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Proto XEP: "Best practices for password hashing…"

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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!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 &lt;<link url='https://owasp.org/'>https://owasp.org/</link>&gt;.</note>" >
<!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 &lt;<link url='https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html'>https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html</link>&gt;.</note>" >
<!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 &lt;<link url='http://tools.ietf.org/html/rfc2195'>http://tools.ietf.org/html/rfc2195</link>&gt;.</note>" >
<!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 &lt;<link url='http://tools.ietf.org/html/rfc7677'>http://tools.ietf.org/html/rfc7677</link>&gt;.</note>" >
<!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 &lt;<link url='http://tools.ietf.org/html/rfc8018'>http://tools.ietf.org/html/rfc8018</link>&gt;.</note>" >
<!ENTITY rfc8265 "<span class='ref'><link url='http://tools.ietf.org/html/rfc8265'>RFC 8265</link></span> <note>RFC 8265: PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application ProtocolePRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocolsration, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords &lt;<link url='http://tools.ietf.org/html/rfc8265'>http://tools.ietf.org/html/rfc8265</link>&gt;.</note>" >
<!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 &lt;<link url='https://doi.org/10.6028/NIST.SP.800-63-3'>https://doi.org/10.6028/NIST.SP.800-63-3</link>&gt;.</note>" >
<!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 &lt;<link url='https://doi.org/10.6028/NIST.SP.800-63b'>https://doi.org/10.6028/NIST.SP.800-63b</link>&gt;.</note>" >
<!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 &lt;<link url='https://doi.org/10.6028/NIST.SP.800-132'>https://doi.org/10.6028/NIST.SP.800-132</link>&gt;.</note>" >
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Best practices for password hashing and storage</title>
<abstract>
This document outlines best practices for handling user passwords on the
public Jabber network for both clients and servers.
</abstract>
&LEGALNOTICE;
<number>xxxx</number>
<status>ProtoXEP</status>
<type>Informational</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies/>
<supersedes/>
<supersededby/>
<shortname>passwords</shortname>
&sam;
<revision>
<version>0.0.1</version>
<date>2020-04-19</date>
<initials>ssw</initials>
<remark><p>First draft.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
Following best practices when hashing and storing passwords and other
authenticator secrets impacts a great deal more than just a users identity.
It also effects usability, and backwards compatibility by determining what
authentication and authorization mechanisms can be used.
Unfortunately, aside from mandating the use of SCRAM-SHA-1 in &rfc6120;, and
recommending at least 4096 rounds of PBKDF2 in &rfc5802; (a
number which is now woefully inadequate), no general recommendations for
best practices in password storage, transmission, or key derivation function
tuning exist in the XMPP ecosystem.
</p>
<p>
Many of the recommendations in this document were taken from
&nistsp800-63b; and &nistsp800-132;.
</p>
</section1>
<section1 topic='Requirements' anchor='reqs'>
<p>
This document makes specific recommendations for best practices on the
public Jabber network for both clients and servers.
It does not attempt to address private networks or proprietary services
which may have different requirements, use cases, and security models.
These recommendations include the hashing and storage of memorized secrets
and other authenticators, authentication, and compatibility between clients
and servers with respect to authentication.
</p>
<p>
To keep the length of this document manageable, we assume basic familiarity
with password storage and handling, common terms, and cryptographic
operations.
For an overview of basic password security see the &owasppasswords;
maintained by &OWASP;.
</p>
</section1>
<section1 topic='Glossary' anchor='glossary'>
<p>
Many terms used in this document are defined in &nistsp800-63-3; Appendix
A.1 and in &nistsp800-132; §3.1.
Throughout this document the term "password" is used to mean any password,
passphrase, PIN, or other memorized secret.
</p>
<dl>
<di>
<dt>Pepper</dt>
<dd>
A secret added to a password hash like a salt.
Unlike a salt, peppers are secret and not unique.
They must not be stored alongside the hashed password.
</dd>
</di>
</dl>
</section1>
<section1 topic='SASL Mechanisms' anchor='required'>
<p>
Clients and servers must already implement the SASL mechanisms listed in RFC
6120 §13.8.1 For Authentication Only.
These mechanisms are:
</p>
<ul>
<li>SCRAM-SHA-1</li>
<li>SCRAM-SHA-1-PLUS</li>
</ul>
<p>
In addition, clients and servers SHOULD support the following SCRAM variants
defined in &rfc7677;:
</p>
<ul>
<li>SCRAM-SHA-256</li>
<li>SCRAM-SHA-256-PLUS</li>
</ul>
<p>
Clients SHOULD NOT invent their own mechanisms that have not been
standardized by the IETF, the XSF, or another reputable standards body.
</p>
<p>
Clients MUST NOT implement any mechanism with a usage status of "OBSOLETE",
"MUST NOT be used", or "LIMITED" in the &ianasasl;.
Similarly, any mechanism that depends on a hash function listed as "MUST
NOT" in &xep0414; MUST NOT be used.
This means that the following mechanisms which were commonly used with XMPP
in the past MUST NOT be supported:
</p>
<ul>
<li>CRAM-MD5 (&rfc2195;)</li>
<li>DIGEST-MD5 (&rfc6331;)</li>
</ul>
</section1>
<section1 topic='Client Best Practices' anchor='client'>
<section2 topic='Mechanism Pinning' anchor='pinning'>
<p>
Clients maintain a list of preferred SASL mechanisms, generally ordered by
perceived strength to enable strong authentication (RFC 6120 §6.3.3
Mechanism Preferences).
To prevent downgrade attacks by a malicious actor that has successfully
man in the middled a connection, or compromised a trusted server's
configuration, clients SHOULD implement "mechanism pinning".
That is, after the first successful authentication with a strong
mechanism, clients SHOULD make a record of the authentication and
thereafter only advertise and use mechanisms of equal or higher perceived
strength.
</p>
<p>
For reference, the following mechanisms are ordered by their perceived
strength from strongest to weakest with mechanisms of equal strength on
the same line.
This list is a non-normative example and does not indicate that these
mechanisms should or should not be supported:
</p>
<ol>
<li>EXTERNAL</li>
<li>SCRAM-SHA-1-PLUS, SCRAM-SHA-256-PLUS</li>
<li>SCRAM-SHA-1, SCRAM-SHA-256</li>
<li>PLAIN</li>
<li>DIGEST-MD5, CRAM-MD5</li>
</ol>
</section2>
<section2 topic='Storage' anchor='client-storage'>
<p>
Clients SHOULD always store authenticators in a trusted and encrypted
keystore such as the system keystore, or an encrypted store created
specifically for the clients use.
They SHOULD NOT store authenticators as plain text.
</p>
<p>
If clients know that they will only ever authenticate using a mechanism
such as SCRAM where the original password is not needed (for example if
the mechanism has been pinned) they SHOULD store the SCRAM bits or the
hashed and salted password instead of the original password.
However, if backwards compatibility with servers that only support the
PLAIN mechanism or other mechanisms that require using the original
password is required, clients MAY choose to store the original password
so long as an appropriate keystore is used.
</p>
</section2>
</section1>
<section1 topic='Server Best Practices' anchor='server'>
<section2 topic='Additional SASL Requirements' anchor='server-required'>
<p>
Servers MUST NOT support any mechanism that would require authenticators
to be stored in such a way that they could be recovered in plain text from
the stored information.
This includes mechanisms that store authenticators using reversable
encryption, obsolete hashing mechanisms such as MD5, and hashes that are
unsuitable for use with authenticators such as SHA256.
</p>
</section2>
<section2 topic='Storage' anchor='server-storage'>
<p>
Servers MUST always store passwords only after they have been salted and
hashed.
If multiple hashes are supported for use with SCRAM, for example
SCRAM-SHA-1 and SCRAM-SHA-256, separate salted and hashed passwords SHOULD
be calculated and stored for each mechanism so that users can log in with
multiple clients that support only some of the mechanisms.
</p>
<p>
A distinct salt SHOULD be used for each user, and each SCRAM family
supported.
Salts MUST be generated using a cryptographically secure random number
generator.
The salt MAY be stored in the same datastore as the password.
If it is stored alongside the password, it SHOULD be combined with a
pepper stored in the application configuration, an environment variable,
or some other location other than the datastore containing the salts.
</p>
</section2>
<section2 topic='Authentication and Rotation' anchor='auth'>
<p>
When authenticating using PLAIN or similar mechanisms that involve
transmitting the original password to the server the password MUST
be hashed and compared against the salted and hashed password in the
database using a constant time comparison.
</p>
<p>
Each time a password is changed or reset, a new random salt should be
created and the iteration count and pepper (if applicable) should be
updated to the latest value required by server policy.
</p>
<p>
If a pepper is used, consideration should be taken to ensure that it can
be easily rotated.
For example, multiple peppers could be stored with new passwords and
reset passwords using the latest pepper.
A hash of the pepper using a cryptographically secure hash function such
as SHA256 could then be stored in the database next to the salt so that
future logins can identify which pepper in the list was used.
This is just one example, pepper rotation schemes are outside the scope of
this document.
</p>
</section2>
</section1>
<section1 topic='PBKDF2 Parameters' anchor='pbkdf2'>
<p>
Because the PBKDF2 key derivation function (&rfc8018;) is used by
SCRAM-SHA-1 which is mandated for use in XMPP, this document recommends it
for password storage.
Servers SHOULD use the following parameters when applying PBKDF2:
</p>
<dl>
<di>
<dt>Minimum iterations</dt>
<dd>10,000 (100,000 for higher security environments)</dd>
</di>
<di>
<dt>Minimum salt length</dt>
<dd>16 bytes</dd>
</di>
<di>
<dt>Minimum pepper length</dt>
<dd>32 bytes</dd>
</di>
</dl>
<p>
The minimum iteration count may be tuned to the specific system on which
password hashing is taking place.
</p>
</section1>
<section1 topic='Password Complexity Requirements' anchor='passwordcomplexity'>
<p>
Clients and servers SHOULD enforce a minimum length of 8 characters for user
passwords.
If using a mechanism such as PLAIN where the server performs hashing on the
original password, a maximum length between 64 and 128 characters MAY be
imposed to prevent denial of service (DoS) attacks.
Passwords SHOULD be required to conform to the Opaque String profile of
&rfc8265;.
No other password restrictions should be applied.
</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>
The SCRAM suite of SASL mechanisms are recommended in this document,
however, there is currently no way to force a password reset.
This reduces upgrade agility if a weakness is discovered in SCRAM and means
that new, untested, SCRAM-based or SCRAM-like mechanisms should be added
with caution.
</p>
<p>
This document mentions many hash functions that are already in
use in the XMPP ecosystem, or that have been used in the past.
It does not make recommendations for what functions should or should not be
used in new applications.
For recommendations about the use of hash functions and their security
implications, see &xep0414;
</p>
<p>
This document contains recommendations that are likely to change over time.
It should be reviewed yearly to ensure that it remains accurate and up to
date.
Many of the recommendations in this document were taken from the
&owasppasswords;, which can be used as a reference when making updates.
</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 &REGISTRAR; as a
result of this document.
</p>
</section1>
</xep>