Technical Reports

L2/01-460

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 for internal use within systems processing Unicode in order to provide an ASCII-compatible 8-bit encoding that is similar to UTF-8 but 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 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 This is a draft document which may be updated, replaced, or superseded by other documents at any time.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 11111xxx 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 a different encoding scheme.  It 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 to avoid possible confusion with UTF-8.

5 References

Copyright © 2001 Unicode, Inc. All Rights Reserved. The Unicode Consortium makes no expressed or implied warranty of any kind, and assumes no liability for errors or omissions. No liability is assumed for incidental and consequential damages in connection with or arising out of the use of the information or programs contained or accompanying this technical report.

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