rfc9839.original   rfc9839.txt 
Network Working Group T. Bray Internet Engineering Task Force (IETF) T. Bray
Internet-Draft Textuality Services Request for Comments: 9839 Textuality Services
Intended status: Standards Track P. Hoffman Category: Standards Track P. Hoffman
Expires: 28 November 2025 ICANN ISSN: 2070-1721 ICANN
27 May 2025 August 2025
Unicode Character Repertoire Subsets Unicode Character Repertoire Subsets
draft-bray-unichars-15
Abstract Abstract
This document discusses subsets of the Unicode character repertoire This document discusses subsets of the Unicode character repertoire
for use in protocols and data formats, and specifies three subsets for use in protocols and data formats and specifies three subsets
recommended for use in IETF specifications. recommended for use in IETF specifications.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
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and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9839.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Notation
2. Characters and Code Points . . . . . . . . . . . . . . . . . 3 2. Characters and Code Points
2.1. Encoding forms . . . . . . . . . . . . . . . . . . . . . 4 2.1. Encoding Forms
2.2. Problematic Code Points . . . . . . . . . . . . . . . . . 4 2.2. Problematic Code Points
2.2.1. Surrogates . . . . . . . . . . . . . . . . . . . . . 5 2.2.1. Surrogates
2.2.2. Control Codes . . . . . . . . . . . . . . . . . . . . 5 2.2.2. Control Codes
2.2.3. Noncharacters . . . . . . . . . . . . . . . . . . . . 5 2.2.3. Noncharacters
3. Dealing With Problematic Code Points . . . . . . . . . . . . 6 3. Dealing with Problematic Code Points
4. Subsets . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Subsets
4.1. Unicode Scalars . . . . . . . . . . . . . . . . . . . . . 7 4.1. Unicode Scalars
4.2. XML Characters . . . . . . . . . . . . . . . . . . . . . 7 4.2. XML Characters
4.3. Unicode Assignables . . . . . . . . . . . . . . . . . . . 8 4.3. Unicode Assignables
5. Using Subsets . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Using Subsets
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations
8. Normative References . . . . . . . . . . . . . . . . . . . . 9 8. References
9. Informative References . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgements
Authors' Addresses
1. Introduction 1. Introduction
Protocols and data formats frequently contain or are made up of Protocols and data formats frequently contain or are made up of
textual data. Such text is normally composed of Unicode [UNICODE] textual data. Such text is normally composed of Unicode [UNICODE]
characters, to support use by speakers of many languages. Unicode characters, to support use by speakers of many languages. Unicode
characters are represented by numeric code points, and the "set of characters are represented by numeric code points, and the "set of
all Unicode code points" is generally not a good choice for use in all Unicode code points" is generally not a good choice for use in
text fields. Unicode recognizes different types of code points, not text fields. Unicode recognizes different types of code points, not
all of which are appropriate in protocols, or even associated with all of which are appropriate in protocols or even associated with
characters. Therefore, even if the desire is to support "all Unicode characters. Therefore, even if the desire is to support "all Unicode
characters" a subset of the Unicode code point repertoire should be characters", a subset of the Unicode code point repertoire should be
specified. Subsets such as those discussed in this document are specified. Subsets such as those discussed in this document are
appropriate choices when more-specific limitations do not apply. appropriate choices when more-specific limitations do not apply.
In this document, "subset" means a subset of the Unicode character In this document, "subset" means a subset of the Unicode character
repertoire. This document specifies subsets that exclude some or all repertoire. This document specifies subsets that exclude some or all
of the code points that are "problematic" as defined in Section 2.2. 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 Authors should have a way to concisely and exactly reference a stable
specification that identifies which subset a protocol or data format specification that identifies which subset a protocol or data format
accepts. accepts.
This document discusses issues that apply in choosing subsets, names This document discusses issues that apply in choosing subsets, names
two subsets that have been popular in practice, and suggests one new two subsets that have been popular in practice, and suggests one new
subset. The intended use is to serve as a convenient target for subset. The intended use is to serve as a convenient target for
cross-reference from other specifications whose authors wish to cross-reference from other specifications whose authors wish to
exclude problematic code points from the data format or protocol exclude problematic code points from the data format or protocol
being specified. being specified.
Note that this document only provides guidance on avoiding the use of Note that this document only provides guidance on avoiding the use of
code points which cannot be used for interoperable interchange of code points that cannot be used for interoperable interchange of
Unicode textual data. Dealing with strings, particularly in the Unicode textual data. Dealing with strings, particularly in the
context of user interfaces, requires addressing language, text context of user interfaces, requires addressing language, text
rendering direction, alternate representations of the same abstract rendering direction, alternate representations of the same abstract
character, and so on. These issues, among many others, led to many character, and so on. These issues, among many others, led to many
efforts by the Unicode Consortium, IETF efforts like [IDN] and efforts by the Unicode Consortium, efforts by the IETF such as [IDN]
[PRECIS], and W3C internationalization efforts such as [W3C-CHAR]. and [PRECIS], and internationalization efforts by W3C such as
The results of these efforts should be consulted by anyone engaging [W3C-CHAR]. The results of these efforts should be consulted by
in such work. anyone engaging in such work.
1.1. Notation 1.1. Notation
In this document, the numeric values assigned to Unicode characters In this document, the numeric values assigned to Unicode characters
are provided in hexadecimal. This document uses Unicode's standard are provided in hexadecimal. This document uses Unicode's standard
notation of "U+" followed by four or more hexadecimal digits. For notation of "U+" followed by four or more hexadecimal digits. For
example, "A", decimal 65, is expressed as U+0041, and "🖤" (Black example, "A", decimal 65, is expressed as U+0041, and "🖤" (Black
Heart), decimal 128,420, is U+1F5A4. Heart), decimal 128,420, is U+1F5A4.
Groups of numeric values described in Section 4 are given in ABNF Groups of numeric values described in Section 4 are given in ABNF
[RFC5234]. In ABNF, hexadecimal values are preceded by "%x" rather [RFC5234]. In ABNF, hexadecimal values are preceded by "%x" rather
than "U+". than "U+".
All the numeric ranges in this document are inclusive. All the numeric ranges in this document are inclusive.
The subsets are described in ABNF. The subsets are described in ABNF.
2. Characters and Code Points 2. Characters and Code Points
Definition D9 in section 3.4 of [UNICODE] defines "Unicode codespace" Definition D9 in Section 3.4 of [UNICODE] defines "Unicode codespace"
as "a range of integers from 0 to 10FFFF_16". Definition D10 defines as "a range of integers from 0 to 10FFFF_16". Definition D10 defines
"code point" as "Any value in the Unicode codespace". "code point" as "Any value in the Unicode codespace".
The Unicode Standard's definition of "Unicode character" is The Unicode Standard's definition of "Unicode character" is
conceptual. However, each Unicode character is assigned a code conceptual. However, each Unicode character is assigned a code
point, used to represent the characters in computer memory and point, used to represent the characters in computer memory and
storage systems and, in specifications, to specify allowed subsets. storage systems and to specify allowed subsets in specifications.
There are 1,114,112 (17 ⨉ 2^16) code points; as of Unicode 16.0 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 (2024), about 155,000 have been assigned to characters. Since
unassigned code points regularly become assigned when new characters unassigned code points regularly become assigned when new characters
are added to Unicode, it is usually not a good practice to specify are added to Unicode, it is usually not a good practice to specify
that unassigned code points should be avoided. that unassigned code points should be avoided.
2.1. Encoding forms 2.1. Encoding Forms
Unicode describes a variety of encoding forms, ways to marshal code Unicode describes a variety of encoding forms, ways to marshal code
points into byte sequences. A survey of these is beyond the scope of points into byte sequences. A survey of these is beyond the scope of
this document. However, it is useful to note that "UTF-16" this document. However, it is useful to note that "UTF-16"
represents each code point with one or two 16-bit chunks, while "UTF- represents each code point with one or two 16-bit chunks, while "UTF-
8" uses variable-length byte sequences [RFC3629]. 8" uses variable-length byte sequences [RFC3629].
The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277], The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277],
says "Protocols MUST be able to use the UTF-8 charset", which becomes 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 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 a single encoding form. UTF-8 is widely used for interoperable data
formats such as JSON, YAML, CBOR, and XML. formats such as JSON, YAML, CBOR, and XML.
2.2. Problematic Code Points 2.2. Problematic Code Points
This section classifies as "problematic" all the code points which This section classifies all the code points that can never represent
can never represent useful text and in some cases can lead to useful text and, in some cases, can lead to software misbehavior as
software misbehavior. This is a low bar; the PRECIS [RFC8264] "problematic". This is a low bar; the PRECIS [RFC8264] framework's
framework's "IdentifierClass" and "FreeformClass" exclude many more "IdentifierClass" and "FreeformClass" exclude many more code points
code points which can cause problems when displayed to humans, in that can cause problems when displayed to humans, in some cases
some cases presenting security risks. Specifications of fields in presenting security risks. Specifications of fields in protocols and
protocols and data formats whose contents are designed for display to data formats whose contents are designed for display to and
and interactions with humans would benefit from careful consideration interactions with humans would benefit from careful consideration of
of the issues described by PRECIS; its more-restrictive subsets might the issues described by PRECIS; its more-restrictive subsets might be
be better choices than those specified in this document. better choices than those specified in this document.
Definition D10a in section 3.4 of [UNICODE] defines seven code point Definition D10a in Section 3.4 of [UNICODE] defines seven code point
types. Three types of code points are assigned to entities which are types. Three types of code points are assigned to entities that are
not actually characters or whose value as Unicode characters in text not actually characters or whose value as Unicode characters in text
fields is questionable: "Surrogate", "Control", and "Noncharacter". fields is questionable: "Surrogate", "Control", and "Noncharacter".
In this document, "problematic" refers to code points whose type is In this document, "problematic" refers to code points whose type is
"Surrogate" or "Noncharacter", and to "legacy controls" as defined in "Surrogate" or "Noncharacter" and to "legacy controls" as defined in
Section 2.2.2.2 below. Section 2.2.2.2 below.
Unicode's definition D49 concerns the "private-use" type and section Definition D49 in [UNICODE] concerns the "private-use" type, and
3.5.10 states that they "are considered to be assigned characters". Section 3.5.10 states that they "are considered to be assigned
Section 23.5 further states that these characters' "use may be characters". Section 23.5 further states that these characters' "use
determined by private agreement among cooperating users". Because may be determined by private agreement among cooperating users".
private-use code points may have uses based on private agreements, Because private-use code points may have uses based on private
this document does not classify them as "problematic". agreements, this document does not classify them as "problematic".
2.2.1. Surrogates 2.2.1. Surrogates
A total of 2,048 code points, the range U+D800-U+DFFF, is divided 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"; into two blocks called "high surrogates" and "low surrogates";
collectively the 2,048 code points are referred to as "surrogates". collectively, the 2,048 code points are referred to as "surrogates".
[UNICODE] section 23.6 specifies how surrogates may be used in Section 23.6 of [UNICODE] specifies how surrogates may be used in
Unicode texts encoded in UTF-16, where a high-surrogate/low-surrogate Unicode texts encoded in UTF-16, where a high-surrogate/low-surrogate
pair represents a code point greater than U+FFFF. pair represents a code point greater than U+FFFF.
A surrogate which occurs in text encoded in any encoding form other A surrogate that occurs in text encoded in any encoding form other
than UTF-16 has no meaning. In particular, [UNICODE] section 3.9.3 than UTF-16 has no meaning. In particular, Section 3.9.3 of
forbids representing a surrogate in UTF-8. [UNICODE] forbids representing a surrogate in UTF-8.
2.2.2. Control Codes 2.2.2. Control Codes
Section 23.1 of [UNICODE] introduces the control codes for Section 23.1 of [UNICODE] introduces the control codes for
compatibility with legacy pre-Unicode standards. They comprise 65 compatibility with legacy pre-Unicode standards. They comprise 65
code points in the ranges U+0000-U+001F ("C0 controls") and code points in the ranges U+0000-U+001F ("C0 controls") and
U+0080-U+009F ("C1 controls"), plus U+007F, "DEL". U+0080-U+009F ("C1 controls"), plus U+007F, "DEL".
2.2.2.1. Useful Controls 2.2.2.1. Useful Controls
skipping to change at page 6, line 7 skipping to change at line 233
asserts repeatedly that they are not designed or used for open asserts repeatedly that they are not designed or used for open
interchange. interchange.
Code points are organized into 17 "planes", each containing 2^16 code Code points are organized into 17 "planes", each containing 2^16 code
points. The last two code points in each plane are noncharacters: 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+FFFE, U+FFFF, U+1FFFE, U+1FFFF, U+2FFFE, U+2FFFF, and so on, up to
U+10FFFE, U+10FFFF. U+10FFFE, U+10FFFF.
The code points in the range U+FDD0-U+FDEF are noncharacters. The code points in the range U+FDD0-U+FDEF are noncharacters.
3. Dealing With Problematic Code Points 3. Dealing with Problematic Code Points
[RFC9413], "Maintaining Robust Protocols", provides a thorough [RFC9413], "Maintaining Robust Protocols", provides a thorough
discussion of strategies for dealing with issues in input data. discussion of strategies for dealing with issues in input data.
Different types of problematic code points cause different issues. Different types of problematic code points cause different issues.
Noncharacters and legacy controls are unlikely to cause software Noncharacters and legacy controls are unlikely to cause software
failures, but they cannot usefully be displayed to humans, and can be failures, but they cannot usefully be displayed to humans, and they
used in attacks based on attempting to display text that includes can be used in attacks based on attempting to display text that
them. includes them.
The behavior of software which encounters surrogates is unpredictable The behavior of software that encounters surrogates is unpredictable
and differs among programming-language implementations, even between and differs among programming-language implementations, even between
different API calls in the same language. different API calls in the same language.
Section 3.9 of [UNICODE] makes it clear that a UTF-8 byte sequence 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 that 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 requires that input data be encoded with UTF-8, and if all input were
well-formed, implementors would never have to concern themselves with well-formed, implementors would never have to concern themselves with
surrogates. surrogates.
Unfortunately, industry experience teaches that problematic code Unfortunately, industry experience teaches that problematic code
points, including surrogates, can and do occur in program input where points, including surrogates, can and do occur in program input where
the source of input data is not controlled by the implementor. In the source of input data is not controlled by the implementor. In
particular, the specification of JSON allows any code point to appear particular, the specification of JSON allows any code point to appear
in object member names and string values [RFC8259]. in object member names and string values [RFC8259].
For example, the following is a conforming JSON text: For example, the following is a conforming JSON text:
{"example": "\u0000\u0089\uDEAD\uD9BF\uDFFF"} {"example": "\u0000\u0089\uDEAD\uD9BF\uDFFF"}
The value of the "example" field contains the C0 control NUL, the C1 The value of the "example" field contains the C0 control NUL, the C1
control "CHARACTER TABULATION WITH JUSTIFICATION", an unpaired control "CHARACTER TABULATION WITH JUSTIFICATION", an unpaired
surrogate, and the noncharacter U+7FFFF encoded per JSON rules as two surrogate, and the noncharacter U+7FFFF encoded per JSON rules as two
escaped UTF-16 surrogate code points as described in [RFC8259] escaped UTF-16 surrogate code points as described in Section 7 of
section 7. It is unlikely to be useful as the value of a text field. [RFC8259]. 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 That value cannot be serialized into well-formed UTF-8, but the
behavior of libraries asked to parse the sample is unpredictable; behavior of libraries asked to parse the sample is unpredictable;
some will silently parse this and generate an ill-formed UTF-8 some will silently parse this and generate an ill-formed UTF-8
string. string.
Two reasonable options for dealing with problematic input are either Two reasonable options for dealing with problematic input are either
rejecting text containing problematic code points, or replacing the rejecting text containing problematic code points or replacing the
problematic code points with placeholders. problematic code points with placeholders.
Silently deleting an ill-formed part of a string is a known security Silently deleting an ill-formed part of a string is a known security
risk. Responding to that risk, [UNICODE] section 3.2 recommends risk. Responding to that risk, Section 3.2 of [UNICODE] recommends
dealing with ill-formed byte sequences by signaling an error, or dealing with ill-formed byte sequences by signaling an error or
replacing problematic code points, ideally with "�" (U+FFFD, replacing problematic code points, ideally with "�" (U+FFFD,
REPLACEMENT CHARACTER). REPLACEMENT CHARACTER).
4. Subsets 4. Subsets
This section describes three increasingly restrictive subsets that This section describes three increasingly restrictive subsets that
can be used in specifying acceptable content for text fields in can be used in specifying acceptable content for text fields in
protocols and data types. Specifications can refer to these subsets protocols and data types. Specifications can refer to these subsets
by the names "Unicode Scalars", "XML Characters", and "Unicode by the names "Unicode Scalars", "XML Characters", and "Unicode
Assignables". Assignables".
4.1. Unicode Scalars 4.1. Unicode Scalars
Definition D76 in section 3.9 of [UNICODE] defines the term "Unicode Definition D76 in Section 3.9 of [UNICODE] defines the term "Unicode
scalar value" as "Any Unicode code point except high-surrogate and scalar value" as "Any Unicode code point except high-surrogate and
low-surrogate code points." low-surrogate code points".
The "Unicode Scalars" subset can be expressed as an ABNF production: The "Unicode Scalars" subset can be expressed as an ABNF production:
unicode-scalar = unicode-scalar =
%x0-D7FF / ; exclude surrogates %x0-D7FF / ; exclude surrogates
%xE000-10FFFF %xE000-10FFFF
This subset is the default for CBOR [RFC8949], and has the advantage This subset is the default for Concise Binary Object Representation
of excluding surrogates. However, it includes legacy controls and (CBOR) [RFC8949] and has the advantage of excluding surrogates.
noncharacters. However, it includes legacy controls and noncharacters.
4.2. XML Characters 4.2. XML Characters
The XML 1.0 Specification [XML], in its grammar production labeled The XML 1.0 Specification [XML], in its grammar production labeled
"Char", specifies a subset of Unicode code points that excludes "Char", specifies a subset of Unicode code points that excludes
surrogates, legacy C0 controls, and the noncharacters U+FFFE and surrogates, legacy C0 controls, and the noncharacters U+FFFE and
U+FFFF. U+FFFF.
The "XML Characters" subset can be expressed as an ABNF production: The "XML Characters" subset can be expressed as an ABNF production:
xml-character = xml-character =
%x9 / %xA / %xD / ; useful controls %x9 / %xA / %xD / ; useful controls
%x20-D7FF / ; exclude surrogates %x20-D7FF / ; exclude surrogates
%xE000-FFFD / ; exclude FFFE and FFFF nonchars %xE000-FFFD / ; exclude FFFE and FFFF nonchars
%x100000-10FFFF %x10000-10FFFF
While this subset does not exclude all the problematic code points, While this subset does not exclude all the problematic code points,
the C1 controls are less likely than the C0 controls to appear the C1 controls are less likely than the C0 controls to appear
erroneously in data, and have not been observed to be a frequent erroneously in data and have not been observed to be a frequent
source of problems. Also, the noncharacters greater in value than source of problems. Also, the noncharacters greater in value than
U+FFFF are rarely encountered. U+FFFF are rarely encountered.
4.3. Unicode Assignables 4.3. Unicode Assignables
This document defines the "Unicode Assignables" subset as all the This document defines the "Unicode Assignables" subset as all the
Unicode code points that are not problematic. This, a proper subset Unicode code points that are not problematic. This, a proper subset
of each of the others, comprises all code points that are currently of each of the others, comprises all code points that are currently
assigned, excluding legacy control codes, or that might in future be assigned, excluding legacy control codes, or that might be assigned
assigned. in the future.
Unicode Assignables can be expressed as an ABNF production: Unicode Assignables can be expressed as an ABNF production:
unicode-assignable = unicode-assignable =
%x9 / %xA / %xD / ; useful controls %x9 / %xA / %xD / ; useful controls
%x20-7E / ; exclude C1 controls and DEL %x20-7E / ; exclude C1 controls and DEL
%xA0-D7FF / ; exclude surrogates %xA0-D7FF / ; exclude surrogates
%xE000-FDCF / ; exclude FDD0 nonchars %xE000-FDCF / ; exclude FDD0 nonchars
%xFDF0-FFFD / ; exclude FFFE and FFFF nonchars %xFDF0-FFFD / ; exclude FFFE and FFFF nonchars
%x10000-1FFFD / %x20000-2FFFD / ; (repeat per plane) %x10000-1FFFD / %x20000-2FFFD / ; (repeat per plane)
skipping to change at page 8, line 39 skipping to change at line 357
%x50000-5FFFD / %x60000-6FFFD / %x50000-5FFFD / %x60000-6FFFD /
%x70000-7FFFD / %x80000-8FFFD / %x70000-7FFFD / %x80000-8FFFD /
%x90000-9FFFD / %xA0000-AFFFD / %x90000-9FFFD / %xA0000-AFFFD /
%xB0000-BFFFD / %xC0000-CFFFD / %xB0000-BFFFD / %xC0000-CFFFD /
%xD0000-DFFFD / %xE0000-EFFFD / %xD0000-DFFFD / %xE0000-EFFFD /
%xF0000-FFFFD / %x100000-10FFFD %xF0000-FFFFD / %x100000-10FFFD
5. Using Subsets 5. Using Subsets
Many IETF specifications rely on well-known data formats such as Many IETF specifications rely on well-known data formats such as
JSON, I-JSON, CBOR, YAML, and XML. These formats specify default JSON, Internet JSON (I-JSON), CBOR, YAML, and XML. These formats
subsets. For example, JSON allows object member names and string specify default subsets. For example, JSON allows object member
values to include any Unicode code point, including all the names and string values to include any Unicode code point, including
problematic types. all the problematic types.
A protocol based on JSON can be made more robust and implementor- A protocol based on JSON can be made more robust and implementor-
friendly by restricting the contents of object member names and friendly by restricting the contents of object member names and
string values to one of the subsets described in Section 4. string values to one of the subsets described in Section 4.
Equivalent restrictions are possible for other packaging formats such Equivalent restrictions are possible for other packaging formats such
as I-JSON, XML, YAML, and CBOR. as I-JSON, XML, YAML, and CBOR.
Note that escaping techniques such as those in the JSON example in Note that escaping techniques such as those in the JSON example in
Section 3 cannot be used to circumvent this sort of restriction, Section 3 cannot be used to circumvent this sort of restriction,
which applies to data content, not textual representation in which applies to data content, not textual representation in
packaging formats. If a specification restricted a JSON field value packaging formats. If a specification restricted a JSON field value
to the Unicode Assignables, the example would remain a conforming to the Unicode Assignables, the example would remain a conforming
JSON Text but the data it represents would not constitute Unicode JSON text but the data it represents would not constitute Unicode
Assignable code points. Assignable code points.
6. IANA Considerations 6. IANA Considerations
This document has no actions for IANA. This document has no IANA actions.
7. Security Considerations 7. Security Considerations
Section 3 of this document discusses security issues. Section 3 of this document discusses security issues.
Unicode Security Considerations [TR36] is a wide-ranging survey of Unicode Security Considerations [TR36] is a wide-ranging survey of
the issues implementors should consider while writing software to the issues implementors should consider while writing software to
process Unicode text. Unicode Source Code Handling [TR55] discusses process Unicode text. Unicode Source Code Handling [TR55] discusses
use of Unicode in programming languages, with a focus on security use of Unicode in programming languages, with a focus on security
issues. Many of the attacks they discuss are aimed at deceiving issues. Many of the attacks they discuss are aimed at deceiving
human readers, but vulnerabilities involving issues such as human readers, but vulnerabilities involving issues such as
surrogates and noncharacters are also covered, and in fact can surrogates and noncharacters are also covered and, in fact, can
contribute to human-deceiving exploits. contribute to human-deceiving exploits.
The Security Considerations in Section 12 of [RFC8264] generally The security considerations in Section 12 of [RFC8264] generally
applies to this document as well. apply to this document as well.
Note that the Unicode-character subsets specified in this document Note that the Unicode-character subsets specified in this document
are increasingly restrictive, omitting more and more problematic code are increasingly restrictive, omitting more and more problematic code
points, and thus should be less and less susceptible to many of these points, and thus should be less and less susceptible to many of these
exploits. The Section 4.3 subset, "Unicode Assignables", excludes exploits. The subset in Section 4.3, "Unicode Assignables", excludes
all of these code points. all of these code points.
8. Normative References 8. References
8.1. Normative References
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[TR36] The Unicode Consortium, "Unicode Security Considerations", [TR36] Davis, M., Ed. and M. Suignard, Ed., "Unicode Security
<https://www.unicode.org/reports/tr36/>. Note that this Considerations", <https://www.unicode.org/reports/tr36/>.
reference is to the latest version of this document, Note that this reference is to the latest version of this
rather than to a specific release. It is not expected document, rather than to a specific release. It is not
that future updates will affect the referenced expected that future updates will affect the referenced
discussions. discussions.
[TR55] The Unicode Consortium, "Unicode Source Code Handling", [TR55] Leroy, R., Ed. and M. Davis, Ed., "Unicode Source Code
<https://www.unicode.org/reports/tr55/>. Note that this Handling", <https://www.unicode.org/reports/tr55/>. Note
reference is to the latest version of this document, that this reference is to the latest version of this
rather than to a specific release. It is not expected document, rather than to a specific release. It is not
that future updates will affect the referenced expected that future updates will affect the referenced
discussions. discussions.
[UNICODE] The Unicode Consortium, "The Unicode Standard", [UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>. Note that this <http://www.unicode.org/versions/latest/>. Note that this
reference is to the latest version of Unicode, rather than reference is to the latest version of Unicode, rather than
to a specific release. It is not expected that future to a specific release. It is not expected that future
changes in the Unicode Standard will affect the referenced changes in the Unicode Standard will affect the referenced
definitions. definitions.
9. Informative References 8.2. Informative References
[IDN] "Internationalized Domain Name Working Group", [IDN] "Internationalized Domain Name Working Group",
<https://datatracker.ietf.org/group/idn/>. <https://datatracker.ietf.org/group/idn/>.
[PRECIS] "PRECIS Working Group", [PRECIS] "PRECIS Working Group",
<https://datatracker.ietf.org/group/precis/>. <https://datatracker.ietf.org/group/precis/>.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277, Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
January 1998, <https://www.rfc-editor.org/info/rfc2277>. January 1998, <https://www.rfc-editor.org/info/rfc2277>.
skipping to change at page 11, line 9 skipping to change at line 472
<https://www.rfc-editor.org/info/rfc8949>. <https://www.rfc-editor.org/info/rfc8949>.
[RFC9413] Thomson, M. and D. Schinazi, "Maintaining Robust [RFC9413] Thomson, M. and D. Schinazi, "Maintaining Robust
Protocols", RFC 9413, DOI 10.17487/RFC9413, June 2023, Protocols", RFC 9413, DOI 10.17487/RFC9413, June 2023,
<https://www.rfc-editor.org/info/rfc9413>. <https://www.rfc-editor.org/info/rfc9413>.
[W3C-CHAR] W3C, "Character encodings: Essential concepts", [W3C-CHAR] W3C, "Character encodings: Essential concepts",
<https://www.w3.org/International/articles/definitions- <https://www.w3.org/International/articles/definitions-
characters/>. characters/>.
[XML] Bray, T., Paoli, J., McQueen, C.M., Maler, E., and F. [XML] Bray, T., Ed., Paoli, J., Ed., McQueen, C.M., Ed., Maler,
Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth E., Ed., and F. Yergeau, Ed., "Extensible Markup Language
Edition)", 26 November 2008, (XML) 1.0 (Fifth Edition)", W3C Recommendation, 26
November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126/>. Note that <http://www.w3.org/TR/2008/REC-xml-20081126/>. Note that
this reference is to a specific release, based on a this reference is to a specific release, based on a
history of previous "Edition" releases having changed this history of previous "Edition" releases having changed this
production. production.
Acknowledgements Acknowledgements
Thanks are due to Guillaume Fortin-Debigaré, who filed an Errata Thanks are due to Guillaume Fortin-Debigaré, who filed an errata
Report against RFC 8259, The JavaScript Object Notation, noting report against RFC 8259, "The JavaScript Object Notation (JSON) Data
frequent references to "Unicode characters", when in fact the RFC Interchange Format", noting frequent references to "Unicode
formally specifies the use of Unicode Code Points. 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 Thanks also to Asmus Freytag for careful review and many constructive
suggestions aimed at making the language more consistent with the suggestions aimed at making the language more consistent with the
structure of the Unicode Standard. structure of the Unicode Standard.
Thanks also to James Manger for the correctness of the ABNF and JSON Thanks also to James Manger for the correctness of the ABNF and JSON
samples. samples.
Thanks also to Addison Phillips and the W3C Internationalization Thanks also to Addison Phillips and the W3C Internationalization
Working Group for helpful suggestions on language and references. Working Group for helpful suggestions on language and references.
Thoughtful comments during the many iterations of this draft, which Thoughtful comments during the many draft versions of this document,
helped tighten up wording and make difficult points clearer, were which helped tighten up wording and make difficult points clearer,
contributed by Harald Alvestrand, Martin J Dürst, Donald E. were contributed by Harald Alvestrand, Martin J. Dürst, Donald
Eastlake, John Klensin, Barry Leiba, Glyn Normington, Peter Saint- E. Eastlake, John Klensin, Barry Leiba, Glyn Normington, Peter Saint-
Andre, and Rob Sayre. Andre, and Rob Sayre.
Authors' Addresses Authors' Addresses
Tim Bray Tim Bray
Textuality Services Textuality Services
Email: tbray@textuality.com Email: tbray@textuality.com
Paul Hoffman Paul Hoffman
ICANN ICANN
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