Unicode Character Repertoire Subsets

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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, "character repertoire" describes subsets of the Unicode character repertoire that exclude some or all of the entities that are "problematic" as defined in Section 2.2. 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 specificying 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.¶

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 when new characters are added to Unicode.¶

3. Dealing With Problematic Code Points

[RFC9413], "Maintaining Robust Protocols", provides a thorough discussion of strategies for dealing with issues in input data, for example problematic code points.¶

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. 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).¶

RFC19413 emphasizes that when encountering problematic input, software should consider the field as a whole, not individual code points or bytes.¶

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 Scalars", "XML Characters", and "Unicode Assignables".¶

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

xml-character =
   %x9 / %xA / %xD /   ; useful controls
   %x20-D7FF /         ; exclude surrogates
   %xE000-FFFD/        ; exclude FFFE and FFFF nonchars
   %x100000-10FFFF

unicode-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

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 Unicode Assignables, the example would remain a conforming JSON Text but the data it represents would not constitute Unicode 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, "Unicode Assignables", excludes all of them.¶

Acknowledgements

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