RFC 8265
Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords
4. Passwords
4.1. Definition
This document specifies that a password is a string of Unicode code points [Unicode] that is conformant to the OpaqueString profile (specified below) of the PRECIS FreeformClass defined in Section 4.3 of [RFC8264] and expressed in a standard Unicode Encoding Form (such as UTF-8 [RFC3629]). The syntax for a password is defined as follows, using the Augmented Backus-Naur Form (ABNF) [RFC5234]. password = 1*(freepoint) ; ; a "freepoint" is a Unicode code point that ; can be contained in a string conforming to ; the PRECIS FreeformClass ; 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 [RFC4013] (when corrected per [Err1812]). 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.4.2. OpaqueString Profile
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 [RFC8264]).4.2.1. Preparation
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 [RFC8264].4.2.2. Enforcement
An entity that performs enforcement according to this profile MUST prepare a string as described in Section 4.2.1 and MUST also apply the rules specified below for the OpaqueString profile (these rules MUST be applied in the order shown): 1. Width Mapping Rule: Fullwidth and halfwidth code points MUST NOT be mapped to their decomposition mappings (see Unicode Standard Annex #11 [UAX11]). 2. 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. 3. 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). 4. Normalization Rule: Unicode Normalization Form C (NFC) MUST be applied to all strings. 5. Directionality Rule: There is no directionality rule. The "Bidi Rule" (defined in [RFC5893]) 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).
4.2.3. Comparison
An entity that performs comparison of two strings according to this
profile MUST prepare each string as specified in Section 4.2.1 and
then MUST enforce the rules specified in Section 4.2.2. 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 Section 8.2 regarding comparison of passwords and passphrases.
4.3. Examples
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).
+------------------------------------+------------------------------+ | # | Password | Notes | +------------------------------------+------------------------------+ | 12| <correct horse battery staple> | SPACE (U+0020) is allowed | +------------------------------------+------------------------------+ | 13| <Correct Horse Battery Staple> | Differs by case from | | | | example 12 | +------------------------------------+------------------------------+ | 14| <πβå> | 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 SPACE (U+0020); | | | | thus, the full string is | | | | mapped to <foo bar> | +------------------------------------+------------------------------+ Table 3: A Sample of Legal Passwords The following examples illustrate strings that are not valid passwords because they violate the format defined above. +------------------------------------+------------------------------+ | # | Password | Notes | +------------------------------------+------------------------------+ | 17| <> | Zero-length passwords are | | | | disallowed | +------------------------------------+------------------------------+ | 18| <my cat is a 	by> | Control characters like TAB | | | | (U+0009) are disallowed | +------------------------------------+------------------------------+ Table 4: A Sample of Strings That Violate the Password Rules Note: Following the "XML Notation" used in [RFC3987], the character TAB (U+0009) in example 18 is represented as 	 because otherwise it could not be shown in running text.5. Use in Application Protocols
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 [RFC8264] 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 Section 3.5), 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 [RFC6943]. 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.6. Migration
The rules defined in this specification differ slightly from those defined by the SASLprep specification [RFC4013] (but not from [RFC7613]). 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.6.1. Usernames
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: o 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:
1. "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
equivalent to "s" (LATIN SMALL LETTER S, U+0073).
2. "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).
3. "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.
o SASLprep mapped the "characters commonly mapped to nothing" (from
Appendix B.1 of [RFC3454]) 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 [RFC8264]. 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.
o 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).
6.2. Passwords
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:
o 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:
1. "ſ" (LATIN SMALL LETTER LONG S, U+017F) is compatibility
equivalent to "s" (LATIN SMALL LETTER S, U+0073).
2. "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).
3. "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.
o SASLprep mapped the "characters commonly mapped to nothing" (from
Appendix B.1 of [RFC3454]) 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 [RFC8264]. 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.
7. IANA Considerations
IANA has made the updates described below.
7.1. UsernameCaseMapped Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: UsernameCaseMapped.
Base Class: IdentifierClass.
Applicability: Usernames in security and application protocols.
Replaces: The SASLprep profile of Stringprep.
Width Mapping Rule: Map fullwidth and halfwidth code points to their
decomposition mappings.
Additional Mapping Rule: None.
Case Mapping Rule: Map uppercase and titlecase code points to
lowercase.
Normalization Rule: NFC.
Directionality Rule: The "Bidi Rule" defined in RFC 5893 applies.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 3.3 of RFC 8265.
7.2. UsernameCasePreserved Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: UsernameCasePreserved.
Base Class: IdentifierClass.
Applicability: Usernames in security and application protocols.
Replaces: The SASLprep profile of Stringprep.
Width Mapping Rule: Map fullwidth and halfwidth code points to their
decomposition mappings.
Additional Mapping Rule: None.
Case Mapping Rule: None.
Normalization Rule: NFC.
Directionality Rule: The "Bidi Rule" defined in RFC 5893 applies.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 3.4 of RFC 8265.
7.3. OpaqueString Profile
IANA has added the following entry to the "PRECIS Profiles" registry.
Name: OpaqueString.
Base Class: FreeformClass.
Applicability: Passwords and other opaque strings in security and
application protocols.
Replaces: The SASLprep profile of Stringprep.
Width Mapping Rule: None.
Additional Mapping Rule: Map non-ASCII space code points to SPACE
(U+0020).
Case Mapping Rule: None.
Normalization Rule: NFC.
Directionality Rule: None.
Enforcement: To be defined by security or application protocols that
use this profile.
Specification: Section 4.2 of RFC 8265.
7.4. Stringprep Profile
The Stringprep specification [RFC3454] 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.8. Security Considerations
8.1. Password/Passphrase Strength
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.8.2. Password/Passphrase Comparison
In systems that conform to modern best practices for security, verification of passwords during authentication will not use the comparison defined in Section 4.2.3. Instead, because the system performs cryptographic calculations to verify the password, it will prepare the password as defined in Section 4.2.1 and enforce the rules as defined in Section 4.2.2 before performing the relevant calculations.8.3. Identifier Comparison
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 [RFC6943].8.4. Reuse of PRECIS
The security considerations described in [RFC8264] apply to the IdentifierClass and FreeformClass string classes used in this document for usernames and passwords, respectively.8.5. Reuse of Unicode
The security considerations described in [UTS39] apply to the use of Unicode code points in usernames and passwords.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, <https://www.rfc-editor.org/info/rfc5890>. [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, <https://www.rfc-editor.org/info/rfc6365>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8264] Saint-Andre, P. and M. Blanchet, "PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocols", RFC 8264, DOI 10.17487/RFC8264, October 2017, <https://www.rfc-editor.org/info/rfc8264>. [UAX11] Unicode Standard Annex #11, "East Asian Width", edited by Ken Lunde. An integral part of The Unicode Standard, <http://unicode.org/reports/tr11/>. [Unicode] The Unicode Consortium, "The Unicode Standard", <http://www.unicode.org/versions/latest/>.
9.2. Informative References
[Err1812] RFC Errata, Erratum ID 1812, RFC 4013, <https://www.rfc-editor.org/errata/eid1812>. [RFC20] Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, <https://www.rfc-editor.org/info/rfc20>. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, DOI 10.17487/RFC3454, December 2002, <https://www.rfc-editor.org/info/rfc3454>. [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, <https://www.rfc-editor.org/info/rfc3501>. [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987, January 2005, <https://www.rfc-editor.org/info/rfc3987>. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, DOI 10.17487/RFC4013, February 2005, <https://www.rfc-editor.org/info/rfc4013>. [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006, <https://www.rfc-editor.org/info/rfc4422>. [RFC4616] Zeilenga, K., Ed., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, DOI 10.17487/RFC4616, August 2006, <https://www.rfc-editor.org/info/rfc4616>. [RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, DOI 10.17487/RFC5802, July 2010, <https://www.rfc-editor.org/info/rfc5802>. [RFC5893] Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010, <https://www.rfc-editor.org/info/rfc5893>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, March 2011, <https://www.rfc-editor.org/info/rfc6120>. [RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May 2013, <https://www.rfc-editor.org/info/rfc6943>. [RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542, DOI 10.17487/RFC7542, May 2015, <https://www.rfc-editor.org/info/rfc7542>. [RFC7613] Saint-Andre, P. and A. Melnikov, "Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords", RFC 7613, DOI 10.17487/RFC7613, August 2015, <https://www.rfc-editor.org/info/rfc7613>. [RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015, <https://www.rfc-editor.org/info/rfc7616>. [RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015, <https://www.rfc-editor.org/info/rfc7617>. [RFC7622] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", RFC 7622, DOI 10.17487/RFC7622, September 2015, <https://www.rfc-editor.org/info/rfc7622>. [RFC8266] Saint-Andre, P., "Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames", RFC 8266, DOI 10.17487/RFC8266, October 2017, <https://www.rfc-editor.org/info/rfc8266>. [UTS39] Unicode Technical Standard #39, "Unicode Security Mechanisms", edited by Mark Davis and Michel Suignard, <http://unicode.org/reports/tr39/>.Appendix A. Changes from RFC 7613
The following changes were made from [RFC7613]. o Corrected the order of operations for the UsernameCaseMapped profile to ensure consistency with [RFC8264].
o In accordance with working group discussions and updates to
[RFC8264], removed the use of the Unicode toCaseFold() operation
in favor of the Unicode toLowerCase() operation.
o Modified the presentation (but not the content) of the rules.
o Removed UTF-8 as a mandatory encoding, because that is a matter
for the application.
o Clarified several editorial matters.
o Updated references.
See [RFC7613] for a description of the differences from [RFC4013].
Acknowledgements
Thanks to Christian Schudt and Sam Whited for their bug reports and
feedback.
See [RFC7613] for acknowledgements related to the specification that
this document supersedes.
Authors' Addresses
Peter Saint-Andre
Jabber.org
P.O. Box 787
Parker, CO 80134
United States of America
Phone: +1 720 256 6756
Email: stpeter@jabber.org
URI: https://www.jabber.org/
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
United Kingdom
Email: Alexey.Melnikov@isode.com