Security Guidelines
===================

Security best practices for the Nexus Embedded Platform.

.. contents:: Table of Contents
   :local:
   :depth: 3

Overview
--------

Security is critical in embedded systems. This guide covers:

* **Input Validation**: Prevent injection attacks
* **Memory Safety**: Prevent buffer overflows
* **Cryptography**: Secure data and communications
* **Access Control**: Protect sensitive resources
* **Secure Boot**: Verify firmware integrity
* **Update Security**: Secure firmware updates

Secure Coding Practices
-----------------------

Input Validation
~~~~~~~~~~~~~~~~

**Always Validate Inputs**

.. code-block:: c

   /* Bad: No validation */
   void process_data(uint8_t* buffer, size_t length) {
       memcpy(internal_buffer, buffer, length);  /* Overflow! */
   }

   /* Good: Validate inputs */
   hal_status_t process_data(const uint8_t* buffer, size_t length) {
       if (buffer == NULL) {
           return HAL_ERROR_PARAM;
       }
       if (length == 0 || length > MAX_BUFFER_SIZE) {
           return HAL_ERROR_PARAM;
       }
       memcpy(internal_buffer, buffer, length);
       return HAL_OK;
   }

**Range Checking**

.. code-block:: c

   /* Validate array indices */
   hal_status_t set_value(size_t index, uint32_t value) {
       if (index >= ARRAY_SIZE) {
           return HAL_ERROR_PARAM;
       }
       array[index] = value;
       return HAL_OK;
   }

**Integer Overflow Checks**

.. code-block:: c

   /* Check for overflow before allocation */
   void* safe_alloc(size_t count, size_t size) {
       if (count > 0 && size > SIZE_MAX / count) {
           return NULL;  /* Would overflow */
       }
       return malloc(count * size);
   }

Buffer Overflow Prevention
~~~~~~~~~~~~~~~~~~~~~~~~~~

**Use Safe String Functions**

.. code-block:: c

   /* Bad: Unsafe string copy */
   void copy_string_unsafe(char* dest, const char* src) {
       strcpy(dest, src);  /* Buffer overflow risk! */
   }

   /* Good: Safe string copy */
   void copy_string_safe(char* dest, size_t dest_size, const char* src) {
       if (dest == NULL || src == NULL || dest_size == 0) {
           return;
       }
       strncpy(dest, src, dest_size - 1);
       dest[dest_size - 1] = '\0';
   }

**Bounds Checking**

.. code-block:: c

   /* Always check buffer bounds */
   hal_status_t write_buffer(uint8_t* buffer, size_t buffer_size,
                             const uint8_t* data, size_t data_size) {
       if (buffer == NULL || data == NULL) {
           return HAL_ERROR_PARAM;
       }
       if (data_size > buffer_size) {
           return HAL_ERROR_NO_MEMORY;
       }
       memcpy(buffer, data, data_size);
       return HAL_OK;
   }

Memory Safety
-------------

Pointer Safety
~~~~~~~~~~~~~~

**Null Pointer Checks**

.. code-block:: c

   /* Always check pointers */
   hal_status_t process_config(const config_t* config) {
       if (config == NULL) {
           return HAL_ERROR_PARAM;
       }
       /* Use config */
       return HAL_OK;
   }

**Avoid Use-After-Free**

.. code-block:: c

   /* Bad: Use after free */
   void bad_example(void) {
       uint8_t* buffer = malloc(256);
       free(buffer);
       buffer[0] = 0;  /* Use after free! */
   }

   /* Good: Clear pointer after free */
   void good_example(void) {
       uint8_t* buffer = malloc(256);
       free(buffer);
       buffer = NULL;  /* Prevent use after free */
   }

**Double-Free Prevention**

.. code-block:: c

   /* Safe free macro */
   #define SAFE_FREE(ptr) \
       do { \
           if (ptr != NULL) { \
               free(ptr); \
               ptr = NULL; \
           } \
       } while(0)

Stack Protection
~~~~~~~~~~~~~~~~

**Stack Canaries**

.. code-block:: c

   #define STACK_CANARY 0xDEADBEEF

   void protected_function(void) {
       uint32_t canary = STACK_CANARY;

       /* Function code */

       /* Check canary */
       if (canary != STACK_CANARY) {
           /* Stack overflow detected */
           handle_security_violation();
       }
   }

Cryptography
------------

Random Number Generation
~~~~~~~~~~~~~~~~~~~~~~~~

**Use Hardware RNG**

.. code-block:: c

   /* Use hardware random number generator */
   uint32_t get_random(void) {
       /* Enable RNG clock */
       RCC->AHB2ENR |= RCC_AHB2ENR_RNGEN;

       /* Enable RNG */
       RNG->CR |= RNG_CR_RNGEN;

       /* Wait for random data */
       while (!(RNG->SR & RNG_SR_DRDY));

       return RNG->DR;
   }

**Seed PRNG Properly**

.. code-block:: c

   /* Seed with hardware RNG */
   void seed_prng(void) {
       uint32_t seed = get_random();
       srand(seed);
   }

Encryption
~~~~~~~~~~

**AES Encryption**

.. code-block:: c

   /* Encrypt data with AES */
   hal_status_t encrypt_data(const uint8_t* plaintext, size_t length,
                             const uint8_t* key, uint8_t* ciphertext) {
       if (plaintext == NULL || key == NULL || ciphertext == NULL) {
           return HAL_ERROR_PARAM;
       }

       /* Initialize AES */
       aes_context_t ctx;
       aes_init(&ctx, key, AES_KEY_SIZE_256);

       /* Encrypt */
       aes_encrypt(&ctx, plaintext, ciphertext, length);

       /* Clear sensitive data */
       memset(&ctx, 0, sizeof(ctx));

       return HAL_OK;
   }

Secure Storage
--------------

Key Storage
~~~~~~~~~~~

**Never Hardcode Keys**

.. code-block:: c

   /* Bad: Hardcoded key */
   const uint8_t encryption_key[] = {
       0x01, 0x02, 0x03, /* ... */
   };

   /* Good: Load from secure storage */
   hal_status_t load_encryption_key(uint8_t* key, size_t key_size) {
       return secure_storage_read(KEY_ID, key, key_size);
   }

**Clear Sensitive Data**

.. code-block:: c

   /* Clear sensitive data after use */
   void process_password(const char* password) {
       /* Use password */
       authenticate(password);

       /* Clear from memory */
       memset((void*)password, 0, strlen(password));
   }

Secure Boot
-----------

Firmware Verification
~~~~~~~~~~~~~~~~~~~~~

**Verify Signature**

.. code-block:: c

   /* Verify firmware signature */
   bool verify_firmware(const uint8_t* firmware, size_t length,
                       const uint8_t* signature) {
       uint8_t hash[32];

       /* Calculate firmware hash */
       sha256(firmware, length, hash);

       /* Verify signature */
       return rsa_verify(hash, sizeof(hash), signature, public_key);
   }

**Secure Boot Flow**

.. code-block:: c

   void secure_boot(void) {
       /* Read firmware */
       uint8_t* firmware = (uint8_t*)FIRMWARE_ADDRESS;
       size_t length = get_firmware_length();

       /* Read signature */
       uint8_t* signature = (uint8_t*)SIGNATURE_ADDRESS;

       /* Verify firmware */
       if (!verify_firmware(firmware, length, signature)) {
           /* Verification failed */
           enter_recovery_mode();
           return;
       }

       /* Jump to application */
       jump_to_application(FIRMWARE_ADDRESS);
   }

Secure Communication
--------------------

TLS/SSL
~~~~~~~

**Use TLS for Network Communication**

.. code-block:: c

   /* Establish secure connection */
   hal_status_t connect_secure(const char* host, uint16_t port) {
       tls_context_t ctx;

       /* Initialize TLS */
       tls_init(&ctx);

       /* Set CA certificate */
       tls_set_ca_cert(&ctx, ca_cert, ca_cert_len);

       /* Connect */
       if (tls_connect(&ctx, host, port) != TLS_OK) {
           return HAL_ERROR;
       }

       return HAL_OK;
   }

Access Control
--------------

Privilege Separation
~~~~~~~~~~~~~~~~~~~~

**Use MPU for Memory Protection**

.. code-block:: c

   /* Configure MPU region */
   void configure_mpu(void) {
       /* Disable MPU */
       MPU->CTRL = 0;

       /* Configure region 0: Flash (read-only) */
       MPU->RBAR = FLASH_BASE | MPU_RBAR_VALID_Msk | 0;
       MPU->RASR = MPU_RASR_ENABLE_Msk |
                   MPU_RASR_SIZE_512KB |
                   MPU_RASR_AP_RO_RO;

       /* Configure region 1: RAM (read-write) */
       MPU->RBAR = RAM_BASE | MPU_RBAR_VALID_Msk | 1;
       MPU->RASR = MPU_RASR_ENABLE_Msk |
                   MPU_RASR_SIZE_128KB |
                   MPU_RASR_AP_RW_RW;

       /* Enable MPU */
       MPU->CTRL = MPU_CTRL_ENABLE_Msk;
   }

Security Checklist
------------------

Development Checklist
~~~~~~~~~~~~~~~~~~~~~

☐ All inputs validated
☐ Buffer bounds checked
☐ No hardcoded secrets
☐ Sensitive data cleared after use
☐ Secure random number generation
☐ Encryption for sensitive data
☐ Firmware signature verification
☐ TLS for network communication
☐ Memory protection enabled
☐ Stack protection enabled

Code Review Checklist
~~~~~~~~~~~~~~~~~~~~~

☐ No buffer overflows
☐ No integer overflows
☐ No format string vulnerabilities
☐ No SQL injection (if applicable)
☐ No command injection
☐ Proper error handling
☐ No information leakage
☐ Secure defaults

See Also
--------

* :doc:`coding_standards` - Coding standards
* :doc:`code_review_guidelines` - Code review
* :doc:`testing` - Security testing

Summary
-------

Key security practices:

* **Validate all inputs**
* **Prevent buffer overflows**
* **Use cryptography correctly**
* **Protect sensitive data**
* **Verify firmware integrity**
* **Use secure communication**
* **Implement access control**

Security must be considered throughout the development lifecycle.