Unicode Character Repertoire Subsets
draft-bray-unichars-15

Network Working Group                                            T. Bray
Internet-Draft                                       Textuality Services
Intended status: Standards Track                              P. Hoffman
Expires: 28 November 2025                                          ICANN
                                                             27 May 2025

                  Unicode Character Repertoire Subsets
                         draft-bray-unichars-15

Abstract

   This document discusses subsets of the Unicode character repertoire
   for use in protocols and data formats, and specifies three subsets
   recommended for use in IETF specifications.

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   This Internet-Draft will expire on 28 November 2025.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Characters and Code Points  . . . . . . . . . . . . . . . . .   3
     2.1.  Encoding forms  . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Problematic Code Points . . . . . . . . . . . . . . . . .   4
       2.2.1.  Surrogates  . . . . . . . . . . . . . . . . . . . . .   5
       2.2.2.  Control Codes . . . . . . . . . . . . . . . . . . . .   5
       2.2.3.  Noncharacters . . . . . . . . . . . . . . . . . . . .   5
   3.  Dealing With Problematic Code Points  . . . . . . . . . . . .   6
   4.  Subsets . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Unicode Scalars . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  XML Characters  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Unicode Assignables . . . . . . . . . . . . . . . . . . .   8
   5.  Using Subsets . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Protocols and data formats frequently contain or are made up of
   textual data.  Such text is normally composed of Unicode [UNICODE]
   characters, to support use by speakers of many languages.  Unicode
   characters are represented by numeric code points, and the "set of
   all Unicode code points" is generally not a good choice for use in
   text fields.  Unicode recognizes different types of code points, not
   all of which are appropriate in protocols, or even associated with
   characters.  Therefore, even if the desire is to support "all Unicode
   characters" a subset of the Unicode code point repertoire should be
   specified.  Subsets such as those discussed in this document are
   appropriate choices when more-specific limitations do not apply.

   In this document, "subset" means a subset of the Unicode character
   repertoire.  This document specifies subsets that exclude some or all
   of the code points that are "problematic" as defined in Section 2.2.
   Authors should have a way to concisely and exactly reference a stable
   specification that identifies which subset a protocol or data format
   accepts.

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   This document discusses issues that apply in choosing subsets, names
   two subsets that have been popular in practice, and suggests one new
   subset.  The intended use is to serve as a convenient target for
   cross-reference from other specifications whose authors wish to
   exclude problematic code points from the data format or protocol
   being specified.

   Note that this document only provides guidance on avoiding the use of
   code points which cannot be used for interoperable interchange of
   Unicode textual data.  Dealing with strings, particularly in the
   context of user interfaces, requires addressing language, text
   rendering direction, alternate representations of the same abstract
   character, and so on.  These issues, among many others, led to many
   efforts by the Unicode Consortium, IETF efforts like [IDN] and
   [PRECIS], and W3C internationalization efforts such as [W3C-CHAR].
   The results of these efforts should be consulted by anyone engaging
   in such work.

1.1.  Notation

   In this document, the numeric values assigned to Unicode characters
   are provided in hexadecimal.  This document uses Unicode's standard
   notation of "U+" followed by four or more hexadecimal digits.  For
   example, "A", decimal 65, is expressed as U+0041, and "🖤" (Black
   Heart), decimal 128,420, is U+1F5A4.

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

   All the numeric ranges in this document are inclusive.

   The subsets are described in ABNF.

2.  Characters and Code Points

   Definition D9 in section 3.4 of [UNICODE] defines "Unicode codespace"
   as "a range of integers from 0 to 10FFFF_16".  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 allowed subsets.

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   There are 1,114,112 (17 ⨉ 2^16) code points; as of Unicode 16.0
   (2024), about 155,000 have been assigned to characters.  Since
   unassigned code points regularly become assigned when new characters
   are added to Unicode, it is usually not a good practice to specify
   that unassigned code points should be avoided.

2.1.  Encoding forms

   Unicode describes a variety of encoding forms, ways to marshal code
   points into byte sequences.  A survey of these is beyond the scope of
   this document.  However, it is useful to note that "UTF-16"
   represents each code point with one or two 16-bit chunks, while "UTF-
   8" uses variable-length byte sequences [RFC3629].

   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 encoding form.  UTF-8 is widely used for interoperable data
   formats such as JSON, YAML, CBOR, and XML.

2.2.  Problematic Code Points

   This section classifies as "problematic" all the code points which
   can never represent useful text and in some cases can lead to
   software misbehavior.  This is a low bar; the PRECIS [RFC8264]
   framework's "IdentifierClass" and "FreeformClass" exclude many more
   code points which can cause problems when displayed to humans, in
   some cases presenting security risks.  Specifications of fields in
   protocols and data formats whose contents are designed for display to
   and interactions with humans would benefit from careful consideration
   of the issues described by PRECIS; its more-restrictive subsets might
   be better choices than those specified in this document.

   Definition D10a in section 3.4 of [UNICODE] defines seven code point
   types.  Three types of code points are assigned to entities which are
   not actually characters or whose value as Unicode characters in text
   fields is questionable: "Surrogate", "Control", and "Noncharacter".
   In this document, "problematic" refers to code points whose type is
   "Surrogate" or "Noncharacter", and to "legacy controls" as defined in
   Section 2.2.2.2 below.

   Unicode's definition D49 concerns the "private-use" type and section
   3.5.10 states that they "are considered to be assigned characters".
   Section 23.5 further states that these characters' "use may be
   determined by private agreement among cooperating users".  Because
   private-use code points may have uses based on private agreements,
   this document does not classify them as "problematic".

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2.2.1.  Surrogates

   A total of 2,048 code points, the range U+D800-U+DFFF, is divided
   into two blocks called "high surrogates" and "low surrogates";
   collectively the 2,048 code points are referred to as "surrogates".
   [UNICODE] section 23.6 specifies how surrogates may 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 encoding form other
   than UTF-16 has no meaning.  In particular, [UNICODE] section 3.9.3
   forbids representing a surrogate in 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".

2.2.2.1.  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".

2.2.2.2.  Legacy Controls

   Aside from the useful controls, both the C0 and C1 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.

   Because 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 2^16 code
   points.  The last two code points in each plane are noncharacters:
   U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, U+2FFFE, U+2FFFF, and so on, up to
   U+10FFFE, U+10FFFF.

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

3.  Dealing With Problematic Code Points

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

   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 attempting to display text that includes
   them.

   The behavior of software which encounters surrogates is unpredictable
   and differs among 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.  If a specification
   requires that input data be encoded with UTF-8, and if all input were
   well-formed, implementors would 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].

   For example, 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 as described in [RFC8259]
   section 7.  It is unlikely to be useful as the value of a text field.
   That value 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.

   Two reasonable options for dealing with problematic input are either
   rejecting text containing problematic code points, or replacing the
   problematic code points with placeholders.

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   Silently deleting 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 signaling an error, or
   replacing problematic code points, ideally with "�" (U+FFFD,
   REPLACEMENT CHARACTER).

4.  Subsets

   This section describes three increasingly restrictive subsets that
   can be used in specifying acceptable content for text fields in
   protocols and data types.  Specifications can refer to these subsets
   by the names "Unicode Scalars", "XML Characters", and "Unicode
   Assignables".

4.1.  Unicode Scalars

   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 Scalars" subset can be expressed as an ABNF production:

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

   This subset is the default for 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
      %x100000-10FFFF

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   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.  Unicode Assignables

   This document defines the "Unicode Assignables" subset as all the
   Unicode code points that are not problematic.  This, a proper subset
   of each of the others, comprises all code points that are currently
   assigned, excluding legacy control codes, or that might in future be
   assigned.

   Unicode Assignables can be expressed as an ABNF production:

   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.  Using Subsets

   Many IETF specifications rely on well-known data formats such as
   JSON, I-JSON, CBOR, YAML, and XML.  These formats specify default
   subsets.  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 restriction,
   which applies to data content, not textual representation in

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   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 has no actions for IANA.

7.  Security Considerations

   Section 3 of this document discusses security issues.

   Unicode Security Considerations [TR36] is a wide-ranging survey of
   the issues implementors should consider while writing software to
   process Unicode text.  Unicode Source Code Handling [TR55] discusses
   use of Unicode in programming languages, with a focus on security
   issues.  Many of the attacks they discuss 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.

   The Security Considerations in Section 12 of [RFC8264] generally
   applies to this document as well.

   Note that the Unicode-character subsets specified in this document
   are increasingly restrictive, omitting more and more problematic code
   points, and thus should be less and less susceptible to many of these
   exploits.  The Section 4.3 subset, "Unicode Assignables", excludes
   all of these code points.

8.  Normative References

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [TR36]     The Unicode Consortium, "Unicode Security Considerations",
              <https://www.unicode.org/reports/tr36/>.  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.

   [TR55]     The Unicode Consortium, "Unicode Source Code Handling",
              <https://www.unicode.org/reports/tr55/>.  Note that this
              reference is to the latest version of this document,

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              rather than to a specific release.  It is not expected
              that future updates will affect the referenced
              discussions.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.  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

   [IDN]      "Internationalized Domain Name Working Group",
              <https://datatracker.ietf.org/group/idn/>.

   [PRECIS]   "PRECIS Working Group",
              <https://datatracker.ietf.org/group/precis/>.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <https://www.rfc-editor.org/info/rfc2277>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8264]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 8264, DOI 10.17487/RFC8264, October 2017,
              <https://www.rfc-editor.org/info/rfc8264>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9413]  Thomson, M. and D. Schinazi, "Maintaining Robust
              Protocols", RFC 9413, DOI 10.17487/RFC9413, June 2023,
              <https://www.rfc-editor.org/info/rfc9413>.

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   [W3C-CHAR] W3C, "Character encodings: Essential concepts",
              <https://www.w3.org/International/articles/definitions-
              characters/>.

   [XML]      Bray, T., Paoli, J., McQueen, C.M., Maler, E., and F.
              Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", 26 November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126/>.  Note that
              this reference is to a specific release, based on a
              history of previous "Edition" releases having changed this
              production.

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.

   Thanks also to Addison Phillips and the W3C Internationalization
   Working Group for helpful suggestions on language and references.

   Thoughtful comments during the many iterations of this draft, which
   helped tighten up wording and make difficult points clearer, were
   contributed by Harald Alvestrand, Martin J Dürst, Donald E.
   Eastlake, John Klensin, Barry Leiba, Glyn Normington, Peter Saint-
   Andre, and Rob Sayre.

Authors' Addresses

   Tim Bray
   Textuality Services
   Email: tbray@textuality.com

   Paul Hoffman
   ICANN
   Email: paul.hoffman@icann.org

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