Technical Reports

Proposed Draft Unicode Technical Report #26

Compatibility Encoding Scheme for UTF-16: 8-Bit (CESU-8)

Summary

This document specifies an 8-bit Compatibility Encoding Scheme for UTF-16 (CESU) that is intended as an alternate encoding to UTF-8 for internal use within systems processing Unicode in order to provide an ASCII-compatible 8-bit encoding that preserves UTF-16 binary collation. It is not intended nor recommended as an encoding used for open information exchange. The Unicode Consortium, does not encourage the use of CESU-8, but does recognize the existence of data in this encoding and supplies this technical report to clearly define the format and to distinguish it from UTF-8. This encoding does not replace or amend the definition of UTF-8.

Status

This document has been approved by the Unicode Technical Committee for public review as a Proposed Draft Unicode Technical Report. Publication does not imply endorsement by the Unicode Consortium. This is a draft document which may be updated, replaced, or superseded by other documents at any time. This is not a stable document; it is inappropriate to cite this document as other than a work in progress.

A list of current Unicode Technical Reports is found on http://www.unicode.org/unicode/reports/. For more information about versions of the Unicode Standard and how to reference this document, see http://www.unicode.org/unicode/standard/versions/.

Contents

• 1 Introduction
• 2 Definitions
• 3 Relation to ISO/IEC 10646 and UTF-8
• 4 IANA Registration
• 5 References

1 Introduction

CESU-8 defines an encoding scheme for Unicode identical to UTF-8 except for its representation of supplementary characters. In CESU-8, supplementary characters are represented as six-byte sequences resulting from the transformation of each UTF-16 surrogate code unit into an eight-bit form similar to the UTF-8 transformation, but without first converting the input surrogate pairs to a scalar value. 

CESU-8 is useful in 8-bit processing environments where binary collation with UTF-16 is required. It is designed and recommended for use only within products requiring this UTF-16 binary collation equivalence. It is not intended nor recommended for open interchange. 

The following lists the important features of this encoding form: 

• The CESU-8 representation of characters on the Basic Multilingual Plane (BMP) is identical to the representation of these characters in UTF-8. Only the representation of supplementary characters differs. 

• Only the six-byte form of supplementary characters is legal in CESU-8; the four-byte UTF-8 style supplementary character sequence is illegal. 

• A binary collation of data encoded in CESU-8 is identical to the binary collation of the same data encoded in UTF-16. 

As a very small percentage of characters in a typical data stream are expected to be supplementary characters, there is a strong possibility that CESU-8 data may be misinterpreted as UTF-8. Therefore, all use of CESU-8 outside closed implementations is strongly discouraged, such as the emittance of CESU-8 in output files, markup language or other open transmission forms.

2 Definitions

The following define the CESU-8 encoding scheme. CESU-8 is not a normative part of the Unicode Standard, and therefore the definitions below do not form part of the standard. Instead, they are encapsulated in this Unicode Technical Report as an implementation-specific encoding scheme for use by implementers of the Unicode Standard. 

2.1 CESU-8 is a Compatibility Encoding Scheme for UTF-16 (CESU) that serializes a Unicode code point as a sequence of one, two, three or six bytes. 

1. Prior to transforming data into CESU-8, supplementary characters must first be converted to their surrogate pair UTF-16 representation. For example, U+F0000 must first be converted to U+DB80 U+DC00. 

2. The resulting data stream is encoded into an eight-bit form using the bit distribution table in definition 2.2. It should be noted that this bit distribution table is identical to that of UTF-8 except that the input value is a sequence of UTF-16 code units, not a scalar value, and that a four-byte transformation is disallowed. 

3. The bit pattern 11110xxx is illegal in any CESU-8 byte, effectively prohibiting the occurrence of UTF-8 four-byte surrogates in CESU-8. Thus, a data stream may not contain both CESU-8 six-byte and UTF-8 four-byte supplementary character sequences. 

4. The shortest form rules applied to UTF-8 in the Unicode Standard Definition D36 will also apply to CESU-8. 

5. Data encoded in CESU-8 should only be exchanged when it is labeled as such in a higher-level protocol or is agreed upon in an API definition. It should not be auto-detected. Use of this encoding in the absence of encoding tags or a higher level protocol describing the encoding is invalid and strongly discouraged. 

• CESU-8 encoding example: 

In CESU-8, <U+004D, U+0061, U+10000> is serialized as <4D 61 ED AE 80 ED B0 80>  

2.2 CESU-8 Bit Distribution

3 Relation to ISO/IEC 10646 and UTF-8

ISO/IEC 10646 and the Unicode Standard define the UTF-8 encoding form, which is very similar in definition to CESU-8 other than its treatment of supplementary characters. CESU-8 is an additional encoding scheme that supplements these definitions, but does not form part of either ISO/IEC 10646 or the Unicode Standard. It is intended only for use in compatibility situations where binary collation with UTF-16 is required.

4 IANA Registration

CESU-8 will be registered with the Internet Assigned Numbers Authority. This section will be updated with the IANA registered name. 

Note: CESU-8 was originally proposed and discussed with the name UTF-8S, but was renamed CESU-8 by the Unicode Technical Committee to avoid possible confusion with UTF-8.

5 References

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