Internet-Draft Specifying Unicode September 2023
Bray & Hoffman Expires 22 March 2024 [Page]
Network Working Group
Intended Status:
Standards Track
T. Bray
Textuality Services
P. Hoffman

Unicode Character Repertoire Subsets


This document discusses specifying subsets of the Unicode character repertoire for use in protocols and data formats.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 22 March 2024.

Table of Contents

1. Introduction

When a protocol or data format has text fields, that text is normally composed of Unicode [UNICODE] characters, to support use by speakers of many languages. The "set of all Unicode characters" is generally not a good choice for use in text fields. Instead, subsets such as those discussed in this document are typically used.

The term "character repertoire" is a well-understood concept when applied to an encoding standard; in this document it describes subsets of the Unicode characters. Authors should have a way to concisely and exactly reference a stable specification that identifies a protocol or data format's character repertoire

This document discusses issues that apply in choosing subsets, names two subsets that have been popular choices in specification character repertoires, and suggests one new subset. The goal is to provide a convenient target for cross-reference from other specifications which discuss character repertoires.

1.1. Notation

In this document, the numeric values assigned to Unicode characters are provided in hexadecimal. In the text, Unicode’s standard "U+", zero-padded to four places [RFC5137], is used. For example, "A", decimal 65, would be expressed as U+0041, and "😉" (Winking Face), decimal 128,521, would be U+1F609.

Groups of numeric values described in Section 4 are given in ABNF [RFC5234]. In ABNF, the hexadecimal values for characters are preceded by "%x" rather than "U+".

All the numeric ranges in this document are inclusive.

2. Characters and Code Points

Definition D9 in section 3.4 of [UNICODE] defines "Unicode codespace" as "a range of integers from 0 to 10FFFF16". Definition D10 defines "code point" as "Any value in the Unicode codespace".

The Unicode Standard's definition of "Unicode character" is conceptual. However, each Unicode character is assigned a code point, used to represent the characters in computer memory and storage systems and, in specifications, to specify the allowed repertoires of Unicode characters.

There are 1,114,112 code points; as of 2023, fewer than 150,000 have been assigned to characters. It is difficult to specify that unassigned code points should be avoided, because they regularly become assigned as new characters are added to Unicode.

2.1. Transformation Formats

Unicode describes a variety of "transformation formats", ways to marshal code points into byte sequences. A survey of transformation formats is beyond the scope of this document. However, it is useful to note that the "UTF-16" format represents each code point with one or two 16-bit chunks, and the “UTF-8” format uses variable-length byte sequences.

The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277], says "Protocols MUST be able to use the UTF-8 charset", which becomes a mandate to use UTF-8 for any protocol or data format that specifies a single transformation format. UTF-8 is widely used for interoperable data formats such as JSON, YAML, and XML.

2.2. Problematic Code Point Types

Definition D10a in section 3.4 of [UNICODE] defines seven code point types. Three types of code points are assigned to constructs which are not actually characters or whose value as Unicode characters in text fields is questionable: "Surrogate", "Control", and "Noncharacter".

2.2.1. Surrogates

A total of 2,048 code points, in the range U+D800-U+DFFF, are divided into two blocks called "high surrogates" and "low surrogates"; collectively the 2,048 code points are referred to as "surrogates". Surrogates may only be used in Unicode texts encoded in UTF-16, where a high-surrogate/low-surrogate pair represents a code point greater than U+FFFF.

A surrogate which occurs in text encoded in any transformation format other than UTF-16 has no meaning and may cause malfunction in software that encounters it. In particular, it is impossible to represent a surrogate in well-formed UTF-8.

2.2.2. Control Codes

Section 23.1 of [UNICODE] introduces the "Control Codes", for compatibility with legacy pre-Unicode standards. They comprise 65 code points in the ranges U+0000-U+001F ("C0 Controls") and U+0080-U+009F (“C1 Controls”), plus U+007F, "DEL". Useful Controls

The C0 Controls include newline (U+000A), carriage return (U+000D), and tab (U+0009); this document refers to these three characters as the "useful controls". Legacy Controls

Aside from the useful controls, the control codes are mostly obsolete and generally lack interoperable semantics. This document uses the phrase "legacy controls" to describe control codes that are not useful controls.

Since the code points for C0 Controls include the 32 smallest integers including zero, they are likely to occur in data as a result of programming errors.

2.2.3. Noncharacters

Certain code points are classified as "noncharacters", and [UNICODE] asserts repeatedly that they are not designed or used for open interchange.

Code points are organized into 17 "planes", each containing 216 code points. The last two code points in each plane are noncharacters: U+00FFFE, U+00FFFF, U+01FFFE, U+01FFFF, U+02FFFE, U+02FFFF, and so on, up to U+10FFFE, U+10FFFF.

The code points in the range U+FDD0-U+FDEF are noncharacters.

3. Dealing With Problematic Code Points

Problematic code points are an example of problematic input. [RFC9413], "Maintaining Robust Protocols", provides a thorough discussion of error-handling options when choosing a strategy for dealing with problematic input.

Different types of problematic code points cause different issues. Noncharacters and legacy controls are unlikely to cause software failures, but they cannot usefully be displayed to humans, and can be used in attacks based on misleading human readers of text that display them. [TR36]

Surrogate characters have been observed to cause software failures. The behavior of software which encounters them is unpredictable and differs in programming-language implementations, even between different API calls in the same language.

Section 3.9 of [UNICODE] makes it clear that a UTF-8 byte sequence which would map to a surrogate is ill-formed. Thus, in theory, if a specification requires that input data be encoded with UTF-8, implementors should never have to concern themselves with surrogates.

Unfortunately, industry experience teaches that problematic code points, including surrogates, can and do occur in program input where the source of input data is not controlled by the implementor. In particular, the specification of JSON allows any code point to appear in object member names and string values [RFC8259]; the following is a conforming JSON text:

{"example": "\u0000\U0089\uDEAD\uD9BF\uDFFF"}

The value of the "example" field contains the C0 Control NUL, the C1 Control "CHARACTER TABULATION WITH JUSTIFICATION", an unpaired surrogate, and the noncharacter U+7FFFF encoded per JSON rules as two escaped UTF-16 surrogate code points. It is unlikely to be useful as the value of a text field. It cannot be serialized into well-formed UTF-8, but the behavior of libraries asked to parse the sample is unpredictable; some will silently parse this and generate an ill-formed UTF-8 string.

Reasonable options for dealing with problematic input include, first, rejecting text containing problematic code points, and second, replacing them with placeholders. RFC19413 emphasizes that when encountering problematic input, software should consider the field as a whole, not individual code points or bytes.

Silently ignoring an ill-formed part of a string is a known security risk. Responding to that risk, [UNICODE] section 3.2 recommends dealing with ill-formed byte sequences by by signaling an error, or replacing problematic code points with "�" (U+FFFD, REPLACEMENT CHARACTER).

4. Subset Character Repertoires

This section describes subsets of the code points that can be used in specifying character repertoires for text fields in protocols and data types. Specifications can refer to these subsets by the names "Unicode Scalar Values", "XML Characters", and "Useful Assignables".

4.1. Unicode Scalar Values

Definition D76 in section 3.9 of [UNICODE] defines the term "Unicode scalar value" as "Any Unicode code point except high-surrogate and low-surrogate code points."

The "Unicode Scalar Values" subset can be expressed as an ABNF production:

unicode-scalar-value =
   %x0-D7FF / %xE000-10FFFF  ; exclude surrogates

This subset is the default character repertoire for I-JSON [RFC7493] and CBOR [RFC8949], and has the advantage of excluding surrogates. However, it includes legacy controls and noncharacters.

4.2. XML Characters

The XML 1.0 Specification [XML], in its grammar production labeled "Char", specifies a subset of Unicode code points that excludes surrogates, legacy C0 Controls, and the noncharacters U+FFFE and U+FFFF.

The "XML Characters" subset can be expressed as an ABNF production:

xml-character =
   %x9 / %xA / %xD /   ; useful controls
   %x20-D7FF /         ; exclude surrogates
   %xE000-FFFD/        ; exclude FFFE and FFFF nonchars

While this subset does not exclude all the problematic code points, the C1 Controls are less likely than the C0 Controls to appear erroneously in data, and have not been observed to be a frequent source of problems. Also, the noncharacters greater in value than U+FFFF are rarely encountered.

4.3. Useful Assignables

This document defines the "Useful Assignables" subset as the Unicode code points, excluding the legacy controls, surrogates, and noncharacters. This subset comprises all code points that are currently assigned, or might in future be assigned, to characters that are not legacy control codes.

Useful Assignables can be expressed as an ABNF production:

useful-assignable =
   %x9 / %xA / %xD /               ; useful controls
   %x20-7E /                       ; exclude C1 Controls and DEL
   %xA0-D7FF /                     ; exclude surrogates
   %xE000-FDCF                     ; exclude FDD0 nonchars
   %xFDF0-FFFD /                   ; exclude FFFE and FFFF nonchars
   %x10000-1FFFD / %x20000-2FFFD / ; (repeat per plane)
   %x30000-3FFFD / %x40000-4FFFD /
   %x50000-5FFFD / %x60000-6FFFD /
   %x70000-7FFFD / %x80000-8FFFD /
   %x90000-9FFFD / %xA0000-AFFFD /
   %xB0000-BFFFD / %xC0000-CFFFD /
   %xD0000-DFFFD / %xE0000-EFFFD /
   %xF0000-FFFFD / %x100000-10FFFD

This subset excludes all code points whose types are identified as problematic above.

5. Restricting Character Repertoires

Many IETF specifications rely on well-known data formats such as JSON, I-JSON, CBOR, YAML, and XML. These formats have default character repertoires. For example, JSON allows object member names and string values to include any Unicode code point, including all the problematic types.

A protocol based on JSON can be made more robust and implementor-friendly by restricting the contents of object member names and string values to one of the subsets described in Section 4. Equivalent restrictions are possible for other packaging formats such as I-JSON, XML, YAML, and CBOR.

Note that escaping techniques such as those in the JSON example in Section 3 cannot be used to circumvent this sort of character-repertoire restriction, which applies to data content, not textual representation in packaging formats. If a specification restricted a JSON field value to the Useful Assignables, the example would remain a conforming JSON Text but the data it represents would not constitute Useful Assignable code points.

6. IANA Considerations

This document makes no requests of IANA.

7. Security Considerations

Unicode Security Considerations [TR36] is a wide-ranging survey of the issues implementors should consider while writing software to process Unicode text. Many of the exploits it discusses are aimed at deceiving human readers, but vulnerabilities involving issues such as surrogates and noncharacters are also covered, and in fact can contribute to human-deceiving exploits.

Note that the Unicode-character subsets specified in this document include a successively-decreasing number of surrogates and noncharacters, and thus should be less and less susceptible to vulnerabilities. The Section 4.3 subset, "Useful Assignables", excludes all of them.

8. Normative References

Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
The Unicode Consortium, "Unicode Security Considerations", <>. Note that this reference is to the latest version of this document, rather than to a specific release. It is not expected that future updates will affect the referenced discussions.
The Unicode Consortium, "The Unicode Standard", <>. Note that this reference is to the latest version of Unicode, rather than to a specific release. It is not expected that future changes in the Unicode Standard will affect the referenced definitions.

9. Informative References

Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277, , <>.
Klensin, J., "ASCII Escaping of Unicode Characters", BCP 137, RFC 5137, DOI 10.17487/RFC5137, , <>.
Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, , <>.
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <>.
Thomson, M. and D. Schinazi, "Maintaining Robust Protocols", RFC 9413, DOI 10.17487/RFC9413, , <>.
Bray, T., Paoli, J., McQueen, C.M., Maler, E., and F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", , <>. Note that this reference is to a specific release, based on a history of previous "Edition" releases having changed this production.


Thanks are due to Guillaume Fortin-Debigaré, who filed an Errata Report against RFC 8259, The JavaScript Object Notation, noting frequent references to "Unicode characters", when in fact the RFC formally specifies the use of Unicode Code Points.

Thanks also to Asmus Freytag for careful review and many constructive suggestions aimed at making the language more consistent with the structure of the Unicode Standard.

Thanks also to James Manger for the correctness of the ABNF and JSON samples.

Authors' Addresses

Tim Bray
Textuality Services
Paul Hoffman