Extensions to Salted Challenge Response (SCRAM) for 2 factor authentication
draft-melnikov-scram-2fa-03

Document Type Active Internet-Draft (individual)
Author Alexey Melnikov 
Last updated 2021-05-24
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Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                            May 24, 2021
Expires: November 25, 2021

      Extensions to Salted Challenge Response (SCRAM) for 2 factor
                             authentication
                      draft-melnikov-scram-2fa-03

Abstract

   This specification describes an extension to family of Simple
   Authentication and Security Layer (SASL; RFC 4422) authentication
   mechanisms called the Salted Challenge Response Authentication
   Mechanism (SCRAM), which provides support for 2 factor
   authentication.

   This specification also gives an example of how TOTP (RFC 6238) can
   be used as the second factor.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 25, 2021.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SCRAM Extension for 2FA . . . . . . . . . . . . . . . . . . .   3
   4.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Use of TOTP with SCRAM  . . . . . . . . . . . . . . . . . . .   4
   6.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   SCRAM [RFC5802] is a password based SASL [RFC4422] 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 [RFC6238]).

   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.

2.  Conventions Used in This Document

   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 [RFC2119].

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   Formal syntax is defined by [RFC5234] including the core rules
   defined in Appendix B of [RFC5234].

   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.

2.1.  Terminology

   This document uses several terms defined in [RFC4949] ("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 [RFC5802] are also used in this
   document.

2.2.  Notation

   This document reuses notation defined in SCRAM.

3.  SCRAM Extension for 2FA

   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 [RFC5802] If the client included
   "type" and "second-factor" attributes defined in this document (see
   Section 4) 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.]]

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4.  Formal Syntax

   This document defines the following new SCRAM attributes:

   o  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.

   o  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 [RFC5234].

      type            = "t=" type-value
                    ; Complies with "attr-val" syntax.
      type-value      = "totp" / value
                    ; Type of second factor.
                    ; Should be registered with IANA.
      second-factor   = "f=" second-factor-value
                    ; Complies with "attr-val" syntax.
      second-factor-value = 6DIGIT / value
                    ; 6DIGIT when "t=totp"
      server-error-value-ext =
              "replayed-second-factor" /
              "invalid-second-factor" /
              "second-factor-value-missing"

      value = <as defined in RFC 5802>

5.  Use of TOTP with SCRAM

   When TOTP is used with SCRAM, the following values for "t" and "f"
   attributes (see Section 4 for their generic syntax) are used:

   o  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.

   o  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.

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   A TOTP URI is specified with the following ABNF:

   totp-uri = "otpauth" "://" "totp/" label "?secret=" secret
              "&issuer=" issuer
   label = issuer (":" / "%3A") identity
   identity = 1*CHAR ; URI-encoded SASL identity
   secret = 40 * HEXCHAR ; Base32 (hex) encoded secret with no padding.
   issuer = 1*CHAR ; Issuer name.

6.  Example

   The following example extends the example from Section 5 of [RFC5802]
   to demonstrate use of TOTP:

     C: n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
     S: r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,
        i=4096
     C: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
        t=totp,f=776804,p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
     S: v=lz59pqV8S7suAoZWja4dJRkFsKQ=

   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.

7.  Open Issues

   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?

8.  Security Considerations

   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.

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   TBD

9.  IANA Considerations

   IANA is requested to update the definition of the SASL family SCRAM
   in the SASL Mechanism registry established by [RFC4422] 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:

   SCRAM Second Factor Scheme Name:
   Pointer to specification text:
   Notes (optional):

   The registration procedure for the above subregistry is Expert
   Review.

   IANA is requested to register a new value in the subregistry defined
   above:

   SCRAM Second Factor Scheme Name: TOTP
   Pointer to specification text: [[ this document ]]
   Notes (optional): (none)

10.  Acknowledgements

   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.

11.  Normative References

   [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>.

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   [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>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC6238]  M'Raihi, D., Machani, S., Pei, M., and J. Rydell, "TOTP:
              Time-Based One-Time Password Algorithm", RFC 6238,
              DOI 10.17487/RFC6238, May 2011,
              <https://www.rfc-editor.org/info/rfc6238>.

Author's Address

   Alexey Melnikov
   Isode Ltd

   Email: Alexey.Melnikov@isode.com

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