RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track J. O'Donoghue
Expires: 12 January 2023 Qualcomm Technologies Inc.
N. Cam-Winget
Cisco Systems
C. Bormann
Universität Bremen TZI
11 July 2022
A CBOR Tag for Unprotected CWT Claims Sets
draft-ietf-rats-uccs-03
Abstract
CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the
protection afforded by wrapping them into COSE, as is required for a
true CWT. This specification defines a CBOR tag for such unprotected
CWT Claims Sets (UCCS) and discusses conditions for its proper use.
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-ietf-rats-uccs/.
Discussion of this document takes place on the Remote ATtestation
ProcedureS (rats) Working Group mailing list (mailto:rats@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/rats/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats-wg/draft-ietf-rats-uccs.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 January 2023.
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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 4
3. Characteristics of a Secure Channel . . . . . . . . . . . . . 4
3.1. UCCS and Remote ATtestation procedureS (RATS) . . . . . . 5
3.2. Privacy Preserving Channels . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. General Considerations . . . . . . . . . . . . . . . . . 7
5.2. AES-CBC_MAC . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.4. AES-CCM . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.5. ChaCha20 and Poly1305 . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. CDDL . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Example . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
A CBOR Web Token (CWT) as specified by [RFC8392] is always wrapped in
a CBOR Object Signing and Encryption (COSE, [RFC8152]) envelope.
COSE provides -- amongst other things -- the end-to-end data origin
authentication and integrity protection employed by RFC 8392 and
optional encryption for CWTs. Under the right circumstances
(Section 3), though, a signature providing proof for authenticity and
integrity can be provided through the transfer protocol and thus
omitted from the information in a CWT without compromising the
intended goal of authenticity and integrity. In other words, if
communicating parties have a pre-existing security association they
can reuse it to provide authenticity and integrity for their
messages, enabling the basic principle of using resources
parsimoniously. Specifically, if a mutually Secured Channel is
established between two remote peers, and if that Secure Channel
provides the required properties (as discussed below), it is possible
to omit the protection provided by COSE, creating a use case for
unprotected CWT Claims Sets. Similarly, if there is one-way
authentication, the party that did not authenticate may be in a
position to send authentication information through this channel that
allows the already authenticated party to authenticate the other
party.
This specification allocates a CBOR tag to mark Unprotected CWT
Claims Sets (UCCS) as such and discusses conditions for its proper
use in the scope of Remote ATtestation procedureS (RATS) and the
conveyance of Evidence from an Attester to a Verifier.
This specification does not change [RFC8392]: A true CWT does not
make use of the tag allocated here; the UCCS tag is an alternative to
using COSE protection and a CWT tag. Consequently, within the well-
defined scope of a secured channel, it can be acceptable and economic
to use the contents of a CWT without its COSE container and tag it
with a UCCS CBOR tag for further processing within that scope -- or
to use the contents of a UCCS CBOR tag for building a CWT to be
signed by some entity that can vouch for those contents.
1.1. Terminology
The term Claim is used as in [RFC7519].
The terms Claim Key, Claim Value, and CWT Claims Set are used as in
[RFC8392].
The terms Attester, Attesting Environment and Verifier are used as in
[I-D.ietf-rats-architecture].
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UCCS: Unprotected CWT Claims Set(s); CBOR map(s) of Claims as
defined by the CWT Claims Registry that are composed of pairs of
Claim Keys and Claim Values.
Secure Channel: A protected communication channel between two peers
that can ensure the same qualities associated for UCCS conveyance
as CWT conveyance without any additional protection.
All terms referenced or defined in this section are capitalized in
the remainder of 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.
2. Example Use Cases
Use cases involving the conveyance of Claims, in particular, remote
attestation procedures (RATS, see [I-D.ietf-rats-architecture])
require a standardized data definition and encoding format that can
be transferred and transported using different communication
channels. As these are Claims, [RFC8392] is a suitable format.
However, the way these Claims are secured depends on the deployment,
the security capabilities of the device, as well as their software
stack. For example, a Claim may be securely stored and conveyed
using a device's Trusted Execution Environment (TEE, see
[I-D.ietf-teep-architecture]) or especially in some resource
constrained environments, the same process that provides the secure
communication transport is also the delegate to compose the Claim to
be conveyed. Whether it is a transfer or transport, a Secure Channel
is presumed to be used for conveying such UCCS. The following
sections further describe the RATS usage scenario and corresponding
requirements for UCCS deployment.
3. Characteristics of a Secure Channel
A Secure Channel for the conveyance of UCCS needs to provide the
security properties that would otherwise be provided by COSE for a
CWT. In this regard, UCCS is similar in security considerations to
JWTs [RFC8725] using the algorithm "none". RFC 8725 states:
| [...] if a JWT is cryptographically protected end-to-end by a
| transport layer, such as TLS using cryptographically current
| algorithms, there may be no need to apply another layer of
| cryptographic protections to the JWT. In such cases, the use of
| the "none" algorithm can be perfectly acceptable.
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The security considerations discussed, e.g., in Sections 2.1, 3.1,
and 3.2 of [RFC8725] apply in an analogous way to the use of UCCS as
elaborated on in this document.
Secure Channels are often set up in a handshake protocol that
mutually derives a session key, where the handshake protocol
establishes the (identity and thus) authenticity of one or both ends
of the communication. The session key can then be used to provide
confidentiality and integrity of the transfer of information inside
the Secure Channel. A well-known example of a such a Secure Channel
setup protocol is the TLS [RFC8446] handshake; the TLS record
protocol can then be used for secure conveyance.
As UCCS were initially created for use in Remote ATtestation
procedureS (RATS) Secure Channels, the following subsection provides
a discussion of their use in these channels. Where other
environments are intended to be used to convey UCCS, similar
considerations need to be documented before UCCS can be used.
3.1. UCCS and Remote ATtestation procedureS (RATS)
For the purposes of this section, the Verifier is the receiver of the
UCCS and the Attester is the provider of the UCCS.
Secure Channels can be transient in nature. For the purposes of this
specification, the mechanisms used to establish a Secure Channel are
out of scope.
As a minimum requirement in the scope of RATS Claims, the Verifier
MUST authenticate the Attester as part of the establishment of the
Secure Channel. Furthermore, the channel MUST provide integrity of
the communication from the Attester to the Verifier. If
confidentiality is also required, the receiving side needs to be
authenticated as well; this can be achieved if the Verifier and the
Attester mutually authenticate when establishing the Secure Channel.
The extent to which a Secure Channel can provide assurances that UCCS
originate from a trustworthy attesting environment depends on the
characteristics of both the cryptographic mechanisms used to
establish the channel and the characteristics of the attesting
environment itself.
A Secure Channel established or maintained using weak cryptography
may not provide the assurance required by a relying party of the
authenticity and integrity of the UCCS.
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Ultimately, it is up to the Verifier's policy to determine whether to
accept a UCCS from the Attester and to the type of Secure Channel it
must negotiate. While the security considerations of the
cryptographic algorithms used are similar to COSE, the considerations
of the secure channel should also adhere to the policy configured at
each of the Attester and the Verifier. However, the policy controls
and definitions are out of scope for this document.
Where the security assurance required of an attesting environment by
a relying party requires it, the attesting environment may be
implemented using techniques designed to provide enhanced protection
from an attacker wishing to tamper with or forge UCCS. A possible
approach might be to implement the attesting environment in a
hardened environment such as a TEE [I-D.ietf-teep-architecture] or a
TPM [TPM2].
When UCCS emerge from the Secure Channel and into the Verifier, the
security properties of the Secure Channel no longer apply and UCCS
have the same properties as any other unprotected data in the
Verifier environment. If the Verifier subsequently forwards UCCS,
they are treated as though they originated within the Verifier.
As with EATs nested in other EATs (Section 4.2.19.1.2 (Nested Token)
of [I-D.ietf-rats-eat]), the Secure Channel does not endorse fully
formed CWTs transferred through it. Effectively, the COSE envelope
of a CWT shields the CWT Claims Set from the endorsement of the
Secure Channel. (Note that EAT might add a nested UCCS Claim, and
this statement does not apply to UCCS nested into UCCS, only to fully
formed CWTs)
3.2. Privacy Preserving Channels
A Secure Channel which preserves the privacy of the Attester may
provide security properties equivalent to COSE, but only inside the
life-span of the session established. In general, a Verifier cannot
correlate UCCS received in different sessions from the same attesting
environment based on the cryptographic mechanisms used when a privacy
preserving Secure Channel is employed.
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In the case of a Remote Attestation, the attester must consider
whether any UCCS it returns over a privacy preserving Secure Channel
compromises the privacy in unacceptable ways. As an example, the use
of the EAT UEID [I-D.ietf-rats-eat] Claim in UCCS over a privacy
preserving Secure Channel allows a verifier to correlate UCCS from a
single attesting environment across many Secure Channel sessions.
This may be acceptable in some use-cases (e.g. if the attesting
environment is a physical sensor in a factory) and unacceptable in
others (e.g. if the attesting environment is a device belonging to a
child).
4. IANA Considerations
In the registry [IANA.cbor-tags], IANA is requested to allocate the
tag in Table 1 from the FCFS space, with the present document as the
specification reference.
+========+===========+======================================+
| Tag | Data Item | Semantics |
+========+===========+======================================+
| TBD601 | map | Unprotected CWT Claims Set [RFCthis] |
+--------+-----------+--------------------------------------+
Table 1: Values for Tags
5. Security Considerations
The security considerations of [RFC8949] apply. The security
considerations of [RFC8392] need to be applied analogously, replacing
the role of COSE with that of the Secured Channel.
Section 3 discusses security considerations for Secure Channels, in
which UCCS might be used. This document provides the CBOR tag
definition for UCCS and a discussion on security consideration for
the use of UCCS in Remote ATtestation procedureS (RATS). Uses of
UCCS outside the scope of RATS are not covered by this document. The
UCCS specification - and the use of the UCCS CBOR tag,
correspondingly - is not intended for use in a scope where a scope-
specific security consideration discussion has not been conducted,
vetted and approved for that use.
5.1. General Considerations
Implementations of Secure Channels are often separate from the
application logic that has security requirements on them. Similar
security considerations to those described in
[I-D.ietf-cose-rfc8152bis-struct] for obtaining the required levels
of assurance include:
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* Implementations need to provide sufficient protection for private
or secret key material used to establish or protect the Secure
Channel.
* Using a key for more than one algorithm can leak information about
the key and is not recommended.
* An algorithm used to establish or protect the Secure Channel may
have limits on the number of times that a key can be used without
leaking information about the key.
The Verifier needs to ensure that the management of key material used
establish or protect the Secure Channel is acceptable. This may
include factors such as:
* Ensuring that any permissions associated with key ownership are
respected in the establishment of the Secure Channel.
* Cryptographic algorithms are used appropriately.
* Key material is used in accordance with any usage restrictions
such as freshness or algorithm restrictions.
* Ensuring that appropriate protections are in place to address
potential traffic analysis attacks.
5.2. AES-CBC_MAC
* A given key should only be used for messages of fixed or known
length.
* Different keys should be used for authentication and encryption
operations.
* A mechanism to ensure that IV cannot be modified is required.
Section 3.2.1 of [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
explanation of these considerations.
5.3. AES-GCM
* The key and nonce pair are unique for every encrypted message.
* The maximum number of messages to be encrypted for a given key is
not exceeded.
Section 4.1.1 of [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
explanation of these considerations.
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5.4. AES-CCM
* The key and nonce pair are unique for every encrypted message.
* The maximum number of messages to be encrypted for a given block
cipher is not exceeded.
* The number of messages both successfully and unsuccessfully
decrypted is used to determine when rekeying is required.
Section 4.2.1 of [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
explanation of these considerations.
5.5. ChaCha20 and Poly1305
* The nonce is unique for every encrypted message.
* The number of messages both successfully and unsuccessfully
decrypted is used to determine when rekeying is required.
Section 4.3.1 of [I-D.ietf-cose-rfc8152bis-algs] contains a detailed
explanation of these considerations.
6. References
6.1. Normative References
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
19 September 2013,
<https://www.iana.org/assignments/cbor-tags>.
[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>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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>.
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[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8725] Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
Current Practices", BCP 225, RFC 8725,
DOI 10.17487/RFC8725, February 2020,
<https://www.rfc-editor.org/info/rfc8725>.
[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>.
6.2. Informative References
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-12.txt>.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
18, 14 June 2022, <https://www.ietf.org/archive/id/draft-
ietf-rats-architecture-18.txt>.
[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
Attestation Token (EAT)", Work in Progress, Internet-
Draft, draft-ietf-rats-eat-14, 10 July 2022,
<https://www.ietf.org/archive/id/draft-ietf-rats-eat-
14.txt>.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
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ietf-teep-architecture-17, 19 April 2022,
<https://www.ietf.org/archive/id/draft-ietf-teep-
architecture-17.txt>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[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>.
[RFC8693] Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
DOI 10.17487/RFC8693, January 2020,
<https://www.rfc-editor.org/info/rfc8693>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[TPM2] "Trusted Platform Module Library Specification, Family
“2.0”, Level 00, Revision 01.59 ed., Trusted Computing
Group", 2019.
Appendix A. CDDL
[RFC8392] does not define CDDL for CWT Claims sets.
This specification proposes using the definitions in Figure 1 for the
claims set defined in [RFC8392]. Note that these definitions have
been built such that they also can describe [RFC7519] claims sets by
disabling feature "cbor" and enabling feature "json", but this
flexibility is not the subject of the present specification.
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Claims-Set = {
* $$Claims-Set-Claims
* Claim-Label .feature "extended-claims-label" => any
}
Claim-Label = int / text
string-or-uri = text
$$Claims-Set-Claims //= ( iss-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( sub-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( aud-claim-label => string-or-uri )
$$Claims-Set-Claims //= ( exp-claim-label => ~time )
$$Claims-Set-Claims //= ( nbf-claim-label => ~time )
$$Claims-Set-Claims //= ( iat-claim-label => ~time )
$$Claims-Set-Claims //= ( cti-claim-label => bytes )
iss-claim-label = JC<"iss", 1>
sub-claim-label = JC<"sub", 2>
aud-claim-label = JC<"aud", 3>
exp-claim-label = JC<"exp", 4>
nbf-claim-label = JC<"nbf", 5>
iat-claim-label = JC<"iat", 6>
cti-claim-label = CBOR-ONLY<7> ; jti in JWT: different name and text
JSON-ONLY<J> = J .feature "json"
CBOR-ONLY<C> = C .feature "cbor"
JC<J,C> = JSON-ONLY<J> / CBOR-ONLY<C>
Figure 1: CDDL definition for Claims-Set
Specifications that define additional claims should also supply
additions to the $$Claims-Set-Claims socket, e.g.:
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; [RFC8747]
$$Claims-Set-Claims //= ( 8: CWT-cnf ) ; cnf
CWT-cnf = {
(1: CWT-COSE-Key) //
(2: CWT-Encrypted_COSE_Key) //
(3: CWT-kid)
}
CWT-COSE-Key = COSE_Key
CWT-Encrypted_COSE_Key = COSE_Encrypt / COSE_Encrypt0
CWT-kid = bytes
; [RFC8693]
$$Claims-Set-Claims //= ( 9: CWT-scope ) ; scope
; TO DO: understand what this means:
; scope The scope of an access token as defined in [RFC6749].
; scope 9 byte string or text string [IESG] [RFC8693, Section 4.2]
CWT-scope = bytes / text
; [RFC-ietf-ace-oauth-authz-45, Section 5.10]
$$Claims-Set-Claims //= ( 38: CWT-ace-profile ) ; ace_profile
CWT-ace-profile = $CWT-ACE-Profiles /
int .feature "ace_profile-extend"
; fill in from IANA registry
; https://www.iana.org/assignments/ace/ace.xhtml#ace-profiles :
$CWT-ACE-Profiles /= 1 ; coap_dtls
$$Claims-Set-Claims //= ( 39: CWT-cnonce ) ; cnonce
CWT-cnonce = bytes
$$Claims-Set-Claims //= ( 40: CWT-exi ) ; exi
CWT-exi = uint ; in seconds (5.10.3)
;;; insert CDDL from 9052-to-be to complete these CDDL definitions.
Appendix B. Example
The example CWT Claims Set from Appendix A.1 of [RFC8392] can be
turned into an UCCS by enclosing it with a tag number TBD601:
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<TBD601>(
{
/ iss / 1: "coap://as.example.com",
/ sub / 2: "erikw",
/ aud / 3: "coap://light.example.com",
/ exp / 4: 1444064944,
/ nbf / 5: 1443944944,
/ iat / 6: 1443944944,
/ cti / 7: h'0b71'
}
)
Acknowledgements
Laurence Lundblade suggested some improvements to the CDDL.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Jeremy O'Donoghue
Qualcomm Technologies Inc.
279 Farnborough Road
Farnborough
GU14 7LS
United Kingdom
Email: jodonogh@qti.qualcomm.com
Nancy Cam-Winget
Cisco Systems
3550 Cisco Way
San Jose, CA 95134
United States of America
Email: ncamwing@cisco.com
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Birkholz, et al. Expires 12 January 2023 [Page 14]
Internet-Draft Unprotected CWT Claims Sets July 2022
Phone: +49-421-218-63921
Email: cabo@tzi.org
Birkholz, et al. Expires 12 January 2023 [Page 15]