Change security_strength of HMAC_DRBG

Looking into SP808-90A, it mentions that the max
security strength is the security strength
of the hash function for pre-image resistance.
The current implementation however always only
uses half of the output length, therefore only providing
half of the security strength possible.
However, SP800-90A supports only up to 256 bit
security strength, so we fix it to this upper limit.

In a second change, add_entropy() now also resets
the reseed counter if enough entropy input provided.

3 files changed, 22 insertions, 13 deletions

diff --git a/src/lib/rng/hmac_drbg/hmac_drbg.cpp b/src/lib/rng/hmac_drbg/hmac_drbg.cpp
index e47d49628..03ea2013a 100644
--- a/src/lib/rng/hmac_drbg/hmac_drbg.cpp
+++ b/src/lib/rng/hmac_drbg/hmac_drbg.cpp
@@ -158,12 +158,18 @@ void HMAC_DRBG::update(const uint8_t input[], size_t input_len)
 void HMAC_DRBG::add_entropy(const uint8_t input[], size_t input_len)
    {
    update(input, input_len);
+
+   if(8*input_len >= security_level())
+      {
+      m_reseed_counter = 1;
+      }
    }
 
 size_t HMAC_DRBG::security_level() const
    {
-   // sqrt of hash size
-   return m_mac->output_length() * 8 / 2;
+   // security strength of the hash function
+   // for pre-image resistance (see NIST SP800-57),
+   // but NIST SP800-90A only supports up to 256 bits
+   return std::min(m_mac->output_length(), size_t(32)) * 8;
    }
-
 }
diff --git a/src/lib/rng/stateful_rng/stateful_rng.h b/src/lib/rng/stateful_rng/stateful_rng.h
index 982747e01..ed51aac6a 100644
--- a/src/lib/rng/stateful_rng/stateful_rng.h
+++ b/src/lib/rng/stateful_rng/stateful_rng.h
@@ -121,14 +121,15 @@ class BOTAN_DLL Stateful_RNG : public RandomNumberGenerator
       Entropy_Sources* m_entropy_sources = nullptr;
 
       const size_t m_reseed_interval;
+      uint32_t m_last_pid = 0;
 
+   protected:
       /*
-      * Set to 1 after a sucessful seeding, then incremented.  Reset
+      * Set to 1 after a successful seeding, then incremented.  Reset
       * to 0 by clear() or a fork. This logic is used even if
       * automatic reseeding is disabled (via m_reseed_interval = 0)
       */
       size_t m_reseed_counter = 0;
-      uint32_t m_last_pid = 0;
    };
 
 }
diff --git a/src/tests/test_rng.cpp b/src/tests/test_rng.cpp
index 0ebc091e2..3fb5bd461 100644
--- a/src/tests/test_rng.cpp
+++ b/src/tests/test_rng.cpp
@@ -144,16 +144,18 @@ class HMAC_DRBG_Unit_Tests : public Test
          Request_Counting_RNG counting_rng;
          Botan::HMAC_DRBG rng(std::move(mac), counting_rng, Botan::Entropy_Sources::global_sources(), 2);
          Botan::secure_vector<uint8_t> seed_input(
-               {0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0xAA,0xBB,0xCC,0xDD,0xEE,0xFF});
+               {0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0xAA,0xBB,0xCC,0xDD,0xEE,0xFF,
+                  0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0xAA,0xBB,0xCC,0xDD,0xEE,0xFF});
          Botan::secure_vector<uint8_t> output_after_initialization(
-               {0x26,0x06,0x95,0xF4,0xB8,0x96,0x0D,0x0B,0x27,0x4E,0xA2,0x9E,0x8D,0x2B,0x5A,0x35});
+               {0x48,0xD3,0xB4,0x5A,0xAB,0x65,0xEF,0x92,0xCC,0xFC,0xB9,0x42,0x7E,0xF2,0x0C,0x90,
+               0x29,0x70,0x65,0xEC,0xC1,0xB8,0xA5,0x25,0xBF,0xE4,0xDC,0x6F,0xF3,0x6D,0x0E,0x38});
          Botan::secure_vector<uint8_t> output_without_reseed(
                {0xC4,0x90,0x04,0x5B,0x35,0x4F,0x50,0x09,0x68,0x45,0xF0,0x4B,0x11,0x03,0x58,0xF0});
          result.test_eq("is_seeded",rng.is_seeded(),false);
 
          rng.initialize_with(seed_input.data(), seed_input.size());
 
-         Botan::secure_vector<uint8_t> out(16);
+         Botan::secure_vector<uint8_t> out(32);
 
          rng.randomize(out.data(), out.size());
          result.test_eq("underlying RNG calls", counting_rng.randomize_count(), size_t(0));
@@ -337,8 +339,8 @@ class HMAC_DRBG_Unit_Tests : public Test
 
          // make sure the nonce has at least 1/2*security_strength bits
 
-         // SHA-256 -> 128 bits security strength
-         for( auto nonce_size : { 0, 4, 15, 16, 17, 32 } )
+         // SHA-256 -> 256 bits security strength
+         for( auto nonce_size : { 0, 4, 8, 16, 31, 32, 34 } )
             {
             if(!mac)
                {
@@ -350,15 +352,15 @@ class HMAC_DRBG_Unit_Tests : public Test
             std::vector<uint8_t> nonce(nonce_size);
             rng.initialize_with(nonce.data(), nonce.size());
 
-            if(nonce_size < 16)
+            if(nonce_size < 32)
                {
                result.test_eq("not seeded", rng.is_seeded(), false);
-               result.test_throws("invalid nonce size", [&rng, &nonce] () { rng.random_vec(16); });
+               result.test_throws("invalid nonce size", [&rng, &nonce] () { rng.random_vec(32); });
                }
             else
                {
                result.test_eq("is seeded", rng.is_seeded(), true);
-               rng.random_vec(16);
+               rng.random_vec(32);
                }
             }