HMAC

The HMAC (Hash-based Message Authentication Code) module provides hardware acceleration for SHA256-HMAC generation using a key burned into an eFuse block. HMACs work with pre-shared secret keys and provide authenticity and integrity to a message.

Look into the ESP32-S2 Technical Reference Manual (PDF) for more detailed information about the application workflow and the HMAC calculation process.

Generalized Application Scheme

Let there be two parties, A and B. They want to verify the authenticity and integrity of messages sent between each other. Before they can start sending messages, they need to exchange the secret key via a secure channel. To verify A’s messages, B can do the following:

  • A calculates the HMAC of the message it wants to send.

  • A sends the message and the HMAC to B.

  • B calculates HMAC of the received message itself.

  • B checks wether the received and calculated HMACs match. If they do match, the message is authentic.

However, the HMAC itself isn’t bound to this use case. It can also be used for challenge-response protocols supporting HMAC or as a key input for further security modules (see below), etc.

HMAC on the ESP32-S2

On the ESP32-S2, the HMAC module works with a secret key burnt into the eFuses. This eFuse key can be made completely inaccessible for any resources outside the cryptographic modules, thus avoiding key leakage.

Furthermore, the ESP32-S2 has three different application scenarios for its HMAC module:

  1. HMAC is generated for software use

  2. HMAC is used as a key for the Digital Signature (DS) module

  3. HMAC is used for enabling the soft-disabled JTAG interface

The first mode is also called Upstream mode, while the last two modes are also called Downstream modes.

eFuse Keys for HMAC

Six physical eFuse blocks can be used as keys for the HMAC module: block 4 up to block 9. The enum hmac_key_id_t in the API maps them to HMAC_KEY0 … HMAC_KEY5. Each key has a corresponding eFuse parameter key purpose determining for which of the three HMAC application scenarios (see below) the key may be used:

Key Purpose

Application Scenario

8

HMAC generated for software use

7

HMAC used as a key for the Digital Signature (DS) module

6

HMAC used for enabling the soft-disabled JTAG interface

5

HMAC both as a key for the DS module and for enabling JTAG

This is to prevent the usage of a key for a different function than originally intended.

To calculate an HMAC, the software has to provide the ID of the key block containing the secret key as well as the key purpose (see chapter eFuse Controller in the ESP32-S2 Technical Reference Manual). Before the HMAC key calculation, the HMAC module looks up the purpose of the provided key block. The calculation only proceeds if the provided key purpose matches the purpose stored in the eFuses of the key block provided by the ID.

HMAC Generation for Software

Key Purpose value: 8

In this case, the HMAC is given out to the software (e.g. to authenticate a message).

The API to calculate the HMAC is esp_hmac_calculate(). Only the message, message length and the eFuse key block ID have to be provided to that function. The rest, like setting the key purpose, is done automatically.

HMAC for Digital Signature

Key Purpose values: 7, 5

The HMAC can be used as a key derivation function to decrypt private key parameters which are used by the Digital Signature module. A standard message is used by the hardware in that case. The user only needs to provide the eFuse key block and purpose on the HMAC side (additional parameters are required for the Digital Signature component in that case). Neither the key nor the actual HMAC are ever exposed to outside the HMAC module and DS component. The calculation of the HMAC and its hand-over to the DS component happen internally.

For more details, check the chapter Digital Signature in the ESP32-S2 Technical Reference Manual.

HMAC for Enabling JTAG

Key Purpose values: 6, 5

The third application is using the HMAC as a key to enable JTAG if it was soft-disabled before. Following is the procedure to re-enable the JTAG

Setup

  1. Generate a 256-bit HMAC secret key to use for JTAG re-enable.

  2. Write the key to an eFuse block with key purpose HMAC_DOWN_ALL (5) or HMAC_DOWN_JTAG (6). This can be done using the ets_efuse_write_key() function in the firmware or using espefuse.py from the host.

  3. Configure the eFuse key block to be read protected using the esp_efuse_set_read_protect(), so that software cannot read back the value.

  4. Burn the “soft JTAG disable” bit by esp_efuse_write_field_bit(ESP_EFUSE_SOFT_DIS_JTAG). This will permanently disable JTAG unless the correct key value is provided by software.

JTAG enable

  1. The key to re-enable JTAG is the output of the HMAC-SHA256 function using the secret key in eFuse and 32 0x00 bytes as the message.

  2. Pass this key value when calling the esp_hmac_jtag_enable() function from the firmware.

  3. To re-disable JTAG in the firmware, reset the system or call esp_hmac_jtag_disable().

For more details, check the chapter HMAC Module in the ESP32-S2 Technical Reference Manual.

Application Outline

Following code is an outline of how to set an eFuse key and then use it to calculate an HMAC for software usage. We use ets_efuse_write_key to set physical key block 4 in the eFuse for the HMAC module together with its purpose. ETS_EFUSE_KEY_PURPOSE_HMAC_UP (8) means that this key can only be used for HMAC generation for software usage:

#include "esp32s2/rom/efuse.h"

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

int ets_status = ets_efuse_write_key(ETS_EFUSE_BLOCK_KEY4,
                    ETS_EFUSE_KEY_PURPOSE_HMAC_UP,
                    key_data, sizeof(key_data));

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

Now we can use the saved key to calculate an HMAC for software usage.

#include "esp_hmac.h"

uint8_t hmac[32];

const char *message = "Hello, HMAC!";
const size_t msg_len = 12;

esp_err_t result = esp_hmac_calculate(HMAC_KEY4, message, msg_len, hmac);

if (result == ESP_OK) {
    // HMAC written to hmac now
} else {
    // failure calculating HMAC
}

API Reference

Functions

esp_err_t esp_hmac_calculate(hmac_key_id_t key_id, const void *message, size_t message_len, uint8_t *hmac)

Calculate the HMAC of a given message.

Calculate the HMAC hmac of a given message message with length message_len. SHA256 is used for the calculation (fixed on ESP32S2).

Note

Uses the HMAC peripheral in “upstream” mode.

Return

  • ESP_OK, if the calculation was successful,

  • ESP_FAIL, if the hmac calculation failed

Parameters
  • key_id: Determines which of the 6 key blocks in the efuses should be used for the HMAC calcuation. The corresponding purpose field of the key block in the efuse must be set to the HMAC upstream purpose value.

  • message: the message for which to calculate the HMAC

  • message_len: message length return ESP_ERR_INVALID_STATE if unsuccessful

  • [out] hmac: the hmac result; the buffer behind the provided pointer must be 32 bytes long

esp_err_t esp_hmac_jtag_enable(hmac_key_id_t key_id, const uint8_t *token)

Use HMAC peripheral in Downstream mode to re-enable the JTAG, if it is not permanently disable by HW. In downstream mode HMAC calculations perfomred by peripheral used internally and not provided back to user.

Return

  • ESP_OK, if the calculation was successful, if the calculated HMAC value matches with provided token, JTAG will be re-enable otherwise JTAG will remain disabled. Return value does not indicate the JTAG status.

  • ESP_FAIL, if the hmac calculation failed or JTAG is permanently disabled by EFUSE_HARD_DIS_JTAG eFuse parameter.

  • ESP_ERR_INVALID_ARG, invalid input arguments

Parameters
  • key_id: Determines which of the 6 key blocks in the efuses should be used for the HMAC calculation. The corresponding purpose field of the key block in the efuse must be set to HMAC downstream purpose.

  • token: Pre calculated HMAC value of the 32-byte 0x00 using SHA-256 and the known private HMAC key. The key is already programmed to a eFuse key block. The key block number is provided as the first parameter to this function.

esp_err_t esp_hmac_jtag_disable(void)

Disable the JTAG which might be enable using the HMAC downstream mode. This function just clear the result generated by JTAG key by calling esp_hmac_jtag_enable() API.

Return

  • ESP_OK return ESP_OK after writing the HMAC_SET_INVALIDATE_JTAG_REG with value 1.

Enumerations

enum hmac_key_id_t

The possible efuse keys for the HMAC peripheral

Values:

HMAC_KEY0 = 0
HMAC_KEY1
HMAC_KEY2
HMAC_KEY3
HMAC_KEY4
HMAC_KEY5
HMAC_KEY_MAX