esp-idf/components/esp32c3/esp_hmac.c

// Copyright 2015-2020 Espressif Systems (Shanghai) PTE LTD
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at

//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include <string.h>
#include "driver/periph_ctrl.h"
#include "esp32c3/rom/hmac.h"
#include "esp32c3/rom/ets_sys.h"
#include "esp_hmac.h"
#include "esp_crypto_lock.h"

#include "hal/hmac_hal.h"

#define SHA256_BLOCK_SZ 64
#define SHA256_PAD_SZ 8

/**
 * @brief Apply the HMAC padding without the embedded length.
 *
 * @note This function does not check the data length, it is the responsability of the other functions in this
 * module to make sure that \c data_len is at most SHA256_BLOCK_SZ - 1 so the padding fits in.
 * Otherwise, this function has undefined behavior.
 * Note however, that for the actual HMAC implementation on ESP32C3, the length also needs to be applied at the end
 * of the block. This function alone deosn't do that.
 */
static void write_and_padd(uint8_t *block, const uint8_t *data, uint16_t data_len)
{
    memcpy(block, data, data_len);
    // Apply a one bit, followed by zero bits (refer to the ESP32C3 TRM).
    block[data_len] = 0x80;
    bzero(block + data_len + 1, SHA256_BLOCK_SZ - data_len - 1);
}

esp_err_t esp_hmac_calculate(hmac_key_id_t key_id,
                             const void *message,
                             size_t message_len,
                             uint8_t *hmac)
{
    const uint8_t *message_bytes = (const uint8_t *)message;

    if (!message || !hmac) {
        return ESP_ERR_INVALID_ARG;
    }
    if (key_id >= HMAC_KEY_MAX) {
        return ESP_ERR_INVALID_ARG;
    }

    esp_crypto_hmac_lock_acquire();

    // We also enable SHA and DS here. SHA is used by HMAC, DS will otherwise hold SHA in reset state.
    periph_module_enable(PERIPH_HMAC_MODULE);
    periph_module_enable(PERIPH_SHA_MODULE);
    periph_module_enable(PERIPH_DS_MODULE);

    hmac_hal_start();

    uint32_t conf_error = hmac_hal_configure(HMAC_OUTPUT_USER, key_id);
    if (conf_error) {
        esp_crypto_hmac_lock_release();
        return ESP_FAIL;
    }

    if (message_len + 1 + SHA256_PAD_SZ <= SHA256_BLOCK_SZ) {
        // If message including padding is only one block...
        // Last message block, so apply SHA-256 padding rules in software
        uint8_t block[SHA256_BLOCK_SZ];
        uint64_t bit_len = __builtin_bswap64(message_len * 8 + 512);

        write_and_padd(block, message_bytes, message_len);
        // Final block: append the bit length in this block and signal padding to peripheral
        memcpy(block + SHA256_BLOCK_SZ - sizeof(bit_len),
               &bit_len, sizeof(bit_len));
        hmac_hal_write_one_block_512(block);
    } else {
        // If message including padding is needs more than one block

        // write all blocks without padding except the last one
        size_t remaining_blocks = message_len / SHA256_BLOCK_SZ;
        for (int i = 1; i < remaining_blocks; i++) {
            hmac_hal_write_block_512(message_bytes);
            message_bytes += SHA256_BLOCK_SZ;
            hmac_hal_next_block_normal();
        }

        // If message fits into one block but without padding, we must not write another block.
        if (remaining_blocks) {
            hmac_hal_write_block_512(message_bytes);
            message_bytes += SHA256_BLOCK_SZ;
        }

        size_t remaining = message_len % SHA256_BLOCK_SZ;
        // Last message block, so apply SHA-256 padding rules in software
        uint8_t block[SHA256_BLOCK_SZ];
        uint64_t bit_len = __builtin_bswap64(message_len * 8 + 512);

        // If the remaining message and appended padding doesn't fit into a single block, we have to write an
        // extra block with the rest of the message and potential padding first.
        if (remaining >= SHA256_BLOCK_SZ - SHA256_PAD_SZ) {
            write_and_padd(block, message_bytes, remaining);
            hmac_hal_next_block_normal();
            hmac_hal_write_block_512(block);
            bzero(block, SHA256_BLOCK_SZ);
        } else {
            write_and_padd(block, message_bytes, remaining);
        }
        memcpy(block + SHA256_BLOCK_SZ - sizeof(bit_len),
               &bit_len, sizeof(bit_len));
        hmac_hal_next_block_padding();
        hmac_hal_write_block_512(block);
    }

    // Read back result (bit swapped)
    hmac_hal_read_result_256(hmac);

    periph_module_disable(PERIPH_DS_MODULE);
    periph_module_disable(PERIPH_SHA_MODULE);
    periph_module_disable(PERIPH_HMAC_MODULE);

    esp_crypto_hmac_lock_release();

    return ESP_OK;
}