# RSA Digital Signature Peripheral (RSA_DS)

[中文]

The RSA Digital Signature Peripheral (RSA_DS) provides hardware acceleration of signing messages based on RSA. It uses pre-encrypted parameters to calculate a signature. The parameters are encrypted using HMAC as a key-derivation function. In turn, the HMAC uses eFuses as the input key.

The whole process happens in hardware so that neither the decryption key for the RSA parameters nor the input key for the HMAC key derivation function can be seen by the software while calculating the signature.

For more detailed information on the hardware involved in the signature calculation and the registers used, see ESP32-S2 Technical Reference Manual > RSA Digital Signature Peripheral (RSA_DS) [PDF].

## Private Key Parameters

The private key parameters for the RSA signature are stored in flash. To prevent unauthorized access, they are AES-encrypted. The HMAC module is used as a key-derivation function to calculate the AES encryption key for the private key parameters. In turn, the HMAC module uses a key from the eFuses key block which can be read-protected to prevent unauthorized access as well.

Upon signature calculation invocation, the software only specifies which eFuse key to use, the corresponding eFuse key purpose, the location of the encrypted RSA parameters, and the message.

## Key Generation

Both the HMAC key and the RSA private key have to be created and stored before the RSA_DS peripheral can be used. This needs to be done in software on the ESP32-S2 or alternatively on a host. For this context, ESP-IDF provides `esp_efuse_write_block()` to set the HMAC key and `esp_hmac_calculate()` to encrypt the private RSA key parameters.

You can find instructions on how to calculate and assemble the private key parameters in ESP32-S2 Technical Reference Manual > RSA Digital Signature Peripheral (RSA_DS) [PDF].

## Signature Calculation with ESP-IDF

For more detailed information on the workflow and the registers used, see ESP32-S2 Technical Reference Manual > RSA Digital Signature Peripheral (RSA_DS) [PDF].

Three parameters need to be prepared to calculate the digital signature:

1. The eFuse key block ID which is used as the key for the HMAC

2. The location of the encrypted private key parameters

3. The message to be signed

Low-level API (plain RSA)

Since the signature calculation takes some time, there are two possible API versions to use in ESP-IDF. The first one is `esp_ds_sign()` and simply blocks until the calculation is finished. If software needs to do something else during the calculation, `esp_ds_start_sign()` can be called, followed by periodic calls to `esp_ds_is_busy()` to check when the calculation has finished. Once the calculation has finished, `esp_ds_finish_sign()` can be called to get the resulting signature.

The APIs `esp_ds_sign()` and `esp_ds_start_sign()` calculate a plain RSA signature with the help of the RSA_DS peripheral. This signature must be converted to an appropriate format (e.g., PKCS#1 v1.5 or PSS) for use in TLS or other protocols.

Note

This is only the basic RSA_DS building block; the message length is fixed. To create signatures of arbitrary messages, the input is normally a hash of the actual message, padded up to the required length.

PSA Crypto driver

The RSA_DS peripheral is also exposed via the PSA Crypto RSA_DS driver, so you can use standard PSA APIs for signing (PKCS#1 v1.5 or PSS) and RSA decryption (PKCS#1 v1.5 or OAEP). Enable `CONFIG_MBEDTLS_HARDWARE_RSA_DS_PERIPHERAL` in `Component config` > `mbedTLS`. For using the RSA_DS peripheral with ESP-TLS (e.g. TLS client authentication), see Digital Signature with ESP-TLS in the ESP-TLS documentation.

## Configure the RSA_DS Peripheral for a TLS Connection

The RSA_DS peripheral on ESP32-S2 chip must be configured before it can be used for a TLS connection. The configuration involves the following steps:

1. Randomly generate a 256-bit value called the `Initialization Vector` (IV).

2. Randomly generate a 256-bit value called the `HMAC_KEY`.

3. Calculate the encrypted private key parameters from the client private key (RSA) and the parameters generated in the above steps.

4. Then burn the 256-bit `HMAC_KEY` on the eFuse, which can only be read by the RSA_DS peripheral.

For more details, see ESP32-S2 Technical Reference Manual > RSA Digital Signature Peripheral (RSA_DS) [PDF].

To configure the RSA_DS peripheral for development purposes, you can use the esp-secure-cert-tool.

The encrypted private key parameters obtained after the RSA_DS peripheral configuration should be stored in flash. The application needs to read the RSA_DS data from flash (e.g., through the APIs provided by the esp_secure_cert_mgr component; see the component/README for more details). For using the RSA_DS peripheral with ESP-TLS, see Digital Signature with ESP-TLS.

## Using RSA_DS with PSA Crypto

To use the RSA_DS peripheral for signing or decryption in application code (outside of ESP-TLS), enable `CONFIG_MBEDTLS_HARDWARE_RSA_DS_PERIPHERAL`. Populate an `esp_ds_data_ctx_t` with the encrypted key data (`esp_ds_data_t`), the eFuse key block ID, and the RSA key length in bits. Ensure the `rsa_length` field of `esp_ds_data_t` is set when the key is created (e.g. via `esp_ds_encrypt_params()` or the RSA_DS provisioning tool). Then wrap the context in an `esp_rsa_ds_opaque_key_t`, import it as a PSA opaque key using `PSA_KEY_LIFETIME_ESP_RSA_DS_VOLATILE`, and call `psa_sign_hash()` or `psa_asymmetric_decrypt()`:

    #include "psa/crypto.h"
    #include "psa_crypto_driver_esp_rsa_ds.h"

    // ds_ctx points to esp_ds_data_ctx_t (e.g. from secure cert or NVS)
    esp_ds_data_ctx_t *ds_ctx = ...;
    esp_rsa_ds_opaque_key_t rsa_ds_opaque_key = {
        .ds_data_ctx = ds_ctx,
    };

    psa_key_attributes_t attrs = PSA_KEY_ATTRIBUTES_INIT;
    psa_set_key_type(&attrs, PSA_KEY_TYPE_RSA_KEY_PAIR);
    psa_set_key_bits(&attrs, ds_ctx->rsa_length_bits);
    psa_set_key_usage_flags(&attrs, PSA_KEY_USAGE_SIGN_HASH);
    psa_set_key_algorithm(&attrs, PSA_ALG_RSA_PKCS1V15_SIGN(PSA_ALG_SHA_256));
    psa_set_key_lifetime(&attrs, PSA_KEY_LIFETIME_ESP_RSA_DS_VOLATILE);

    psa_key_id_t key_id;
    psa_status_t status = psa_import_key(&attrs,
                                         (const uint8_t *)&rsa_ds_opaque_key,
                                         sizeof(rsa_ds_opaque_key),
                                         &key_id);
    psa_reset_key_attributes(&attrs);
    if (status != PSA_SUCCESS) {
        // handle error
    }

    // Sign a hash (e.g. SHA-256 of your message)
    uint8_t hash[32] = { ... };
    uint8_t signature[256];
    size_t sig_len;
    status = psa_sign_hash(key_id, PSA_ALG_RSA_PKCS1V15_SIGN(PSA_ALG_SHA_256),
                           hash, sizeof(hash), signature, sizeof(signature), &sig_len);

    psa_destroy_key(key_id);

### Persistent vs. Volatile RSA_DS PSA Keys

The driver supports two PSA key lifetimes for RSA_DS keys:

- `PSA_KEY_LIFETIME_ESP_RSA_DS_VOLATILE` (used in the example above) stores only pointers to the caller-supplied `esp_ds_data_ctx_t` and any Key Manager recovery info in the PSA key slot. The referenced buffers must remain valid until `psa_destroy_key()` is called. This avoids deep-copying large blobs such as `esp_ds_data_t` (≈1200-1600 bytes, chip-dependent) when they already live in mmap'd flash via `esp_secure_cert_mgr`.

- `PSA_KEY_LIFETIME_ESP_RSA_DS` (persistent) deep-copies the encrypted key material into the PSA key slot at `psa_import_key()` time and PSA persists it to NVS together with the rest of the key attributes. The caller is free to release the import-time buffers once `psa_import_key()` returns; subsequent `psa_sign_hash()` / `psa_asymmetric_decrypt()` calls retrieve the bytes back from NVS automatically. Use this lifetime when the application wants the key to survive reboots without having to reload the `esp_ds_data_ctx_t` from external storage on every boot.

## Example for SSL Mutual Authentication Using RSA_DS

The SSL mutual authentication example that previously lived under `examples/protocols/mqtt/ssl_ds` is now shipped with the standalone espressif/mqtt component. Follow the component documentation to fetch the SSL RSA_DS example and build it together with ESP-MQTT. The example continues to use `mqtt_client` (implemented by ESP-MQTT) to connect to `test.mosquitto.org` over mutual-authenticated TLS, with the TLS portion handled by ESP-TLS.

## API Reference

### Header File

- components/mbedtls/port/psa_driver/include/psa_crypto_driver_esp_rsa_ds_contexts.h

- This header file can be included with:

  >     #include "psa_crypto_driver_esp_rsa_ds_contexts.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

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