Merge remote-tracking branch 'public/feature17694_strongest_027'

changes/bug17694_strongest

@@ -0,0 +1,6 @@
+  o Minor features (security):
+    - Never use the system entropy output directly for anything besides
+      seeding the PRNG.  When we want to generate important keys, instead
+      of using system entropy directly, hash it with the PRNG stream.
+      This may help resist certain attacks based on broken OS entropy
+      implementations. Closes part of ticket 17694.

src/common/crypto.c

@@ -2481,7 +2481,7 @@ crypto_strongest_rand_fallback(uint8_t *out, size_t out_len)
  * request size of 256 bytes is imposed.
  */
 int
-crypto_strongest_rand(uint8_t *out, size_t out_len)
+crypto_strongest_rand_raw(uint8_t *out, size_t out_len)
 {
   static const size_t sanity_min_size = 16;
   static const int max_attempts = 3;
@@ -2523,6 +2523,43 @@ crypto_strongest_rand(uint8_t *out, size_t out_len)
   return -1;
 }
 
+/** Try to get <b>out_len</b> bytes of the strongest entropy we can generate,
+ * storing it into <b>out</b>.
+ */
+void
+crypto_strongest_rand(uint8_t *out, size_t out_len)
+{
+  const unsigned DLEN = SHA512_DIGEST_LENGTH;
+  /* We're going to hash DLEN bytes from the system RNG together with some
+   * bytes from the openssl PRNG, in order to yield DLEN bytes.
+   */
+  uint8_t inp[DLEN*2];
+  uint8_t tmp[DLEN];
+  tor_assert(out);
+  while (out_len) {
+    crypto_rand((char*) inp, DLEN);
+    if (crypto_strongest_rand_raw(inp+DLEN, DLEN) < 0) {
+      log_err(LD_CRYPTO, "Failed to load strong entropy when generating an "
+              "important key. Exiting.");
+      /* Die with an assertion so we get a stack trace. */
+      tor_assert(0);
+    }
+    if (out_len >= DLEN) {
+      SHA512(inp, sizeof(inp), out);
+      out += DLEN;
+      out_len -= DLEN;
+    } else {
+      SHA512(inp, sizeof(inp), tmp);
+      memcpy(out, tmp, out_len);
+      out += DLEN;
+      out_len -= DLEN;
+      break;
+    }
+  }
+  memwipe(tmp, 0, sizeof(tmp));
+  memwipe(inp, 0, sizeof(inp));
+}
+
 /** Seed OpenSSL's random number generator with bytes from the operating
  * system.  Return 0 on success, -1 on failure.
  */
@@ -2539,7 +2576,7 @@ crypto_seed_rng(void)
   if (rand_poll_ok == 0)
     log_warn(LD_CRYPTO, "RAND_poll() failed.");
 
-  load_entropy_ok = !crypto_strongest_rand(buf, sizeof(buf));
+  load_entropy_ok = !crypto_strongest_rand_raw(buf, sizeof(buf));
   if (load_entropy_ok) {
     RAND_seed(buf, sizeof(buf));
   }
@@ -2580,6 +2617,9 @@ crypto_rand_unmocked(char *to, size_t n)
   tor_assert(n < INT_MAX);
   tor_assert(to);
   r = RAND_bytes((unsigned char*)to, (int)n);
+  /* We consider a PRNG failure non-survivable. Let's assert so that we get a
+   * stack trace about where it happened.
+   */
   tor_assert(r >= 0);
 }
 

src/common/crypto.h

@@ -273,7 +273,7 @@ int crypto_expand_key_material_rfc5869_sha256(
 int crypto_seed_rng(void) ATTR_WUR;
 MOCK_DECL(void,crypto_rand,(char *to, size_t n));
 void crypto_rand_unmocked(char *to, size_t n);
-int crypto_strongest_rand(uint8_t *out, size_t out_len);
+void crypto_strongest_rand(uint8_t *out, size_t out_len);
 int crypto_rand_int(unsigned int max);
 int crypto_rand_int_range(unsigned int min, unsigned int max);
 uint64_t crypto_rand_uint64_range(uint64_t min, uint64_t max);

src/common/crypto_curve25519.c

@@ -111,18 +111,11 @@ curve25519_public_key_is_ok(const curve25519_public_key_t *key)
 int
 curve25519_rand_seckey_bytes(uint8_t *out, int extra_strong)
 {
-  uint8_t k_tmp[CURVE25519_SECKEY_LEN];
+  if (extra_strong)
+    crypto_strongest_rand(out, CURVE25519_SECKEY_LEN);
+  else
+    crypto_rand((char*)out, CURVE25519_SECKEY_LEN);
 
-  crypto_rand((char*)out, 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 entropy rather than using it directly,
-     * just in case the extra-strong entropy is less amazing than we hoped. */
-    crypto_hmac_sha256((char*) out,
-                       (const char *)k_tmp, sizeof(k_tmp),
-                       (const char *)out, CURVE25519_SECKEY_LEN);
-  }
-  memwipe(k_tmp, 0, sizeof(k_tmp));
   return 0;
 }
 

src/common/crypto_ed25519.c

@@ -107,7 +107,9 @@ ed25519_secret_key_generate(ed25519_secret_key_t *seckey_out,
 {
   int r;
   uint8_t seed[32];
-  if (! extra_strong || crypto_strongest_rand(seed, sizeof(seed)) < 0)
+  if (extra_strong)
+    crypto_strongest_rand(seed, sizeof(seed));
+ else
     crypto_rand((char*)seed, sizeof(seed));
 
   r = get_ed_impl()->seckey_expand(seckey_out->seckey, seed);

src/ext/ed25519/donna/ed25519_tor.c

@@ -148,8 +148,7 @@ ed25519_donna_seckey(unsigned char *sk)
 {
   ed25519_secret_key seed;
 
-  if (crypto_strongest_rand(seed, 32))
-    return -1;
+  crypto_strongest_rand(seed, 32);
 
   ed25519_extsk(sk, seed);
 

src/ext/ed25519/ref10/randombytes.h

@@ -1,4 +1,4 @@
 /* Added for Tor. */
 #include "crypto.h"
 #define randombytes(b, n) \
-  (crypto_strongest_rand((b), (n)))
+  (crypto_strongest_rand((b), (n)), 0)