Symmetric key algorithm based Initial Attestation

Author: David Hu
Organization: Arm Limited
Contact: david.hu@arm.com

Introduction

This document proposes a design of symmetric key algorithm based Initial Attestation in TF-M.

Symmetric key algorithm based Initial Attestation (symmetric Initial Attestation for short) signs and verifies Initial Attestation Token (IAT) with a symmetric cryptography signature scheme, such as HMAC. It can reduce TF-M binary size and memory footprint on ultra-constrained devices without integrating asymmetric ciphers.

This proposal follows PSA Attestation API document 1.

Note

As pointed out by PSA Attestation API 1, the use cases of Initial Attestation based on symmetric key algorithms can be limited due to the associated infrastructure costs for key management and operational complexities. It may also restrict the ability to interoperate with scenarios that involve third parties.

Design overview

The symmetric Initial Attestation follows the existing IAT generation sequence for Initial Attestation based on asymmetric key algorithm (asymmetric Initial Attestation for short).

As Profile Small design 2 requests, a configuration flag SYMMETRIC_INITIAL_ATTESTATION selects symmetric initial attestation during build.

The top-level design is shown in Overall design diagram below.

Figure 13: Overall design diagram

Symmetric Initial Attestation adds its own implementations of some steps in IAT generation in Initial Attestation secure service. More details are covered in IAT generation in Initial Attestation secure service.

The interfaces and procedures of Initial Attestation secure service are not affected. Refer to Initial Attestation Service Integration Guide 3 for details of the implementation of Initial Attestation secure service.

Symmetric Initial Attestation invokes t_cose library to build up COSE_Mac0 structure. COSE_Mac0 support was originally added to the t_cose library fork in TF-M, however since t_cose 2.0 it is part of the upstream library 4 which is already used by TF-M too.

Several HAL APIs are defined to fetch platform specific assets required by Symmetric Initial Attestation. For example, tfm_plat_get_symmetric_iak() fetches symmetric Initial Attestation Key (IAK). Those HAL APIs are summarized in HAL APIs.

Decoding and verification of symmetric Initial Attestation is also included in this proposal for regression test. The test suites and IAT decoding are discussed in TF-M Test suite.

QCBOR library and Crypto service are also invoked. But this proposal doesn’t require any modification to either QCBOR or Crypto service. Therefore, descriptions of QCBOR and Crypto service are skipped in this document.

IAT generation in Initial Attestation secure service

The sequence of IAT generation of symmetric Initial Attestation is shown in Symmetric IAT generation flow in Initial Attestation secure service below. Note that the Register symmetric IAK stage is no longer required due to changes in the Crypto partition (attest_symmetric_key.c is now responsible only for calculating the instance ID).

Figure 14: Symmetric IAT generation flow in Initial Attestation secure service

In Initial Attestation secure service, symmetric Initial Attestation implements the following steps in attest_create_token(), which are different from those of asymmetric Initial Attestation.

attest_token_start(),

Instance ID claims,

attest_token_finish().

If SYMMETRIC_INITIAL_ATTESTATION is selected, symmetric Initial Attestation dedicated implementations of those steps are included in build. Otherwise, asymmetric Initial Attestation dedicated implementations are included instead. Symmetric Initial Attestation implementation resides a new file attest_symmetric_key.c to handle symmetric Instance ID related operations. Symmetric Initial Attestation dedicated attest_token_start() and attest_token_finish() are added in attestation_token.c. The details are covered in following sections.

Symmetric Instance ID

Symmetric Initial Attestation dedicated attest_symmetric_key.c implements the attest_get_instance_id() function. This function returns the Instance ID value, calculating it if it has not already been calculated. Refer to Instance ID claim for more details.

Note

Only symmetric IAK for HMAC algorithm is allowed so far.

Instance ID calculation

In symmetric Initial Attestation, Instance ID is also calculated the first time it is requested. It can protect critical symmetric IAK from being frequently fetched, which increases the risk of asset disclosure.

The Instance ID value is the output of hashing symmetric IAK raw data twice, as requested in PSA Attestation API 1. HMAC-SHA256 may be hard-coded as the hash algorithm of Instance ID calculation.

Note

According to RFC2104 5, if a HMAC key is longer than the HMAC block size, the key will be first hashed. The hash output is used as the key in HMAC computation.

In current design, HMAC is used to calculate the authentication tag of COSE_Mac0. Assume that symmetric IAK is longer than HMAC block size (HMAC-SHA256 by default), the Instance ID is actually the HMAC key for COSE_Mac0 authentication tag generation, if Instance ID value is the output of hashing IAK only once. Therefore, attackers may request an valid IAT from device and fake malicious ones by using Instance ID to calculate valid authentication tags, to cheat others.

As a result, symmetric IAK raw data should be hashed twice to generate the Instance ID value.

The Instance ID calculation result is stored in a static buffer. Token generation process can call attest_get_instance_id() to fetch the data from that static buffer.

attest_token_start()

Symmetric Initial Attestation dedicated attest_token_start() initializes the COSE_Mac0 computation context and builds up the COSE_Mac0 Header.

The workflow inside attest_token_start() is shown in Workflow in symmetric Initial Attestation attest_token_start() below.

Figure 15: Workflow in symmetric Initial Attestation attest_token_start()

Descriptions of each step are listed below:

t_cose_mac0_sign_init() is invoked to initialize COSE_Mac0 signing context in t_cose.
The symmetric IAK handle is set into COSE_Mac0 signing context via t_cose_mac0_set_signing_key().
Initialize QCBOR encoder.
The header parameters are encoded into COSE_Mac0 structure in t_cose_mac0_encode_parameters().
QCBOREncode_OpenMap() prepares for encoding the COSE_Mac0 payload, which is filled with IAT claims.

For detailed description and documentation of the COSE_Mac0 functionalities please refer to the t_cose repository 4.

Instance ID claim

Symmetric Initial Attestation also implements Instance ID claims in attest_add_instance_id_claim(). The Instance ID value is fetched via attest_get_instance_id(). The value has already been calculated during symmetric IAK registration. See Instance ID calculation for details.

The other steps are the same as those in asymmetric Initial Attestation implementation. The UEID type byte is set to 0x01.

attest_token_finish()

Symmetric Initial Attestation dedicated attest_token_finish() calls t_cose_mac0_encode_tag() to calculate and encode the authentication tag of COSE_Mac0 structure. The whole COSE and CBOR encoding are completed in attest_token_finish(). The simplified flow in attest_token_finish() is shown in Workflow in symmetric Initial Attestation attest_token_finish() below.

Figure 16: Workflow in symmetric Initial Attestation attest_token_finish()

TF-M Test suite

Symmetric Initial Attestation adds dedicated non-secure and secure test suites. The test suites also follow asymmetric Initial Attestation test suites implementation but optimize the memory footprint. Symmetric Initial Attestation non-secure and secure test suites request Initial Attestation secure service to generate IATs. After IATs are generated successfully, test suites decode IATs and parse the claims. Secure test suite also verifies the authentication tag in COSE_Mac0 structure.

Symmetric Initial Attestation implements its dedicated attest_token_decode_validate_token() in attest_symmetric_iat_decoded.c to perform IAT decoding required by test suites. If SYMMETRIC_INITIAL_ATTESTATION is selected, attest_symmetric_iat_decoded.c is included in build. Otherwise, asymmetric Initial Attestation dedicated implementations are included instead.

The workflow of symmetric Initial Attestation dedicated attest_token_decode_validate_token() is shown below.

Figure 17: Workflow in symmetric Initial Attestation attest_token_decode_validate_token()

If the decoding is required from secure test suite, attest_token_decode_validate_token() will fetch symmetric IAK to verify the authentication tag in COSE_Mac0 structure. If the decoding is required from non-secure test suite, attest_token_decode_validate_token() will only decode COSE_Mac0 by setting T_COSE_OPT_DECODE_ONLY option flag. Non-secure must not access the symmetric IAK.

HAL APIs

HAL APIs are summarized below.

Fetch device symmetric IAK

tfm_plat_get_symmetric_iak() fetches device symmetric IAK.

enum tfm_plat_err_t tfm_plat_get_symmetric_iak(uint8_t *key_buf,
                                               size_t buf_len,
                                               size_t *key_len,
                                               psa_algorithm_t *key_alg);

Parameters:

`key_buf`	Buffer to store the symmetric IAK.
`buf_len`	The length of `key_buf`.
`key_len`	The length of the symmetric IAK.
`key_alg`	The key algorithm. Only HMAC SHA-256 is supported so far.

It returns error code specified in enum tfm_plat_err_t.

Get symmetric IAK key identifier

attest_plat_get_symmetric_iak_id() gets the key identifier of the symmetric IAK as the kid parameter in COSE Header.

Optional if device doesn’t install a key identifier for symmetric IAK.

enum tfm_plat_err_t attest_plat_get_symmetric_iak_id(void *kid_buf,
                                                     size_t buf_len,
                                                     size_t *kid_len);

Parameters:

`kid_buf`	Buffer to store the IAK identifier.
`buf_len`	The length of `kid_buf`.
`kid_len`	The length of the IAK identifier.

It returns error code specified in enum tfm_plat_err_t.

Reference

1(1,2,3): PSA Attestation API 1.0 (ARM IHI 0085)
2: Trusted Firmware-M Profile Small Design
3: Initial Attestation Service Integration Guide
4(1,2): t_cose library
5: HMAC: Keyed-Hashing for Message Authentication