From daa44905eed56f240d7ec685a8bc229895b20ec2 Mon Sep 17 00:00:00 2001
From: Daniel Micay <danielmicay@gmail.com>
Date: Thu, 23 Aug 2018 16:30:44 -0400
Subject: [PATCH] implement a cache for the CSPRNG

---
 random.c | 24 ++++++++++++++++++++----
 random.h |  7 ++++++-
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/random.c b/random.c
index 3fa1096..0cbb74d 100644
--- a/random.c
+++ b/random.c
@@ -1,4 +1,5 @@
 #include <errno.h>
+#include <string.h>
 
 #include <sys/random.h>
 
@@ -22,12 +23,27 @@ void get_random_seed(void *buf, size_t size) {
     }
 }
 
-void random_state_init(UNUSED struct random_state *state) {
+void random_state_init(struct random_state *state) {
+    state->index = RANDOM_CACHE_SIZE;
 }
 
-// TODO: add ChaCha20-based CSPRNG, for now avoid using this other than during initialization...
-void get_random_bytes(UNUSED struct random_state *state, void *buf, size_t size) {
-    get_random_seed(buf, size);
+void get_random_bytes(struct random_state *state, void *buf, size_t size) {
+    if (size > RANDOM_CACHE_SIZE / 2) {
+        get_random_seed(buf, size);
+        return;
+    }
+
+    while (size) {
+        if (state->index == RANDOM_CACHE_SIZE) {
+            state->index = 0;
+            get_random_seed(state->cache, 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;
+    }
 }
 
 size_t get_random_size(struct random_state *state) {
diff --git a/random.h b/random.h
index 613e75f..40f97ee 100644
--- a/random.h
+++ b/random.h
@@ -1,8 +1,13 @@
 #ifndef RANDOM_H
 #define RANDOM_H
 
+#include <stdint.h>
+
+#define RANDOM_CACHE_SIZE 4096
+
 struct random_state {
-    char unused;
+    size_t index;
+    uint8_t cache[RANDOM_CACHE_SIZE];
 };
 
 void get_random_seed(void *buf, size_t size);