sha204 library extended with CRC calculator

Locking operations are recommended to include CRC
and since sha204 library internally carries the proper
algorithm for this, it has now been refactored to support
both static and incremental calculations needed for both
command marchalling and locking.

Author: fallberg

libraries/sha204/sha204_library.cpp

@@ -50,6 +50,33 @@ uint8_t atsha204Class::getSerialNumber(uint8_t * response)
 	return returnCode;
 }
 
+/*  Calculates CRC16 value of provided data (and optionally including provided existing CRC16 data) 
+  returns the calculated CRC16 value */
+uint16_t atsha204Class::calculateAndUpdateCrc(uint8_t length, uint8_t *data, uint16_t current_crc)
+{
+  uint8_t counter;
+  uint16_t crc_register = current_crc;
+  uint16_t polynom = 0x8005;
+  uint8_t shift_register;
+  uint8_t data_bit, crc_bit;
+
+  for (counter = 0; counter < length; counter++)
+  {
+    for (shift_register = 0x01; shift_register > 0x00; shift_register <<= 1) 
+    {
+      data_bit = (data[counter] & shift_register) ? 1 : 0;
+      crc_bit = crc_register >> 15;
+
+      // Shift CRC to the left by 1.
+      crc_register <<= 1;
+
+      if ((data_bit ^ crc_bit) != 0)
+        crc_register ^= polynom;
+    }
+  }
+  return crc_register;
+}
+
 /* SWI bit bang functions */
 
 void atsha204Class::swi_set_signal_pin(uint8_t is_high)
@@ -772,26 +799,9 @@ uint8_t atsha204Class::sha204m_check_parameters(uint8_t op_code, uint8_t param1,
 
 void atsha204Class::sha204c_calculate_crc(uint8_t length, uint8_t *data, uint8_t *crc) 
 {
-  uint8_t counter;
   uint16_t crc_register = 0;
-  uint16_t polynom = 0x8005;
-  uint8_t shift_register;
-  uint8_t data_bit, crc_bit;
-
-  for (counter = 0; counter < length; counter++)
-  {
-    for (shift_register = 0x01; shift_register > 0x00; shift_register <<= 1) 
-    {
-      data_bit = (data[counter] & shift_register) ? 1 : 0;
-      crc_bit = crc_register >> 15;
-
-      // Shift CRC to the left by 1.
-      crc_register <<= 1;
 
-      if ((data_bit ^ crc_bit) != 0)
-        crc_register ^= polynom;
-    }
-  }
+  crc_register = calculateAndUpdateCrc(length, data, crc_register);
   crc[0] = (uint8_t) (crc_register & 0x00FF);
   crc[1] = (uint8_t) (crc_register >> 8);
 }

libraries/sha204/sha204_library.h

@@ -314,6 +314,7 @@ class atsha204Class
 			uint8_t tx_size, uint8_t *tx_buffer, uint8_t rx_size, uint8_t *rx_buffer);
 
 	uint8_t getSerialNumber(uint8_t *response);
+	uint16_t calculateAndUpdateCrc(uint8_t length, uint8_t *data, uint16_t current_crc);
 
 };