Atlanta GA USA sam@samwhited.com https://blog.samwhited.com/

Internet Common Authentication Technology Next Generation 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 . 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 26 May 2021.

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Table of Contents

• .  Introduction
  • .  Conventions and Terminology
• .  SASL Mechanisms
• .  Client Best Practices
  • .  Mechanism Pinning
  • .  Storage
• .  Server Best Practices
  • .  Additional SASL Requirements
  • .  Storage
  • .  Authentication and Rotation
• .  KDF Recommendations
  • .  Argon2
  • .  Bcrypt
  • .  PBKDF2
  • .  Scrypt
• .  Password Complexity Requirements
• .  Internationalization Considerations
• .  Security Considerations
• .  IANA Considerations
• . References
  • .  Normative References
  • .  Informative References
• .  Acknowledgments
• Author's Address

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.

Conventions and Terminology Various security-related terms are to be understood in the sense defined in . Some may also be defined in Appendix A.1 and in 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:

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.

Pepper

A secret added to a password hash like a salt. Unlike a salt, peppers are secret and the same pepper may be reused for many hashed passwords. They must not be stored alongside the hashed password.

Salt

In this document salt is used as defined in .

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 when, and only when, they appear in all capitals, as shown here.

SASL Mechanisms For clients and servers that support password based authentication using SASL it is RECOMMENDED that the following mechanisms be implemented:

• SCRAM-SHA-256
• SCRAM-SHA-256-PLUS

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. The SASL mechanisms discussed in this document do not negotiate a security layer. Because of this a strong security layer such as TLS MUST be negotiated before SASL mechanisms can be advertised or negotiated.

Client Best Practices

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. 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-256-PLUS
3. SCRAM-SHA-1-PLUS
4. SCRAM-SHA-256
5. SCRAM-SHA-1
6. PLAIN
7. DIGEST-MD5, CRAM-MD5

The EXTERNAL mechanism defined in 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 services are being used. 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, but does allow for the use of a strong key derivation function (KDF) 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 KDF. For a list of problems with DIGEST-MD5 see .

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 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.

Server Best Practices

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 or hashing mechanisms that are cryptographically secure but designed for speed such as SHA256.

Storage Servers MUST always store passwords only after they have been salted, peppered (if possible with the given authentication mechanism), and hashed using a strong KDF. A distinct salt SHOULD be used for each user, and each SCRAM family supported. Salts SHOULD be generated using a cryptographically secure random number generator. The salt MAY be stored in the same datastore as the password. A pepper stored in the application configuration, or a secure location other than the datastore containing the salts, SHOULD be combined with the password before hashing if possible with the given authentication mechanism. Peppers SHOULD NOT be combined with the salt because the salt is not secret and may appear in the final hash output. The following restrictions MUST be observed when generating salts and peppers, more up to date numbers may be found in . Common Parameters

Parameter	Value
Minimum Salt Length	16 bytes
Minimum Pepper Length	32 bytes

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.

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.

Argon2 Argon2 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 . They are copied here for easier reference. Argon Parameters

Parameter	Value
Degree of parallelism (p)	1
Minimum memory size (m)	32*1024
Minimum number of iterations (t)	1
Algorithm type (y)	Argon2id (2)

Bcrypt bcrypt is a Blowfish-based KDF that is the current OWASP recommendation for password hashing. Bcrypt Parameters

Parameter	Value
Recommended Cost	12
Maximum Password Length	50-72 bytes depending on the implementation

PBKDF2 PBKDF2 is used by the SCRAM family of SASL mechanisms. PBKDF2 Parameters

Parameter	Value
Minimum iteration count (c)	10,000
Hash	SHA256
Output length (dkLen)	hLen (length of the chosen hash)

Scrypt The 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 . Scrypt Parameters

Parameter	Value
Minimum CPU/Memory cost parameter (N)	32768 (N=2^15)
Blocksize (r)	8
Parallelization parameter (p)	1
Output length (dkLen)	hLen (length of the chosen hash)

Password Complexity Requirements Before any other password complexity requirements are checked, the preparation and enforcement steps of the OpaqueString profile of 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 blocklist and reject attempts by a claimant to use passwords on the blocklist during registration or password reset. The contents of this blocklist are a matter of server policy. Some common recommendations include lists of common passwords that are not otherwise prevented by length requirements, and passwords present in known breaches.

Internationalization Considerations The PRECIS framework (Preparation, Enforcement, and Comparison of Internationalized Strings) defined in 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 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.

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 , , and . The "-PLUS" variants of SCRAM support channel binding to their underlying security layer, but lack a mechanism for negotiating what type of channel binding to use. In the tls-unique 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 and the renegotiation indication TLS extension 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 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.

IANA Considerations This document has no actions for IANA.

References Normative References IETF In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This Glossary provides definitions, abbreviations, and explanations of terminology for information system security. The 334 pages of entries offer recommendations to improve the comprehensibility of written material that is generated in the Internet Standards Process (RFC 2026). The recommendations follow the principles that such writing should (a) use the same term or definition whenever the same concept is mentioned; (b) use terms in their plainest, dictionary sense; (c) use terms that are already well-established in open publications; and (d) avoid terms that either favor a particular vendor or favor a particular technology or mechanism over other, competing techniques that already exist or could be developed. This memo provides information for the Internet community. Secure Socket Layer (SSL) and Transport Layer Security (TLS) renegotiation are vulnerable to an attack in which the attacker forms a TLS connection with the target server, injects content of his choice, and then splices in a new TLS connection from a client. The server treats the client's initial TLS handshake as a renegotiation and thus believes that the initial data transmitted by the attacker is from the same entity as the subsequent client data. This specification defines a TLS extension to cryptographically tie renegotiations to the TLS connections they are being performed over, thus preventing this attack. [STANDARDS-TRACK] This document defines three channel binding types for Transport Layer Security (TLS), tls-unique, tls-server-end-point, and tls-unique-for-telnet, in accordance with RFC 5056 (On Channel Binding). Note that based on implementation experience, this document changes the original definition of 'tls-unique' channel binding type in the channel binding type IANA registry. [STANDARDS-TRACK] The Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters such as the server certificate. Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same. Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the-middle attack, where the attacker can simply forward messages back and forth between the client and server. This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document describes the Argon2 memory-hard function for password hashing and proof-of-work applications. We provide an implementer- oriented description with test vectors. The purpose is to simplify adoption of Argon2 for Internet protocols. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. Work in Progress National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology Altmode NetworksLos Altos, CA National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. It provides a structured interface between protocols and mechanisms. The resulting framework allows new protocols to reuse existing mechanisms and allows old protocols to make use of new mechanisms. The framework also provides a protocol for securing subsequent protocol exchanges within a data security layer. This document describes how a SASL mechanism is structured, describes how protocols include support for SASL, and defines the protocol for carrying a data security layer over a connection. In addition, this document defines one SASL mechanism, the EXTERNAL mechanism. This document obsoletes RFC 2222. [STANDARDS-TRACK] The secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment, and have had success only in limited use. This specification describes a family of Simple Authentication and Security Layer (SASL; RFC 4422) authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM), which addresses the security concerns and meets the deployability requirements. When used in combination with TLS or an equivalent security layer, a mechanism from this family could improve the status quo for application protocol authentication and provide a suitable choice for a mandatory-to-implement mechanism for future application protocol standards. [STANDARDS-TRACK] This memo describes problems with the DIGEST-MD5 Simple Authentication and Security Layer (SASL) mechanism as specified in RFC 2831. It marks DIGEST-MD5 as OBSOLETE in the IANA Registry of SASL mechanisms and moves RFC 2831 to Historic status. This document is not an Internet Standards Track specification; it is published for informational purposes. This document registers the Simple Authentication and Security Layer (SASL) mechanisms SCRAM-SHA-256 and SCRAM-SHA-256-PLUS, provides guidance for secure implementation of the original SCRAM-SHA-1-PLUS mechanism, and updates the SCRAM registration procedures of RFC 5802. This document specifies the password-based key derivation function scrypt. The function derives one or more secret keys from a secret string. It is based on memory-hard functions, which offer added protection against attacks using custom hardware. The document also provides an ASN.1 schema. This document provides recommendations for the implementation of password-based cryptography, covering key derivation functions, encryption schemes, message authentication schemes, and ASN.1 syntax identifying the techniques. This document represents a republication of PKCS #5 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 2898. Application protocols using Unicode code points in protocol strings need to properly handle such strings in order to enforce internationalization rules for strings placed in various protocol slots (such as addresses and identifiers) and to perform valid comparison operations (e.g., for purposes of authentication or authorization). This document defines a framework enabling application protocols to perform the preparation, enforcement, and comparison of internationalized strings ("PRECIS") in a way that depends on the properties of Unicode code points and thus is more agile with respect to versions of Unicode. As a result, this framework provides a more sustainable approach to the handling of internationalized strings than the previous framework, known as Stringprep (RFC 3454). This document obsoletes RFC 7564. This document describes updated methods for handling Unicode strings representing usernames and passwords. The previous approach was known as SASLprep (RFC 4013) and was based on Stringprep (RFC 3454). The methods specified in this document provide a more sustainable approach to the handling of internationalized usernames and passwords. This document obsoletes RFC 7613. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations.

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

Atlanta GA USA sam@samwhited.com https://blog.samwhited.com/