Diff: crypt.h

Revision:

0:027b7829b46a

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/crypt.h	Fri Jan 19 02:38:38 2018 +0000
@@ -0,0 +1,150 @@
+#include "nrf_soc.h"
+#include "nrf_delay.h"
+#include "app_error.h"
+
+#define ECB_KEY_LEN            (16UL)
+#define COUNTER_BYTE_LEN       (4UL)
+#define NONCE_RAND_BYTE_LEN    (12UL)
+
+// The RNG wait values are typical and not guaranteed. See Product Specifications for more info.
+#ifdef NRF51
+#define RNG_BYTE_WAIT_US       (677UL)
+#elif defined NRF52
+#define RNG_BYTE_WAIT_US       (124UL)
+#else
+#error "Either NRF51 or NRF52 must be defined."
+#endif
+
+/**
+ * @brief Uses the RNG to write a 12-byte nonce to a buffer
+ * @details The 12 bytes will be written to the buffer starting at index 4 to leave
+ *          space for the 4-byte counter value.
+ *
+ * @param[in]    p_buf    An array of length 16
+ */
+void nonce_generate(uint8_t * p_buf)
+{
+    uint8_t i         = COUNTER_BYTE_LEN;
+    uint8_t remaining = NONCE_RAND_BYTE_LEN;
+
+    // The random number pool may not contain enough bytes at the moment so
+    // a busy wait may be necessary.
+    while(0 != remaining)
+    {
+        uint32_t err_code;
+        uint8_t  available = 0;
+
+        err_code = sd_rand_application_bytes_available_get(&available);
+        APP_ERROR_CHECK(err_code);
+
+        available = ((available > remaining) ? remaining : available);
+        if (0 != available)
+        {
+            err_code = sd_rand_application_vector_get((p_buf + i), available);
+            APP_ERROR_CHECK(err_code);
+
+            i         += available;
+            remaining -= available;
+        }
+
+        if (0 != remaining)
+        {
+            nrf_delay_us(RNG_BYTE_WAIT_US * remaining);
+        }
+    }    
+}
+
+static bool m_initialized = false;
+
+// NOTE: The ECB data must be located in RAM or a HardFault will be triggered.
+static nrf_ecb_hal_data_t m_ecb_data;
+
+/**
+ * @brief Initializes the module with the given nonce and key
+ * @details The nonce will be copied to an internal buffer so it does not need to
+ *          be retained after the function returns. Additionally, a 32-bit counter
+ *          will be initialized to zero and placed into the least-significant 4 bytes
+ *          of the internal buffer. The nonce value should be generated in a
+ *          reasonable manner (e.g. using this module's nonce_generate function).
+ *
+ * @param[in]    p_nonce    An array of length 16 containing 12 random bytes
+ *                          starting at index 4
+ * @param[in]    p_ecb_key  An array of length 16 containing the ECB key
+ */
+void ctr_init(const uint8_t * p_nonce, const uint8_t * p_ecb_key)
+{
+    m_initialized = true;
+
+    // Save the key.
+    memcpy(&m_ecb_data.key[0], p_ecb_key, ECB_KEY_LEN);
+
+    // Copy the nonce.
+    memcpy(&m_ecb_data.cleartext[COUNTER_BYTE_LEN],
+              &p_nonce[COUNTER_BYTE_LEN],
+              NONCE_RAND_BYTE_LEN);
+
+    // Zero the counter value.
+    memset(&m_ecb_data.cleartext[0], 0x00, COUNTER_BYTE_LEN);
+}
+
+static uint32_t crypt(uint8_t * buf)
+{
+    uint8_t  i;
+    uint32_t err_code;
+
+    if (!m_initialized)
+    {
+        return NRF_ERROR_INVALID_STATE;
+    }
+
+    err_code = sd_ecb_block_encrypt(&m_ecb_data);
+    if (NRF_SUCCESS != err_code)
+    {
+        return err_code;
+    }
+
+    for (i=0; i < ECB_KEY_LEN; i++)
+    {
+        buf[i] ^= m_ecb_data.ciphertext[i];
+    }
+
+    // Increment the counter.
+    (*((uint32_t*) m_ecb_data.cleartext))++;
+
+    return NRF_SUCCESS;
+}
+
+/**
+ * @brief Encrypts the given buffer in-situ
+ * @details The encryption step is done separately (using the nonce, counter, and
+ *          key) and then the result from the encryption is XOR'd with the given
+ *          buffer in-situ. The counter will be incremented only if no error occurs.
+ *
+ * @param[in]    p_clear_text    An array of length 16 containing the clear text
+ *
+ * @retval    NRF_SUCCESS                         Success
+ * @retval    NRF_ERROR_INVALID_STATE             Module has not been initialized
+ * @retval    NRF_ERROR_SOFTDEVICE_NOT_ENABLED    SoftDevice is present, but not enabled
+ */
+uint32_t ctr_encrypt(uint8_t * p_clear_text)
+{
+    return crypt(p_clear_text);
+}
+
+/**
+ * @brief Decrypts the given buffer in-situ
+ * @details The encryption step is done separately (using the nonce, counter, and
+ *          key) and then the result from the encryption is XOR'd with the given
+ *          buffer in-situ. The counter will be incremented only if no error occurs.
+ *
+ * @param[in]    p_cipher_text    An array of length 16 containing the cipher text
+ *
+ * @retval    NRF_SUCCESS                         Succeess
+ * @retval    NRF_ERROR_INVALID_STATE             Module has not been initialized
+ * @retval    NRF_ERROR_SOFTDEVICE_NOT_ENABLED    SoftDevice is present, but not enabled
+ */
+uint32_t ctr_decrypt(uint8_t * p_cipher_text)
+{
+    return crypt(p_cipher_text);
+}
+