Stringprep Revision and Problem Statement for the Preparation and Comparison of Internationalized Strings (PRECIS)
RFC 6885
Internet Engineering Task Force (IETF) M. Blanchet
Request for Comments: 6885 Viagenie
Category: Informational A. Sullivan
ISSN: 2070-1721 Dyn, Inc.
March 2013
Stringprep Revision and Problem Statement
for the Preparation and Comparison of Internationalized Strings (PRECIS)
Abstract
If a protocol expects to compare two strings and is prepared only for
those strings to be ASCII, then using Unicode code points in those
strings requires they be prepared somehow. Internationalizing Domain
Names in Applications (here called IDNA2003) defined and used
Stringprep and Nameprep. Other protocols subsequently defined
Stringprep profiles. A new approach different from Stringprep and
Nameprep is used for a revision of IDNA2003 (called IDNA2008). Other
Stringprep profiles need to be similarly updated, or a replacement of
Stringprep needs to be designed. This document outlines the issues
to be faced by those designing a Stringprep replacement.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6885.
Blanchet & Sullivan Informational [Page 1]
RFC 6885 Stringprep Revision Problem Statement March 2013
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Blanchet & Sullivan Informational [Page 2]
RFC 6885 Stringprep Revision Problem Statement March 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Stringprep Profiles Limitations . . . . . . . . . . . . . . . 6
5. Major Topics for Consideration . . . . . . . . . . . . . . . . 8
5.1. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1.1. Types of Identifiers . . . . . . . . . . . . . . . . . 8
5.1.2. Effect of Comparison . . . . . . . . . . . . . . . . . 8
5.2. Dealing with Characters . . . . . . . . . . . . . . . . . 9
5.2.1. Case Folding, Case Sensitivity, and Case
Preservation . . . . . . . . . . . . . . . . . . . . . 9
5.2.2. Stringprep and NFKC . . . . . . . . . . . . . . . . . 9
5.2.3. Character Mapping . . . . . . . . . . . . . . . . . . 10
5.2.4. Prohibited Characters . . . . . . . . . . . . . . . . 10
5.2.5. Internal Structure, Delimiters, and Special
Characters . . . . . . . . . . . . . . . . . . . . . . 10
5.2.6. Restrictions Because of Glyph Similarity . . . . . . . 11
5.3. Where the Data Comes from and Where It Goes . . . . . . . 11
5.3.1. User Input and the Source of Protocol Elements . . . . 11
5.3.2. User Output . . . . . . . . . . . . . . . . . . . . . 12
5.3.3. Operations . . . . . . . . . . . . . . . . . . . . . . 12
6. Considerations for Stringprep Replacement . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
9. Informative References . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Classification of Stringprep Profiles . . . . . . . . 19
Appendix B. Evaluation of Stringprep Profiles . . . . . . . . . . 19
B.1. iSCSI Stringprep Profile: RFC 3720, RFC 3721, RFC 3722 . . 19
B.2. SMTP/POP3/ManageSieve Stringprep Profiles: RFC 4954,
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