Internet DRAFT - draft-bft-rats-kat
draft-bft-rats-kat
Remote ATtestation ProcedureS M. Brossard
Internet-Draft arm
Intended status: Standards Track T. Fossati
Expires: 5 September 2024 Linaro
H. Tschofenig
4 March 2024
An EAT-based Key Attestation Token
draft-bft-rats-kat-03
Abstract
This document defines an evidence format for key attestation based on
the Entity Attestation Token (EAT).
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Remote ATtestation
ProcedureS Working Group mailing list (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/thomas-fossati/draft-kat.
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
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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 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Key Attestation Token Format . . . . . . . . . . . . . . . . 6
4.1. Proof-of-Possession . . . . . . . . . . . . . . . . . . . 6
5. Platform Attestation Token Format . . . . . . . . . . . . . . 7
5.1. KAT-PAT Bundle . . . . . . . . . . . . . . . . . . . . . 7
5.1.1. KAT-PAT linkage . . . . . . . . . . . . . . . . . . . 8
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Amalgamated CDDL . . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
This document defines an evidence format for key attestation based on
EAT [I-D.ietf-rats-eat].
2. Terminology
The following terms are used in this document:
Root of Trust (RoT):
A set of software and/or hardware components that need to be
trusted to act as a security foundation required for accomplishing
the security goals of a system. In our case, the RoT is expected
to offer the functionality for attesting to the state of the
platform, and indirectly also to attest the integrity of the IK
(public as well as private key) and the confidentiality of the IK
private key.
Attestation Key (AK):
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Cryptographic key belonging to the RoT that is only used to sign
attestation tokens.
Platform Attestation Key (PAK):
An AK used specifically for signing attestation tokens relating to
the state of the platform.
Key Attestation:
Evidence containing properties of the environment(s) in which the
private keys are stored. For example, a Relying Party may want to
know whether a private key is stored in a hardware security module
and cannot be exported in unencrypted fashion.
Key Attestation Key (KAK):
An AK used specifically for signing KATs. In some systems only a
single AK is used. In that case the AK is used as a PAK and a
KAK.
Identity Key (IK):
The IK consists of a private and a public key. The private key is
used by the usage protocol. The public key is included in the Key
Attestation Token. The IK is protected by the RoT.
Usage Protocol:
A (security) protocol that requires demonstrating possession of
the private IK.
Attestation Token (AT):
A collection of claims that a RoT assembles (and signs) with the
purpose of informing - in a verifiable way - relying parties about
the identity and state of the platform. Essentially a type of
Evidence as per the RATS architecture terminology [RFC9334].
Platform Attestation Token (PAT):
An AT containing claims relating to the security state of the
platform, including software constituting the platform trusted
computing base (TCB). The process of generating a PAT typically
involves gathering data during measured boot.
Key Attestation Token (KAT):
An AT containing a claim with a public key. The KAT may also
contain other claims, such as those indicating its validity. The
KAT is signed by the KAK. The key attestation service, which is
part of the platform root of trust (RoT), conceptually acts as a
local certification authority since the KAT behaves like a
certificate.
Combined Attestation Bundle (CAB):
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A structure used to bundle a KAT and a PAT together for transport
in the usage protocol. If the KAT already includes a PAT, in form
of a nested token, then it already corresponds to a CAB. A CAB is
equivalent to a certificate that binds the identity of the
platform's TCB with the IK public key.
Presenter:
Party that proves possession of a private key to a recipient of a
KAT.
Recipient:
Party that receives the KAT containing the proof-of-possession key
information from the presenter.
Key Attestation Service (KAS):
The issuer that creates the KAT and bundles a KAT together with a
PAT in a CAB.
The reader is assumed to be familiar with the vocabulary and concepts
defined in [RFC9334].
CDDL [RFC8610] [RFC9165] is used to describe the data formats and the
examples in Section 6 use CBOR diagnostic notation defined in
Section 8 of [STD94] and Appendix G of [RFC8610].
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.
3. Architecture
Key attestation is an extension to the attestation functionality
described in [RFC9334]. We describe this conceptually by splitting
the internals of the attester into two parts, platform attestation
and key attestation. This is shown in Figure 1. These are logical
roles and implementations may combine them into a single physical
entity.
Security-sensitive functionality, like attestation, has to be placed
into the trusted computing base. Since the trusted computing base
itself may support different isolation layers, the design allows
platform attestation to be separated from key attestation whereby
platform attestation requires more privilege than the key attestation
code. Cryptographic services, used by key attestation and by
platform attestation, are separated although not shown in the figure.
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The protocol used for communication between the Presenter and the
Recipient is referred as usage protocol. The usage protocol, which
is outside the scope of this specification, needs to support proof-
of-possession of the private key (explained further below). An
example usage protocol is TLS with the extension defined in
[I-D.fossati-tls-attestation].
.------------------------------------.
| .----------------------------------. |
| | Attester | |
| | .-------------. .-------------. | |
| | | Key | | Platform | | |
| | | Attestation | | Attestation | | |
| | | Service | | Service | | |
| | '-------------' '-------------' | |
| '----------------------------------' |
| ^ |
| | Trusted Computing Base |
'------+-----------------------------'
|
|
v
.-----------------. .-----------------.
| | Usage Protocol | |
| Presenter +---------------->| Recipient |
| | | |
'-----------------' '-----------------'
Figure 1: Architecture
The Presenter triggers the generation of the IK. The IK consists of
a public key (pIK) and a private key (sIK). The Presenter may, for
example, use the following API call to trigger the generation of the
key pair for a given algorithm and to obtain a key handle (key_id).
key_id = GenerateKeyPair(alg_id)
The private key is created and stored such that it is only accessible
to the KAS rather than to the Presenter.
Next, the KAS needs to trigger the creation of the Platform
Attestation Token (PAT) by the Platform Attestation Service. The PAT
needs to be linked to the Key Attestation Token (KAT) and this
linkage can occur in a number of ways. One approach is described in
this specification in Section 5.1. The Key Attestation Token (KAT)
includes the public key of the IK (pIK) and is then signed with the
Key Attestation Key (KAK).
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To ensure freshness of the PAT and the KAT a nonce is used, as
suggested by the RATS architecture [RFC9334]. Here is the symbolic
API call to request a KAT and a PAT, which are concatenated together
as the CAB.
cab = createCAB(key_id, nonce)
Once the CAB has been sent by the Presenter to the Recipient, the
Presenter has to demonstrate possession of the private key. The
signature operation uses the private key of the IK (sIK). How this
proof-of-possession of the private key is accomplished depends on the
details of the usage protocol and is outside the scope of this
specification.
The Recipient of the CAB and the proof-of-possession data (such as a
digital signature) first extracts the PAT and the KAT. The PAT and
the KAT may need to be conveyed to a Verifier. If the PAT is in the
form of attestation results the checks can be performed locally at
the Recipient, whereby the following checks are made:
* The signature protecting the PAT MUST pass verification when using
available trust anchor(s).
* The chaining of PAT and KAT MUST be verified. The detailed
verification procedure depends on the chaining mechanism utilized.
* The claims in the PAT MUST be matched against stored reference
values.
* The signature protecting the KAT MUST pass verification.
* The KAT MUST be checked for replays using the nonce included in
the KAT definition (see Figure 2).
Once all these steps are completed, the verifier produces the
attestation result and includes (if needed) the IK public key (pIK).
4. Key Attestation Token Format
4.1. Proof-of-Possession
The KAT utilizes the proof-of-possession functionality defined in
[RFC8747] to encode the public key of the IK (pIK).
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;# import rfc9052
kat = {
&(eat_nonce: 10) => bstr .size (8..64)
&(cnf: 8) => ak-pub
&(kak-pub: 2500) => COSE_Key
}
ak-pub = cnf-map
cnf-map = {
&(cose-key: 1) => COSE_Key
}
Figure 2: KAT Definition
The claims in the KAT are as follows:
* eat_nonce: challenge from the relying party
* cnf: the key confirmation [RFC8747] of the pIK, encoded as
COSE_Key [STD96]
* kak-pub: the public part of the KAK (used for verification of the
KAT), encoded as COSE_Key
5. Platform Attestation Token Format
There are no strict requirements regarding the composition of the
platform attestation token's claims-set, except for the presence of
the eat_nonce claim used for binding (Section 5.1.1).
An example of PAT could be the PSA Token
[I-D.tschofenig-rats-psa-token].
5.1. KAT-PAT Bundle
The KAT and PAT tokens are combined in a CMW "collection"
[I-D.ietf-rats-msg-wrap] as shown in Figure 3.
cab = {
"kat": [ "application/eat+cwt", bytes .cbor cose_sign1<kat> ]
"pat": [ "application/eat+cwt", bytes .cbor cose_sign1<pat> ]
"__cmwc_t": "tag:ietf.org,2024-02-29:rats/kat"
}
Figure 3: KAT Bundle Definition
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5.1.1. KAT-PAT linkage
KAT and PAT are a form of layered attestation (Section 3.2 of
[RFC9334]). For the scheme to be secure, it is crucial that their
linkage is captured in their combined wire image. The way this is
achieved is by hashing the CBOR-encoded COSE_Key corresponding to the
KAK (i.e., the kak-pub claim in the KAT) and using it to populate the
eat_nonce claim in the PAT. The signature on the PAT seals the image
of the used KAK and therefore the linkage between the two layers.
6. Examples
{
/ nonce / 10: h'B91B03129222973C214E42BF31D6872A3EF2DBDDA401FBD1
F725D48D6BF9C817',
/ kak-pub / 2500: {
/ kty / 1: 2, / EC2 /
/ crv / -1: 1, / P-256 /
/ x / -2: h'F0FFFA7BA35E76E44CA1F5446D327C8382A5A40E5F2974
5DF948346C7C88A5D3',
/ y / -3: h'7CB4C4873CBB6F097562F61D5280768CD2CFE35FBA97E9
97280DBAAAE3AF92FE'
},
/ cnf / 8: {
/ COSE_Key / 1: {
/ kty / 1: 2, / EC2 /
/ crv / -1: 1, / P-256 /
/ x / -2: h'D7CC072DE2205BDC1537A543D53C60A6ACB62ECCD8
90C7FA27C9E354089BBE13',
/ y / -3: h'F95E1D4B851A2CC80FFF87D8E23F22AFB725D535E5
15D020731E79A3B4E47120'
}
}
}
Figure 4: KAT
{
/ eat_nonce / 10: h'FAF2BCA754DFD3F309FCED20791DC1173B1BF61CF5
49145A6EDD4AD4CE2DC4F2'
}
Figure 5: Minimal PAT
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{
"kat": [
"application/eat+cwt",
<< [
/ protected / h'A10126',
/ unprotected / {},
/ payload (KAT Claims-Set) / << {
/ nonce / 10: h'B91B03129222973C214E42BF31D6872A3EF2DBDD
A401FBD1F725D48D6BF9C817',
/ kak-pub / 2500: {
/ kty / 1: 2, / EC2 /
/ crv / -1: 1, / P-256 /
/ x / -2: h'F0FFFA7BA35E76E44CA1F5446D327C8382A5A40E
5F29745DF948346C7C88A5D3',
/ y / -3: h'7CB4C4873CBB6F097562F61D5280768CD2CFE35F
BA97E997280DBAAAE3AF92FE'
},
/ cnf / 8: {
/ COSE_Key / 1: {
/ kty / 1: 2, / EC2 /
/ crv / -1: 1, / P-256 /
/ x / -2: h'D7CC072DE2205BDC1537A543D53C60A6ACB62E
CCD890C7FA27C9E354089BBE13',
/ y / -3: h'F95E1D4B851A2CC80FFF87D8E23F22AFB725D5
35E515D020731E79A3B4E47120'
}
}
} >>,
/ signature / h'56F50D131FA83979AE064E76E70DC75C070B6D991A
EC08ADF9F41CAB7F1B7E2C47F67DACA8BB49E3119B7BAE77AEC6C89162713E0C
C6D0E7327831E67F32841A'
] >>
],
"pat": [
"application/eat+cwt",
<< [
/ protected / h'A10126',
/ unprotected / {},
/ payload (PAT Claims-Set) / << {
/ eat_nonce / 10: h'5CA3750DAF829C30C20797EDDB7949B1FD028C
5408F2DD8650AD732327E3FB64'
/ further platform specific claims /
} >>,
/ signature / h'F9F41CAB7F1B7E2C47F67DACA8BB49E3119B7BAE77AE
C6C89162713E0CC6D0E7327831E67F32841A56F50D131FA83979AE064E76E70D
C75C070B6D991AEC08AD'
] >>
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],
"__cmwc_t": "tag:ietf.org,2024-02-29:rats/kat"
}
Figure 6: CMW Collection combining KAT and PAT
7. Security Considerations
TODO Security
8. IANA Considerations
TODO IANA
9. References
9.1. Normative References
[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>.
[I-D.ietf-rats-msg-wrap]
Birkholz, H., Smith, N., Fossati, T., and H. Tschofenig,
"RATS Conceptual Messages Wrapper (CMW)", Work in
Progress, Internet-Draft, draft-ietf-rats-msg-wrap-04, 27
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-msg-wrap-04>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
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[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/rfc/rfc8747>.
[RFC9165] Bormann, C., "Additional Control Operators for the Concise
Data Definition Language (CDDL)", RFC 9165,
DOI 10.17487/RFC9165, December 2021,
<https://www.rfc-editor.org/rfc/rfc9165>.
[STD94] 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>.
[STD96] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
9.2. Informative References
[I-D.fossati-tls-attestation]
Tschofenig, H., Sheffer, Y., Howard, P., Mihalcea, I., and
Y. Deshpande, "Using Attestation in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", Work in Progress, Internet-Draft, draft-fossati-
tls-attestation-04, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-fossati-tls-
attestation-04>.
[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>.
[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. Amalgamated CDDL
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cab = {
"kat": [ "application/eat+cwt", bytes .cbor cose_sign1<kat> ]
"pat": [ "application/eat+cwt", bytes .cbor cose_sign1<pat> ]
"__cmwc_t": "tag:ietf.org,2024-02-29:rats/kat"
}
;# import rfc9052
kat = {
&(eat_nonce: 10) => bstr .size (8..64)
&(cnf: 8) => ak-pub
&(kak-pub: 2500) => COSE_Key
}
ak-pub = cnf-map
cnf-map = {
&(cose-key: 1) => COSE_Key
}
pat = {
&(eat_nonce: 10) => bstr .size (8..64)
* (int / text) => any
}
cose_sign1<C> = [
Headers
payload: bytes .cbor C
signature: bytes
]
Acknowledgments
TODO acknowledge.
Authors' Addresses
Mathias Brossard
arm
Email: mathias.brossard@arm.com
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org
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Hannes Tschofenig
Email: hannes.tschofenig@gmx.net
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