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Network Working Group                                       D. GoldsmithRequest for Comments: 2152                          Apple Computer, Inc.Obsoletes: RFC1642                                             M. DavisCategory: Informational                                   Taligent, Inc.                                                                May 1997UTF-7              A Mail-Safe Transformation Format of UnicodeStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   The Unicode Standard, version 2.0, and ISO/IEC 10646-1:1993(E) (as   amended) jointly define a character set (hereafter referred to as   Unicode) which encompasses most of the world's writing systems.   However, Internet mail (STD 11,RFC 822) currently supports only 7-   bit US ASCII as a character set. MIME (RFC 2045 through 2049) extends   Internet mail to support different media types and character sets,   and thus could support Unicode in mail messages. MIME neither defines   Unicode as a permitted character set nor specifies how it would be   encoded, although it does provide for the registration of additional   character sets over time.   This document describes a transformation format of Unicode that   contains only 7-bit ASCII octets and is intended to be readable by   humans in the limiting case that the document consists of characters   from the US-ASCII repertoire. It also specifies how this   transformation format is used in the context of MIME andRFC 1641,   "Using Unicode with MIME".Motivation   Although other transformation formats of Unicode exist and could   conceivably be used in this context (most notably UTF-8, also known   as UTF-2 or UTF-FSS), they suffer the disadvantage that they use   octets in the range decimal 128 through 255 to encode Unicode   characters outside the US-ASCII range. Thus, in the context of mail,   those octets must themselves be encoded. This requires putting text   through two successive encoding processes, and leads to a significant   expansion of characters outside the US-ASCII range, putting non-   English speakers at a disadvantage. For example, using UTF-8 togetherGoldsmith & Davis            Informational                      [Page 1]

RFC 2152                         UTF-7                          May 1997   with the Quoted-Printable content transfer encoding of MIME   represents US-ASCII characters in one octet, but other characters may   require up to nine octets.Overview   UTF-7 encodes Unicode characters as US-ASCII octets, together with   shift sequences to encode characters outside that range. For this   purpose, one of the characters in the US-ASCII repertoire is reserved   for use as a shift character.   Many mail gateways and systems cannot handle the entire US-ASCII   character set (those based on EBCDIC, for example), and so UTF-7   contains provisions for encoding characters within US-ASCII in a way   that all mail systems can accomodate.   UTF-7 should normally be used only in the context of 7 bit   transports, such as mail. In other contexts, straight Unicode or   UTF-8 is preferred.   SeeRFC 1641, "Using Unicode with MIME" for the overall specification   on usage of Unicode transformation formats with MIME.Definitions   First, the definition of Unicode:      The 16 bit character set Unicode is defined by "The Unicode      Standard, Version 2.0". This character set is identical with the      character repertoire and coding of the international standard      ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2;      Subset=300; Implementation Level=3, including the first 7      amendments to 10646 plus editorial corrections.      Note. Unicode 2.0 further specifies the use and interaction of      these character codes beyond the ISO standard. However, any valid      10646 sequence is a valid Unicode sequence, and vice versa;      Unicode supplies interpretations of sequences on which the ISO      standard is silent as to interpretation.   Next, some handy definitions of US-ASCII character subsets:      Set D (directly encoded characters) consists of the following      characters (derived fromRFC 1521, Appendix B, which no longer      appears inRFC 2045): the upper and lower case letters A through Z      and a through z, the 10 digits 0-9, and the following nine special      characters (note that "+" and "=" are omitted):Goldsmith & Davis            Informational                      [Page 2]

RFC 2152                         UTF-7                          May 1997               Character   ASCII & Unicode Value (decimal)                  '           39                  (           40                  )           41                  ,           44                  -           45                  .           46                  /           47                  :           58                  ?           63      Set O (optional direct characters) consists of the following      characters (note that "\" and "~" are omitted):               Character   ASCII & Unicode Value (decimal)                  !           33                  "           34                  #           35                  $           36                  %           37                  &           38                  *           42                  ;           59                  <           60                  =           61                  >           62                  @           64                  [           91                  ]           93                  ^           94                  _           95                  '           96                  {           123                  |           124                  }           125   Rationale. The characters "\" and "~" are omitted because they are   often redefined in variants of ASCII.   Set B (Modified Base 64) is the set of characters in the Base64   alphabet defined inRFC 2045, excluding the pad character "="   (decimal value 61).Goldsmith & Davis            Informational                      [Page 3]

RFC 2152                         UTF-7                          May 1997   Rationale. The pad character = is excluded because UTF-7 is designed   for use within header fields as set forth inRFC 2047. Since the only   readable encoding inRFC 2047 is "Q" (based onRFC 2045's Quoted-   Printable), the "=" character is not available for use (without a lot   of escape sequences). This was very unfortunate but unavoidable. The   "=" character could otherwise have been used as the UTF-7 escape   character as well (rather than using "+").   Note that all characters in US-ASCII have the same value in Unicode   when zero-extended to 16 bits.UTF-7 Definition   A UTF-7 stream represents 16-bit Unicode characters using 7-bit US-   ASCII octets as follows:      Rule 1: (direct encoding) Unicode characters in set D above may be      encoded directly as their ASCII equivalents. Unicode characters in      Set O may optionally be encoded directly as their ASCII      equivalents, bearing in mind that many of these characters are      illegal in header fields, or may not pass correctly through some      mail gateways.      Rule 2: (Unicode shifted encoding) Any Unicode character sequence      may be encoded using a sequence of characters in set B, when      preceded by the shift character "+" (US-ASCII character value      decimal 43). The "+" signals that subsequent octets are to be      interpreted as elements of the Modified Base64 alphabet until a      character not in that alphabet is encountered. Such characters      include control characters such as carriage returns and line      feeds; thus, a Unicode shifted sequence always terminates at the      of a line. As a special case, if the sequence terminates with the      character "-" (US-ASCII decimal 45) then that character is      absorbed; other terminating characters are not absorbed and are      processed normally.      Note that if the first character after the shifted sequence is "-"      then an extra "-" must be present to terminate the shifted      sequence so that the actual "-" is not itself absorbed.      Rationale. A terminating character is necessary for cases where      the next character after the Modified Base64 sequence is part of      character set B or is itself the terminating character. It can      also enhance readability by delimiting encoded sequences.Goldsmith & Davis            Informational                      [Page 4]

RFC 2152                         UTF-7                          May 1997      Also as a special case, the sequence "+-" may be used to encode      the character "+". A "+" character followed immediately by any      character other than members of set B or "-" is an ill-formed      sequence.      Unicode is encoded using Modified Base64 by first converting      Unicode 16-bit quantities to an octet stream (with the most      significant octet first). Surrogate pairs (UTF-16) are converted      by treating each half of the pair as a separate 16 bit quantity      (i.e., no special treatment). Text with an odd number of octets is      ill-formed. ISO 10646 characters outside the range addressable via      surrogate pairs cannot be encoded.      Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters      the UCS-2 form are serialized as octets, that the most significant      octet appear first.  This is also in keeping with common network      practice of choosing a canonical format for transmission.      Rationale. The policy for code point allocation within ISO 10646      and Unicode is that the repertoires be kept synchronized. No code      points will be allocated in ISO 10646 outside the range      addressable by surrogate pairs.      Next, the octet stream is encoded by applying the Base64 content      transfer encoding algorithm as defined inRFC 2045, modified to      omit the "=" pad character. Instead, when encoding, zero bits are      added to pad to a Base64 character boundary. When decoding, any      bits at the end of the Modified Base64 sequence that do not      constitute a complete 16-bit Unicode character are discarded. If      such discarded bits are non-zero the sequence is ill-formed.      Rationale. The pad character "=" is not used when encoding      Modified Base64 because of the conflict with its use as an escape      character for the Q content transfer encoding inRFC 2047 header      fields, as mentioned above.      Rule 3: The space (decimal 32), tab (decimal 9), carriage return      (decimal 13), and line feed (decimal 10) characters may be      directly represented by their ASCII equivalents. However, note      that MIME content transfer encodings have rules concerning the use      of such characters. Usage that does not conform to the      restrictions ofRFC 822, for example, would have to be encoded      using MIME content transfer encodings other than 7bit or 8bit,      such as quoted-printable, binary, or base64.   Given this set of rules, Unicode characters which may be encoded via   rules 1 or 3 take one octet per character, and other Unicode   characters are encoded on average with 2 2/3 octets per characterGoldsmith & Davis            Informational                      [Page 5]

RFC 2152                         UTF-7                          May 1997   plus one octet to switch into Modified Base64 and an optional octet   to switch out.      Example. The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>."      (hexadecimal 0041,2262,0391,002E) may be encoded as follows:            A+ImIDkQ.      Example. The Unicode sequence "Hi Mom -<WHITE SMILING FACE>-!"      (hexadecimal 0048, 0069, 0020, 004D, 006F, 006D, 0020, 002D, 263A,       002D, 0021) may be encoded as follows:            Hi Mom -+Jjo--!      Example. The Unicode sequence representing the Han characters for      the Japanese word "nihongo" (hexadecimal 65E5,672C,8A9E) may be      encoded as follows:            +ZeVnLIqe-Use of Character Set UTF-7 Within MIME   Character set UTF-7 is safe for mail transmission and therefore may   be used with any content transfer encoding in MIME (except where line   length and line break restrictions are violated). Specifically, the 7   bit encoding for bodies and the Q encoding for headers are both   acceptable. The MIME character set tag is UTF-7. This signifies any   version of Unicode equal to or greater than 2.0.      Example. Here is a text portion of a MIME message containing the      Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (hexadecimal 0048,      0069, 0020, 004D, 006F, 006D, 0020, 263A, 0021).      Content-Type: text/plain; charset=UTF-7      Hi Mom +Jjo-!      Example. Here is a text portion of a MIME message containing the      Unicode sequence representing the Han characters for the Japanese      word "nihongo" (hexadecimal 65E5,672C,8A9E).      Content-Type: text/plain; charset=UTF-7      +ZeVnLIqe-      Example. Here is a text portion of a MIME message containing the      Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal      0041,2262,0391,002E).Goldsmith & Davis            Informational                      [Page 6]

RFC 2152                         UTF-7                          May 1997      Content-Type: text/plain; charset=utf-7      A+ImIDkQ.      Example. Here is a text portion of a MIME message containing the      Unicode sequence "Item 3 is <POUND SIGN>1."  (hexadecimal 0049,      0074, 0065, 006D, 0020, 0033, 0020, 0069, 0073, 0020, 00A3, 0031,      002E).      Content-Type: text/plain; charset=UTF-7      Item 3 is +AKM-1.   Note that to achieve the best interoperability with systems that may   not support Unicode or MIME, when preparing text for mail   transmission line breaks should follow Internet conventions. This   means that lines should be short and terminated with the proper SMTP   CRLF sequence. Unicode LINE SEPARATOR (hexadecimal 2028) and   PARAGRAPH SEPARATOR (hexadecimal 2029) should be converted to SMTP   line breaks. Ideally, this would be handled transparently by a   Unicode-aware user agent.   This preparation is not absolutely necessary, since UTF-7 and the   appropriate MIME content transfer encoding can handle text that does   not follow Internet conventions, but readability by systems without   Unicode or MIME will be impaired. SeeRFC 2045 for a discussion of   mail interoperability issues.   Lines should never be broken in the middle of a UTF-7 shifted   sequence, since such sequences may not cross line breaks. Therefore,   UTF-7 encoding should take place after line breaking. If a line   containing a shifted sequence is too long after encoding, a MIME   content transfer encoding such as Quoted Printable can be used to   encode the text. Another possibility is to perform line breaking and   UTF-7 encoding at the same time, so that lines containing shifted   sequences already conform to length restrictions.Discussion   In this section we will motivate the introduction of UTF-7 as opposed   to the alternative of using the existing transformation formats of   Unicode (e.g., UTF-8) with MIME's content transfer encodings. Before   discussing this, it will be useful to list some assumptions about   character frequency within typical natural language text strings that   we use to estimate typical storage requirements:   1. Most Western European languages use roughly 7/8 of their letters      from US-ASCII and 1/8 from Latin 1 (ISO-8859-1).Goldsmith & Davis            Informational                      [Page 7]

RFC 2152                         UTF-7                          May 1997   2. Most non-Roman alphabet-based languages (e.g., Greek) use about      1/6 of their letters from ASCII (since white space is in the 7-bit      area) and the rest from their alphabets.   3. East Asian ideographic-based languages (including Japanese) use      essentially all of their characters from the Han or CJK syllabary      area.   4. Non-directly encoded punctuation characters do not occur      frequently enough to affect the results.   Notice that current 8 bit standards, such as ISO-8859-x, require use   of a content transfer encoding. For comparison with the subsequent   discussion, the costs break down as follows (note that many of these   figures are approximate since they depend on the exact composition of   the text):   8859-x in Base64      Text type          Average octets/character      All                      1.33   8859-x in Quoted Printable      Text type          Average octets/character      US-ASCII                 1      Western European         1.25      Other                    2.67   Note also that Unicode encoded in Base64 takes a constant 2.67 octets   per character. For purposes of comparison, we will look at UTF-8 in   Base64 and Quoted Printable, and UTF-7. Also note that fixed overhead   for long strings is relative to 1/n, where n is the encoded string   length in octets.   UTF-8 in Base64      Text type          Average octets/character      US-ASCII                 1.33      Western European         1.5      Some Alphabetics         2.44      All others               4Goldsmith & Davis            Informational                      [Page 8]

RFC 2152                         UTF-7                          May 1997   UTF-8 in Quoted Printable      Text type          Average octets/character      US-ASCII                 1      Western European         1.63      Some Alphabetics         5.17      All others               7-9   UTF-7      Text type          Average octets/character      Most US-ASCII            1      Western European         1.5      All others               2.67+2/n   We feel that the UTF-8 in Quoted Printable option is not viable due   to the very large expansion of all text except Western European. This   would only be viable in texts consisting of large expanses of US-   ASCII or Latin characters with occasional other characters   interspersed. We would prefer to introduce one encoding that works   reasonably well for all users.   We also feel that UTF-8 in Base64 has high expansion for non-   Western-European users, and is less desirable because it cannot be   read directly, even when the content is largely US-ASCII. The base   encoding of UTF-7 gives competitive results and is readable for ASCII   text.   UTF-7 gives results competitive with ISO-8859-x, with access to all   of the Unicode character set. We believe this justifies the   introduction of a new transformation format of Unicode.Goldsmith & Davis            Informational                      [Page 9]

RFC 2152                         UTF-7                          May 1997   As an alternative to use of UTF-7, it might be possible to intermix   Unicode characters with other character sets using an existing MIME   mechanism, the multipart/mixed content type, ignoring for the moment   the issues with line breaks (thanks to Nathaniel Borenstein for   suggesting this). For instance (repeating an earlier example):      Content-type: multipart/mixed; boundary=foo      Content-Disposition: inline      --foo      Content-type: text/plain; charset=us-ascii      Hi Mom      --foo      Content-type: text/plain; charset=UNICODE-2-0      Content-transfer-encoding: base64      Jjo=      --foo      Content-type: text/plain; charset=us-ascii      !      --foo--   Theoretically, this removes the need for UTF-7 in message bodies   (multipart may not be used in header fields). However, we feel that   as use of the Unicode character set becomes more widespread,   intermittent use of specialized Unicode characters (such as dingbats   and mathematical symbols) will occur, and that text will also   typically include small snippets from other scripts, such as   Cyrillic, Greek, or East Asian languages (anything in the Roman   script is already handled adequately by existing MIME character   sets). Although the multipart technique works well for large chunks   of text in alternating character sets, we feel it does not adequately   support the kinds of uses just discussed, and so we still believe the   introduction of UTF-7 is justified.Summary   The UTF-7 encoding allows Unicode characters to be encoded within the   US-ASCII 7 bit character set. It is most effective for Unicode   sequences which contain relatively long strings of US-ASCII   characters interspersed with either single Unicode characters or   strings of Unicode characters, as it allows the US-ASCII portions to   be read on systems without direct Unicode support.   UTF-7 should only be used with 7 bit transports such as mail. In   other contexts, use of straight Unicode or UTF-8 is preferred.Goldsmith & Davis            Informational                     [Page 10]

RFC 2152                         UTF-7                          May 1997Acknowledgements   Many thanks to the following people for their contributions,   comments, and suggestions. If we have omitted anyone it was through   oversight and not intentionally.         Glenn Adams         Harald T. Alvestrand         Nathaniel Borenstein         Lee Collins         Jim Conklin         Dave Crocker         Steve Dorner         Dana S. Emery         Ned Freed         Kari E. Hurtta         John H. Jenkins         John C. Klensin         Valdis Kletnieks         Keith Moore         Masataka Ohta         Einar Stefferud         Erik M. van der PoelGoldsmith & Davis            Informational                     [Page 11]

RFC 2152                         UTF-7                          May 1997Appendix A -- Examples   Here is a longer example, taken from a document originally in Big5   code. It has been condensed for brevity. There are two versions: the   first uses optional characters from set O (and so may not pass   through some mail gateways), and the second does not.   Content-type: text/plain; charset=utf-7   Below is the full Chinese text of the Analects (+itaKng-).   The sources for the text are:   "The sayings of Confucius," James R. Ware, trans.  +U/BTFw-:   +ZYeB9FH6ckh5Pg-, 1980.  (Chinese text with English translation)   +Vttm+E6UfZM-, +W4tRQ066bOg-, +UxdOrA-:  +Ti1XC2b4Xpc-, 1990.   "The Chinese Classics with a Translation, Critical and Exegetical   Notes, Prolegomena, and Copius Indexes," James Legge, trans., Taipei:   Southern Materials Center Publishing, Inc., 1991.  (Chinese text with   English translation)   Big Five and GB versions of the text are being made available   separately.   Neither the Big Five nor GB contain all the characters used in this   text.  Missing characters have been indicated using their Unicode/ISO   10646 code points.  "U+-" followed by four hexadecimal digits   indicates a Unicode/10646 code (e.g., U+-9F08).  There is no good   solution to the problem of the small size of the Big Five/GB   character sets; this represents the solution I find personally most   satisfactory.   (omitted...)   I have tried to minimize this problem by using variant characters   where they were available and the character actually in the text was   not.  Only variants listed as such in the +XrdxmVtXUXg- were used.   (omitted...)   John H. Jenkins +TpVPXGBG- jenkins@apple.com 5 January 1993   (omitted...)   Content-type: text/plain; charset=utf-7   Below is the full Chinese text of the Analects (+itaKng-).Goldsmith & Davis            Informational                     [Page 12]

RFC 2152                         UTF-7                          May 1997   The sources for the text are:   +ACI-The sayings of Confucius,+ACI- James R. Ware, trans.  +U/BTFw-:   +ZYeB9FH6ckh5Pg-, 1980.  (Chinese text with English translation)   +Vttm+E6UfZM-, +W4tRQ066bOg-, +UxdOrA-:  +Ti1XC2b4Xpc-, 1990.   +ACI-The Chinese Classics with a Translation, Critical and Exegetical   Notes, Prolegomena, and Copius Indexes,+ACI- James Legge, trans.,   Taipei:  Southern Materials Center Publishing, Inc., 1991.  (Chinese   text with English translation)   Big Five and GB versions of the text are being made available   separately.   Neither the Big Five nor GB contain all the characters used in this   text.  Missing characters have been indicated using their Unicode/ISO   10646 code points.  +ACI-U+-+ACI- followed by four hexadecimal digits   indicates a Unicode/10646 code (e.g., U+-9F08).  There is no good   solution to the problem of the small size of the Big Five/GB   character sets+ADs- this represents the solution I find personally   most satisfactory.   (omitted...)   I have tried to minimize this problem by using variant characters   where they were available and the character actually in the text was   not.  Only variants listed as such in the +XrdxmVtXUXg- were used.   (omitted...)   John H. Jenkins +TpVPXGBG- jenkins+AEA-apple.com 5 January 1993   (omitted...)Goldsmith & Davis            Informational                     [Page 13]

RFC 2152                         UTF-7                          May 1997Security Considerations   Security issues are not discussed in this memo.References[UNICODE 2.0]  "The Unicode Standard, Version 2.0", The Unicode               Consortium, Addison-Wesley, 1996. ISBN 0-201-48345-9.[ISO 10646]    ISO/IEC 10646-1:1993(E) Information Technology--Universal               Multiple-octet Coded Character Set (UCS). See also               amendments 1 through 7, plus editorial corrections.[RFC-1641]     Goldsmith, D., and M. Davis, "Using Unicode with MIME",RFC 1641, Taligent, Inc., July 1994.[US-ASCII]     Coded Character Set--7-bit American Standard Code for               Information Interchange, ANSI X3.4-1986.[ISO-8859]     Information Processing -- 8-bit Single-Byte Coded Graphic               Character Sets -- Part 1: Latin Alphabet No. 1, ISO               8859-1:1987.  Part 2: Latin alphabet No.  2, ISO 8859-2,               1987.  Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.               Part 4: Latin alphabet No.  4, ISO 8859-4, 1988.  Part 5:               Latin/Cyrillic alphabet, ISO 8859-5, 1988.  Part 6:               Latin/Arabic alphabet, ISO 8859-6, 1987.  Part 7:               Latin/Greek alphabet, ISO 8859-7, 1987.  Part 8:               Latin/Hebrew alphabet, ISO 8859-8, 1988.  Part 9: Latin               alphabet No. 5, ISO 8859-9, 1990.[RFC822]       Crocker, D., "Standard for the Format of ARPA Internet               Text Messages", STD 11,RFC 822, UDEL, August 1982.[MIME]         Borenstein N., N. Freed, K. Moore, J. Klensin, and J.               Postel, "MIME (Multipurpose Internet Mail Extensions)               Parts One through Five",RFC 2045, 2046, 2047, 2048, and               2049, November 1996.Authors' Addresses   David Goldsmith   Apple Computer, Inc.   2 Infinite Loop, MS: 302-2IS   Cupertino, CA 95014   Phone: 408-974-1957   Fax: 408-862-4566   EMail: goldsmith@apple.comGoldsmith & Davis            Informational                     [Page 14]

RFC 2152                         UTF-7                          May 1997   Mark Davis   Taligent, Inc.   10201 N. DeAnza Blvd.   Cupertino, CA 95014-2233   Phone: 408-777-5116   Fax: 408-777-5081   EMail: mark_davis@taligent.comGoldsmith & Davis            Informational                     [Page 15]