# Digital Signature (DS)

[中文]

The Digital Signature (DS) module 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 > Digital Signature (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 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 > Digital Signature (DS) [PDF].

## Signature Calculation with ESP-IDF

For more detailed information on the workflow and the registers used, see ESP32-S2 Technical Reference Manual > Digital Signature (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

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 DS peripheral. This signature needs to be converted to an appropriate format for further use. For example, the MbedTLS SSL stack supports PKCS#1 format. The API `esp_ds_rsa_sign()` can be used to obtain the signature directly in the PKCS#1 v1.5 format. It internally uses `esp_ds_start_sign()` and converts the signature into PKCS#1 v1.5 format.

Note

This is only the basic 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. An API to do this is planned in the future.

## Configure the DS Peripheral for a TLS Connection

The 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 DS peripheral.

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

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

The encrypted private key parameters obtained after the DS peripheral configuration are then to be kept in flash. Furthermore, they are to be passed to the DS peripheral which makes use of those parameters for the Digital Signature operation. The application then needs to read the DS data from the flash, which has been done through the APIs provided by the esp_secure_cert_mgr component. Please refer to the component/README for more details.

The process of initializing the DS peripheral and then performing the Digital Signature operation is done internally with the help of ESP-TLS. Please refer to Digital Signature with ESP-TLS for more details.

As mentioned in the ESP-TLS documentation, the application only needs to provide the encrypted private key parameters to the esp_tls context (as ds_data), which internally performs all necessary operations for initializing the DS peripheral and then performing the DS operation.

## Example for SSL Mutual Authentication Using DS

The example protocols/mqtt/ssl_ds shows how to use the DS peripheral for mutual authentication. The example uses mqtt_client (Implemented through ESP-MQTT) to connect to broker `test.mosquitto.org` using SSL transport with mutual authentication. The SSL part is internally performed with ESP-TLS. See protocols/mqtt/ssl_ds/README.md for more details.

## API Reference

### Header File

- components/esp_hw_support/include/esp_ds.h

- This header file can be included with:

  >     #include "esp_ds.h"

### Functions

esp_err_t esp_ds_sign(const void *message, const esp_ds_data_t *data, hmac_key_id_t key_id, void *signature)

  Sign the message with a hardware key from specific key slot. The function calculates a plain RSA signature with help of the DS peripheral. The RSA encryption operation is as follows: Z = XY mod M where, Z is the signature, X is the input message, Y and M are the RSA private key parameters.

  This function is a wrapper around `esp_ds_finish_sign()` and `esp_ds_start_sign()`, so do not use them in parallel. It blocks until the signing is finished and then returns the signature.

  Note

  Please see note section of `esp_ds_start_sign()` for more details about the input parameters.

  Parameters:

    - message -- the message to be signed; its length should be (data->rsa_length + 1)*4 bytes, and those bytes must be in little endian format. It is your responsibility to apply your hash function and padding before calling this function, if required. (e.g. message = padding(hash(inputMsg)))

    - data -- the encrypted signing key data (AES encrypted RSA key + IV)

    - key_id -- the HMAC key ID determining the HMAC key of the HMAC which will be used to decrypt the signing key data

    - signature -- the destination of the signature, should be (data->rsa_length + 1)*4 bytes long

  Returns:

    - ESP_OK if successful, the signature was written to the parameter `signature`.

    - ESP_ERR_INVALID_ARG if one of the parameters is NULL or data->rsa_length is too long or 0

    - ESP_ERR_HW_CRYPTO_DS_HMAC_FAIL if there was an HMAC failure during retrieval of the decryption key

    - ESP_ERR_NO_MEM if there hasn't been enough memory to allocate the context object

    - ESP_ERR_HW_CRYPTO_DS_INVALID_KEY if there's a problem with passing the HMAC key to the DS component

    - ESP_ERR_HW_CRYPTO_DS_INVALID_DIGEST if the message digest didn't match; the signature is invalid.

    - ESP_ERR_HW_CRYPTO_DS_INVALID_PADDING if the message padding is incorrect, the signature can be read though since the message digest matches.

esp_err_t esp_ds_start_sign(const void *message, const esp_ds_data_t *data, hmac_key_id_t key_id, esp_ds_context_t **esp_ds_ctx)

  Start the signing process.

  This function yields a context object which needs to be passed to `esp_ds_finish_sign()` to finish the signing process. The function calculates a plain RSA signature with help of the DS peripheral. The RSA encryption operation is as follows: Z = XY mod M where, Z is the signature, X is the input message, Y and M are the RSA private key parameters.

  Note

  This function locks the HMAC, SHA, AES and RSA components, so the user has to ensure to call `esp_ds_finish_sign()` in a timely manner. The numbers Y, M, Rb which are a part of esp_ds_data_t should be provided in little endian format and should be of length equal to the RSA private key bit length The message length in bits should also be equal to the RSA private key bit length. No padding is applied to the message automatically, Please ensure the message is appropriate padded before calling the API.

  Parameters:

    - message -- the message to be signed; its length should be (data->rsa_length + 1)*4 bytes, and those bytes must be in little endian format. It is your responsibility to apply your hash function and padding before calling this function, if required. (e.g. message = padding(hash(inputMsg)))

    - data -- the encrypted signing key data (AES encrypted RSA key + IV)

    - key_id -- the HMAC key ID determining the HMAC key of the HMAC which will be used to decrypt the signing key data

    - esp_ds_ctx -- the context object which is needed for finishing the signing process later

  Returns:

    - ESP_OK if successful, the ds operation was started now and has to be finished with `esp_ds_finish_sign()`

    - ESP_ERR_INVALID_ARG if one of the parameters is NULL or data->rsa_length is too long or 0

    - ESP_ERR_HW_CRYPTO_DS_HMAC_FAIL if there was an HMAC failure during retrieval of the decryption key

    - ESP_ERR_NO_MEM if there hasn't been enough memory to allocate the context object

    - ESP_ERR_HW_CRYPTO_DS_INVALID_KEY if there's a problem with passing the HMAC key to the DS component

bool esp_ds_is_busy(void)

  Return true if the DS peripheral is busy, otherwise false.

  Note

  Only valid if `esp_ds_start_sign()` was called before.

esp_err_t esp_ds_finish_sign(void *signature, esp_ds_context_t *esp_ds_ctx)

  Finish the signing process.

  Parameters:

    - signature -- the destination of the signature, should be (data->rsa_length + 1)*4 bytes long, the resultant signature bytes shall be written in little endian format.

    - esp_ds_ctx -- the context object retrieved by `esp_ds_start_sign()`

  Returns:

    - ESP_OK if successful, the ds operation has been finished and the result is written to signature.

    - ESP_ERR_INVALID_ARG if one of the parameters is NULL

    - ESP_ERR_HW_CRYPTO_DS_INVALID_DIGEST if the message digest didn't match; the signature is invalid. This means that the encrypted RSA key parameters are invalid, indicating that they may have been tampered with or indicating a flash error, etc.

    - ESP_ERR_HW_CRYPTO_DS_INVALID_PADDING if the message padding is incorrect, the signature can be read though since the message digest matches (see TRM for more details).

esp_err_t esp_ds_encrypt_params(esp_ds_data_t *data, const void *iv, const esp_ds_p_data_t *p_data, const void *key)

  Encrypt the private key parameters.

  The encryption is a prerequisite step before any signature operation can be done. It is not strictly necessary to use this encryption function, the encryption could also happen on an external device.

  Note

  The numbers Y, M, Rb which are a part of esp_ds_data_t should be provided in little endian format and should be of length equal to the RSA private key bit length The message length in bits should also be equal to the RSA private key bit length. No padding is applied to the message automatically, Please ensure the message is appropriate padded before calling the API.

  Parameters:

    - data -- Output buffer to store encrypted data, suitable for later use generating signatures.

    - iv -- Pointer to 16 byte IV buffer, will be copied into 'data'. Should be randomly generated bytes each time.

    - p_data -- Pointer to input plaintext key data. The expectation is this data will be deleted after this process is done and 'data' is stored.

    - key -- Pointer to 32 bytes of key data. Type determined by key_type parameter. The expectation is the corresponding HMAC key will be stored to efuse and then permanently erased.

  Returns:

    - ESP_OK if successful, the ds operation has been finished and the result is written to signature.

    - ESP_ERR_INVALID_ARG if one of the parameters is NULL or p_data->rsa_length is too long

### Structures

struct esp_ds_context

  Context object used for non-blocking digital signature operations.

  This object is allocated by `esp_ds_start_sign()` and must be passed to `esp_ds_finish_sign()` to complete the digital signature operation.

  Public Members

  const ets_ds_data_t *data

    Pointer to the encrypted private key data

struct esp_digital_signature_data

  Encrypted private key data. Recommended to store in flash in this format.

  Note

  This struct has to match to one from the ROM code! This documentation is mostly taken from there.

  Public Members

  esp_digital_signature_length_t rsa_length

    RSA LENGTH register parameters (number of words in RSA key & operands, minus one).

    This value must match the length field encrypted and stored in 'c', or invalid results will be returned. (The DS peripheral will always use the value in 'c', not this value, so an attacker can't alter the DS peripheral results this way, it will just truncate or extend the message and the resulting signature in software.)

    Note

    In IDF, the enum type length is the same as of type unsigned, so they can be used interchangeably. See the ROM code for the original declaration of struct `ets_ds_data_t`.

  uint32_t iv[ESP_DS_IV_BIT_LEN / 32]

    IV value used to encrypt 'c'

  uint8_t c[ESP_DS_C_LEN]

    Encrypted Digital Signature parameters. Result of AES-CBC encryption of plaintext values. Includes an encrypted message digest.

struct esp_ds_p_data_t

  Plaintext parameters used by Digital Signature.

  This is only used for encrypting the RSA parameters by calling esp_ds_encrypt_params(). Afterwards, the result can be stored in flash or in other persistent memory. The encryption is a prerequisite step before any signature operation can be done.

  Note

  Y, M, Rb, & M_Prime must all be in little endian format.

  Public Members

  uint32_t Y[ESP_DS_SIGNATURE_MAX_BIT_LEN / 32]

    RSA exponent.

  uint32_t M[ESP_DS_SIGNATURE_MAX_BIT_LEN / 32]

    RSA modulus.

  uint32_t Rb[ESP_DS_SIGNATURE_MAX_BIT_LEN / 32]

    RSA r inverse operand.

  uint32_t M_prime

    RSA M prime operand.

  uint32_t length

    RSA length in words (32 bit)

### Macros

ESP_DS_IV_BIT_LEN

ESP_DS_IV_LEN

ESP_DS_SIGNATURE_MAX_BIT_LEN

ESP_DS_SIGNATURE_MD_BIT_LEN

ESP_DS_SIGNATURE_M_PRIME_BIT_LEN

ESP_DS_SIGNATURE_L_BIT_LEN

ESP_DS_SIGNATURE_PADDING_BIT_LEN

ESP_DS_C_LEN

### Type Definitions

typedef struct esp_ds_context esp_ds_context_t

  Context object used for non-blocking digital signature operations.

  This object is allocated by `esp_ds_start_sign()` and must be passed to `esp_ds_finish_sign()` to complete the digital signature operation.

typedef struct esp_digital_signature_data esp_ds_data_t

  Encrypted private key data. Recommended to store in flash in this format.

  Note

  This struct has to match to one from the ROM code! This documentation is mostly taken from there.

### Enumerations

enum esp_digital_signature_length_t

  Values:

  enumerator ESP_DS_RSA_1024
  enumerator ESP_DS_RSA_2048
  enumerator ESP_DS_RSA_3072
  enumerator ESP_DS_RSA_4096

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