| Internet-Draft | PSA Attestation Token | February 2021 |
| Tschofenig, et al. | Expires 5 August 2021 | [Page] |
The Platform Security Architecture (PSA) is a family of hardware and firmware security specifications, as well as open-source reference implementations, to help device makers and chip manufacturers build best-practice security into products. Devices that are PSA compliant are able to produce attestation tokens as described in this memo, which are the basis for a number of different protocols, including secure provisioning and network access control. This document specifies the PSA attestation token structure and semantics.¶
At its core, the CWT (COSE Web Token) format is used and populated with a set of claims in a way similar to EAT (Entity Attestation Token). This specification describes what claims are used by PSA compliant systems.¶
Source for this draft and an issue tracker can be found at https://github.com/thomas-fossati/draft-psa-token.¶
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/.¶
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 August 2021.¶
Copyright (c) 2021 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
Trusted execution environments are now present in many devices, which provide a safe environment to place security sensitive code such as cryptography, secure boot, secure storage, and other essential security functions. These security functions are typically exposed through a narrow and well-defined interface, and can be used by operating system libraries and applications. Various APIs have been developed by Arm as part of the Platform Security Architecture [PSA] framework. This document focuses on the output provided by PSA's Initial Attestation API. Since the tokens are also 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 PSA Security Model documentation [PSA-SM].¶
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.¶
This section describes the claims to be used in a PSA attestation token.¶
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:¶
psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64¶
The Nonce claim is a challenge from the caller. The length must be 32, 48, or 64 bytes.¶
This claim MUST be present in a PSA attestation token.¶
psa-nonce = (
psa-nonce-key => psa-hash-type
)
¶
The Client ID claim represents the security domain of the caller.¶
In PSA, a security domain is represented by a signed integer whereby negative values represent callers from the NSPE and where positive IDs represent callers from the SPE. The value 0 is not permitted.¶
For an example definition of client IDs, see the PSA Firmware Framework [PSA-FF].¶
It is essential that this claim is checked in the verification process to ensure that a security domain, i.e., an attestation endpoint, cannot spoof a report from another security domain.¶
This claim MUST be present in a PSA attestation token.¶
Note that the CDDL label used to be called arm_psa_partition_id.¶
psa-client-id-nspe-type = -2147483648...0
psa-client-id-spe-type = 1..2147483647
psa-client-id-type = psa-client-id-nspe-type / psa-client-id-spe-type
psa-client-id = (
psa-client-id-key => psa-client-id-type
)
¶
The Instance ID claim represents the unique identifier of the device instance. It is a 32 bytes hash of the public key corresponding to the Initial Attestation Key (IAK). If the IAK is a symmetric key then the Instance ID is a hash of the hash of the IAK itself. It is encoded as a Universal Entity ID of type RAND [I-D.ietf-rats-eat], i.e., prepending a 0x01 type byte to the key hash. The full definition is in [PSA-SM].¶
This claim MUST be present in a PSA attestation token.¶
psa-instance-id-type = bytes .size 33
psa-instance-id = (
psa-instance-id-key => psa-instance-id-type
)
¶
The Implementation ID claim uniquely identifies the underlying immutable PSA RoT. A verification service can use this claim to locate the details of the verification process. Such details include the implementation's origin and associated certification state. The full definition is in [PSA-SM].¶
This claim MUST be present in a PSA attestation token.¶
psa-implementation-id-type = bytes .size 32
psa-implementation-id = (
psa-implementation-id-key => psa-implementation-id-type
)
¶
The Certification Reference claim is used to link the class of chip and PSA RoT of the attesting device to an associated entry in the PSA Certification database. It MUST be represented as a thirteen-digit [EAN-13].¶
Linking to the PSA Certification entry can still be achieved if this claim is not present in the token by making an association at a Verifier between the reference value and other token claim values - for example, the Implementation ID.¶
psa-certification-reference-type = text .regexp "[0-9]{13}"
psa-certification-reference = (
? psa-certification-reference-key =>
psa-certification-reference-type
)
¶
The Security Lifecycle claim represents the current lifecycle state of the PSA RoT. The state is represented by an integer that is divided to convey a major state and a minor state. A major state is mandatory and defined by [PSA-SM]. A minor state is optional and 'IMPLEMENTATION DEFINED'. The PSA security lifecycle state and implementation state are encoded as follows:¶
The PSA lifecycle states are illustrated in Figure 1. For PSA, a remote verifier can only trust reports from the PSA RoT when it is in SECURED or NON_PSA_ROT_DEBUG major states.¶
This claim MUST be present in a PSA attestation token.¶
psa-lifecycle-unknown-type = 0x0000..0x00ff
psa-lifecycle-assembly-and-test-type = 0x1000..0x10ff
psa-lifecycle-psa-rot-provisioning-type = 0x2000..0x20ff
psa-lifecycle-secured-type = 0x3000..0x30ff
psa-lifecycle-non-psa-rot-debug-type = 0x4000..0x40ff
psa-lifecycle-recoverable-psa-rot-debug-type = 0x5000..0x50ff
psa-lifecycle-decommissioned-type = 0x6000..0x60ff
psa-lifecycle-type =
psa-lifecycle-unknown-type /
psa-lifecycle-assembly-and-test-type /
psa-lifecycle-psa-rot-provisioning-type /
psa-lifecycle-secured-type /
psa-lifecycle-non-psa-rot-debug-type /
psa-lifecycle-recoverable-psa-rot-debug-type /
psa-lifecycle-decommissioned-type
psa-lifecycle = (
psa-lifecycle-key => psa-lifecycle-type
)
¶
The Boot Seed claim represents a random value created at system boot time that will allow differentiation of reports from different boot sessions.¶
This claim MUST be present in a PSA attestation token.¶
psa-boot-seed-type = bytes .size 32
psa-boot-seed = (
psa-boot-seed-key => psa-boot-seed-type
)
¶
The Software Components claim is a list of software components that includes all the software loaded by the PSA RoT. This claim SHALL be included in attestation tokens produced by an implementation conformant with [PSA-SM]. If the Software Components claim is present, then the No Software Measurement claim (Section 3.4.2) MUST NOT be present.¶
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 [I-D.ietf-rats-architecture], a relying party will typically see the result of the verification process from the Verifier in form of an attestation result, rather than the "naked" PSA 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 endorsements and provide an answer in form of an "high level" attestation result, which may or may not include the original Software Components claim.¶
psa-software-component = {
? 1 => text, ; measurement type
2 => psa-hash-type, ; measurement value
? 4 => text, ; version
5 => psa-hash-type, ; signer id
? 6 => text, ; measurement description
}
psa-software-components = (
psa-software-components-key => [ + psa-software-component ]
)
¶
The Measurement Type attribute (key=1) is short string representing the role of this software component.¶
The following measurement types MAY be used:¶
The Measurement Value attribute (key=2) represents a hash of the invariant software component in memory at startup time. 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 value of this attribute will correspond to the entry in the original signed manifest of the component.¶
The Signer ID attribute (key=5) is the hash of a signing authority public key for the software component. 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 PSA software component to be compliant with [PSA-SM].¶
The Measurement Description attribute (key=6) is the description of the way in which the measurement value of the software component is computed. The value will be a text string containing an abbreviated description (or name) of the measurement method which can be used to lookup the details of the method in a profile document. This attribute will normally be excluded, unless there was an exception to the default measurement described in the profile for a specific component.¶
In the event that the implementation does not contain any software measurements then the Software Components claim Section 3.4.1 can be omitted but instead the token MUST include this claim to indicate this is a deliberate state. The value SHOULD be 1. This claim is intended for devices that are not compliant with [PSA-SM].¶
psa-no-sw-measurements-type = 1
psa-no-sw-measurement = (
psa-no-sw-measurement-key => psa-no-sw-measurements-type
)
¶
The Verification Service Indicator claim is a hint used by a relying party to locate a validation service for the token. The value is a text string that can be used to locate the service or a URL specifying the address of the service. A verifier may choose to ignore this claim in favor of other information.¶
psa-verification-service-indicator-type = text
psa-verification-service-indicator = (
? psa-verification-service-indicator-key =>
psa-verification-service-indicator-type
)
¶
The Profile Definition claim contains the name of a document that describes the "profile" of the report. The document name may include versioning. The value for this specification MUST be PSA_IOT_PROFILE_1.¶
psa-profile-type = "PSA_IOT_PROFILE_1"
psa-profile = (
? psa-profile-key => psa-profile-type
)
¶
The report is encoded as a COSE Web Token (CWT) [RFC8392], similar to the Entity Attestation Token (EAT) [I-D.ietf-rats-eat]. The token consists of a series of claims declaring evidence as to the nature of the instance of hardware and software. The claims are encoded in CBOR [RFC7049] format. For asymmetric key algorithms, the signature structure MUST be COSE_Sign1. For symmetric key algorithms, the structure MUST be COSE_Mac0.¶
psa-token = {
psa-nonce,
psa-instance-id,
psa-verification-service-indicator,
psa-profile,
psa-implementation-id,
psa-client-id,
psa-lifecycle,
psa-certification-reference,
psa-boot-seed,
( psa-software-components // psa-no-sw-measurement ),
}
psa-profile-key = -75000
psa-client-id-key = -75001
psa-lifecycle-key = -75002
psa-implementation-id-key = -75003
psa-boot-seed-key = -75004
psa-certification-reference-key = -75005
psa-software-components-key = -75006
psa-no-sw-measurement-key = -75007
psa-nonce-key = -75008
psa-instance-id-key = -75009
psa-verification-service-indicator-key = -75010
psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
psa-boot-seed-type = bytes .size 32
psa-boot-seed = (
psa-boot-seed-key => psa-boot-seed-type
)
psa-client-id-nspe-type = -2147483648...0
psa-client-id-spe-type = 1..2147483647
psa-client-id-type = psa-client-id-nspe-type / psa-client-id-spe-type
psa-client-id = (
psa-client-id-key => psa-client-id-type
)
psa-certification-reference-type = text .regexp "[0-9]{13}"
psa-certification-reference = (
? psa-certification-reference-key =>
psa-certification-reference-type
)
psa-implementation-id-type = bytes .size 32
psa-implementation-id = (
psa-implementation-id-key => psa-implementation-id-type
)
psa-instance-id-type = bytes .size 33
psa-instance-id = (
psa-instance-id-key => psa-instance-id-type
)
psa-no-sw-measurements-type = 1
psa-no-sw-measurement = (
psa-no-sw-measurement-key => psa-no-sw-measurements-type
)
psa-nonce = (
psa-nonce-key => psa-hash-type
)
psa-profile-type = "PSA_IOT_PROFILE_1"
psa-profile = (
? psa-profile-key => psa-profile-type
)
psa-lifecycle-unknown-type = 0x0000..0x00ff
psa-lifecycle-assembly-and-test-type = 0x1000..0x10ff
psa-lifecycle-psa-rot-provisioning-type = 0x2000..0x20ff
psa-lifecycle-secured-type = 0x3000..0x30ff
psa-lifecycle-non-psa-rot-debug-type = 0x4000..0x40ff
psa-lifecycle-recoverable-psa-rot-debug-type = 0x5000..0x50ff
psa-lifecycle-decommissioned-type = 0x6000..0x60ff
psa-lifecycle-type =
psa-lifecycle-unknown-type /
psa-lifecycle-assembly-and-test-type /
psa-lifecycle-psa-rot-provisioning-type /
psa-lifecycle-secured-type /
psa-lifecycle-non-psa-rot-debug-type /
psa-lifecycle-recoverable-psa-rot-debug-type /
psa-lifecycle-decommissioned-type
psa-lifecycle = (
psa-lifecycle-key => psa-lifecycle-type
)
psa-software-component = {
? 1 => text, ; measurement type
2 => psa-hash-type, ; measurement value
? 4 => text, ; version
5 => psa-hash-type, ; signer id
? 6 => text, ; measurement description
}
psa-software-components = (
psa-software-components-key => [ + psa-software-component ]
)
psa-verification-service-indicator-type = text
psa-verification-service-indicator = (
? psa-verification-service-indicator-key =>
psa-verification-service-indicator-type
)
¶
This specification re-uses the CWT and the EAT specification. Hence, the security and privacy considerations of those specifications apply here as well.¶
Since CWTs offer different ways to protect the token, this specification profiles those options and allows signatures based on use of public key cryptography as well as MAC authentication. The token MUST be signed following the structure of the COSE specification [RFC8152]. The COSE type MUST be COSE_Sign1 for public key signatures or COSE_Mac0 for MAC authentication. Note however that use of MAC authentication is NOT RECOMMENDED due to the associated infrastructure costs for key management and protocol complexities. It may also restrict the ability to interoperate with third parties.¶
Attestation tokens contain information that may be unique to a device and therefore they may allow to single out an individual device for tracking purposes. Implementations that have privacy requirements must take appropriate measures to ensure that the token is only used to provision anonymous/pseudonym keys.¶
To verify the token, the primary need is to check correct formation and signing as for any CWT token. In addition though, the verifier can 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:¶
This specification registers the following claims in the IANA "CBOR Web Token (CWT) Claims" registry [IANA-CWT], established by [RFC8392].¶
IANA is requested to register the "application/psa-attestation-token" media type [RFC2046] in the "Media Types" registry [IANA-MediaTypes] in the manner described in RFC 6838 [RFC6838], which can be used to indicate that the content is a PSA Attestation Token.¶
Additional information:¶
IANA is requested to register the CoAP Content-Format ID for the "application/psa-attestation-token" media type in the "CoAP Content-Formats" registry [IANA-CoAP-Content-Formats].¶
A reference implementation is provided by the Trusted Firmware project [TF-M].¶
The following example shows a PSA attestation token for an hypothetical system comprising two measured software components (a boot loader and a trusted RTOS). The attesting device is in a lifecycle state Section 3.3.1 of SECURED. The attestation has been requested from a client residing in the SPE:¶
{
/ psa-profile / -75000: "PSA_IOT_PROFILE_1",
/ psa-client-id / -75001: 1,
/ psa-lifecycle / -75002: 12288,
/ psa-implementation-id / -75003: h'50515253545556575051
52535455565750515253545556575051525354555657',
/ psa-boot-seed / -75004: h'DEADBEEFDEADBEEFDEAD
BEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEF',
/ psa-certification-reference / -75005: "1234567890123",
/ psa-software-components / -75006: [
{
/ measurement type / 1: "BL",
/ measurement value / 2: h'0001020400010204000102040001020
400010204000102040001020400010204',
/ signer ID / 5: h'519200FF519200FF519200FF519200F
F519200FF519200FF519200FF519200FF'
},
{
/ measurement type / 1: "PRoT",
/ measurement value / 2: h'0506070805060708050607080506070
805060708050607080506070805060708',
/ signer ID / 5: h'519200FF519200FF519200FF519200F
F519200FF519200FF519200FF519200FF'
}
],
/ psa-nonce / -75008: h'00010203000102030001020300010203
00010203000102030001020300010203',
/ psa-instance-id / -75009: h'01A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2
A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3',
/ psa-verification-service-indicator / -75010: "https://psa-ve
rifier.org"
}
¶
The JWK representation of the IAK used for creating the COSE Sign1 signature over the PSA token is:¶
{
"kty": "EC",
"crv": "P-256",
"x": "MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4",
"y": "4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM",
"d": "870MB6gfuTJ4HtUnUvYMyJpr5eUZNP4Bk43bVdj3eAE",
"use": "enc",
"kid": "1"
}
¶
The resulting COSE object is:¶
18(
[
/ protected / h'A10126',
/ unprotected / {},
/ payload / h'AA3A000124F7715053415F494F545F50524F46494C
455F313A000124F8013A000124F91930003A000124FA58205051525354555657
5051525354555657505152535455565750515253545556573A000124FB5820DE
ADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEF3A
000124FC6D313233343536373839303132333A000124FD82A30162424C025820
0001020400010204000102040001020400010204000102040001020400010204
055820519200FF519200FF519200FF519200FF519200FF519200FF519200FF51
9200FFA3016450526F5402582005060708050607080506070805060708050607
08050607080506070805060708055820519200FF519200FF519200FF519200FF
519200FF519200FF519200FF519200FF3A000124FF5820000102030001020300
01020300010203000102030001020300010203000102033A00012500582101A0
A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A33A
00012501781868747470733A2F2F7073612D76657269666965722E6F7267',
/ signature / h'7C0FA38F80E5EA2A5C710A4BB37ABE63B26B25F17D
B6BE9489587F9B3F8FEB80E0E410D8CDAAFAE5588024CB3E18D60C1F96CED9E0
6743824614019E99BF13FE'
]
)
¶
We would like to thank the following colleagues for their contributions:¶
* Laurence Lundblade Security Theory LLC lgl@securitytheory.com¶
* Tamas Ban Arm Limited Tamas.Ban@arm.com¶
* Sergei Trofimov Arm Limited Sergei.Trofimov@arm.com¶
Thanks to Carsten Bormann for help with the CDDL and Nicholas Wood for ideas and comments.¶