Crypt module (#549)

- Moved cryptography code out of Tactility project and into a kernel module.
- Converted C++ code to C style interface
- Hardened security

Modules/crypt-module/include/tactility/crypt.h

@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: Apache-2.0
+/** @file crypt.h
+ *
+ * @brief Encryption helper functions.
+ *
+ * Offers AES 256 CBC encryption with built-in key.
+ * The key is built from data including:
+ *  - the internal factory MAC address
+ *  - random data stored in NVS
+ *
+ * It's important to use flash encryption to avoid an attacker to get
+ * access to your encrypted data. If flash encryption is disabled,
+ * someone can fetch the key from the partitions.
+ *
+ * See:
+ * https://docs.espressif.com/projects/esp-idf/en/latest/esp32/security/secure-boot-v2.html
+ * https://docs.espressif.com/projects/esp-idf/en/latest/esp32/security/flash-encryption.html
+ */
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Deterministically derives an IV from the given data (the first 16 bytes of its SHA-256 hash).
+ *
+ * Calling this again with the same data always produces the same IV - use this when there's nowhere
+ * to store a per-encryption IV alongside the ciphertext, and the caller can supply the same associated
+ * data (e.g. an identifier that's known at both encrypt and decrypt time, not the secret itself) on both
+ * ends. Because the IV doesn't change between encryptions with the same associated data, encrypting the
+ * same plaintext twice produces the same ciphertext - prefer crypt_generate_iv() when ciphertext can be
+ * stored alongside a random IV instead.
+ *
+ * @param[in] data input data
+ * @param[in] dataLength input data length
+ * @param[out] iv output IV
+ */
+void crypt_get_iv(const void* data, size_t dataLength, uint8_t iv[16]);
+
+/**
+ * @brief Fills the IV with cryptographically secure random bytes.
+ *
+ * Use this when the IV can be stored alongside the ciphertext (e.g. prefixed to it) and read back for
+ * decryption. This gives every encryption operation a unique, unpredictable IV, which is the standard
+ * and strongest way to use crypt_encrypt()/crypt_decrypt().
+ *
+ * @param[out] iv output IV
+ */
+void crypt_generate_iv(uint8_t iv[16]);
+
+/**
+ * @brief Encrypt data.
+ *
+ * Important: Use flash encryption to increase security.
+ * Important: input and output data must be aligned to 16 bytes.
+ *
+ * @param[in] iv the AES IV
+ * @param[in] inData input data
+ * @param[out] outData output data
+ * @param[in] dataLength data length, a multiple of 16 (for both inData and outData)
+ * @return the result of esp_aes_crypt_cbc() (MBEDTLS_ERR_*), or -1 if dataLength is not a positive multiple of 16
+ */
+int crypt_encrypt(const uint8_t iv[16], const uint8_t* inData, uint8_t* outData, size_t dataLength);
+
+/**
+ * @brief Decrypt data.
+ *
+ * Important: Use flash encryption to increase security.
+ * Important: input and output data must be aligned to 16 bytes.
+ *
+ * @param[in] iv AES IV
+ * @param[in] inData input data
+ * @param[out] outData output data
+ * @param[in] dataLength data length, a multiple of 16 (for both inData and outData)
+ * @return the result of esp_aes_crypt_cbc() (MBEDTLS_ERR_*), or -1 if dataLength is not a positive multiple of 16
+ */
+int crypt_decrypt(const uint8_t iv[16], const uint8_t* inData, uint8_t* outData, size_t dataLength);
+
+#ifdef __cplusplus
+}
+#endif

Modules/crypt-module/include/tactility/crypt_module.h

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: Apache-2.0
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern struct Module crypt_module;
+
+#ifdef __cplusplus
+}
+#endif

Modules/crypt-module/include/tactility/hash.h

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: Apache-2.0
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Implementation of DJB2 hashing algorithm.
+ * @param[in] str the string to calculate the hash for
+ * @return the hash
+ */
+uint32_t djb2_str(const char* str);
+
+/**
+ * Implementation of DJB2 hashing algorithm.
+ * @param[in] data the bytes to calculate the hash for
+ * @param[in] length the size of data
+ * @return the hash
+ */
+uint32_t djb2_data(const void* data, size_t length);
+
+#ifdef __cplusplus
+}
+#endif