Refactor strong os-RNG into its own function

Previously, we only used the strong OS entropy source as part of
seeding OpenSSL's RNG.  But with curve25519, we'll have occasion to
want to generate some keys using extremely-good entopy, as well as the
means to do so.  So let's!

This patch refactors the OS-entropy wrapper into its own
crypto_strongest_rand() function, and makes our new
curve25519_secret_key_generate function try it as appropriate.

1 files changed, 45 insertions, 32 deletions

diff --git a/src/common/crypto.c b/src/common/crypto.c
index 2147738b4..4d61f2d93 100644
--- a/src/common/crypto.c
+++ b/src/common/crypto.c
@@ -2344,22 +2344,16 @@ seed_weak_rng(void)
   tor_init_weak_random(seed);
 }
 
-/** Seed OpenSSL's random number generator with bytes from the operating
- * system.  <b>startup</b> should be true iff we have just started Tor and
- * have not yet allocated a bunch of fds.  Return 0 on success, -1 on failure.
+/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
+ * storing it into <b>out</b>.
  */
 int
-crypto_seed_rng(int startup)
+crypto_strongest_rand(uint8_t *out, size_t out_len)
 {
-  int rand_poll_status = 0;
-
-  /* local variables */
 #ifdef _WIN32
-  unsigned char buf[ADD_ENTROPY];
   static int provider_set = 0;
   static HCRYPTPROV provider;
 #else
-  char buf[ADD_ENTROPY];
   static const char *filenames[] = {
     "/dev/srandom", "/dev/urandom", "/dev/random", NULL
   };
@@ -2367,58 +2361,77 @@ crypto_seed_rng(int startup)
   size_t n;
 #endif
 
-  /* OpenSSL has a RAND_poll function that knows about more kinds of
-   * entropy than we do.  We'll try calling that, *and* calling our own entropy
-   * functions.  If one succeeds, we'll accept the RNG as seeded. */
-  if (startup || RAND_POLL_IS_SAFE) {
-    rand_poll_status = RAND_poll();
-    if (rand_poll_status == 0)
-      log_warn(LD_CRYPTO, "RAND_poll() failed.");
-  }
-
 #ifdef _WIN32
   if (!provider_set) {
     if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
                              CRYPT_VERIFYCONTEXT)) {
       if ((unsigned long)GetLastError() != (unsigned long)NTE_BAD_KEYSET) {
         log_warn(LD_CRYPTO, "Can't get CryptoAPI provider [1]");
-        return rand_poll_status ? 0 : -1;
+        return -1;
       }
     }
     provider_set = 1;
   }
-  if (!CryptGenRandom(provider, sizeof(buf), buf)) {
+  if (!CryptGenRandom(provider, out_len, out)) {
     log_warn(LD_CRYPTO, "Can't get entropy from CryptoAPI.");
-    return rand_poll_status ? 0 : -1;
+    return -1;
   }
-  RAND_seed(buf, sizeof(buf));
-  memwipe(buf, 0, sizeof(buf));
-  seed_weak_rng();
+
   return 0;
 #else
   for (i = 0; filenames[i]; ++i) {
     fd = open(filenames[i], O_RDONLY, 0);
     if (fd<0) continue;
-    log_info(LD_CRYPTO, "Seeding RNG from \"%s\"", filenames[i]);
-    n = read_all(fd, buf, sizeof(buf), 0);
+    log_info(LD_CRYPTO, "Reading entropy from \"%s\"", filenames[i]);
+    n = read_all(fd, (char*)out, out_len, 0);
     close(fd);
-    if (n != sizeof(buf)) {
+    if (n != out_len) {
       log_warn(LD_CRYPTO,
                "Error reading from entropy source (read only %lu bytes).",
                (unsigned long)n);
       return -1;
     }
-    RAND_seed(buf, (int)sizeof(buf));
-    memwipe(buf, 0, sizeof(buf));
-    seed_weak_rng();
+
     return 0;
   }
 
-  log_warn(LD_CRYPTO, "Cannot seed RNG -- no entropy source found.");
-  return rand_poll_status ? 0 : -1;
+  log_warn(LD_CRYPTO, "Cannot get strong entropy: no entropy source found.");
+  return -1;
 #endif
 }
 
+/** Seed OpenSSL's random number generator with bytes from the operating
+ * system.  <b>startup</b> should be true iff we have just started Tor and
+ * have not yet allocated a bunch of fds.  Return 0 on success, -1 on failure.
+ */
+int
+crypto_seed_rng(int startup)
+{
+  int rand_poll_ok = 0, load_entropy_ok = 0;
+  uint8_t buf[ADD_ENTROPY];
+
+  /* OpenSSL has a RAND_poll function that knows about more kinds of
+   * entropy than we do.  We'll try calling that, *and* calling our own entropy
+   * functions.  If one succeeds, we'll accept the RNG as seeded. */
+  if (startup || RAND_POLL_IS_SAFE) {
+    rand_poll_ok = RAND_poll();
+    if (rand_poll_ok == 0)
+      log_warn(LD_CRYPTO, "RAND_poll() failed.");
+  }
+
+  load_entropy_ok = !crypto_strongest_rand(buf, sizeof(buf));
+  if (load_entropy_ok) {
+    RAND_seed(buf, sizeof(buf));
+  }
+
+  memwipe(buf, 0, sizeof(buf));
+  seed_weak_rng();
+  if (rand_poll_ok || load_entropy_ok)
+    return 0;
+  else
+    return -1;
+}
+
 /** Write <b>n</b> bytes of strong random data to <b>to</b>. Return 0 on
  * success, -1 on failure.
  */