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443 lines
20 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY OWASP "the <span class='ref'><link url='https://owasp.org/'>Open Web Application Security Project (OWASP)</link></span> <note>The Open Web Application Security Project (OWASP, or OWASP Foundation) is a nonprofit foundation that works to improve the security of software. For further information, see <<link url='https://owasp.org/'>https://owasp.org/</link>>.</note>" >
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<!ENTITY owasppasswords "<span class='ref'><link url='https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html'>OWASP Password Storage Cheat Sheet</link></span> <note>OWASP Cheat Sheet Series for password storage <<link url='https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html'>https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html</link>>.</note>" >
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<!ENTITY rfc2195 "<span class='ref'><link url='http://tools.ietf.org/html/rfc2195'>RFC 2195</link></span> <note>RFC 2195: IMAP/POP AUTHorize Extension for Simple Challenge/Response <<link url='http://tools.ietf.org/html/rfc2195'>http://tools.ietf.org/html/rfc2195</link>>.</note>" >
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<!ENTITY rfc5746 "<span class='ref'><link url='http://tools.ietf.org/html/rfc5746'>RFC 5746</link></span> <note>RFC 5746: Transport Layer Security (TLS) Renegotiation Indication Extension <<link url='http://tools.ietf.org/html/rfc5746'>http://tools.ietf.org/html/rfc5746</link>>.</note>" >
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<!ENTITY rfc7677 "<span class='ref'><link url='http://tools.ietf.org/html/rfc7677'>RFC 7677</link></span> <note>RFC 7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms <<link url='http://tools.ietf.org/html/rfc7677'>http://tools.ietf.org/html/rfc7677</link>>.</note>" >
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<!ENTITY rfc8018 "<span class='ref'><link url='http://tools.ietf.org/html/rfc8018'>RFC 8018</link></span> <note>RFC 8018: PKCS #5: Password-Based Cryptography Specification Version 2.1 <<link url='http://tools.ietf.org/html/rfc8018'>http://tools.ietf.org/html/rfc8018</link>>.</note>" >
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<!ENTITY rfc8265 "<span class='ref'><link url='http://tools.ietf.org/html/rfc8265'>RFC 8265</link></span> <note>RFC 8265: Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords <<link url='http://tools.ietf.org/html/rfc8265'>http://tools.ietf.org/html/rfc8265</link>>.</note>" >
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<!ENTITY nistsp800-63-3 "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-63-3'>Digital Identity Guidelines</link></span> <note>Digital Identity Guidelines, NIST Special Publication 800-63-3 <<link url='https://doi.org/10.6028/NIST.SP.800-63-3'>https://doi.org/10.6028/NIST.SP.800-63-3</link>>.</note>" >
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<!ENTITY nistsp800-63b "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-63b'>Digital Identity Guidelines: Authentication and Lifecycle Management</link></span> <note>Digital Identity Guidelines: Authentication and Lifecycle Management, NIST Special Publication 800-63B <<link url='https://doi.org/10.6028/NIST.SP.800-63b'>https://doi.org/10.6028/NIST.SP.800-63b</link>>.</note>" >
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<!ENTITY nistsp800-132 "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-132'>Recommendation for Password-Based Key Derivation, Part 1: Storage Applications</link></span> <note>Recommendation for Password-Based Key Derivation, Part 1: Storage Applications, NIST Special Publication 800-132 <<link url='https://doi.org/10.6028/NIST.SP.800-132'>https://doi.org/10.6028/NIST.SP.800-132</link>>.</note>" >
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Best practices for password hashing and storage</title>
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<abstract>
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This document outlines best practices for handling user passwords on the
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public Jabber network for both clients and servers.
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</abstract>
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&LEGALNOTICE;
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<number>0438</number>
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<status>Experimental</status>
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<type>Informational</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies/>
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<supersedes/>
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<supersededby/>
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<shortname>passwords</shortname>
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&sam;
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<revision>
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<version>0.2.0</version>
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<date>2020-10-30</date>
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<initials>ssw</initials>
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<remark><p>Update to match draft-ietf-kitten-password-storage-01.</p></remark>
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</revision>
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<revision>
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<version>0.1.1</version>
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<date>2020-05-05</date>
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<initials>ssw</initials>
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<remark><p>Fix reference to external document.</p></remark>
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</revision>
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<revision>
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<version>0.1.0</version>
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<date>2020-05-05</date>
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<initials>XEP Editor (jsc)</initials>
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<remark>Accepted by vote of Council on 2020-04-22.</remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2020-04-19</date>
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<initials>ssw</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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Following best practices when hashing and storing passwords and other
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authenticator secrets impacts a great deal more than just a users identity.
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It also effects usability, and backwards compatibility by determining what
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authentication and authorization mechanisms can be used.
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Unfortunately, aside from mandating the use of SCRAM-SHA-1 in &rfc6120;, and
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recommending at least 4096 rounds of PBKDF2 in &rfc5802; (a
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number which is now woefully inadequate), no general recommendations for
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best practices in password storage, transmission, or key derivation function
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tuning exist in the XMPP ecosystem.
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</p>
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<p>
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Many of the recommendations in this document were taken from
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&nistsp800-63b; and &nistsp800-132;.
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</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<p>
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This document makes specific recommendations for best practices on the
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public Jabber network for both clients and servers.
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It does not attempt to address private networks or proprietary services
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which may have different requirements, use cases, and security models.
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These recommendations include the hashing and storage of memorized secrets
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and other authenticators, authentication, and compatibility between clients
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and servers with respect to authentication.
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</p>
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<p>
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To keep the length of this document manageable, we assume basic familiarity
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with password storage and handling, common terms, and cryptographic
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operations.
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For an overview of basic password security see the &owasppasswords;
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maintained by &OWASP;.
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</p>
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</section1>
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<section1 topic='Glossary' anchor='glossary'>
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<p>
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Various security-related terms are to be understood in the sense
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defined in &rfc4949;
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Some may also be defined in defined in &nistsp800-63-3; Appendix A.1 and in
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&nistsp800-132; §3.1.
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</p>
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<p>
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Throughout this document the term "password" is used to mean any password,
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passphrase, PIN, or other memorized secret.
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</p>
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<p>
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Other common terms used throughout this document include:
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</p>
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<dl>
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<di>
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<dt>Mechanism Pinning</dt>
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<dd>
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Mechanism pinning A security mechanism which allows SASL clients to
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resist downgrade attacks.
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Clients that implement mechanism pinning remember the perceived strength
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of the SASL mechanism used in a previous successful authentication
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attempt and thereafter only authenticate using mechanisms of equal or
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higher perceived strength.
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</dd>
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</di>
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<di>
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<dt>Pepper</dt>
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<dd>
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A secret added to a password hash like a salt.
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Unlike a salt, peppers are secret and not unique.
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They must not be stored alongside the hashed password.
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</dd>
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</di>
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<di>
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<dt>Salt</dt>
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<dd>
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In this document salt is used as defined in &rfc4949;.
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</dd>
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</di>
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</dl>
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</section1>
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<section1 topic='SASL Mechanisms' anchor='required'>
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<p>
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Clients and servers must already implement the SASL mechanisms listed in RFC
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6120 §13.8.1 For Authentication Only.
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These mechanisms are:
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</p>
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<ul>
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<li>SCRAM-SHA-1</li>
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<li>SCRAM-SHA-1-PLUS</li>
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</ul>
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<p>
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In addition, clients and servers SHOULD support the following SCRAM variants
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defined in &rfc7677;:
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</p>
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<ul>
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<li>SCRAM-SHA-256</li>
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<li>SCRAM-SHA-256-PLUS</li>
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</ul>
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<p>
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Clients SHOULD NOT invent their own mechanisms that have not been
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standardized by the IETF, the XSF, or another reputable standards body.
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</p>
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<p>
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Clients MUST NOT implement any mechanism with a usage status of "OBSOLETE",
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"MUST NOT be used", or "LIMITED" in the &ianasasl;.
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Similarly, any mechanism that depends on a hash function listed as "MUST
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NOT" in &xep0414; MUST NOT be used.
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This means that the following mechanisms which were commonly used with XMPP
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in the past MUST NOT be supported:
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</p>
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<ul>
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<li>CRAM-MD5 (&rfc2195;)</li>
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<li>DIGEST-MD5 (&rfc6331;)</li>
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</ul>
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</section1>
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<section1 topic='Client Best Practices' anchor='client'>
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<section2 topic='Mechanism Pinning' anchor='pinning'>
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<p>
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Clients maintain a list of preferred SASL mechanisms, generally ordered by
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perceived strength to enable strong authentication (&rfc6120; §6.3.3
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Mechanism Preferences).
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To prevent downgrade attacks by a malicious actor that has successfully
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man in the middled a connection, or compromised a trusted server's
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configuration, clients SHOULD implement "mechanism pinning".
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That is, after the first successful authentication with a strong
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mechanism, clients SHOULD make a record of the authentication and
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thereafter only advertise and use mechanisms of equal or higher perceived
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strength.
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</p>
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<p>
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For reference, the following mechanisms are ordered by their perceived
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strength from strongest to weakest with mechanisms of equal strength on
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the same line.
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This list is a non-normative example and does not indicate that these
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mechanisms should or should not be supported:
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</p>
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<ol>
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<li>EXTERNAL</li>
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<li>SCRAM-SHA-1-PLUS, SCRAM-SHA-256-PLUS</li>
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<li>SCRAM-SHA-1, SCRAM-SHA-256</li>
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<li>PLAIN</li>
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<li>DIGEST-MD5, CRAM-MD5</li>
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</ol>
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<p>
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The EXTERNAL mechanism defined in &rfc4422; appendix A is placed at
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the top of the list.
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However, its perceived strength depends on the underlying authentication
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protocol.
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In this example, we assume that TLS (&rfc8446;) services are being used.
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</p>
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<p>
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The channel binding ("-PLUS") variants of SCRAM (&rfc5802;) are listed
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above their non-channel binding cousins, but may not always be
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available depending on the type of channel binding data available to
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the SASL negotiator.
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</p>
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<p>
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The PLAIN mechanism sends the username and password in plain text,
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but does allow for the use of a strong key derivation function (KDF)
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for the stored version of the password on the server.
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</p>
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<p>
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Finally, the DIGEST-MD5 and CRAM-MD5 mechanisms are listed last
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because they use weak hashes and ciphers and prevent the server from
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storing passwords using a KDF. For a list of problems with DIGEST-
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MD5 see &rfc6331;.
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</p>
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</section2>
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<section2 topic='Storage' anchor='client-storage'>
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<p>
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Clients SHOULD always store authenticators in a trusted and encrypted
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keystore such as the system keystore, or an encrypted store created
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specifically for the clients use.
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They SHOULD NOT store authenticators as plain text.
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</p>
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<p>
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If clients know that they will only ever authenticate using a mechanism
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such as SCRAM where the original password is not needed (for example if
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the mechanism has been pinned) they SHOULD store the SCRAM bits or the
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hashed and salted password instead of the original password.
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However, if backwards compatibility with servers that only support the
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PLAIN mechanism or other mechanisms that require using the original
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password is required, clients MAY choose to store the original password
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so long as an appropriate keystore is used.
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</p>
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</section2>
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</section1>
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<section1 topic='Server Best Practices' anchor='server'>
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<section2 topic='Additional SASL Requirements' anchor='server-required'>
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<p>
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Servers MUST NOT support any mechanism that would require authenticators
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to be stored in such a way that they could be recovered in plain text from
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the stored information.
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This includes mechanisms that store authenticators using reversable
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encryption, obsolete hashing mechanisms such as MD5, and hashes that are
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unsuitable for use with authenticators such as SHA256.
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</p>
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</section2>
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<section2 topic='Storage' anchor='server-storage'>
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<p>
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Servers MUST always store passwords only after they have been salted and
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hashed using a strong KDF.
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If multiple hashes are supported for use with SCRAM, for example
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SCRAM-SHA-1 and SCRAM-SHA-256, separate salted and hashed passwords SHOULD
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be calculated and stored for each mechanism so that users can log in with
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multiple clients that support only some of the mechanisms.
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</p>
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<p>
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A distinct salt SHOULD be used for each user, and each SCRAM family
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supported.
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Salts MUST be generated using a cryptographically secure random number
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generator.
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The salt MAY be stored in the same datastore as the password.
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If it is stored alongside the password, it SHOULD be combined with a
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pepper stored in the application configuration, an environment variable,
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or some other location other than the datastore containing the salts.
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</p>
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<p>
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The following minimum restrictions MUST be observed when generating salts
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and peppers.
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More up to date numbers may be found in &owasppasswords;
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</p>
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<dl>
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<di>
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<dt>Minimum Salt Length</dt>
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<dd>
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16 bytes
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</dd>
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</di>
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<di>
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<dt>Minimum Pepper Length</dt>
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<dd>
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32 bytes
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</dd>
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</di>
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</dl>
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</section2>
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<section2 topic='Authentication and Rotation' anchor='auth'>
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<p>
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When authenticating using PLAIN or similar mechanisms that involve
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transmitting the original password to the server the password MUST
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be hashed and compared against the salted and hashed password in the
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database using a constant time comparison.
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</p>
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<p>
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Each time a password is changed or reset, a new random salt should be
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created and the iteration count and pepper (if applicable) should be
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updated to the latest value required by server policy.
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</p>
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<p>
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If a pepper is used, consideration should be taken to ensure that it can
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be easily rotated.
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For example, multiple peppers could be stored with new passwords and
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reset passwords using the latest pepper.
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A hash of the pepper using a cryptographically secure hash function such
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as SHA256 could then be stored in the database next to the salt so that
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future logins can identify which pepper in the list was used.
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This is just one example, pepper rotation schemes are outside the scope of
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this document.
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</p>
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</section2>
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</section1>
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<section1 topic='PBKDF2 Parameters' anchor='pbkdf2'>
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<p>
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Because the PBKDF2 key derivation function (&rfc8018;) is used by
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SCRAM-SHA-1 which is mandated for use in XMPP, this document recommends it
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for password storage.
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Servers SHOULD use the following parameters when applying PBKDF2:
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</p>
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<dl>
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<di>
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<dt>Minimum iteration count (c)</dt>
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<dd>10,000 (100,000 for higher security environments)</dd>
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</di>
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<di>
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<dt>Hash</dt>
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<dd>SHA256</dd>
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</di>
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<di>
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<dt>Output length (dkLen)</dt>
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<dd>hLen (length of the chosen hash)</dd>
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</di>
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</dl>
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<p>
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The minimum iteration count may be tuned to the specific system on which
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password hashing is taking place.
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</p>
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</section1>
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<section1 topic='Password Complexity Requirements' anchor='passwordcomplexity'>
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<p>
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Before any other password complexity requirements are checked, the
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preparation and enforcement steps of the OpaqueString profile of &rfc8265;
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SHOULD be applied (for more information see the Internationalization
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Considerations section).
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Entities SHOULD enforce a minimum length of 8 characters for user passwords.
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If using a mechanism such as PLAIN where the server performs hashing on the
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original password, a maximum length between 64 and 128 characters MAY be
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imposed to prevent denial of service (DoS) attacks.
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Entities SHOULD NOT apply any other password restrictions.
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</p>
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<p>
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In addition to these password complexity requirements, servers SHOULD
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maintain a password blocklist and reject attempts by a claimant to use
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passwords on the blocklist during registration or password reset.
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The contents of this blocklist are a matter of server policy.
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Some common recommendations include lists of common passwords that are not
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otherwise prevented by length requirements, and passwords present in known
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breaches.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>
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This document contains recommendations that are likely to change over time.
|
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It should be reviewed regularly to ensure that it remains accurate and up to
|
|
date.
|
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Many of the recommendations in this document were taken from
|
|
&owasppasswords;, &nistsp800-63b;, and &nistsp800-132;.
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</p>
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<p>
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The SCRAM suite of SASL mechanisms are recommended in this document,
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however, there is currently no way to force a password reset.
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This reduces upgrade agility if a weakness is discovered in SCRAM and means
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that new, untested, SCRAM-based or SCRAM-like mechanisms should be added
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with caution.
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</p>
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<p>
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The "-PLUS" variants of SCRAM support channel binding to their underlying
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security layer, but lack a mechanism for negotiating what type of channel
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binding to use.
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In &rfc5802; the tls-unique (&rfc5929;) channel
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|
binding mechanism is specified as the default, and it is therefore likely to
|
|
be used in most applications that support channel binding.
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However, in the absence of the TLS extended master secret fix (&rfc7627;)
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and the renegotiation indication TLS extension (&rfc5746;) the tls-unique
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|
and tls-server-endpoint channel binding data can be forged by an attacker
|
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that can MITM the connection.
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Before advertising a channel binding SASL mechanism, entities MUST ensure
|
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that both the TLS extended master secret fix and the renegotiation
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indication extension are in place and that the connection has not been
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renegotiated.
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</p>
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<p>
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This document mentions many hash functions that are already in
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use in the XMPP ecosystem, or that have been used in the past.
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It does not make recommendations for what functions should or should not be
|
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used in new applications.
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For recommendations about the use of hash functions and their security
|
|
implications, see &xep0414;
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</p>
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<p>
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For TLS 1.3 no channel binding types are currently defined. Channel binding
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SASL mechanisms MUST NOT be advertised or negotiated over a TLS 1.3 channel
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until such types are defined.
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</p>
|
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</section1>
|
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<section1 topic='Internationalization Considerations' anchor='security'>
|
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<p>
|
|
The PRECIS framework (Preparation, Enforcement, and Comparison of
|
|
Internationalized Strings) defined in &rfc8264; is used to enforce
|
|
internationalization rules on strings and to prevent common application
|
|
security issues arrising from allowing the full range of Unicode codepoints
|
|
in usernames, passwords, and other identifiers.
|
|
The OpaqueString profile of &rfc8265; is used in this document to ensure
|
|
that codepoints in passwords are treated carefully and consistently.
|
|
This ensures that users typing certain characters on different keyboards
|
|
that may provide different versions of the same character will still be able
|
|
to log in.
|
|
For example, some keyboards may output the full-width version of a character
|
|
while other keyboards output the half-width version of the same character.
|
|
The Width Mapping rule of the OpaqueString profile addresses this and
|
|
ensures that comparison succeeds and the claimant is able to be
|
|
authenticated.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='IANA Considerations' anchor='iana'>
|
|
<p>
|
|
This document requires no interaction with &IANA;.
|
|
</p>
|
|
</section1>
|
|
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
|
|
<p>
|
|
No namespaces or parameters need to be registered with the ®ISTRAR; as a
|
|
result of this document.
|
|
</p>
|
|
</section1>
|
|
</xep>
|