalent to "iso-8859-1", and the
 encoding named "Q" may be spelled either "Q" or "q".
 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 CRLF SPACE) may be
 While there is no limit to the length of a multiple-line header
 field, each line of a header field that contains one or more
 encoded-words is limited to 76 characters.
 The length restrictions are included not only to ease
 interoperability 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.
 The characters which may appear in encoded-text are further
 restricted by the rules in section 5.

Moore [Page 3] RFC 1522 MIME Part Two September 1993

3. Character sets

 The "charset" portion of an encoded-word specifies the character set
 associated with the unencoded text.  A charset can be any of the
 character set names allowed in an RFC 1521 "charset" parameter of a
 "text/plain" body part, or any character set name registered with
 IANA for use with the MIME text/plain content-type [3].  (See section
 7.1.1 of RFC 1521 for a list of charsets defined in that document).
 Some character sets use code-switching techniques to switch between
 "ASCII mode" and other modes.  If unencoded text in an encoded-word
 contains control codes to switch out of ASCII mode, it must also
 contain additional control codes such that ASCII mode is again
 selected at the end of the encoded-word.  (This rule applies
 separately to each encoded-word, including adjacent encoded-words
 within a single header field.)
 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.

4. Encodings

 Initially, the legal values for "encoding" are "Q" and "B".  These
 encodings are described below.  The "Q" encoding is recommended for
 use when most of the characters to be encoded are in the ASCII
 character set; otherwise, the "B" encoding should be used.
 Nevertheless, a mail reader which claims to recognize encoded-words
 MUST be able to accept either encoding for any character set which it
 Only a subset of the printable ASCII characters may be used in
 encoded-text.  Space and tab characters are 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" preceding 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

Moore [Page 4] RFC 1522 MIME Part Two September 1993

 fewer characters available for use in other locations.

4.1. The "B" encoding

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

4.2. The "Q" encoding

 The "Q" encoding is similar to the "Quoted-Printable" content-
 transfer-encoding defined in RFC 1521.  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".  (Upper case should be used for
     hexadecimal digits "A" through "F".)
 (2) The 8-bit hexadecimal value 20 (e.g., ISO-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 section 5 for restrictions.)

5. Use of encoded-words in message headers

 An encoded-word may appear in a message header or body part header
 according to the following rules:
 (1) An encoded-word may replace a "text" token (as defined by RFC
     822) in any Subject or Comments header field, any extension
     message header field, or any RFC 1521 body part field for which
     the field body is defined as "*text".  An encoded-word may also
     appear in any user-defined ("X-") message or body part header
     Ordinary ASCII text and encoded-words may appear together in the
     same header field.  However, an encoded-word that appears in a
     header field defined as "*text" MUST be separated from any
     adjacent encoded-word or "text" by linear-white-space.

Moore [Page 5] RFC 1522 MIME Part Two September 1993

 (2) An encoded-word may appear within a comment delimited by "(" and
     ")", i.e., wherever a "ctext" is allowed.  More precisely, the
     RFC 822 ABNF 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 " encoded-word that appears in
     a "comment" MUST be separated from any adjacent encoded-word or
     "ctext" by linear-white-space.
 (3) 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 ABNF definition for phrase from RFC 822 thus
     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.)>.  An encoded-word that appears
     within a "phrase" MUST be separated from any adjacent "word",
     "text" or "special" by linear-white-space.
 These are the ONLY locations where an encoded-word may appear.  In
 particular, an encoded-word MUST NOT appear in any portion of an
 "addr-spec".  In addition, an encoded-word MUST NOT be used in a
 Received header field.
 Each encoded-word MUST encode an integral number of octets.  The
 encoded-text in each encoded-word must be well-formed according to
 the encoding specified; the encoded-text may not be continued in the
 next encoded-word.  (For example, "=?charset?Q?=?= =?charset?Q?AB?="
 would be illegal, because the two hex digits "AB" must follow the "="
 in the same encoded-word.)
 Each encoded-word MUST represent an integral number of characters. A
 multi-octet character may not be split across adjacent encoded-words.
 Only printable and white space character data should be encoded using
 this scheme.  However, since these encoding schemes allow the
 encoding of arbitrary octet 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

Moore [Page 6] RFC 1522 MIME Part Two September 1993

 represent strings of purely ASCII characters is allowed, but
 discouraged.  In rare cases it may be necessary to encode ordinary
 text that looks like an encoded-word.

6. Support of encoded-words by mail readers

6.1. Recognition of encoded-words in message headers

 A mail reader must parse the message and body part headers according
 to the rules in RFC 822 to correctly recognize encoded-words.
 Encoded-words are to be recognized as follows:
 (1) Any message or body part header field defined as "*text", or any
     user-defined header field, should be parsed as follows: Beginning
     at the start of the field-body and immediately following each
     occurrence of linear-white-space, each sequence of up to 75
     printable characters (not containing any linear-white-space)
     should be examined to see if it is an encoded-word according to
     the syntax rules in section 2.  Any other sequence of printable
     characters should be treated as ordinary ASCII text.
 (2) Any header field not defined as "*text" should be parsed
     according to the syntax rules for that header field.  However,
     any "word" that appears within a "phrase" should be treated as an
     encoded-word if it meets the syntax rules in section 2.
     Otherwise it should be treated as an ordinary "word".
 (3) Within a "comment", any sequence of up to 75 printable characters
     (not containing linear-white-space), that meets the syntax rules
     in section 2, should be treated as an encoded-word.  Otherwise it
     should be treated as normal comment text.

6.2. Display of encoded-words

 Any encoded-words so recognized are decoded, and if possible, the
 resulting unencoded text is displayed in the original character set.
 When displaying a particular header field that contains multiple
 encoded-words, any linear-white-space that separates a pair of
 adjacent encoded-words is ignored.  (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.)
 In the event other encodings are defined in the future, and the mail
 reader does not support the encoding used, it may either (a) display
 the encoded-word as ordinary text, or (b) substitute an appropriate

Moore [Page 7] RFC 1522 MIME Part Two September 1993

 message indicating that the text could not be decoded.
 If the mail reader does not support the character set used, it may
 (a) display the encoded-word as ordinary text (i.e., as it appears in
 the header), (b) make a "best effort" to display using such
 characters as are available, or (c) substitute an appropriate message
 indicating that the decoded text could not be displayed.
 If the character set being used employs code-switching techniques,
 display of the encoded text implicitly begins in "ASCII mode".  In
 addition, the mail reader must ensure that the output device is once
 again in "ASCII mode" after the encoded-word is displayed.

6.3. Mail reader handling of incorrectly formed encoded-words

 It is possible that an encoded-word that is legal according to the
 syntax defined in section 2, is incorrectly formed according to the
 rules for the encoding being used.   For example:
 (1) An encoded-word which contains characters which are not legal for
     a particular encoding (for example, a '-' in the "B" encoding),
     is incorrectly formed.
 (2) Any encoded-word which encodes a non-integral number of
     characters or octets is incorrectly formed.
 A mail reader need not attempt to display the text associated with an
 encoded-word that is incorrectly formed.  However, a mail reader MUST
 NOT prevent the display or handling of a message because an encoded-
 word is incorrectly formed.

7. Conformance

 A mail composing program claiming compliance with this specification
 MUST ensure that any string of non-white-space printable ASCII
 characters within a "*text" or "*ctext" that begins with "=?" and
 ends with "?=" be a valid encoded-word.  ("begins" means: at the
 start of the field-body or immediately following linear-white-space;
 "ends" means: at the end of the field-body or immediately preceding
 linear-white-space.) In addition, any "word" within a "phrase" that
 begins with "=?" and ends with "?=" must 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, according to the rules in section 6, anytime they appear in
 appropriate places in message headers.  It must support both the "B"
 and "Q" encodings for any character set which it supports.  The
 program must be able to display the unencoded text if the character

Moore [Page 8] RFC 1522 MIME Part Two September 1993

 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.

8. Examples

    From: =?US-ASCII?Q?Keith_Moore?= <>
    To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <>
    CC: =?ISO-8859-1?Q?Andr=E9_?= Pirard <>
    Subject: =?ISO-8859-1?B?SWYgeW91IGNhbiByZWFkIHRoaXMgeW8=?=
    From: =?ISO-8859-1?Q?Olle_J=E4rnefors?= <>
    Subject: Time for ISO 10646?
    To: Dave Crocker <>
    From: =?ISO-8859-1?Q?Patrik_F=E4ltstr=F6m?= <>
    Subject: Re: RFC-HDR care and feeding
    From: Nathaniel Borenstein <>
    To: Greg Vaudreuil <gvaudre@NRI.Reston.VA.US>, Ned Freed
       <>, Keith Moore <>
    Subject: Test of new header generator
    MIME-Version: 1.0
    Content-type: text/plain; charset=ISO-8859-1

9. References

 [1] Borenstein N., and N. Freed, "MIME (Multipurpose Internet Mail
     Extensions) Part One:  Mechanisms for Specifying and Describing
     the Format of Internet Message Bodies", RFC 1521, Bellcore,
     Innosoft, September 1993.
 [2] Crocker, D., "Standard for the Format of ARPA Internet Text
     Messages", STD 11, RFC 822, UDEL, August 1982.
 [3] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
     RFC 1340, USC/Information Sciences Institute, July 1992.

Moore [Page 9] RFC 1522 MIME Part Two September 1993

10. Security Considerations

 Security issues are not discussed in this memo.

11. Author's Address

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

Moore [Page 10]

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