# Attestation

## Overview

Attestation is the process of securely providing information about a device to external parties through a cryptographically secured token. It enables IoT devices, especially those without user interfaces or in remote locations, to introduce themselves securely to networks and IoT platforms by offering attestation information.

Thus, the attestation service is employed by the device to communicate evidence regarding its credentials and operational state to a remote relying party. The collected information is structured into the Entity Attestation Token (EAT) format.

To ensure security, the EAT is cryptographically protected. The remote relying party can then verify the authenticity of the EAT and make decisions about engaging with the device based on its contents.

Note

- Support for Attestation can be toggled using the option CONFIG_SECURE_TEE_ATTESTATION (enabled by default).

## Attestation Flow

ESP-TEE Attestation Flow

1. The Relying Party sends an Attestation Request (AR) to the device REE with some challenge or nonce (usually a random number).

2. The device REE forwards the request to the device TEE attestation service through the secure service call interface.

3. The TEE assembles the Entity Attestation Token (EAT) (the attestation evidence) by gathering all the relevant claim information as outlined in the EAT claim table. It then generates a signature on the claim data, appending it to the token.

4. The evidence is transmitted back to the remote attestation service (verifier) through the device REE and the relying party. In certain cases, the Relying Party itself may act as the attestation service.

5. The attestation service assesses the received evidence and, upon successful verification, compares the EAT claim data with the available reference values.

6. The relying party determines the trustworthiness of the device based on the attestation result and its predefined policy. Subsequently, it communicates this determination to the device.

## Entity Attestation Token

The Entity Attestation Token (EAT) is a small blob of data that contains claims and is cryptographically signed. The cryptographic signing secures the token, so the means to convey the token does not need to provide any security.

When an attestation request is sent to the device, it generates the EAT in the predefined JSON format as given below. The token contains all the necessary information about the hardware and software entities on device. Here, an ECDSA key-pair from the TEE secure storage is used for signing the EAT, utilizing the slot ID mentioned in the attestation request. The EAT contains the compressed ECDSA public key as well as the `R` and `S` components of the signature.

> ESP-TEE: Attestation Evidence

### EAT: Header

Claim | Description | Comments
--- | --- | ---
Magic | Identifies the commencement of the EAT |
Encryption Algorithm | Specifies encryption algorithm details if the EAT is encrypted |
Signing Algorithm | Specifies signing algorithm details if the EAT is signed | Currently, only `ecdsa_secp256r1_sha256` is supported
Key ID | Identifies the key to be utilized by the device for encryption and signing | TEE secure storage key ID (string)

### EAT: Claim Table

Claim | Description | Comments
--- | --- | ---
Nonce | For protection from Reply Attack. If attestation is initiated by the device, it provides the nonce as part of the attestation request to the Relying Party. |
Client ID | Relying Party identification |
Chip ID | SoC chip identifier |
UEID | Universal Entity Identifiers, factory-burnt in eFuse | Device MAC address and the Optional Unique ID from eFuse
Device ID | Device identification (should be unique) | SHA256 digest of the device MAC address
Device Version | SoC H/W revision from eFuse |
Instance ID | Instance identification | Public-key hash (SHA256) from the current secure storage slot for the active session
Certification Reference | PSA certification ID |
Device State | Development/Production Mode |
Software Measurement | Details of the active firmware components - for e.g. app version, ESP-IDF version, digest, etc. There will be one entry of this type in the EAT for each firmware in the device. |

### EAT: Software Measurement

Key | Description | Comments
--- | --- | ---
Version | Entity image version | Not applicable to the bootloader
ESP-IDF Version |  | Not applicable to the bootloader
Secure Version | For firmware anti-rollback support | Applicable only to the REE application
Digest: Type |  | Currently, only `SHA256` is supported
Digest: Calculated Value |  |
Digest: Validation | Whether the calculated digest matches the one embedded in the firmware image |
Secure Boot signature verification status |  |
Secure Padding status |  | For more details, refer here.

### Sample EAT in JSON format

    {
      "header": {
        "magic": "44fef7cc",
        "encr_alg": "",
        "sign_alg": "ecdsa_secp256r1_sha256",
        "key_id": "tee_att_key0"
      },
      "eat": {
        "nonce": -1582119980,
        "client_id": 262974944,
        "chip_id": 13,
        "device_ver": 1,
        "ueid": {
          "mac": "d885ac67c978",
          "optional_id": "94fa4d7e305682714d48e7bbd710c961"
        },
        "device_id": "e8cddb2a7f9a5a7c61735d6dda26e4bd153c6d772a9be6f26bd321dfe25e0ac8",
        "instance_id": "1adba85e0df997fd961f25a9e312430cef162b5c69466cd5b172f1e65ac7360c",
        "psa_cert_ref": "0716053550477-10100",
        "device_status": 255,
        "sw_claims": {
          "tee": {
            "type": 1,
            "ver": "v0.3.0",
            "idf_ver": "v5.1.2-139-g07d83a7ced",
            "secure_ver": 0,
            "part_chip_rev": {
              "min": 0,
              "max": 99
            },
            "part_digest": {
              "type": 0,
              "calc_digest": "f732e7f285b7de7ac3167a867711eddbf17a2a05513d35e41cd1ebf2e0958b2e",
              "digest_validated": true,
              "sign_verified": true,
              "secure_padding": true
            }
          },
          "app": {
            "type": 2,
            "ver": "v0.1.0",
            "idf_ver": "v5.1.2-139-g07d83a7ced",
            "secure_ver": 0,
            "part_chip_rev": {
              "min": 0,
              "max": 99
            },
            "part_digest": {
              "type": 0,
              "calc_digest": "21e114fd30b9234c501525990dfab71d00348c531bb64224feff9deb32e66f9f",
              "digest_validated": true,
              "sign_verified": true,
              "secure_padding": true
            }
          },
          "bootloader": {
            "type": 0,
            "ver": "",
            "idf_ver": "",
            "secure_ver": -1,
            "part_chip_rev": {
              "min": 0,
              "max": 99
            },
            "part_digest": {
              "type": 0,
              "calc_digest": "516148649a7f670b894391ded9d64a0e8604c5cec9a1eeb0014d2549cdaa4725",
              "digest_validated": true,
              "sign_verified": true
            }
          }
        }
      },
      "public_key": {
        "compressed": "02a45c6c94c4be7722bd2513f4ccbc4daa369747e6e96e0f9f7a2eba055dee6d46"
      },
      "sign": {
        "r": "37bcc8ed9c15a4712c18fe20b257992e5d9ec273b6261675f247667b4575495b",
        "s": "28ce15da73880f7d5ee303948769b197077208f1f242aaee448e9ed23f9085fa"
      }
    }

## Application Example

The tee_attestation example demonstrates how to generate an entity attestation token containing validation details for all active firmware components (bootloader, TEE, and REE app).

## API Reference

Note

To use the TEE Attestation APIs in your project, ensure that the tee_attestation component is listed as a local dependency in the component manager manifest file idf_component.yml. Refer to the tee_attestation example for guidance.

### Header File

- components/esp_tee/subproject/components/tee_attestation/esp_tee_attestation.h

### Functions

esp_err_t esp_tee_att_generate_token(const uint32_t nonce, const uint32_t client_id, const char *psa_cert_ref, uint8_t *token_buf, const size_t token_buf_size, uint32_t *token_len)

  Generate and return an entity attestation token (EAT) from the TEE.

  The EAT consists of the below details:

  - For all firmware images (Bootloader, active TEE and non-secure app)

    - Project and ESP-IDF version

    - Digest (SHA256)

  - Public key corresponding to the private key used for signing (in compressed format)

  - Signature generated using the ECDSA key stored in the configured slot of the TEE's Secure Storage (`r` and `s` components)

  Parameters:

    - nonce -- [in] Attestation request identification

    - client_id -- [in] Relying Party identification

    - psa_cert_ref -- [in] PSA certification ID

    - token_buf -- [in] Output buffer which will hold the resultant EAT in JSON format

    - token_buf_size -- [in] Size of the output buffer

    - token_len -- [out] Actual length of the output EAT JSON

  Returns:

    - `ESP_OK` on success

    - `ESP_ERR_INVALID_ARG` in case token_buf or token_len are NULL or token_buf_size is 0

    - `ESP_ERR_NO_MEM` in case memory could not be allocated for the internal structures

    - `ESP_FAIL` other errors

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