RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Standards Track J. O'Donoghue
Expires: June 5, 2021 Qualcomm Technologies Inc.
N. Cam-Winget
Cisco Systems
C. Bormann
Universitaet Bremen TZI
December 02, 2020
A CBOR Tag for Unprotected CWT Claims Sets
draft-birkholz-rats-uccs-02
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation and Requirements . . . . . . . . . . . . . . . . . 3
3. Characteristics of a Secure Channel . . . . . . . . . . . . . 4
3.1. UCCS and Remote ATtestation procedureS (RATS) . . . . . . 4
3.2. Privacy Preserving Channels . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. General Considerations . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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 integrity protection
mandated by RFC 8392 and optional encryption for CWTs. Under the
right circumstances, 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. 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
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using COSE protection and a CWT tag. Consequently, in a well-defined
scope, it might be acceptable to use the contents of a CWT without
its COSE container and tag it with a UCCS CBOR tag for further
processing - 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 [RFC8725].
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].
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. Motivation and Requirements
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
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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.". Analogously, the considerations discussed in Sections
2.1, 3.1, and 3.2 of RFC 8725 apply 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 authenticity of one of 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 be
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authenticated as well, i.e., the Verifier and the Attester SHOULD
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.
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].
As with EATs nested in other EATs (Section 3.12.1.2 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.
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).
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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 [RFC7049] and [RFC8392] apply.
Section 3 discusses security considerations for Secure Channels, in
which UCCS might be used. This documents 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:
6. References
6.1. Normative References
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<http://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>.
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[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[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>.
[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>.
[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>.
[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>.
[TPM2] "Trusted Platform Module Library Specification, Family
"2.0", Level 00, Revision 01.59 ed., Trusted Computing
Group", 2019.
6.2. Informative References
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
(work in progress), September 2020.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis-
struct-14 (work in progress), September 2020.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-07 (work in progress),
October 2020.
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[I-D.ietf-rats-eat]
Mandyam, G., Lundblade, L., Ballesteros, M., and J.
O'Donoghue, "The Entity Attestation Token (EAT)", draft-
ietf-rats-eat-05 (work in progress), December 2020.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", draft-ietf-teep-architecture-13 (work in
progress), November 2020.
Appendix A. 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:
<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'
}
)
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
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
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Nancy Cam-Winget
Cisco Systems
3550 Cisco Way
San Jose, CA 95134
USA
Email: ncamwing@cisco.com
Carsten Bormann
Universitaet Bremen TZI
Bibliothekstrasse 1
Bremen 28369
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.de
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