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IETF RFC 6885
Last modified on Thursday, March 14th, 2013
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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/RFC 6885.
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RFC 6885 Stringprep Revision Problem Statement March 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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,
RFC 5034, RFC 5804 . . . . . . . . . . . . . . . . . . . . 21
B.3. IMAP Stringprep Profiles for Usernames: RFC 4314, RFC
5738 . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
B.4. IMAP Stringprep Profiles for Passwords: RFC 5738 . . . . . 26
B.5. Anonymous SASL Stringprep Profiles: RFC 4505 . . . . . . . 28
B.6. XMPP Stringprep Profiles for Nodeprep: RFC 3920 . . . . . 30
B.7. XMPP Stringprep Profiles for Resourceprep: RFC 3920 . . . 31
B.8. EAP Stringprep Profiles: RFC 3748 . . . . . . . . . . . . 33
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1. Introduction
Internationalizing Domain Names in Applications (here called
IDNA2003) [RFC 3490] [RFC 3491] [RFC 3492] and [RFC 3454] describes a
mechanism for encoding Unicode labels that make up the
Internationalized Domain Names (IDNs) as standard DNS labels. The
labels were processed using a method called Nameprep [RFC 3491] and
Punycode [RFC 3492]. That method was specific to IDNA2003 but is
generalized as Stringprep [RFC 3454]. The general mechanism is used
by other protocols with similar needs but with different constraints
than IDNA2003.
Stringprep defines a framework within which protocols define their
Stringprep profiles. Some known IETF specifications using Stringprep
are listed below:
o The Nameprep profile [RFC 3490] for use in Internationalized Domain
Names (IDNs);
o The Inter-Asterisk eXchange (IAX) using Nameprep [RFC 5456];
o NFSv4 [RFC 3530] and NFSv4.1 [RFC 5661];
o The Internet Small Computer System Interface (iSCSI) profile
[RFC 3722] for use in iSCSI names;
o The Extensible Authentication Protocol (EAP) [RFC 3748];
o The Nodeprep and Resourceprep profiles [RFC 3920] (which was
obsoleted by [RFC 6120]) for use in the Extensible Messaging and
Presence Protocol (XMPP), and the XMPP to Common Presence and
Instant Messaging (CPIM) mapping [RFC 3922] (the latter of these
relies on the former);
o The Internationalized Resource Identifier (IRI) and URI in XMPP
[RFC 5122];
o The Policy MIB profile [RFC 4011] for use in the Simple Network
Management Protocol (SNMP);
o Transport Layer Security (TLS) [RFC 4279];
o The Lightweight Directory Access Protocol (LDAP) profile [RFC 4518]
for use with LDAP [RFC 4511] and its authentication methods
[RFC 4513];
o PKIX subject identification using LDAPprep [RFC 4683];
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o PKIX Certificate Revocation List (CRL) using LDAPprep [RFC 5280];
o The Simple Authentication and Security Layer (SASL) [RFC 4422] and
SASLprep profile [RFC 4013] for use in SASL;
o Plain SASL using SASLprep [RFC 4616];
o SMTP Auth using SASLprep [RFC 4954];
o The Post Office Protocol (POP3) Auth using SASLprep [RFC 5034];
o TLS Secure Remote Password (SRP) using SASLprep [RFC 5054];
o SASL Salted Challenge Response Authentication Mechanism (SCRAM)
using SASLprep [RFC 5802];
o Remote management of Sieve using SASLprep [RFC 5804];
o The Network News Transfer Protocol (NNTP) using SASLprep
[RFC 4643];
o IMAP4 using SASLprep [RFC 4314];
o The trace profile [RFC 4505] for use with the SASL ANONYMOUS
mechanism;
o Internet Application Protocol Collation Registry [RFC 4790];
o The unicode-casemap Unicode Collation [RFC 5051].
However, a review (see [78PRECIS]) of these protocol specifications
found that they are very similar and can be grouped into a short
number of classes. Moreover, many reuse the same Stringprep profile,
such as the SASL one.
IDNA2003 was replaced because of some limitations described in
[RFC 4690]. The new IDN specification, called IDNA2008 [RFC 5890],
[RFC 5891], [RFC 5892], [RFC 5893] was designed based on the
considerations found in [RFC 5894]. One of the effects of IDNA2008 is
that Nameprep and Stringprep are not used at all. Instead, an
algorithm based on Unicode properties of code points is defined.
That algorithm generates a stable and complete table of the supported
Unicode code points for each Unicode version. This algorithm uses an
inclusion-based approach, instead of the exclusion-based approach of
Stringprep/Nameprep. That is, IDNA2003 created an explicit list of
excluded or mapped-away characters; anything in Unicode 3.2 that was
not so listed could be assumed to be allowed under the protocol.
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IDNA2008 begins instead from the assumption that code points are
disallowed and then relies on Unicode properties to derive whether a
given code point actually is allowed in the protocol.
This document lists the shortcomings and issues found by protocols
listed above that defined Stringprep profiles. It also lists the
requirements for any potential replacement of Stringprep.
2. Keywords
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].
This document uses various internationalization terms, which are
defined and discussed in [RFC 6365].
Additionally, this document defines the following keyword:
PRECIS: Preparation and Comparison of Internationalized Strings
3. Conventions
A single Unicode code point in this memo is denoted by "U+" followed
by four to six hexadecimal digits, as used in [Unicode61],
Appendix A.
4. Stringprep Profiles Limitations
During IETF 77 (March 2010), a BOF discussed the current state of the
protocols that have defined Stringprep profiles [NEWPREP]. The main
conclusions from that discussion were as follows:
o Stringprep is bound to Version 3.2 of Unicode. Stringprep has not
been updated to new versions of Unicode. Therefore, the protocols
using Stringprep are stuck at Unicode 3.2, and their
specifications need to be updated to support new versions of
Unicode.
o The protocols would like to not be bound to a specific version of
Unicode, but rather have better Unicode version agility in the way
of IDNA2008. This is important partly because it is usually
impossible for an application to require Unicode 3.2; the
application gets whatever version of Unicode is available on the
host.
o The protocols require better bidirectional support (bidi) than
currently offered by Stringprep.
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o If the protocols are updated to use a new version of Stringprep or
another framework, then backward compatibility is an important
requirement. For example, Stringprep normalization is based on
and profiles may use Unicode Normalization Form KC (NFKC) [UAX15],
while IDNA2008 mostly uses Unicode Normalization Form C (NFC)
[UAX15].
o Identifiers are passed between protocols. For example, the same
username string of code points may be passed between SASL, XMPP,
LDAP, and EAP. Therefore, a common set of rules or classes of
strings are preferred over specific rules for each protocol.
Without real planning in advance, many Stringprep profiles reuse
other profiles, so this goal was accomplished by accident with
Stringprep.
Protocols that use Stringprep profiles use strings for different
purposes:
o XMPP uses a different Stringprep profile for each part of the XMPP
address Jabber Identifier (JID): a localpart, which is similar to
a username and used for authentication; a domainpart, which is a
domain name; and a resourcepart, which is less restrictive than
the localpart.
o iSCSI uses a Stringprep profile for the names of protocol
participants (called initiators and targets). The iSCSI Qualified
Name (IQN) format of iSCSI names contains a reversed DNS domain
name.
o SASL and LDAP use a Stringprep profile for usernames.
o LDAP uses a set of Stringprep profiles.
The apparent judgement of the BOF attendees [NEWPREP] was that it
would be highly desirable to have a replacement of Stringprep, with
similar characteristics to IDNA2008. That replacement should be
defined so that the protocols could use internationalized strings
without a lot of specialized internationalization work, since
internationalization expertise is not available in the respective
protocols or working groups. Accordingly, the IESG formed the PRECIS
working group to undertake the task.
Notwithstanding the desire evident in [NEWPREP] and the chartering of
a working group, IDNA2008 may be a poor model for what other
protocols ought to do, because it is designed to support an old
protocol that is designed to operate on the scale of the entire
Internet. Moreover, IDNA2008 is intended to be deployed without any
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change to the base DNS protocol. Other protocols may aim at
deployment in more local environments, or may have protocol version
negotiation built in.
5. Major Topics for Consideration
This section provides an overview of major topics that a Stringprep
replacement needs to address. The headings correspond roughly with
categories under which known Stringprep-using protocol RFCs have been
evaluated. For the details of those evaluations, see Appendix A.
5.1. Comparison
5.1.1. Types of Identifiers
Following [ID-COMP], it is possible to organize identifiers into
three classes in respect of how they may be compared with one
another:
Absolute Identifiers: Identifiers that can be compared byte-by-byte
for equality.
Definite Identifiers: Identifiers that have a well-defined
comparison algorithm on which all parties agree.
Indefinite Identifiers: Identifiers that have no single comparison
algorithm on which all parties agree.
Definite Identifiers include cases like the comparison of Unicode
code points in different encodings: they do not match byte for byte
but can all be converted to a single encoding which then does match
byte for byte. Indefinite Identifiers are sometimes algorithmically
comparable by well-specified subsets of parties. For more discussion
of these categories, see [ID-COMP].
The section on treating the existing known cases, Appendix A, uses
the categories above.
5.1.2. Effect of Comparison
The three classes of comparison style outlined in Section 5.1.1 may
have different effects when applied. It is necessary to evaluate the
effects if a comparison results in a false positive or a false
negative, especially in terms of the consequences to security and
usability.
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5.2. Dealing with Characters
This section outlines a range of issues having to do with characters
in the target protocols, the ways in which IDNA2008 might be a good
analogy to other protocols, and ways in which it might be a poor one.
5.2.1. Case Folding, Case Sensitivity, and Case Preservation
In IDNA2003, labels are always mapped to lowercase before the
Punycode transformation. In IDNA2008, there is no mapping at all:
input is either a valid U-label or it is not. At the same time,
uppercase characters are by definition not valid U-labels, because
they fall into the Unstable category (category B) of [RFC 5892].
If there are protocols that require case be preserved, then the
analogy with IDNA2008 will break down. Accordingly, existing
protocols are to be evaluated according to the following criteria:
1. Does the protocol use case folding? For all blocks of code
points or just for certain subsets?
2. Is the system or protocol case-sensitive?
3. Does the system or protocol preserve case?
5.2.2. Stringprep and NFKC
Stringprep profiles may use normalization. If they do, they use NFKC
[UAX15] (most profiles do). It is not clear that NFKC is the right
normalization to use in all cases. In [UAX15], there is the
following observation regarding Normalization Forms KC and KD: "It is
best to think of these Normalization Forms as being like uppercase or
lowercase mappings: useful in certain contexts for identifying core
meanings, but also performing modifications to the text that may not
always be appropriate." In general, it can be said that NFKC is more
aggressive about finding matches between code points than NFC. For
things like the spelling of users' names, NFKC may not be the best
form to use. At the same time, one of the nice things about NFKC is
that it deals with the width of characters that are otherwise
similar, by canonicalizing half-width to full-width. This mapping
step can be crucial in practice. A replacement for Stringprep
depends on analyzing the different use profiles and considering
whether NFKC or NFC is a better normalization for each profile.
For the purposes of evaluating an existing example of Stringprep use,
it is helpful to know whether it uses no normalization, NFKC, or NFC.
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5.2.3. Character Mapping
Along with the case mapping issues raised in Section 5.2.1, there is
the question of whether some characters are mapped either to other
characters or to nothing during Stringprep. [RFC 3454], Section 3,
outlines a number of characters that are mapped to nothing, and also
permits Stringprep profiles to define their own mappings.
5.2.4. Prohibited Characters
Along with case folding and other character mappings, many protocols
have characters that are simply disallowed. For example, control
characters and special characters such as "@" or "/" may be
prohibited in a protocol.
One of the primary changes of IDNA2008 is in the way it approaches
Unicode code points, using the new inclusion-based approach (see
Section 1).
Because of the default assumption in IDNA2008 that a code point is
not allowed by the protocol, it has more than one class of "allowed
by the protocol"; this is unlike IDNA2003. While some code points
are disallowed outright, some are allowed only in certain contexts.
The reasons for the context-dependent rules have to do with the way
some characters are used. For instance, the ZERO WIDTH JOINER and
ZERO WIDTH NON-JOINER (ZWJ, U+200D and ZWNJ, U+200C) are allowed with
contextual rules because they are required in some circumstances, yet
are considered punctuation by Unicode and would therefore be
DISALLOWED under the usual IDNA2008 derivation rules. The goal of
IDNA2008 is to provide the widest repertoire of code points possible
and consistent with the traditional DNS "LDH" (letters, digits,
hyphen) rule (see [RFC 952]), trusting to the operators of individual
zones to make sensible (and usually more restrictive) policies for
their zones.
5.2.5. Internal Structure, Delimiters, and Special Characters
IDNA2008 has a special problem with delimiters, because the delimiter
"character" in the DNS wire format is not really part of the data.
In DNS, labels are not separated exactly; instead, a label carries
with it an indicator that says how long the label is. When the label
is displayed in presentation format as part of a fully qualified
domain name, the label separator FULL STOP, U+002E (.) is used to
break up the labels. But because that label separator does not
travel with the wire format of the domain name, there is no way to
encode a different, "internationalized" separator in IDNA2008.
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Other protocols may include characters with similar special meaning
within the protocol. Common characters for these purposes include
FULL STOP, U+002E (.); COMMERCIAL AT, U+0040 (@); HYPHEN-MINUS,
U+002D (-); SOLIDUS, U+002F (/); and LOW LINE, U+005F (_). The mere
inclusion of such a character in the protocol is not enough for it to
be considered similar to another protocol using the same character;
instead, handling of the character must be taken into consideration
as well.
An important issue to tackle here is whether it is valuable to map to
or from these special characters as part of the Stringprep
replacement. In some locales, the analogue to FULL STOP, U+002E is
some other character, and users may expect to be able to substitute
their normal stop for FULL STOP, U+002E. At the same time, there are
predictability arguments in favor of treating identifiers with FULL
STOP, U+002E in them just the way they are treated under IDNA2008.
5.2.6. Restrictions Because of Glyph Similarity
Homoglyphs are similarly (or identically) rendered glyphs of
different code points. For DNS names, homoglyphs may enable
phishing. If a protocol requires some visual comparison by end-
users, then the issue of homoglyphs is to be considered. In the DNS
context, these issues are documented in [RFC 5894] and [RFC 4690].
However, IDNA2008 does not have a mechanism to deal with them,
trusting DNS zone operators to enact sensible policies for the subset
of Unicode they wish to support, given their user community. A
similar policy/protocol split may not be desirable in every protocol.
5.3. Where the Data Comes from and Where It Goes
5.3.1. User Input and the Source of Protocol Elements
Some protocol elements are provided by users, and others are not.
Those that are not may presumably be subject to greater restrictions,
whereas those that users provide likely need to permit the broadest
range of code points. The following questions are helpful:
1. Do users input the strings directly?
2. If so, how? (keyboard, stylus, voice, copy-paste, etc.)
3. Where do we place the dividing line between user interface and
protocol? (see [RFC 5895])
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5.3.2. User Output
Just as only some protocol elements are expected to be entered
directly by users, only some protocol elements are intended to be
consumed directly by users. It is important to know how users are
expected to be able to consume the protocol elements, because
different environments present different challenges. An element that
is only ever delivered as part of a vCard remains in machine-readable
format, so the problem of visual confusion is not a great one. Is
the protocol element published as part of a vCard, a web directory,
on a business card, or on "the side of a bus"? Do users use the
protocol element as an identifier (which means that they might enter
it again in some other context)? (See also Section 5.2.6.)
5.3.3. Operations
Some strings are useful as part of the protocol but are not used as
input to other operations (for instance, purely informative or
descriptive text). Other strings are used directly as input to other
operations (such as cryptographic hash functions), or are used
together with other strings to (such as concatenating a string with
some others to form a unique identifier).
5.3.3.1. String Classes
Strings often have a similar function in different protocols. For
instance, many different protocols contain user identifiers or
passwords. A single profile for all such uses might be desirable.
Often, a string in a protocol is effectively a protocol element from
another protocol. For instance, different systems might use the same
credentials database for authentication.
5.3.3.2. Community Considerations
A Stringprep replacement that does anything more than just update
Stringprep to the latest version of Unicode will probably entail some
changes. It is important to identify the willingness of the
protocol-using community to accept backwards-incompatible changes.
By the same token, it is important to evaluate the desire of the
community for features not available under Stringprep.
5.3.3.3. Unicode Incompatible Changes
IDNA2008 uses an algorithm to derive the validity of a Unicode code
point for use under IDNA2008. It does this by using the properties
of each code point to test its validity.
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This approach depends crucially on the idea that code points, once
valid for a protocol profile, will not later be made invalid. That
is not a guarantee currently provided by Unicode. Properties of code
points may change between versions of Unicode. Rarely, such a change
could cause a given code point to become invalid under a protocol
profile, even though the code point would be valid with an earlier
version of Unicode. This is not merely a theoretical possibility,
because it has occurred [RFC 6452].
Accordingly, as in IDNA2008, a Stringprep replacement that intends to
be Unicode version agnostic will need to work out a mechanism to
address cases where incompatible changes occur because of new Unicode
versions.
6. Considerations for Stringprep Replacement
The above suggests the following guidance:
o A Stringprep replacement should be defined.
o The replacement should take an approach similar to IDNA2008 (e.g.,
by using properties of code points instead of whitelisting of code
points), in that it enables better Unicode agility.
o Protocols share similar characteristics of strings. Therefore,
defining internationalization preparation algorithms for the
smallest set of string classes may be sufficient for most cases,
providing coherence among a set of related protocols or protocols
where identifiers are exchanged.
o The sets of string classes need to be evaluated according to the
considerations that make up the headings in Section 5
o It is reasonable to limit scope to Unicode code points and rule
the mapping of data from other character encodings outside the
scope of this effort.
o The replacement ought to at least provide guidance to applications
using the replacement on how to handle protocol incompatibilities
resulting from changes to Unicode. In an ideal world, the
Stringprep replacement would handle the changes automatically, but
it appears that such automatic handling would require magic and
cannot be expected.
o Compatibility within each protocol between a technique that is
Stringprep-based and the technique's replacement has to be
considered very carefully.
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Existing deployments already depend on Stringprep profiles.
Therefore, a replacement must consider the effects of any new
strategy on existing deployments. By way of comparison, it is worth
noting that some characters were acceptable in IDNA labels under
IDNA2003, but are not protocol-valid under IDNA2008 (and conversely);
disagreement about what to do during the transition has resulted in
different approaches to mapping. Different implementers may make
different decisions about what to do in such cases; this could have
interoperability effects. It is necessary to trade better support
for different linguistic environments against the potential side
effects of backward incompatibility.
7. Security Considerations
This document merely states what problems are to be solved and does
not define a protocol. There are undoubtedly security implications
of the particular results that will come from the work to be
completed. Moreover, the Stringprep Security Considerations
[RFC 3454] Section applies. See also the analysis in the subsections
of Appendix B, below.
8. Acknowledgements
This document is the product of the PRECIS IETF Working Group, and
participants in that working group were helpful in addressing issues
with the text.
Specific contributions came from David Black, Alan DeKok, Simon
Josefsson, Bill McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave
Thaler, and Yoshiro Yoneya.
Dave Thaler provided the "buckets" insight in Section 5.1.1, central
to the organization of the problem.
Evaluations of Stringprep profiles that are included in Appendix B
were done by David Black, Alexey Melnikov, Peter Saint-Andre, and
Dave Thaler.
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9. Informative References
[78PRECIS] Blanchet, M., "PRECIS Framework", Proceedings of IETF
78, July 2010, <http://www.ietf.org/proceedings/78/
slides/precis-2.pdf>.
[ID-COMP] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", Work in Progress, March 2013.
[NEWPREP] "Newprep BoF Meeting Minutes", March 2010,
<http://www.ietf.org/proceedings/77/minutes/
newprep.txt>.
[RFC 952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD
Internet host table specification", RFC 952,
October 1985.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC 3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[RFC 3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC 3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.
[RFC 3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File
System (NFS) version 4 Protocol", RFC 3530, April 2003.
[RFC 3722] Bakke, M., "String Profile for Internet Small Computer
Systems Interface (iSCSI) Names", RFC 3722, April 2004.
[RFC 3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
Blanchet & Sullivan Informational PAGE 15
RFC 6885 Stringprep Revision Problem Statement March 2013
[RFC 3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[RFC 3922] Saint-Andre, P., "Mapping the Extensible Messaging and
Presence Protocol (XMPP) to Common Presence and Instant
Messaging (CPIM)", RFC 3922, October 2004.
[RFC 4011] Waldbusser, S., Saperia, J., and T. Hongal, "Policy
Based Management MIB", RFC 4011, March 2005.
[RFC 4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC 4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, December 2005.
[RFC 4314] Melnikov, A., "IMAP4 Access Control List (ACL)
Extension", RFC 4314, December 2005.
[RFC 4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC 4505] Zeilenga, K., "Anonymous Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4505, June 2006.
[RFC 4511] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC 4513] Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
[RFC 4518] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): Internationalized String Preparation", RFC 4518,
June 2006.
[RFC 4616] Zeilenga, K., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616, August 2006.
[RFC 4643] Vinocur, J. and K. Murchison, "Network News Transfer
Protocol (NNTP) Extension for Authentication", RFC 4643,
October 2006.
[RFC 4683] Park, J., Lee, J., Lee, H., Park, S., and T. Polk,
"Internet X.509 Public Key Infrastructure Subject
Identification Method (SIM)", RFC 4683, October 2006.
Blanchet & Sullivan Informational PAGE 16
RFC 6885 Stringprep Revision Problem Statement March 2013
[RFC 4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
and Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC 4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790,
March 2007.
[RFC 4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC 5034] Siemborski, R. and A. Menon-Sen, "The Post Office
Protocol (POP3) Simple Authentication and Security Layer
(SASL) Authentication Mechanism", RFC 5034, July 2007.
[RFC 5051] Crispin, M., "i;unicode-casemap - Simple Unicode
Collation Algorithm", RFC 5051, October 2007.
[RFC 5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T.
Perrin, "Using the Secure Remote Password (SRP) Protocol
for TLS Authentication", RFC 5054, November 2007.
[RFC 5122] Saint-Andre, P., "Internationalized Resource Identifiers
(IRIs) and Uniform Resource Identifiers (URIs) for the
Extensible Messaging and Presence Protocol (XMPP)",
RFC 5122, February 2008.
[RFC 5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC 5456] Spencer, M., Capouch, B., Guy, E., Miller, F., and K.
Shumard, "IAX: Inter-Asterisk eXchange Version 2",
RFC 5456, February 2010.
[RFC 5661] Shepler, S., Eisler, M., and D. Noveck, "Network File
System (NFS) Version 4 Minor Version 1 Protocol",
RFC 5661, January 2010.
[RFC 5802] Newman, C., Menon-Sen, A., Melnikov, A., and N.
Williams, "Salted Challenge Response Authentication
Mechanism (SCRAM) SASL and GSS-API Mechanisms",
RFC 5802, July 2010.
[RFC 5804] Melnikov, A. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, July 2010.
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[RFC 5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC 5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC 5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC 5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC 5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC 5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010.
[RFC 6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[RFC 6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
September 2011.
[RFC 6452] Faltstrom, P. and P. Hoffman, "The Unicode Code Points
and Internationalized Domain Names for Applications
(IDNA) - Unicode 6.0", RFC 6452, November 2011.
[UAX15] "Unicode Standard Annex #15: Unicode Normalization
Forms", UAX 15, September 2009.
[Unicode61] The Unicode Consortium. The Unicode Standard, Version
6.1.0, (Mountain View, CA: The Unicode Consortium, 2012.
ISBN 978-1-936213-02-3).
<http://www.unicode.org/versions/Unicode6.1.0/>.
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Appendix A. Classification of Stringprep Profiles
A number of the known cases of Stringprep use were evaluated during
the preparation of this document. The known cases are here described
in two ways. The types of identifiers the protocol uses is first
called out in the ID type column (from Section 5.1.1) using the short
forms "a" for Absolute, "d" for Definite, and "i" for Indefinite.
Next, there is a column that contains an "i" if the protocol string
comes from user input, an "o" if the protocol string becomes user-
facing output, "b" if both are true, and "n" if neither is true.
+------+--------+-------+
| RFC | IDtype | User? |
+------+--------+-------+
| 3722 | a | b |
| 3748 | - | - |
| 3920 | a,d | b |
| 4505 | a | i |
| 4314 | a,d | b |
| 4954 | a,d | b |
| 5034 | a,d | b |
| 5804 | a,d | b |
+------+--------+-------+
Table 1
Appendix B. Evaluation of Stringprep Profiles
This section is a summary of evaluation of Stringprep profiles that
was done to get a good understanding of the usage of Stringprep.
This summary is by no means normative nor the actual evaluations
themselves. A template was used for reviewers to get a coherent view
of all evaluations.
B.1. iSCSI Stringprep Profile: RFC 3720, RFC 3721, RFC 3722
Description: An iSCSI session consists of an initiator (i.e., host
or server that uses storage) communicating with a target (i.e., a
storage array or other system that provides storage). Both the
iSCSI initiator and target are named by iSCSI names. The iSCSI
Stringprep profile is used for iSCSI names.
How it is used: iSCSI initiators and targets (see above). They can
also be used to identify SCSI ports (these are software entities
in the iSCSI protocol, not hardware ports) and iSCSI logical units
(storage volumes), although both are unusual in practice.
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What entities create these identifiers? Generally, a human user (1)
configures an automated system (2) that generates the names.
Advance configuration of the system is required due to the
embedded use of external unique identifier (from the DNS or IEEE).
How is the string input in the system? Keyboard and copy-paste are
common. Copy-paste is common because iSCSI names are long enough
to be problematic for humans to remember, causing use of email,
sneaker-net, text files, etc., to avoid mistype mistakes.
Where do we place the dividing line between user interface and
protocol? The iSCSI protocol requires that all
internationalization string preparation occur in the user
interface. The iSCSI protocol treats iSCSI names as opaque
identifiers that are compared byte-by-byte for equality. iSCSI
names are generally not checked for correct formatting by the
protocol.
What entities enforce the rules? There are no iSCSI-specific
enforcement entities, although the use of unique identifier
information in the names relies on DNS registrars and the IEEE
Registration Authority.
Comparison: Byte-by-byte.
Case Folding, Sensitivity, Preservation: Case folding is required
for the code blocks specified in RFC 3454, Table B.2. The overall
iSCSI naming system (UI + protocol) is case-insensitive.
What is the impact if the comparison results in a false positive?
Potential access to the wrong storage.
- If the initiator has no access to the wrong storage, an
authentication failure is the probable result.
- If the initiator has access to the wrong storage, the resulting
misidentification could result in use of the wrong data and
possible corruption of stored data.
What is the impact if the comparison results in a false negative?
Denial of authorized storage access.
What are the security impacts? iSCSI names may be used as the
authentication identities for storage systems. Comparison
problems could result in authentication problems, although note
that authentication failure ameliorates some of the false positive
cases.
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Normalization: NFKC, as specified by RFC 3454.
Mapping: Yes, as specified by Table B.1 in RFC 3454.
Disallowed Characters: Only the following characters are allowed:
- ASCII dash, dot, colon
- ASCII lowercase letters and digits
- Unicode lowercase characters as specified by RFC 3454.
All other characters are disallowed.
Which other strings or identifiers are these most similar to?
None -- iSCSI names are unique to iSCSI.
Are these strings or identifiers sometimes the same as strings or
identifiers from other protocols? No.
Does the identifier have internal structure that needs to be
respected? Yes. ASCII dot, dash, and colon are used for internal
name structure. These are not reserved characters, in that they
can occur in the name in locations other than those used for
structuring purposes (e.g., only the first occurrence of a colon
character is structural, others are not).
How are users exposed to these strings? How are they published?
iSCSI names appear in server and storage system configuration
interfaces. They also appear in system logs.
Is the string / identifier used as input to other operations?
Effectively, no. The rarely used port and logical unit names
involve concatenation, which effectively extends a unique iSCSI
name for a target to uniquely identify something within that
target.
How much tolerance for change from existing Stringprep approach?
Good tolerance; the community would prefer that
internationalization experts solve internationalization problems.
How strong a desire for change (e.g., for Unicode agility)? Unicode
agility is desired, in principle, as long as nothing significant
breaks.
B.2. SMTP/POP3/ManageSieve Stringprep Profiles: RFC 4954, RFC 5034,
RFC 5804
Description: Authorization identity (user identifier) exchanged
during SASL authentication: AUTH (SMTP/POP3) or AUTHENTICATE
(ManageSieve) command.
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How It's Used: Used for proxy authorization, e.g., to [lawfully]
impersonate a particular user after a privileged authentication.
Who Generates It:
- Typically generated by email system administrators using some
tools/conventions, sometimes from some backend database.
- In some setups, human users can register their own usernames
(e.g., webmail self-registration).
User Input Methods:
- typing or selecting from a list
- copy and paste
- voice input
- in configuration files or on the command line
Enforcement: Rules enforced by server / add-on service (e.g.,
gateway service) on registration of account.
Comparison Method: "Type 1" (byte-for-byte) or "Type 2" (compare by
a common algorithm that everyone agrees on (e.g., normalize and
then compare the result byte-by-byte).
Case Folding, Sensitivity, Preservation: Most likely case-sensitive.
Exact requirements on case-sensitivity/case-preservation depend on
a specific implementation, e.g., an implementation might treat all
user identifiers as case-insensitive (or case-insensitive for
US-ASCII subset only).
Impact of Comparison: False positives: an unauthorized user is
allowed email service access (login). False negatives: an
authorized user is denied email service access.
Normalization: NFKC (as per RFC 4013).
Mapping: (see Section 2 of RFC 4013 for the full list) Non-ASCII
spaces are mapped to space, etc.
Disallowed Characters: (see Section 2 of RFC 4013 for the full list)
Unicode Control characters, etc.
String Classes: Simple username. See Section 2 of RFC 4013 for
details on restrictions. Note that some implementations allow
spaces in these. While implementations are not required to use a
specific format, an authorization identity frequently has the same
format as an email address (and Email Address Internationalization
(EAI) email address in the future), or as a left hand side of an
email address. Note: whatever is recommended for SMTP/POP/
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ManageSieve authorization identity should also be used for IMAP
authorization identities, as IMAP/POP3/SMTP/ManageSieve are
frequently implemented together.
Internal Structure: None
User Output: Unlikely, but possible. For example, if it is the same
as an email address.
Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function.
How much tolerance for change from existing Stringprep approach? Not
sure.
Background Information:
In RFC 5034, when describing the POP3 AUTH command:
The authorization identity generated by the SASL exchange is a
simple username, and SHOULD use the SASLprep profile (see
[RFC 4013]) of the StringPrep algorithm (see [RFC 3454]) to
prepare these names for matching. If preparation of the
authorization identity fails or results in an empty string
(unless it was transmitted as the empty string), the server
MUST fail the authentication.
In RFC 4954, when describing the SMTP AUTH command:
The authorization identity generated by this [SASL] exchange is
a "simple username" (in the sense defined in [SASLprep]), and
both client and server SHOULD (*) use the [SASLprep] profile of
the [StringPrep] algorithm to prepare these names for
transmission or comparison. If preparation of the
authorization identity fails or results in an empty string
(unless it was transmitted as the empty string), the server
MUST fail the authentication.
(*) Note: Future revision of this specification may change this
requirement to MUST. Currently, the SHOULD is used in order to
avoid breaking the majority of existing implementations.
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In RFC 5804, when describing the ManageSieve AUTHENTICATE command:
The authorization identity generated by this [SASL] exchange is
a "simple username" (in the sense defined in [SASLprep]), and
both client and server MUST use the [SASLprep] profile of the
[StringPrep] algorithm to prepare these names for transmission
or comparison. If preparation of the authorization identity
fails or results in an empty string (unless it was transmitted
as the empty string), the server MUST fail the authentication.
B.3. IMAP Stringprep Profiles for Usernames: RFC 4314, RFC 5738
Evaluation Note: These documents have 2 types of strings (usernames
and passwords), so there are two separate templates.
Description: "username" parameter to the IMAP LOGIN command,
identifiers in IMAP Access Control List (ACL) commands. Note that
any valid username is also an IMAP ACL identifier, but IMAP ACL
identifiers can include other things like the name of a group of
users.
How It's Used: Used for authentication (Usernames), or in IMAP
Access Control Lists (Usernames or Group names).
Who Generates It:
- Typically generated by email system administrators using some
tools/conventions, sometimes from some backend database.
- In some setups, human users can register own usernames (e.g.,
webmail self-registration).
User Input Methods:
- typing or selecting from a list
- copy and paste
- voice input
- in configuration files or on the command line
Enforcement: Rules enforced by server / add-on service (e.g.,
gateway service) on registration of account.
Comparison Method: "Type 1" (byte-for-byte) or "Type 2" (compare by
a common algorithm that everyone agrees on (e.g., normalize and
then compare the result byte-by-byte).
Case Folding, Sensitivity, Preservation: Most likely case-sensitive.
Exact requirements on case-sensitivity/case-preservation depend on
a specific implementation, e.g., an implementation might treat all
user identifiers as case-insensitive (or case-insensitive for
US-ASCII subset only).
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Impact of Comparison: False positives: an unauthorized user is
allowed IMAP access (login), privileges improperly granted (e.g.,
access to a specific mailbox, ability to manage ACLs for a
mailbox). False negatives: an authorized user is denied IMAP
access, unable to use granted privileges (e.g., access to a
specific mailbox, ability to manage ACLs for a mailbox).
Normalization: NFKC (as per RFC 4013)
Mapping: (see Section 2 of RFC 4013 for the full list) Non-ASCII
spaces are mapped to space.
Disallowed Characters: (see Section 2 of RFC 4013 for the full list)
Unicode Control characters, etc.
String Classes: Simple username. See Section 2 of RFC 4013 for
details on restrictions. Note that some implementations allow
spaces in these. While IMAP implementations are not required to
use a specific format, an IMAP username frequently has the same
format as an email address (and EAI email address in the future),
or as a left hand side of an email address. Note: whatever is
recommended for the IMAP username should also be used for
ManageSieve, POP3 and SMTP authorization identities, as IMAP/POP3/
SMTP/ManageSieve are frequently implemented together.
Internal Structure: None.
User Output: Unlikely, but possible. For example, if it is the same
as an email address, access control lists (e.g. in IMAP ACL
extension), both when managing membership and listing membership
of existing access control lists. Often shows up as mailbox names
(under Other Users IMAP namespace).
Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function.
How much tolerance for change from existing Stringprep approach? Not
sure. Non-ASCII IMAP usernames are currently prohibited by IMAP
(RFC 3501). However, they are allowed when used in IMAP ACL
extension.
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B.4. IMAP Stringprep Profiles for Passwords: RFC 5738
Description: "Password" parameter to the IMAP LOGIN command.
How It's Used: Used for authentication (Passwords).
Who Generates It: Either generated by email system administrators
using some tools/conventions, or specified by the human user.
User Input Methods:
- typing or selecting from a list
- copy and paste
- voice input
- in configuration files or on the command line
Enforcement: Rules enforced by server / add-on service (e.g.,
gateway service or backend database) on registration of account.
Comparison Method: "Type 1" (byte-for-byte).
Case Folding, Sensitivity, Preservation: Most likely case-sensitive.
Impact of Comparison: False positives: an unauthorized user is
allowed IMAP access (login). False negatives: an authorized user
is denied IMAP access.
Normalization: NFKC (as per RFC 4013).
Mapping: (see Section 2 of RFC 4013 for the full list) Non-ASCII
spaces are mapped to space.
Disallowed Characters: (see Section 2 of RFC 4013 for the full list)
Unicode Control characters, etc.
String Classes: Currently defined as "simple username" (see Section
2 of RFC 4013 for details on restrictions); however, this is
likely to be a different class from usernames. Note that some
implementations allow spaces in these. Password in all email
related protocols should be treated in the same way. Same
passwords are frequently shared with web, IM, and etc.
applications.
Internal Structure: None.
User Output: Text of email messages (e.g. in "you forgot your
password" email messages), web page / directory, side of the bus /
in ads -- possible.
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Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function. Frequently stored as is,
or hashed.
How much tolerance for change from existing Stringprep approach? Not
sure. Non-ASCII IMAP passwords are currently prohibited by IMAP
(RFC 3501); however, they are likely to be in widespread use.
Background Information:
RFC 5738, Section 5 ("UTF8=USER Capability"):
If the "UTF8=USER" capability is advertised, that indicates the
server accepts UTF-8 user names and passwords and applies
SASLprep [RFC 4013] to both arguments of the LOGIN command. The
server MUST reject UTF-8 that fails to comply with the formal
syntax in RFC 3629 [RFC 3629] or if it encounters Unicode
characters listed in Section 2.3 of SASLprep RFC 4013
[RFC 4013].
RFC 4314, Section 3 ("Access control management commands and
responses"):
Servers, when processing a command that has an identifier as a
parameter (i.e., any of SETACL, DELETEACL, and LISTRIGHTS
commands), SHOULD first prepare the received identifier using
"SASLprep" profile [SASLprep] of the "stringprep" algorithm
[Stringprep]. If the preparation of the identifier fails or
results in an empty string, the server MUST refuse to perform
the command with a BAD response. Note that Section 6
recommends additional identifier's verification steps.
RFC 4314, Section 6 ("Security Considerations"):
This document relies on [SASLprep] to describe steps required
to perform identifier canonicalization (preparation). The
preparation algorithm in SASLprep was specifically designed
such that its output is canonical, and it is well-formed.
However, due to an anomaly [PR29] in the specification of
Unicode normalization, canonical equivalence is not guaranteed
for a select few character sequences. Identifiers prepared
with SASLprep can be stored and returned by an ACL server. The
anomaly affects ACL manipulation and evaluation of identifiers
containing the selected character sequences. These sequences,
however, do not appear in well-formed text. In order to
address this problem, an ACL server MAY reject identifiers
containing sequences described in [PR29] by sending the tagged
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BAD response. This is in addition to the requirement to reject
identifiers that fail SASLprep preparation as described in
Section 3.
B.5. Anonymous SASL Stringprep Profiles: RFC 4505
Description: RFC 4505 defines a "trace" field:
Comparison: this field is not intended for comparison (only used for
logging)
Case folding; case-sensitivity, preserve case: No case folding/
case-sensitive
Do users input the strings directly? Yes. Possibly entered in
configuration UIs, or on a command line. Can also be stored in
configuration files. The value can also be automatically
generated by clients (e.g., a fixed string is used, or a user's
email address).
How users input strings? Keyboard/voice, stylus (pick from a list).
Copy-paste - possibly.
Normalization: None.
Disallowed Characters: Control characters are disallowed. (See
Section 3 of RFC 4505).
Which other strings or identifiers are these most similar to?
RFC 4505 says that the trace "should take one of two forms: an
Internet email address, or an opaque string that does not contain
the '@' (U+0040) character and that can be interpreted by the
system administrator of the client's domain". In practice, this
is a free-form text, so it belongs to a different class from
"email address" or "username".
Are these strings or identifiers sometimes the same as strings or
identifiers from other protocols (e.g., does an IM system
sometimes use the same credentials database for authentication as
an email system)? Yes: see above. However, there is no strong
need to keep them consistent in the future.
How are users exposed to these strings, how are they published? No.
However, the value can be seen in server logs.
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Impacts of false positives and false negatives:
False positive: a user can be confused with another user.
False negative: two distinct users are treated as the same user.
But note that the trace field is not authenticated, so it can be
easily falsified.
Tolerance of changes in the community: The community would be
flexible.
Delimiters: No internal structure, but see comments above about
frequent use of email addresses.
Background Information:
RFC 4505, Section 2 ("The Anonymous Mechanism"):
The mechanism consists of a single message from the client to the
server. The client may include in this message trace information
in the form of a string of [UTF-8]-encoded [Unicode] characters
prepared in accordance with [StringPrep] and the "trace"
stringprep profile defined in Section 3 of this document. The
trace information, which has no semantical value, should take one
of two forms: an Internet email address, or an opaque string that
does not contain the '@' (U+0040) character and that can be
interpreted by the system administrator of the client's domain.
For privacy reasons, an Internet email address or other
information identifying the user should only be used with
permission from the user.
RFC 4505, Section 3 ('The "trace" Profile of "Stringprep"'):
This section defines the "trace" profile of [StringPrep]. This
profile is designed for use with the SASL ANONYMOUS Mechanism.
Specifically, the client is to prepare the <message> production in
accordance with this profile.
The character repertoire of this profile is Unicode 3.2 [Unicode].
No mapping is required by this profile.
No Unicode normalization is required by this profile.
The list of unassigned code points for this profile is that
provided in Appendix A of [StringPrep]. Unassigned code points
are not prohibited.
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Characters from the following tables of [StringPrep] are
prohibited:
- C.2.1 (ASCII control characters)
- C.2.2 (Non-ASCII control characters)
- C.3 (Private use characters)
- C.4 (Non-character code points)
- C.5 (Surrogate codes)
- C.6 (Inappropriate for plain text)
- C.8 (Change display properties are deprecated)
- C.9 (Tagging characters)
No additional characters are prohibited.
This profile requires bidirectional character checking per Section 6
of [StringPrep].
B.6. XMPP Stringprep Profiles for Nodeprep: RFC 3920
Description: Localpart of JabberID ("JID"), as in:
localpart@domainpart/resourcepart
How It's Used:
- Usernames (e.g., stpeter@jabber.org)
- Chatroom names (e.g., precis@jabber.ietf.org)
- Publish-subscribe nodes
- Bot names
Who Generates It:
- Typically, end users via an XMPP client
- Sometimes created in an automated fashion
User Input Methods:
- typing
- copy and paste
- voice input
- clicking a URI/IRI
Enforcement: Rules enforced by server / add-on service (e.g.,
chatroom service) on registration of account, creation of room,
etc.
Comparison Method: "Type 2" (common algorithm)
Case Folding, Sensitivity, Preservation:
- Strings are always folded to lowercase
- Case is not preserved
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Impact of Comparison:
False positives:
- unable to authenticate at server (or authenticate to wrong
account)
- add wrong person to buddy list
- join the wrong chatroom
- improperly grant privileges (e.g., chatroom admin)
- subscribe to wrong pubsub node
- interact with wrong bot
- allow communication with blocked entity
False negatives:
- unable to authenticate
- unable to add someone to buddy list
- unable to join desired chatroom
- unable to use granted privileges (e.g., chatroom admin)
- unable to subscribe to desired pubsub node
- unable to interact with desired bot
- disallow communication with unblocked entity
Normalization: NFKC
Mapping: Spaces are mapped to nothing
Disallowed Characters: ",&,',/,:,<,>,@
String Classes:
- Often similar to generic username
- Often similar to localpart of email address
- Sometimes same as localpart of email address
Internal Structure: None
User Output:
- vCard
- email signature
- web page / directory
- text of message (e.g., in a chatroom)
Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function
B.7. XMPP Stringprep Profiles for Resourceprep: RFC 3920
Description:
- Resourcepart of JabberID ("JID"), as in:
localpart@domainpart/resourcepart
- Typically free-form text
Blanchet & Sullivan Informational PAGE 31
RFC 6885 Stringprep Revision Problem Statement March 2013
How It's Used:
- Device / session names (e.g., stpeter@jabber.org/Home)
- Nicknames (e.g., precis@jabber.ietf.org/StPeter)
Who Generates It:
- Often human users via an XMPP client
- Often generated in an automated fashion by client or server
User Input Methods:
- typing
- copy and paste
- voice input
- clicking a URI/IRI
Enforcement: Rules enforced by server / add-on service (e.g.,
chatroom service) on account login, joining a chatroom, etc.
Comparison Method: "Type 2" (byte-for-byte)
Case Folding, Sensitivity, Preservation:
- Strings are never folded
- Case is preserved
Impact of Comparison:
False positives:
- interact with wrong device (e.g., for file transfer or voice
call)
- interact with wrong chatroom participant
- improperly grant privileges (e.g., chatroom moderator)
- allow communication with blocked entity
False negatives:
- unable to choose desired chatroom nickname
- unable to use granted privileges (e.g., chatroom moderator)
- disallow communication with unblocked entity
Normalization: NFKC
Mapping: Spaces are mapped to nothing
Disallowed Characters: None
String Classes: Basically a free-form identifier
Internal Structure: None
User Output:
- text of message (e.g., in a chatroom)
- device names often not exposed to human users
Blanchet & Sullivan Informational PAGE 32
RFC 6885 Stringprep Revision Problem Statement March 2013
Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function
B.8. EAP Stringprep Profiles: RFC 3748
Description: RFC 3748, Section 5, references Stringprep, but the WG
did not agree with the text (was added by IESG) and there are no
known implementations that use Stringprep. The main problem with
that text is that the use of strings is a per-method concept, not
a generic EAP concept and so RFC 3748 itself does not really use
Stringprep, but individual EAP methods could. As such, the
answers to the template questions are mostly not applicable, but a
few answers are universal across methods. The list of IANA
registered EAP methods is at
<http://www.iana.org/assignments/eap-numbers/eap-numbers.xml>.
Comparison Methods: n/a (per-method)
Case Folding, Case-Sensitivity, Case Preservation: n/a (per-method)
Impact of comparison: A false positive results in unauthorized
network access (and possibly theft of service if some else is
billed). A false negative results in lack of authorized network
access (no connectivity).
User input: n/a (per-method)
Normalization: n/a (per-method)
Mapping: n/a (per-method)
Disallowed characters: n/a (per-method)
String classes: Although some EAP methods may use a syntax similar
to other types of identifiers, EAP mandates that the actual values
must not be assumed to be identifiers usable with anything else.
Internal structure: n/a (per-method)
User output: Identifiers are never human displayed except perhaps as
they're typed by a human.
Operations: n/a (per-method)
Blanchet & Sullivan Informational PAGE 33
RFC 6885 Stringprep Revision Problem Statement March 2013
Community considerations: There is no resistance to change for the
base EAP protocol (as noted, the WG didn't want the existing
text). However, actual use of Stringprep, if any, within specific
EAP methods may have resistance. It is currently unknown whether
any EAP methods use Stringprep.
Authors' Addresses
Marc Blanchet
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
EMail: Marc.Blanchet@viagenie.ca
URI: http://viagenie.ca
Andrew Sullivan
Dyn, Inc.
150 Dow St
Manchester, NH 03101
U.S.A.
EMail: asullivan@dyn.com
Blanchet & Sullivan Informational PAGE 34
RFC TOTAL SIZE: 72167 bytes
PUBLICATION DATE: Thursday, March 14th, 2013
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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