Internet DRAFT - draft-whited-kitten-password-storage
draft-whited-kitten-password-storage
Common Authentication Technology Next Generation S. Whited
Internet-Draft 12 May 2020
Intended status: Best Current Practice
Expires: 13 November 2020
Best practices for password hashing and storage
draft-whited-kitten-password-storage-04
Abstract
This document outlines best practices for handling user passwords and
other authenticator secrets in client-server systems making use of
SASL.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 13 November 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 2
2. SASL Mechanisms . . . . . . . . . . . . . . . . . . . . . . . 3
3. Client Best Practices . . . . . . . . . . . . . . . . . . . . 3
3.1. Mechanism Pinning . . . . . . . . . . . . . . . . . . . . 3
3.2. Storage . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Server Best Practices . . . . . . . . . . . . . . . . . . . . 5
4.1. Additional SASL Requirements . . . . . . . . . . . . . . 5
4.2. Storage . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Authentication and Rotation . . . . . . . . . . . . . . . 6
5. KDF Recommendations . . . . . . . . . . . . . . . . . . . . . 6
5.1. Argon2 . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Bcrypt . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. PBKDF2 . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Scrypt . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Password Complexity Requirements . . . . . . . . . . . . . . 8
7. Internationalization Considerations . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Following best practices when hashing and storing passwords for use
with SASL impacts a great deal more than just a user's identity. It
also affects usability, backwards compatibility, and interoperability
by determining what authentication and authorization mechanisms can
be used.
1.1. Conventions and Terminology
Various security-related terms are to be understood in the sense
defined in [RFC4949]. Some may also be defined in [NISTSP63-3]
Appendix A.1 and in [NISTSP132] section 3.1.
Throughout this document the term "password" is used to mean any
password, passphrase, PIN, or other memorized secret.
Other common terms used throughout this document include:
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Pepper 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.
Mechanism pinning A security mechanism which allows SASL clients to
resist downgrade attacks. Clients that implement mechanism
pinning remember the perceived strength of the SASL mechanism used
in a previous successful authentication attempt and thereafter
only authenticate using mechanisms of equal or higher perceived
strength.
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. SASL Mechanisms
For clients and servers that support password based authentication
using SASL [RFC4422] it is RECOMMENDED that the following mechanisms
be implemented:
* SCRAM-SHA-256 [RFC7677]
* SCRAM-SHA-256-PLUS [RFC7677]
System entities SHOULD NOT invent their own mechanisms that have not
been standardized by the IETF or another reputable standards body.
Similarly, entities SHOULD NOT implement any mechanism with a usage
status of "OBSOLETE", "MUST NOT be used", or "LIMITED" in the IANA
SASL Mechanisms Registry [IANA.sasl.mechanisms].
The SASL mechanisms discussed in this document do not negotiate a
security layer. Because of this a strong security layer such as TLS
[RFC8446] MUST be negotiated before SASL mechanisms can be advertised
or negotiated.
3. Client Best Practices
3.1. Mechanism Pinning
Clients often maintain a list of preferred SASL mechanisms, generally
ordered by perceived strength to enable strong authentication. 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
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mechanism, clients SHOULD make a record of the authentication and
thereafter only advertise and use mechanisms of equal or higher
perceived strength.
The following mechanisms are ordered by their perceived strength from
strongest to weakest with mechanisms of equal strength on the same
line. The remainder of this section is merely informative. In
particular this example does not imply that mechanisms in this list
should or should not be implemented.
1. EXTERNAL
2. SCRAM-SHA-1-PLUS, SCRAM-SHA-256-PLUS
3. SCRAM-SHA-1, SCRAM-SHA-256
4. PLAIN
5. DIGEST-MD5, CRAM-MD5
The EXTERNAL mechanism defined in [RFC4422] appendix A is placed at
the top of the list. However, its perceived strength depends on the
underlying authentication protocol. In this example, we assume that
TLS [RFC8446] services are being used which can provide a strong
authenticator assurance level.
The channel binding ("-PLUS") variants of SCRAM [RFC5802] are listed
above their non-channel binding cousins, but may not always be
available depending on the type of channel binding data available to
the SASL negotiator.
The PLAIN mechanism sends the username and password in plain text,
but does allow for the use of a strong key derivation function for
the stored version of the password on the server.
Finally, the DIGEST-MD5 and CRAM-MD5 mechanisms are listed last
because they use weak hashes and ciphers and prevent the server from
storing passwords using a strong key derivation function. For a list
of problems with DIGEST-MD5 see [RFC6331].
3.2. Storage
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.
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If clients know that they will only ever authenticate using a
mechanism such as SCRAM [RFC5802] where the original password is not
needed after the first authentication attempt 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.
4. Server Best Practices
4.1. Additional SASL Requirements
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.
4.2. Storage
Servers MUST always store passwords only after they have been salted
and hashed. 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
location other than the datastore containing the salts.
The following restrictions MUST be observed when generating salts and
peppers:
+-----------------------+----------+
| Parameter | Value |
+=======================+==========+
| Minimum Salt Length | 16 bytes |
+-----------------------+----------+
| Minimum Pepper Length | 32 bytes |
+-----------------------+----------+
Table 1: Common Parameters
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4.3. Authentication and Rotation
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.
Each time a password is changed a new random salt MUST be created and
the iteration count and pepper (if applicable) MUST be updated to the
latest value required by server policy.
If a pepper is used, consideration should be taken to ensure that it
can be easily rotated. For example, multiple peppers could be
stored. New passwords and reset passwords would use the newest
pepper and 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.
5. KDF Recommendations
When properly configured, the following commonly used KDFs create
suitable password hash results for server side storage. The
recommendations in this section may change depending on the hardware
being used and the security level required for the application.
With all KDFs proper tuning is required to ensure that it meets the
needs of the specific application or service. For persistent login
an iteration count or work factor that adds approximately a quarter
of a second to login may be an acceptable tradeoff since logins are
relatively rare. By contrast, verification tokens that are generated
many times per second may need to use a much lower work factor.
5.1. Argon2
Argon2 [ARGON2ESP] is the 2015 winner of the Password Hashing
Competition and has been recomended by OWASP for password hashing.
Security considerations, test vectors, and parameters for tuning
argon2 can be found in [I-D.irtf-cfrg-argon2]. They are copied here
for easier reference.
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+---------------------------+--------------+
| Parameter | Value |
+===========================+==============+
| Degree of parallelism (p) | 1 |
+---------------------------+--------------+
| Memory size (m) | 32*1024 |
+---------------------------+--------------+
| Number of iterations (t) | 1 |
+---------------------------+--------------+
| Algorithm type (y) | Argon2id (2) |
+---------------------------+--------------+
Table 2: Argon Parameters
5.2. Bcrypt
bcrypt [BCRYPT] is a Blowfish-based KDF that is the current OWASP
recommendation for password hashing.
+-------------------------+-------+
| Parameter | Value |
+=========================+=======+
| Recommended Cost | 12 |
+-------------------------+-------+
| Maximum Password Length | 64 |
+-------------------------+-------+
Table 3: Bcrypt Parameters
5.3. PBKDF2
PBKDF2 [RFC8018] is used by the SCRAM [RFC5802] family of SASL
mechanisms.
+-----------------------------+-------------------------------------+
| Parameter | Value |
+=============================+=====================================+
| Minimum iteration count (c) | 10,000 |
+-----------------------------+-------------------------------------+
| Hash | SHA256 |
+-----------------------------+-------------------------------------+
| Output length (dkLen) | 64 (or length of |
| | chosen hash, hLen) |
+-----------------------------+-------------------------------------+
Table 4: PBKDF2 Parameters
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5.4. Scrypt
The [SCRYPT] KDF is designed to be memory-hard and sequential memory-
hard to prevent against custom hardware based attacks.
Security considerations, test vectors, and further notes on tuning
scrypt may be found in [RFC7914].
+-----------+----------------+
| Parameter | Value |
+===========+================+
| N | 32768 (N=2^15) |
+-----------+----------------+
| r | 8 |
+-----------+----------------+
| p | 1 |
+-----------+----------------+
Table 5: Scrypt Parameters
6. Password Complexity Requirements
Before any other password complexity requirements are checked, the
preparation and enforcement steps of the OpaqueString profile of
[RFC8265] SHOULD be applied (for more information see the
Internationalization Considerations section). Entities 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. Entities
SHOULD NOT apply any other password restrictions.
In addition to these password complexity requirements, servers SHOULD
maintain a password blacklist and reject attempts by a claimant to
use passwords on the blacklist during registration or password reset.
The contents of this blacklist are a matter of server policy. Some
common recommendations include lists of common passwords that are not
otherwise prevented by length requirements, passwords present in
known breaches (when paired with the same email or other uniquely
identifying information) to prevent reuse of compromised passwords,
and password that match commonly used patterns such as "any single
repeated character".
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7. Internationalization Considerations
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.
8. Security Considerations
This document contains recommendations that are likely to change over
time. It should be reviewed regularly to ensure that it remains
accurate and up to date. Many of the recommendations in this
document were taken from [OWASP.CS.passwords], [NISTSP63b], and
[NISTSP132].
The "-PLUS" variants of SCRAM [RFC5802] support channel binding to
their underlying security layer, but lack a mechanism for negotiating
what type of channel binding to use. In [RFC5802] the tls-unique
[RFC5929] channel binding mechanism is specified as the default, and
it is therefore likely to be used in most applications that support
channel binding. However, in the absence of the TLS extended master
secret fix [RFC7627] and the renegotiation indication TLS extension
[RFC5746] the tls-unique and tls-server-endpoint channel binding data
can be forged by an attacker that can MITM the connection. Before
advertising a channel binding SASL mechanism, entities MUST ensure
that both the TLS extended master secret fix and the renegotiation
indication extension are in place and that the connection has not
been renegotiated.
For TLS 1.3 [RFC8446] no channel binding types are currently defined.
Channel binding SASL mechanisms MUST NOT be advertised or negotiated
over a TLS 1.3 channel until such types are defined.
9. IANA Considerations
This document has no actions for IANA.
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10. References
10.1. Normative References
[IANA.sasl.mechanisms]
IETF, "Simple Authentication and Security Layer (SASL)
Mechanisms", November 2015,
<https://www.iana.org/assignments/sasl-mechanisms/sasl-
mechanisms.xhtml>.
[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>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
[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>.
10.2. Informative References
[ARGON2ESP]
Biryukov, A., Dinu, D., and D. Khovratovich, "Argon2: New
Generation of Memory-Hard Functions for Password Hashing
and Other Applications", Euro SnP 2016, March 2016,
<https://www.cryptolux.org/images/d/d0/Argon2ESP.pdf>.
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[BCRYPT] Provos, N. and D. Mazières, "A Future-Adaptable Password
Scheme", USENIX 1999
https://www.usenix.org/legacy/event/usenix99/provos/
provos.pdf, June 1999.
[I-D.irtf-cfrg-argon2]
Biryukov, A., Dinu, D., Khovratovich, D., and S.
Josefsson, "The memory-hard Argon2 password hash and
proof-of-work function", Work in Progress, Internet-Draft,
draft-irtf-cfrg-argon2-10, 25 March 2020,
<https://tools.ietf.org/html/draft-irtf-cfrg-argon2-10>.
[NISTSP132]
Turan, M., Barker, E., Burr, W., and L. Chen,
"Recommendation for Password-Based Key Derivation Part 1:
Storage Applications", NIST Special Publication SP
800-132, DOI 10.6028/NIST.SP.800-132, December 2010,
<https://nvlpubs.nist.gov/nistpubs/Legacy/SP/
nistspecialpublication800-132.pdf>.
[NISTSP63-3]
Grassi, P., Garcia, M., and J. Fenton, "Digital Identity
Guidelines", NIST Special Publication SP 800-63-3,
DOI 10.6028/NIST.SP.800-63-3, June 2017,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-63-3.pdf>.
[NISTSP63b]
Grassi, P., Fenton, J., Newton, E., Perlner, R.,
Regenscheid, A., Burr, W., Richer, J., Lefkovitz, N.,
Danker, J., Choong, Y., Greene, K., and M. Theofanos,
"Digital Identity Guidelines: Authentication and Lifecycle
Management", NIST Special Publication SP 800-63b,
DOI 10.6028/NIST.SP.800-63b, June 2017,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-63b.pdf>.
[OWASP.CS.passwords]
Manico, J., Saad, E., Maćkowski, J., and R. Bailey,
"Password Storage", OWASP Cheat Sheet Password Storage,
April 2020,
<https://cheatsheetseries.owasp.org/cheatsheets/
Password_Storage_Cheat_Sheet.html>.
[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>.
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[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>.
[RFC6331] Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331,
DOI 10.17487/RFC6331, July 2011,
<https://www.rfc-editor.org/info/rfc6331>.
[RFC7677] Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
Authentication and Security Layer (SASL) Mechanisms",
RFC 7677, DOI 10.17487/RFC7677, November 2015,
<https://www.rfc-editor.org/info/rfc7677>.
[RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based
Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
August 2016, <https://www.rfc-editor.org/info/rfc7914>.
[RFC8018] Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
Password-Based Cryptography Specification Version 2.1",
RFC 8018, DOI 10.17487/RFC8018, January 2017,
<https://www.rfc-editor.org/info/rfc8018>.
[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>.
[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[SCRYPT] Percival, C., "Stronger key derivation via sequential
memory-hard functions",
BSDCan'09 http://www.tarsnap.com/scrypt/scrypt.pdf, May
2009.
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Appendix A. Acknowledgments
The author would like to thank the civil servants at the National
Institute of Standards and Technology for their work on the Special
Publications series. U.S. executive agencies are an undervalued
national treasure, and they deserve our thanks.
Thanks also to Cameron Paul and Thomas Copeland for their reviews and
suggestions.
Author's Address
Sam Whited
Atlanta, GA
United States of America
Email: sam@samwhited.com
URI: https://blog.samwhited.com/
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