[Unicode]  Technical Reports
 

Unicode® Technical Standard #39

Unicode Security Mechanisms

Version 8.0
Editors Mark Davis (markdavis@google.com),
Michel Suignard (michel@suignard.com)
Date 2015-06-04
This Version http://www.unicode.org/reports/tr39/tr39-11.html
Previous Version http://www.unicode.org/reports/tr39/tr39-9.html
Latest Version http://www.unicode.org/reports/tr39/
Latest Proposed Update http://www.unicode.org/reports/tr39/proposed.html
Revision 11


Summary

Because Unicode contains such a large number of characters and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This document specifies mechanisms that can be used to detect possible security problems.

Status

This document has been reviewed by Unicode members and other interested parties, and has been approved for publication by the Unicode Consortium. This is a stable document and may be used as reference material or cited as a normative reference by other specifications.

A Unicode Technical Standard (UTS) is an independent specification. Conformance to the Unicode Standard does not imply conformance to any UTS.

Please submit corrigenda and other comments with the online reporting form [Feedback]. Related information that is useful in understanding this document is found in the References. For the latest version of the Unicode Standard, see [Unicode]. For a list of current Unicode Technical Reports, see [Reports]. For more information about versions of the Unicode Standard, see [Versions].

Contents



1 Introduction

Unicode Technical Report #36, "Unicode Security Considerations" [UTR36] provides guidelines for detecting and avoiding security problems connected with the use of Unicode. This document specifies mechanisms that are used in that document, and can be used elsewhere. Readers should be familiar with [UTR36] before continuing. See also the Unicode FAQ on Security Issues [FAQSec].

2 Conformance

An implementation claiming conformance to this specification must do so in conformance to the following clauses:

C1 An implementation claiming to implement the General Profile for Identifiers shall do so in accordance with the specifications in Section 3.1, General Security Profile for Identifiers.

Alternatively, it shall declare that it uses a modification, and provide a precise list of characters that are added to or removed from the profile.
C2 An implementation claiming to implement any of the following confusable-detection functions must do so in accordance with the specifications in Section 4, Confusable Detection.
  1. X and Y are single-script confusables
  2. X and Y are mixed-script confusables
  3. X and Y are whole-script confusables
  4. X has any simple single-script confusables
  5. X has any mixed-script confusable
  6. X has any whole-script confusable

Alternatively, it shall declare that it uses a modification, and provide a precise list of character mappings that are added to or removed from the provided ones.
C3 An implementation claiming to detect mixed scripts must do so in accordance with the specifications in Section 5.1, Mixed-Script Detection.

Alternatively, it shall declare that it uses a modification, and provide a precise specification of the differences in behavior.
C4 An implementation claiming to detect Restriction Levels must do so in accordance with the specifications in Section 5.2, Restriction-Level Detection.

Alternatively, it shall declare that it uses a modification, and provide a precise specification of the differences in behavior.
C5 An implementation claiming to detect mixed numbers must do so in accordance with the specifications in Section 5.3, Mixed-Number Detection.

Alternatively, it shall declare that it uses a modification, and provide a precise specification of the differences in behavior.

3 Identifier Characters

Identifiers are special-purpose strings used for identification—strings that are deliberately limited to particular repertoires for that purpose. Exclusion of characters from identifiers does not affect the general use of those characters, such as within documents. Unicode Standard Annex #31, "Identifier and Pattern Syntax" [UAX31] provides a recommended method of determining which strings should qualify as identifiers. The UAX #31 specification extends the common practice of defining identifiers in terms of letters and numbers to the Unicode repertoire.

That specification also permits other protocols to use that method as a base, and to define a profile that adds or removes characters. For example, identifiers for specific programming languages typically add some characters like "$", and remove others like "-" (because of the use as minus), while IDNA removes "_" (among others)—see Unicode Technical Standard #46, "Unicode IDNA Compatibility Processing" [UTS46], as well as [IDNA2003], and [IDNA2008].

This document provides for additional identifier profiles for environments where security is an issue. These are profiles of the extended identifiers based on properties and specifications of the Unicode Standard [Unicode], including:

The data files used in defining these profiles follow the UCD File Format, which has a semicolon-delimited list of data fields associated with given characters, with each field referenced by number. For more details, see [UCDFormat].

3.1 General Security Profile for Identifiers

The file [idmod] provides data for a profile of identifiers in environments where security is at issue. The file contains a set of characters recommended to be restricted from use. It also contains a small set of characters that are recommended as additions to the list of characters defined by the XID_Start and XID_Continue properties, because they may be used in identifiers in a broader context than programming identifiers.

The Restricted characters are characters not in common use, and are can be blocked to further reduce the possibilities for visual confusion. They include the following:

The principle has been to be more conservative initially, allowing for the set to be modified in the future as requirements for characters are refined. For information on handling modifications over time, see Section 2.9.1, Backward Compatibility in Unicode Technical Report #36, "Unicode Security Considerations" [UTR36].

An implementation following the General Security Profile does not permit Restricted characters, unless it documents the additional characters that it does allow. Common candidates for such additions include characters for scripts listed in Table 6, Aspirational Use Scripts and Table 7, Limited Use Scripts of [UAX31]. However, characters from these scripts have not been a priority for examination for confusables or to determine specialized, non-modern, or uncommon-use characters.

Canonical equivalence is applied when testing candidate identifiers for inclusion of Allowed characters. For example, suppose the candidate string is the sequence

<u, combining-diaeresis>

The target string would be Allowed in either of the following 2 situations:

  1. u is Allowed and ¨ is Allowed, or
  2. ü is Allowed

In the file [idmod], Field 1 is the character in question, Field 2 is a Status value (either Restricted or Allowed), and Field 3 is a Type value. The Types are subcategories of the Status value, and are listed in Table 1. Identifier Status and Type:

Table 1. Identifier Status and Type
Status Type Description
Restricted Not_Character Unassigned characters, private use characters, surrogates, most control characters
Deprecated Characters with the Unicode property Deprecated=Yes
Default_Ignorable Characters with the Unicode property Default_Ignorable_Code_Point=Yes
Not_NFKC Characters that cannot occur in strings normalized to NFKC.
Not_XID Other characters that do not qualify as default Unicode identifiers; that is, they do not have the Unicode property XID_Continue=True.
Exclusion
Characters from scripts that are not in customary modern use: Table 4, Candidate Characters for Exclusion from Identifiers from [UAX31]
Obsolete Characters that are no longer in modern use.
Technical Specialized usage: technical, liturgical, etc.
Uncommon_Use Characters whose status is uncertain, or that are not commonly used in modern text.
Limited_Use Characters from scripts that are in limited use: Table 7, Limited Use Scripts in [UAX31].
Aspirational Characters from scripts would otherwise qualify as Limited Use, but have strong current efforts to increase their usage: Table 6, Aspirational Use Scripts in [UAX31].
Allowed Inclusion Exceptional allowed characters, including Table 3, Candidate Characters for Inclusion in Identifiers in [UAX31], and some characters for IDNA2008.
Recommended Table 5, Recommended Scripts in [UAX31]

The distinctions among the Type values is not strict; if there are multiple Types for restricting a character only one is given. The important characteristic is the Status: whether or not the character is Restricted. As more information is gathered about characters, this data may change in successive versions. That can cause either the Status or Type to change for a particular character. Thus users of this data should be prepared for changes in successive versions, such as by having a grandfathering policy in place for previously supported characters or registrations. Both Status and Type values are to be compared case-insensitively and ignoring hyphens and underbars.

Restricted characters should be treated with caution in registration, and disallowed unless there is good reason to allow them in the environment in question. However, the set of Status=Allowed characters are not typically used as-is by implementations. Instead, they are applied as filters to the set of characters C that are supported by the identifier syntax, generating a new set C′. Typically there are also particular characters or classes of characters from C that are retained as Exception characters.

C′ = (C ∩ {Status=Allowed}) ∪ Exceptions

The implementation may simply restrict use of new identifiers to C′, or may apply some other strategy. For example, there might be an appeal process for registrations of ids that contain characters outside of C' (but still inside of C), or in user interfaces for lookup of identifiers, warnings of some kind may be appropriate. For more information, see [UTR36].

The Exception characters would be implementation-specific. For example, a particular implementation might extend the default Unicode identifier syntax by adding Exception characters with the Unicode property XID_Continue=False, such as “$”, “-”, and “.”. Those characters are specific to that identifier syntax, and would be retained even though they are not in the Status=Allowed set. Some implementations may also wish to add some [CLDR] exemplar characters for particular supported languages that have unusual characters.

The Type=Inclusion characters already contain some characters that are not letters or numbers, but that are used within words in some languages. For example, it is recommended that U+00B7 (·) MIDDLE DOT be allowed in identifiers, because it is required for Catalan.

The implementation may also apply other restrictions discussed in this document, such as checking for confusable characters or doing mixed-script detection.

3.2 IDN Security Profiles for Identifiers

Version 1 of this document defined operations and data that apply to [IDNA2003], which has been superseded by [IDNA2008] and Unicode Technical Standard #46, "Unicode IDNA Compatibility Processing" [UTS46]. The identifier modification data can be applied to whichever specification of IDNA is being used. For more information, see the [IDN FAQ].

4 Confusable Detection

The data in [confusables] provide a mechanism for determining when two strings are visually confusable. The data in these files may be refined and extended over time. For information on handling modifications over time, see Section 2.9.1, Backward Compatibility in Unicode Technical Report #36, "Unicode Security Considerations" [UTR36] and the Migration section of this document.

Collection of data for detecting gatekeeper-confusable strings is not currently a goal for the confusable detection mechanism in this document. For more information, see Section 2 Visual Security Issues in [UTR36].

The data provides a mapping from source characters to target strings.

To see whether two strings X and Y are confusable (abbreviated as X ≅ Y), an implementation uses a transform of X called a skeleton(X) defined by:

  1. Converting X to NFD format, as described in [UAX15].
  2. Successively mapping each source character in X to the target string according to the specified data.
  3. Reapplying NFD.

The resulting strings skeleton(X) and skeleton(Y) are then compared. If they are identical (codepoint-for-codepoint), then X ≅ Y.

Note: The strings skeleton(X) and skeleton(Y) are not intended for display, storage or transmission. They should be thought of as an intermediate processing form, similar to a hashcode. The characters in skeleton(X) and skeleton(Y) are not guaranteed to be identifier characters.

Definitions

X and Y are single-script confusables if they are confusable, and each of them is a single script string according to Section 5, Mixed-Script Detection, and it is the same script for each. Examples: "so̷s" and "søs" in Latin, where the first word has the character "o" followed by the character U+0337 (  ̷ ) COMBINING SHORT SOLIDUS OVERLAY.

X and Y are mixed-script confusables if they are confusable but they are not single-script confusables. Examples: "paypal" and "pаypаl", where the second word has the character U+0430 ( а ) CYRILLIC SMALL LETTER A.

X and Y are whole-script confusables if they are mixed-script confusables, and each of them is a single script string. Example: "scope" in Latin and "ѕсоре" in Cyrillic.

Characters with the Script_Extension property values COMMON or INHERITED are ignored when testing for differences in script.

Each line in the data file has the following format: Field 1 is the source, Field 2 is the target, and Field 3 is obsolete . Field 3 used to contain different types, but now only has the value MA, which stands for "Mixed-Script, Any-Case".

For example:

0441 ; 0063 ; MA # ( с → c ) CYRILLIC SMALL LETTER ES → LATIN SMALL LETTER C #

2CA5 ; 0063 ; MA # ( ⲥ → c ) COPTIC SMALL LETTER SIMA → LATIN SMALL LETTER C # →ϲ→

Everything after the # is a comment and is purely informative. A asterisk after the comment indicates that the character is not an XID character [UAX31]. The comments provide the character names. If the data was derived via transitivity, there is an extra comment at the end. For instance, in the above example the derivation was:

  1. ⲥ (U+2CA5 COPTIC SMALL LETTER SIMA)
  2. → ϲ (U+03F2 GREEK LUNATE SIGMA SYMBOL)
  3. → c (U+0063 LATIN SMALL LETTER C)

To reduce security risks, it is advised that identifiers use casefolded forms, thus eliminating uppercase variants where possible.

The data may change between versions. Even where the data is the same, the order of lines in the files may change between versions. For more information, see Migration.

Implementations that use the confusable data do not have to recursively apply the mappings, because the transforms are idempotent. That is,

skeleton(skeleton(X)) = skeleton(X)

This mechanism imposes transitivity on the data, so if X ≅ Y and Y ≅ Z, then X ≅ Z. It is possible to provide a more sophisticated confusable detection, by providing a metric between given characters, indicating their "closeness." However, that is computationally much more expensive, and requires more sophisticated data, so at this point in time the simpler mechanism has been chosen. That means that in some cases the test may be overly inclusive. However the frequency of such cases in real data should be small.

4.1 Whole-Script Confusables

Data is also provided for testing a string to see if a string X has any whole-script confusable, using the file [confusablesWS]. This file consists of a list of lines of the form: 

<range>; <sourceScript>; <targetScript>; <type> #comment

The types are either L for lowercase-only, or A for any-case, where the any-case ranges are broader (including uppercase and lowercase characters). If the string is only lowercase, use the lowercase-only table. Otherwise, first test according to the any-case table, then casefold the string and test according to the lowercase-only table.

In using the data, all lines with the same sourceScript and targetScript are collected together to form a set of Unicode characters, after filtering to the Allowed characters from Section 3.1, General Security Profile for Identifiers . Logically, the file is a set of tuples of the form <sourceScript, unicodeSet, targetScript>. For example, the following lines are present for Latin to Cyrillic: 

0061       ; Latn; Cyrl; L #     (a)    LATIN SMALL LETTER A
0063..0065 ; Latn; Cyrl; L # [3] (c..e) LATIN SMALL LETTER C..LATIN SMALL LETTER E
...
0292       ; Latn; Cyrl; L #     (ʒ)    LATIN SMALL LETTER EZH

They logically form a tuple <Latin, [a c-e ... \u0292], Cyrillic>, which indicates that a Latin string containing characters only from that Unicode set can have a whole-script confusable in Cyrillic (lowercase-only). Note that if the implementation needs a set of allowed characters that is different from those in Section 3.1, General Security Profile for Identifiers, this process needs to be used to generate a different set of data.

To test whether a single-script string givenString has a whole-script confusable in targetScript, the following process is used:

  1. Convert the givenString to NFD format, as specified in [UAX15]
  2. Let givenSet be the set of all characters in givenString
  3. Remove all [:script=common:] and [:script=inherited:] characters from givenSet
  4. Let givenScript be the script of the characters in givenSet
    • (if there is more than one script, fail with error).
  5. See if there is a tuple <sourceScript, unicodeSet, targetScript> where
    • sourceScript = givenScript
    • unicodeSet ⊇ givenSet
  6. If so, then there is a whole-script confusable in targetScript

The test is actually slightly broader than a whole-script confusable test. It tests whether the given string has a whole-script confusable string in another script, possibly with the addition or removal of common/inherited characters such as numbers and combining marks characters to both strings. In practice, however, this broadening has no significant impact.

Implementations would normally read the data into appropriate data structures in memory for processing. A quick additional optimization is to keep, for each script, a fastReject set, containing characters in the script contained in none of the unicodeSet values.

The following Java sample shows how this can be done (using the Java version of [ICU]): 

/*
 * For this routine, we do not care what the target scripts are,
 * just whether there is at least one whole-script confusable.
 */ 
boolean hasWholeScriptConfusable(String s) {
	int givenScript = getSingleScript(s);
	if (givenScript == UScript.INVALID_CODE) {
		throw new IllegalArgumentException("Not single script string");
	}
	UnicodeSet givenSet = new UnicodeSet()
		.addAll(s)
		.removeAll(commonAndInherited);
	if (fastReject[givenScript].containsSome(givenSet)) return false;
	UnicodeSet[] possibles = scriptToUnicodeSets[givenScript];
	for (int i = 0; i < possibles.length; ++i) {
		if (possibles[i].containsAll(givenSet)) return true;
	}
	return false;
}

The data in [confusablesWS] is built using the data in [confusables], and subject to the same caveat: The data in these files may be refined and extended over time. For information on handling that, see Section 2.9.1, Backward Compatibility of [UTR36].

4.2 Mixed-Script Confusables

To test for mixed-script confusables, use the following process:

  1. Convert the given string to NFD format, as specified in [UAX15].
  2. For each script found in the given string, see if all the characters in the string outside of that script have whole-script confusables for that script (according to Section 4.1, Whole-Script Confusables) .

Example 1: "pаypаl", with Cyrillic "а"s.

There are two scripts, Latin and Cyrillic. The set of Cyrillic characters {a}  has a whole-script confusable in Latin. Thus the string is a mixed-script confusable.

Example 2: "toys-я-us", with one Cyrillic character "я".

The set of Cyrillic characters {я} does not have a whole-script confusable in Latin (there is no Latin character that looks like "я", nor does the set of Latin characters {o s t u y} have a whole-script confusable in Cyrillic (there is no Cyrillic character that looks like "t" or "u"). Thus this string is not a mixed-script confusable.

Example 3: "1iνе", with a Greek "ν" and Cyrillic "е".

There are three scripts, Latin, Greek, and Cyrillic. The set of Cyrillic characters {е} and the set of Greek characters {ν} each have a whole-script confusable in Latin. Thus the string is a mixed-script confusable.

5 Detection Mechanisms

5.1 Mixed-Script Detection

The Unicode Standard supplies information that can be used for determining the script of characters and detecting mixed-script text. The determination of script is according to the Unicode Standard Annex #24, "Unicode Script Property" [UAX24] , using data from the Unicode Character Database [UCD]. For a given input string, the logical process is the following:

Define a set of sets of scripts SOSS.

For each character in the string:

  1. Use the Script_Extensions property to find the set of scripts that the character has.
  2. Remove Common and Inherited from that set of scripts.
  3. If the result is not empty, add that set to SOSS.

If no single script is common to all of the sets in SOSS, then the string contains mixed scripts.

Characters with the script values Common and Inherited are ignored, because they are used with more than one script. For example, "abc-def" counts as a single script Latin because the script of "-" is ignored.

A set of scripts S is said to cover a SOSS if S intersects each element of SOSS. For example, {Latin, Greek} covers {{Latin, Georgian}, {Greek, Cyrillic}}, because:

  1. {Latin, Greek} intersects {Latin, Georgian} (the intersection being {Latin}).
  2. {Latin, Greek} intersects {Greek, Cyrillic} (the intersection being {Greek}).

The actual implementation of this algorithm can be optimized; as usual, the specification only depends on the results. The following Java sample using [ICU] shows how the above process can be implemented: 

    public static boolean isMultiScript(String identifier) {
        // Non-optimized code, for simplicity
        Set<BitSet> setOfScriptSets = new HashSet<BitSet>();
        BitSet temp = new BitSet();
        int cp;
        for (int i = 0; i < identifier.length(); i += Character.charCount(i)) {
            cp = Character.codePointAt(identifier, i);
            UScript.getScriptExtensions(cp, temp);
            if (temp.cardinality() == 0) {
                // HACK for older version of ICU
                final int script = UScript.getScript(cp);
                temp.set(script);
            }
            temp.andNot(COMMON_AND_INHERITED);
            if (temp.cardinality() != 0 && setOfScriptSets.add(temp)) {
                // If the set hasn't been added already,
                // add it and create new temporary for the next pass,
                // so we don't rewrite what's already in the set.
                temp = new BitSet();
            }
        }
        if (setOfScriptSets.size() == 0) {
            return true; // trivially true
        }
        temp.clear();
        // check to see that there is at least one script common to all the sets
        boolean first = true;
        for (BitSet other : setOfScriptSets) {
            if (first) {
                temp.or(other);
                first = false;
            } else {
                temp.and(other);
            }
        }
        return temp.cardinality() != 0;
    }

This formulation ignores Common and Inherited scripts, and returns an error when a string contains mixed scripts.

5.2 Restriction-Level Detection

Restriction Levels 1-5 are defined here for use in implementations. These place restrictions on the use of identifiers according to the appropriate Identifier Profile as specified in Section 3, Identifier Characters. The lists of Recommended and Aspirational scripts are taken from Table 5, Recommended Scripts and Table 6, Aspirational Use Scripts of [UAX31]. For more information on the use of Restriction Levels, see Section 2.9 Restriction Levels and Alerts in [UTR36].

Whenever scripts are tested for in the following definitions, characters with Script_Extension=Common and Script_Extension=Inherited are ignored.

  1. ASCII-Only
    • All characters in each identifier must be ASCII
  2. Single Script
    • All characters in each identifier must be from a single script.
  3. Highly Restrictive
    • All characters in each identifier must be from a single script, or from the combinations:
      • Latin + Han + Hiragana + Katakana;
      • Latin + Han + Bopomofo; or
      • Latin + Han + Hangul
    • No characters in the identifier can be outside of the Identifier Profile
    Note that this level will satisfy the vast majority of users.
  4. Moderately Restrictive
    • Allow Latin with other Recommended or Aspirational scripts except Cyrillic and Greek
    • Otherwise, the same as Highly Restrictive
  5. Minimally Restrictive
    • Allow arbitrary mixtures of scripts, such as Ωmega, Teχ, HλLF-LIFE, Toys-Я-Us.
    • Otherwise, the same as Moderately Restrictive
  6. Unrestricted
    • Any valid identifiers, including characters outside of the Identifier Profile, such as I♥NY.org

These levels can be detected by reusing some of the mechanisms of Section 5.1. For a given input string, the Restriction Level is determined by the following logical process:

  1. If the string contains any characters outside of the identifer profile, return Unrestricted.
  2. If no character in the string is above 0x7F, return ASCII.
  3. Compute SOSS as in Mixed Script Detection.
  4. If a single script covers SOSS, return Single Script.
  5. If any of the following sets cover SOSS, return Highly Restrictive.
    • {Latin, Han, Hiragana, Katakana}
    • {Latin, Han, Bopomofo}
    • {Latin, Han, Hangul}
  6. Remove Latin from each element of SOSS. Then if SOSS contains any single Recommended or Aspirational script except Cyrillic or Greek, return Moderately Restrictive.
  7. Otherwise, return Minimally Restrictive.

The actual implementation of this algorithm can be optimized; as usual, the specification only depends on the results.

5.3 Mixed-Number Detection

There are three different types of numbers in Unicode. Only numbers with General_Category = Decimal_Numbers (Nd) should be allowed in identifiers. However, characters from different decimal number systems can be easily confused. For example, U+0660 ( ٠ ) ARABIC-INDIC DIGIT ZERO can be confused with U+06F0 ( ۰ ) EXTENDED ARABIC-INDIC DIGIT ZERO, and U+09EA ( ৪ ) BENGALI DIGIT FOUR can be confused with U+0038 ( 8 ) DIGIT EIGHT.

For a given input string which does not contain non-decimal numbers, the logical process of detecting mixed numbers is the following:

For each character in the string:

  1. Find the decimal number value for that character, if any.
  2. Map the value to the unique zero character for that number system.

If there is more than one such zero character, then the string contains multiple decimal number systems.

The actual implementation of this algorithm can be optimized; as usual, the specification only depends on the results. The following Java sample using [ICU] shows how this can be done : 

    public UnicodeSet getNumberRepresentatives(String identifier) {
        int cp;
        UnicodeSet numerics = new UnicodeSet();
        for (int i = 0; i < identifier.length(); i += Character.charCount(i)) {
            cp = Character.codePointAt(identifier, i);
            // Store a representative character for each kind of decimal digit
            switch (UCharacter.getType(cp)) {
            case UCharacterCategory.DECIMAL_DIGIT_NUMBER:
                // Just store the zero character as a representative for comparison. 
                // Unicode guarantees it is cp - value.
                numerics.add(cp - UCharacter.getNumericValue(cp));
                break;
            case UCharacterCategory.OTHER_NUMBER:
            case UCharacterCategory.LETTER_NUMBER:
                throw new IllegalArgumentException("Should not be in identifiers.");
            }
        } 
        return numerics;
    }
...
    UnicodeSet numerics = getMixedNumbers(String identifier);
    if (numerics.size() > 1) reject(identifer, numerics);

5.4 Optional Detection

There are additional enhancements that may be useful in spoof detection. This includes such mechanisms as marking strings as "mixed script" where they contain both simplified-only and traditional-only Chinese characters, using the Unihan data in the Unicode Character Database [UCD], or detecting sequences of the same nonspacing mark.

Other enhancements useful in spoof detection include the following:

  1. Mark Chinese strings as "mixed script" if they contain both simplified (S) and traditional (T) Chinese characters, using the Unihan data in the Unicode Character Database [UCD].
    1. The criterion can only be applied if the language of the string is known to be Chinese. So, for example, the string “写真だけの結婚式 ” is Japanese, and should not be marked as mixed script because of a mixture of S and T characters.
    2. Testing for whether a character is S or T needs to be based not on whether the character has a S or T variant , but whether the character is an S or T variant.
  2. Forbid sequences of the same nonspacing mark
  3. Check to see that all the characters are in the sets of exemplar characters for at least one language in the Unicode Common Locale Data Repository [CLDR].

6 Development Process

As discussed in Unicode Technical Report #36, "Unicode Security Considerations" [UTR36], confusability among characters cannot be an exact science. There are many factors that make confusability a matter of degree:

In-script confusability is extremely user-dependent. For example, in the Latin script, characters with accents or appendices may look similar to the unadorned characters for some users, especially if they are not familiar with their meaning in a particular language. However, most users will have at least a minimum understanding of the range of characters in their own script, and there are separate mechanisms available to deal with other scripts, as discussed in [UTR36].

As described elsewhere, there are cases where the confusable data may be different than expected. Sometimes this is because two characters or two strings may only be confusable in some fonts. In other cases, it is because of transitivity. For example, the dotless and dotted I are considered equivalent (ı ↔ i), because they look the same when accents such as an acute are applied to each. However, for practical implementation usage, transitivity is sufficiently important that some oddities are accepted.

The data may be enhanced in future versions of this specification. For information on handling changes in data over time, see Section 2.9.1, Backward Compatibility of [UTR36].

6.1 Confusables Data Collection

The confusability data was created by collecting a number of prospective confusables, examining those confusables according to a set of common fonts, and processing the result for transitive closure.

The primary goal is to include characters that would be Status=Allowed as in Table 1. Identifier Status and Type. Other characters, such as NFKC variants, are not a primary focus for data collection. However, such variants may certainly be included in the data, and may be submitted using the online forms at [Feedback].

The prospective confusables were gathered from a number of sources. Erik van der Poel contributed a list derived from running a program over a large number of fonts to catch characters that shared identical glyphs within a font, and Mark Davis did the same more recently for fonts on Windows and the Macintosh. Volunteers from Google, IBM, Microsoft and other companies gathered other lists of characters. These included native speakers for languages with different writing systems. The Unicode compatibility mappings were also used as a source. The process of gathering visual confusables is ongoing: the Unicode Consortium welcomes submission of additional mappings. The complex scripts of South and Southeast Asia need special attention. The focus is on characters that can be in the Recommended profile for identifiers, because they are of most concern.

The fonts used to assess the confusables included those used by the major operating systems in user interfaces. In addition, the representative glyphs used in the Unicode Standard were also considered. Fonts used for the user interface in operating systems are an important source, because they are the ones that will usually be seen by users in circumstances where confusability is important, such such as when using IRIS (Internationalized Resource Identifiers) and their sub-elements (such as domain names). These fonts have a number of other relevant characteristics:

Pairs of prospective confusables were removed if they were always visually distinct at common sizes, both within and across fonts. The data was then closed under transitivity, so that if X≅Y and Y≅Z, then X≅Z. In addition, the data was closed under substring operations, so that if X≅Y then AXB≅AYB. It was then processed to produce the in-script and cross-script data, so that a single data table can be used to map an input string to a resulting skeleton.

A skeleton is intended only for internal use for testing confusability of strings; the resulting text is not suitable for display to users, because it will appear to be a hodgepodge of different scripts. In particular, the result of mapping an identifier will not necessary be an identifier. Thus the confusability mappings can be used to test whether two identifiers are confusable (if their skeletons are the same), but should definitely not be used as a "normalization" of identifiers.

6.2 Identifier Modification Data Collection

The idmod data is gathered in the following way. The basic assignments are derived based on UCD character properties, information in [UAX31], and a curated list of exceptions based on information from various sources, including the core specification of the Unicode Standard, annotations in the code charts, information regarding CLDR exemplar characters, and external feedback.

The first condition that matches in the order of the items from top to bottom in Table 1. Identifier Status and Type is used, with a few exceptions:

  1. When a character is in Table 3, Candidate Characters for Inclusion in Identifiers in [UAX31], then it is given the Type Inclusion, regardless of other properties.
  2. When the Script_Extensions property value for a character contains multiple Script property values, the Script used for the derivation is the first in the following list:
    1. Table 5, Recommended Scripts 
    2. Table 6, Aspirational Use Scripts
    3. Table 7, Limited Use Scripts
    4. Table 4, Candidate Characters for Exclusion from Identifiers
      • Table 4 also has some conditions that are not dependent on script; those conditions are applied regardless of Script_Extensions property value.

The script information in Table 4, Table 5, Table 6 and Table 7 are in machine-readable form in CLDR, as scriptMetadata.txt.

7 Data Files

The following files provide data used to implement the recommendations in this document. The data may be refined in future versions of this specification. For more information, see Section 2.9.1, Backward Compatibility of [UTR36].

The Unicode Consortium welcomes feedback on additional confusables or identifier restrictions. There are online forms at [Feedback] where you can suggest additional characters or corrections.

The files are in http://www.unicode.org/Public/security/. The directories there contain data files associated with a given version. The directory for this version is:

http://www.unicode.org/Public/security/8.0.0

The data files for the latest approved version are also in the directory:

http://www.unicode.org/Public/security/latest

Data File List
[idmod] xidmodifications.txt Identifier Modifications: Provides the list of additions and restrictions recommended for building a profile of identifiers for environments where security is at issue.
[confusables] confusables.txt Visually Confusable Characters: Provides a mapping for visual confusables for use in detecting possible security problems. The usage of the file is described in Section 4, Confusable Detection.
[confusablesSummary] confusablesSummary.txt A summary view of the confusables: Groups each set of confusables together, listing them first on a line starting with #, then individually with names and code points. See Section 4, Confusable Detection
[confusablesWS] confusablesWholeScript.txt Whole Script Confusables: Data for testing for the possible existence of whole-script and mixed-script confusables. See Section 4, Confusable Detection
[intentional] intentional.txt Intentional Confusable Mappings: A selection of characters whose glyphs in any particular typeface would probably be designed to be identical in shape when using a harmonized typeface design.

Migration

Beginning with version 6.3.0, the version numbering of this document has been changed to indicate the version of the UCD that the data is based on. For versions up to and including 6.3.0, the following table shows the correspondence between the versions of this document and UCD versions that they were based on.

Version Correspondence
Version Release Date Data File Directory UCD Version UCD Date
Version 1 2006-08-15 /Public/security/revision-02/ 5.1.0 2008-04
draft only 2006-08-11 /Public/security/revision-03/ n/a n/a
Version 2 2010-08-05 /Public/security/revision-04/ 6.0.0 2010-10
Version 3 2012-07-23 /Public/security/revision-05/ 6.1.0 2012-01
6.3.0 2013-11-11 /Public/security/6.3.0/ 6.3.0 2013-09


If an update version of this standard is required between the associated UCD versions, the version numbering will include an update number in the 3rd field. For example, if a version of this document and its associated data is needed between UCD 6.3.0 and UCD 7.0.0, then a version 6.3.1 could be used.

Migrating Persistent Data

Implementations must migrate their persistent data stores (such as database indexes) whenever those implementations update to use the data files from a new version of this specification.

Stability is never guaranteed between versions, although it is maintained where feasible. In particular, an updated version of confusable mapping data may use a mapping for a particular character that is different from the mapping used for that character in an earlier version. Thus there may be cases where X → Y in Version N, and X → Z in Version N+1, where Z may or may not have mapped to Y in Version N. Even in cases where the logical data has not changed between versions, the order of lines in the data files may have been changed.

Implementations should therefore have a strategy for migrating their persistent data stores (such as database indexes) that use any of the confusable mapping data or other data files.

Version 8.0 Migration

In Version 8.0, the following changes were made to the Identifier Status and Type:

Version 7.0 Migration

Due to production problems, versions of the confusable mapping tables before 7.0 did not maintain idempotency in all cases, so updating to version 8.0 is strongly advised.

Anyone using the skeleton mappings needs to rebuild any persistent uses of skeletons, such as in database indexes.

The SL, SA, and ML mappings in 7.0 were significantly changed to address the idempotency problem. However, the tables SL, SA, and ML were still problematic, and discouraged from use in 7.0. They were thus removed from version 8.0.

All of the data necessary for an implementation to recreate the removed tables is available in the remaining data (MA) plus the Unicode Character Database properties (script, casing, etc.). Such a recreation would examine each of the equivalence classes from the MA data, and filter out instances that didn't fit the constraints (of script or casing). For the target character, it would choose the most neutral character, typically a symbol. However, the reasons for deprecating them still stand, so it is not recommended that implementations recreate them.

Note also that as the Script_Extensions data is made more complete, it may cause characters in the whole-script confusables data file to no longer match. For more information, see Section 4 Confusable Detection.

Acknowledgments

Mark Davis and Michel Suignard authored the bulk of the text, under direction from the Unicode Technical Committee. Steven Loomis and other people on the ICU team were very helpful in developing the original proposal for this technical report. Thanks also to the following people for their feedback or contributions to this document or earlier versions of it, or to the source data for confusables or idmod: Julie Allen, Andrew Arnold, David Corbett, Douglas Davidson, Chris Fynn, Martin Dürst, Asmus Freytag, Deborah Goldsmith, Paul Hoffman, Denis Jacquerye, Cibu Johny, Patrick L. Jones, Peter Karlsson, Mike Kaplinskiy, Gervase Markham, Eric Muller, Erik van der Poel, Roozbeh Pournader, Michael van Riper, Marcos Sanz, Alexander Savenkov, Dominikus Scherkl, Chris Weber, Kenneth Whistler, and Waïl Yahyaoui. Thanks to Peter Peng for his assistance with font confusables.

References

[CLDR] Unicode Locales Project (Unicode Common Locale Data Repository)
http://www.unicode.org/cldr/
[DCore] Derived Core Properties
http://www.unicode.org/Public/UNIDATA/DerivedCoreProperties.txt
[DemoConf] http://unicode.org/cldr/utility/confusables.jsp
[DemoIDN] http://unicode.org/cldr/utility/idna.jsp
[DemoIDNChars] http://unicode.org/cldr/utility/list-unicodeset.jsp?a=\p{age%3D3.2}-\p{cn}-\p{cs}-\p{co}&abb=on&uts46+idna+idna2008
[FAQSec] Unicode FAQ on Security Issues
http://www.unicode.org/faq/security.html
[ICANN] ICANN Documents:
Internationalized Domain Names
http://www.icann.org/en/topics/idn/
The IDN Variant Issues Project
http://www.icann.org/en/topics/new-gtlds/idn-vip-integrated-issues-23dec11-en.pdf
Maximal Starting Repertoire Version 2 (MSR-2)
https://www.icann.org/news/announcement-2-2015-04-27-en
[ICU] International Components for Unicode
http://site.icu-project.org/
[IDNA2003] The IDNA2003 specification is defined by a cluster of IETF RFCs:
[IDNA2008] The IDNA2008 specification is defined by a cluster of IETF RFCs: There are also informative documents:
[IDN-FAQ] http://www.unicode.org/faq/idn.html
[Feedback] To suggest additions or changes to confusables or identifier restriction data, please see:
http://unicode.org/reports/tr39/suggestions.html

For issues in the text, please see:
Reporting Errors and Requesting Information Online
http://www.unicode.org/reporting.html
[Reports] Unicode Technical Reports
http://www.unicode.org/reports/
For information on the status and development process for technical reports, and for a list of technical reports.
[RFC3454] P. Hoffman, M. Blanchet. "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002.
http://ietf.org/rfc/rfc3454.txt
[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003.
http://ietf.org/rfc/rfc3490.txt
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003.
http://ietf.org/rfc/rfc3491.txt
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003.
http://ietf.org/rfc/rfc3492.txt
[Security-FAQ] http://www.unicode.org/faq/security.html
[UCD] Unicode Character Database.
http://www.unicode.org/ucd/
For an overview of the Unicode Character Database and a list of its associated files.
[UCDFormat] UCD File Format
http://www.unicode.org/reports/tr44/#Format_Conventions
[UAX15] UAX #15: Unicode Normalization Forms
http://www.unicode.org/reports/tr15/
[UAX24] UAX #24: Unicode Script Property
http://www.unicode.org/reports/tr24/
[UAX29] UAX #29: Unicode Text Segmentation
http://www.unicode.org/reports/tr29/
[UAX31] UAX #31: Unicode Identifier and Pattern Syntax
http://www.unicode.org/reports/tr31/
[Unicode] The Unicode Standard
For the latest version, see:
http://www.unicode.org/versions/latest/
[UTR36] UTR #36: Unicode Security Considerations
http://www.unicode.org/reports/tr36/
[UTS18] UTS #18: Unicode Regular Expressions
 http://www.unicode.org/reports/tr18/
[UTS39] UTS #39: Unicode Security Mechanisms
http://www.unicode.org/reports/tr39/
[UTS46] Unicode IDNA Compatibility Processing
http://www.unicode.org/reports/tr46/
[Versions] Versions of the Unicode Standard
http://www.unicode.org/standard/versions/
For information on version numbering, and citing and referencing the Unicode Standard, the Unicode Character Database, and Unicode Technical Reports.

 

Modifications

The following summarizes modifications from the previous revision of this document.

Revision 11

Revision 10 being a proposed update, only changes between revisions 9 and 11 are noted here.

Revision 9

Revision 8 being a proposed update, only changes between revisions 7 and 9 are noted here.

Revision 7

Revision 6 being a proposed update, only changes between revisions 5 and 7 are noted here.

Revision 5

Revision 4

Revision 3 being a proposed update, only changes between revisions 2 and 4 are noted here.

Revision 2

Revision 1

Copyright © 2004-2015 Unicode, Inc. All Rights Reserved. The Unicode Consortium makes no expressed or implied warranty of any kind, and assumes no liability for errors or omissions. No liability is assumed for incidental and consequential damages in connection with or arising out of the use of the information or programs contained or accompanying this technical report. The Unicode Terms of Use apply.

Unicode and the Unicode logo are trademarks of Unicode, Inc., and are registered in some jurisdictions.