One document matched: draft-ietf-precis-7613bis-03.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc sortrefs="yes"?>
<?rfc strict="yes"?>
<?rfc symrefs="yes"?>
<?rfc toc="yes"?>
<?rfc tocdepth="4"?>
<rfc category="std" ipr="trust200902" docName="draft-ietf-precis-7613bis-03" obsoletes="7613">
<front>
<title abbrev="PRECIS: Usernames and Passwords">Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords</title>
<author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre">
<organization>Filament</organization>
<address>
<email>peter@filament.com</email>
<uri>https://filament.com/</uri>
</address>
</author>
<author initials="A." surname="Melnikov" fullname="Alexey Melnikov">
<organization>Isode Ltd</organization>
<address>
<postal>
<street>5 Castle Business Village</street>
<street>36 Station Road</street>
<city>Hampton</city>
<region>Middlesex</region>
<code>TW12 2BX</code>
<country>United Kingdom</country>
</postal>
<email>Alexey.Melnikov@isode.com</email>
</address>
</author>
<date/>
<keyword>Username</keyword>
<keyword>Password</keyword>
<keyword>Unicode</keyword>
<keyword>Internationalization</keyword>
<keyword>i18n</keyword>
<keyword>Authentication</keyword>
<keyword>SASLprep</keyword>
<abstract>
<t>This document describes updated methods for handling Unicode strings
representing usernames and passwords. The previous approach was known
as SASLprep (RFC 4013) and was based on stringprep (RFC 3454). The
methods specified in this document provide a more sustainable approach
to the handling of internationalized usernames and passwords. The
preparation, enforcement, and comparison of internationalized strings
(PRECIS) framework, RFC 7564, obsoletes RFC 3454, and this document
obsoletes RFC 7613.</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="intro">
<t>Usernames and passwords are widely used for authentication and
authorization on the Internet, either directly when provided in plaintext
(as in the PLAIN Simple Authentication and Security Layer (SASL)
mechanism <xref target='RFC4616'/> and the HTTP Basic scheme
<xref target='RFC7617'/>) or indirectly
when provided as the input to a cryptographic algorithm such as a hash
function (as in the Salted Challenge Response Authentication Mechanism
(SCRAM) SASL mechanism <xref target='RFC5802'/> and the
HTTP Digest scheme <xref target='RFC7616'/>).</t>
<t>To increase the likelihood that the input and comparison of usernames
and passwords will work in ways that make sense for typical users
throughout the world, this document defines rules for preparing,
enforcing, and comparing internationalized strings that represent
usernames and passwords. Such strings consist of characters from the
Unicode character set <xref target="Unicode"/>, with special attention to
characters outside the ASCII range <xref target='RFC20'/>. The rules for
handling such strings are specified through profiles of the string classes
defined in the preparation, enforcement, and comparison of
internationalized strings (PRECIS) framework specification <xref
target='RFC7564'/>.</t>
<t>Profiles of the PRECIS framework enable software to handle Unicode
characters outside the ASCII range in an automated way, so that such
characters are treated carefully and consistently in application
protocols. In large measure, these profiles are designed to protect
application developers from the potentially negative consequences of
supporting the full range of Unicode characters. For instance, in almost
all application protocols it would be dangerous to treat the Unicode
character SUPERSCRIPT ONE (U+00B9) as equivalent to DIGIT ONE (U+0031),
because that would result in false positives during comparison,
authentication, and authorization (e.g., an attacker could easy spoof an
account "user1@example.com").</t>
<t>Whereas a naive use of Unicode would make such attacks trivially easy,
the PRECIS profile defined here for usernames generally protects
applications from inadvertently causing such problems. (Similar
considerations apply to passwords, although here it is desirable to
support a wider range of characters so as to maximize entropy for purposes
of authentication.)</t>
<t>The methods defined here might be applicable wherever usernames or
passwords are used. However, the methods are not intended for use in
preparing strings that are not usernames (e.g., Lightweight Directory
Access Protocol (LDAP) distinguished names), nor in cases where
identifiers or secrets are not strings (e.g., keys and
certificates) or require specialized handling.</t>
<t>This document obsoletes RFC 4013 (the SASLprep profile of stringprep
<xref target='RFC3454'/>) but can be used by technologies other than
SASL <xref target='RFC4422'/>, such as HTTP authentication as
specified in <xref target='RFC7617'/> and <xref
target='RFC7616'/>.</t>
<t>This document does not modify the handling of internationalized strings
in usernames and passwords as prescribed by existing application protocols
that use SASLprep. If the community that uses such an application
protocol wishes to modernize its handling of internationalized strings to
use PRECIS instead of stringprep, it needs to explicitly update the
existing application protocol definition (one example is <xref
target='RFC7622'/>.
Non-coordinated updates to protocol implementations are discouraged
because they can have a negative impact on interoperability and
security.</t>
</section>
<section title="Terminology" anchor="terms">
<t>Many important terms used in this document are defined in <xref
target='RFC5890'/>, <xref target='RFC6365'/>, <xref target='RFC7564'/>,
and <xref target='Unicode'/>. The term "non-ASCII space" refers to any
Unicode code point having a Unicode general category of "Zs", with the
exception of U+0020 (here called "ASCII space").</t>
<t>As used here, the term "password" is not literally limited to a word;
i.e., a password could be a passphrase consisting of more than one word,
perhaps separated by spaces, punctuation, or other non-alphanumeric
characters.</t>
<t>Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
that the authentication identity used in the context of such mechanisms is
a "simple user name" (see Section 2 of <xref target='RFC4422'/> as well as
<xref target='RFC4013'/>). Various application technologies also assume
that the identity of a user or account takes the form of a username (e.g.,
authentication for the Hypertext Transfer Protocol as specified in <xref
target='RFC7617'/> and <xref
target='RFC7616'/>), whether or not they use SASL. Note
well that the exact form of a username in any particular SASL mechanism or
application technology is a matter for implementation and deployment, and
that a username does not necessarily map to any particular application
identifier (such as the localpart of an email address).</t>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in <xref
target='RFC2119'/>.</t>
</section>
<section title='Usernames' anchor='username'>
<section title='Definition' anchor='username-definition'>
<t>This document specifies that a username is a string of Unicode code
points <xref target='Unicode'/> that is
structured as an ordered sequence of "userparts"
and expressed in a standard Unicode Encoding Form (such as
UTF-8 <xref target='RFC3629'/>).
A userpart is allowed to contain only code points that
are allowed by the PRECIS IdentifierClass defined in
Section 4.2 of <xref target="RFC7564"/>, and thus consists almost
exclusively of letters and digits. A username can consist of a
single userpart or a space-separated sequence of userparts.</t>
<t>The syntax for a username is defined as follows, using the Augmented
Backus-Naur Form (ABNF) <xref target="RFC5234"/>.</t>
<figure>
<artwork><![CDATA[
username = userpart *(1*SP userpart)
userpart = 1*(idbyte)
;
; an "idbyte" is a byte used to encode a
; Unicode code point that can be contained
; in a string that conforms to the PRECIS
; IdentifierClass
;
]]></artwork>
</figure>
<t>All code points and blocks not explicitly allowed in the PRECIS
IdentifierClass are disallowed; this includes private use characters,
surrogate code points, and the other code points and blocks that were
defined as "Prohibited Output" in <xref target='RFC4013'/>. In
addition, common constructions such as "user@example.com" (e.g., the
Network Access Identifier from <xref target='RFC7542'/>) are allowed as
usernames under this specification, as they were under <xref
target='RFC4013'/>.</t>
<t><list style='empty'><t>Implementation Note: The username construct
defined in this document does not necessarily match what all deployed
applications might refer to as a "username" or "userid" but instead
provides a relatively safe subset of Unicode characters that can be
used in existing SASL mechanisms and in application protocols that use
SASL, and even in most application protocols that do not currently use
SASL.</t></list></t>
<t>A username MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.</t>
<t>In protocols that provide usernames as input to a cryptographic
algorithm such as a hash function, the client will need to perform
enforcement of the rules for the UsernameCaseMapped or
UsernameCasePreserved profile before applying the algorithm.</t>
<t>This specification defines two profiles for usernames: one that
performs case mapping and one that performs case preservation (see
further discussion under <xref target='username-case'/>).</t>
</section>
<section title='UsernameCaseMapped Profile' anchor='usernamecasemapped'>
<section title="Rules" anchor="usernamecasemapped-rules">
<t>The following rules apply within the UsernameCaseMapped profile of the PRECIS IdentifierClass.</t>
<t>
<list style='numbers'>
<t>Width-Mapping Rule: Map fullwidth and halfwidth characters to their decomposition mappings (see Unicode Standard Annex #11 <xref target='UAX11'/>).</t>
<t>Additional Mapping Rule: There is no additional mapping rule.</t>
<t>Case-Mapping Rule: Map uppercase and titlecase characters to their lowercase equivalents, preferably using the Unicode toLower() operation as defined in the Unicode Standard <xref target='Unicode'/>; see further discussion in <xref target='username-case'/>.</t>
<t>Normalization Rule: Apply Unicode Normalization Form C (NFC) to all characters.</t>
<t>Directionality Rule: Apply the "Bidi Rule" defined in <xref target='RFC5893'/> to strings that contain right-to-left characters (i.e., each of the six conditions of the Bidi Rule must be satisfied); for strings that do not contain right-to-left characters, there is no special processing for directionality.</t>
</list>
</t>
</section>
<section title="Preparation" anchor="usernamecasemapped-preparation">
<t>An entity that prepares a string for subsequent enforcement according to this profile MUST proceed as follows (applying the steps in the order shown).</t>
<t>
<list style='numbers'>
<t>Apply the width-mapping rule specified in <xref target='usernamecasemapped-rules'/>. It is necessary to apply the rule at this point because otherwise the PRECIS "HasCompat" category specified in Section 9.17 of <xref target='RFC7564'/> would forbid fullwidth and halfwidth characters.</t>
<t>Ensure that the string consists only of Unicode code points that conform to the PRECIS IdentifierClass defined in Section 4.2 of <xref target='RFC7564'/>.</t>
</list>
</t>
</section>
<section title="Enforcement" anchor="usernamecasemapped-enforcement">
<t>An entity that performs enforcement according to this profile MUST prepare a string as described in <xref target='usernamecasemapped-preparation'/> and MUST also apply the following rules specified in <xref target='usernamecasemapped-rules'/> in the order shown:</t>
<t>
<list style="numbers">
<t>Case-Mapping Rule</t>
<t>Normalization Rule</t>
<t>Directionality Rule</t>
</list>
</t>
<t>After all of the foregoing rules have been enforced, the entity MUST ensure that the username is not zero bytes in length (this is done after enforcing the rules to prevent applications from mistakenly omitting a username entirely, because when internationalized characters are accepted, a non-empty sequence of characters can result in a zero-length username after canonicalization).</t>
</section>
<section title="Comparison" anchor="usernamecasemapped-comparison">
<t>An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
<xref target="usernamecasemapped-preparation"/> and then MUST
enforce the rules specified in
<xref target="usernamecasemapped-enforcement"/>.
The two strings are to be considered equivalent if they are an exact
octet-for-octet match (sometimes called "bit-string identity").</t>
</section>
</section>
<section title='UsernameCasePreserved Profile' anchor='usernamecasepreserved'>
<section title="Rules" anchor="usernamecasepreserved-rules">
<t>The following rules apply within the UsernameCasePreserved profile of the PRECIS IdentifierClass.</t>
<t>
<list style='numbers'>
<t>Width-Mapping Rule: Map fullwidth and halfwidth characters to their decomposition mappings (see Unicode Standard Annex #11 <xref target='UAX11'/>).</t>
<t>Additional Mapping Rule: There is no additional mapping rule.</t>
<t>Case-Mapping Rule: There is no case-mapping rule.</t>
<t>Normalization Rule: Apply Unicode Normalization Form C (NFC) to all characters.</t>
<t>Directionality Rule: Apply the "Bidi Rule" defined in <xref target='RFC5893'/> to strings that contain right-to-left characters (i.e., each of the six conditions of the Bidi Rule must be satisfied); for strings that do not contain right-to-left characters, there is no special processing for directionality.</t>
</list>
</t>
</section>
<section title="Preparation" anchor="usernamecasepreserved-preparation">
<t>An entity that prepares a string for subsequent enforcement according to this profile MUST proceed as follows (applying the steps in the order shown).</t>
<t>
<list style='numbers'>
<t>Apply the width-mapping rule specified in <xref target='usernamecasemapped-rules'/>. It is necessary to apply the rule at this point because otherwise the PRECIS "HasCompat" category specified in Section 9.17 of <xref target='RFC7564'/> would forbid fullwidth and halfwidth characters.</t>
<t>Ensure that the string consists only of Unicode code points that conform to the PRECIS IdentifierClass defined in Section 4.2 of <xref target='RFC7564'/>.</t>
</list>
</t>
</section>
<section title="Enforcement" anchor="usernamecasepreserved-enforcement">
<t>An entity that performs enforcement according to this profile MUST prepare a string as described in <xref target='usernamecasepreserved-preparation'/> and MUST also apply the following rules specified in <xref target='usernamecasepreserved-rules'/> in the order shown:</t>
<t>
<list style="numbers">
<t>Normalization Rule</t>
<t>Directionality Rule</t>
</list>
</t>
<t>After all of the foregoing rules have been enforced, the entity MUST ensure that the username is not zero bytes in length (this is done after enforcing the rules to prevent applications from mistakenly omitting a username entirely, because when internationalized characters are accepted, a non-empty sequence of characters can result in a zero-length username after canonicalization).</t>
</section>
<section title="Comparison" anchor="usernamecasepreserved-comparison">
<t>An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
<xref target="usernamecasepreserved-preparation"/> and then MUST
enforce the rules specified in
<xref target="usernamecasepreserved-enforcement"/>.
The two strings are to be considered equivalent if they are an exact
octet-for-octet match (sometimes called "bit-string identity").</t>
</section>
</section>
<section title='Case Mapping vs. Case Preservation' anchor='username-case'>
<t>In order to accommodate the widest range of username constructs in
applications, this document defines two username profiles:
UsernameCaseMapped and UsernameCasePreserved. These two profiles differ
only in the Case-Mapping Rule and are otherwise identical.</t>
<t>Case mapping is a matter for the application protocol, protocol
implementation, or end deployment. In general, this document suggests
that it is preferable to apply the UsernameCaseMapped profile and
therefore perform case mapping, because not doing so can lead to false
positives during authentication and authorization (as described in <xref
target='RFC6943'/>) and can result in confusion among end users,
given the prevalence of case mapping in many existing protocols and
applications. However, there can be good reasons to apply the
UsernameCasePreserved profile and thus not perform case mapping, such as
backward compatibility with deployed infrastructure.</t>
<t>In particular:</t>
<t>
<list style='symbols'>
<t>SASL mechanisms that follow the recommendations in this document
MUST specify whether and when case mapping is to be applied to
authentication identifiers. SASL mechanisms SHOULD delay any
case mapping to the last possible moment, such as when doing a
lookup by username, performing username comparisons, or generating a
cryptographic salt from a username (if the last possible moment
happens on the server, then decisions about case mapping can be a
matter of deployment policy). In keeping with
<xref target='RFC4422'/>, SASL mechanisms are not to apply this or
any other profile to authorization identifiers, only to
authentication identifiers.</t>
<t>Application protocols that use SASL (such as IMAP <xref
target='RFC3501'/> and the Extensible Messaging and Presence
Protocol (XMPP) <xref target='RFC6120'/>) and that
directly reuse this profile MUST specify whether or not
case mapping is to be applied to authorization identifiers.
Such "SASL application protocols" SHOULD delay any case-mapping
of authorization identifiers to the last possible moment, which
happens to necessarily be on the server side (this enables
decisions about case mapping to be a matter of deployment
policy). In keeping with <xref target='RFC4422'/>, SASL
application protocols are not to apply this or any other
profile to authentication identifiers, only to authorization
identifiers.</t>
<t>Application protocols that do not use SASL (such as HTTP
authentication with the HTTP Basic and Digest schemes as specified
in <xref target='RFC7617'/> and <xref
target='RFC7616'/>) but that directly reuse this
profile MUST specify whether and when case mapping is to be applied
to authentication identifiers or authorization identifiers, or both.
Such "non-SASL application protocols" SHOULD delay any
case mapping to the last possible moment, such as when doing a
lookup by username, performing username comparisons, or
generating a cryptographic salt from a username (if the
last possible moment happens on the server, then decisions
about case mapping can be a matter of deployment policy).</t>
</list>
</t>
<t>If the specification for a SASL mechanism, SASL application protocol,
or non-SASL application protocol uses the UsernameCaseMapped profile, it
MUST clearly describe whether case mapping is to be applied at the level
of the protocol itself, implementations thereof, or service deployments
(each of these approaches can be legitimate, depending on the
application in question).</t>
</section>
<section title="Application-Layer Constructs" anchor="username-app">
<t>Both the UsernameCaseMapped and UsernameCasePreserved profiles enable
an application protocol, implementation, or deployment to create
application-layer constructs such as a username that is a
space-separated set of userparts like "Firstname Middlename Lastname".
Although such a construct is not a profile of the PRECIS IdentifierClass
(because U+0020 SPACE is not allowed in the IdentifierClass), it can be
created at the application layer because U+0020 SPACE can be used as a
separator between instances of the PRECIS IdentifierClass (e.g.,
userparts as defined in this specification).</t>
</section>
<section title="Examples" anchor="username-examples">
<t>The following examples illustrate a small number of userparts (not
usernames) that are consistent with the format defined above (note that
the characters "<" and ">" are used here to
delineate the actual userparts and are not part of the userpart
strings).</t>
<figure align="center">
<artwork align="center"><![CDATA[
+--------------------------+---------------------------------+
| # | Userpart | Notes |
+--------------------------+---------------------------------+
| 1 | <juliet@example.com> | The at-sign is allowed in the |
| | | PRECIS IdentifierClass |
+--------------------------+---------------------------------+
| 2 | <fussball> | |
+--------------------------+---------------------------------+
| 3 | <fußball> | The third character is LATIN |
| | | SMALL LETTER SHARP S (U+00DF) |
+--------------------------+---------------------------------+
| 4 | <π> | A userpart of GREEK SMALL |
| | | LETTER PI (U+03C0) |
+--------------------------+---------------------------------+
| 5 | <Σ> | A userpart of GREEK CAPITAL |
| | | LETTER SIGMA (U+03A3) |
+--------------------------+---------------------------------+
| 6 | <σ> | A userpart of GREEK SMALL |
| | | LETTER SIGMA (U+03C3) |
+--------------------------+---------------------------------+
| 7 | <ς> | A userpart of GREEK SMALL |
| | | LETTER FINAL SIGMA (U+03C2) |
+--------------------------+---------------------------------+
]]></artwork>
<postamble>Table 1: A Sample of Legal Userparts</postamble>
</figure>
<t>Several points are worth noting. Regarding examples 2 and 3:
although in German the character eszett (LATIN SMALL LETTER SHARP S
(U+00DF)) can mostly be used interchangeably with the two characters
"ss", the userparts in these examples are different and (if desired) a
server would need to enforce a registration policy that disallows one
of them if the other is registered. Regarding examples 5, 6, and 7:
optional case-mapping of GREEK CAPITAL LETTER SIGMA (U+03A3) to
lowercase (i.e., to GREEK SMALL LETTER SIGMA (U+03C3)) during
comparison would result in matching the userparts in examples 5
and 6; however, because the PRECIS mapping rules do not account for
the special status of GREEK SMALL LETTER FINAL SIGMA (U+03C2), the
userparts in examples 5 and 7 or examples 6 and 7 would not be
matched during comparison.</t>
<t>The following examples illustrate strings that are not valid
userparts (not usernames) because they violate the format defined
above.</t>
<figure align="center">
<artwork align="center"><![CDATA[
+--------------------------+---------------------------------+
| # | Non-Userpart String | Notes |
+--------------------------+---------------------------------+
| 8 | <foo bar> | Space (U+0020) is disallowed in |
| | | the userpart |
+--------------------------+---------------------------------+
| 9 | <> | Zero-length userpart |
+--------------------------+---------------------------------+
| 10| <henryⅣ> | The sixth character is ROMAN |
| | | NUMERAL FOUR (U+2163) |
+--------------------------+---------------------------------+
| 11| <♚> | A user part of BLACK CHESS KING |
| | | (U+265A) |
+--------------------------+---------------------------------+
]]></artwork>
<postamble>Table 2: A Sample of Strings That Violate the Userpart Rule</postamble>
</figure>
<t>Here again, several points are worth noting. Regarding example 8:
although this is not a valid userpart, it is a valid username because
it is a space-separated sequence of userparts. Regarding example 10:
the Unicode character ROMAN NUMERAL FOUR (U+2163) has a compatibility
equivalent of the string formed of LATIN CAPITAL LETTER I (U+0049) and
LATIN CAPITAL LETTER V (U+0056), but characters with compatibility
equivalents are not allowed in the PRECIS IdentifierClass. Regarding
example 11: symbol characters such as BLACK CHESS KING (U+265A) are not
allowed in the PRECIS IdentifierClass.</t>
</section>
</section>
<section title='Passwords' anchor='password'>
<section title='Definition' anchor='password-definition'>
<t>This document specifies that a password is a string of Unicode code
points <xref target='Unicode'/> that is conformant to the OpaqueString
profile (specified below) of the PRECIS FreeformClass defined in
Section 4.3 of <xref target="RFC7564"/>, and that is expressed in a
standard Unicode Encoding Form (such as UTF-8 <xref target='RFC3629'/>).</t>
<t>The syntax for a password is defined as follows, using the Augmented
Backus-Naur Form (ABNF) <xref target="RFC5234"/>.</t>
<figure>
<artwork><![CDATA[
password = 1*(freebyte)
;
; a "freebyte" is a byte used to encode a
; Unicode code point that can be contained
; in a string that conforms to the PRECIS
; FreeformClass
;
]]></artwork>
</figure>
<t>All code points and blocks not explicitly allowed in the PRECIS
FreeformClass are disallowed; this includes private use characters,
surrogate code points, and the other code points and blocks defined as
"Prohibited Output" in Section 2.3 of RFC 4013 (when corrected per
<xref target="Err1812" />).</t>
<t>A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.</t>
<t><list style='empty'><t>Note: Some existing systems allow an empty
string in places where a password would be expected (e.g., command-line
tools that might be called from an automated script, or servers that
might need to be restarted without human intervention). From the
perspective of this document (and RFC 4013 before it), these empty
strings are not passwords but are workarounds for the practical
difficulty of using passwords in certain scenarios. The prohibition of
zero-length passwords is not a recommendation regarding password
strength (because a password of only one byte is highly insecure) but is
meant to prevent applications from mistakenly omitting a password
entirely; such an outcome is possible when internationalized
characters are accepted, because a non-empty sequence of
characters can result in a zero-length password after
canonicalization.</t></list></t>
<t>In protocols that provide passwords as input to a cryptographic
algorithm such as a hash function, the client will need to perform
enforcement of the rules for the OpaqueString profile before
applying the algorithm, because the password is not available to
the server in plaintext form.</t>
</section>
<section title='OpaqueString Profile' anchor='opaquestring-profile'>
<t>The definition of the OpaqueString profile is provided in the
following sections, including detailed information about preparation,
enforcement, and comparison (for details on the distinction
between these actions, refer to <xref target='RFC7564'/>).</t>
<section title="Preparation" anchor="opaquestring-profile-preparation">
<t>An entity that prepares a string according to this profile MUST
ensure that the string consists only of Unicode code points that
conform to the FreeformClass base string class defined in <xref
target='RFC7564'/>.</t>
</section>
<section title="Enforcement" anchor="opaquestring-profile-enforcement">
<t>An entity that performs enforcement according to this profile MUST
prepare a string as described in
<xref target="opaquestring-profile-preparation"/> and MUST also
apply the rules specified below for the OpaqueString profile
(these rules MUST be applied in the order shown):</t>
<t>
<list style='numbers'>
<t>Width-Mapping Rule: Fullwidth and halfwidth characters MUST NOT
be mapped to their decomposition mappings (see Unicode Standard
Annex #11 <xref target='UAX11'/>).</t>
<t>Additional Mapping Rule: Any instances of non-ASCII space MUST
be mapped to ASCII space (U+0020); a non-ASCII space is any
Unicode code point having a Unicode general category of "Zs"
(with the exception of U+0020).</t>
<t>Case-Mapping Rule: There is no case mapping rule (because mapping
uppercase and titlecase characters to their lowercase equivalents
would lead to false positives and thus to reduced security).</t>
<t>Normalization Rule: Unicode Normalization Form C (NFC) MUST be
applied to all characters.</t>
<t>Directionality Rule: There is no directionality rule. The "Bidi
Rule" (defined in <xref target='RFC5893'/>) and similar rules are
unnecessary and inapplicable to passwords, because they can reduce
the range of characters that are allowed in a string and therefore
reduce the amount of entropy that is possible in a password. Such
rules are intended to minimize the possibility that the same
string will be displayed differently on a layout system set for
right-to-left display and a layout system set for left-to-right
display; however, passwords are typically not displayed at all and
are rarely meant to be interoperable across different layout
systems in the way that non-secret strings like domain names and
usernames are. Furthermore, it is perfectly acceptable for opaque
strings other than passwords to be presented differently in
different layout systems, as long as the presentation is
consistent in any given layout system.</t>
</list>
</t>
</section>
<section title="Comparison" anchor="opaquestring-profile-comparison">
<t>An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
<xref target="opaquestring-profile-preparation"/> and then MUST
enforce the rules specified in
<xref target="opaquestring-profile-enforcement"/>.
The two strings are to be considered equivalent if they are an exact
octet-for-octet match (sometimes called "bit-string identity").</t>
</section>
</section>
<section title="Examples" anchor="password-examples">
<t>The following examples illustrate a small number of passwords that
are consistent with the format defined above (note that the characters
"<" and ">" are used here to delineate the
actual passwords and are not part of the password strings).</t>
<figure align="center">
<artwork align="center"><![CDATA[
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 12| <correct horse battery staple> | ASCII space is allowed |
+------------------------------------+------------------------------+
| 13| <Correct Horse Battery Staple> | Differs by case from |
| | | example 12 |
+------------------------------------+------------------------------+
| 14| <πßå> | Non-ASCII letters are OK |
| | | (e.g., GREEK SMALL LETTER |
| | | PI (U+03C0)) |
+------------------------------------+------------------------------+
| 15| <Jack of ♦s> | Symbols are OK (e.g., BLACK |
| | | DIAMOND SUIT (U+2666)) |
+------------------------------------+------------------------------+
| 16| <foo bar> | OGHAM SPACE MARK (U+1680) is |
| | | mapped to U+0020, and thus |
| | | the full string is mapped to |
| | | <foo bar> |
+------------------------------------+------------------------------+
]]></artwork>
<postamble>Table 3: A Sample of Legal Passwords</postamble>
</figure>
<t>The following example illustrates a string that is not a valid
password because it violates the format defined above.</t>
<figure align="center">
<artwork align="center"><![CDATA[
+------------------------------------+------------------------------+
| # | Password | Notes |
+------------------------------------+------------------------------+
| 17| <my cat is a by> | Controls are disallowed |
+------------------------------------+------------------------------+
]]></artwork>
<postamble>Table 4: A String That Violates the Password Rules</postamble>
</figure>
</section>
</section>
<section title="Use in Application Protocols" anchor="use">
<t>This specification defines only the PRECIS-based rules for the handling
of strings conforming to the UsernameCaseMapped and UsernameCasePreserved
profiles of the PRECIS IdentifierClass, and strings conforming to the
OpaqueString profile of the PRECIS FreeformClass. It is the
responsibility of an application protocol to specify the protocol slots in
which such strings can appear, the entities that are expected to enforce
the rules governing such strings, and at what points during
protocol processing or interface handling the rules need to be enforced.
See Section 6 of <xref target='RFC7564'/> for guidelines on using
PRECIS profiles in applications.</t>
<t>Above and beyond the PRECIS-based rules specified here, application
protocols can also define application-specific rules governing such
strings (rules regarding minimum or maximum length, further restrictions
on allowable characters or character ranges, safeguards to mitigate the
effects of visually similar characters, etc.), application-layer
constructs (see <xref target='username-app'/>), and related matters.</t>
<t>Some PRECIS profile definitions encourage entities that enforce the
rules to be liberal in what they accept. However, for usernames and
passwords such a policy can be problematic, because it can lead to false
positives. An in-depth discussion can be found in
<xref target='RFC6943'/>.</t>
</section>
<section title="Migration" anchor='migration'>
<t>The rules defined in this specification differ slightly from those
defined by the SASLprep specification <xref target='RFC4013'/>. The
following sections describe these differences, along with their
implications for migration, in more detail.</t>
<section title='Usernames' anchor='migration-usernames'>
<t>Deployments that currently use SASLprep for handling usernames might
need to scrub existing data when they migrate to the rules defined
in this specification. In particular:</t>
<t>
<list style='symbols'>
<t>SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas the UsernameCaseMapped and UsernameCasePreserved
profiles employ Unicode Normalization Form C (NFC). In practice,
this change is unlikely to cause significant problems, because NFKC
provides methods for mapping Unicode code points with compatibility
equivalents to those equivalents, whereas the PRECIS IdentifierClass
entirely disallows Unicode code points with compatibility
equivalents (i.e., during comparison, NFKC is more "aggressive" about
finding matches than NFC). A few examples might suffice to indicate
the nature of the problem:
<list style='numbers'>
<t>LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent
to LATIN SMALL LETTER S (U+0073).</t>
<t>ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent
to LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V
(U+0056).</t>
<t>LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent
to LATIN SMALL LETTER F (U+0066) and LATIN SMALL LETTER I
(U+0069).</t>
</list>
Under SASLprep, the use of NFKC also handled the mapping of
fullwidth and halfwidth code points to their decomposition mappings.
<vspace blankLines='1'/>
For migration purposes, operators might want to search their database
of usernames for names containing Unicode code points with
compatibility equivalents and, where there is no conflict, map those
code points to their equivalents. Naturally, it is possible that
during this process the operator will discover conflicting usernames
(e.g., HENRYIV with the last two characters being LATIN CAPITAL
LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056)
vs. "HENRYIV" with the last character being ROMAN NUMERAL FOUR
(U+2163), which is compatibility equivalent to U+0049 and U+0056);
in these cases, the operator will need to determine how to proceed --
for instance, by disabling the account whose name contains a
Unicode code point with a compatibility equivalent. Such cases
are probably rare, but it is important for operators to be aware
of them.</t>
<t>SASLprep mapped the "characters commonly mapped to nothing" from
Appendix B.1 of <xref target='RFC3454'/>) to nothing, whereas the
PRECIS IdentifierClass entirely disallows most of these characters,
which correspond to the code points from the PRECIS "M" category
defined under Section 9.13 of <xref target='RFC7564'/>. For migration
purposes, the operator might want to remove from usernames any
code points contained in the PRECIS "M" category (e.g.,
SOFT HYPHEN (U+00AD)). Because these code points would have
been "mapped to nothing" in stringprep, in practice a user would not
notice the difference if, upon migration to PRECIS, the code points
are removed.</t>
<t>SASLprep allowed uppercase and titlecase characters, whereas the
UsernameCaseMapped profile maps uppercase and titlecase characters
to their lowercase equivalents (by contrast, the
UsernameCasePreserved profile matches SASLprep in this regard). For
migration purposes, the operator can use either the
UsernameCaseMapped profile (thus losing the case information) or the
UsernameCasePreserved profile (thus ignoring case difference when
comparing usernames).</t>
</list>
</t>
</section>
<section title='Passwords' anchor='migration-passwords'>
<t>Depending on local service policy, migration from RFC 4013 to this
specification might not involve any scrubbing of data (because passwords
might not be stored in the clear anyway); however, service providers
need to be aware of possible issues that might arise during migration.
In particular:</t>
<t>
<list style='symbols'>
<t>SASLprep specified the use of Unicode Normalization Form KC
(NFKC), whereas the OpaqueString profile employs Unicode
Normalization Form C (NFC). Because NFKC is more aggressive about
finding matches than NFC, in practice this change is unlikely to
cause significant problems and indeed has the security benefit of
probably resulting in fewer false positives when comparing
passwords. A few examples might suffice to indicate the nature of
the problem:
<list style='numbers'>
<t>LATIN SMALL LETTER LONG S (U+017F) is compatibility equivalent
to LATIN SMALL LETTER S (U+0073).</t>
<t>ROMAN NUMERAL FOUR (U+2163) is compatibility equivalent to
LATIN CAPITAL LETTER I (U+0049) and LATIN CAPITAL LETTER V
(U+0056).</t>
<t>LATIN SMALL LIGATURE FI (U+FB01) is compatibility equivalent
to LATIN SMALL LETTER F (U+0066) and LATIN SMALL LETTER I
(U+0069).</t>
</list>
Under SASLprep, the use of NFKC also handled the mapping of
fullwidth and halfwidth code points to their decomposition mappings.
Although it is expected that code points with compatibility
equivalents are rare in existing passwords, some passwords that
matched when SASLprep was used might no longer work when the rules
in this specification are applied.</t>
<t>SASLprep mapped the "characters commonly mapped to nothing" from
Appendix B.1 of <xref target='RFC3454'/>) to nothing, whereas the
PRECIS FreeformClass entirely disallows such characters, which
correspond to the code points from the PRECIS "M" category defined
under Section 9.13 of <xref target='RFC7564'/>.
In practice, this change will probably have no effect
on comparison, but user-oriented software might reject such code
points instead of ignoring them during password preparation.</t>
</list>
</t>
</section>
</section>
<section title="IANA Considerations" anchor="iana">
<t>IANA has made the updates described below. </t>
<section title="UsernameCaseMapped Profile" anchor="iana-usernamecasemapped">
<t>
IANA has added the following entry to the "PRECIS Profiles"
registry.
<list style='hanging'>
<t hangText='Name:'>UsernameCaseMapped.</t>
<t hangText='Base Class:'>IdentifierClass.</t>
<t hangText='Applicability:'>Usernames in security and application
protocols.</t>
<t hangText='Replaces:'>The SASLprep profile of stringprep.</t>
<t hangText='Width-Mapping Rule:'>Map fullwidth and halfwidth
characters to their decomposition mappings.</t>
<t hangText='Additional Mapping Rule:'>None.</t>
<t hangText='Case-Mapping Rule:'>Map uppercase and titlecase
characters to lowercase.</t>
<t hangText='Normalization Rule:'>NFC.</t>
<t hangText='Directionality Rule:'>The "Bidi Rule" defined in RFC
5893 applies.</t>
<t hangText='Enforcement:'>To be defined by security or application
protocols that use this profile.</t>
<t hangText='Specification:'>RFC 7613 (this document), Section 3.2.</t>
</list>
</t>
</section>
<section title="UsernameCasePreserved Profile" anchor="iana-usernamecasepreserved">
<t>
IANA has added the following entry to the "PRECIS Profiles"
registry.
<list style='hanging'>
<t hangText='Name:'>UsernameCasePreserved.</t>
<t hangText='Base Class:'>IdentifierClass.</t>
<t hangText='Applicability:'>Usernames in security and application
protocols.</t>
<t hangText='Replaces:'>The SASLprep profile of stringprep.</t>
<t hangText='Width-Mapping Rule:'>Map fullwidth and halfwidth
characters to their decomposition mappings.</t>
<t hangText='Additional Mapping Rule:'>None.</t>
<t hangText='Case-Mapping Rule:'>None.</t>
<t hangText='Normalization Rule:'>NFC.</t>
<t hangText='Directionality Rule:'>The "Bidi Rule" defined in RFC
5893 applies.</t>
<t hangText='Enforcement:'>To be defined by security or application
protocols that use this profile.</t>
<t hangText='Specification:'>RFC 7613 (this document), Section 3.3.</t>
</list>
</t>
</section>
<section title="OpaqueString Profile" anchor="iana-password">
<t>
IANA has added the following entry to the "PRECIS Profiles"
registry.
<list style='hanging'>
<t hangText='Name:'>OpaqueString.</t>
<t hangText='Base Class:'>FreeformClass.</t>
<t hangText='Applicability:'>Passwords and other opaque strings in
security and application protocols.</t>
<t hangText='Replaces:'>The SASLprep profile of stringprep.</t>
<t hangText='Width-Mapping Rule:'>None.</t>
<t hangText='Additional Mapping Rule:'>Map non-ASCII space
characters to ASCII space.</t>
<t hangText='Case-Mapping Rule:'>None.</t>
<t hangText='Normalization Rule:'>NFC.</t>
<t hangText='Directionality Rule:'>None.</t>
<t hangText='Enforcement:'>To be defined by security or application
protocols that use this profile.</t>
<t hangText='Specification:'>RFC 7613 (this document), Section 4.2.</t>
</list>
</t>
</section>
<section title="Stringprep Profile">
<t>
The stringprep specification <xref target="RFC3454"/> did not
provide for entries in the "Stringprep Profiles" registry to have
any state except "Current" or "Not Current". Because this document
obsoletes RFC 4013, which registered the SASLprep profile of
stringprep, IANA has marked that profile as "Not Current"
and cited this document as an additional reference.
</t>
</section>
</section>
<section title="Security Considerations" anchor="security">
<section title="Password/Passphrase Strength" anchor="security-passwords">
<t>The ability to include a wide range of characters in passwords and
passphrases can increase the potential for creating a strong password
with high entropy. However, in practice, the ability to include such
characters ought to be weighed against the possible need to reproduce
them on various devices using various input methods.</t>
</section>
<section title="Identifier Comparison" anchor="security-id">
<t>The process of comparing identifiers (such as SASL simple user names,
authentication identifiers, and authorization identifiers) can lead to
either false negatives or false positives, both of which have security
implications. A more detailed discussion can be found in <xref
target='RFC6943'/>.</t>
</section>
<section title="Reuse of PRECIS" anchor="security-precis">
<t>The security considerations described in <xref target="RFC7564"/>
apply to the IdentifierClass and FreeformClass base string classes
used in this document for usernames and passwords, respectively.</t>
</section>
<section title="Reuse of Unicode" anchor="security-unicode">
<t>The security considerations described in <xref target='UTS39'/> apply
to the use of Unicode characters in usernames and passwords.</t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.3629" ?>
<?rfc include="reference.RFC.5234" ?>
<?rfc include="reference.RFC.5890" ?>
<?rfc include="reference.RFC.6365" ?>
<?rfc include="reference.RFC.7564" ?>
<reference anchor="UAX11" target='http://unicode.org/reports/tr11/'>
<front>
<title>East Asian Width</title>
<author>
<organization>Unicode Standard Annex #11</organization>
</author>
<!-- <date month="June" year="2015" /> -->
<date/>
</front>
<seriesInfo name='edited by Ken Lunde. ' value='An integral part of The
Unicode Standard'/>
</reference>
<reference anchor="Unicode" target="http://www.unicode.org/versions/latest/">
<front>
<title>The Unicode Standard</title>
<author>
<organization>The Unicode Consortium</organization>
</author>
<date/>
<!-- <date year="2015-present" /> -->
</front>
</reference>
</references>
<references title="Informative References">
<reference anchor='RFC20' target='http://www.rfc-editor.org/info/rfc20'>
<front>
<title>ASCII format for network interchange</title>
<author initials='V.G.' surname='Cerf' fullname='V.G. Cerf'><organization /></author>
<date year='1969' month='October' />
</front>
<seriesInfo name='STD' value='80'/>
<seriesInfo name='RFC' value='20'/>
<seriesInfo name='DOI' value='10.17487/RFC0020'/>
</reference>
<?rfc include="reference.RFC.3454" ?>
<?rfc include="reference.RFC.3501" ?>
<?rfc include="reference.RFC.4013" ?>
<?rfc include="reference.RFC.4422" ?>
<?rfc include="reference.RFC.4616" ?>
<?rfc include="reference.RFC.5802" ?>
<?rfc include="reference.RFC.5893" ?>
<?rfc include="reference.RFC.6120" ?>
<?rfc include="reference.RFC.6943" ?>
<?rfc include="reference.RFC.7542" ?>
<?rfc include="reference.RFC.7613" ?>
<?rfc include="reference.RFC.7616" ?>
<?rfc include="reference.RFC.7617" ?>
<?rfc include="reference.RFC.7622" ?>
<reference anchor="UTS39" target='http://unicode.org/reports/tr39/'>
<front>
<title>Unicode Security Mechanisms</title>
<author>
<organization>Unicode Technical Standard #39</organization>
</author>
<!-- <date month="June" year="2015" /> -->
<date/>
</front>
<seriesInfo name='edited by Mark Davis' value='and Michel Suignard'/>
</reference>
<reference anchor="Err1812" target="http://www.rfc-editor.org">
<front>
<title>Erratum ID 1812</title>
<author>
<organization>RFC Errata</organization>
</author>
<date month="" year="" />
</front>
<seriesInfo name="RFC" value="4013" />
</reference>
</references>
<section title="Differences from RFC 7613" anchor="diffs">
<t>The following modifications were made from <xref target='RFC7613'/>.</t>
<t>
<list style='symbols'>
<t>Corrected the order of operations for the UsernameCaseMapped profile to ensure consistency with RFC 7564.</t>
<t>In accordance with working group discussions and updates to <xref target='RFC7564'/>, removed the use of the Unicode CaseFold() operation in favor of the Unicode toLower() operation.</t>
<t>Modified the presentation (but not the content) of the rules.</t>
<t>Clarified several editorial matters.</t>
</list>
</t>
<t>See <xref target='RFC7613'/> for a description of the differences from <xref target='RFC4013'/>.</t>
</section>
<section title="Acknowledgements" anchor="acks">
<t>Thanks to Christian Schudt and Sam Whited for their bug reports and feedback.</t>
<t>See <xref target='RFC7613'/> for acknowledgements related to the specification that this document supersedes.</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 20:44:12 |