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Elliptic Curve Digital Signature Algorithm (ECDSA)

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The Elliptic Curve Digital Signature Algorithm (ECDSA) offers a variant of the Digital Signature Algorithm (DSA) which uses elliptic-curve cryptography.

ESP32-C5's ECDSA peripheral provides a secure and efficient environment for computing ECDSA signatures. It offers fast computations while ensuring the confidentiality of the signing process to prevent information leakage. ECDSA private key used in the signing process is accessible only to the hardware peripheral, and it is not readable by software.

ECDSA peripheral can help to establish Secure Device Identity for TLS mutual authentication and similar use-cases.

Supported Features

  • ECDSA digital signature generation and verification

  • Two different elliptic curves, namely P-192 and P-256 (FIPS 186-3 specification)

  • Two hash algorithms for message hash in the ECDSA operation, namely SHA-224 and SHA-256 (FIPS PUB 180-4 specification)

ECDSA on ESP32-C5

On ESP32-C5, the ECDSA module works with a secret key burnt into an eFuse block. This eFuse key is made completely inaccessible (default mode) for any resources outside the cryptographic modules, thus avoiding key leakage.

ECDSA key can be programmed externally through idf.py script. Here is an example of how to program the ECDSA key:

idf.py efuse-burn-key <BLOCK_NUM> </path/to/ecdsa_private_key.pem> ECDSA_KEY

Note

Six physical eFuse blocks can be used as keys for the ECDSA module: block 4 ~ block 9. E.g., for block 4 (which is the first key block) , the argument should be BLOCK_KEY0.

Alternatively the ECDSA key can also be programmed through the application running on the target.

Following code snippet uses esp_efuse_write_key() to set physical key block 0 in the eFuse with key purpose as esp_efuse_purpose_t::ESP_EFUSE_KEY_PURPOSE_ECDSA_KEY:

#include "esp_efuse.h"

const uint8_t key_data[32] = { ... };

esp_err_t status = esp_efuse_write_key(EFUSE_BLK_KEY0,
                    ESP_EFUSE_KEY_PURPOSE_ECDSA_KEY,
                    key_data, sizeof(key_data));

if (status == ESP_OK) {
    // written key
} else {
    // writing key failed, maybe written already
}

Non-Determinisitic Signature Generation

Dependency on TRNG

ECDSA peripheral relies on the hardware True Random Number Generator (TRNG) for its internal entropy requirement for generating non-deterministic signatures. During ECDSA signature creation, the algorithm requires a random integer to be generated as specified in the RFC 6090 section 5.3.2.

Please ensure that hardware RNG is enabled before starting ECDSA computations (primarily signing) in the application.

Application Outline

Please refer to the ECDSA Peripheral with ESP-TLS guide for details on how-to use ECDSA peripheral for establishing a mutually authenticated TLS connection.

The ECDSA peripheral in Mbed TLS stack is integrated by overriding the ECDSA signing and verifying APIs. Please note that, the ECDSA peripheral does not support all curves or hash algorithms, and hence for cases where the hardware requirements are not met, the implementation falls back to the software.

For a particular TLS context, additional APIs have been supplied to populate certain fields (e.g., private key ctx) to differentiate routing to hardware. ESP-TLS layer integrates these APIs internally and hence no additional work is required at the application layer. However, for custom use-cases please refer to API details below.

API Reference

Header File

  • components/mbedtls/port/include/ecdsa/ecdsa_alt.h

  • This header file can be included with:

    #include "ecdsa/ecdsa_alt.h"

  • This header file is a part of the API provided by the mbedtls component. To declare that your component depends on mbedtls, add the following to your CMakeLists.txt:

    REQUIRES mbedtls

    or

    PRIV_REQUIRES mbedtls

Functions

int esp_ecdsa_load_pubkey(mbedtls_ecp_keypair *keypair, int efuse_blk)

Populate the public key buffer of the mbedtls_ecp_keypair context.

Parameters

  • keypair -- The mbedtls ECP key-pair structure

  • efuse_blk -- The efuse key block that should be used as the private key. The key purpose of this block must be ECDSA_KEY

Returns

- 0 if successful

  • MBEDTLS_ERR_ECP_BAD_INPUT_DATA if invalid ecp group id specified

  • MBEDTLS_ERR_ECP_INVALID_KEY if efuse block with purpose ECDSA_KEY is not found

  • -1 if invalid efuse block is specified

int esp_ecdsa_privkey_load_mpi(mbedtls_mpi *key, int efuse_blk)

Initialize MPI to notify mbedtls_ecdsa_sign to use the private key in efuse We break the MPI struct of the private key in order to differentiate between hardware key and software key.

Parameters

  • key -- The MPI in which this functions stores the hardware context. This must be uninitialized

  • efuse_blk -- The efuse key block that should be used as the private key. The key purpose of this block must be ECDSA_KEY

Returns

- 0 if successful

  • -1 otherwise

int esp_ecdsa_privkey_load_pk_context(mbedtls_pk_context *key_ctx, int efuse_blk)

Initialize PK context to notify mbedtls_ecdsa_sign to use the private key in efuse We break the MPI struct used to represent the private key d in ECP keypair in order to differentiate between hardware key and software key.

Parameters

  • key_ctx -- The context in which this functions stores the hardware context. This must be uninitialized

  • efuse_blk -- The efuse key block that should be used as the private key. The key purpose of this block must be ECDSA_KEY

Returns

- 0 if successful

  • -1 otherwise

int esp_ecdsa_set_pk_context(mbedtls_pk_context *key_ctx, esp_ecdsa_pk_conf_t *conf)

Initialize PK context and completely populate mbedtls_ecp_keypair context. We break the MPI struct used to represent the private key d in ECP keypair in order to differentiate between hardware key and software key. We also populate the ECP group field present in the mbedtls_ecp_keypair context. If the ECDSA peripheral of the chip supports exporting the public key, we can also populate the public key buffer of the mbedtls_ecp_keypair context if the load_pubkey flag is set in the esp_ecdsa_pk_conf_t config argument.

Parameters

  • key_ctx -- The context in which this functions stores the hardware context. This must be uninitialized

  • conf -- ESP-ECDSA private key context initialization config structure

Returns

- 0 if successful

  • -1 otherwise

Structures

struct esp_ecdsa_pk_conf_t

ECDSA private key context initialization config structure.

Note

Contains configuration information like the efuse key block that should be used as the private key, EC group ID of the private key and if the export public key operation is supported by the peripheral, a flag load_pubkey that is used specify if the public key has to be populated

Public Members

mbedtls_ecp_group_id grp_id

MbedTLS ECP group identifier

uint8_t efuse_block

EFuse block id for ECDSA private key

bool load_pubkey

Export ECDSA public key from the hardware

bool use_km_key

Use key deployed in the key manager for ECDSA operation. Note: The key must be already deployed by the application and it must be activated for the lifetime of this context

Macros

USE_ECDSA_KEY_FROM_KEY_MANAGER

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