Common Authentication Technology Next Generation S. Whited Internet-Draft 28 April 2020 Intended status: Experimental Expires: 30 October 2020 Best practices for password hashing and storage draft-whited-kitten-password-storage-01 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- Drafts is at https://datatracker.ietf.org/drafts/current/. 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 30 October 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Whited Expires 30 October 2020 [Page 1] Internet-DrafBest practices for password hashing and storage April 2020 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 . . . . . . . . . . . . . . . 5 5. KDF Recommendations . . . . . . . . . . . . . . . . . . . . . 6 5.1. Argon2 . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. Bcrypt . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.3. PBKDF2 . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.4. Scrypt . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Password Complexity Requirements . . . . . . . . . . . . . . 7 7. Internationalization Considerations . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction Following best practices when hashing and storing passwords for use with SASL impacts a great deal more than just a users identity. It also effects usability, backwards compatibility, and interoperability by determining what authentication and authorization mechanisms can be used. Many of the recommendations in this document were taken from [NIST.SP.800-63b] and [NIST.SP.800-132]. 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 [NIST.SP.800-63-3] Appendix A.1 and in [NIST.SP.800-132] section 3.1. Throughout this document the term "password" is used to mean any password, passphrase, PIN, or other memorized secret. Whited Expires 30 October 2020 [Page 2] Internet-DrafBest practices for password hashing and storage April 2020 The term "pepper" is used to mean a secret added to a password hash like a salt. Unlike a salt, peppers are secret, not unique, and are not stored alongside the hashed password. Mechanism pinning is 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 [RFC4422] it is RECOMMENDED that the following SCRAM based 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]. 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 mechanism, clients SHOULD make a record of the authentication and thereafter only advertise and use mechanisms of equal or higher perceived strength. Whited Expires 30 October 2020 [Page 3] Internet-DrafBest practices for password hashing and storage April 2020 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. In particular this example does not imply that mechanisms in this list should or should not be supported. 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 it should be noted that its perceived strength is equal to that of its 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 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. It is therefore REQUIRED that a strong security layer such as TLS [RFC8446] be negotiated before using PLAIN. 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. If clients know that they will only ever authenticate using a mechanism such as SCRAM 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 Whited Expires 30 October 2020 [Page 4] Internet-DrafBest practices for password hashing and storage April 2020 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 other 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 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. Whited Expires 30 October 2020 [Page 5] Internet-DrafBest practices for password hashing and storage April 2020 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 The recomendations in this section may change depending on the type of hardware being used and the security level required for the application. With all Key Derivation Functions proper tuning is required to ensure that it meets the needs of the specific application or service. 5.1. Argon2 Argon2 [ARGON2ESP] is a 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]. 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 2: Bcrypt Parameters 5.3. PBKDF2 PBKDF2 [RFC8018] is the key derivation function used by the SCRAM family of SASL mechanisms. Whited Expires 30 October 2020 [Page 6] Internet-DrafBest practices for password hashing and storage April 2020 +-----------------------+--------+ | Parameter | Value | +=======================+========+ | Minimum Iterations | 10,000 | +-----------------------+--------+ | Recommended HMAC Hash | SHA256 | +-----------------------+--------+ Table 3: PBKDF2 Parameters 5.4. Scrypt The [SCRYPT] key derivation function 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 4: 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 Whited Expires 30 October 2020 [Page 7] Internet-DrafBest practices for password hashing and storage April 2020 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". 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 the [OWASP.CS.passwords]. 9. IANA Considerations This document has no actions for IANA. 10. References 10.1. Normative References [IANA.sasl.mechanisms] IETF, "Simple Authentication and Security Layer (SASL) Mechanisms", November 2015, . Whited Expires 30 October 2020 [Page 8] Internet-DrafBest practices for password hashing and storage April 2020 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 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, . [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, . [NIST.SP.800-132] 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, . [NIST.SP.800-63-3] 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, . Whited Expires 30 October 2020 [Page 9] Internet-DrafBest practices for password hashing and storage April 2020 [NIST.SP.800-63b] 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, . [OWASP.CS.passwords] Manico, J., Saad, E., Maćkowski, J., and R. Bailey, "Password Storage", OWASP Cheat Sheet Password Storage, April 2020, . [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006, . [RFC6331] Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331, DOI 10.17487/RFC6331, July 2011, . [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, . [RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914, August 2016, . [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, . [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, . Whited Expires 30 October 2020 [Page 10] Internet-DrafBest practices for password hashing and storage April 2020 [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, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [SCRYPT] Percival, C., "Stronger key derivation via sequential memory-hard functions", BSDCan'09 http://www.tarsnap.com/scrypt/scrypt.pdf, May 2009. Appendix A. Acknowledgments U.S. executive agencies are an undervalued national treasure, so 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. Author's Address Sam Whited Atlanta, GA United States of America Email: sam@samwhited.com URI: https://blog.samwhited.com/ Whited Expires 30 October 2020 [Page 11]