draft-ietf-precis-problem-statement-04.txt   draft-ietf-precis-problem-statement-05.txt 
Network Working Group M. Blanchet Network Working Group M. Blanchet
Internet-Draft Viagenie Internet-Draft Viagenie
Intended status: Informational A. Sullivan Intended status: Informational A. Sullivan
Expires: July 16, 2012 January 13, 2012 Expires: September 13, 2012 Dyn, Inc.
March 12, 2012
Stringprep Revision Problem Statement Stringprep Revision Problem Statement
draft-ietf-precis-problem-statement-04.txt draft-ietf-precis-problem-statement-05.txt
Abstract Abstract
Using Unicode codepoints in protocol strings that expect comparison Using Unicode codepoints in protocol strings that expect comparison
with other strings requires preparation of the string that contains with other strings requires preparation of the string that contains
the Unicode codepoints. Internationalizing Domain Names in the Unicode codepoints. Internationalizing Domain Names in
Applications (IDNA2003) defined and used Stringprep and Nameprep. Applications (IDNA2003) defined and used Stringprep and Nameprep.
Other protocols subsequently defined Stringprep profiles. A new Other protocols subsequently defined Stringprep profiles. A new
approach different from Stringprep and Nameprep is used for a approach different from Stringprep and Nameprep is used for a
revision of IDNA2003 (called IDNA2008). Other Stringprep profiles revision of IDNA2003 (called IDNA2008). Other Stringprep profiles
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 16, 2012. This Internet-Draft will expire on September 13, 2012.
Copyright Notice Copyright Notice
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document authors. All rights reserved. document authors. All rights reserved.
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than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Issues raised during newprep BOF . . . . . . . . . . . . . . . 5 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Major Topics for Consideration . . . . . . . . . . . . . . . . 6 3. Stringprep Profiles Limitations . . . . . . . . . . . . . . . 5
3.1. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Major Topics for Consideration . . . . . . . . . . . . . . . . 6
3.1.1. Types of Identifiers . . . . . . . . . . . . . . . . . 6 4.1. Comparison . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Effect of comparison . . . . . . . . . . . . . . . . . 7 4.1.1. Types of Identifiers . . . . . . . . . . . . . . . . . 6
3.2. Dealing with characters . . . . . . . . . . . . . . . . . 7 4.1.2. Effect of comparison . . . . . . . . . . . . . . . . . 7
3.2.1. Case folding, case sensitivity, and case 4.2. Dealing with characters . . . . . . . . . . . . . . . . . 7
4.2.1. Case folding, case sensitivity, and case
preservation . . . . . . . . . . . . . . . . . . . . . 7 preservation . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Stringprep and NFKC . . . . . . . . . . . . . . . . . 7 4.2.2. Stringprep and NFKC . . . . . . . . . . . . . . . . . 8
3.2.3. Character mapping . . . . . . . . . . . . . . . . . . 8 4.2.3. Character mapping . . . . . . . . . . . . . . . . . . 8
3.2.4. Prohibited characters . . . . . . . . . . . . . . . . 8 4.2.4. Prohibited characters . . . . . . . . . . . . . . . . 8
3.2.5. Internal structure, delimiters, and special 4.2.5. Internal structure, delimiters, and special
characters . . . . . . . . . . . . . . . . . . . . . . 9 characters . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Where the data comes from and where it goes . . . . . . . 9 4.2.6. Restrictions because of glyph similarity . . . . . . . 10
3.3.1. User input and the source of protocol elements . . . . 9 4.3. Where the data comes from and where it goes . . . . . . . 10
3.3.2. User output . . . . . . . . . . . . . . . . . . . . . 10 4.3.1. User input and the source of protocol elements . . . . 10
3.3.3. Operations . . . . . . . . . . . . . . . . . . . . . . 10 4.3.2. User output . . . . . . . . . . . . . . . . . . . . . 10
3.3.4. Some useful classes of strings . . . . . . . . . . . . 11 4.3.3. Operations . . . . . . . . . . . . . . . . . . . . . . 10
4. Considerations for Stringprep replacement . . . . . . . . . . 12 4.3.4. Some useful classes of strings . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 5. Considerations for Stringprep replacement . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Discussion home for this draft . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 8. Discussion home for this draft . . . . . . . . . . . . . . . . 13
9. Informative References . . . . . . . . . . . . . . . . . . . . 13 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Protocols known to be using Stringprep . . . . . . . 16 10. Informative References . . . . . . . . . . . . . . . . . . . . 13
Appendix B. Detailed discussion of protocols under Appendix A. Classification of Stringprep Profiles . . . . . . . . 17
consideration . . . . . . . . . . . . . . . . . . . . 17 Appendix B. Evaluation of Stringprep Profiles . . . . . . . . . . 18
Appendix C. Changes between versions . . . . . . . . . . . . . . 17 B.1. iSCSI Stringprep Profiles: RFC3722, RFC3721, RFC3720 . . . 18
C.1. 00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 B.2. SMTP/POP3/ManageSieve Stringprep Profiles:
C.2. 01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 RFC4954,RFC5034,RFC 5804 . . . . . . . . . . . . . . . . . 20
C.3. 02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 B.3. IMAP Stringprep Profiles: RFC5738, RFC4314: Usernames . . 21
C.4. 03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 B.4. IMAP Stringprep Profiles: RFC5738: Passwords . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 B.5. Anonymous SASL Stringprep Profiles: RFC4505 . . . . . . . 24
B.6. XMPP Stringprep Profiles: RFC3920 Nodeprep . . . . . . . . 26
B.7. XMPP Stringprep Profiles: RFC3920 Resourceprep . . . . . . 27
B.8. EAP Stringprep Profiles: RFC3748 . . . . . . . . . . . . . 27
Appendix C. Changes between versions . . . . . . . . . . . . . . 28
C.1. 00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
C.2. 01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
C.3. 02 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
C.4. 03 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
C.5. 04 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
C.6. 05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
Internationalizing Domain Names in Applications (IDNA2003) [RFC3490], Internationalizing Domain Names in Applications (IDNA2003) [RFC3490],
[RFC3491], [RFC3492], [RFC3454] described a mechanism for encoding [RFC3491], [RFC3492], [RFC3454] describes a mechanism for encoding
Unicode labels making up Internationalized Domain Names (IDNs) as Unicode labels making up Internationalized Domain Names (IDNs) as
standard DNS labels. The labels were processed using a method called standard DNS labels. The labels were processed using a method called
Nameprep [RFC3491] and Punycode [RFC3492]. That method was specific Nameprep [RFC3491] and Punycode [RFC3492]. That method was specific
to IDNA2003, but is generalized as Stringprep [RFC3454]. The general to IDNA2003, but is generalized as Stringprep [RFC3454]. The general
mechanism can be used to help other protocols with similar needs, but mechanism is used by other protocols with similar needs, but with
with different constraints than IDNA2003. different constraints than IDNA2003.
Stringprep defines a framework within which protocols define their Stringprep defines a framework within which protocols define their
Stringprep profiles. Known IETF specifications using Stringprep are Stringprep profiles. Known IETF specifications using Stringprep are
listed below: listed below:
o The Nameprep profile [RFC3490] for use in Internationalized Domain o The Nameprep profile [RFC3490] for use in Internationalized Domain
Names (IDNs); Names (IDNs);
o NFSv4 [RFC3530] and NFSv4.1 [RFC5661]; o NFSv4 [RFC3530] and NFSv4.1 [RFC5661];
o The iSCSI profile [RFC3722] for use in Internet Small Computer o The iSCSI profile [RFC3722] for use in Internet Small Computer
Systems Interface (iSCSI) Names; Systems Interface (iSCSI) Names;
o EAP [RFC3748]; o EAP [RFC3748];
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o PKIX subject identification using LDAPprep [RFC4683]; o PKIX subject identification using LDAPprep [RFC4683];
o Internet Application Protocol Collation Registry [RFC4790]; o Internet Application Protocol Collation Registry [RFC4790];
o SMTP Auth using SASLprep [RFC4954]; o SMTP Auth using SASLprep [RFC4954];
o POP3 Auth using SASLprep [RFC5034]; o POP3 Auth using SASLprep [RFC5034];
o TLS SRP using SASLprep [RFC5054]; o TLS SRP using SASLprep [RFC5054];
o IRI and URI in XMPP [RFC5122]; o IRI and URI in XMPP [RFC5122];
o PKIX CRL using LDAPprep [RFC5280]; o PKIX CRL using LDAPprep [RFC5280];
o IAX using Nameprep [RFC5456]; o IAX using Nameprep [RFC5456];
o SASL SCRAM using SASLprep [RFC5802]; o SASL SCRAM using SASLprep [RFC5802];
o Remote management of Sieve using SASLprep [RFC5804]; o Remote management of Sieve using SASLprep [RFC5804];
o The i;unicode-casemap Unicode Collation [RFC5051]. o The unicode-casemap Unicode Collation [RFC5051].
There turned out to be some difficulties with IDNA2003, documented in However, a review [1] of these protocol specifications found that
[RFC4690]. These difficulties led to a new IDN specification, called they are very similar and can be grouped into a short number of
IDNA2008 [RFC5890], [RFC5891], [RFC5892], [RFC5893]. Additional classes. Moreover, many reuse the same Stringprep profile, such as
background and explanations of the decisions embodied in IDNA2008 is the SASL one.
presented in [RFC5894]. One of the effects of IDNA2008 is that
Nameprep and Stringprep are not used at all. Instead, an algorithm IDNA2003 was replaced because of some limitations described in
based on Unicode properties of codepoints is defined. That algorithm [RFC4690]. The new IDN specification, called IDNA2008 [RFC5890],
generates a stable and complete table of the supported Unicode [RFC5891], [RFC5892], [RFC5893] was designed based on the
codepoints. This algorithm is based on an inclusion-based approach, considerations found in [RFC5894]. One of the effects of IDNA2008 is
instead of the exclusion-based approach of Stringprep/Nameprep. that Nameprep and Stringprep are not used at all. Instead, an
algorithm based on Unicode properties of codepoints is defined. That
algorithm generates a stable and complete table of the supported
Unicode codepoints for each Unicode version. This algorithm is based
on an inclusion-based approach, instead of the exclusion-based
approach of Stringprep/Nameprep.
This document lists the shortcomings and issues found by protocols This document lists the shortcomings and issues found by protocols
listed above that defined Stringprep profiles. It also lists some listed above that defined Stringprep profiles. It also lists the
early conclusions and requirements for a potential replacement of requirements for any potential replacement of Stringprep.
Stringprep.
2. Issues raised during newprep BOF 2. Conventions
During IETF 77, a BOF discussed the current state of the protocols This document uses the Unicode convention [2] to specify Unicode
that have defined Stringprep profiles [NEWPREP]. The main codepoint with the following syntax: U+ABCD where ABCD is the
codepoint in hexadecimal.
3. Stringprep Profiles Limitations
During IETF 77, a BOF [3] discussed the current state of the
protocols that have defined Stringprep profiles [NEWPREP]. The main
conclusions from that discussion were as follows: conclusions from that discussion were as follows:
o Stringprep is bound to a specific version of Unicode: 3.2. o Stringprep is bound to version 3.2 of Unicode. Stringprep has not
Stringprep has not been updated to new versions of Unicode. been updated to new versions of Unicode. Therefore, the protocols
Therefore, the protocols using Stringprep are stuck to Unicode using Stringprep are stuck to Unicode 3.2.
3.2.
o The protocols need to be updated to support new versions of o The protocols need to be updated to support new versions of
Unicode. The protocols would like to not be bound to a specific Unicode. The protocols would like to not be bound to a specific
version of Unicode, but rather have better Unicode agility in the version of Unicode, but rather have better Unicode agility in the
way of IDNA2008. This is important partly because it is usually way of IDNA2008. This is important partly because it is usually
impossible for an application to require Unicode 3.2; the impossible for an application to require Unicode 3.2; the
application gets whatever version of Unicode is available on the application gets whatever version of Unicode is available on the
host. host.
o The protocols require better bidirectional support (bidi) than o The protocols require better bidirectional support (bidi) than
currently offered by Stringprep. currently offered by Stringprep.
o If the protocols are updated to use a new version of Stringprep or o If the protocols are updated to use a new version of Stringprep or
another framework, then backward compatibility is an important another framework, then backward compatibility is an important
requirement. For example, Stringprep is based on and may use NFKC requirement. For example, Stringprep is based on and profiles may
[UAX15], while IDNA2008 mostly uses NFC [UAX15]. use NFKC [UAX15], while IDNA2008 mostly uses NFC [UAX15].
o Protocols use each other; for example, a protocol can use user o Identifiers are passed between protocols. For example, the same
identifiers that are later passed to SASL, LDAP or another username string of codepoints may be passed between SASL, XMPP,
authentication mechanism. Therefore, common set of rules or LDAP and EAP. Therefore, common set of rules or classes of
classes of strings are preferred over specific rules for each strings are preferred over specific rules for each protocol.
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 Protocols that use Stringprep profiles use strings for different
purposes: purposes:
o XMPP uses a different Stringprep profile for each part of the XMPP o XMPP uses a different Stringprep profile for each part of the XMPP
address (JID): a localpart which is similar to a username and used address (JID): a localpart which is similar to a username and used
for authentication, a domainpart which is a domain name and a for authentication, a domainpart which is a domain name and a
resource part which is less restrictive than the localpart. resource part which is less restrictive than the localpart.
o iSCSI uses a Stringprep profile for the IQN, which is very similar o iSCSI uses a Stringprep profile for the IQN, which is very similar
to (often is) a DNS domain name. to (often is) a DNS domain name.
o SASL and LDAP uses a Stringprep profile for usernames. o SASL and LDAP uses a Stringprep profile for usernames.
o LDAP uses a set of Stringprep profiles. o LDAP uses a set of Stringprep profiles.
During the newprep BOF, it was the consensus of the attendees that it The consensus [4] of the BOF attendees is that it would be highly
would be highly desirable to have a replacement of Stringprep, with desirable to have a replacement of Stringprep, with similar
similar characteristics to IDNA2008. That replacement should be characteristics to IDNA2008. That replacement should be defined so
defined so that the protocols could use internationalized strings that the protocols could use internationalized strings without a lot
without a lot of specialized internationalization work, since of specialized internationalization work, since internationalization
internationalization expertise is not available in the respective expertise is not available in the respective protocols or working
protocols or working groups. groups.
3. Major Topics for Consideration 4. Major Topics for Consideration
This section provides an overview of major topics that a Stringprep This section provides an overview of major topics that a Stringprep
replacement needs to address. The headings correspond roughly with replacement needs to address. The headings correspond roughly with
categories under which known Stringprep-using protocol RFCs have been categories under which known Stringprep-using protocol RFCs have been
evaluated. For the details of those evaluations, see Appendix A. evaluated. For the details of those evaluations, see Appendix A.
3.1. Comparison 4.1. Comparison
3.1.1. Types of Identifiers 4.1.1. Types of Identifiers
Following [I-D.iab-identifier-comparison], we can organize Following [I-D.iab-identifier-comparison], it is possible to organize
identifiers into three classes in respect of how they may be compared identifiers into three classes in respect of how they may be compared
with one another: with one another:
Absolute Identifiers Identifiers that can be compared byte-by-byte Absolute Identifiers Identifiers that can be compared byte-by-byte
for equality. for equality.
Definite Identifiers Identifiers that have a well-defined comparison Definite Identifiers Identifiers that have a well-defined comparison
algorithm on which all parties agree. algorithm on which all parties agree.
Indefinite Identifiers Identifiers that have no single comparison Indefinite Identifiers Identifiers that have no single comparison
algorithm on which all parties agree. algorithm on which all parties agree.
Definite Identifiers include cases like the comparison of Unicode Definite Identifiers include cases like the comparison of Unicode
code points in different encodings: they do not match byte for byte, 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 but can all be converted to a single encoding which then does match
byte for byte. Indefinite Identifiers are sometimes algorithmically byte for byte. Indefinite Identifiers are sometimes algorithmically
comparable by well-specified subsets of parties. For more discussion comparable by well-specified subsets of parties. For more discussion
of these categories, see [I-D.iab-identifier-comparison]. of these categories, see [I-D.iab-identifier-comparison].
The section on treating the existing known cases, Appendix A uses The section on treating the existing known cases, Appendix A uses the
these categories. categories above.
3.1.2. Effect of comparison 4.1.2. Effect of comparison
The three classes of comparison style outlined in Section 3.1.1 may The three classes of comparison style outlined in Section 4.1.1 may
have different effects when applied. It is necessary to evaluate the have different effects when applied. It is necessary to evaluate the
effects if a comparison results in a false positive, and what the effects if a comparison results in a false positive, and what the
effects are if a comparison results in a false negative, especially effects are if a comparison results in a false negative, especially
in terms of the consequences to security and usability. in terms of the consequences to security and usability.
3.2. Dealing with characters 4.2. Dealing with characters
This section outlines a range of issues having to do with characters This section outlines a range of issues having to do with characters
in the target protocols, and spends some effort to outline the ways in the target protocols, and outlines the ways in which IDNA2008
in which IDNA2008 might be a good analogy to other protocols, and might be a good analogy to other protocols, and ways in which it
ways in which it might be a poor one. might be a poor one.
3.2.1. Case folding, case sensitivity, and case preservation 4.2.1. Case folding, case sensitivity, and case preservation
In IDNA2003, labels are always mapped to lower case before the In IDNA2003, labels are always mapped to lower case before the
Punycode transformation. In IDNA2008, there is no mapping at all: 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, input is either a valid U-label or it is not. At the same time,
upper-case characters are by definition not valid U-labels, because upper-case characters are by definition not valid U-labels, because
they fall into the Unstable category (category B) of [RFC5892]. they fall into the Unstable category (category B) of [RFC5892].
If there are protocols that require upper and lower cases be If there are protocols that require upper and lower cases be
preserved, then the analogy with IDNA2008 will break down. preserved, then the analogy with IDNA2008 will break down.
Accordingly, existing protocols are to be evaluated according to the Accordingly, existing protocols are to be evaluated according to the
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upper-case characters are by definition not valid U-labels, because upper-case characters are by definition not valid U-labels, because
they fall into the Unstable category (category B) of [RFC5892]. they fall into the Unstable category (category B) of [RFC5892].
If there are protocols that require upper and lower cases be If there are protocols that require upper and lower cases be
preserved, then the analogy with IDNA2008 will break down. preserved, then the analogy with IDNA2008 will break down.
Accordingly, existing protocols are to be evaluated according to the Accordingly, existing protocols are to be evaluated according to the
following criteria: following criteria:
1. Does the protocol use case folding? For all blocks of code 1. Does the protocol use case folding? For all blocks of code
points, or just for certain subsets? points, or just for certain subsets?
2. Is the system or protocol case sensitive? 2. Is the system or protocol case sensitive?
3. Does the system or protocol preserve case? 3. Does the system or protocol preserve case?
3.2.2. Stringprep and NFKC 4.2.2. Stringprep and NFKC
Stringprep profiles may use normalization. If they do, they use NFKC Stringprep profiles may use normalization. If they do, they use NFKC
[UAX15]. It is not clear that NFKC is the right normalization to use [UAX15] (most profiles do). It is not clear that NFKC is the right
in all cases. In [UAX15], there is the following observation normalization to use in all cases. In [UAX15], there is the
regarding Normalization Forms KC and KD: "It is best to think of following observation regarding Normalization Forms KC and KD: "It is
these Normalization Forms as being like uppercase or lowercase best to think of these Normalization Forms as being like uppercase or
mappings: useful in certain contexts for identifying core meanings, lowercase mappings: useful in certain contexts for identifying core
but also performing modifications to the text that may not always be meanings, but also performing modifications to the text that may not
appropriate." For things like the spelling of users' names, then, always be appropriate." For things like the spelling of users'
NFKC may not be the best form to use. At the same time, one of the names, then, NFKC may not be the best form to use. At the same time,
nice things about NFKC is that it deals with the width of characters one of the nice things about NFKC is that it deals with the width of
that are otherwise similar, by canonicalizing half-width to full- characters that are otherwise similar, by canonicalizing half-width
width. This mapping step can be crucial in practice. The WG will to full-width. This mapping step can be crucial in practice. A
need to analyze the different use profiles and consider whether NFKC replacement for stringprep depends on analyzing the different use
or NFC is a better normalization for each profile. 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, For the purposes of evaluating an existing example of Stringprep use,
it is helpful to know whether it uses no normalization, NFKC, or NFC. it is helpful to know whether it uses no normalization, NFKC, or NFC.
3.2.3. Character mapping 4.2.3. Character mapping
Along with the case mapping issues raised in Section 3.2.1, there is Along with the case mapping issues raised in Section 4.2.1, there is
the question of whether some characters are mapped either to other the question of whether some characters are mapped either to other
characters or to nothing during Stringprep. [RFC3454], Section 3, characters or to nothing during Stringprep. [RFC3454], Section 3,
outlines a number of characters that are mapped to nothing, and also outlines a number of characters that are mapped to nothing, and also
permits Stringprep profiles to define their own mappings. permits Stringprep profiles to define their own mappings.
3.2.4. Prohibited characters 4.2.4. Prohibited characters
Along with case folding and other character mappings, many protocols Along with case folding and other character mappings, many protocols
have characters that are simply disallowed. For example, control have characters that are simply disallowed. For example, control
characters and special characters such as "@" or "/" may be characters and special characters such as "@" or "/" may be
prohibited in a protocol. prohibited in a protocol.
One of the primary changes of IDNA2008 is in the way it approaches One of the primary changes of IDNA2008 is in the way it approaches
Unicode code points. IDNA2003 created an explicit list of excluded Unicode code points. IDNA2003 created an explicit list of excluded
or mapped-away characters; anything in Unicode 3.2 that was not so or mapped-away characters; anything in Unicode 3.2 that was not so
listed could be assumed to be allowed under the protocol. IDNA2008 listed could be assumed to be allowed under the protocol. IDNA2008
begins instead from the assumption that code points are disallowed, begins instead from the assumption that code points are disallowed,
and then relies on Unicode properties to derive whether a given code and then relies on Unicode properties to derive whether a given code
point actually is allowed in the protocol. point actually is allowed in the protocol.
Moreover, there is more than one class of "allowed in the protocol". Moreover, there is more than one class of "allowed in the protocol"
While some code points are disallowed outright, some are allowed only in IDNA2008 (but not in IDNA2003). While some code points are
in certain contexts. The reasons for the context-dependent rules disallowed outright, some are allowed only in certain contexts. The
have to do with the way some characters are used. For instance, the reasons for the context-dependent rules have to do with the way some
ZERO WIDTH JOINER and ZERO WIDTH NON-JOINER (ZWJ, U+200D and ZWNJ, characters are used. For instance, the ZERO WIDTH JOINER and ZERO
U+200C) are allowed with contextual rules because they are required WIDTH NON-JOINER (ZWJ, U+200D and ZWNJ, U+200C) are allowed with
in some circumstances, yet are considered punctuation by Unicode and contextual rules because they are required in some circumstances, yet
would therefore be DISALLOWED under the usual IDNA2008 derivation are considered punctuation by Unicode and would therefore be
rules. The goal is to provide the widest possible repertoire of code DISALLOWED under the usual IDNA2008 derivation rules. The goal of
points possible and consistent with the traditional DNS, trusting to IDNA2008 is to provide the widest repertoire of code points possible
the operators of individual zones to make sensible (and usually more and consistent with the traditional DNS LDH rule, trusting to the
operators of individual zones to make sensible (and usually more
restrictive) policies for their zones. restrictive) policies for their zones.
IDNA2008 may be a poor model for what other protocols ought to do in IDNA2008 may be a poor model for what other protocols ought to do in
this case, because it is designed to support an old protocol that is this case, because it is designed to support an old protocol that is
designed to operate on the scale of the entire Internet. Moreover, designed to operate on the scale of the entire Internet. Moreover,
IDNA2008 is intended to be deployed without any change to the base IDNA2008 is intended to be deployed without any change to the base
DNS protocol. Other protocols may aim at deployment in more local DNS protocol. Other protocols may aim at deployment in more local
environments, or may have protocol version negotiation built in. environments, or may have protocol version negotiation built in.
3.2.5. Internal structure, delimiters, and special characters 4.2.5. Internal structure, delimiters, and special characters
IDNA2008 has a special problem with delimiters, because the delimiter IDNA2008 has a special problem with delimiters, because the delimiter
"character" in the DNS wire format is not really part of the data. "character" in the DNS wire format is not really part of the data.
In DNS, labels are not separated exactly; instead, a label carries 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 with it an indicator that says how long the label is. When the label
is presented in presentation format as part of a fully qualified is presented in presentation format as part of a fully qualified
domain name, the label separator FULL STOP, U+002E (.) is used to domain name, the label separator FULL STOP, U+002E (.) is used to
break up the labels. But because that label separator does not 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 travel with the wire format of the domain name, there is no way to
encode a different, "internationalized" separator in IDNA2008. encode a different, "internationalized" separator in IDNA2008.
skipping to change at page 9, line 33 skipping to change at page 9, line 50
inclusion of such a character in the protocol is not enough for it to inclusion of such a character in the protocol is not enough for it to
be considered similar to another protocol using the same character; be considered similar to another protocol using the same character;
instead, handling of the character must be taken into consideration instead, handling of the character must be taken into consideration
as well. as well.
An important issue to tackle here is whether it is valuable to map to An important issue to tackle here is whether it is valuable to map to
or from these special characters as part of the Stringprep or from these special characters as part of the Stringprep
replacement. In some locales, the analogue to FULL STOP, U+002E is replacement. In some locales, the analogue to FULL STOP, U+002E is
some other character, and users may expect to be able to substitute 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 their normal stop for FULL STOP, U+002E. At the same time, there are
predictability arguments in favour of treating names with FULL STOP, predictability arguments in favour of treating identifiers with FULL
U+002E in them just the way they are treated under IDNA2008. STOP, U+002E in them just the way they are treated under IDNA2008.
3.3. Where the data comes from and where it goes 4.2.6. Restrictions because of glyph similarity
3.3.1. User input and the source of protocol elements Homoglyphs are similarly (or identically) rendered glyphs of
different codepoints. For DNS names, homoglyphs may enable phishing.
If a protocol requires some visual comparison by end-users, then the
issue of homoglyphs are to be considered. In the DNS context, theses
issues are documented in [RFC5894] and [RFC4690]. IDNA2008 does not,
however, have a mechanism to deal with them, trusting to 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.
4.3. Where the data comes from and where it goes
4.3.1. User input and the source of protocol elements
Some protocol elements are provided by users, and others are not. Some protocol elements are provided by users, and others are not.
Those that are not may presumably be subject to greater restrictions, Those that are not may presumably be subject to greater restrictions,
whereas those that users provide likely need to permit the broadest whereas those that users provide likely need to permit the broadest
range of code points. The following questions are helpful: range of code points. The following questions are helpful:
1. Do users input the strings directly? 1. Do users input the strings directly?
2. If so, how? (keyboard, stylus, voice, copy-paste, etc.) 2. If so, how? (keyboard, stylus, voice, copy-paste, etc.)
3. Where do we place the dividing line between user interface and 3. Where do we place the dividing line between user interface and
protocol? (see [RFC5895]) protocol? (see [RFC5895])
3.3.2. User output 4.3.2. User output
Just as only some protocol elements are expected to be entered Just as only some protocol elements are expected to be entered
directly by users, only some protocol elements are intended to be directly by users, only some protocol elements are intended to be
consumed directly by users. It is important to know how users are consumed directly by users. It is important to know how users are
expected to be able to consume the protocol elements, because expected to be able to consume the protocol elements, because
different environments present different challenges. An element that different environments present different challenges. An element that
is only ever delivered as part of a vCard remains in machine-readable 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 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, 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 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 protocol element as an identifier (which means that they might enter
it again in some other context)? it again in some other context)? (See also Section 4.2.6.)
3.3.3. Operations 4.3.3. Operations
Some strings are useful as part of the protocol but are not used as Some strings are useful as part of the protocol but are not used as
input to other operations (for instance, purely informative or input to other operations (for instance, purely informative or
descriptive text). Other strings are used directly as input to other descriptive text). Other strings are used directly as input to other
operations (such as cryptographic hash functions), or are used operations (such as cryptographic hash functions), or are used
together with other strings to (such as concatenating a string with together with other strings to (such as concatenating a string with
some others to form a unique identifier). some others to form a unique identifier).
3.3.3.1. String classes 4.3.3.1. String classes
Strings often have a similar function in different protocols. For Strings often have a similar function in different protocols. For
instance, many different protocols contain user identifiers or instance, many different protocols contain user identifiers or
passwords. A single profile for all such uses might be desirable. passwords. A single profile for all such uses might be desirable.
Often, a string in a protocol is effectively a protocol element from Often, a string in a protocol is effectively a protocol element from
another protocol. For instance, different systems might use the same another protocol. For instance, different systems might use the same
credentials database for authentication. credentials database for authentication.
3.3.3.2. Community considerations 4.3.3.2. Community Considerations
A Stringprep replacement that does anything more than just update A Stringprep replacement that does anything more than just update
Stringprep to the latest version of Unicode will probably entail some Stringprep to the latest version of Unicode will probably entail some
changes. It is important to identify the willingness of the changes. It is important to identify the willingness of the
protocol-using community to accept backwards-incompatible changes. protocol-using community to accept backwards-incompatible changes.
By the same token, it is important to evaluate the desire of the By the same token, it is important to evaluate the desire of the
community for features not available under Stringprep. community for features not available under Stringprep.
3.3.3.3. What to do about Unicode changes 4.3.3.3. Unicode Incompatible Changes
IDNA2008 uses an algorithm to derive the validity of a Unicode code IDNA2008 uses an algorithm to derive the validity of a Unicode code
point for use under IDNA2008. It does this by using the properties point for use under IDNA2008. It does this by using the properties
of each code point to test its validity. of each code point to test its validity.
This approach depends crucially on the idea that code points, once This approach depends crucially on the idea that code points, once
valid for a protocol profile, will not later be made invalid. That valid for a protocol profile, will not later be made invalid. That
is not a guarantee currently provided by Unicode. Properties of code is not a guarantee currently provided by Unicode. Properties of code
points may change between versions of Unicode. Rarely, such a change points may change between versions of Unicode. Rarely, such a change
could cause a given code point to become invalid under a protocol could cause a given code point to become invalid under a protocol
profile, even though the code point would be valid with an earlier profile, even though the code point would be valid with an earlier
version of Unicode. This is not merely a theoretical possibility, version of Unicode. This is not merely a theoretical possibility,
because it has occurred ([I-D.faltstrom-5892bis]). because it has occurred ([RFC6452]).
Accordingly, a Stringprep replacement that intends to be Unicode Accordingly, as IDNA2008,a Stringprep replacement that intends to be
version agnostic will need to work out a mechanism to address cases Unicode version agnostic will need to work out a mechanism to address
where incompatible changes occur because of new Unicode versions. cases where incompatible changes occur because of new Unicode
versions.
3.3.4. Some useful classes of strings 4.3.4. Some useful classes of strings
With the above considerations in hand, we can usefully classify With the above considerations in hand, we can usefully classify
strings into the following categories, inspired by those outlined in strings into the following categories:
[I-D.saintandre-xmpp-i18n]:
Domainy strings Strings that are intended for use in a domain name DomainClass Strings that are intended for use in a domain name slot,
slot, as defined in [RFC5890]. Note that domainy strings could be as defined in [RFC5890]. Note that strings of DomainClass could
used outside a domain name slot: the question here is what the be used outside a domain name slot: the question here is what the
eventual intended use for the string is, and not whether the eventual intended use for the string is, and not whether the
string is actually functioning as a domain name at any moment. string is actually functioning as a domain name at any moment.
Namey strings Strings that are intended for use as identifiers but NameClass Strings that are intended for use as identifiers but that
that are not domainy strings. Namey strings are normally public are not DomainClass strings. NameClass strings are normally
data within the protocol where they are used: these are intended public data within the protocol where they are used: these are
as identifiers that can be passed around to identify something. intended as identifiers that can be passed around to identify
Secretish strings Strings that are intended for use as passwords or something.
passphrases or other such type of token. Secretish strings are FreeClass Strings that are intended to be used by the protocol as
normally not public data within the protocol where they are used: free-form strings, but that have some significant handling within
they function as a token for authorization, and normally should the protocol. This includes things that are normally not public
not be shared publicly. data in a protocol (like passwords), and things that might have
Protocolish strings Strings that are intended to be used by the additional restrictions within the protocol in question, such as a
protocol as free-form strings, but that have some significant friendly name in a chat room.
handling within the protocol. For instance, a protocol slot that
allows free-form text where case is not preserved would need to
have case mapping rules applied; in this case, the string would be
a protocolish string.
String blobs Elements of the protocol that look like strings to
users, but that are passed around in the protocol unchanged and
that cannot be used for comparison or other purposes. In effect,
these are strings that are part of a protocol payload, and are not
themselves part of the protocol at all.
4. Considerations for Stringprep replacement 5. Considerations for Stringprep replacement
The above suggests the following direction for the working group: The above suggests the following guidance for replacing Stringprep:
o A stringprep replacement should be defined. o A stringprep replacement should be defined.
o The replacement should take an approach similar to IDNA2008, in o The replacement should take an approach similar to IDNA2008, (e.g.
that it enables Unicode agility. by using codepoint properties instead of codepoint whitelisting)
in that it enables better Unicode agility.
o Protocols share similar characteristics of strings. Therefore, o Protocols share similar characteristics of strings. Therefore,
defining i18n preparation algorithms for a (small) set of string defining i18n preparation algorithms for the smallest set of
classes may be sufficient for most cases and provides the string classes may be sufficient for most cases, providing
coherence among a set of protocol friends. 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 o The sets of string classes need to be evaluated according to the
considerations that make up the headings in Section 3 considerations that make up the headings in Section 4
o It is reasonable to limit scope to Unicode code points, and rule o It is reasonable to limit scope to Unicode code points, and rule
the mapping of data from other character encodings outside the the mapping of data from other character encodings outside the
scope of this effort. scope of this effort.
o Recommendations for handling protocol incompatibilities resulting o Recommendations for handling protocol incompatibilities resulting
from changes to Unicode are required. from changes to Unicode are required.
o Comptability within each protocol between a technique that is
stringprep-based and the technique's replacement has to be
considered very carefully.
Existing deployments already depend on Stringprep profiles. Existing deployments already depend on Stringprep profiles.
Therefore, the working group will need to consider the effects of any Therefore, a replacement must consider the effects of any new
new strategy on existing deployments. By way of comparison, it is strategy on existing deployments. By way of comparison, it is worth
worth noting that some characters were acceptable in IDNA labels noting that some characters were acceptable in IDNA labels under
under IDNA2003, but are not protocol-valid under IDNA2008 (and IDNA2003, but are not protocol-valid under IDNA2008 (and conversely);
conversely). Different implementers may make different decisions disagreement about what to do during the transition has resulted in
about what to do in such cases; this could have interoperability different approaches to mapping. Different implementers may make
effects. The working group will need to trade better support for different decisions about what to do in such cases; this could have
different linguistic environments against the potential side effects interoperability effects. It is necessary to trade better support
of backward incompatibility. for different linguistic environments against the potential side
effects of backward incompatibility.
5. Security Considerations 6. Security Considerations
This document merely states what problems are to be solved, and does This document merely states what problems are to be solved, and does
not define a protocol. There are undoubtedly security implications not define a protocol. There are undoubtedly security implications
of the particular results that will come from the work to be of the particular results that will come from the work to be
completed. completed.
6. IANA Considerations 7. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
7. Discussion home for this draft 8. Discussion home for this draft
Note: RFC-Editor, please remove this section before publication.
This document is intended to define the problem space discussed on This document is intended to define the problem space discussed on
the precis@ietf.org mailing list. the precis@ietf.org mailing list.
8. Acknowledgements 9. Acknowledgements
This document is the product of the PRECIS IETF Working Group, and This document is the product of the PRECIS IETF Working Group, and
participants in that Working Group were helpful in addressing issues participants in that Working Group were helpful in addressing issues
with the text. with the text.
Specific contributions came from David Black, Alan DeKok, Bill Specific contributions came from David Black, Alan DeKok, Bill
McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave Thaler, and McQuillan, Alexey Melnikov, Peter Saint-Andre, Dave Thaler, and
Yoshiro Yoneya. Yoshiro Yoneya.
Dave Thaler provided the "buckets" insight in Section 3.1.1, central Dave Thaler provided the "buckets" insight in Section 4.1.1, central
to the organization of the problem. to the organization of the problem.
9. Informative References Evaluations of Stringprep profiles that are included in Appendix B
were done by: David Black, Alexey Melnikov, Peter Saint-Andre, Dave
Thaler.
[I-D.faltstrom-5892bis] 10. Informative References
Faltstrom, P. and P. Hoffman, "The Unicode code points and
IDNA - Unicode 6.0", draft-faltstrom-5892bis-05 (work in
progress), June 2011.
[I-D.iab-identifier-comparison] [I-D.iab-identifier-comparison]
Thaler, D., "Issues in Identifier Comparison for Security Thaler, D., "Issues in Identifier Comparison for Security
Purposes", draft-iab-identifier-comparison-00 (work in Purposes", draft-iab-identifier-comparison-00 (work in
progress), July 2011. progress), July 2011.
[I-D.saintandre-xmpp-i18n]
Saint-Andre, P., "Internationalized Addresses in XMPP",
draft-saintandre-xmpp-i18n-03 (work in progress),
March 2011.
[NEWPREP] "Newprep BoF Meeting Minutes", March 2010. [NEWPREP] "Newprep BoF Meeting Minutes", March 2010.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454, Internationalized Strings ("stringprep")", RFC 3454,
December 2002. December 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003. RFC 3490, March 2003.
skipping to change at page 16, line 37 skipping to change at page 17, line 11
RFC 5893, August 2010. RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for [RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010. Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA) Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010. 2008", RFC 5895, September 2010.
[RFC6452] 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", [UAX15] "Unicode Standard Annex #15: Unicode Normalization Forms",
UAX 15, September 2009. UAX 15, September 2009.
Appendix A. Protocols known to be using Stringprep [1] <http://www.ietf.org/proceedings/78/slides/precis-2.pdf>
The known cases are here described in two ways. The types of [2] <http://www.unicode.org/standard/principles.html>
identifiers the protocol uses is first called out in the ID type
column (from Section 3.1.1), 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. The remaining columns have an
"x" if and only if the protocol uses that class, as described in
Section 3.3.4. Values marked "-" indicate that an answer is not
useful; in this case, see detailed discussion in Appendix B.
+------+--------+-------+-------+-------+---------+---------+------+ [3] <http://www.ietf.org/proceedings/77/newprep.html>
| RFC | IDtype | User? | Dom'y | Nam'y | Sec'ish | Pro'ish | Blob |
+------+--------+-------+-------+-------+---------+---------+------+ [4] <http://www.ietf.org/proceedings/77/minutes/newprep.txt>
| 3722 | a | o | | x | x | x | |
| 3748 | - | - | - | x | - | - | - | [5] <http://trac.tools.ietf.org/wg/precis/trac/report/6>
| 3920 | a,d | b | | x | | x | |
| 4314 | a,d | b | | x | x | x | | 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 4.1.1), 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. The remaining columns have an "x" if and only if the
protocol uses that class, as described in Section 4.3.4. Values
marked "-" indicate that an answer is not useful; in this case, see
detailed discussion in Appendix B.
+------+--------+-------+-------------+-----------+-----------+
| RFC | IDtype | User? | DomainClass | NameClass | FreeClass |
+------+--------+-------+-------------+-----------+-----------+
| 3722 | a | o | | x | x |
| 3748 | - | - | - | x | - |
| 3920 | a,d | b | | x | x |
| 4505 | a | i | | | x |
| 4314 | a,d | b | | x | x |
| 4954 | a,d | b | | x | |
| 5034 | a,d | b | | x | |
| 5804 | a,d | b | | x | |
+------+--------+-------+-------------+-----------+-----------+
Table 1 Table 1
[[anchor21: The table still needs to be filled in, I am aware. [[anchor22: This table now contains results of any reviews the WG
did. Unreviewed things in the tracker are not reflected here.
--ajs@anvilwalrusden.com]] --ajs@anvilwalrusden.com]]
Appendix B. Detailed discussion of protocols under consideration Appendix B. Evaluation of Stringprep Profiles
Below are detailed reviews of the protocols under consideration This section is a summary of the evaluation of Stringprep
(where such reviews are available). [[anchor22: These are to be cut profiles [5] that was done to get a good understanding of the usage
and pasted from the wiki. --ajs@anvilwalrusden.com]] 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 Profiles: RFC3722, RFC3721, RFC3720
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.
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 i18n 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 worng
storage, the resulting mis-identificaiton 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 are often 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.
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 lower case letters and digits -
Unicode lower case 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 i18n experts solve
i18n 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: RFC4954,RFC5034,RFC
5804
Description: Authorization identity (user identifier) exchanged
during SASL authentication: AUTH (SMTP/POP3) or AUTHENTICATE
(ManageSieve) command.
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 own usernames
(e.g. webmail self registration)
User Input Methods: - Typed by user / selected from a list - Copy-
and-paste - Perhaps voice input - Can also be specified in
configuration files or on a 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 EAI email address in the future),
or as a left hand side of an email address. Note: whatever is
recommended for SMTP/POP/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
[RFC4013]) of the StringPrep algorithm (see [RFC3454]) 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. 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: RFC5738, RFC4314: Usernames
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 ACL commands. Note that any valid username is
also an IMAP ACL identifier, but IMAP ACL identifiers can include
other things like name of 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: - Typed by user / selected from a list - Copy-
and-paste - Perhaps voice input - Can also be specified in
configuration files or on a 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 IMAP access (login) - improperly grant privileges (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 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 show 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.
B.4. IMAP Stringprep Profiles: RFC5738: Passwords
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: - Typed by user - Copy-and-paste - Perhaps voice
input - Can also be specified in configuration files or on a
command line
Enforcement: Rules enforced by server / add-on service (e.g.,
gateway service or backend databse) 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, 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
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 (IMAP I18N): 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 [RFC4013] to both arguments of the LOGIN command.
The server MUST reject UTF-8 that fails to comply with the formal
syntax in RFC 3629 [RFC3629] or if it encounters Unicode
characters listed in Section 2.3 of SASLprep RFC 4013 [RFC4013].
RFC 4314 (IMAP4 Access Control List (ACL) Extension): 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.
and in Section 6: 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 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: RFC4505
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 freeform 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
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: 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. 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. 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: RFC3920 Nodeprep
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: - Typed by user - Copy-and-paste - Perhaps 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
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: RFC3920 Resourceprep
Description: - Resourcepart of JabberID ("JID"), as in:
localpart@domainpart/resourcepart - Typically free-form text
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: - Typed by user - Copy-and-paste - Perhaps 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 nick - 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
Operations: Sometimes concatenated with other data and then used as
input to a cryptographic hash function
B.8. EAP Stringprep Profiles: RFC3748
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#eap-numbers-3
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)
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.
Appendix C. Changes between versions Appendix C. Changes between versions
Note to RFC Editor: This section should be removed prior to Note to RFC Editor: This section should be removed prior to
publication. publication.
C.1. 00 C.1. 00
First WG version. Based on First WG version. Based on
draft-blanchet-precis-problem-statement-00. draft-blanchet-precis-problem-statement-00.
skipping to change at page 18, line 16 skipping to change at page 29, line 24
o Cleared up details of comparison classes o Cleared up details of comparison classes
o Added a section on changes in Unicode o Added a section on changes in Unicode
C.4. 03 C.4. 03
o Aligned comparison discussion with identifier discussion from o Aligned comparison discussion with identifier discussion from
draft-iab-identifier-comparison-00 draft-iab-identifier-comparison-00
o Added section on classes of strings ("Namey" and so on) o Added section on classes of strings ("Namey" and so on)
C.5. 04
Keepalive version
C.6. 05
o Changed classes of strings to align with framework doc
o Altered table in Appendix A
o Added all profiles evaluations from the wg wiki in appendix B
Authors' Addresses Authors' Addresses
Marc Blanchet Marc Blanchet
Viagenie Viagenie
2600 boul. Laurier, suite 625 246 Aberdeen
Quebec, QC G1V 4W1 Quebec, QC G1R 2E1
Canada Canada
Email: Marc.Blanchet@viagenie.ca Email: Marc.Blanchet@viagenie.ca
URI: http://viagenie.ca URI: http://viagenie.ca
Andrew Sullivan Andrew Sullivan
519 Maitland St. Dyn, Inc.
London, ON N6B 2Z5 150 Dow St
Canada Manchester, NH 03101
U.S.A.
Email: ajs@anvilwalrusden.com Email: asullivan@dyn.com
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