Mix input to randLIB_add_seed calls

Don't just xor in the 64-bit number given, splitmix64 it across the entire
state. Also, make sure we don't accidentally set the state to all-zero.

RANDLIB_ASSUME_GOOD_SEED dropped - we now assume splitmix64 will be
present for the seed additions.

(Note that add_seed will be somewhat slow on platforms like MSP430 with
slow multipliers - don't call it too much.)

source/randLIB.c

@@ -69,7 +69,6 @@ static inline uint64_t rol(uint64_t n, int bits)
     return (n << bits) | (n >> (64 - bits));
 }
 
-#ifndef RANDLIB_ASSUME_GOOD_SEED
 /* Lower-quality generator used only for initial seeding, if platform
  * isn't returning multiple seeds itself. Multiplies are rather heavy
  * for lower-end platforms, but this is initialisation only.
@@ -81,7 +80,6 @@ static uint64_t splitmix64(uint64_t *seed)
     z = (z ^ (z >> 27)) * UINT64_C(0x94D049BB133111EB);
     return z ^ (z >> 31);
 }
-#endif // RANDLIB_ASSUME_GOOD_SEED
 #endif // RANDOM_DEVICE
 
 void randLIB_seed_random(void)
@@ -94,7 +92,8 @@ void randLIB_seed_random(void)
     arm_random_module_init();
 
     /* We exclusive-OR with the current state, in case they make this call
-     * multiple times. We don't want to potentially lose entropy.
+     * multiple times,or in case someone has called randLIB_add_seed before
+     * this. We don't want to potentially lose entropy.
      */
 
     /* Spell out expressions so we get known ordering of 4 seed calls */
@@ -104,30 +103,24 @@ void randLIB_seed_random(void)
     s = (uint64_t) arm_random_seed_get() << 32;
     state[1] ^= s | arm_random_seed_get();
 
-#ifdef RANDLIB_ASSUME_GOOD_SEED
-    /* Can avoid significant code overhead of splitmix64(), but we do still
-     * have to check for the theoretically possible illegal case of all-zero.
-     */
-    if (state[0] == 0 && state[1] == 0) {
-        state[1] = 1;
-    }
-#else
     /* This check serves to both to stir the state if the platform is returning
      * constant seeding values, and to avoid the illegal all-zero state.
      */
     if (state[0] == state[1]) {
-        uint64_t seed = state[0];
+        randLIB_add_seed(state[0]);
-        state[0] = splitmix64(&seed);
-        state[1] = splitmix64(&seed);
     }
-#endif // RANDLIB_ASSUME_GOOD_SEED
 #endif // RANDOM_DEVICE
 }
 
 void randLIB_add_seed(uint64_t seed)
 {
 #ifndef RANDOM_DEVICE
-    state[1] += seed;
+    state[0] ^= splitmix64(&seed);
+    state[1] ^= splitmix64(&seed);
+    /* This is absolutely necessary, but I challenge you to add it to line coverage */
+    if (state[1] == 0 && state[0] == 0) {
+        state[0] = 1;
+    }
 #endif
 }
 

test/mbed-client-randlib/unittest/randlib/test_randlib.c

@@ -111,18 +111,18 @@ bool test_randLIB_get_64bit()
 
     /* If passed all "4" seeds, seeding should detect this
      * and use splitmix64 to create the actual seed
-     * (0x03910b0aab9b37e5, 0x0b309ab13d42b2a6),
+     * (0x03910b0eab9b37e1, 0x0b309ab53d42b2a2),
      * and produce this output:
      */
     static const uint64_t expected4444[] = {
-        UINT64_C(0x0ec1a5bbe8ddea8b),
+        UINT64_C(0x0ec1a5c3e8ddea83),
-        UINT64_C(0x0be710b8fcf5a491),
+        UINT64_C(0x09e710b8faf5a491),
-        UINT64_C(0xb21127f7159348b4),
+        UINT64_C(0xd4102776f79448b4),
-        UINT64_C(0xdf31900d21f92182),
+        UINT64_C(0x5d61988b60091900),
-        UINT64_C(0xd5a797507d94daa9),
+        UINT64_C(0xf6c8a72a9c72cb4b),
-        UINT64_C(0x66a1c5a4fb04be3d),
+        UINT64_C(0xb06923e0cf0f2fb1),
-        UINT64_C(0x259e5385f48353be),
+        UINT64_C(0x24bbed475153f573),
-        UINT64_C(0x5d3e3286cd4eae19)
+        UINT64_C(0xfff0b4bd08c5581f),
     };
 
     if (!test_output(4, false, expected4444)) {