Extensions to Salted Challenge Response (SCRAM) for 2 factor authentication

SCRAM is a password based SASL authentication mechanism that provides (among other things) mutual authentication and binding to an external security layer such as TLS. Two-factor authentication (2FA) is a way to add additional security to an authentication exchange. The first "factor" is a password. The second "factor" is a verification code retrieved from an application on a mobile device or computer. 2FA is conceptually similar to a security token device that banks in some countries require for online banking. Other names for 2FA systems include OTP (one-time password) and TOTP (Time-based One-time Password algorithm, such as ). This specification describes an extension to SCRAM to provide 2 factor authentication. SCRAM already relies on passwords for authentication. This document specifies how second "factors" can be incorporated into SCRAM authentication.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Formal syntax is defined by including the core rules defined in Appendix B of . Example lines prefaced by "C:" are sent by the client and ones prefaced by "S:" by the server. If a single "C:" or "S:" label applies to multiple lines, then the line breaks between those lines are for editorial clarity only, and are not part of the actual protocol exchange.

This document uses several terms defined in ("Internet Security Glossary") including the following: authentication, authentication exchange, authentication information, brute force, challenge-response, cryptographic hash function, dictionary attack, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, one-way encryption function, password, replay attack and salt. Readers not familiar with these terms should use that glossary as a reference. Other terms defined in are also used in this document.

This document reuses notation defined in SCRAM.

This extension doesn't add any extra roundtrips to SCRAM authentication. SCRAM was designed to be extensible, so it allows for optional and mandatory attributes, which covered by MAC codes. Second "factors" are conveyed in the second message ("client-final-message-without-proof" ABNF production) sent from the client to the server. This extension doesn't change how the client authenticates the server. The server authenticates the client after receiving the second message as described in Section 3 of If the client included "type" and "second-factor" attributes defined in this document (see ) and the server supports the specified second factor type, the server verifies content of the "second-factor" according to the "type". If the second factor verification fails, the server MUST fail authentication and SHOULD return either "replayed-second-factor" or "invalid-second-factor" error in the "e" attribute. [[It would be possible to make the extra attributes mandatory by using SCRAM's "m=", but the text above doesn't do that.]]

This document defines the following new SCRAM attributes: t: This attribute specifies the type of second factor. [[Create IANA registry for these?]] This document defines one type: "totp". If this attribute is specified, the "f" attribute MUST also be specified. f: This attribute specifies the value of the second factor. For "t=totp" it is 6 digit decimal number. [[Use 8 digits per Rick van Rein?]] This attribute MUST be ignored unless the "t" attribute is also specified. The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in .

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When TOTP is used with SCRAM, the following values for "t" and "f" attributes (see for their generic syntax) are used: t: This attribute specifies the type of second factor. For TOTP the value is "totp". If this attribute is specified, the "f" attribute MUST also be specified. f: This attribute specifies the value of the second factor. For "t=totp" it is 6 digit decimal number. This attribute MUST be ignored unless the "t" attribute is also specified. A TOTP URI is specified with the following ABNF:

The following example extends the example from Section 5 of to demonstrate use of TOTP:

Please note that TOTP extension described in this document works in the same way with SCRAM-SHA-256/SCRAM-SHA-256-PLUS and SCRAM-SHA-512/SCRAM-SHA-512-PLUS variants of SCRAM.

Simon Josefsson: should this be a new SASL mechanism name, e.g. CROTP-SHA-256? Simon Josefsson: cookie option for fast reauthentication? Alexey: can do or just used CLIENT-KEY (draft-cridland-kitten-clientkey)? Rick van Rein: specify a HOTP variant as well? Rick van Rein: use TOTP with 6 or 8 digits? Register both variants?

Unless an external security layer (such as TLS) is also used, the OTP value is sent in unencrypted/unhashed form from the client to the server, which allows an attacker to read the OTP value and perform a race with the server to validate the OTP. TBD

IANA is requested to update the definition of the SASL family SCRAM in the SASL Mechanism registry established by to also point to this document. IANA is also requested to create a new subregistry of "SASL mechanism" for registering second factor schemes used in the "t" attribute as specified in this document. The registration template is as follows:

The registration procedure for the above subregistry is Expert Review. IANA is requested to register a new value in the subregistry defined above:

Thank you to Stephen Farrell for motivating creation of this document and to Dave Cridland for describing how TOTP can be used with XMPP in XEP-400. Thank you to Rick van Rein and Simon Josefsson for comments and corrections, but all final errors in this document remain mine.