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TF-M Initial Attestation Service Integration Guide

Introduction

TF-M Initial Attestation Service allows the application to prove the device identity during an authentication process to a verification entity. The initial attestation service can create a token on request, which contains a fix set of device specific data. Device must contain an attestation key pair, which is unique per device. The token is signed with the private part of attestation key pair. The public part of the key pair is known by the verification entity. The public key is used to verify the token authenticity. The data items in the token used to verify the device integrity and assess its trustworthiness. Attestation key provisioning is out of scope for the attestation service and is expected to take part during manufacturing of the device.

Current service limitations

Signing of token - In the current implementation the token is not properly signed. Signature is generated according to the COSE format. But its actual value is not a correct ECDSA P256 signature, due to the lack of support of the ECDSA algorithm in the current implementation of the TF-M Crypto service. A fake signature is created, which is the concatenation of the token's hash value twice.

Claims in the initial attestation token

The initial attestation token is formed of claims. A claim is a data item, which is represented in a key - value structure. The following fixed set of claims are included in the token:

Challenge: Input object from caller. Can be a single nonce from server or hash of nonce and attested data. It is intended to provide freshness to reports and the caller has responsibility to arrange this. Allowed length: 32, 48, 64 bytes. The claim is modeled to be eventually represented by the EAT standard claim nonce. Until such a time as that standard exists, the claim will be represented by a custom claim. Value is encoded as byte string.
Instance ID: It represents the unique identifier of the instance. In the PSA definition it is a hash of the public attestation key of the instance. The claim is modeled to be eventually represented by the EAT standard claim UEID of type GUID. Until such a time as that standard exists, the claim will be represented by a custom claim Value is encoded as byte string.
Verification service indicator: Optional, recommended claim. It is 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. The claim is modeled to be eventually represented by the EAT standard claim origination. Until such a time as that standard exists, the claim will be represented by a custom claim. Value is encoded as text string.
Profile definition: Optional, recommended claim. It contains the name of a document that describes the 'profile' of the token, being a full description of the claims, their usage, verification and token signing. The document name may include versioning. Custom claim with a value encoded as text string.
Implementation ID: It represents the original implementation signer of the attestation key and identifies the contract between the report and verification. A verification service will use this claim to locate the details of the verification process. Custom claim with a value encoded as byte string.
Security lifecycle: It represents the current lifecycle state of the instance. Custom claim with a value encoded as an integer.
Client ID: The partition ID of that secure partition or non-secure thread who called the initial attestation API. Custom claim with a value encoded as a signed integer. Negative number represents non-secure caller, positive numbers represents secure callers, zero is invalid.
HW version: Optional claim. Globally unique number in EAN-13 format identifying the GDSII that went to fabrication, HW and ROM. It can be used to reference the security level of the PSA-ROT via a certification website. Custom claim with a value is encoded as text string.
Boot seed: It represents a random value created at system boot time that will allow differentiation of reports from different system sessions. The size is 32 bytes. Custom claim with a value is encoded as byte string.
Software components: Optional, recommended claim. It represents the software state of the system. The value of the claim is an array of CBOR map entries, with one entry per software component within the device. Each map contains multiple claims that describe evidence about the details of the software component.
Measurement type: Optional claim. It represents the role of the software component. Value is encoded as short(!) text string.
Measurement value: It represents a hash of the invariant software component in memory at start-up time. The value must be a cryptographic hash of 256 bits or stronger. Value is encoded as byte string.
Security epoch: Optional claim. It represents the security control point of the software component. Value is encoded as unsigned integer.
Version: Optional claim. It represents the issued software version. Value is encoded as text string.
Signer ID: It represents the hash of a signing authority public key. Value is encoded as byte string.
Measurement description: Optional claim. It represents the way in which the measurement value of the software component is computed. Value is encoded as text string containing an abbreviated description (name) of the measurement method.
No software measurements: In the event that the implementation does not contain any software measurements then the software components claim above can be omitted but instead it is mandatory to include this claim to indicate this is a deliberate state. Custom claim a value is encoded as an unsigned integer set to 1.

Initial attestation token (IAT) data encoding

The initial attestation token is planned to be aligned with future version of Entity Attestation Token format. The token is encoded according to the CBOR format and signed according to COSE standard.

Code structure

The PSA interface for the Initial Attestation Service is located in interface/include. The only header to be included by applications that want to use functions from the PSA API is psa_initial_attestation.h. The TF-M Initial Attestation Service source files are located in secure_fw/services/initial_attestation. The CBOR library is located in lib/ext/qcbor folder.

Service source files

CBOR library:
lib/ext/qcbor: This library is used to create a proper CBOR token. It can be used on 32-bit and 64-bit machines. It was designed to suite constrained devices with low memory usage and without dynamic memory allocation. It is a fork of this external QCBOR library.
lib/ext/qcbor/inc/qcbor.h: Public API documentation of CBOR library.
COSE library:
lib/t_cose: This library is used to sign a CBOR token and create the COSE header and signature around the initial attestation token. Only a subset of the COSE standard is implemented. Only the cose_sign1 signature schema is supported.
lib/t_cose/src/t_cose_crypto.h: Expose an API to bind t_cose library with available crypto library in the device.
lib/t_cose/src/t_cose_psa_crypto.c: Implements the exposed API and ports t_cose to psa_crypto library.
Initial Attestation Service:
attestation_core.c : Implements core functionalities such as implementation of APIs, retrieval of claims and token creation.
attest_token.c: Implements the token creation function such as start and finish token creation and adding claims to the token.
attestation_crypto_stub.c: Temporary file, it implements the missing psa_crypto APIs.
attestation_key.c: Get the attestation key from platform layer and register it to psa_crypto service for further usage.
tfm_attestation.c: Implements the SPM abstraction layer, and bind the attestation service to the SPM implementation in TF-M project.
tfm_attestation_secure_api.c: Implements the secure API layer to allow other services in the secure domain to request functionalities from the attestation service using the PSA API interface.

Service interface definitions

Boot loader interface: The attestation service might include data in the token about the distinct software components in the device. This data is provided by the boot loader and must be encoded in the TLV format, definition is described below in the boot loader interface paragraph. Possible claims in the boot status are describe above in the software components paragraph.
Hardware abstraction layer:
- Headers are located in platform/include folder.
- tfm_attest_hal.h: Expose an API to get the following claims: security lifecycle, verification service indicator, profile definition.
- tfm_plat_boot_seed.h: Expose an API to get the boot seed claim.
- tfm_plat_device_id.h: Expose an API to get the following claims: implementation ID, hardware version, instance ID.
SPM interface:
attestation.h: Expose an API to bind attestation service to an SPM implementation.
PSA interface:
- psa_initial_attestation.h: Public API definition of initial attestation service.
Crypto interface:
- t_cose_crypto.h: Expose an API to bind the t_cose implementation to any cryptographic library.
- tfm_plat_crypto_keys.h: Expose an API to get the attestation key from platform layer.

PSA interface

The TF-M Initial Attestation Service exposes the following PSA interface:

enum psa_attest_err_t
psa_initial_attest_get_token(const uint8_t *challenge_obj,
                             uint32_t       challenge_size,
                             uint8_t       *token,
                             uint32_t      *token_size);

enum psa_attest_err_t
psa_initial_attest_get_token_size(uint32_t  challenge_size,
                                 uint32_t *token_size);

The caller must allocate a large enough buffer, where the token is going to be created by Initial Attestation Service. The size of the created token is highly dependent on the number of software components in the system and the provided attributes of these. The psa_initial_attest_get_token_size() function can be called to get the exact size of the created token.

System integrators might need to port these interfaces to a custom secure partition manager implementation (SPM). Implementation in TF-M project can be found here:

interface/src/tfm_initial_attestation_api.c: non-secure interface implementation
secure_fw/services/initial_attestation/tfm_attestation_secure_api.c: secure interface implementation

Secure Partition Manager (SPM) interface

The Initial Attestation Service defines the following interface towards the secure partition manager (SPM). System integrators must port this interface according to their SPM implementation.

enum psa_attest_err_t
attest_get_boot_data(uint8_t major_type, void *ptr, uint32_t len);

enum psa_attest_err_t
attest_get_caller_client_id(int32_t *caller_id);

enum psa_attest_err_t
attest_check_memory_access(void *addr,
                           uint32_t size,
                           enum attest_memory_access access);

attest_get_boot_data(): Service can retrieve the relevant data from shared memory area between boot loader and runtime software. It might be the case that only SPM has direct access to the shared memory area, therefore this function can be used to copy the service related data from shared memory to a local memory buffer. In TF-M implementation this function must be called during service initialization phase, because the shared memory region is deliberately overlapping with secure main stack to spare some memory and reuse this area during execution. If boot loader is not available in the system to provide attributes of software components then this function must be implemented in a way that just initialize service's memory buffer to:
```
struct shared_data_tlv_header *tlv_header =
        (struct shared_data_tlv_header *)ptr;
tlv_header->tlv_magic   = 2016;
tlv_header->tlv_tot_len = sizeof(struct shared_data_tlv_header *tlv_header);
```
attest_get_caller_client_id(): Retrieves the ID of the caller thread.
attest_check_memory_access(): Validates the availability and access rights of memory regions received as input data: challenge object, token buffer, etc.

tfm_client.h: Service relies on the following external definitions, which must be present or included in this header file:

typedef struct psa_invec {
    const void *base;
    size_t len;
} psa_invec;

typedef struct psa_outvec {
    void *base;
    size_t len;
} psa_outvec;

Hardware abstraction layer:

The following API definitions are intended to retrieve the platform specific claims. System integrators must implement these interface according to their SoC and software design. Detailed definition of the claims are above in the claims in the initial attestation token paragraph.

tfm_attest_hal_get_security_lifecycle(): Get the security lifecycle of the device.
tfm_attest_hal_get_verification_service(): Get the verification service indicator for initial attestation.
tfm_attest_hal_get_profile_definition(): Get the name of the profile definition document for initial attestation.
tfm_plat_get_boot_seed(): Get the boot seed, which is a constant random number during a boot cycle.
tfm_plat_get_instance_id(): Get the UEID of the device.
tfm_plat_get_implementation_id: Get the implementation ID of the device.
tfm_plat_get_hw_version: Get the hardware version of the device.

Boot loader interface

It is recommended to have a secure boot loader in the boot chain, which is capable of measuring the runtime firmware components (calculates the hash value of firmware images) and provide other attributes of these (version, type, etc).

The shared data between boot loader and runtime software is TLV encoded. The definition of TLV structure is described in bl2/include/tfm_boot_status.h. The shared data is stored in a well known location in secure internal memory and this is a contract between boot loader and runtime SW.

The structure of shared data must be the following:

At the beginning there must be a header: struct shared_data_tlv_header This contains a magic number and a size field which covers the entire size of the shared data area including this header.
```
struct shared_data_tlv_header {
    uint16_t tlv_magic;
    uint16_t tlv_tot_len;
};
```
After the header there come the entries which are composed from an entry header structure: struct shared_data_tlv_entry and the data. In the entry header is a type field tlv_type which identify the consumer of the entry in the runtime software and specify the subtype of that data item. There is a size field tlv_len which covers the size of the entry header and the data. After this structure comes the actual data.
```
struct shared_data_tlv_entry {
    uint16_t tlv_type;
    uint16_t tlv_len;
};
```
Arbitrary number and size of data entry can be in the shared memory area.

The table below gives of overview about the tlv_type field in the entry header. The tlv_type always composed from a major and minor number. Major number identifies the addressee in runtime software, which the data entry is sent to. Minor number used to encode more info about the data entry. The actual definition of minor number could change per major number. In case of boot status data, which is going to be processed by initial attestation service the minor number is split further to two part: sw_module and claim. The sw_module identifies the SW component in the system which the data item belongs to and the claim part identifies the exact type of the data.

tlv_type description:

|------------------------------------------------ |
|                  tlv_type (16 bits)             |
|-------------------------------------------------|
|   tlv_major(4 bits)   |   tlv_minor(12 bits)    |
|-------------------------------------------------|
| MAJOR_IAS   | sw_module(6 bits) | claim(6 bits) |
|-------------------------------------------------|
| MAJOR_CORE  |          TBD                      |
|-------------------------------------------------|

Overall structure of shared data:

---------------------------------------------------------------
| Magic number(uint16_t) | Shared data total length(uint16_t) |
---------------------------------------------------------------
| Major_type(4 bits) | Minor_type(12 bits) | Length(uint16_t) |
---------------------------------------------------------------
|                         Raw data                            |
---------------------------------------------------------------
|                              .                              |
|                              .                              |
|                              .                              |
---------------------------------------------------------------
| Major_type(4 bits) | Minor_type(12 bits) | Length(uint16_t) |
---------------------------------------------------------------
|                         Raw data                            |
---------------------------------------------------------------

Crypto interface

Device must contain an asymmetric key pair. The private part of it is used to sign the initial attestation token. Current implementation supports only the ECDSA P256 signature over SHA256. The public part of the key pair is used to create the key identifier (kid) in the unprotected part of the COSE header. The kid is used by verification entity to look up the corresponding public key to verify the signature in the token. The t_cose part of the initial attestation service implements the signature generation and kid creation. But the actual calculation of token's hash and signature is done by the Crypto service in the device. System integrators might need to re-implement the following functions if they want to use initial attestation service with a different cryptographic library than Crypto service:

t_cose_crypto_pub_key_sign(): Calculates the signature over a hash value.
t_cose_crypto_get_ec_pub_key(): Get the public key to create the key identifier.
t_cose_crypto_hash_start(): Start a multipart hash operation.
t_cose_crypto_hash_update(): Add a message fragment to a multipart hash operation.
t_cose_crypto_hash_finish():Finish the calculation of the hash of a message.

Interface needed by verification code:

t_cose_crypto_pub_key_verify(): Verify the signature over a hash value.

Key handling

The provisioning of the initial attestation key is out of scope of the service and this document. It is assumed that device maker provisions the unique asymmetric key pair during the manufacturing process. The following API is defined to retrieve the attestation key pair from platform layer. Software integrators must port this interface according to their SoC design and make sure that key pair is available by Crypto service:

tfm_plat_get_initial_attest_key(): Retrieve the initial attestation key pair from platform layer.

In TF-M project the attestation key is retrieved by initial attestation service. The key is registered and unregistered to the Crypto service by attestation service with psa_import_key() and psa_destroy_key() API calls for further usage. See in attestation_key.c. In other implementation if the attestation key is directly retrieved by the Crypto service then this key handling is not necessary.

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