Extensions to Salted Challenge Response (SCRAM) for 2 factor authentication
draft-ietf-kitten-scram-2fa-04

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                            4 March 2024
Expires: 5 September 2024

      Extensions to Salted Challenge Response (SCRAM) for 2 factor
                             authentication
                     draft-ietf-kitten-scram-2fa-04

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.  It also includes a separate extension for quick
   reauthentication.

   This specification also gives 2 examples of second factors: TOTP (RFC
   6238) and FIDO CTAP1/U2F (Passkey).

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 5 September 2024.

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   Copyright (c) 2024 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
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SCRAM Extension for 2FA . . . . . . . . . . . . . . . . . . .   3
   4.  SCRAM Extension for reauthentication  . . . . . . . . . . . .   4
   5.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Use of TOTP with SCRAM  . . . . . . . . . . . . . . . . . . .   5
   7.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Use of FIDO CTAP1/U2F with SCRAM  . . . . . . . . . . . . . .   6
   9.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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.  Some examples of 2FA systems are 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.  It also includes a separate
   (but frequently used together with the 2 factor authentication)
   extension for quick reauthentication.

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2.  Conventions Used in This Document

   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.

   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.

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.  This extension also doesn't cover enrollment with a 2FA
   system, such enrollment happends out-of-band.

   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 5) 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,

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   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 is one of open issues to resolve.]]

4.  SCRAM Extension for reauthentication

   This reauthentication extension to SCRAM allows the server to return
   a token that can be used for quick reauthentication and bypasses 2
   factor authentication prompt to the user.  The reauthentication token
   is a randomly generated value [RFC4086].  The reauthentication token
   is returned in the "o" attribute that is appended to the end of the
   "server-final-message".

   [[Note: it would be possible to extend SCRAM itself to do
   reauthentication, by including an earlier received reauthentication
   token in the "client-first-message" of a subsequent SCRAM
   authentication.  This will also turn off the server checking for 2
   factor authentication information, unless the reauthentication
   attempt is rejected by the server.  In the meantime, this document
   presents a couple of other alternatives on how to use other SASL
   mechanisms with the reauthentication token.]]

   When the HT-* mechanism [draft-schmaus-kitten-sasl-ht] is used for
   the reauthentication after a successful SCRAM authentication, the
   reauthentication token is the SASL-HT token (see
   [draft-schmaus-kitten-sasl-ht]).  [[Note that the HT hash should
   probably match the SCRAM hash used or "be better".]]

5.  Formal Syntax

   This document defines the following new SCRAM attributes:

   *  t: This attribute specifies the type of second factor.  This
      document defines two possible types: "totp" (see Section 6) and
      "ctap1" (see Section 8).  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.

   *  l: This attribute is used by some second factors (e.g.  CTAP1) to
      specify the challenge returned by the SCRAM server.

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   *  o: This attribute specifies the base64-encoded value of the
      reauthentication token.

   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.
                 ; Returned in client-final-message.
   type-value      = "totp" / "ctap1" / value
                 ; Type of second factor.
                 ; Should be registered with IANA.
   second-factor   = "f=" second-factor-value
                 ; Complies with "attr-val" syntax.
                 ; Returned in client-final-message.
   second-factor-value = 6DIGIT / value
                 ; 6DIGIT when "t=totp".
   server-challenge = "l=" base64
                 ; Data returned by SCRAM server in server-first-message
                 ; to be used for construction of second-factor-value.

   server-error-value-ext =
           "replayed-second-factor" /
           "invalid-second-factor" /
           "second-factor-value-missing"

   value = <as defined in RFC 5802>

   reauth-token = "o=" base64
                  ; base64 encoding of reauthentication
                  ; token.
                  ; Returned in server-final-message.

6.  Use of TOTP with SCRAM

   When TOTP is used with SCRAM, the following values for "t" and "f"
   attributes (see Section 5 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.

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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.

7.  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, SCRAM-SHA-512/
   SCRAM-SHA-512-PLUS or any other SCRAM variants that use other hash
   functions.

8.  Use of FIDO CTAP1/U2F with SCRAM

   Note that this section describes steps to achieve Client-to-
   Authenticator Protocols v1 (CTAP1)/U2F 2FA in terms of CTAP2 API.

   When FIDO CTAP1/U2F is used with SCRAM, the following values for "t",
   "l" and "f" attributes (see Section 5 for their generic syntax) are
   used:

   *  t: This attribute specifies the type of second factor.  For FIDO
      CTAP1/U2F the value is "ctap1".  If this attribute is specified,
      the "f" attribute MUST also be specified.

   *  l: base64-encoded challenge as returned by SCRAM server.

   *  f: This attribute specifies the value of the second factor.  For
      "t=ctap1" it is the CBOR-serialized [RFC8949] value of
      authenticatorGetAssertionResponse map described below.  This
      attribute MUST be ignored unless the "t" attribute is also
      specified.

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   SCRAM client sends U2F_AUTHENTICATE command formatted as specified in
   [FIDO-U2F-Raw-Message-Formats] to the authenticator (e.g. a USB or
   NFC device).

   The "P1" framing parameter to U2F_AUTHENTICATE is a single octet
   parameter defined as follows:

   *  If "up" is set to false, set it to 0x08 (dont-enforce-user-
      presence-and-sign).

   *  For USB, set it to 0x07 (check-only).  This should prevent call
      getting blocked on waiting for user input.  If response returns
      success, then call again setting the enforce-user-presence-and-
      sign.

   *  For NFC, set it to 0x03 (enforce-user-presence-and-sign).  The tap
      has already provided the presence and won't block.

   The "P2" framing parameter to U2F_AUTHENTICATE is a single octet that
   always has the value 0x00.

   The "request-data" framing parameter (u2fAuthenticateRequest) to
   U2F_AUTHENTICATE is constructed as follows:

   *  The clientData structure (see Section 5.8.1 of [W3C_webauthn_3])
      is filled in as follows:

      -  The "type" field has value "webauthn.get".

      -  The "challenge" field contains the base64url encoding of the
         challenge provided by the Relying Party.  This is the value of
         the "l" attribute described above.  (Note, possibly need to
         base64-decode and base64url-encode.)

      -  The "origin" field contains the origin of the SCRAM client in
         the format specified in [RFC6454], for example
         "smtp://client.example.net:587".

      -  Other fields are specified in Section 5.8.1 of
         [W3C_webauthn_3].

   *  Use clientDataHash parameter of [CTAP2] request as CTAP1/U2F
      challenge parameter (32 bytes).

   *  Let rpIdHash be a byte string of size 32 initialized with SHA-256
      hash of rp.id parameter as CTAP1/U2F application parameter (32
      bytes).  (The rp.id parameter is the hostname of the SCRAM
      server.)

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   *  Let credentialId is the byte string initialized with the id for
      this PublicKeyCredentialDescriptor.

   *  Let keyHandleLength be a byte initialized with length of
      credentialId byte string.

   *  Let u2fAuthenticateRequest be a byte string with the following
      structure:

   +===================+=============+=================================+
   | Length (in        | Description | Value                           |
   | bytes)            |             |                                 |
   +===================+=============+=================================+
   | 32                | Challenge   | Initialized with clientDataHash |
   |                   | parameter   | parameter bytes.                |
   +-------------------+-------------+---------------------------------+
   | 32                | Application | Initialized with rpIdHash       |
   |                   | parameter   | bytes.                          |
   +-------------------+-------------+---------------------------------+
   | 1                 | Key handle  | Initialized with                |
   |                   | length      | keyHandleLength's value.        |
   +-------------------+-------------+---------------------------------+
   | keyHandleLength   | Key handle  | Initialized with credentialId   |
   |                   |             | bytes.                          |
   +-------------------+-------------+---------------------------------+

                                  Table 1

   Generate authenticatorData from the U2F authentication response
   message received from the authenticator:

   *  Copy bits 0 (the UP bit) and bit 1 from the CTAP2/U2F response
      user presence byte to bits 0 and 1 of the CTAP2 flags,
      respectively.  Set all other bits of flags to zero.  Note: bit
      zero is the least significant bit.  See also Authenticator Data
      section of [W3C_webauthn_3].

   *  Let signCount be a 4-byte unsigned integer initialized with CTAP1/
      U2F response counter field.

   Let authenticatorData is a byte string of following structure:

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   +===================+============================+==================+
   | Length (in bytes) | Description                | Value            |
   +===================+============================+==================+
   | 32                | SHA-256 hash of            | Initialized with |
   |                   | the rp.id.                 | rpIdHash bytes.  |
   +-------------------+----------------------------+------------------+
   | 1                 | Flags                      | Initialized with |
   |                   |                            | flags' value.    |
   +-------------------+----------------------------+------------------+
   | 4                 | Signature counter          | Initialized with |
   |                   | (signCount)                | signCount bytes. |
   +-------------------+----------------------------+------------------+

                                  Table 2

   Let authenticatorGetAssertionResponse be a CBOR map with the
   following keys whose values are as follows:

   *  Set 0x01 with the credential from allowList that whose response
      succeeded.

   *  Set 0x02 with authenticatorData bytes.

   *  Set 0x03 with signature field from CTAP1/U2F authentication
      response message.  Note: An ASN.1-encoded ECDSA signature value
      ranges over 8-72 bytes in length.

9.  Open Issues

   Simon Josefsson: should this be a new SASL mechanism name, e.g.
   CROTP-SHA-XXX?

   Should we pick between TOTP and FIDO2 (CTAP1)?

   Rick van Rein: specify a HOTP variant as well?

   Rick van Rein: use TOTP with 6 or 8 digits?  Register both variants?

10.  Security Considerations

   An OTP value is a sensitive piece of data and thus should only be
   sent over a secure channel (such as TLS).  Otherwise an attacker
   might be able to intercept it and reuse it for authentication to the
   SCRAM server.

   TBD

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11.  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)

12.  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-0400.  Thank you to Rick van Rein, Simon Josefsson and Ludovic
   Bocquet for comments and corrections, but all final errors in this
   document remain mine.

13.  Normative References

   [CTAP2]    Bradley, J., Hodges, J., Jones, M., Kumar, A., Lindemann,
              R., and J. Verrept, "Client to Authenticator Protocol
              (CTAP)", FIDO fido-client-to-authenticator-protocol-v2.1-
              ps-errata-20220621, June 2022,
              <https://fidoalliance.org/specs/fido-v2.1-ps-20210615/
              fido-client-to-authenticator-protocol-v2.1-ps-errata-
              20220621.html>.

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   [draft-schmaus-kitten-sasl-ht]
              Schmaus, F. and C. Egger, "The Hashed Token SASL
              Mechanism", Work in Progress, Internet-Draft, draft-
              schmaus-kitten-sasl-ht-09, 6 November 2022,
              <https://www.ietf.org/archive/id/draft-schmaus-kitten-
              sasl-ht-09.txt>.

   [FIDO-U2F-Raw-Message-Formats]
              Balfanz, D., Ehrensvard, J., and J. Lang, "FIDO U2F Raw
              Message Formats", FIDO fido-u2f-raw-message-formats-v1.2-
              ps-20170411, April 2017, <https://fidoalliance.org/specs/
              fido-u2f-v1.2-ps-20170411/fido-u2f-raw-message-formats-
              v1.2-ps-20170411.pdf>.

   [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>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [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>.

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   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [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>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [W3C_webauthn_3]
              "Web Authentication: An API for accessing Public Key
              Credentials - Level 3", W3C WD webauthn-3, W3C webauthn-3,
              <https://www.w3.org/TR/webauthn-3/>.

Author's Address

   Alexey Melnikov
   Isode Ltd
   Email: Alexey.Melnikov@isode.com

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