| Internet-Draft | CCA Reference Attestation Token | July 2024 |
| Frost, et al. | Expires 5 January 2025 | [Page] |
The Arm Confidential Compute Architecture (CCA) is series of hardware and software innovations that enhance Arm’s support for Confidential Computing for large, compute-intensive workloads. Devices that implement CCA can produce attestation tokens as described in this memo, which are the basis for trustworthiness assessment of the Confidential Compute environment. This document specifies the CCA attestation token structure and semantics.¶
The CCA attestation token is a profile of the Entity Attestation Token (EAT). This specification describes what claims are used in an attestation token generated by CCA compliant systems, how these claims get serialized to the wire, and how they are cryptographically protected.¶
This informational document is published as an independent submission to improve interoperability with Arm's architecture. It is not a standard nor a product of the IETF.¶
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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This Internet-Draft will expire on 5 January 2025.¶
Copyright (c) 2024 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/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.¶
The Arm Confidential Compute Architecture (CCA) [CCA-ARCH] is a set of hardware [RME] and firmware [RMM] specifications, backed by a reference implementation [TF-RMM] .¶
CCA provides confidential compute environments, called Realms, that can be dynamically allocated by the Normal world host. The initial state of a Realm, and of the platform on which it executes, can be attested. Attestation allows the Realm owner to establish trust in the Realm, before provisioning any secrets to it. The Realm does not have to inherit the trust from the Non-secure hypervisor which controls it.¶
As outlined in the RATS Architecture [RFC9334], an Attester produces a signed collection of Claims that constitutes Evidence about its target environment. This document focuses on the output provided by requests from the Realm to the Realm Management Monitor (RMM) management component for an attestation token that covers the state of that Realm and the CCA Platform. This output corresponds to Evidence in [RFC9334] and, as a design decision, the CCA attestation token is a profile of the Entity Attestation Token (EAT) [EAT]. Note that there are other profiles of EAT available, such as [I-D.kdyxy-rats-tdx-eat-profile] and [I-D.mandyam-rats-qwestoken], for use with different use cases and by different attestation technologies.¶
Since the CCA tokens are consumed by services outside the device, there is an actual need to ensure interoperability. Interoperability needs are addressed here by describing the exact syntax and semantics of the attestation claims, and defining the way these claims are encoded and cryptographically protected.¶
Further details on concepts expressed below can be found in the Realm Management Monitor specification 1.0 [RMM].¶
As mentioned in the abstract, this memo documents a vendor extension to the RATS architecture, and is not a standard.¶
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 terms Attester, Relying Party, Verifier, Attestation Result, Target Environment, Attesting Environment and Evidence are defined in [RFC9334]. We use the term "receiver" to refer to Relying Parties and Verifiers.¶
We use the terms Evidence, "CCA attestation token", and "CCA token" interchangeably. The terms "sender" and Attester are used interchangeably. Likewise, we use the terms Verifier and "verification service" interchangeably.¶
Root of Trust, the minimal set of software, hardware and data that has to be implicitly trusted in the platform - there is no software or hardware at a deeper level that can verify that the Root of Trust is authentic and unmodified. An example of a RoT suitable for CCA would be an isolated Trusted subsystem responsible for initial measurements, lifecycle state management, identity and attestation services. The services that the RoT provides for securitization of the CCA environment are descibed as Hardware-Enforced Security (HES) - see Section B4.1.5 of [RME].¶
Realm World, provides a security state and physical address range that provides an execution environment for VMs that is isolated from the Normal and Secure worlds. The controlling firmware running in the Realm world can access memory in the Normal world to allow shared buffers. (This is similar to Trusted Execution Environment (TEE), "secure world", or "secure enclave".)¶
the Realm execution environment, is an Arm CCA environment that can be dynamically allocated by the Normal world Host.¶
Normal world host, refers to the security domain outside of the restricted Root, Secure and Realm worlds. This typically contains the host hypervisor and supervisory services. The NW-Host can allocate and manage resource allocation and can manage the scheduling for other worlds.¶
In this document, the structure of data is specified in Concise Data Definition Language (CDDL) [RFC8610].¶
Figure 1 outlines the structure of the CCA Attester according to the conceptual model described in Section 3.1 of [RFC9334].¶
The CCA Attester is a relatively straightforward embodiment of the RATS Attester with exactly one Attesting Environment and one or more Target Environments.¶
The Attesting Environment is responsible for collecting the information to be represented in CCA claims and to assemble them into Evidence. It is made of three cooperating components:¶
The Main Bootloader, executing at boot-time, measures the trusted computing base (TCB) of the Realm World¶
i.e., loaded firmware components and sends them to the HES RoT to be stored isolated. (CCA Platform Boot State). See Figure 2.¶
The Realm Management Monitor (RMM), executing at run-time, maintains measurements for the state of a Realm. It can respond to requests issued from a Realm for an attestation token relevant for that Realm by obtaining a CCA Platform attestation token from the HES RoT and combining that with an attestation token containing Evidence reflecting Realm state.¶
The HES RoT, executing at run-time, maintains measurements for the state of the CCA platform TCB, including the lifecycle state of the CCA platform. It can answer requests coming from the RMM to collect and format claims corresponding to that state and use a CCA Platform Attestation Key (CPAK) to sign them. How the CPAK is derived is implementation-specific.¶
The Target Environment consists of two elements:¶
Realm World TCB - hardware, firmware and configuration contributions.¶
Individual state of a Realm - measurements of the initial state of the Realm and dynamic measurements provided by Realm guest code.¶
A reference implementation of the CCA Attester is provided by [TF-RMM].¶
This section describes the claims to be used in a CCA reference attestation token.¶
There are two logical sections within the CCA attestation token, relating to the two Target Environment elements:¶
The two sections use inter-related claims to bind together into a single logical unit. See Section 9 for more details.¶
The above tokens are presented to the requester within a top level Conceptual Message WWrapper (CMW) collection [CMW].¶
CDDL [RFC8610] along with text descriptions is used to define each claim independent of encoding. The following CDDL type(s) are reused by different claims:¶
arm-platform-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64¶
Two conventions are used to encode the Right-Hand-Side (RHS) of a claim: the postfix -label is used for EAT-defined claims, and the postfix -key for PSA-originated claims.¶
The above tokens are presented to the requester within a top level CMW collection [CMW]. The collection map has two entries, one for a bstr encoding of the CCA Platform token and the other for a bstr encoding of the Realm state token/¶
cca-token = #6.399(cca-token-collection) ; CMW (draft-ietf-rats-msg-wrap) Collection
cca-platform-token = bstr .cbor COSE_Sign1_Tagged
cca-realm-delegated-token = bstr .cbor COSE_Sign1_Tagged
cca-token-collection = {
44234 => cca-platform-token ; 44234 = 0xACCA
44241 => cca-realm-delegated-token
}
; EAT standard definitions
COSE_Sign1_Tagged = #6.18(COSE_Sign1)
; Deliberately shortcut these definitions until EAT is finalised and able to
; pull in the full set of definitions
COSE_Sign1 = "COSE-Sign1 placeholder"
¶
The Nonce claim is used to carry a challenge provided by the caller to demonstrate freshness of the generated token.¶
The EAT [EAT] nonce (claim key 10) is used. Since the EAT nonce claim offers flexiblity for different
attestation technologies, this specifications applies the following constraints
to the nonce-type:¶
The length MUST be either 32, 48, or 64 bytes.¶
Only a single nonce value is conveyed. The array notation MUST NOT be used for encoding the nonce value.¶
Where the CCA Platform implementation uses the Delegated Token signing model Section 4.10, the value of the Nonce claim will be a hash of the Realm Public Key claim of the CCA Realm State token Section 4.8.7.¶
This claim MUST be present in a CCA Platform attestation token.¶
arm-platform-challenge-label = 10
arm-platform-challenge = (
arm-platform-challenge-label => arm-platform-hash-type
)
¶
The Instance ID claim represents the unique identifier of the Platform
Attestation Key (PAK).
The EAT ueid (claim key 256) of type RAND is used. The following constraints
apply to the ueid-type:¶
The length MUST be 33 bytes.¶
The first byte MUST be 0x01 (RAND) followed by the 32-byte unique identifier of the PAK.¶
This claim MUST be present in a CCA Platform attestation token.¶
arm-platform-instance-id-label = 256 ; EAT ueid
; TODO: require that the first byte of arm-platform-instance-id-type is 0x01
; EAT UEIDs need to be 7 - 33 bytes
arm-platform-instance-id-type = bytes .size 33
arm-platform-instance-id = (
arm-platform-instance-id-label => arm-platform-instance-id-type
)
¶
The Implementation ID claim uniquely identifies the implementation of the CCA Platform. A verification service uses this claim to locate the details of the CCA Platform implementation from an Endorser or manufacturer. Such details are used by a verification service to determine the security properties or certification status of the CCA Platform implementation.¶
The value and format of the ID is decided by the manufacturer or a particular certification scheme. For example, the ID could take the form of a product serial number, database ID, or other appropriate identifier.¶
This claim MUST be present in a CCA Platform attestation token.¶
Note that this identifies the CCA Platform implementation, not a particular instance. To uniquely identify an instance, see the Instance ID claim Section 4.4.1.¶
arm-platform-implementation-id-label = 2396 ; PSA implementation ID
arm-platform-implementation-id-type = bytes .size 32
arm-platform-implementation-id = (
arm-platform-implementation-id-label => arm-platform-implementation-id-type
)
¶
The CCA platform profile claim identifies the EAT profile to which the CCA platform token conforms. This allows a receiver to assign the intended semantics to the rest of the claims found in the token.¶
The EAT eat_profile (claim key 265) is used.¶
The format of the CCA platform profile claim is defined as a text string of value "tag:arm.com,2023:cca#1.0.0".¶
This claim MUST be present in a CCA Platform attestation token.¶
See Section 4.9, for considerations about backwards compatibility with previous versions of the CCA Platform attestation token format.¶
cca-platform-profile-label = 265 ; EAT profile
cca-platform-profile-type = "tag:arm.com,2023:cca_platform#1.0.0"
cca-platform-profile = (
cca-platform-profile-label => cca-platform-profile-type
)
¶
The Security Lifecycle claim represents the current lifecycle state of the CCA Platform.¶
The state is represented by an integer that is divided as follows:¶
major[15:8] - CCA Platform security lifecycle state, and¶
minor[7:0] - IMPLEMENTATION DEFINED state.¶
The CCA Platform lifecycle states are illustrated in Figure 4. A non debugged CCA platform will be in psa-lifecycle-secured state. Realm Management Security Domain debug is always recoverable, and would therefore be represented by psa-lifecycle-non-psa-rot-debug state. Root world debug is recoverable on a HES system and would be represented by psa-lifecycle-recoverable-psa-rot state. On a non-HES system Root world debug is usually non-recoverable, and would be represented by psa-lifecycle-lifecycle-decommissioned state¶
This claim MUST be present in a CCA Platform attestation token.¶
The CDDL representation is shown below. Table 1 provides the mappings between Figure 4 and the data model.¶
arm-platform-lifecycle-label = 2395 ; PSA lifecycle
arm-platform-lifecycle-unknown-type = 0x0000..0x00ff
arm-platform-lifecycle-assembly-and-test-type = 0x1000..0x10ff
arm-platform-lifecycle-arm-platform-rot-provisioning-type = 0x2000..0x20ff
arm-platform-lifecycle-secured-type = 0x3000..0x30ff
arm-platform-lifecycle-non-arm-platform-rot-debug-type = 0x4000..0x40ff
arm-platform-lifecycle-recoverable-arm-platform-rot-debug-type = 0x5000..0x50ff
arm-platform-lifecycle-decommissioned-type = 0x6000..0x60ff
arm-platform-lifecycle-type =
arm-platform-lifecycle-unknown-type /
arm-platform-lifecycle-assembly-and-test-type /
arm-platform-lifecycle-arm-platform-rot-provisioning-type /
arm-platform-lifecycle-secured-type /
arm-platform-lifecycle-non-arm-platform-rot-debug-type /
arm-platform-lifecycle-recoverable-arm-platform-rot-debug-type /
arm-platform-lifecycle-decommissioned-type
arm-platform-lifecycle = (
arm-platform-lifecycle-label => arm-platform-lifecycle-type
)
¶
psa-lifecycle-unknown-type is not shown in Figure 4; it represents an invalid state that must not occur in a system.¶
| CDDL | Lifecycle States |
|---|---|
psa-lifecycle-unknown-type
|
|
psa-lifecycle-assembly-and-test-type
|
Assembly and Test |
psa-lifecycle-psa-rot-provisioning-type
|
CCA Platform Provisioning |
psa-lifecycle-secured-type
|
Secured |
psa-lifecycle-non-psa-rot-debug-type
|
Non-Recoverable CCA Platform Debug |
psa-lifecycle-recoverable-psa-rot-debug-type
|
Recoverable CCA Platform Debug |
psa-lifecycle-decommissioned-type
|
Decommissioned |
The CCA platform config claim describes the set of chosen implementation options of the CCA platform. As an example, these may include a description of the level of physical memory protection which is provided.¶
The CCA platform config claim is expected to contain the System Properties field which is present in the Root Non-volatile Storage (RNVS) public parameters.¶
This claim MUST be present in a CCA Platform attestation token.¶
arm-platform-config-label = 2401 ; PSA platform range
; TBD: add to IANA registration
arm-platform-config-type = bytes
arm-platform-config = (
arm-platform-config-label => arm-platform-config-type
)
¶
The Software Components claim is a list of software components which can affect the behavior of the CCA platform.¶
This claim MUST be present in a CCA Platform attestation token.¶
Each entry in the Software Components list describes one software component using the attributes described in the following subsections. Unless explicitly stated, the presence of an attribute is OPTIONAL.¶
Note that, as described in [RFC9334], a relying party will typically see the result of the appraisal process from the Verifier in form of an Attestation Result, rather than the CCA Platform token from the attesting endpoint. Therefore, a relying party is not expected to understand the Software Components claim. Instead, it is for the Verifier to check this claim against the available Reference Values and provide an answer in form of an "high level" Attestation Result, which may or may not include the original Software Components claim.¶
arm-platform-sw-components-label = 2399 ; PSA software components
arm-platform-sw-component = {
? 1 => text, ; component type
2 => arm-platform-hash-type, ; measurement value
? 4 => text, ; version
5 => arm-platform-hash-type, ; signer id
? 6 => text, ; hash algorithm identifier
}
arm-platform-sw-components = (
arm-platform-sw-components-label => [ + arm-platform-sw-component ]
)
¶
The Component Type attribute (key=1) is a short string representing the role of this software component. This attribute is intended for use as a hint to help the verifier understand how to evaluate the CCA platform software component measurement value.¶
This attribute is optional in a CCA Platform software component.¶
The Measurement Value attribute (key=2) represents a hash of the invariant software component in memory at the time it was initialized. The value MUST be a cryptographic hash of 256 bits or stronger.¶
This attribute MUST be present in a PSA software component.¶
The Version attribute (key=4) is the issued software version in the form of a text string. The meaning of this string is defined by the software component vendor.¶
This attribute is optional in a CCA Platform software component.¶
The Signer ID attribute (key=5) uniquely identifies the signer of the software component. The identification is typically accomplished by hashing the signer's public key. The value of this attribute will correspond to the entry in the original manifest for the component. This can be used by a Verifier to ensure the components were signed by an expected trusted source.¶
This attribute MUST be present in a CCA Platform software component.¶
The Measurement Description attribute (key=6) contains a string identifying the hash algorithm used to compute the corresponding Measurement Value. The string SHOULD be encoded according to "Hash Name String" in the "Named Information Hash Algorithm Registry" [IANA.named-information].¶
The Measurement Description attribute (key=6) contains a string identifying the hash algorithm used to compute the corresponding Measurement Value. The string SHOULD be encoded according to "Hash Name String" in the "Named Information Hash Algorithm Registry" [IANA.named-information].¶
The following claims are part of the CCA Platform token (and therefore still Evidence) but aim to help receivers, including relying parties, with the processing of the received attestation Evidence.¶
The Verification Service Indicator claim is a hint used by a relying party to locate a verification service for the token. The value is a text string that can be used to locate the service (typically, a URL specifying the address of the verification service API). A Relying Party may choose to ignore this claim in favor of other information.¶
arm-platform-verification-service-label = 2400 ; PSA verification service
arm-platform-verification-service-type = text
arm-platform-verification-service = (
arm-platform-verification-service-label =>
arm-platform-verification-service-type
)
¶
It is assumed that the relying party is pre-configured with a list of trusted verification services and that the contents of this hint can be used to look up the correct one. Under no circumstances must the relying party be tricked into contacting an unknown and untrusted verification service since the returned Attestation Result cannot be relied on.¶
Note: This hint requires the relying party to parse the content of the CCA Platform token. Since the relying party may not be in possession of a trust anchor to verify the digital signature, it uses the hint in the same way as it would treat any other information provided by an external party, which includes attacker-provided data.¶
The CCA platform hash algorithm ID claim is a text string that identifies the algorithm used to calculate the extended measurements in the CCA platform token.¶
The string SHOULD be encoded according to "Hash Name String" in the "Named Information Hash Algorithm Registry" [IANA.named-information].¶
The CCA platform hash algorithm ID claim MUST be present in a CCA platform token.¶
arm-platform-hash-algo-id-label = 2402 ; PSA platform range
; TBD: add to IANA registration
arm-platform-hash-algo-id = (
arm-platform-hash-algo-id-label => text
)
¶
The CCA Realm state token contains claims that represent the Target Environment that is the Realm that requested the attestation report.¶
The Nonce claim is used to carry a challenge provided by the caller to demonstrate freshness of the generated token.¶
The EAT [EAT] nonce (claim key 10) is used. Since the EAT nonce claim offers flexiblity for different
attestation technologies, this specification applies the following constraints
to the nonce-type:¶
The length MUST be either 32, 48, or 64 bytes.¶
Only a single nonce value is conveyed. The array notation MUST NOT be used for encoding the nonce value.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-challenge-label = 10
cca-realm-challenge-type = bytes .size 64
cca-realm-challenge = (
cca-realm-challenge-label => cca-realm-challenge-type
)
¶
The Realm profile claim identifies the EAT profile to which the Realm token conforms. This allows a receiver to assign the intended semantics to the rest of the claims found in the token.¶
The EAT eat_profile (claim key 265) is used.¶
The format of the CCA platform profile claim is defined as a text string of value "tag:arm.com,2023:realm#1.0.0".¶
This claim is OPTIONAL in a CCA Realm attestation token. If the Realm profile is not included in a CCA Realm token then the profile value used in the CCA Platform token should refer to a profile that describes both Platform and Realm claims.¶
cca-realm-profile-label = 265 ; EAT profile
cca-realm-profile-type = "tag:arm.com,2023:realm#1.0.0"
cca-realm-profile = (
cca-realm-profile-label => cca-realm-profile-type
)
¶
The Realm Personalization Value (RPV) claim contains the RPV which was provided at Realm creation.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-personalization-value-label = 44235
cca-realm-personalization-value-type = bytes .size 64
cca-realm-personalization-value = (
cca-realm-personalization-value-label => cca-realm-personalization-value-type
)
¶
The Realm Initial Measurement claim contains the measurements taken of Realm state before the Realm is activated.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-initial-measurement-label = 44238
cca-realm-initial-measurement = (
cca-realm-initial-measurement-label => cca-realm-measurement-type
)
¶
The Realm Extensible Measurements claim contains measurements provided by Realm guest software and extended to the set of Realm Extensible Measurements maintained by the RMM.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-extensible-measurements-label = 44239
cca-realm-extensible-measurements = (
cca-realm-extensible-measurements-label => [ 4*4 cca-realm-measurement-type ]
)
¶
The Realm hash algorithm ID claim identifies the algorithm used to calculate all hash values which are present in the Realm token.¶
The string value of the claim SHOULD be encoded according to "Hash Name String" in the "Named Information Hash Algorithm Registry" [IANA.named-information].¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-hash-algo-id-label = 44236
cca-realm-hash-algo-id = (
cca-realm-hash-algo-id-label => text
)
¶
The Realm public key claim identifies the attestation key which is used to sign the Realm token¶
The value of the Realm public key claim is a byte string representation of a COSE_Key.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-public-key-label = 44237
; See RFC8152 for definition of COSE_Key
cca-realm-public-key-type = bstr .cbor COSE_Key
cca-realm-public-key = (
cca-realm-public-key-label => cca-realm-public-key-type
)
¶
The Realm public key hash algorithm identifier claim identifies the algorithm used to hash the value of the Realm Public Key claim Section 4.8.7 such that it can be presented as a Challenge for the bound CCA Platform token Section 4.10.¶
This claim MUST be present in a CCA Realm state attestation token.¶
cca-realm-public-key-hash-algo-id-label = 44240
cca-realm-public-key-hash-algo-id = (
cca-realm-public-key-hash-algo-id-label => text
)
¶
This profile conforms to the claims in the Beta2 release of the 1.0 release of the Realm Management Monitor specification. [RMM]. There has not been a prior release of this specification to the 1.0 release. Hence this section is a place holder for claim changes introduced in future releases.¶
The reference implementation uses a 'Delegated Model' for token signing. In this model, the completion of signing operations for the CCA token is delegated from the CCA Platform RoT to the RMM. When the RMM initialises, it obtains a 'Realm Attestation Token' (RAK) signing key pair from the CCA Platform RoT. The public part of that key pair is hashed and used as a challenge to obtain a CCA Platform token (signed by the CCA Platform RoT). When guest code in a Realm requests a CCA Attestation token, the RMM prepares a Realm state token, signed by the RAK private key, then wraps both tokens in a CMW Collection. The two tokens are bound together by the Nonce claim in the CCA Platform token having the same value as a hash of the Realm Public key claim in the Realm state token (using the hash algorithm identified by the Realm Public Key Hash Algorithm ID claim).¶
A verifier MUST check this binding is valid when verifying a CCA Attestation token.¶
An implementation may choose instead a 'Direct Model'. In this model, when guest code in a Realm requests a CCA Attestation token, the RMM prepares a Realm state claim set, but does not wrap it in a CMW. Instead, the claim set is hashed and this value is used as a Challenge to obtain a CCA Platform token, signed by the CCA Platform RoT. The CCA Platform and Realm state claim set are presented within a CMW Collection as in the Delegated model. The two parts of the collection are bound together by the Nonce claim in the CCA Platform token having the same value as the hash of the Realm state claim set. If the Direct Model is used, the CCA Platform profile claim Section 4.5.1 MUST have a different value from the reference profile. The map value within the CCA Attestation token CMW Collection for the Realm state claim set MUST also have a different value to that used for a Realm state CMW token. In such a profile, the Realm Public Key Section 4.8.7 and Realm Public Key Hash Algorithm ID Section 4.8.8 claims will not be used.¶
The CCA attestation token is encoded in CBOR [STD94] format. The CBOR representation of a CCA attestation token MUST be "valid" according to the definition in Section 1.2 of [STD94]. Besides, only definite-length string, arrays, and maps are allowed.¶
Given that a PSA Attester is typically found in a constrained device, it MAY NOT emit CBOR preferred serializations (Section 4.1 of [STD94]). Therefore, the Verifier MUST be a variation-tolerant CBOR decoder. TODO.... need different narrative from IoT reasons...¶
Cryptographic protection is obtained by wrapping the CCA Platform and Realm state claims-set in a COSE Web Token (CWT) [RFC8392]. The signature structure MUST be a tagged (18) COSE_Sign1.¶
Acknowledging the variety of markets, regulations and use cases in which the CCA attestation token can be used, the baseline profile does not impose any strong requirement on the cryptographic algorithms that need to be supported by Attesters and Verifiers. The flexibility provided by the COSE format should be sufficient to deal with the level of cryptographic agility needed to adapt to specific use cases. It is RECOMMENDED that commonly adopted algorithms are used, such as those discussed in [COSE-ALGS]. It is expected that receivers will accept a wider range of algorithms, while Attesters would produce CCA tokens using only one such algorithm.¶
The CCA Platform token is always directly signed by the CCA Platform RoT. Therefore, the CCA claims-set is never carried in a Detached EAT bundle (Section 5 of [EAT]).¶
The CCA token supports the freshness models for attestation Evidence based on
nonces and epoch handles (Section 10.2 and Section 10.3 of [RFC9334]) using
the nonce claim to convey the nonce or epoch handle supplied by the Verifier.
No further assumption on the specific remote attestation protocol is made.¶
Note that use of epoch handles is constrained by the type restrictions imposed by the eat_nonce syntax.
For use in CCA tokens, it must be possible to encode the epoch handle as an opaque binary string between 8 and 64 octets.¶
Table 2 presents a concise view of the requirements described in the preceding sections.¶
| Issue | Profile Definition |
|---|---|
| CBOR/JSON | CBOR MUST be used |
| CBOR Encoding | Definite length maps and arrays MUST be used |
| CBOR Encoding | Definite length strings MUST be used |
| CBOR Serialization | Variant serialization MAY be used |
| COSE Protection | COSE_Sign1 MUST be used |
| Algorithms | [COSE-ALGS] SHOULD be used |
| Detached EAT Bundle Usage | Detached EAT bundles MUST NOT be sent |
| Verification Key Identification | Any identification method listed in Appendix F.1 of [EAT] |
| Endorsements | See Section 7.2 |
| Freshness | nonce or epoch ID based |
| Claims | Those defined in Section 4. As per general EAT rules, the receiver MUST NOT error out on claims it does not understand. |
TODO...include cddl/cca-attestation.cddl¶
In the CCA Platform reference design, PAKs (Section 3, Paragraph 8) are raw public keys.¶
Some implementations may choose to use an PAK that is a certified public key. If this option is taken, the value of the CCA Platform Profile Definition claim Section 4.5.1 MUST be altered from the reference implementation value.¶
TODO... perhaps lose this justification section as...¶
Certified public keys require the manufacturer to run the certification authority (CA) that issues X.509 certs for the PAKs. (Note that operating a CA is a complex and expensive task that may be unaffordable to certain manufacturers.)¶
Using certified public keys offers better scalability properties when compared to using raw public keys, namely:¶
storage requirements for the Verifier are minimised - the same manufacturer's trust anchor is used for any number of devices,¶
the provisioning model is simpler and more robust since there is no need to notify the Verifier about each newly manufactured device,¶
Furthermore, existing and well-understood revocation mechanisms can be readily used.¶
TODO... ...to here¶
The PAK's X.509 cert can be inlined in the CCA Platform token using the x5chain COSE
header parameter [COSE-X509] at the cost of an increase in the CCA Platform token
size.
Note that the exact split between pre-provisioned and inlined certs may vary
depending on the specific deployment. In that respect, x5chain is quite
flexible: it can contain the end-entity (EE) cert only, the EE and a partial
chain, or the EE and the full chain up to the trust anchor (see Section 2 of [COSE-X509] for the details).¶
TODO...lose following as IoT centric?? :: Constraints around network bandwidth and computing resources available to endpoints, such as network buffers, may dictate a reasonable split point.¶
To verify the token for the reference profile, the initial need is to check correct encoding for the token. Primary trust is established by checking the signing of the CCA Platform token CWT. The key used for verification is supplied to the Verifier by an authorized Endorser along with the corresponding Attester's Instance ID. For the verifier, the CCA Platform Instance ID Section 4.4.1 claim is used to assist locating the key used to verify the signature covering the CCA Platform CWT token. The verifier can also be supplied with the information that the key instance has been revoked and is no longer valid.¶
Additional validation checks on the token are:¶
Checking that the binding between the CCA Platform token and the Realm state token is valid Section 4.10}. This has the side effect of establishing the trustworthiness of the RAK public key.¶
Validating that the Realm state token is correctly signed by the RAK.¶
Checking that the value of the lll claim is psa-lifecycle-secured state. Note that some other values of this claim (psa-lifecycle-non-psa-rot-debug and psa-lifecycle-recoverable-psa-rot states) may indicate that the attester is only temporarily unsuitable and the verifier may choose the to indicate this as a contraindication rather than a full verification failure. See discussion of the CCA platform lifecycle in [RMM].¶
The Verifier will typically operate a policy where values of some of the claims in this profile can be compared to reference values, registered with the Verifier for a given deployment, in order to confirm that the device is endorsed by the manufacturer supply chain. The policy may require that the relevant claims must have a match to a registered reference value. All claims may be worthy of additional appraisal. It is likely that most deployments would include a policy with appraisal for the following claims:¶
Implementation ID - the value of the Implementation ID can be used to identify the verification requirements of the deployment.¶
Software Component, Measurement Value - this value can uniquely identify a firmware release from the supply chain. In some cases, a Verifier may maintain a record for a series of firmware releases, being patches to an original baseline release. A verification policy may then allow this value to match any point on that release sequence or expect some minimum level of maturity related to the sequence.¶
Software Component, Signer ID - where present in a deployment, this could allow a Verifier to operate a more general policy than that for Measurement Value as above, by allowing a token to contain any firmware entries signed by a known Signer ID, without checking for a uniquely registered version.¶
[RATS-AR4SI] defines an information model that Verifiers can employ to produce Attestation Results. AR4SI provides a set of standardized appraisal categories and tiers that greatly simplifies the task of writing Relying Party policies in multi-attester environments.¶
The contents of Table 3 are intended as guidance for implementing a PSA Verifier that computes its results using AR4SI. The table describes which PSA Evidence claims (if any) are related to which AR4SI trustworthiness claim, and therefore what the Verifier must consider when deciding if and how to appraise a certain feature associated with the PSA Attester.¶
| Trustworthiness Vector claims | Related PSA claims |
|---|---|
configuration
|
Software Components (Section 4.6.1) |
executables
|
ditto |
file-system
|
N/A |
hardware
|
Implementation ID (Section 4.4.2) and CCA Platform config (TODO) |
instance-identity
|
Instance ID (Section 4.4.1). The Security Lifecycle (Section 4.5.2) can also impact the derived identity. |
runtime-opaque
|
Indirectly derived from executables, hardware, and instance-identity. The Security Lifecycle (Section 4.5.2) can also be relevant: for example, any debug state will expose otherwise protected memory. |
sourced-data
|
N/A |
storage-opaque
|
Indirectly derived from executables, hardware, and instance-identity. |
This document does not prescribe what value must be chosen based on each possible situation: when assigning specific Trustworthiness Claim values, an implementation is expected to follow the algorithm described in Section 2.3.3 of [RATS-AR4SI].¶
The TODO ref-to-CCA-Endorsements defines a protocol based on the [RATS-CoRIM] data model that can be used to convey CCA Endorsements, Reference Values and verification key material to the Verifier. TODO... perhaps redact this section until a cca-endorsements draft is available?¶
RFC Editor: please remove this section before pubblication.¶
Implementations of this specification are provided by the Trusted Firmware-RMM project [TF-RMM] and the Veraison project [Veraison]. These implementations are released as open-source software.¶
This specification re-uses the EAT specification and therefore the CWT specification. Hence, the security and privacy considerations of those specifications apply here as well.¶
TODO... questionable ability to execute on this as anyone can call RSI?? A PSA Attester MUST NOT provide Evidence to an untrusted challenger, as it may allow attackers to interpose and trick the Verifier into believing the attacker is a legitimate Attester. This is especially relevant to protocols that use PSA attestation tokens to authenticate the attester to a relying party.¶
Attestation tokens contain information that may be unique to a device and therefore they may allow singling out an individual device for tracking purposes. Deployments that have privacy requirements must take appropriate measures to ensure that the token is only used to provision anonymous/pseudonym keys.¶
IANA is requested to make permanent the following claims that have been assigned via early allocation in the "CBOR Web Token (CWT) Claims" registry [IANA-CWT].¶
Claim Name: arm-platform-security-lifecycle¶
Claim Description: Arm Platform Security Lifecycle¶
JWT Claim Name: N/A¶
Claim Key: 2395¶
Claim Value Type(s): unsigned integer¶
Change Controller: Hannes Tschofenig TODO... find document centric change controller...¶
Specification Document(s): Section 4.5.2 of RFCthis¶
No new media type registration is requested.
To indicate that the transmitted content is a CCA attestation token,
applications can use the application/eat+cwt media type defined in
[EAT-MEDIATYPES] with the eat_profile parameter set to
tag:arm.com,2023:cca#1.0.0.¶
IANA is requested to register a CoAP Content-Format ID in the "CoAP Content-Formats" registry [IANA-CoAP-Content-Formats]:¶
A registration for the application/eat+cwt media type with the eat_profile parameter
equal to "tag:arm.com,2023:cca#1.0.0"¶
The Content-Formats should be allocated from the Expert review range (0-255).¶
The following examples show CCA attestation tokens for an hypothetical system comprising a single measured software component. The attesting device is in a lifecycle state (Section 4.5.2) of SECURED.¶
TODO...include cddl/example/sign1-claims.diag¶
The JWK representation of the PAK used for creating the COSE Sign1 signature over the PSA token is:¶
TODO...include cddl/example/tfm-es-iak.json¶
The resulting COSE object is:¶
TODO...include cddl/example/psa-sign1.diag¶
which has the following base16 encoding:¶
TODO...include cddl/example/psa-sign1.hex¶
TODO¶