11. IANA Considerations 11.1. PRECIS Derived Property Value Registry IANA has created and now maintains the "PRECIS Derived Property Value" registry that records the derived properties for the versions of Unicode that are released after (and including) version 7.0. The derived property value is to be calculated in cooperation with a designated expert [RFC5226] according to the rules specified under Sections 8 and 9. The IESG is to be notified if backward-incompatible changes to the table of derived properties are discovered or if other problems arise during the process of creating the table of derived property values or during expert review. Changes to the rules defined under Sections 8 and 9 require IETF Review. 11.2. PRECIS Base Classes Registry IANA has created the "PRECIS Base Classes" registry. In accordance with [RFC5226], the registration policy is "RFC Required". The registration template is as follows: Base Class: [the name of the PRECIS string class] Description: [a brief description of the PRECIS string class and its intended use, e.g., "A sequence of letters, numbers, and symbols that is used to identify or address a network entity."] Specification: [the RFC number] The initial registrations are as follows: Base Class: FreeformClass. Description: A sequence of letters, numbers, symbols, spaces, and other code points that is used for free-form strings. Specification: Section 4.3 of RFC 7564. Base Class: IdentifierClass. Description: A sequence of letters, numbers, and symbols that is used to identify or address a network entity. Specification: Section 4.2 of RFC 7564.
11.3. PRECIS Profiles Registry IANA has created the "PRECIS Profiles" registry to identify profiles that use the PRECIS string classes. In accordance with [RFC5226], the registration policy is "Expert Review". This policy was chosen in order to ease the burden of registration while ensuring that "customers" of PRECIS receive appropriate guidance regarding the sometimes complex and subtle internationalization issues related to profiles of PRECIS string classes. The registration template is as follows: Name: [the name of the profile] Base Class: [which PRECIS string class is being profiled] Applicability: [the specific protocol elements to which this profile applies, e.g., "Localparts in XMPP addresses."] Replaces: [the Stringprep profile that this PRECIS profile replaces, if any] Width Mapping Rule: [the behavioral rule for handling of width, e.g., "Map fullwidth and halfwidth characters to their compatibility variants."] Additional Mapping Rule: [any additional mappings that are required or recommended, e.g., "Map non-ASCII space characters to ASCII space."] Case Mapping Rule: [the behavioral rule for handling of case, e.g., "Unicode Default Case Folding"] Normalization Rule: [which Unicode normalization form is applied, e.g., "NFC"] Directionality Rule: [the behavioral rule for handling of right-to- left code points, e.g., "The 'Bidi Rule' defined in RFC 5893 applies."] Enforcement: [which entities enforce the rules, and when that enforcement occurs during protocol operations] Specification: [a pointer to relevant documentation, such as an RFC or Internet-Draft] In order to request a review, the registrant shall send a completed template to the firstname.lastname@example.org list or its designated successor.
Factors to focus on while defining profiles and reviewing profile registrations include the following: o Would an existing PRECIS string class or profile solve the problem? If not, why not? (See Section 5.1 for related considerations.) o Is the problem being addressed by this profile well defined? o Does the specification define what kinds of applications are involved and the protocol elements to which this profile applies? o Is the profile clearly defined? o Is the profile based on an appropriate dividing line between user interface (culture, context, intent, locale, device limitations, etc.) and the use of conformant strings in protocol elements? o Are the width mapping, case mapping, additional mappings, normalization, and directionality rules appropriate for the intended use? o Does the profile explain which entities enforce the rules, and when such enforcement occurs during protocol operations? o Does the profile reduce the degree to which human users could be surprised or confused by application behavior (the "Principle of Least Astonishment")? o Does the profile introduce any new security concerns such as those described under Section 12 of this document (e.g., false positives for authentication or authorization)? 12. Security Considerations 12.1. General Issues If input strings that appear "the same" to users are programmatically considered to be distinct in different systems, or if input strings that appear distinct to users are programmatically considered to be "the same" in different systems, then users can be confused. Such confusion can have security implications, such as the false positives and false negatives discussed in [RFC6943]. One starting goal of work on the PRECIS framework was to limit the number of times that users are confused (consistent with the "Principle of Least Astonishment"). Unfortunately, this goal has been difficult to achieve given the large number of application protocols already in existence. Despite these difficulties, profiles should not be
multiplied beyond necessity (see Section 5.1). In particular, application protocol designers should think long and hard before defining a new profile instead of using one that has already been defined, and if they decide to define a new profile then they should clearly explain their reasons for doing so. The security of applications that use this framework can depend in part on the proper preparation, enforcement, and comparison of internationalized strings. For example, such strings can be used to make authentication and authorization decisions, and the security of an application could be compromised if an entity providing a given string is connected to the wrong account or online resource based on different interpretations of the string (again, see [RFC6943]). Specifications of application protocols that use this framework are strongly encouraged to describe how internationalized strings are used in the protocol, including the security implications of any false positives and false negatives that might result from various enforcement and comparison operations. For some helpful guidelines, refer to [RFC6943], [RFC5890], [UTR36], and [UTS39]. 12.2. Use of the IdentifierClass Strings that conform to the IdentifierClass and any profile thereof are intended to be relatively safe for use in a broad range of applications, primarily because they include only letters, digits, and "grandfathered" non-space characters from the ASCII range; thus, they exclude spaces, characters with compatibility equivalents, and almost all symbols and punctuation marks. However, because such strings can still include so-called confusable characters (see Section 12.5), protocol designers and implementers are encouraged to pay close attention to the security considerations described elsewhere in this document. 12.3. Use of the FreeformClass Strings that conform to the FreeformClass and many profiles thereof can include virtually any Unicode character. This makes the FreeformClass quite expressive, but also problematic from the perspective of possible user confusion. Protocol designers are hereby warned that the FreeformClass contains code points they might not understand, and are encouraged to profile the IdentifierClass wherever feasible; however, if an application protocol requires more code points than are allowed by the IdentifierClass, protocol designers are encouraged to define a profile of the FreeformClass that restricts the allowable code points as tightly as possible.
(The PRECIS Working Group considered the option of allowing "superclasses" as well as profiles of PRECIS string classes, but decided against allowing superclasses to reduce the likelihood of security and interoperability problems.) 12.4. Local Character Set Issues When systems use local character sets other than ASCII and Unicode, this specification leaves the problem of converting between the local character set and Unicode up to the application or local system. If different applications (or different versions of one application) implement different rules for conversions among coded character sets, they could interpret the same name differently and contact different application servers or other network entities. This problem is not solved by security protocols, such as Transport Layer Security (TLS) [RFC5246] and the Simple Authentication and Security Layer (SASL) [RFC4422], that do not take local character sets into account. 12.5. Visually Similar Characters Some characters are visually similar and thus can cause confusion among humans. Such characters are often called "confusable characters" or "confusables". The problem of confusable characters is not necessarily caused by the use of Unicode code points outside the ASCII range. For example, in some presentations and to some individuals the string "ju1iet" (spelled with DIGIT ONE, U+0031, as the third character) might appear to be the same as "juliet" (spelled with LATIN SMALL LETTER L, U+006C), especially on casual visual inspection. This phenomenon is sometimes called "typejacking". However, the problem is made more serious by introducing the full range of Unicode code points into protocol strings. For example, the characters U+13DA U+13A2 U+13B5 U+13AC U+13A2 U+13AC U+13D2 from the Cherokee block look similar to the ASCII characters "STPETER" as they might appear when presented using a "creative" font family. In some examples of confusable characters, it is unlikely that the average human could tell the difference between the real string and the fake string. (Indeed, there is no programmatic way to distinguish with full certainty which is the fake string and which is the real string; in some contexts, the string formed of Cherokee characters might be the real string and the string formed of ASCII characters might be the fake string.) Because PRECIS-compliant strings can contain almost any properly encoded Unicode code point, it can be relatively easy to fake or mimic some strings in systems that use the PRECIS framework. The fact that some strings are easily
confused introduces security vulnerabilities of the kind that have also plagued the World Wide Web, specifically the phenomenon known as phishing. Despite the fact that some specific suggestions about identification and handling of confusable characters appear in the Unicode Security Considerations [UTR36] and the Unicode Security Mechanisms [UTS39], it is also true (as noted in [RFC5890]) that "there are no comprehensive technical solutions to the problems of confusable characters." Because it is impossible to map visually similar characters without a great deal of context (such as knowing the font families used), the PRECIS framework does nothing to map similar- looking characters together, nor does it prohibit some characters because they look like others. Nevertheless, specifications for application protocols that use this framework are strongly encouraged to describe how confusable characters can be abused to compromise the security of systems that use the protocol in question, along with any protocol-specific suggestions for overcoming those threats. In particular, software implementations and service deployments that use PRECIS-based technologies are strongly encouraged to define and implement consistent policies regarding the registration, storage, and presentation of visually similar characters. The following recommendations are appropriate: 1. An application service SHOULD define a policy that specifies the scripts or blocks of characters that the service will allow to be registered (e.g., in an account name) or stored (e.g., in a filename). Such a policy SHOULD be informed by the languages and scripts that are used to write registered account names; in particular, to reduce confusion, the service SHOULD forbid registration or storage of strings that contain characters from more than one script and SHOULD restrict registrations to characters drawn from a very small number of scripts (e.g., scripts that are well understood by the administrators of the service, to improve manageability). 2. User-oriented application software SHOULD define a policy that specifies how internationalized strings will be presented to a human user. Because every human user of such software has a preferred language or a small set of preferred languages, the software SHOULD gather that information either explicitly from the user or implicitly via the operating system of the user's device. Furthermore, because most languages are typically represented by a single script or a small set of scripts, and because most scripts are typically contained in one or more blocks of characters, the software SHOULD warn the user when
presenting a string that mixes characters from more than one script or block, or that uses characters outside the normal range of the user's preferred language(s). (Such a recommendation is not intended to discourage communication across different communities of language users; instead, it recognizes the existence of such communities and encourages due caution when presenting unfamiliar scripts or characters to human users.) The challenges inherent in supporting the full range of Unicode code points have in the past led some to hope for a way to programmatically negotiate more restrictive ranges based on locale, script, or other relevant factors; to tag the locale associated with a particular string; etc. As a general-purpose internationalization technology, the PRECIS framework does not include such mechanisms. 12.6. Security of Passwords Two goals of passwords are to maximize the amount of entropy and to minimize the potential for false positives. These goals can be achieved in part by allowing a wide range of code points and by ensuring that passwords are handled in such a way that code points are not compared aggressively. Therefore, it is NOT RECOMMENDED for application protocols to profile the FreeformClass for use in passwords in a way that removes entire categories (e.g., by disallowing symbols or punctuation). Furthermore, it is NOT RECOMMENDED for application protocols to map uppercase and titlecase code points to their lowercase equivalents in such strings; instead, it is RECOMMENDED to preserve the case of all code points contained in such strings and to compare them in a case-sensitive manner. That said, software implementers need to be aware that there exist tradeoffs between entropy and usability. For example, allowing a user to establish a password containing "uncommon" code points might make it difficult for the user to access a service when using an unfamiliar or constrained input device. Some application protocols use passwords directly, whereas others reuse technologies that themselves process passwords (one example of such a technology is the Simple Authentication and Security Layer [RFC4422]). Moreover, passwords are often carried by a sequence of protocols with backend authentication systems or data storage systems such as RADIUS [RFC2865] and the Lightweight Directory Access Protocol (LDAP) [RFC4510]. Developers of application protocols are encouraged to look into reusing these profiles instead of defining new ones, so that end-user expectations about passwords are consistent no matter which application protocol is used.
In protocols that provide passwords as input to a cryptographic algorithm such as a hash function, the client will need to perform proper preparation of the password before applying the algorithm, since the password is not available to the server in plaintext form. Further discussion of password handling can be found in [PRECIS-Users-Pwds]. 13. Interoperability Considerations 13.1. Encoding Although strings that are consumed in PRECIS-based application protocols are often encoded using UTF-8 [RFC3629], the exact encoding is a matter for the application protocol that uses PRECIS, not for the PRECIS framework. 13.2. Character Sets It is known that some existing systems are unable to support the full Unicode character set, or even any characters outside the ASCII range. If two (or more) applications need to interoperate when exchanging data (e.g., for the purpose of authenticating a username or password), they will naturally need to have in common at least one coded character set (as defined by [RFC6365]). Establishing such a baseline is a matter for the application protocol that uses PRECIS, not for the PRECIS framework. 13.3. Unicode Versions Changes to the properties of Unicode code points can occur as the Unicode Standard is modified from time to time. For example, three code points underwent changes in their GeneralCategory between Unicode 5.2 (current at the time IDNA2008 was originally published) and Unicode 6.0, as described in [RFC6452]. Implementers might need to be aware that the treatment of these characters differs depending on which version of Unicode is available on the system that is using IDNA2008 or PRECIS. Other such differences might arise between the version of Unicode current at the time of this writing (7.0) and future versions. 13.4. Potential Changes to Handling of Certain Unicode Code Points As part of the review of Unicode 7.0 for IDNA, a question was raised about a newly added code point that led to a re-analysis of the normalization rules used by IDNA and inherited by this document (Section 5.2.4). Some of the general issues are described in [IAB-Statement] and pursued in more detail in [IDNA-Unicode].
At the time of writing, these issues have yet to be settled. However, implementers need to be aware that this specification is likely to be updated in the future to address these issues. The potential changes include the following: o The range of characters in the LetterDigits category (Sections 4.2.1 and 9.1) might be narrowed. o Some characters with special properties that are now allowed might be excluded. o More "Additional Mapping Rules" (Section 5.2.2) might be defined. o Alternative normalization methods might be added. Nevertheless, implementations and deployments that are sensitive to the advice given in this specification are unlikely to encounter significant problems as a consequence of these issues or potential changes -- specifically, the advice to use the more restrictive IdentifierClass whenever possible or, if using the FreeformClass, to allow only a restricted set of characters, particularly avoiding characters whose implications they do not actually understand. 14. References 14.1. Normative References [RFC20] Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, <http://www.rfc-editor.org/info/rfc20>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008, <http://www.rfc-editor.org/info/rfc5198>. [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, <http://www.rfc-editor.org/info/rfc6365>.
[Unicode] The Unicode Consortium, "The Unicode Standard", <http://www.unicode.org/versions/latest/>. [Unicode7.0] The Unicode Consortium, "The Unicode Standard, Version 7.0.0", (Mountain View, CA: The Unicode Consortium, 2014 ISBN 978-1-936213-09-2), <http://www.unicode.org/versions/Unicode7.0.0/>. 14.2. Informative References [DerivedCoreProperties] The Unicode Consortium, "DerivedCoreProperties-7.0.0.txt", Unicode Character Database, February 2014, <http://www.unicode.org/Public/UCD/latest/ucd/ DerivedCoreProperties.txt>. [IAB-Statement] Internet Architecture Board, "IAB Statement on Identifiers and Unicode 7.0.0", February 2015, <https://www.iab.org/ documents/correspondence-reports-documents/ 2015-2/iab-statement-on-identifiers-and-unicode-7-0-0/>. [IDNA-Unicode] Klensin, J. and P. Faltstrom, "IDNA Update for Unicode 7.0.0", Work in Progress, draft-klensin-idna-5892upd-unicode70-04, March 2015. [PRECIS-Mappings] Yoneya, Y. and T. Nemoto, "Mapping characters for PRECIS classes", Work in Progress, draft-ietf-precis-mappings-10, May 2015. [PRECIS-Nickname] Saint-Andre, P., "Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames", Work in Progress, draft-ietf-precis-nickname-17, April 2015. [PRECIS-Users-Pwds] Saint-Andre, P. and A. Melnikov, "Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords", Work in Progress, draft-ietf-precis-saslprepbis-17, May 2015.
[PropertyAliases] The Unicode Consortium, "PropertyAliases-7.0.0.txt", Unicode Character Database, November 2013, <http://www.unicode.org/Public/UCD/latest/ucd/ PropertyAliases.txt>. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, DOI 10.17487/RFC2865, June 2000, <http://www.rfc-editor.org/info/rfc2865>. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, DOI 10.17487/RFC3454, December 2002, <http://www.rfc-editor.org/info/rfc3454>. [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, DOI 10.17487/RFC3490, March 2003, <http://www.rfc-editor.org/info/rfc3490>. [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, DOI 10.17487/RFC3491, March 2003, <http://www.rfc-editor.org/info/rfc3491>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>. [RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006, <http://www.rfc-editor.org/info/rfc4422>. [RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, DOI 10.17487/RFC4510, June 2006, <http://www.rfc-editor.org/info/rfc4510>. [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and Recommendations for Internationalized Domain Names (IDNs)", RFC 4690, DOI 10.17487/RFC4690, September 2006, <http://www.rfc-editor.org/info/rfc4690>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, <http://www.rfc-editor.org/info/rfc5890>. [RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010, <http://www.rfc-editor.org/info/rfc5891>. [RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA)", RFC 5892, DOI 10.17487/RFC5892, August 2010, <http://www.rfc-editor.org/info/rfc5892>. [RFC5893] Alvestrand, H., Ed., and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, <http://www.rfc-editor.org/info/rfc5893>. [RFC5894] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010, <http://www.rfc-editor.org/info/rfc5894>. [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008", RFC 5895, DOI 10.17487/RFC5895, September 2010, <http://www.rfc-editor.org/info/rfc5895>.
[RFC6452] Faltstrom, P., Ed., and P. Hoffman, Ed., "The Unicode Code Points and Internationalized Domain Names for Applications (IDNA) - Unicode 6.0", RFC 6452, DOI 10.17487/RFC6452, November 2011, <http://www.rfc-editor.org/info/rfc6452>. [RFC6885] Blanchet, M. and A. Sullivan, "Stringprep Revision and Problem Statement for the Preparation and Comparison of Internationalized Strings (PRECIS)", RFC 6885, DOI 10.17487/RFC6885, March 2013, <http://www.rfc-editor.org/info/rfc6885>. [RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May 2013, <http://www.rfc-editor.org/info/rfc6943>. [UAX11] Unicode Standard Annex #11, "East Asian Width", edited by Ken Lunde. An integral part of The Unicode Standard, <http://unicode.org/reports/tr11/>. [UAX15] Unicode Standard Annex #15, "Unicode Normalization Forms", edited by Mark Davis and Ken Whistler. An integral part of The Unicode Standard, <http://unicode.org/reports/tr15/>. [UAX9] Unicode Standard Annex #9, "Unicode Bidirectional Algorithm", edited by Mark Davis, Aharon Lanin, and Andrew Glass. An integral part of The Unicode Standard, <http://unicode.org/reports/tr9/>. [UTR36] Unicode Technical Report #36, "Unicode Security Considerations", by Mark Davis and Michel Suignard, <http://unicode.org/reports/tr36/>. [UTS39] Unicode Technical Standard #39, "Unicode Security Mechanisms", edited by Mark Davis and Michel Suignard, <http://unicode.org/reports/tr39/>. [XMPP-Addr-Format] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", Work in Progress, draft-ietf-xmpp-6122bis-22, May 2015.
Acknowledgements The authors would like to acknowledge the comments and contributions of the following individuals during working group discussion: David Black, Edward Burns, Dan Chiba, Mark Davis, Alan DeKok, Martin Duerst, Patrik Faltstrom, Ted Hardie, Joe Hildebrand, Bjoern Hoehrmann, Paul Hoffman, Jeffrey Hutzelman, Simon Josefsson, John Klensin, Alexey Melnikov, Takahiro Nemoto, Yoav Nir, Mike Parker, Pete Resnick, Andrew Sullivan, Dave Thaler, Yoshiro Yoneya, and Florian Zeitz. Special thanks are due to John Klensin and Patrik Faltstrom for their challenging feedback and detailed reviews. Charlie Kaufman, Tom Taylor, and Tim Wicinski reviewed the document on behalf of the Security Directorate, the General Area Review Team, and the Operations and Management Directorate, respectively. During IESG review, Alissa Cooper, Stephen Farrell, and Barry Leiba provided comments that led to further improvements. Some algorithms and textual descriptions have been borrowed from [RFC5892]. Some text regarding security has been borrowed from [RFC5890], [PRECIS-Users-Pwds], and [XMPP-Addr-Format]. Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for employing him during his work on earlier draft versions of this document. Authors' Addresses Peter Saint-Andre &yet EMail: email@example.com URI: https://andyet.com/ Marc Blanchet Viagenie 246 Aberdeen Quebec, QC G1R 2E1 Canada EMail: Marc.Blanchet@viagenie.ca URI: http://www.viagenie.ca/