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
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
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
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, RFC2119
[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.
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.
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.
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
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.
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.
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.
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
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]
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
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
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.
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
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
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.
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, RFC2119, 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, RFC2434, 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.
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
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
- RFCin 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.
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
A. Unicode repertoires
The following is the only repertoire covered in this document:
Unicode 3.2, as defined in [Unicode3.2].
A.1 Unassigned code points in Unicode 3.2
----- Start Table A.1 -----
0221
0234-024F
02AE-02AF
02EF-02FF
0350-035F
0370-0373
0376-0379
037B-037D
037F-0383
038B
038D
03A2
03CF
03F7-03FF
0487
04CF
04F6-04F7
04FA-04FF
0510-0530
0557-0558
0560
0588
058B-0590
05A2
05BA
05C5-05CF
05EB-05EF
05F5-060B
060D-061A
061C-061E
0620
063B-063F
0656-065F
06EE-06EF
06FF
070E
072D-072F
074B-077F
07B2-0900
0904
093A-093B
094E-094F
0955-0957
0971-0980
0984
098D-098E
0991-0992
09A9
09B1
09B3-09B5
09BA-09BB
09BD
09C5-09C6
09C9-09CA
09CE-09D6
09D8-09DB
09DE
09E4-09E5
09FB-0A01
0A03-0A04
0A0B-0A0E
0A11-0A12
0A29
0A31
0A34
0A37
0A3A-0A3B
0A3D
0A43-0A46
0A49-0A4A
0A4E-0A58
0A5D
0A5F-0A65
0A75-0A80
0A84
0A8C
0A8E
0A92
0AA9
0AB1
0AB4
0ABA-0ABB
0AC6
0ACA
0ACE-0ACF
0AD1-0ADF
0AE1-0AE5
0AF0-0B00
0B04
0B0D-0B0E
0B11-0B12
0B29
0B31
0B34-0B35
0B3A-0B3B
0B44-0B46
0B49-0B4A
0B4E-0B55
0B58-0B5B
0B5E
0B62-0B65
0B71-0B81
0B84
0B8B-0B8D
0B91
0B96-0B98
0B9B
0B9D
0BA0-0BA2
0BA5-0BA7
0BAB-0BAD
0BB6
0BBA-0BBD
0BC3-0BC5
0BC9
0BCE-0BD6
0BD8-0BE6
0BF3-0C00
0C04
0C0D
0C11
0C29
0C34
0C3A-0C3D
0C45
0C49
0C4E-0C54
0C57-0C5F
0C62-0C65
0C70-0C81
0C84
0C8D
0C91
0CA9
0CB4
0CBA-0CBD
0CC5
0CC9
0CCE-0CD4
0CD7-0CDD
0CDF
0CE2-0CE5
0CF0-0D01
0D04
0D0D
0D11
0D29
0D3A-0D3D
0D44-0D45
0D49
0D4E-0D56
0D58-0D5F
0D62-0D65
0D70-0D81
0D84
0D97-0D99
0DB2
0DBC
0DBE-0DBF
0DC7-0DC9
0DCB-0DCE
0DD5
0DD7
0DE0-0DF1
0DF5-0E00
0E3B-0E3E