Crypt module (#549)
- Moved cryptography code out of Tactility project and into a kernel module. - Converted C++ code to C style interface - Hardened security
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Ken Van Hoeylandt
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GitHub
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// SPDX-License-Identifier: Apache-2.0
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#include <tactility/crypt.h>
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#include <tactility/check.h>
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#include <tactility/log.h>
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#include <mbedtls/aes.h>
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#include <mbedtls/platform_util.h>
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#include <mbedtls/sha256.h>
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#include <cstring>
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#include <cstdint>
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#ifdef ESP_PLATFORM
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#include "esp_mac.h"
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#include "esp_random.h"
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#include "nvs_flash.h"
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#else
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#include <random>
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#endif
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constexpr auto* TAG = "crypt";
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#define TT_NVS_NAMESPACE "tt_secure"
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/**
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* Fills a buffer with cryptographically secure random bytes.
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* @param[out] out output buffer
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* @param[in] length number of bytes to fill
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*/
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static void fill_random(uint8_t* out, size_t length) {
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#ifdef ESP_PLATFORM
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esp_fill_random(out, length);
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#else
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static std::random_device rd;
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static std::mt19937 gen(rd());
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std::uniform_int_distribution<int> dist(0, 255);
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for (size_t i = 0; i < length; ++i) {
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out[i] = static_cast<uint8_t>(dist(gen));
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}
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#endif
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}
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#ifdef ESP_PLATFORM
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/**
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* Get a key based on hardware parameters.
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* @param[out] key the output key
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*/
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static void get_hardware_key(uint8_t key[32]) {
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uint8_t mac[8];
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// MAC can be 6 or 8 bytes
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size_t mac_length = esp_mac_addr_len_get(ESP_MAC_EFUSE_FACTORY);
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LOG_I(TAG, "Using MAC with length %zu", mac_length);
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check(mac_length <= 8);
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ESP_ERROR_CHECK(esp_read_mac(mac, ESP_MAC_EFUSE_FACTORY));
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// Fill buffer with repeating MAC
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for (size_t i = 0; i < 32; ++i) {
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key[i] = mac[i % mac_length];
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}
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}
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#endif
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#ifdef ESP_PLATFORM
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/**
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* The key is built up as follows:
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* - Fetch 32 bytes from NVS storage and store as key data
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* - Fetch 6-8 MAC bytes and overwrite the first 6-8 bytes of the key with this info
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*
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* When flash encryption is disabled:
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* Without the MAC data, an attack would look like this:
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* - Retrieve all the partitions from the ESP32
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* - Read the key from NVS flash
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* - Use the key to decrypt
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* With the MAC data added, an attacker would have to do much more:
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* - Retrieve all the partitions from the ESP32 (copy app)
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* - Upload custom app to retrieve internal MAC
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* - Read the key from NVS flash
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* - Re-flash original app and combine it with the MAC
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* - Use the key to decrypt
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* - Re-flash the device with original firmware.
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*
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* Adding the MAC doesn't add a lot of extra security, but I think it's worth it.
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*
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* @param[out] key the output key
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*/
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static void get_nvs_key(uint8_t key[32]) {
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nvs_handle_t handle;
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esp_err_t result = nvs_open(TT_NVS_NAMESPACE, NVS_READWRITE, &handle);
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if (result != ESP_OK) {
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LOG_E(TAG, "Failed to get key from NVS (%s)", esp_err_to_name(result));
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check(false, "NVS error");
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}
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size_t length = 32;
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if (nvs_get_blob(handle, "key", key, &length) == ESP_OK) {
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LOG_I(TAG, "Fetched key from NVS (%zu bytes)", length);
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check(length == 32);
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} else {
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fill_random(key, 32);
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ESP_ERROR_CHECK(nvs_set_blob(handle, "key", key, 32));
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ESP_ERROR_CHECK(nvs_commit(handle));
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LOG_I(TAG, "Stored new key in NVS");
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}
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nvs_close(handle);
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}
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#endif
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/**
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* Performs XOR on 2 memory regions and stores it in a third
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* @param[in] inLeft input buffer for XOR
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* @param[in] inRight second input buffer for XOR
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* @param[out] out output buffer for result of XOR
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* @param[in] length data length (all buffers must be at least this size)
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*/
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static void xorKey(const uint8_t* inLeft, const uint8_t* inRight, uint8_t* out, size_t length) {
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for (size_t i = 0; i < length; ++i) {
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out[i] = inLeft[i] ^ inRight[i];
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}
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}
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/**
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* Combines a stored key and a hardware key into a single reliable key value.
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* @param[out] key the key output
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*/
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static void getKey(uint8_t key[32]) {
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#if !defined(CONFIG_SECURE_BOOT) || !defined(CONFIG_SECURE_FLASH_ENC_ENABLED)
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LOG_W(TAG, "Using tt_secure_* code with secure boot and/or flash encryption disabled.");
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LOG_W(TAG, "An attacker with physical access to your ESP32 can decrypt your secure data.");
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#endif
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#ifdef ESP_PLATFORM
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uint8_t hardware_key[32];
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uint8_t nvs_key[32];
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get_hardware_key(hardware_key);
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get_nvs_key(nvs_key);
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xorKey(hardware_key, nvs_key, key, 32);
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mbedtls_platform_zeroize(hardware_key, sizeof(hardware_key));
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mbedtls_platform_zeroize(nvs_key, sizeof(nvs_key));
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#else
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LOG_W(TAG, "Using unsafe key for debugging purposes.");
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memset(key, 0, 32);
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#endif
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}
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void crypt_get_iv(const void* data, size_t dataLength, uint8_t iv[16]) {
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uint8_t hash[32];
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mbedtls_sha256(static_cast<const unsigned char*>(data), dataLength, hash, 0);
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memcpy(iv, hash, 16);
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mbedtls_platform_zeroize(hash, sizeof(hash));
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}
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void crypt_generate_iv(uint8_t iv[16]) {
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fill_random(iv, 16);
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}
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static int aes256CryptCbc(
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const uint8_t key[32],
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int mode,
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size_t length,
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unsigned char iv[16],
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const unsigned char* input,
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unsigned char* output
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) {
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check(key && iv && input && output);
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if ((length % 16) || (length == 0)) {
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return -1; // TODO: Proper error code from mbed lib?
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}
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mbedtls_aes_context master;
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mbedtls_aes_init(&master);
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if (mode == MBEDTLS_AES_ENCRYPT) {
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mbedtls_aes_setkey_enc(&master, key, 256);
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} else {
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mbedtls_aes_setkey_dec(&master, key, 256);
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}
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int result = mbedtls_aes_crypt_cbc(&master, mode, length, iv, input, output);
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mbedtls_aes_free(&master);
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return result;
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}
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int crypt_encrypt(const uint8_t iv[16], const uint8_t* inData, uint8_t* outData, size_t dataLength) {
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uint8_t key[32];
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getKey(key);
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// TODO: Is this still needed after switching to regular AES functions?
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uint8_t iv_copy[16];
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memcpy(iv_copy, iv, sizeof(iv_copy));
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int result = aes256CryptCbc(key, MBEDTLS_AES_ENCRYPT, dataLength, iv_copy, inData, outData);
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mbedtls_platform_zeroize(key, sizeof(key));
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mbedtls_platform_zeroize(iv_copy, sizeof(iv_copy));
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return result;
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}
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int crypt_decrypt(const uint8_t iv[16], const uint8_t* inData, uint8_t* outData, size_t dataLength) {
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uint8_t key[32];
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getKey(key);
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// TODO: Is this still needed after switching to regular AES functions?
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uint8_t iv_copy[16];
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memcpy(iv_copy, iv, sizeof(iv_copy));
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int result = aes256CryptCbc(key, MBEDTLS_AES_DECRYPT, dataLength, iv_copy, inData, outData);
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mbedtls_platform_zeroize(key, sizeof(key));
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mbedtls_platform_zeroize(iv_copy, sizeof(iv_copy));
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return result;
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}
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