Internet DRAFT - draft-schmaus-kitten-sasl-ht
draft-schmaus-kitten-sasl-ht
Common Authentication Technology Next Generation F. Schmaus
Internet-Draft C. Egger
Intended status: Experimental FAU Erlangen-Nürnberg
Expires: 11 May 2023 7 November 2022
The Hashed Token SASL Mechanism
draft-schmaus-kitten-sasl-ht-09
Abstract
This document specifies the family of Hashed Token SASL mechanisms
which enable a proof-of-possession-based authentication scheme and
are meant to be used for quick re-authentication of a previous
session. The Hashed Token SASL mechanism's authentication sequence
consists of only one round-trip. The usage of short-lived,
exclusively ephemeral hashed tokens is achieving the single round-
trip property. The SASL mechanism specified herin further provides
hash agility, mutual authentication and support for channel binding.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-schmaus-kitten-sasl-ht/.
Source for this draft and an issue tracker can be found at
https://github.com/flowdalic/xeps/tree/master/draft-schmaus-kitten-
sasl-ht.
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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This Internet-Draft will expire on 11 May 2023.
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Copyright Notice
Copyright (c) 2022 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 3
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
2. The HT Family of Mechanisms . . . . . . . . . . . . . . . . . 4
3. The HT Authentication Exchange . . . . . . . . . . . . . . . 5
3.1. Initiator First Message . . . . . . . . . . . . . . . . . 5
3.2. Initiator Authentication . . . . . . . . . . . . . . . . 6
3.3. Final Responder Message . . . . . . . . . . . . . . . . . 6
4. Compliance with SASL Mechanism Requirements . . . . . . . . . 7
5. Requirements for the Application-Protocol Extension . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This specification describes the family of Hashed Token (HT) Simple
Authentication and Security Layer (SASL) [RFC4422] mechanisms, which
enable a proof-of-possession-based authentication scheme. The HT
mechanism is designed to be used with short-lived, exclusively
ephemeral tokens, called SASL-HT tokens, and allow for quick, one
round-trip, re-authentication of a previous session.
Further properties of the HT mechanism are 1) hash agility, 2) mutual
authentication, and 3) support for channel binding.
Clients are supposed to request SASL-HT tokens from the server after
being authenticated using a "strong" SASL mechanism like SCRAM
[RFC5802]. Hence a typical sequence of actions using HT may look
like the following:
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A) Client authenticates using a strong mechanism (e.g., SCRAM)
B) Client requests secret SASL-HT token
C) Service returns SASL-HT token
<normal client-server interaction here>
D) Connection between client and server gets interrupted,
for example because of a WiFi ↔ GSM switch
E) Client resumes the previous session using HT and token from C)
F) Service revokes the successfully used SASL-HT token
[goto B]
The HT mechanism requires an accompanying, application protocol
specific, extension, which allows clients to requests a new SASL-HT
token (see Section 5 (Section 5)). One example for such an
application protocol specific extension based on HT is [XEP-0397].
This XMPP [RFC6120] extension protocol allows, amongst other things,
B) and C),
Since the SASL-HT token is not salted, and only one hash iteration is
used, the HT mechanism is not suitable to protect long-lived shared
secrets (e.g. "passwords"). You may want to look at [RFC5802] for
that.
1.1. Conventions and Terminology
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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
1.2. Applicability
Because this mechanism transports information that should not be
controlled by an attacker, the HT mechanism *MUST* only be used over
channels protected by Transport Layer Security (TLS, see [RFC8446]),
or over similar integrity-protected and authenticated channels.
Also, the application protcol specific extension which requests a new
SASL-HT token *SHOULD* only be used over similarly protected
channels.
Also, when TLS is used, the client *MUST* successfully validate the
server's certificate ([RFC5280], [RFC6125]).
The family of HT mechanisms is not applicable for proxy
authentication since they can not carry an authorization identity
string (authzid).
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2. The HT Family of Mechanisms
Each mechanism in this family differs by choice of the hash algorithm
and the choice of the channel binding [RFC5929] type.
An HT mechanism name is a string beginning with "HT-" followed by the
capitalised name of the used hash, followed by "-", and suffixed by
one of 'ENDP', 'UNIQ', 'EXPR' or 'NONE'.
Hence each HT mechanism has a name of the following form:
HT-<hash-alg>-<cb-type>
Where <hash-alg> is the capitalised "Hash Name String" of the IANA
"Named Information Hash Algorithm Registry" [iana-hash-alg] as
specified in [RFC6920], and <cb-type> is one of 'ENDP', 'UNIQ',
'EXPR' or 'NONE' denoting the channel binding type. In the case of
'ENDP', the tls-server-end-point channel binding type is used. In
the case of 'UNIQ', the tls-unique channel binding type is used. In
the case of 'EXPR', the tls-exporter [RFC9266] channel binding type
is used. Valid channel binding types are defined in the IANA
"Channel-Binding Types" registry [iana-cbt] as specified in
[RFC5056].
In the special case of 'NONE', no channel binding is to be used (cb-
data is to be an empty string).
+=========+======================+
| cb-type | Channel Binding Type |
+=========+======================+
| ENDP | tls-server-end-point |
+---------+----------------------+
| UNIQ | tls-unique |
+---------+----------------------+
| EXPR | tls-exporter |
+---------+----------------------+
Table 1: Mapping of cb-type to
Channel Binding Types
The following table lists some examples of HT SASL mechanisms
registered by this document.
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+==================+===================+======================+
| Mechanism Name | HT Hash Algorithm | Channel-binding |
| | | unique prefix |
+==================+===================+======================+
| HT-SHA-512-ENDP | SHA-512 | tls-server-end-point |
+------------------+-------------------+----------------------+
| HT-SHA-512-UNIQ | SHA-512 | tls-unique |
+------------------+-------------------+----------------------+
| HT-SHA3-512-ENDP | SHA3-512 | tls-server-end-point |
+------------------+-------------------+----------------------+
| HT-SHA-256-UNIQ | SHA-256 | tls-unique |
+------------------+-------------------+----------------------+
| HT-SHA-256-NONE | SHA-256 | N/A |
+------------------+-------------------+----------------------+
Table 2: Examples of HT SASL mechanisms
3. The HT Authentication Exchange
The mechanism consists of a simple exchange of precisely two messages
between the initiator and responder.
The following syntax specifications use the Augmented Backus-Naur
form (ABNF) notation as specified in [RFC5234].
3.1. Initiator First Message
The HT mechanism starts with the initiator-msg, send by the initiator
to the responder. The following lists the ABNF grammar for the
initiator-msg:
initiator-msg = authcid NUL initiator-hashed-token
authcid = 1*SAFE ; MUST accept up to 255 octets
initiator-hashed-token = 1*OCTET
NUL = %0x00 ; The null octet
SAFE = UTF1 / UTF2 / UTF3 / UTF4
;; any UTF-8 encoded Unicode character except NUL
UTF1 = %x01-7F ;; except NUL
UTF2 = %xC2-DF UTF0
UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
%xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
%xF4 %x80-8F 2(UTF0)
UTF0 = %x80-BF
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The initiator first message starts with the authentication identity
(authcid, see[RFC4422]) as UTF-8 [RFC3629] encoded string. It is
followed by initiator-hashed-token separated by as single null octet.
The value of the initiator-hashed-token is defined as follows:
initiator-hashed-token := HMAC(token, "Initiator" || cb-data)
HMAC() is the function defined in [RFC2104] with H being the selected
HT hash algorithm, 'cb-data' represents the data provided by the
selected channel binding type, and 'token' are the UTF-8 encoded
octets of the SASL-HT token string which acts as a shared secret
between initiator and responder.
The initiator-msg *MAY* be included in TLS 1.3 0-RTT early data, as
specified in [RFC8446]. If this is the case, then the initiating
entity *MUST NOT* include any further application protocol payload in
the early data besides the HT initiator-msg and potential required
framing of the SASL profile. The responder *MUST* abort the SASL
authentication if the early data contains additional application
protocol payload.
SASL-HT hence allows exploiting TLS 1.3 early data for "0.5 Round
Trip Time (RTT)" resumption of the application protocol's session.
Using TLS early data requires extra care when implementing: The
early data should only contain the SASL-HT payload, i.e., the
initiator-msg, and not an application protocol specific payload.
The reason for this is that the early data could be replayed, and
thus needs to carry an idempotent operation. On the other hand,
if the responding entity can verify the early data, then it can
send additional application protocol payload together with the
"resumption successful" response to the initiating entity.
3.2. Initiator Authentication
Upon receiving the initiator-msg, the responder calculates itself the
value of initiator-hashed-token and compares it with the received
value found in the initiator-msg. If both values are equal, then the
initiator has been successfully authenticated. Otherwise, if both
values are not equal, then authentication *MUST* fail.
3.3. Final Responder Message
After the initiator was authenticated the responder continues the
SASL authentication by sending the responder-msg to the initiator.
The ABNF for responder-msg is:
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responder-msg = 1*OCTET
The responder-msg value is defined as follows:
responder-msg := HMAC(token, "Responder" || cb-data)
The initiating entity *MUST* verify the responder-msg to achieve
mutual authentication.
4. Compliance with SASL Mechanism Requirements
This section describes compliance with SASL mechanism requirements
specified in Section 5 of [RFC4422].
1. "HT-SHA-256-ENDP", "HT-SHA-256-UNIQ", "HT-SHA-3-512-ENDP" and
"HT-SHA-3-512-UNIQ".
2. Definition of server-challenges and client-responses: a) HT is a
client-first mechanism. b) HT does send additional data with
success (the responder-msg).
3. HT is not capable of transferring authorization identities from
the client to the server.
4. HT does not offer any security layers (HT offers channel binding
instead).
5. HT does not protect the authorization identity.
5. Requirements for the Application-Protocol Extension
It is *REQUIRED* that the application-protocol specific extension
provides a mechanism to request a SASL-HT token in form of a Unicode
string. The returned token *MUST* have been newly generated by a
cryptographically secure random number generator and MUST contain at
least 128 bit of entropy.
It is *RECOMMENDED* that the protocol allows the requestor to signal
the name of the SASL mechanism which he intends to use with the
token. If a token is used with a different mechanism than the one
which was signalled upon requesting the token, then the
authentication *MUST* fail. This allows pinning the token to a SASL
mechanism, which increases the security because it makes it
impossible for an attacker to downgrade the SASL mechanism.
It is *RECOMMENDED* that the protocol defines a way for a client to
request rotation or revocation of a token.
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6. Security Considerations
To be secure, the HT mechanism *MUST* be used over a TLS channel that
has had the session hash extension [RFC7627] negotiated, or session
resumption *MUST NOT* have been used.
It is *RECOMMENDED* that implementations periodically require a full
authentication using a strong SASL mechanism which does not use the
SASL-HT token.
It is of vital importance that the SASL-HT token is generated by a
cryptographically secure random generator. See [RFC4086] for more
information about Randomness Requirements for Security. In addition,
comparison of the client's HMAC with the server's calculated HMAC
*SHOULD* be performed using constant-time comparison functions, to
protect against timing attacks.
The tokens used with HT mechanisms *SHOULD* have a limited lifetime,
e.g. based on usage count or time elapsed since issuance.
Due to the additional security properties afforded by channel
binding, it is *RECOMMENDED* that clients use HT mechanisms
supporting channel binding in environments that can support it.
7. IANA Considerations
IANA is requested to add the following family of SASL mechanisms to
the SASL Mechanism registry established by [RFC4422]:
To: iana@iana.org
Subject: Registration of a new SASL family HT
SASL mechanism name (or prefix for the family): HT-*
Security considerations: Section 6 of draft-schmaus-kitten-sasl-ht
Published specification (optional, recommended): draft-schmaus-
kitten-sasl-ht-XX (TODO)
Person & email address to contact for further information: IETF
SASL WG kitten@ietf.org (mailto:kitten@ietf.org)
Intended usage: COMMON
Owner/Change controller: IESG iesg@ietf.org (mailto:iesg@ietf.org)
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Note: Members of this family MUST be explicitly registered using
the "IETF Review" [RFC8126] registration procedure. Reviews MUST
be requested on the Kitten WG mailing list kitten@ietf.org
(mailto:kitten@ietf.org) (or a successor designated by the
responsible Security AD).
8. References
8.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[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>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
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[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9266] Whited, S., "Channel Bindings for TLS 1.3", RFC 9266,
DOI 10.17487/RFC9266, July 2022,
<https://www.rfc-editor.org/info/rfc9266>.
[iana-hash-alg]
Williams, N., "IANA Named Information Hash Algorithm
Registry", 2010, <https://www.iana.org/assignments/named-
information/named-information.xhtml#hash-alg>.
[iana-cbt] Williams, N., "IANA Channel-Binding Types", 2010,
<https://www.iana.org/assignments/channel-binding-types/
channel-binding-types.xhtml>.
[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>.
[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>.
8.2. Informative References
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[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>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[XEP-0397] Schmaus, F., "XEP-0397: Instant Stream Resumption", 3
November 2018,
<https://xmpp.org/extensions/xep-0397.html>.
Acknowledgments
This document benefited from discussions on the KITTEN WG mailing
list. The authors would like to especially thank Thijs Alkemade, Sam
Whited and Alexey Melnikov for their comments on this topic.
Furthermore, we would like to thank Alexander Wuerstlein, who came up
with the idea to pin the token to a SASL mechanism for increased
security. And last but not least, thanks to Matthew Wild for working
on the -NONE variant of SASL-HT.
Authors' Addresses
Florian Schmaus
FAU Erlangen-Nürnberg
Email: flow@cs.fau.de
Christoph Egger
FAU Erlangen-Nürnberg
Email: egger@cs.fau.de
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