Remote attestation over EDHOC
draft-song-lake-ra-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | SONG Yuxuan | ||
| Last updated | 2024-03-04 | ||
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draft-song-lake-ra-00
Lightweight Authenticated Key Exchange Y. Song
Internet-Draft Inria
Intended status: Standards Track 4 March 2024
Expires: 5 September 2024
Remote attestation over EDHOC
draft-song-lake-ra-00
Abstract
This document specifies how to perform remote attestation as part of
the lightweight authenticated Diffie-Hellman key exchange protocol
EDHOC (Ephemeral Diffie-Hellman Over COSE), based on the Remote
ATtestation procedureS (RATS) architecture.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://ysong02.github.io/RemoteAttestation_overEDHOC/draft-song-
lake-ra.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-song-lake-ra/.
Discussion of this document takes place on the Lightweight
Authenticated Key Exchange Working Group mailing list
(mailto:lake@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/lake/. Subscribe at
https://www.ietf.org/mailman/listinfo/lake/.
Source for this draft and an issue tracker can be found at
https://github.com/ysong02/RemoteAttestation_overEDHOC.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Problem Description . . . . . . . . . . . . . . . . . . . . . 4
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. The Protocol . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2. External Authorization Data 1 . . . . . . . . . . . . . . 5
5.3. External Authorization Data 2 . . . . . . . . . . . . . . 6
5.4. External Authorization Data 3 . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7.1. EDHOC External Authorization Data Registry . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Example: Remote Attestation Flow . . . . . . . . . . 9
Appendix B. Example: Firmware Version . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Remote attestation is a security process which verifies and confirms
the integrity and trustworthiness of a remote device or system in the
network. This process helps establish a level of trust in the remote
system before allowing the device to e.g. join the network or access
some sensitive information and resources. The use cases that require
remote attestation include secure boot and firmware management, cloud
computing, network access control, etc.
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The IETF working group Remote ATtestation procedureS (RATS) has
defined an architecture [RFC9334] for remote attestation. The three
main roles in the RATS architecture are the Attester, the Verifier
and the Relying Party. The Attester generates the evidence
concerning its identity and integrity, which must be appraised by the
Verifier for its validity. Then, the Verifier produces the
attestation result, which is consequently used by the Relying Party
for the purposes of reliably applying application-specific actions.
One type of interaction model defined in the RATS architecture is
called the background-check model. It resembles the procedure of how
employers perform background checks to determine the prospective
employee's trustworthiness, by contacting the respective organization
that issues a report. In this case, the employer acts as the Relying
Party, the employee acts as the Attester and the organization acts as
the Verifier. The Attester conveys evidence directly to the Relying
Party and the Relying Party forwards the evidence to the Verifier for
appraisal. Once the attestation result is computed by the Verifier,
it is sent back to the Relying Party to decide what action to take
based on the attestation result. This specification employs the RATS
background check model.
One way of conveying attestation evidence is the Entity Attestation
Token (EAT) [I-D.ietf-rats-eat]. It provides an attested claims set
that describes the state and the characteristics of the Attester,
which can be used to determine its level of trustworthiness. This
specification relies on the EAT as the attestation evidence.
Ephemeral Diffie-Hellman over COSE (EDHOC) [I-D.ietf-lake-edhoc] is a
lightweight authenticated key exchange protocol for highly
constrained networks. In EDHOC, the two parties involved in the key
exchange are referred to as the Initiator (I) and the Responder (R).
EDHOC supports the transport of external authorization data, through
the dedicated EAD fields. This specification delivers EAT through
EDHOC. Specifically, EAT is transported as an EAD item.
Typically, the Attester incorporates an internal attestation service,
including a specific trusted element known as the "root of trust".
Root of trust serves as the starting point for establishing and
validating the trustworthiness appraisals of other components on the
system. The measurements signed by the attestation service are
referred to as the Evidence. The signing is requested through an
attestation API. How the components are separated between the secure
and non-secure worlds on a device is out of scope of this
specification.
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2. Conventions and Definitions
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.
The reader is assumed to be familiar with the terms and concepts
defined in EDHOC [I-D.ietf-lake-edhoc] and RATS [RFC9334].
3. Problem Description
This specification describes how to perform remote attestation over
the EDHOC protocol according to the RATS architecture. Remote
attestation protocol elements are carried within EDHOC's External
Authorization Data (EAD) fields. More specifically, this
specification supports the RATS background check model. It describes
how the Attester (EDHOC Initiator) and Relying Party (EDHOC
Responder) complete the EDHOC handshake complemented with remote
attestation protocol elements.
4. Assumptions
The details of the protocol between Relying Party and Verifier are
out of the scope. The only assumption is that the Verifier outputs a
fresh nonce and that same nonce is passed on to the EDHOC session.
That is where the link between the two protocols comes in. The
remainder, such as the evidence type selection is just the
negotiation. In general, the Verifier is supposed to know how to
verify more than one format of the evidence type. Therefore, the
Verifier MUST send back at least one format to the Relying Party. We
assume in this specification, the Relying Party also has knowledge
about the Attester, so it can narrow down the type selection and send
to the Attester only one format of evidnece type.
The Attester should have an explicit relation with the Verifier, such
as from device manufacuture, so that the Verifier can evaluate the
Evidence that is produced by the Attester. The authentication
between the Attester and the Relying Party is performed with EDHOC
[I-D.ietf-lake-edhoc] and defines the process of remote attestation
using the External Authorization Data (EAD) fields defined in EDHOC.
5. The Protocol
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5.1. Overview
EDHOC Initiator plays the role of the RATS Attester. EDHOC Responder
plays the role of the RATS Relying Party. An external entity, out of
scope of this specification, plays the role of the RATS Verifier.
+----------+ Attestation +-----------+ +----------+
| | proposal | | Provided | |
| Attester +------------->| Relying | EvidenceTypes | Verifier |
| | | +-------------->| |
| | | Party |<--------------+ |
| |<-------------+ | Selected | |
| | Attestation | |EvidenceType(s)| |
| | request | | | |
| (A) | | (RP) | | (V) |
| | Evidence | | Evidence | |
| +------------->| +-------------->| |
| | | |<--------------+ |
| | | | Attestation | |
| | | | Result | |
+----------+ +-----------+ +----------+
Figure 1: Overview of message flow. EDHOC is used between A and
RP. Remote attestation proposal and request are sent in EDHOC
External Authorization Data (EAD). The link between V and RP is
out of scope of this specification.
The Attester and the Relying Party communicate by transporting
messages within EDHOC's External Authorization Data (EAD) fields.
5.2. External Authorization Data 1
In EAD_1, the Attester transports the Proposed_EvidenceType object.
It signals to the Relying Party the proposal to do remote
attestation, as well as which attestation claims the Attester
supports. The supported attestation claims are encoded in CBOR in
the form of a sequence.
The external authorization data EAD_1 contains an EAD item with
* ead_label = TBD1
* ead_value = Attestation_proposal, which is a CBOR byte string:
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Attestation_proposal = bstr .cbor Proposed_EvidenceType
Proposed_EvidenceType = (
content-format: [ + uint]
)
where
* content-format is an array that contains all the supported
evidence types by the Attester in decreasing order of preference.
* There MUST be at least one item in the array.
* content-format is an indicator of the format type (e.g.,
application/eat+cwt with an appropriate eat_profile parameter
set).
The sign of ead_label MUST be negative to indicate that the EAT item
is critical. If the receiver cannot recognize the critical EAD item,
or cannot process the information in the critical EAD item, then the
receiver MUST send an EDHOC error message back.
5.3. External Authorization Data 2
In EAD_2, the Relying Party signals to the Attester the supported and
requested evidence types. In case none of the evidence types is
supported, the Relying Party rejects the first message_1 with an
error indicating support for another evidence type.
EAD_2 carries the Selected_EvidenceType object. Similarly to EAD_1,
Selected_EvidenceType object is encoded in CBOR.
The external authorization data EAD_2 contains an EAD item with
* ead_label = TBD2
* ead_value = Attestation_request, which is a CBOR byte string:
Attestation_request = bstr .cbor Selected_EvidenceType
Selected_EvidenceType = (
content-format:uint,
nonce:bstr
)
where
* content-format is the selected evidence type by the Relying Party
and supported by the Verifier.
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* nonce is generated by the Verifier and forwarded by the Relying
Party.
5.4. External Authorization Data 3
As a response to the attestation request, the Attester calls its
local attestation service to generate and return the serialized EAT
[I-D.ietf-rats-eat] as Evidence.
The external authorization data EAD_3 contains an EAD item with
* ead_label = TBD3
* ead_value is a serialized EAT.
6. Security Considerations
TODO: Security considerations
7. IANA Considerations
7.1. EDHOC External Authorization Data Registry
IANA is requested to register the following entry in the "EDHOC
External Authorization Data" registry under the group name "Ephemeral
Diffie-Hellman Over Cose (EDHOC)". The ead_label = TBD1 corresponds
to the ead_value Attestation_proposal in EAD_1 with processing
specified in Section 5.2. The ead_label = TBD2 corresponds to the
ead_value Attestation_request in Section 5.3. The ead_label = TBD3
corresponds to the ead_value which carries the EAT, as specified in
Section 5.4.
+=======+============+=================================+
| Label | Value Type | Description |
+=======+============+=================================+
| TBD1 | bstr | Attestation Proposal |
+-------+------------+---------------------------------+
| TBD2 | bstr | Attestation Request |
+-------+------------+---------------------------------+
| TBD3 | bstr | Evidence for remote attestation |
+-------+------------+---------------------------------+
Table 1: Addition to the EDHOC EAD registry
8. References
8.1. Normative References
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[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-23, 22
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lake-edhoc-23>.
[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Wallace, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-25, 15
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-25>.
[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/rfc/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/rfc/rfc8174>.
8.2. Informative References
[I-D.tschofenig-rats-psa-token]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and
T. Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-22, 21 February 2024,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-22>.
[IANA-CoAP-Content-Formats]
"CoAP Content-Formats", n.d.,
<https://www.iana.org/assignments/core-parameters>.
[IANA.CWT.Claims]
IANA, "CBOR Web Token (CWT) Claims",
<http://www.iana.org/assignments/cwt>.
[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/rfc/rfc8392>.
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[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/rfc/rfc8949>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
Appendix A. Example: Remote Attestation Flow
.--------------------------.
| Attestation | Attester | .---------------. .----------.
| Service | | | Relying Party | | Verifier |
'--+----------------+------' '-------+-------' '-----+----'
| | | |
.--+------------. | | |
| EDHOC session | | | |
+--+------------+---+------------------------+-------------------+---.
| | | | | |
| | |EDHOC message_1 | | |
| | | {...} | | |
| | | EAD_1( | | |
| | | types(a,b,c) | | |
| | | ) | | |
| | +----------------------->| | |
| | | | | |
| | | | | |
| | | +------------------>| |
| | | | | |
| | | | | |
| | | | Body: { | |
| | | | nonce, | |
| | | EDHOC message_2 | types(a,b) | |
| | | {...} | } | |
| | | EAD_2( |<------------------+ |
| | | nonce, | | |
| | | type(a) | | |
| | | ) | | |
| | | Auth_CRED(Sig/MAC) | | |
| | |<-----------------------+ | |
| | Body:{ | | | |
| | nonce, | | | |
| | type(a) | | | |
| | } | | | |
| |<---------------+ | | |
| | Body:{ | | | |
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| | nonce, | | | |
| | Evidence | | | |
| | } | | | |
| +--------------->| | | |
| | | EDHOC message_3 | | |
| | | {...} | | |
| | | EAT(nonce,Evidence) | | |
| | | Auth_CRED(sig/MAC) | | |
| | +----------------------->| | |
| | | | | |
'--+----------------+------------------------+-------------------+----'
| | | |
| | | Body: { |
| | | EAT} |
| | +------------------>|
| | | Body: { |
| | | att-result: AR{} |
| | | } |
| | |<------------------+
| | +---. |
| | | | verify AR{} |
| | |<--' |
| | | |
| | | |
'----------------+------------------------+-------------------'
| application data |
|<---------------------->|
| |
{: #figure-iot-example title="Example of remote attestation."
Appendix B. Example: Firmware Version
The goal in this example is to verify that the firmware running on
the device is the latest version, and is neither tampered or
compromised. A device acts as the Attester, currently in an
untrusted state. The Attester needs to generate the Evidence to
attest itself. A gateway that can communicate with the Attester and
can control its access to the network acts as the Relying Party. The
gateway will finally decide whether the device can join the network
or not depending on the Attestation Result. The Attestation Result
is produced by the Verifier, which is a web server that can be seen
as the manufacturer of the device. Therefore it can appraise the
Evidence that is sent by the Attester. The remote attestation
session starts with the Attester sending EAD_1 in EDHOC message 1, as
specified in Section 5.2. In EAD_1 field, the Attester indicates
that the format of EAT is in CWT and the profile of EAT is Platform
Security Architecture (PSA) attestation token
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[I-D.tschofenig-rats-psa-token]. PSA attestation token contains the
claims relating to the security state of the platform, which are
provided by PSA's Initial Attestation API.
Therefore, the EAD_1 in EDHOC message_1 is:
TBD
According to [I-D.tschofenig-rats-psa-token], IANA is requested to
register the Content-Format ID in the "CoAP Content-Formats" registry
[IANA-CoAP-Content-Formats], for the application/eat+cwt media type
with the eat_profile parameter equal to
tag:psacertified.org,2023:psa#tfm.
The Media Type equivalent is:
media-type: application/eat+cwt; eat_profile="tag:psacertified.org,2023:psa#tfm"
If the Verifier and the Relying Party can support this evidence type
that is proposed by the Attester, the Relying Party will include in
the EAD_2 field the same evidence type, alongside a nonce for message
freshness.
TBD
The Evidence in EAD_3 field is the Platform Security Architecture
(PSA) attestation token, which is the attestation of the platform
state to assure the firmware integrity. This can be generated from
Measured boot, which creates the measurements of loaded code and data
during the boot process and make them part of an overall chain of
trust. Each stage of the chain of trust stores the measurements in a
local root of trust, then the Root of Trust for Report (RTR) of the
device can use them as materials to generate the Evidence. The
components of the Evidence should at least be:
TBD
The Relying Party (co-located with the gateway) then treats the
Evidence as opaque and sends it to the Verifier. Once the Verifier
sends back the Attestation Result, the Relying Party can be assured
on the version of the firmware that the device is running.
Acknowledgments
The author would like to thank Thomas Fossati, Goran Selander, and
Malisa Vucinic for the provided feedback.
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Author's Address
Yuxuan Song
Inria
Email: yuxuan.song@inria.fr
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