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.

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.
  */

src/common/crypto.h

@@ -252,6 +252,7 @@ int crypto_expand_key_material_rfc5869_sha256(
 /* random numbers */
 int crypto_seed_rng(int startup);
 int crypto_rand(char *to, size_t n);
+int crypto_strongest_rand(uint8_t *out, size_t out_len);
 int crypto_rand_int(unsigned int max);
 uint64_t crypto_rand_uint64(uint64_t max);
 double crypto_rand_double(void);

src/common/crypto_curve25519.c

@@ -59,9 +59,18 @@ void
 curve25519_secret_key_generate(curve25519_secret_key_t *key_out,
                                int extra_strong)
 {
-  (void)extra_strong;
+  uint8_t k_tmp[CURVE25519_SECKEY_LEN];
 
-  crypto_rand((char*)key_out->secret_key, 32);
+  crypto_rand((char*)key_out->secret_key, CURVE25519_SECKEY_LEN);
+  if (extra_strong && !crypto_strongest_rand(k_tmp, CURVE25519_SECKEY_LEN)) {
+    /* If they asked for extra-strong entropy and we have some, use it as an
+     * HMAC key to improve not-so-good entopy rather than using it directly,
+     * just in case the extra-strong entropy is less amazing than we hoped. */
+    crypto_hmac_sha256((char *)key_out->secret_key,
+                    (const char *)k_tmp, sizeof(k_tmp),
+                    (const char *)key_out->secret_key, CURVE25519_SECKEY_LEN);
+  }
+  memwipe(k_tmp, 0, sizeof(k_tmp));
   key_out->secret_key[0] &= 248;
   key_out->secret_key[31] &= 127;
   key_out->secret_key[31] |= 64;

src/test/test_crypto.c

@@ -993,7 +993,7 @@ test_crypto_curve25519_wrappers(void *arg)
 
   /* Test a simple handshake, serializing and deserializing some stuff. */
   curve25519_secret_key_generate(&seckey1, 0);
-  curve25519_secret_key_generate(&seckey2, 0);
+  curve25519_secret_key_generate(&seckey2, 1);
   curve25519_public_key_generate(&pubkey1, &seckey1);
   curve25519_public_key_generate(&pubkey2, &seckey2);
   test_assert(curve25519_public_key_is_ok(&pubkey1));