Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and PasswordsJabber.orgP.O. Box 787ParkerCO80134United States of America+1 720 256 6756stpeter@jabber.orghttps://www.jabber.org/Isode Ltd5 Castle Business Village36 Station RoadHamptonMiddlesexTW12 2BXUnited KingdomAlexey.Melnikov@isode.comUsernamePasswordUnicodeInternationalizationi18nAuthenticationSASLprepThis 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. This
document obsoletes RFC 7613.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 and the HTTP Basic scheme
) 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 and the
HTTP Digest scheme ).To increase the likelihood that the input and comparison of usernames
and passwords will work in ways that make sense for typical users
throughout the world, this document defines rules for handling
internationalized strings that represent
usernames and passwords. Such strings consist of code points from the
Unicode coded character set , with special attention to
code points outside the ASCII range . 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 .Profiles of the PRECIS framework enable software to handle Unicode
code points outside the ASCII range in an automated way, so that such
code points 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 code points. For instance, in almost
all application protocols it would be dangerous to treat the Unicode
code point "¹" (SUPERSCRIPT ONE, U+00B9) as equivalent to "1" (DIGIT ONE, U+0031),
because that would result in false accepts during comparison,
authentication, and authorization (e.g., an attacker could easily spoof an
account "user1@example.com").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.)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.Although the historical predecessor of this document was the
SASLprep profile of Stringprep ), the
approach defined here can be used by technologies other than
SASL , such as HTTP authentication as
specified in and .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 ).
Non-coordinated updates to protocol implementations are discouraged
because they can have a negative impact on interoperability and
security.A "username" or "user identifier" is a string of characters designating
an account on a computing device or system, often but not necessarily for
use by a person. Although some devices and systems might allow a username
to be part or all of a person's name and a person might want their account
designator to be part or all of their name, because of the complexities
involved, that outcome is not guaranteed for all human names on all
computing devices or systems that follow the rules defined in this
specification. Protocol designers and application developers who wish
to allow a wider range of characters are encouraged to consider a
separation between more restrictive account identifiers and more
expressive display names or nicknames (see ).A "password" is a string of characters that allows access to a computing
device or system, often associated with a particular username. A password
is not literally limited to a word, because a password could be a passphrase
consisting of more than one word, perhaps separated by spaces, punctuation,
or other non-alphanumeric characters.Some SASL mechanisms (e.g., CRAM-MD5, DIGEST-MD5, and SCRAM) specify
that the authentication identity used in the context of such mechanisms is
a "simple username" (see Section 2 of as well as
). Various application technologies also assume
that the identity of a user or account takes the form of a username (e.g.,
authentication for the Hypertext Transfer Protocol as specified in and ), 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; note
also that a username does not necessarily map to any particular application
identifier.Many important terms used in this document are defined in , , ,
and . The term "non-ASCII space" refers to any
Unicode code point having a Unicode general category of "Zs", naturally with the
exception of SPACE (U+0020).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 BCP 14
when,
and only when, they appear in all capitals, as shown here.This document specifies that a username is a string of Unicode code
points that is
structured as an ordered sequence of "userparts"
and expressed in a standard Unicode Encoding Form (such as
UTF-8 ).
A userpart is allowed to contain only code points that
are allowed by the PRECIS IdentifierClass defined in
Section 4.2 of and thus consists almost
exclusively of letters and digits. A username can consist of a
single userpart or a space-separated sequence of userparts.The syntax for a username is defined as follows, using the Augmented
Backus-Naur Form (ABNF) .All code points and blocks not explicitly allowed in the PRECIS
IdentifierClass are disallowed; this includes private-use code points,
surrogate code points, and the other code points and blocks that were
defined as "Prohibited Output" in Section 2.3 of
(when corrected per ).
In addition, common constructions such as "user@example.com" (e.g., the
Network Access Identifier from ) are allowed as
usernames under this specification, as they were under .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 code points 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.A username MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.This specification defines two profiles for usernames: the
UsernameCaseMapped profile performs case mapping, and the
UsernameCasePreserved performs case preservation (see
further discussion under ).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.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 their use (or not) of the Case Mapping Rule and are otherwise
identical.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
accepts during authentication and authorization (as described in ) and can result in confusion among end users,
given the prevalence of case mapping in many existing protocols and
applications. However, there can be good reasons to apply the
UsernameCasePreserved profile and thus not perform case mapping, such as
backward compatibility with deployed infrastructure.In particular:SASL mechanisms that follow the recommendations in this document
MUST specify whether and when case mapping is to be applied to
authentication identifiers. Because case mapping results in
information loss, in order to retain that information for as
long as possible during processing, implementations 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 a server, then decisions about case mapping can be a
matter of service deployment policy). In keeping with
, SASL mechanisms are not to apply this or
any other profile to authorization identifiers, only to
authentication identifiers.Application protocols that use SASL (such as IMAP and the Extensible Messaging and Presence
Protocol (XMPP) ) 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 service deployment
policy). In keeping with , SASL
application protocols are not to apply this or any other
profile to authentication identifiers, only to authorization
identifiers.Application protocols that do not use SASL (such as HTTP
authentication with the HTTP Basic and Digest schemes as specified
in and ) 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 service deployment policy).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).The following rules are defined for use within the
UsernameCaseMapped profile of the PRECIS IdentifierClass.Width Mapping Rule: Map fullwidth and halfwidth code points to
their decomposition mappings (see Unicode Standard Annex #11 ).Additional Mapping Rule: There is no additional mapping
rule.Case Mapping Rule: Map uppercase and titlecase code points to
their lowercase equivalents, preferably using the Unicode
toLowerCase() operation as defined in the Unicode Standard ; see further discussion in .Normalization Rule: Apply Unicode Normalization Form C (NFC) to
all strings.Directionality Rule: Apply the "Bidi Rule" defined in to strings that contain right-to-left code
points (i.e., each of the six conditions of the Bidi Rule must be
satisfied); for strings that do not contain right-to-left code
points, there is no special processing for directionality.An entity that prepares an input string for subsequent enforcement
according to this profile MUST proceed as follows (applying the steps
in the order shown).Apply the width mapping rule specified in . It is necessary to apply the
rule at this point because otherwise the PRECIS "HasCompat"
category specified in Section 9.17 of
would forbid fullwidth and halfwidth code points.Ensure that the string consists only of Unicode code points
that are explicitly allowed by the PRECIS IdentifierClass defined
in Section 4.2 of .An entity that performs enforcement according to this profile MUST prepare an input string as described in and MUST also apply the following rules specified in in the order shown:Case Mapping RuleNormalization RuleDirectionality RuleAfter 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 strings are accepted, a non-empty sequence of characters can result in a zero-length username after canonicalization).The result of the foregoing operations is an output string that conforms to the UsernameCaseMapped profile. Until an implementation produces such an output string, it MUST NOT treat the string as conforming (in particular, it MUST NOT assume that an input string is conforming before the enforcement operation has been completed).An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
and then MUST
enforce the rules specified in
.
The two strings are to be considered equivalent if and only if they are an exact
octet-for-octet match (sometimes called "bit-string identity").Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).The following rules are defined for use within the
UsernameCasePreserved profile of the PRECIS IdentifierClass.Width Mapping Rule: Map fullwidth and halfwidth code points to
their decomposition mappings (see Unicode Standard Annex #11 ).Additional Mapping Rule: There is no additional mapping
rule.Case Mapping Rule: There is no case mapping rule.Normalization Rule: Apply Unicode Normalization Form C (NFC) to
all strings.Directionality Rule: Apply the "Bidi Rule" defined in to strings that contain right-to-left code
points (i.e., each of the six conditions of the Bidi Rule must be
satisfied); for strings that do not contain right-to-left code
points, there is no special processing for directionality.An entity that prepares a string for subsequent enforcement
according to this profile MUST proceed as follows (applying the steps
in the order shown).Apply the width mapping rule specified in .
It is necessary to apply
the rule at this point because otherwise the PRECIS "HasCompat"
category specified in Section 9.17 of
would forbid fullwidth and halfwidth code points.Ensure that the string consists only of Unicode code points
that are explicitly allowed by the PRECIS IdentifierClass defined
in Section 4.2 of .An entity that performs enforcement according to this profile MUST
prepare a string as described in and MUST also apply the
following rules specified in in the order shown:Normalization RuleDirectionality RuleAfter 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 strings
are accepted, a non-empty sequence of characters can result in a
zero-length username after canonicalization).The result of the foregoing operations is an output string that
conforms to the UsernameCasePreserved profile. Until an
implementation produces such an output string, it MUST NOT treat the
string as conforming (in particular, it MUST NOT assume that an input
string is conforming before the enforcement operation has been
completed).An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
and then MUST
enforce the rules specified in
.
The two strings are to be considered equivalent if and only if they
are an exact octet-for-octet match (sometimes called "bit-string
identity").Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).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".
Such a construct is not a profile of the PRECIS IdentifierClass,
because SPACE (U+0020) is not allowed in the IdentifierClass; however, it can be
created at the application layer because SPACE (U+0020) can be used as a
separator between instances of the PRECIS IdentifierClass (e.g.,
userparts as defined in this specification).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).Regarding examples 2 and 3:
although in German writing the character eszett "ß" (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 "Σ" (GREEK CAPITAL LETTER SIGMA, U+03A3) to
the lowercase character "σ" (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 the character "ς" (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.The following examples illustrate strings that are not valid
userparts (not usernames) because they violate the format defined
above.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 character "Ⅳ" (ROMAN NUMERAL FOUR, U+2163) has a compatibility
equivalent of the characters "I" (LATIN CAPITAL LETTER I, U+0049) and
"V" (LATIN CAPITAL LETTER V, U+0056), but code points with compatibility
equivalents are not allowed in the PRECIS IdentifierClass.Regarding example 11:
symbol characters such as "∞" (INFINITY, U+221E) are not
allowed in the PRECIS IdentifierClass.This document specifies that a password is a string of Unicode code
points that is conformant to the OpaqueString
profile (specified below) of the PRECIS FreeformClass defined in
Section 4.3 of and expressed in a standard
Unicode Encoding Form (such as UTF-8 ).The syntax for a password is defined as follows, using the Augmented
Backus-Naur Form (ABNF) .All code points and blocks not explicitly allowed in the PRECIS
FreeformClass are disallowed; this includes private-use code points,
surrogate code points, and the other code points and blocks defined as
"Prohibited Output" in Section 2.3 of
(when corrected per ).A password MUST NOT be zero bytes in length. This rule is to be
enforced after any normalization and mapping of code points.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.Note: 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
strings are accepted, because a non-empty sequence of
characters can result in a zero-length password after
canonicalization.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.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 ).An entity that prepares a string according to this profile MUST
ensure that the string consists only of Unicode code points that
are explicitly allowed by the FreeformClass string class defined
in .An entity that performs enforcement according to this profile MUST
prepare a string as described in
and MUST also
apply the rules specified below for the OpaqueString profile
(these rules MUST be applied in the order shown):Width Mapping Rule: Fullwidth and halfwidth code points MUST NOT
be mapped to their decomposition mappings (see Unicode Standard
Annex #11 ).Additional Mapping Rule: Any instances of non-ASCII space MUST
be mapped to SPACE (U+0020); a non-ASCII space is any
Unicode code point having a Unicode general category of "Zs",
with the exception of SPACE (U+0020). As was the case in RFC 4013,
the inclusion of only SPACE (U+0020) prevents confusion with various
non-ASCII space code points, many of which are difficult to
reproduce across different input methods.Case Mapping Rule: There is no case mapping rule (because mapping
uppercase and titlecase code points to their lowercase equivalents
would lead to false accepts and thus to reduced security).Normalization Rule: Unicode Normalization Form C (NFC) MUST be
applied to all strings.Directionality Rule: There is no directionality rule. The "Bidi
Rule" (defined in ) and similar rules are
unnecessary and inapplicable to passwords, because they can reduce
the repertoire 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.The result of the foregoing operations is an output string that conforms to the OpaqueString profile. Until an implementation produces such an output string, it MUST NOT treat the string as conforming (in particular, it MUST NOT assume that an input string is conforming before the enforcement operation has been completed).An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in
and then MUST
enforce the rules specified in
.
The two strings are to be considered equivalent if and only if they are an exact
octet-for-octet match (sometimes called "bit-string identity").Until an implementation determines whether two strings are to be
considered equivalent, it MUST NOT treat them as equivalent (in
particular, it MUST NOT assume that two input strings are equivalent
before the comparison operation has been completed).See regarding comparison
of passwords and passphrases.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).The following examples illustrate strings that are not valid
passwords because they violate the format defined above.Note: Following the "XML Notation" used in ,
the character TAB (U+0009) in example 18 is represented
as 	 because otherwise it could not be shown in running text.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 for guidelines on using
PRECIS profiles in applications.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 code points or character ranges, safeguards to mitigate the
effects of visually similar characters, etc.), application-layer
constructs (see ), and related matters.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
accepts. An in-depth discussion can be found in
.Applying the rules for any given PRECIS profile is not necessarily an
idempotent procedure for all code points. Therefore, an implementation
SHOULD apply the rules repeatedly until the output string is stable;
if the output string does not stabilize after reapplying the rules
three (3) additional times after the first application,
the implementation SHOULD terminate application of the rules
and reject the input string as invalid.The rules defined in this specification differ slightly from those
defined by the SASLprep specification (but
not from ). In order to smooth the process
of migrating from SASLprep to the approach defined herein, the
following sections describe these differences, along with their
implications for migration, in more detail.Deployments that currently use SASLprep for handling usernames might
need to scrub existing data when they migrate to the rules defined
in this specification. In particular: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:
"ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility equivalent
to "s" (LATIN SMALL LETTER S, U+0073)."Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V,
U+0056)."fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility equivalent
to "f" (LATIN SMALL LETTER F, U+0066) and "i" (LATIN SMALL LETTER I,
U+0069).
Under SASLprep, the use of NFKC also handled the mapping of
fullwidth and halfwidth code points to their decomposition mappings.
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;
for instance, "HENRYIV" with the last two code points being LATIN CAPITAL
LETTER I (U+0049) and LATIN CAPITAL LETTER V (U+0056) as opposed to
"HENRYⅣ" with the last character being "Ⅳ" (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.SASLprep mapped the "characters commonly mapped to nothing"
(from Appendix B.1 of ) to nothing,
whereas the PRECIS IdentifierClass entirely disallows most of these
code points, which correspond to the code points from the PRECIS
"M" category defined under Section 9.13 of . 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.SASLprep allowed uppercase and titlecase code points, whereas the
UsernameCaseMapped profile maps uppercase and titlecase code points
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).Depending on local service policy, migration from SASLprep 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: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 accepts when comparing
passwords. A few examples might suffice to indicate the nature of
the problem:
"ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility equivalent
to "s" (LATIN SMALL LETTER S, U+0073)."Ⅳ" (ROMAN NUMERAL FOUR, U+2163) is compatibility equivalent
to "I" (LATIN CAPITAL LETTER I, U+0049) and "V" (LATIN CAPITAL LETTER V,
U+0056)."fi" (LATIN SMALL LIGATURE FI, U+FB01) is compatibility equivalent
to "f" (LATIN SMALL LETTER F, U+0066) and "i" (LATIN SMALL LETTER I,
U+0069).
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.SASLprep mapped the "characters commonly mapped to nothing"
(from Appendix B.1 of ) to nothing,
whereas the PRECIS FreeformClass entirely disallows such code
points, which correspond to the code points from the PRECIS "M"
category defined under Section 9.13 of . 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.IANA has made the updates described below.
IANA has added the following entry to the "PRECIS Profiles"
registry.
UsernameCaseMappedIdentifierClassUsernames in security and application
protocols.The SASLprep profile of Stringprep.Map fullwidth and halfwidth
code points to their decomposition mappings.NoneMap uppercase and titlecase
code points to lowercase.NFCThe "Bidi Rule" defined in RFC
5893 applies.To be defined by security or application
protocols that use this profile.Section 3.3 of RFC 8265
IANA has added the following entry to the "PRECIS Profiles"
registry.
UsernameCasePreservedIdentifierClassUsernames in security and application
protocols.The SASLprep profile of Stringprep.Map fullwidth and halfwidth
code points to their decomposition mappings.NoneNoneNFCThe "Bidi Rule" defined in RFC
5893 applies.To be defined by security or application
protocols that use this profile.Section 3.4 of RFC 8265
IANA has added the following entry to the "PRECIS Profiles"
registry.
OpaqueStringFreeformClassPasswords and other opaque strings in
security and application protocols.The SASLprep profile of Stringprep.NoneMap non-ASCII space
code points to SPACE (U+0020).NoneNFCNoneTo be defined by security or application
protocols that use this profile.Section 4.2 of RFC 8265
The Stringprep specification did not
provide for entries in the "Stringprep Profiles" registry to have
any state except "Current" or "Not Current". Because RFC 7613
obsoleted RFC 4013, which registered the SASLprep profile of
Stringprep, IANA previously marked that profile as "Not Current"
and cited RFC 7613 as an additional reference. IANA has modified
the profile so that the current document is now cited as the additional reference.
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.In systems that conform to modern best practices for security,
verification of passwords during authentication will not use the
comparison defined in .
Instead, because the system performs cryptographic calculations to verify
the password, it will prepare the password as defined in
and enforce the rules
as defined in before
performing the relevant calculations.The process of comparing identifiers (such as SASL simple usernames,
authentication identifiers, and authorization identifiers) can lead to
either false rejects or false accepts, both of which have security
implications. A more detailed discussion can be found in .The security considerations described in
apply to the IdentifierClass and FreeformClass string classes
used in this document for usernames and passwords, respectively.The security considerations described in apply
to the use of Unicode code points in usernames and passwords.PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application ProtocolsEast Asian WidthUnicode Standard Annex #11The Unicode StandardThe Unicode ConsortiumASCII format for network interchangePreparation, Enforcement, and Comparison of Internationalized Strings Representing NicknamesUnicode Security MechanismsUnicode Technical Standard #39Erratum ID 1812RFC ErrataThe following changes were made from .Corrected the order of operations for the UsernameCaseMapped
profile to ensure consistency with .In accordance with working group discussions and updates to , removed the use of the Unicode toCaseFold()
operation in favor of the Unicode toLowerCase() operation.Modified the presentation (but not the content) of the rules.Removed UTF-8 as a mandatory encoding, because that is a matter
for the application.Clarified several editorial matters.Updated references.See for a description of the differences from .Thanks to Christian Schudt and Sam Whited for their bug reports and
feedback.See for acknowledgements related to the
specification that this document supersedes.