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rfc:rfc1642

Network Working Group D. Goldsmith Request for Comments: 1642 M. Davis Category: Experimental Taligent, Inc.

                                                             July 1994
                               UTF-7
            A Mail-Safe Transformation Format of Unicode

Status of this Memo

 This memo defines an Experimental Protocol 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 1.1, and ISO/IEC 10646-1:1993(E)
 jointly define a 16 bit 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 1521 and RFC 1522) 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 new transformation format of Unicode that
 contains only 7-bit ASCII characters 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 RFC 1521, RFC 1522,
 and the document "Using Unicode with MIME".

Motivation

 Although other transformation formats of Unicode exist and could
 conceivably be used in this context (most notably UTF-1 and 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

Goldsmith & Davis [Page 1] RFC 1642 UTF-7 July 1994

 UTF-FSS together 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, 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 and news. In other contexts, straight
 Unicode or UTF-8 is preferred.
 See the document "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 1.1". 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.
    Note. Unicode 1.1 further specifies the use and interaction of
    these character codes beyond the ISO standard. However, any valid
    10646 BMP (Basic Multilingual Plane) 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 from RFC 1521, Appendix B): 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 [Page 2] RFC 1642 UTF-7 July 1994

             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 in RFC 1521, excluding the pad character "="
 (decimal value 61).
 Rationale. The pad character = is excluded because UTF-7 is designed
 for use within header fields as set forth in RFC 1522. Since the only
 readable encoding in RFC 1522 is "Q" (based on RFC 1521's Quoted-
 Printable), the "=" character is not available for use (without a lot
 of escape sequences). This was very unfortunate but unavoidable. The

Goldsmith & Davis [Page 3] RFC 1642 UTF-7 July 1994

 "=" 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 in 7-bit US-ASCII
 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
    end 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.
    Rationale. A terminating character is necessary for cases where
    the next character after the Modified Base64 sequence is part of
    character set B. It can also enhance readability by delimiting
    encoded sequences.
    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). Text with an odd number of octets is
    ill-formed.
    Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters
    in the UCS-2 form are serialized as octets, that the most

Goldsmith & Davis [Page 4] RFC 1642 UTF-7 July 1994

    significant octet appear first.  This is also in keeping with
    common network practice of choosing a canonical format for
    transmission.
    Next, the octet stream is encoded by applying the Base64 content
    transfer encoding algorithm as defined in RFC 1521, 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 in RFC 1522 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 of RFC 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 character
 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, 004D, 0020, 263A, 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-

Goldsmith & Davis [Page 5] RFC 1642 UTF-7 July 1994

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 identifier is UNICODE-1-1-UTF-7.
    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, 004D, 0020, 263A, 0021).
    Content-Type: text/plain; charset=UNICODE-1-1-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=UNICODE-1-1-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).
    Content-Type: text/plain; charset=UNICODE-1-1-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=UNICODE-1-1-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

Goldsmith & Davis [Page 6] RFC 1642 UTF-7 July 1994

 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. See RFC 1521 for an in-depth
 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).
 2. Most non-European 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):

Goldsmith & Davis [Page 7] RFC 1642 UTF-7 July 1994

 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. UTF-1 gives results
 substantially similar to UTF-8.  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               4
 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.

Goldsmith & Davis [Page 8] RFC 1642 UTF-7 July 1994

 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.
 As an alternative to use of UTF-7, it is possible to intermix Unicode
 characters with other character sets using an existing MIME
 mechanism, the multipart/mixed content type (thanks to Nathaniel
 Borenstein for pointing this out). For instance (repeating an earlier
 example):
    Content-type: multipart/mixed; boundary=foo
  1. -foo

Content-type: text/plain; charset=us-ascii

    Hi Mom
    --foo
    Content-type: text/plain; charset=UNICODE-1-1
    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.

Goldsmith & Davis [Page 9] RFC 1642 UTF-7 July 1994

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 and
 news. In other contexts, use of straight Unicode or UTF-8 is
 preferred.

Acknowledgements

 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 Poel

Goldsmith & Davis [Page 10] RFC 1642 UTF-7 July 1994

Appendix 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 thus may not pass
 through some mail gateways), and the second uses no optional
 characters.
 Content-type: text/plain; charset=unicode-1-1-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-
 John_Jenkins@taligent.com
 5 January 1993

Goldsmith & Davis [Page 11] RFC 1642 UTF-7 July 1994

 (omitted...)
 Content-type: text/plain; charset=unicode-1-1-utf-7
 Below is the full Chinese text of the Analects (+itaKng-).
 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-
 John+AF8-Jenkins+AEA-taligent.com
 5 January 1993
 (omitted...)

Goldsmith & Davis [Page 12] RFC 1642 UTF-7 July 1994

Security Considerations

 Security issues are not discussed in this memo.

References

[UNICODE 1.1] "The Unicode Standard, Version 1.1": Version 1.0, Volume

             1 (ISBN 0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-
             201-60845-6), and "Unicode Technical Report #4, The
             Unicode Standard, Version 1.1" (available from The
             Unicode Consortium, and soon to be published by Addison-
             Wesley).

[ISO 10646] ISO/IEC 10646-1:1993(E) Information Technology–Universal

             Multiple-octet Coded Character Set (UCS).

[MIME/UNICODE] 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.

[RFC-1521] 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.

[RFC-1522] Moore, K., "Representation of Non-Ascii Text in Internet

             Message Headers" RFC 1522, University of Tennessee,
             September 1993.

Goldsmith & Davis [Page 13] RFC 1642 UTF-7 July 1994

[UTF-8] X/Open Company Ltd., "File System Safe UCS Transformation

             Format (FSS_UTF)", X/Open Preliminary Specification,
             Document Number: P316. This information also appears in
             Unicode Technical Report #4, and in a forthcoming annex
             to ISO/IEC 10646.

Authors' Addresses

 David Goldsmith
 Taligent, Inc.
 10201 N. DeAnza Blvd.
 Cupertino, CA 95014-2233
 Phone: 408-777-5225
 Fax: 408-777-5081
 EMail: david_goldsmith@taligent.com
 Mark Davis
 Taligent, Inc.
 10201 N. DeAnza Blvd.
 Cupertino, CA 95014-2233
 Phone: 408-777-5116
 Fax: 408-777-5081
 EMail: mark_davis@taligent.com

Goldsmith & Davis [Page 14]

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