RFC2152 - UTF-7 A Mail-Safe Transformation Format of Unicode

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　　Network Working Group D. Goldsmith
Request for Comments: 2152 Apple Computer, Inc.
Obsoletes: RFC1642 M. Davis
Category: Informational Taligent, Inc.
May 1997

UTF-7

A Mail-Safe Transformation Format of Unicode

Status 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, RFC822) currently supports only 7-
bit US ASCII as a character set. MIME (RFC2045 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 and RFC1641,
"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 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 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.

See RFC1641, "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 from RFC1521, Appendix B, which no longer
appears in RFC2045): 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):

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 RFC2045, 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 RFC2047. Since the only
readable encoding in RFC2047 is "Q" (based on RFC2045'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.

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 in RFC2045, 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 RFC2047 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 RFC822, 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, 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).

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. See RFC2045 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).

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 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.

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 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.

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

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 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-).

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...)

Security 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",
RFC1641, 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, RFC822, UDEL, August 1982.

[MIME] Borenstein N., N. Freed, K. Moore, J. Klensin, and J.
Postel, "MIME (Multipurpose Internet Mail Extensions)
Parts One through Five", RFC2045, 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.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

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