Remove EEPROM address argument from RNG.begin()

Always store the seed at the very end of EEPROM memory.

doc/crypto-rng.dox

@@ -133,8 +133,8 @@ all of the application's noise sources:
 \code
 void setup() {
     // Initialize the random number generator with the application tag
-    // "MyApp 1.0" and load the previous seed from EEPROM address 950.
-    RNG.begin("MyApp 1.0", 950);
+    // "MyApp 1.0" and load the previous seed from EEPROM.
+    RNG.begin("MyApp 1.0");
 
     // Add the noise source to the list of sources known to RNG.
     RNG.addNoiseSource(noise);
@@ -143,17 +143,16 @@ void setup() {
 }
 \endcode
 
-The begin() function is passed two arguments: a tag string that should
-be different for every application and an EEPROM address to use to
-load and save the random number seed.  The tag string ensures that
-different applications and versions will generate different random numbers
-upon first boot before the noise source has collected any entropy.
-If the device also has a unique serial number or a MAC address, then
-those can be mixed in during the setup() function after calling begin():
+The begin() function is passed a tag string that should be different for
+every application.  The tag string ensures that different applications and
+versions will generate different random numbers upon first boot before
+the noise source has collected any entropy.  If the device also has a unique
+serial number or a MAC address, then those can be mixed in during the
+setup() function after calling begin():
 
 \code
 void setup() {
-    RNG.begin("MyApp 1.0", 950);
+    RNG.begin("MyApp 1.0");
     RNG.stir(serial_number, sizeof(serial_number));
     RNG.stir(mac_address, sizeof(mac_address));
     RNG.addNoiseSource(noise);
@@ -161,8 +160,8 @@ void setup() {
 }
 \endcode
 
-The random number generator needs 49 bytes of EEPROM space at the
-specified address to store the previous seed.  When the system is started
+The random number generator uses 49 bytes of space at the end of
+EEPROM memory to store the previous seed.  When the system is started
 next time, the previous saved seed is loaded and then deliberately
 overwritten with a new seed.  This ensures that the device will not
 accidentally generate the same sequence of random numbers if it is

host/Crypto/RNG_host.cpp

@@ -57,7 +57,7 @@ RNGClass::~RNGClass()
     clean(stream);
 }
 
-void RNGClass::begin(const char *tag, int eepromAddress)
+void RNGClass::begin(const char *tag)
 {
 }
 

libraries/Crypto/RNG.cpp

@@ -35,6 +35,8 @@
 #define RNG_WATCHDOG 1      // Harvest entropy from watchdog jitter.
 #include <avr/eeprom.h>
 #include <avr/wdt.h>
+#include <avr/io.h>
+#define RNG_EEPROM_ADDRESS (E2END + 1 - RNGClass::SEED_SIZE)
 #endif
 #include <string.h>
 
@@ -73,8 +75,8 @@
  *     Ethernet.begin(mac_address);
  *
  *     // Initialize the random number generator with the application tag
- *     // "MyApp 1.0" and load the previous seed from EEPROM address 950.
- *     RNG.begin("MyApp 1.0", 950);
+ *     // "MyApp 1.0" and load the previous seed from EEPROM.
+ *     RNG.begin("MyApp 1.0");
  *
  *     // Stir in the Ethernet MAC address.
  *     RNG.stir(mac_address, sizeof(mac_address));
@@ -101,8 +103,8 @@
  * \endcode
  *
  * The loop() function will automatically save the random number seed on a
- * regular basis.  By default the seed is saved every hour but this can be
- * changed using setAutoSaveTime().
+ * regular basis to the last SEED_SIZE bytes of EEPROM memory.  By default
+ * the seed is saved every hour but this can be changed using setAutoSaveTime().
  *
  * Keep in mind that saving too often may cause the EEPROM to wear out quicker.
  * It is wise to limit saving to once an hour or once a day depending
@@ -136,6 +138,9 @@ RNGClass RNG;
 /**
  * \var RNGClass::SEED_SIZE
  * \brief Size of a saved random number seed in EEPROM space.
+ *
+ * The seed is saved into the last SEED_SIZE bytes of EEPROM memory.
+ * The address is dependent upon the size of EEPROM fitted in the device.
  */
 
 // Number of ChaCha hash rounds to use for random number generation.
@@ -228,8 +233,7 @@ ISR(WDT_vect)
  * \sa begin()
  */
 RNGClass::RNGClass()
-    : address(0)
-    , credits(0)
+    : credits(0)
     , firstSave(1)
     , timer(0)
     , timeout(3600000UL)    // 1 hour in milliseconds
@@ -344,27 +348,19 @@ static void eraseAndWriteSeed()
  * usually this should be a value that is unique to the application and
  * version such as "MyApp 1.0" so that different applications do not
  * generate the same sequence of values upon first boot.
- * \param eepromAddress The EEPROM address to load the previously saved
- * seed from and to save new seeds when save() is called.  There must be
- * at least SEED_SIZE (49) bytes of EEPROM space available at the address.
  *
  * This function should be followed by calls to addNoiseSource() to
  * register the application's noise sources.
  *
- * The \a eepromAddress is ignored on the Arduino Due.  The seed is instead
- * stored in the last page of system flash memory.
- *
  * \sa addNoiseSource(), stir(), save()
  */
-void RNGClass::begin(const char *tag, int eepromAddress)
+void RNGClass::begin(const char *tag)
 {
-    // Save the EEPROM address for use by save().
-    address = eepromAddress;
-
     // Initialize the ChaCha20 input block from the saved seed.
     memcpy_P(block, tagRNG, sizeof(tagRNG));
     memcpy_P(block + 4, initRNG, sizeof(initRNG));
 #if defined(RNG_EEPROM)
+    int address = RNG_EEPROM_ADDRESS;
     if (eeprom_read_byte((const uint8_t *)address) == 'S') {
         // We have a saved seed: XOR it with the initialization block.
         for (int posn = 0; posn < 12; ++posn) {
@@ -691,6 +687,7 @@ void RNGClass::save()
     ++(block[12]);
     ChaCha::hashCore(stream, block, RNG_ROUNDS);
 #if defined(RNG_EEPROM)
+    int address = RNG_EEPROM_ADDRESS;
     eeprom_write_block(stream, (void *)(address + 1), 48);
     eeprom_update_byte((uint8_t *)address, 'S');
 #elif defined(RNG_DUE_TRNG)
@@ -808,6 +805,7 @@ void RNGClass::destroy()
     clean(block);
     clean(stream);
 #if defined(RNG_EEPROM)
+    int address = RNG_EEPROM_ADDRESS;
     for (int posn = 0; posn < SEED_SIZE; ++posn)
         eeprom_write_byte((uint8_t *)(address + posn), 0xFF);
 #elif defined(RNG_DUE_TRNG)

libraries/Crypto/RNG.h

@@ -34,7 +34,7 @@ public:
     RNGClass();
     ~RNGClass();
 
-    void begin(const char *tag, int eepromAddress);
+    void begin(const char *tag);
     void addNoiseSource(NoiseSource &source);
 
     void setAutoSaveTime(uint16_t minutes);
@@ -55,7 +55,6 @@ public:
 private:
     uint32_t block[16];
     uint32_t stream[16];
-    int address;
     uint16_t credits : 15;
     uint16_t firstSave : 1;
     unsigned long timer;

libraries/Crypto/examples/TestCurve25519/TestCurve25519.ino

@@ -207,7 +207,7 @@ void setup()
     // Start the random number generator.  We don't initialise a noise
     // source here because we don't need one for testing purposes.
     // Real DH applications should of course use a proper noise source.
-    RNG.begin("TestCurve25519 1.0", 950);
+    RNG.begin("TestCurve25519 1.0");
 
     // Perform the tests.
     testEval();

libraries/Crypto/examples/TestEd25519/TestEd25519.ino

@@ -173,7 +173,7 @@ void setup()
     // Start the random number generator.  We don't initialise a noise
     // source here because we don't need one for testing purposes.
     // Real applications should of course use a proper noise source.
-    RNG.begin("TestEd25519 1.0", 950);
+    RNG.begin("TestEd25519 1.0");
 
     // Perform the tests.
     testFixedVectors();

libraries/Crypto/examples/TestP521/TestP521.ino

@@ -503,7 +503,7 @@ void setup()
     // Start the random number generator.  We don't initialise a noise
     // source here because we don't need one for testing purposes.
     // Real DH applications should of course use a proper noise source.
-    RNG.begin("TestP521 1.0", 950);
+    RNG.begin("TestP521 1.0");
 
     // Perform the tests.
     testEval();

libraries/Crypto/examples/TestRNG/TestRNG.ino

@@ -11,9 +11,6 @@
 // even if the input noise or seed data is otherwise identical.
 #define RNG_APP_TAG "MyApp 1.0"
 
-// EEPROM address to save the random number seed at.
-#define RNG_EEPROM_ADDRESS 950
-
 // Noise source to seed the random number generator.
 TransistorNoiseSource noise(A1);
 //RingOscillatorNoiseSource noise;
@@ -28,7 +25,7 @@ void setup() {
     Serial.println("start");
 
     // Initialize the random number generator.
-    RNG.begin(RNG_APP_TAG, RNG_EEPROM_ADDRESS);
+    RNG.begin(RNG_APP_TAG);
 
     // Add the noise source to the list of sources known to RNG.
     RNG.addNoiseSource(noise);

libraries/NewHope/examples/TestNewHope/TestNewHope.ino

@@ -908,7 +908,7 @@ void setup()
     // Start the random number generator.  We don't initialise a noise
     // source here because we don't need one for testing purposes.
     // Real applications should of course use a proper noise source.
-    RNG.begin("TestNewHope 1.0", 950);
+    RNG.begin("TestNewHope 1.0");
 
     // Perform the tests.
     Serial.println();