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RFC 3454

Preparation of Internationalized Strings ("stringprep")

Pages: 91
Obsoleted by:  7564
Part 1 of 3 – Pages 1 to 22
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Network Working Group                                         P. Hoffman
Request for Comments: 3454                                    IMC & VPNC
Category: Standards Track                                    M. Blanchet
                                                                Viagenie
                                                           December 2002


        Preparation of Internationalized Strings ("stringprep")

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

This document describes a framework for preparing Unicode text strings in order to increase the likelihood that string input and string comparison work in ways that make sense for typical users throughout the world. The stringprep protocol is useful for protocol identifier values, company and personal names, internationalized domain names, and other text strings. This document does not specify how protocols should prepare text strings. Protocols must create profiles of stringprep in order to fully specify the processing options.

Table of Contents

1. Introduction....................................................3 1.1 Terminology..................................................4 1.2 Using stringprep in protocols................................4 2. Preparation Overview............................................6 3. Mapping.........................................................7 3.1 Commonly mapped to nothing...................................7 3.2 Case folding.................................................8 4. Normalization...................................................9 5. Prohibited Output..............................................10 5.1 Space characters............................................11 5.2 Control characters..........................................11 5.3 Private use.................................................12
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     5.4 Non-character code points...................................12
     5.5 Surrogate codes.............................................13
     5.6 Inappropriate for plain text................................13
     5.7 Inappropriate for canonical representation..................13
     5.8 Change display properties or deprecated.....................13
     5.9 Tagging characters..........................................14
   6. Bidirectional Characters.......................................14
   7. Unassigned Code Points in Stringprep Profiles..................15
     7.1 Categories of code points...................................16
     7.2 Reasons for difference between stored strings and queries...17
     7.3 Versions of applications and stored strings.................18
   8. References.....................................................19
     8.1 Normative references........................................19
     8.2 Informative references......................................19
   9. Security Considerations........................................19
     9.1 Stringprep-specific security considerations.................19
     9.2 Generic Unicode security considerations.....................20
   10. IANA Considerations...........................................21
   11. Acknowledgements..............................................22
   A. Unicode repertoires............................................23
     A.1 Unassigned code points in Unicode 3.2.......................23
   B. Mapping Tables.................................................31
     B.1 Commonly mapped to nothing..................................31
     B.2 Mapping for case-folding used with NFKC.....................32
     B.3 Mapping for case-folding used with no normalization.........61
   C. Prohibition tables.............................................78
     C.1 Space characters............................................78
       C.1.1 ASCII space characters..................................78
       C.1.2 Non-ASCII space characters..............................79
     C.2 Control characters..........................................79
       C.2.1 ASCII control characters................................79
       C.2.2 Non-ASCII control characters............................79
     C.3 Private use.................................................80
     C.4 Non-character code points...................................80
     C.5 Surrogate codes.............................................80
     C.6 Inappropriate for plain text................................80
     C.7 Inappropriate for canonical representation..................81
     C.8 Change display properties or are deprecated.................81
     C.9 Tagging characters..........................................81
   D. Bidirectional tables...........................................81
     D.1 Characters with bidirectional property "R" or "AL"..........81
     D.2 Characters with bidirectional property "L"..................82
   Authors' Addresses................................................90
   Full Copyright Statement..........................................91
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1. Introduction

Application programs can display text in many different ways. Similarly, a user can enter text into an application program in a myriad of fashions. Internationalized text (that is, text that is not restricted to the narrow set of US-ASCII characters) has many input and display behaviors that make it difficult to compare text in a consistent fashion. This document specifies a framework of processing rules for Unicode text. Other protocols can create profiles of these rules; these profiles will allow users to enter internationalized text strings in applications and have the highest chance of getting the content of the strings correct. In this case, "correct" means that if two different people enter what they think is the same string into two different input mechanisms, the strings should match on a character- by-character basis. This framework does not describe how data is transcoded from other character sets into Unicode. In systems that uses non-Unicode character sets, the transcoding algorithm is a critical part of enabling secure and "correct" operation of internationalized text strings. In addition to helping string matching, profiles of stringprep can also exclude characters that should not normally appear in text that is used in the protocol. The profile can prevent such characters by changing the characters to be excluded to other characters, by removing those characters, or by causing an error if the characters would appear in the output. For example, because the backspace character can cause unpredictable display results, a profile can specify that a string containing a backspace character would cause an error. A profile of stringprep converts a single string of input characters to a string of output characters, or returns an error if the output string would contain a prohibited character. Stringprep profiles cannot both emit a string and return an error. Stringprep profiles cannot account for all of the variations that might occur or that a user might expect. In particular, a profile will not be able to account for choice of spellings in all languages for all scripts because the number of alternative spellings of words and phrases is immense. Users would probably expect all spelling equivalents to be made equivalent, or none of them to be. Examples of spelling equivalents include "theater" vs. "theatre", and "hemoglobin" vs. "h<U+00E6>moglobin" in American vs. British English. Other examples are simplified Chinese spellings of names (for
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   example,"<U+7EDF><U+4E00><U+7801>") vs. the equivalent traditional
   Chinese spelling (for example, "<U+7D71><U+4E00><U+78BC>").
   Language-specific equivalences such as "Aepfel" vs. "<U+00C4>pfel",
   which are sometimes considered equivalent in German, may not be
   considered equivalent in other languages.

1.1 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119]. Note: A glossary of terms used in Unicode and ISO/IEC 10646 can be found in [Glossary]. Information on the 10646/Unicode character encoding model can be found in [CharModel]. Character names in this document use the notation for code points and names from the Unicode Standard [Unicode3.2] and ISO/IEC 10646 [ISO10646]. For example, the letter "a" may be represented as either "U+0061" or "LATIN SMALL LETTER A". In the lists of mappings and the prohibited characters, the "U+" is left off to make the lists easier to read. The comments for character ranges are shown in square brackets (such as "[CONTROL CHARACTERS]") and do not come from the standards.

1.2 Using stringprep in protocols

The stringprep protocol does not stand on its own; it has to be used by other protocols at precisely-defined places in those other protocols. For example, a protocol that has strings that come from the entire ISO/IEC 10646 [ISO10646] character repertoire might specify that only strings that have been processed with a particular profile of stringprep are legal. Another example would be a protocol that does string comparison as a step in the protocol; that protocol might specify that such comparison is done only after processing the strings with a specific profile of stringprep. When two protocols that use different profiles of stringprep interoperate, there may be conflict about what characters are and are not allowed in the final string. Thus, protocol developers should strongly consider re-using existing profiles of stringprep. When developers wish to allow users as wide of a range of characters as possible in input text strings, they should, where possible, cause stringprep to convert characters from the input string to a canonical form instead of prohibiting them.
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   Although it would be easy to use the stringprep process to "correct"
   perceived mis-features or bugs in the current character standards,
   stringprep profiles SHOULD NOT do so.

   A profile of stringprep can create tables different from those in the
   appendixes of this document, but it will be an exception when they
   do.  The intention of stringprep is to define the tables and have the
   profiles of stringprep select among those defined tables.

   A profile of stringprep MUST include all of the following:

   - The intended applicability of the profile

   - The character repertoire that is the input and output to stringprep
     (which is Unicode 3.2 for this version of stringprep)

   - The mapping tables from this document used (as described in section
     3)

   - Any additional mapping tables specific to the profile

   - The Unicode normalization used, if any (as described in section 4)

   - The tables from this document of characters that are prohibited as
     output (as described in section 5)

   - The bidirectional string testing used, if any (as described in
     section 6)

   - Any additional characters that are prohibited as output specific to
     the profile

   Each profile MUST state the character repertoire on which the profile
   will operate.  Appendix A lists the Unicode repertoires that can be
   selected.  No repertoire is ever complete, and it is expected that
   characters will be added to the Unicode repertoire for the
   foreseeable future.  Section 7 of this document describes how to
   handle characters that are assigned in later versions of the Unicode
   repertories.  Subsections of appendix A also list unassigned code
   points for each repertoire.

   This document is for Unicode version 3.2, and should not be
   considered to automatically apply to later Unicode versions.  The
   IETF, through an explicit standards action, may update this document
   as appropriate to handle later Unicode versions.
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   This document lists the unassigned code points in the range 0 to
   10FFFF for Unicode 3.2 in appendix A.  The list in appendix A MUST be
   used by implementations of this specification.  If there are any
   discrepancies between the list in appendix A and the Unicode 3.2
   specification, the list in appendix A always takes precedence.

   Each profile of stringprep MUST be registered with IANA.  The
   registration procedure is described in the IANA Considerations
   appendix; basically, the IESG must review each profile of stringprep.
   Protocol developers are strongly encouraged to look through the IANA
   profile registry when creating new profiles for stringprep, and to
   re-use logic from earlier profiles where possible in new profiles.
   In some cases, an existing profile can be reused by a different
   protocol.

2. Preparation Overview

The steps for preparing strings are: 1) Map -- For each character in the input, check if it has a mapping and, if so, replace it with its mapping. This is described in section 3. 2) Normalize -- Possibly normalize the result of step 1 using Unicode normalization. This is described in section 4. 3) Prohibit -- Check for any characters that are not allowed in the output. If any are found, return an error. This is described in section 5. 4) Check bidi -- Possibly check for right-to-left characters, and if any are found, make sure that the whole string satisfies the requirements for bidirectional strings. If the string does not satisfy the requirements for bidirectional strings, return an error. This is described in section 6. The above steps MUST be performed in the order given to comply with this specification. The mappings described in section 3, and the optional Unicode normalization described in section 4, can be one-to-none, one-to-one, one-to-many, many-to-one, or many-to-many. That is, some characters might be eliminated or replaced by more than one character, and the output of this step might be shorter or longer than the input. Because of this, the system using stringprep MUST be prepared to receive a longer or shorter string than the one input in the stringprep algorithm.
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3. Mapping

Each character in the input stream MUST be checked against a mapping table. The mapping table SHOULD come from this document, although the mapping table MAY be added to or altered by the profile. The mapping tables are subsections of appendix B. The lists in appendix B MUST be used by implementations of this specification. If there are any discrepancies between the lists in appendix B and subsections below, the lists in appendix B always takes precedence. For any individual character, the mapping table MAY specify that a character be mapped to nothing, or mapped to one other character, or mapped to a string of other characters. Mapped characters are not re-scanned during the mapping step. That is, if character A at position X is mapped to character B, character B which is now at position X is not checked against the mapping table.

3.1 Commonly mapped to nothing

The following characters are simply deleted from the input (that is, they are mapped to nothing) because their presence or absence in protocol identifiers should not make two strings different. They are listed in Table B.1. Some characters are only useful in line-based text, and are otherwise invisible and ignored. 00AD; SOFT HYPHEN 1806; MONGOLIAN TODO SOFT HYPHEN 200B; ZERO WIDTH SPACE 2060; WORD JOINER FEFF; ZERO WIDTH NO-BREAK SPACE Some characters affect glyph choice and glyph placement, but do not bear semantics. 034F; COMBINING GRAPHEME JOINER 180B; MONGOLIAN FREE VARIATION SELECTOR ONE 180C; MONGOLIAN FREE VARIATION SELECTOR TWO 180D; MONGOLIAN FREE VARIATION SELECTOR THREE 200C; ZERO WIDTH NON-JOINER 200D; ZERO WIDTH JOINER FE00; VARIATION SELECTOR-1 FE01; VARIATION SELECTOR-2
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   FE02; VARIATION SELECTOR-3
   FE03; VARIATION SELECTOR-4
   FE04; VARIATION SELECTOR-5
   FE05; VARIATION SELECTOR-6
   FE06; VARIATION SELECTOR-7
   FE07; VARIATION SELECTOR-8
   FE08; VARIATION SELECTOR-9
   FE09; VARIATION SELECTOR-10
   FE0A; VARIATION SELECTOR-11
   FE0B; VARIATION SELECTOR-12
   FE0C; VARIATION SELECTOR-13
   FE0D; VARIATION SELECTOR-14
   FE0E; VARIATION SELECTOR-15
   FE0F; VARIATION SELECTOR-16

3.2 Case folding

If a profile is going to map characters for case-insensitive comparison, that profile SHOULD map using either appendix B.2 or appendix B.3. appendix B.2 is for profiles that also use Unicode normalization form KC, while appendix B.3 is for profiles that do not use Unicode normalization. These tables map from uppercase to lowercase characters. Note that this could have been "change all lowercase characters into uppercase characters". However, the upper-to-lower folding was chosen because there is a tradition of using lowercase in current Internet applications and protocols. If a profile creates its own mapping tables for case folding, they SHOULD be based on [UTR21], and SHOULD map from uppercase characters to lowercase. The "CaseFolding.txt" file from the Unicode database SHOULD be used to prepare the mapping table. The profile SHOULD do full case mapping (that is, using statuses C, F, and I). If the profile is using Unicode normalization form KC (as described in section 4 of this document), it is important to note that there are some characters that do not have mappings in [UTR21] but still need processing. These characters include a few Greek characters and many symbols that contain Latin characters. The list of characters to add to the mapping table can determined by the following algorithm: b = NormalizeWithKC(Fold(a)); c = NormalizeWithKC(Fold(b)); if c is not the same as b, add a mapping for "a to c". Because NormalizeWithKC(Fold(c)) always equals c, the table is stable from that point on.
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   Appendix B.3 is derived from the CaseFolding-3.txt file associated
   with Unicode 3.2; appendix B.2 is based on appendix B.3 with the
   additional characters added from the algorithm above.

   Authors of profiles of this document need to consider the effects of
   changing the mapping of any currently-assigned character when
   updating their profiles.  Adding a new mapping for a currently-
   assigned character, or changing an existing mapping, could cause a
   variance between the behavior of systems that have been updated and
   systems that have not been updated.

4. Normalization

The output of the mapping step is optionally normalized using one of the Unicode normalization forms, as described in [UAX15]. A profile can specify one of two options for Unicode normalization: - no normalization - Unicode normalization with form KC A profile MAY choose to do no normalization. However, such a profile can easily yield results that will be surprising to typical users, depending on the input mechanism they use. For example, some input mechanisms enter compatibility characters that look exactly like the underlying characters, but have different code points. Another example of where Unicode normalization helps create predictable results is with characters that have multiple combining diacritics: normalization orders those diacritics in a predictable fashion. On the other hand, Unicode normalization requires fairly large tables and somewhat complicated character reordering logic. The size and complexity should not be considered daunting except in the most restricted of environments, and needs to be weighed against the problems of user surprise from comparing unnormalized strings. Note that the tables used for normalization are not given in this document, but instead must be derived from the Unicode database, as described in [UAX15]. There is a third form of normalization, Unicode normalization with form C. If a profile is going to use a Unicode normalization, it MUST use Unicode normalization form KC. Form KC maps many "compatibility characters" to their equivalents. Some user interface systems make it possible to enter compatibility characters instead of the base equivalents. Thus, using form KC instead of form C will cause more strings that users would expect to match to actually match.
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   A profile that specifies Unicode normalization MUST use the
   normalization in [UAX15] that is associated with the version of the
   Unicode character set specified for the profile.

   The composition process described in [UAX15] requires a fixed
   composition version of Unicode to ensure that strings normalized
   under one version of Unicode remain normalized under all future
   versions of Unicode.

   The IETF is relying on Unicode not to change the normalization of
   currently-assigned characters in future versions of normalization.
   If a future version of the normalization tables changes the
   normalized value of an existing character, authors of profiles of
   this document have to look at the changes very carefully before they
   update their normalization tables.  Such a change could cause a
   variance between the behavior of systems that have been updated and
   systems that have not been updated.

5. Prohibited Output

Before the text can be emitted, it MUST be checked for prohibited code points. There are a variety of prohibited code points, as described in this section. A profile of this document MAY use all or some of the tables in appendix C. The stringprep process never emits both an error and a string. If an error is detected during the checking for prohibited code points, only an error is returned. Note that the subsections below describe how the tables in appendix C were formed. They are here for people who want to understand more, but they should be ignored by implementors. Implementations that use tables MUST map based on the tables themselves, not based on the descriptions in this section of how the tables were created. The lists in appendix C MUST be used by implementations of this specification. If there are any discrepancies between the lists in appendix C and subsections below, the lists in appendix C always take precedence. Some code points listed in one section may also appear in other sections. It is important to note that a profile of this document MAY prohibit additional characters.
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   Each subsection of this section has a matching subsection in appendix
   C.  For example, the characters listed in section 5.1 are listed in
   appendix C.1.

5.1 Space characters

Space characters can make accurate visual transcription of strings nearly impossible and could lead to user entry errors in many ways. Note that the list below is split into two tables in appendix C: Table C.1.1 contains the ASCII code points, while Table C.1.2 contains the non-ASCII code points. Most profiles of this document that want to prohibit space characters will want to include both tables. 0020; SPACE 00A0; NO-BREAK SPACE 1680; OGHAM SPACE MARK 2000; EN QUAD 2001; EM QUAD 2002; EN SPACE 2003; EM SPACE 2004; THREE-PER-EM SPACE 2005; FOUR-PER-EM SPACE 2006; SIX-PER-EM SPACE 2007; FIGURE SPACE 2008; PUNCTUATION SPACE 2009; THIN SPACE 200A; HAIR SPACE 200B; ZERO WIDTH SPACE 202F; NARROW NO-BREAK SPACE 205F; MEDIUM MATHEMATICAL SPACE 3000; IDEOGRAPHIC SPACE

5.2 Control characters

Control characters (or characters with control function) cannot be seen and can cause unpredictable results when displayed. Note that the list below is split into two tables in appendix C: Table C.2.1 contains the ASCII code points, while Table C.2.2 contains the non- ASCII code points. Most profiles of this document that want to prohibit control characters will want to include both tables. 0000-001F; [CONTROL CHARACTERS] 007F; DELETE 0080-009F; [CONTROL CHARACTERS] 06DD; ARABIC END OF AYAH 070F; SYRIAC ABBREVIATION MARK 180E; MONGOLIAN VOWEL SEPARATOR
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   200C; ZERO WIDTH NON-JOINER
   200D; ZERO WIDTH JOINER
   2028; LINE SEPARATOR
   2029; PARAGRAPH SEPARATOR
   2060; WORD JOINER
   2061; FUNCTION APPLICATION
   2062; INVISIBLE TIMES
   2063; INVISIBLE SEPARATOR
   206A-206F; [CONTROL CHARACTERS]
   FEFF; ZERO WIDTH NO-BREAK SPACE
   FFF9-FFFC; [CONTROL CHARACTERS]
   1D173-1D17A; [MUSICAL CONTROL CHARACTERS]

5.3 Private use

Because private-use characters do not have defined meanings, they are likely to be prohibited. The private-use characters are: E000-F8FF; [PRIVATE USE, PLANE 0] F0000-FFFFD; [PRIVATE USE, PLANE 15] 100000-10FFFD; [PRIVATE USE, PLANE 16]

5.4 Non-character code points

Non-character code points are code points that have been allocated in ISO/IEC 10646 but are not characters. Because they are already assigned, they are guaranteed not to later change into characters. FDD0-FDEF; [NONCHARACTER CODE POINTS] FFFE-FFFF; [NONCHARACTER CODE POINTS] 1FFFE-1FFFF; [NONCHARACTER CODE POINTS] 2FFFE-2FFFF; [NONCHARACTER CODE POINTS] 3FFFE-3FFFF; [NONCHARACTER CODE POINTS] 4FFFE-4FFFF; [NONCHARACTER CODE POINTS] 5FFFE-5FFFF; [NONCHARACTER CODE POINTS] 6FFFE-6FFFF; [NONCHARACTER CODE POINTS] 7FFFE-7FFFF; [NONCHARACTER CODE POINTS] 8FFFE-8FFFF; [NONCHARACTER CODE POINTS] 9FFFE-9FFFF; [NONCHARACTER CODE POINTS] AFFFE-AFFFF; [NONCHARACTER CODE POINTS] BFFFE-BFFFF; [NONCHARACTER CODE POINTS] CFFFE-CFFFF; [NONCHARACTER CODE POINTS] DFFFE-DFFFF; [NONCHARACTER CODE POINTS] EFFFE-EFFFF; [NONCHARACTER CODE POINTS] FFFFE-FFFFF; [NONCHARACTER CODE POINTS] 10FFFE-10FFFF; [NONCHARACTER CODE POINTS]
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   The non-character code points are listed in the PropList.txt file
   from the Unicode database.

5.5 Surrogate codes

The following code points are permanently reserved for use as surrogate code values in the UTF-16 encoding, will never be assigned to characters in the Unicode repertoire, and are therefore prohibited: D800-DFFF; [SURROGATE CODES]

5.6 Inappropriate for plain text

The following characters do not appear in regular text. FFF9; INTERLINEAR ANNOTATION ANCHOR FFFA; INTERLINEAR ANNOTATION SEPARATOR FFFB; INTERLINEAR ANNOTATION TERMINATOR FFFC; OBJECT REPLACEMENT CHARACTER Although the replacement character (U+FFFD) might be used when a string is displayed, it doesn't make sense for it to be part of the string itself. It is often displayed by renderers to indicate "there would be some character here, but it cannot be rendered". For example, on a computer with no Asian fonts, a string with three ideographs might be rendered with three replacement characters. FFFD; REPLACEMENT CHARACTER

5.7 Inappropriate for canonical representation

The ideographic description characters allow different sequences of characters to be rendered the same way, which makes them inappropriate for strings that have to have a single canonical representation. 2FF0-2FFB; [IDEOGRAPHIC DESCRIPTION CHARACTERS]

5.8 Change display properties or are deprecated

The following characters can cause changes in display or the order in which characters appear when rendered, or are deprecated in Unicode. 0340; COMBINING GRAVE TONE MARK 0341; COMBINING ACUTE TONE MARK 200E; LEFT-TO-RIGHT MARK 200F; RIGHT-TO-LEFT MARK
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   202A; LEFT-TO-RIGHT EMBEDDING
   202B; RIGHT-TO-LEFT EMBEDDING
   202C; POP DIRECTIONAL FORMATTING
   202D; LEFT-TO-RIGHT OVERRIDE
   202E; RIGHT-TO-LEFT OVERRIDE
   206A; INHIBIT SYMMETRIC SWAPPING
   206B; ACTIVATE SYMMETRIC SWAPPING
   206C; INHIBIT ARABIC FORM SHAPING
   206D; ACTIVATE ARABIC FORM SHAPING
   206E; NATIONAL DIGIT SHAPES
   206F; NOMINAL DIGIT SHAPES

5.9 Tagging characters

The following characters are used for tagging text and are invisible. E0001; LANGUAGE TAG E0020-E007F; [TAGGING CHARACTERS]

6. Bidirectional Characters

Most characters are displayed from left to right, but some are displayed from right to left. This feature of Unicode is called "bidirectional text", or "bidi" for short. The Unicode standard has an extensive discussion of how to reorder glyphs for display when dealing with bidirectional text such as Arabic or Hebrew. See [UAX9] for more information. In particular, all Unicode text is stored in logical order. A profile MAY choose to ignore bidirectional text. However, ignoring bidirectional text can cause display ambiguities. For example, it is quite easy to create two different strings with the same characters (but in different order) that are correctly displayed identically. Therefore, in order to avoid most problems with ambiguous bidirectional text display, profile creators should strongly consider including the bidirectional character handling described in this section in their profile. The stringprep process never emits both an error and a string. If an error is detected during the checking of bidirectional strings, only an error is returned. [Unicode3.2] defines several bidirectional categories; each character has one bidirectional category assigned to it. For the purposes of the requirements below, an "RandALCat character" is a character that has Unicode bidirectional categories "R" or "AL"; an "LCat character" is a character that has Unicode bidirectional category "L". Note
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   that there are many characters which fall in neither of the above
   definitions; Latin digits (<U+0030> through <U+0039>) are examples of
   this because they have bidirectional category "EN".

   In any profile that specifies bidirectional character handling, all
   three of the following requirements MUST be met:

   1) The characters in section 5.8 MUST be prohibited.

   2) If a string contains any RandALCat character, the string MUST NOT
      contain any LCat character.

   3) If a string contains any RandALCat character, a RandALCat
      character MUST be the first character of the string, and a
      RandALCat character MUST be the last character of the string.

   Note that requirement 3 prohibits strings such as <U+0627><U+0031>
   ("aleph 1") but allows strings such as <U+0627><U+0031><U+0628>
   ("aleph 1 beh").  [UAX9] goes into great detail about the display
   order of strings that contain particular categories of characters in
   particular sequences.

   Table D.1 lists the characters that belong to Unicode bidirectional
   categories "R" and "AL".  Table D.2 lists all the characters that
   belong to Unicode bidirectonal category "L".  These tables are
   derived from [Unicode3.2].

7. Unassigned Code Points in Stringprep Profiles

This section describes two different types of strings in typical protocols where internationalized strings are used: "stored strings" and "queries". Of course, different Internet protocols use strings very differently, so these terms cannot be used exactly in every protocol that needs to use stringprep. In general, "stored strings" are strings that are used in protocol identifiers and named entities, such as names in digital certificates and DNS domain name parts. "Queries" are strings that are used to match against strings that are stored identifiers, such as user-entered names for digital certificate authorities and DNS lookups. All code points not assigned in the character repertoire named in a stringprep profile are called "unassigned code points". Stored strings using the profile MUST NOT contain any unassigned code points. Queries for matching strings MAY contain unassigned code points. Note that this is the only part of this document where the requirements for queries differs from the requirements for stored strings.
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   Using two different policies for where unassigned code points can
   appear removes the need for versioning in protocols that use
   stringprep profiles.  This is very useful since it makes the overall
   processing simpler and does not impose a "protocol" to handle
   versioning.  It is expected that the ISO/IEC 10646 and Unicode
   repertoires will be updated fairly frequently; at the time that this
   document is being written, it has happened approximately once a year.
   Each time a new version of a repertoire appears, a new version of a
   profile MAY be created.  Some end users will want to use the new code
   points as soon as they are defined.

   The list of unassigned code points MUST be given in a profile, and
   that list MUST be used by implementations of the profile.

   The goal of the requirements in this section is to prevent
   comparisons between two strings that were both permitted to contain
   unassigned code points.  When two strings X and Y are compared and
   string Y was prepared in a way that permits unassigned code points, a
   negative result to the comparison is not definitive; it's possible
   that the strings don't match even though they would match if a more
   recent version of the profile were used for Y.  However, if both X
   and Y were prepared in a way that permits unassigned code points,
   something worse can happen: even a positive result for the comparison
   is not definitive.  It is possible that the strings do match even
   though they would not match if a more recent version of the profile
   were used (one that prohibits a code point appearing in both X and
   Y).

   Due to the way that versioning is handled in this section, stored
   strings that are embedded in structures that cannot be changed (such
   as the signed parts of digital certificates) MUST NOT contain any
   unassigned code points.

7.1 Categories of code points

Each code point in a repertoire named by a profile of stringprep can be categorized by how it acts in the process described in earlier sections of this document: AO Code points that can be in the output MN Code points that cannot be in the output because they never appear as output from mapping or normalization D Code points that cannot be in the output because they are disallowed in the prohibition step U Unassigned code points
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   A subsequent version of a profile that references a newer version of
   a repertoire with new code points will inherently have some code
   points move from category U to either D, MN, or AO.  For backwards
   compatibility, a subsequent version of a profile MUST NOT move code
   points from any other category.  That is, current AO, MN, or D code
   points MUST NOT ever change to a different category.

   Stored strings MUST NOT contain any code points outside of AO for the
   latest version of a profile.  That is, they are forbidden to contain
   code points from the MN, D, or U categories.

   Applications creating queries MUST treat U code points as if they
   were AO when preparing the query to be entered in the process
   described by a profile of stringprep.  Those applications MAY
   optionally have a preprocessor that provide stricter checks: treating
   unassigned code points in the input as errors, or warning the user
   about the fact that the code point is unassigned in the version of a
   profile that the software is based on; such a choice is a local
   matter for the software.

7.2 Reasons for the difference between stored strings and queries

Different software using different versions of a stringprep profile need to interoperate with maximal compatibility. The scheme described in this section (stored strings MUST NOT contain unassigned code points, queries MAY include unassigned code points) allows that compatibility without introducing any known security or interoperability issues. The list below shows what happens if a query contains a code point from category U that is allowed in a newer version of a profile. The query either matches the string that was intended, or matches no string at all. In this list, the query comes from an application using version "oldVersion" of a profile, the stored string was created using version "newVersion" of the same profile, and the code point X was in category U in oldVersion, and has changed category to AO, MN, or D. There are 3 possible scenarios: 1. X is assigned to AO -- In newVersion, X is in category AO. Because the application passed X through, it gets back a positive match with the stored string. There is one exceptional case, where X is a combining mark. The order of combining marks is normalized, so if another combining mark Y has a lower combining class than X then XY will be put in the canonical order YX. (Unassigned code points are never reordered, so this doesn't happen in oldVersion). If the query contains YX, the query will get positive match with the
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      stored string.  However, no string can be stored with XY, so a
      query with XY will get a negative answer to the test for matching.

   2. X is assigned to MN -- In newVersion, X is normalized to code
      point "nX" and therefore X is now put in category MN.  This cannot
      exist in any stored string, so any query containing X will get a
      negative answer to the test for matching.  Note, however, if the
      query had contained the letter nX, it would have positively
      matched.

   3. X is assigned to D -- In newVersion, X is in category D.  This
      cannot exist in any stored string, so any query containing X will
      get a negative answer to the test for matching.

   In none of the cases does the query get data for a stored string
   other than the one it actually tried to match against.

   Profiles are stable between versions in the following sense: If a
   string S has been prepared using newVersion, then it will not change
   if it is subsequently prepared using oldVersion.

7.3 Versions of applications and stored strings

Another way to see that this versioning system works is to compare what happens when an application uses a newer or older version of a profile. Newer query application -- Suppose that a querying application is using version newVersion and the stored string was created using version oldVersion. This case is simple: there will be no characters in the stored string that cannot be queried by the application because the new profile uses a superset of the code points used for making the stored string. Newer stored string -- Suppose that a querying application is using oldVersion and the stored string was created using a profile that uses newVersion. Because the querying application let unassigned code points pass through, the user can query on stored strings that use code points in newVersion. No stored strings can have code points that are unassigned in newVersion, since that is illegal. In order to get a match, the querying application has to enter the unassigned code points in the proper order, and has to use unassigned code points that would make it through both the mapping and the normalization steps.
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8. References

8.1 Normative references

[UAX15] Mark Davis and Martin Duerst. Unicode Standard Annex #15: Unicode Normalization Forms, Version 3.2.0. <http://www.unicode.org/unicode/reports/tr15/tr15- 22.html>. [Unicode3.2] The Unicode Consortium. The Unicode Standard, Version 3.2.0 is defined by The Unicode Standard, Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as amended by the Unicode Standard Annex #27: Unicode 3.1 (http://www.unicode.org/reports/tr27/) and by the Unicode Standard Annex #28: Unicode 3.2 (http://www.unicode.org/reports/tr28/). [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2 Informative references

[CharModel] Unicode Technical Report;17, Character Encoding Model. <http://www.unicode.org/unicode/reports/tr17/>. [Glossary] Unicode Glossary, <http://www.unicode.org/glossary/>. [ISO10646] ISO/IEC, "Information Technology - Universal Multiple- Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane", ISO/IEC 10646-1:2000, October 2000. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for IANA Considerations", BCP 26, RFC 2434, October 1998. [UAX9] The Unicode Consortium. Unicode Standard Annex #9, The Bidirectional Algorithm, <http://www.unicode.org/unicode/reports/tr9/>. [UTR21] Mark Davis. Case Mappings. Unicode Technical Report 21. <http://www.unicode.org/unicode/reports/tr21/>.

9. Security Considerations

Stringprep is used with Unicode characters. There are security considerations that are specific to stringprep, and others that are generic to using Unicode.
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9.1 Stringprep-specific security considerations

The Unicode and ISO/IEC 10646 repertoires have many characters that look similar. In many cases, users of security protocols might do visual matching, such as when comparing the names of trusted third parties. Because it is impossible to map similar-looking characters without a great deal of context such as knowing the fonts used, stringprep does nothing to map similar-looking characters together nor to prohibit some characters because they look like others. User applications can help disambiguate some similar-looking characters by showing the user when a string changes between scripts. Most profiles of stringprep can cause changes in strings that are input to stringprep. Because of this, protocols that have sets of non-allowed characters or sequences MUST check for the non-allowed characters or sequences after the stringprep processing. This document does not mandate the checking of bidirectional characters in section 6. If the requirements in section 6 are not used in a profile of stringprep, it is easy to create many strings whose characters are in different order but are displayed identically. This can cause security-related user confusion similar to look-alike characters, as described above. Stringprep does not do anything to assure that any algorithms translating characters from non-Unicode into Unicode produce the same output in all implementations. Some Unicode codepoints are invisible. Protocols that allow these characters (that is, do not map them out or prohibit them in stringprep) can cause users confusion when two identical-looking strings do not match.

9.2 Generic Unicode security considerations

Using Unicode characters explicitly forces applications to use multi-octet characters. Converting an application from one that uses single-octet characters to one that uses multi-octet characters must be done very carefully, particularly in an application that checks for values of characters or sorts characters. Protocols that use stringprep usually also use encodings of Unicode, such as UTF-8 or UTF-16. Some applications using those encodings have been known to not check for illegal or ill-formed sequences in the encodings, and thereby have not detected sequences of octets that would have been detected if they used just ASCII. For example, in
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   UTF-8 the octet sequence "0xC0 0xAB" is an illegal formation of
   U+002B (plus sign).  All programs should reject any string that is an
   illegal or ill-formed octet sequence for the encoding being used.

   Both Unicode normalization and conversion between Unicode encodings
   can cause strings to grow or shrink.  Programs that used fixed-size
   buffers, or that make assumptions that buffers will always be greater
   than or less than particular sizes, are likely to fail in insecure
   fashions when using Unicode normalization or encoding conversions.

   Covering an extensive list of security threats and considerations on
   the use of current and future versions of Unicode is outside of the
   scope of this document.

10. IANA Considerations

Stringprep profiles MUST have IETF consensus as described in [RFC2434]. Each profile MUST be reviewed by the IESG before it is registered. The IESG MAY change a profile before registration. IANA has set up a registry of stringprep profiles. This registry is a single text file that lists the known profiles. Each entry in the registry has three fields: - Profile name - RFC in which the profile is defined - Indicator whether or not this is the newest version of the profile Each version of a profile will remain listed in the registry forever. That is, if a new version of a profile supersedes an earlier version, both versions will continue to be listed in the registry, but the current version indicator will be turned off for the earlier version and turned on for the newer version. It is probably harmful if a large number of profiles of stringprep proliferate. Therefore, the IESG may reject proposals for new profiles and instead suggest that protocols reuse existing profiles.
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11. Acknowledgements

Many people from the IETF IDN Working Group and the Unicode Technical Committee contributed ideas that went into the first document of this document. Mark Davis and Patrik Faltstrom were particularly helpful in some of the ideas, such as the versioning description. The IDN nameprep design team made many useful changes to the first document. That team and its advisors include: Asmus Freytag Cathy Wissink Francois Yergeau James Seng Marc Blanchet Mark Davis Martin Duerst Patrik Faltstrom Paul Hoffman Additional significant improvements were proposed by: Jonathan Rosenne Kent Karlsson Scott Hollenbeck Dave Crocker Erik Nordmark Matitiahu Allouche


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