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Network Working Group                                           K. MooreRequest for Comments: 1342                       University of Tennessee                                                               June 1992Representation of Non-ASCII Text in Internet Message HeadersStatus 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 inRFC822 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 supportRFC 1341.IntroductionRFC 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 aRFC 822 [2] message header, in a manner   which is unlikely to confuse existing message handling software.   Like the encoding techniques described inRFC 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 toRFC 822, make use of   backslash-quoting to "hide" special characters such as "<", ",", or   or which exploit other infrequently-used features of that   specification.Moore                                                           [Page 1]

RFC 1342                 Non-ASCII Mail Headers                June 1992   While it is unfortunate that these programs do not correctly   interpretRFC 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 ofRFC 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 ofRFC 822:   encoded-word = "=" "?" charset "?" encoding "?" encoded-text "?" "="   charset = token    ; legal charsets defined byRFC 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)Moore                                                           [Page 2]

RFC 1342                 Non-ASCII Mail Headers                June 1992   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 inRFC 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 byRFC1341.The "Q" encoding   The "Q" encoding is similar to the "Quoted-Printable" content-   transfer-encoding defined inRFC 1341.  It is designed to allow text   containing mostly ASCII characters to be decipherable on an ASCII   terminal without decoding.Moore                                                           [Page 3]

RFC 1342                 Non-ASCII Mail Headers                June 1992   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 anRFC 1341 "charset" parameter of a   "text/plain" body part.  (Seesection 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 theRFC 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.)Moore                                                           [Page 4]

RFC 1342                 Non-ASCII Mail Headers                June 1992   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 byRFC 822) in:  (1) a Subject or Comments header field, (2) any extension message  header field, (3) any user-defined message header field, or (4) anyRFC 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, theRFC 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 fromRFC 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.Moore                                                           [Page 5]

RFC 1342                 Non-ASCII Mail Headers                June 1992Recognition 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 feedingMoore                                                           [Page 6]

RFC 1342                 Non-ASCII Mail Headers                June 1992   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-8859-1References   [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.eduMoore                                                           [Page 7]