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
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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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