Internet DRAFT - draft-ietf-krb-wg-utf8-profile
Internet Draft Jeffrey Altman
draft-ietf-krb-wg-utf8-profile-00.txt Columbia University
February 12, 2002
Expires in six months
Stringprep Profile for Kerberos UTF-8 Strings
Status of this memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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This document describes how to prepare UTF-8 strings
in order to increase the likelihood that name input and name comparison
work in ways that make sense for typical users throughout the world. This
is a profile of the stringprep protocol developed in the IDN working group.
This document specifies processing rules that will allow users to enter
Kerberos Principal Names and input to cryptographic String to Key functions.
It is a profile of stringprep [STRINGPREP].
This profile defines the following, as required by [STRINGPREP]
- The intended applicability of the profile: internationalized
host name parts
- The character repertoire that is the input and output to stringprep:
defined in Section 2
- The list of unassigned code points for the repertoire: defined
in Appendix F.
- The mappings used: defined in Section 3.
- The Unicode normalization used: defined in Section 4
- The characters that are prohibited as output: Defined in section 5
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in RFC 2119
Examples in this document use the notation for code points and names
from the Unicode Standard [Unicode3.1] 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 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 "[SYMBOLS]") and do not come from
2. Character Repertoire
Unicode 3.1 [Unicode3.1] is the repertoire used in this profile.
The reason Unicode 3.1 was chosen instead of a version of
ISO/IEC 10646 is that ISO/IEC 10646 is expected to be updated soon after
this document becomes an RFC. Unicode 3.1 has the exact repertoire that
is expected in the next version of ISO/IEC 10646, and is therefore used
This profile specifies stringprep mapping using the mapping table
in Appendix D. That table includes all the steps described in this
Note that text in this section describe how Appendix D was formed. It is
there for people who want to understand more, but it should be ignored
by implementors. Implementations of this profile MUST map based on
Appendix D, not based on the descriptions in this section of how
Appendix D was created.
3.1 Mapped out
The following characters are simply deleted from the input (that is,
they are mapped to nothing) because their presence or absence should not
make two strings different.
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
FEFF; ZERO WIDTH NO-BREAK SPACE
Variation selectors and cursive connectors select different glyphs, but
do not bear semantics.
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
3.2 Space Character Conversions
The following Unicode spaces are to be mapped to 0020; SPACE:
00A0; NO-BREAK SPACE
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
202F; NARROW NO-BREAK SPACE
3000; IDEOGRAPHIC SPACE
This profile specifies using Unicode normalization form KC, as described
NOTE: There was some discussion on the mailing list that would suggest
that Unicode NFKC does not properly handle the composition of
normalized Hangul strings. Following the lead of the IDN working
group, the Kerberos working group will not attempt to second-guess the
the authors of Unicode 3.1 Annex 15 (formerly Technical Report 15)
[UAX15], which specifies the normalization methods, or the Ideographic
Rappaorteur Group (IRG), which is the formal subgroup of ISO/IEC
JTC1/SC2/WG2 charged with approving all CJKV elements of the Unicode
standards. Such issues are outside the working group's charter and
its area of expertise.
5. Prohibited Output
This profile specifies using the prohibition table in Appendix E.
Note that the subsections below describe how Appendix E was formed. They
are there for people who want to understand more, but they should be
ignored by implementors. Implementations of this profile MUST map based
on Appendix E, not based on the descriptions in this section of how
Appendix E was created.
The collected lists of prohibited code points can be found in Appendix E
of this document. The lists in Appendix E MUST be used by implementations
of this specification. If there are any discrepancies between the lists
in Appendix E and subsections below, the lists in Appendix E always takes
Some code points listed in one section would also appear in other
sections. Each code point is only listed once in the tables in Appendix
5.1 Control characters
Control characters (or characters with control function) cannot be seen
and can cause unpredictable results when displayed.
0000-001F; [CONTROL CHARACTERS]
0080-009F; [CONTROL CHARACTERS]
070F; SYRIAC ABBREVIATION MARK
180E; MONGOLIAN VOWEL SEPARATOR
2028; LINE SEPARATOR
2029; PARAGRAPH SEPARATOR
206A-206F; [CONTROL CHARACTERS]
FFF9-FFFC; [CONTROL CHARACTERS]
1D173-1D17A; [MUSICAL CONTROL CHARACTERS]
5.2 Private use and replacement characters
Because private-use characters do not have defined meanings, they are
prohibited. The private-use characters are:
E000-F8FF; [PRIVATE USE, PLANE 0]
F0000-FFFFD; [PRIVATE USE, PLANE 15]
100000-10FFFD; [PRIVATE USE, PLANE 16]
The replacement character (U+FFFD) has no known semantic definition in a
name, and 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 name with three ideographs might be
rendered with three replacement characters.
FFFD; REPLACEMENT CHARACTER
5.3 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 the PropList.txt file from the
5.4 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, and are therefore prohibited:
D800-DFFF; [SURROGATE CODES]
5.5 Inappropriate for plain text
The following characters should not appear in regular text.
FFF9; INTERLINEAR ANNOTATION ANCHOR
FFFA; INTERLINEAR ANNOTATION SEPARATOR
FFFB; INTERLINEAR ANNOTATION TERMINATOR
FFFC; OBJECT REPLACEMENT CHARACTER
5.6 Inappropriate for canonical representation
The ideographic description characters allow different sequences of
characters to be rendered the same way, which makes them inappropriate
for host names that must have a single canonical representation.
2FF0-2FFB; [IDEOGRAPHIC DESCRIPTION CHARACTERS]
5.7 Change display properties
The following characters, some of which are deprecated in ISO/IEC 10646,
can cause changes in display or the order in which characters appear
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
5.8 Tagging characters
The following characters are used for tagging text and are invisible.
E0001; LANGUAGE TAG
E0020-E007F; [TAGGING CHARACTERS]
6. Unassigned Code Points in Internationalized Host Names
This profile lists the unassigned code points for Unicode 3.1 in
Appendix F. The list in Appendix F MUST be used by implementations of
this specification. If there are any discrepancies between the list in
Appendix F and the Unicode 3.1 specification, the list Appendix F always
7. Security Considerations
ISO/IEC 10646 has 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. This profile does nothing
to map similar-looking characters together.
Principal names and passwords are entered by users and used within the
Kerberos protocol. The
security of the Internet would be compromised if a user entering a
single internationalized string could be connected to different servers
or denied access based on different interpretations of
[CharModel] Unicode Technical Report;17, Character Encoding Model.
[Glossary] Unicode Glossary, <http://www.unicode.org/glossary/>.
[ISO10646] ISO/IEC 10646-1:2000. International Standard -- Information
technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part
1: Architecture and Basic Multilingual Plane.
[RFC2119] Scott Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997, RFC 2119.
[STRINGPREP] Paul Hoffman and Marc Blanchet, "Preparation of
Internationalized Strings ("stringprep")", draft-hoffman-stringprep,
work in progress
[Unicode3.1] The Unicode Standard, Version 3.1.0: The Unicode
Consortium. The Unicode Standard, Version 3.0. Reading, MA,
Addison-Wesley Developers Press, 2000. ISBN 0-201-61633-5, as amended
by: Unicode Standard Annex #27: Unicode 3.1
[UAX15] Mark Davis and Martin Duerst. Unicode Standard Annex #15:
Unicode Normalization Forms, Version 3.1.0.
This draft is based upon the work of the IETF IDN Working Group's
IDN Nameprep design team.
B. IANA Considerations
This is a profile of stringprep. When it becomes an RFC, it
should be registered in the stringprep profile registry.
C. Author Contact Information
612 West 115th Street
New York NY 10025
D. Mapping Tables
The following is the mapping table from Section 3. The table has three
- the character that is mapped from
- the zero or more characters that it is mapped to
- the reason for the mapping
The columns are separated by semicolons. Note that the second column may
be empty, or it may have one character, or it may have more than one
character, with each character separated by a space.
----- Start Mapping Table -----
... to be filled in ...
----- End Mapping Table -----
E. Prohibited Code Point List
----- Start Prohibited Table -----
... to be filled in ...
----- End Prohibited Table -----
NOTE WELL: Software that follows this specification that will be used to
check names before they are put in authoritative name servers MUST add
all unassigned code pints to the list of characters that are prohibited.
See Section 6 of [STRINGPREP] for more details.
F. Unassigned Code Point List
----- Start Unassigned Table -----
... to be filled in ...
----- End Unassigned Table -----
Jeffrey Altman * Sr.Software Designer C-Kermit 8.0 available now!!!
The Kermit Project @ Columbia University includes Telnet, FTP and HTTP
http://www.kermit-project.org/ secured with Kerberos, SRP, and
firstname.lastname@example.org OpenSSL. Interfaces with OpenSSH