Network Working Group T. Bray
Internet-Draft Textuality Services
Intended status: Standards Track P. Hoffman
Expires: 4 September 2025 ICANN
3 March 2025
Unicode Character Repertoire Subsets
draft-bray-unichars-11
Abstract
This document discusses specifying subsets of the Unicode character
repertoire for use in protocols and data formats.
Status of This Memo
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This Internet-Draft will expire on 4 September 2025.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Characters and Code Points . . . . . . . . . . . . . . . . . 3
2.1. Transformation Formats . . . . . . . . . . . . . . . . . 3
2.2. Problematic Code Points . . . . . . . . . . . . . . . . . 4
2.2.1. Surrogates . . . . . . . . . . . . . . . . . . . . . 4
2.2.2. Control Codes . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Noncharacters . . . . . . . . . . . . . . . . . . . . 5
3. Dealing With Problematic Code Points . . . . . . . . . . . . 5
4. Subsets . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Unicode Scalars . . . . . . . . . . . . . . . . . . . . . 7
4.2. XML Characters . . . . . . . . . . . . . . . . . . . . . 7
4.3. Unicode Assignables . . . . . . . . . . . . . . . . . . . 7
5. Using Subsets . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
9. Informative References . . . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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. 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.
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.
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.
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. 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.
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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, 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".
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".
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. In particular, [UNICODE] section
3.9.3 forbids representing a surrogate in UTF-8.
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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+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
[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.
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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. 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.
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 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".
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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 / %xE000-10FFFF ; exclude surrogates
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
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 subset, which is
smaller than 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:
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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
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
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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
successively exclude an increasing number of 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,
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
[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>.
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[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>.
[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.
This document got a great deal of thoughtful discussion during the
late stages of review which helped tighten up wording and make
difficult points clearer.
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Authors' Addresses
Tim Bray
Textuality Services
Email: tbray@textuality.com
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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