Preparation and Comparison of Internationalized Strings Representing Usernames and PasswordsCisco Systems, Inc.1899 Wynkoop Street, Suite 600DenverCO80202USA+1-303-308-3282psaintan@cisco.comIsode Ltd5 Castle Business Village36 Station RoadHamptonMiddlesexTW12 2BXUKAlexey.Melnikov@isode.com
Applications
PRECISUsernamePasswordUnicodeInternationalizationSASLprepThis document describes methods for handling Unicode strings representing usernames and passwords. This document obsoletes RFC 4013.Usernames and passwords are widely used for authentication and authorization on the Internet, either directly when provided in plaintext (as in the SASL PLAIN mechanism or the HTTP Basic scheme ) or indirectly when provided as the input to a cryptographic algorithm such as a hash function (as in the SASL SCRAM mechanism or the HTTP Digest scheme ). To increase the likelihood that the input and comparison of usernames and passwords will work in ways that make sense for typical users throughout the world, this document defines rules for preparing and comparing internationalized strings that represent usernames and passwords.The methods specified in this document define two PRECIS profiles as explained in the PRECIS framework specification . This document assumes that all strings are comprised of characters from the Unicode character set , with special attention to characters outside the ASCII range . The methods defined here might be applicable wherever usernames or passwords are used. However, the methods are not intended for use in preparing strings that are not usernames (e.g., email addresses and LDAP distinguished names), nor in cases where identifiers or secrets are not strings (e.g., keys and certificates) or require specialized handling.This document obsoletes RFC 4013 (the "SASLprep" profile of stringprep ) but can be used by technologies other than the Simple Authentication and Security Layer (SASL) , such as HTTP authentication .Profiles of the PRECIS framework enable software to handle Unicode characters outside the ASCII range in an automated way, so that such characters are treated carefully and consistently in application protocols. In large measure, these profiles are designed to protect application developers from the potentially negative consequences of supporting the full range of Unicode characters. For instance, in almost all application protocols it would be dangerous to treat the Unicode character SUPERSCRIPT ONE (U+0089) as equivalent to DIGIT ONE (U+0031), since that would result in false positives during comparison, authentication, and authorization (e.g., an attacker could easy spoof an account "user1@example.com").Whereas a naive use of Unicode would make such attacks trivially easy, the Username PRECIS profile defined in this document generally protects applications from inadvertently causing such problems. (Similar considerations apply to passwords, although here it is desirable to support a wider range of characters so as to maximize entropy during authentication.)Many important terms used in this document are defined in , , , and . The term "non-ASCII space" refers to any Unicode code point having a general category of "Zs", with the exception of U+0020 (here called "ASCII space").As used here, the term "password" is not literally limited to a word; i.e., a password could be a passphrase consisting of more than one word, perhaps separated by spaces or other such characters.Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify that the authentication identity used in the context of such mechanisms is a "simple user name" (see Section 2 of as well as ). Various application technologies also assume that the identity of a user or account takes the form of a username (e.g., authentication for the HyperText Transfer Protocol ), whether or not they use SASL. Note well that the exact form of a username in any particular SASL mechanism or application technology is a matter for implementation and deployment, and that a username does not necessarily map to any particular application identifier (such as the localpart of an email address).The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .This document specifies that a username is a string of Unicode code points , encoded using UTF-8 , and structured either as an ordered sequence of "userparts" (where the complete username can consist of a single userpart or a space-separated sequence of userparts) or as a userpart@domainpart (where the domainpart is an IP literal, an IPv4 address, or a fully-qualified domain name).The syntax for a username is defined as follows using the Augmented Backus-Naur Form (ABNF) .All code points and blocks not explicitly allowed in the PRECIS IdentifierClass are disallowed; this includes private use characters, surrogate code points, and the other code points and blocks that were defined as "Prohibited Output" in . In addition, common constructions such as "user@example.com" are allowed as usernames under this specification, as they were under .Each userpart of a username MUST conform to the "UsernameIdentifierClass" profile of the PRECIS IdentifierClass, which is defined as follows:The base string class is the "IdentifierClass" specified in .Fullwidth and halfwidth characters MUST be mapped to their decomposition equivalents.So-called additional mappings MAY be applied, such as those defined in .Uppercase and titlecase characters might be mapped to their lowercase equivalents (see below).Unicode Normalization Form C (NFC) MUST be applied to all characters.With regard to directionality, the "Bidi Rule" provided in applies.A username MUST NOT be zero bytes in length. This rule is to be enforced after any normalization and mapping of code points.In protocols that provide usernames as input to a cryptographic algorithm such as a hash function, the client will need to perform proper preparation of the username before applying the algorithm.Case mapping is a matter for the application protocol, protocol implementation, or end deployment. In general, this document suggests that it is preferable to perform case mapping, since not doing so can lead to false positives during authentication and authorization (as described in ) and can result in confusion among end users given the prevalence of case mapping in many existing protocols and applications. However, there can be good reasons to not perform case mapping, such as backward compatibility with deployed infrastructure.In particular:SASL mechanisms that directly re-use this profile MUST specify whether and when case mapping is to be applied to authentication identifiers. SASL mechanisms SHOULD delay any case mapping to the last possible moment, such as when doing a lookup by username, username comparisons, or generating a cryptographic salt from a username (if the last possible moment happens on the server, then decisions about case mapping can be a matter of deployment policy). In keeping with RFC4422, SASL mechanisms are not to apply this or any other profile to authorization identifiers.Application protocols that use SASL (such as IMAP and XMPP ) and that directly re-use this profile MUST specify whether case mapping is to be applied to authorization identifiers. Such "SASL application protocols" SHOULD delay any case mapping of authorization identifiers to the last possible moment, which happens to necessarily be on the server side (this enables decisions about case mapping to be a matter of deployment policy). In keeping with RFC4422, SASL application protocols are not to apply this or any other profile to authentication identifiers.Application protocols that do not use SASL (such as HTTP authentication with the Basic and Digest schemes ) MUST specify whether and when case mapping is to be applied to authentication identifiers and authorization identifiers. Such "non-SASL application protocols" SHOULD delay any case mapping to the last possible moment, such as when doing a lookup by username, username comparisons, or generating a cryptographic salt from a username (if the last possible moment happens on the server, then decisions about case mapping can be a matter of deployment policy).If the specification for a SASL mechanism, SASL application protocol, or non-SASL application protocol specifies the handling of case mapping for strings that conform to the UsernameIdentifierClass, it MUST clearly describe whether case mapping is required, recommended, or optional at the level of the protocol itself, implementations thereof, or service deployments.The following examples illustrate a small number of usernames that are consistent with the format defined above (note that the characters < and > are used here to delineate the actual usernames and are not part of the username strings).Several points are worth noting. Regarding examples 2 and 3: although in German the character esszett (LATIN SMALL LETTER SHARP S, U+00DF) can mostly be used interchangeably with the two characters "ss", the userparts in these examples are different and (if desired) a server would need to enforce a registration policy that disallows one of them if the other is registered. Regarding examples 5, 6, and 7: optional case-mapping of GREEK CAPITAL LETTER SIGMA (U+03A3) to lowercase (i.e., to GREEK SMALL LETTER SIGMA, U+03C3) during comparison would result in matching the usernames in examples 5 and 6; however, because the PRECIS mapping rules do not account for the special status of GREEK SMALL LETTER FINAL SIGMA (U+03C2), the usernames in examples 5 and 7 or examples 6 and 7 would not be matched.The following examples illustrate strings that are not valid usernames because they violate the format defined above.Here again, several points are worth noting. Regarding example 11, the Unicode character ROMAN NUMERAL FOUR (U+2163) has a compatibility equivalent of the string formed of LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056), but characters with compatibility equivalents are not allowed in the PRECIS IdentiferClass. Regarding example 12: symbol characters such as BLACK CHESS KING (U+265A) are not allowed in the PRECIS IdentifierClass.This document specifies that a password is a string of Unicode code points , encoded using UTF-8 , and conformant to the PRECIS FreeformClass.The syntax for a password is defined as follows using the Augmented Backus-Naur Form (ABNF) .All code points and blocks not explicitly allowed in the PRECIS FreeformClass are disallowed; this includes private use characters, surrogate code points, and the other code points and blocks defined as "Prohibited Output" in Section 2.3 of RFC 4013.A password MUST conform to the "PasswordFreeformClass" profile of the PRECIS FreeformClass, which is defined as follows:The base string class is the "FreeformClass" specified in .Fullwidth and halfwidth characters MUST NOT be mapped to their decomposition equivalents.Any instances of non-ASCII space MUST be mapped to ASCII space (U+0020).So-called additional mappings MAY be applied, such as those defined in .Uppercase and titlecase characters MUST NOT be mapped to their lowercase equivalents.Unicode Normalization Form C (NFC) MUST be applied to all characters.With regard to directionality, the "Bidi Rule" (defined in ) and similar rules are unnecessary and inapplicable to passwords, since they can reduce the range of characters that are allowed in a string and therefore reduce the amount of entropy that is possible in a password. Furthermore, such rules are intended to minimize the possibility that the same string will be displayed differently on a system set for right-to-left display and a system set for left-to-right display; however, passwords are typically not displayed at all and are rarely meant to be interoperable across different systems in the way that non-secret strings like domain names and usernames are.A password MUST NOT be zero bytes in length. This rule is to be enforced after any normalization and mapping of code points.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.The following examples illustrate a small number of passwords that are consistent with the format defined above (note that the characters < and > are used here to delineate the actual passwords and are not part of the username strings).The following examples illustrate strings that are not valid passwords because they violate the format defined above.The rules defined in this specification differ slightly from those defined by the SASLprep specification . The following sections describe these differences, along with their implications for migration, in more detail.Deployments that currently use SASLprep for handling usernames might need to scrub existing data when migrating to use of the rules defined in this specification. In particular:SASLprep specified the use of Unicode Normalization Form KC (NFKC), whereas this usage of the PRECIS IdentifierClass employs Unicode Normalization Form C (NFC). In practice this change is unlikely to cause significant problems, because NFKC provides methods for mapping Unicode code points with compatibility equivalents to those equivalents, whereas the PRECIS IdentifierClass entirely disallows Unicode code points with compatibility equivalents (i.e., during comparison NFKC is more "aggressive" about finding matches than is NFC). A few examples might suffice to indicate the nature of the problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also handled the mapping of fullwidth and halfwidth code points to their decomposition equivalents (see ). Although it is expected that code points with compatibility equivalents are rare in existing usernames, for migration purposes deployments might want to search their database of usernames for Unicode code points with compatibility equivalents and map those code points to their compatibility equivalents.SASLprep mapped the "characters commonly mapped to nothing" from Appendix B.1 of ) to nothing, whereas the PRECIS IdentifierClass entirely disallows most of these characters, which correspond to the code points from the "M" category defined under Section 6.13 of (with the exception of U+1806 MONGOLIAN TODO SOFT HYPHEN, which was "commonly mapped to nothing" in Unicode 3.2 but at the time of this writing does not have a derived property of Default_Ignorable_Code_Point in Unicode 6.2). For migration purposes, deployments might want to remove code points contained in the PRECIS "M" category from usernames.SASLprep allowed uppercase and titlecase characters, whereas this usage of the PRECIS IdentifierClass maps uppercase and titlecase characters to their lowercase equivalents. For migration purposes, deployments can either convert uppercase and titlecase characters to their lowercase equivalents in usernames (thus losing the case information) or preserve uppercase and titlecase characters and ignore the case difference when comparing usernames.Depending on local service policy, migration from RFC 4013 to this specification might not involve any scrubbing of data (since passwords might not be stored in the clear anyway); however, service providers need to be aware of possible issues that might arise during migration. In particular:SASLprep specified the use of Unicode Normalization Form KC (NFKC), whereas this usage of the PRECIS FreeformClass employs Unicode Normalization Form C (NFC). Because NFKC is more aggressive about finding matches than NFC, in practice this change is unlikely to cause significant problems and indeed has the security benefit of probably resulting in fewer false positives when comparing passwords. A few examples might suffice to indicate the nature of the problem: (1) U+017F LATIN SMALL LETTER LONG S is compatibility equivalent to U+0073 LATIN SMALL LETTER S (2) U+2163 ROMAN NUMERAL FOUR is compatibility equivalent to U+0049 LATIN CAPITAL LETTER I and U+0056 LATIN CAPITAL LETTER V (3) U+FB01 LATIN SMALL LIGATURE FI is compatibility equivalent to U+0066 LATIN SMALL LETTER F and U+0069 LATIN SMALL LETTER I. Under SASLprep, the use of NFKC also handled the mapping of fullwidth and halfwidth code points to their decomposition equivalents (see ). Although it is expected that code points with compatibility equivalents are rare in existing passwords, some passwords that matched when SASLprep was used might no longer work when the rules in this specification are applied.SASLprep mapped the "characters commonly mapped to nothing" from Appendix B.1 of ) to nothing, whereas the PRECIS FreeformClass entirely disallows such characters, which correspond to the code points from the "M" category defined under Section 6.13 of (with the exception of U+1806 MONGOLIAN TODO SOFT HYPHEN, which was commonly mapped to nothing in Unicode 3.2 but at the time of this writing is allowed by Unicode 6.2). In practice, this change will probably have no effect on comparison, but user-oriented software might reject such code points instead of ignoring them during password preparation.The IANA shall add the following entries to the PRECIS Profiles Registry.UsernameIdentifierClass.Usernames in security and application protocols.IdentifierClass.The SASLprep profile of Stringprep.Map fullwidth and halfwidth characters to their decomposition equivalents.None required or recommended.To be defined by security or application protocols that use this profile.NFC.The "Bidi Rule" defined in RFC 5893 applies.None.To be defined by security or application protocols that use this profile.RFC XXXX. [Note to RFC Editor: please change XXXX to the number issued for this specification.]PasswordFreeformClass.Passwords in security and application protocols.FreeformClassThe SASLprep profile of Stringprep.None.Map non-ASCII space characters to ASCII space.None.NFC.None.None.To be defined by security or application protocols that use this profile.RFC XXXX.The ability to include a wide range of characters in passwords and passphrases can increase the potential for creating a strong password with high entropy. However, in practice, the ability to include such characters ought to be weighed against the possible need to reproduce them on various devices using various input methods.The process of comparing identifiers (such as SASL simple user names, authentication identifiers, and authorization identifiers) can lead to either false negatives or false positives, both of which have security implications. A more detailed discussion can be found in .The security considerations described in apply to the "IdentifierClass" and "FreeformClass" base string classes used in this document for usernames and passwords, respectively.The security considerations described in apply to the use of Unicode characters in usernames and passwords.Precis Framework: Handling Internationalized Strings in ProtocolsCiscoViagenieApplication protocols using Unicode code points in protocol strings need to properly prepare such strings in order to perform valid comparison operations (e.g., for purposes of authentication or authorization). This document defines a framework enabling application protocols to perform the preparation and comparison of internationalized strings (a.k.a. "PRECIS") in a way that depends on the properties of Unicode code points and thus is agile with respect to versions of Unicode. As a result, this framework provides a more sustainable approach to the handling of internationalized strings than the previous framework, known as Stringprep (RFC 3454). This document obsoletes RFC 3454.Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keyword
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
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
RFC 2119.
Note that the force of these words is modified by the requirement
level of the document in which they are used.
UTF-8, a transformation format of ISO 10646ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279.Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]The Unicode Standard, Version 6.1The Unicode ConsortiumMapping characters for PRECIS classesThe framework for preparation and comparison of internationalized strings ("PRECIS") defines several classes of strings for preparation and comparison. In the framework, case mapping is defined because many protocols handle case-sensitive or case-insensitive string comparison and therefore preparation of the string is mandatory. As described in the mapping for Internationalized Domain Names in Applications (IDNA) and the PRECIS problem statement, mappings for internationalized strings are not limited to case, but also width mapping and mapping of delimiters and other specials can be taken into consideration. This document provides guidelines for authors of protocol profiles of the PRECIS framework and describes several mappings that can be applied between receiving user input and passing permitted code points to internationalized protocols. The mappings described here are expected to be applied as Additional mapping in the PRECIS framework.ASCII format for network interchangeUniversity California Los Angeles (UCLA)For concreteness, we suggest the use of standard 7-bit ASCII embedded in an 8 bit byte whose high order bit is always 0.HTTP Authentication: Basic and Digest Access AuthenticationNorthwestern University, Department of MathematicsNorthwestern UniversityEvanstonIL60208-2730USAjohn@math.nwu.eduVerisign Inc.301 Edgewater PlaceSuite 210WakefieldMA01880USApbaker@verisign.comAbiSource, Inc.6 Dunlap CourtSavoyIL61874USAjeff@AbiSource.comAgranat Systems, Inc.5 Clocktower PlaceSuite 400MaynardMA01754USAlawrence@agranat.comMicrosoft Corporation1 Microsoft WayRedmondWA98052USApaulle@microsoft.comNetscape Communications Corporation501 East Middlefield RoadMountain ViewCA94043USAOpen Market, Inc.215 First StreetCambridgeMA02142USAstewart@OpenMarket.com
"HTTP/1.0", includes the specification for a Basic Access
Authentication scheme. This scheme is not considered to be a secure
method of user authentication (unless used in conjunction with some
external secure system such as SSL ), as the user name and
password are passed over the network as cleartext.
This document also provides the specification for HTTP's
authentication framework, the original Basic authentication scheme
and a scheme based on cryptographic hashes, referred to as "Digest
Access Authentication". It is therefore also intended to serve as a
replacement for RFC 2069 . Some optional elements specified by
RFC 2069 have been removed from this specification due to problems
found since its publication; other new elements have been added for
compatibility, those new elements have been made optional, but are
strongly recommended.
Like Basic, Digest access authentication verifies that both parties
to a communication know a shared secret (a password); unlike Basic,
this verification can be done without sending the password in the
clear, which is Basic's biggest weakness. As with most other
authentication protocols, the greatest sources of risks are usually
found not in the core protocol itself but in policies and procedures
surrounding its use.
Preparation of Internationalized Strings ("stringprep")INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev1 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev1 also provides the capability for an offline client to resynchronize with the server. IMAP4rev1 includes operations for creating, deleting, and renaming mailboxes, checking for new messages, permanently removing messages, setting and clearing flags, RFC 2822 and RFC 2045 parsing, searching, and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev1 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev1 supports a single server. A mechanism for accessing configuration information to support multiple IMAP4rev1 servers is discussed in RFC 2244. IMAP4rev1 does not specify a means of posting mail; this function is handled by a mail transfer protocol such as RFC 2821. [STANDARDS-TRACK]SASLprep: Stringprep Profile for User Names and PasswordsThis document describes how to prepare Unicode strings representing usernames and passwords for comparison. The document defines the "SASLprep" profile of the "stringprep" algorithm to be used for both usernames and passwords. This profile is intended to be used by Simple Authentication and Security Layer (SASL) mechanisms (such as PLAIN, CRAM-MD5, and DIGEST-MD5), as well as other protocols exchanging simple user names and/or passwords. [STANDARDS-TRACK]Simple Authentication and Security Layer (SASL)<p>The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. It provides a structured interface between protocols and mechanisms. The resulting framework allows new protocols to reuse existing mechanisms and allows old protocols to make use of new mechanisms. The framework also provides a protocol for securing subsequent protocol exchanges within a data security layer.</p><p> This document describes how a SASL mechanism is structured, describes how protocols include support for SASL, and defines the protocol for carrying a data security layer over a connection. In addition, this document defines one SASL mechanism, the EXTERNAL mechanism.</p><p> This document obsoletes RFC 2222. [STANDARDS TRACK]</p>The PLAIN Simple Authentication and Security Layer (SASL) MechanismThis document defines a simple clear-text user/password Simple Authentication and Security Layer (SASL) mechanism called the PLAIN mechanism. The PLAIN mechanism is intended to be used, in combination with data confidentiality services provided by a lower layer, in protocols that lack a simple password authentication command. [STANDARDS-TRACK]Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API MechanismsThe secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment, and have had success only in limited use.</t><t> This specification describes a family of Simple Authentication and Security Layer (SASL; RFC 4422) authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM), which addresses the security concerns and meets the deployability requirements. When used in combination with TLS or an equivalent security layer, a mechanism from this family could improve the status quo for application protocol authentication and provide a suitable choice for a mandatory-to-implement mechanism for future application protocol standards. [STANDARDS-TRACK]Internationalized Domain Names for Applications (IDNA): Definitions and Document FrameworkThis document is one of a collection that, together, describe the protocol and usage context for a revision of Internationalized Domain Names for Applications (IDNA), superseding the earlier version. It describes the document collection and provides definitions and other material that are common to the set. [STANDARDS TRACK]Internationalized Domain Names in Applications (IDNA): ProtocolThis document is the revised protocol definition for Internationalized Domain Names (IDNs). The rationale for changes, the relationship to the older specification, and important terminology are provided in other documents. This document specifies the protocol mechanism, called Internationalized Domain Names in Applications (IDNA), for registering and looking up IDNs in a way that does not require changes to the DNS itself. IDNA is only meant for processing domain names, not free text. [STANDARDS TRACK]Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)The use of right-to-left scripts in Internationalized Domain Names (IDNs) has presented several challenges. This memo provides a new Bidi rule for Internationalized Domain Names for Applications (IDNA) labels, based on the encountered problems with some scripts and some shortcomings in the 2003 IDNA Bidi criterion. [STANDARDS-TRACK]Internationalized Domain Names for Applications (IDNA): Background, Explanation, and RationaleSeveral years have passed since the original protocol for Internationalized Domain Names (IDNs) was completed and deployed. During that time, a number of issues have arisen, including the need to update the system to deal with newer versions of Unicode. Some of these issues require tuning of the existing protocols and the tables on which they depend. This document provides an overview of a revised system and provides explanatory material for its components. This document is not an Internet Standards Track specification; it is published for informational purposes.Extensible Messaging and Presence Protocol (XMPP): CoreThe Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions. This document obsoletes RFC 3920. [STANDARDS-TRACK]Terminology Used in Internationalization in the IETFThis document provides a list of terms used in the IETF when discussing internationalization. The purpose is to help frame discussions of internationalization in the various areas of the IETF and to help introduce the main concepts to IETF participants. This memo documents an Internet Best Current Practice.Issues in Identifier Comparison for Security PurposesIdentifiers such as hostnames, URIs, IP addresses, and email addresses are often used in security contexts to identify security principals and resources. In such contexts, an identifier presented via some protocol is often compared using some policy to make security decisions such as whether the security principal may access the resource, what level of authentication or encryption is required, etc. If the parties involved in a security decision use different algorithms to compare identifiers, then failure scenarios ranging from denial of service to elevation of privilege can result. This document provides a discussion of these issues that designers should consider when defining identifiers and protocols, and when constructing architectures that use multiple protocols.Unicode Technical Standard #39: Unicode Security MechanismsThe Unicode ConsortiumThis document builds upon the PRECIS framework defined in , which differs fundamentally from the stringprep technology used in SASLprep . The primary difference is that stringprep profiles allowed all characters except those which were explicitly disallowed, whereas PRECIS profiles disallow all characters except those which are explicitly allowed (this "inclusion model" was originally used for internationalized domain names in ; see for further discussion). It is important to keep this distinction in mind when comparing the technology defined in this document to SASLprep .The following substantive modifications were made from RFC 4013.A single SASLprep algorithm was replaced by two separate algorithms: one for usernames and another for passwords.The new preparation algorithms use PRECIS instead of a stringprep profile. The new algorithms work independenctly of Unicode versions.As recommended in the PRECIS framwork, changed the Unicode normalization form to NFC (from NFKC).Some Unicode code points that were mapped to nothing in RFC 4013 are simply disallowed by PRECIS.The following individuals provided helpful feedback on this document: Marc Blanchet, Alan DeKok, Joe Hildebrand, Jeffrey Hutzelman, Simon Josefsson, Jonathan Lennox, Matt Miller, Chris Newman, Yutaka OIWA, Pete Resnick, Andrew Sullivan, and Nico Williams (Nico in particular provided text that was used in ). Thanks also to Yoshiro YONEYA and Takahiro NEMOTO for implementation feedback.This document borrows some text from and .