manually specialize the CSPRNG cache

random.c

@@ -48,33 +48,24 @@ void random_state_init(struct random_state *state) {
     state->reseed = 0;
 }
 
-void get_random_bytes(struct random_state *state, void *buf, size_t size) {
-    if (size > RANDOM_CACHE_SIZE / 2) {
+static void refill(struct random_state *state) {
+    if (state->reseed < RANDOM_RESEED_SIZE) {
         chacha_keystream_bytes(&state->ctx, state->cache, RANDOM_CACHE_SIZE);
-        return;
-    }
-
-    while (size) {
-        if (state->index == RANDOM_CACHE_SIZE) {
-            if (state->reseed >= RANDOM_RESEED_SIZE) {
-                random_state_init(state);
-            } else {
-                chacha_keystream_bytes(&state->ctx, state->cache, RANDOM_CACHE_SIZE);
-                state->index = 0;
-                state->reseed += RANDOM_CACHE_SIZE;
-            }
-        }
-        size_t remaining = RANDOM_CACHE_SIZE - state->index;
-        size_t copy_size = size < remaining ? size : remaining;
-        memcpy(buf, state->cache + state->index, copy_size);
-        state->index += copy_size;
-        size -= copy_size;
+        state->index = 0;
+        state->reseed += RANDOM_CACHE_SIZE;
+    } else {
+        random_state_init(state);
     }
 }
 
 uint16_t get_random_u16(struct random_state *state) {
     uint16_t value;
-    get_random_bytes(state, &value, sizeof(value));
+    size_t remaining = RANDOM_CACHE_SIZE - state->index;
+    if (remaining < sizeof(value)) {
+        refill(state);
+    }
+    memcpy(&value, state->cache + state->index, sizeof(value));
+    state->index += sizeof(value);
     return value;
 }
 
@@ -89,14 +80,19 @@ uint16_t get_random_u16_uniform(struct random_state *state, uint16_t bound) {
     uint16_t r;
     do {
         r = get_random_u16(state);
-    } while (r < min);
+    } while (unlikely(r < min));
 
     return r % bound;
 }
 
 uint64_t get_random_u64(struct random_state *state) {
     uint64_t value;
-    get_random_bytes(state, &value, sizeof(value));
+    size_t remaining = RANDOM_CACHE_SIZE - state->index;
+    if (remaining < sizeof(value)) {
+        refill(state);
+    }
+    memcpy(&value, state->cache + state->index, sizeof(value));
+    state->index += sizeof(value);
     return value;
 }
 
@@ -111,7 +107,7 @@ uint64_t get_random_u64_uniform(struct random_state *state, uint64_t bound) {
     uint64_t r;
     do {
         r = get_random_u64(state);
-    } while (r < min);
+    } while (unlikely(r < min));
 
     return r % bound;
 }

random.h

@@ -16,7 +16,6 @@ struct random_state {
 };
 
 void random_state_init(struct random_state *state);
-void get_random_bytes(struct random_state *state, void *buf, size_t size);
 uint16_t get_random_u16(struct random_state *state);
 uint16_t get_random_u16_uniform(struct random_state *state, uint16_t bound);
 uint64_t get_random_u64(struct random_state *state);