Merge #17573: Seed RNG with precision timestamps on receipt of net messages.

02d8c56a18 Seed RNG with precision timestamps on receipt of net messages. (Matt Corallo)

Pull request description:

  See title. Exposes a generic dead-simple "SeedEvent" interface, but currently just used for net messages.

ACKs for top commit:
  sipa:
    utACK https://github.com/bitcoin/bitcoin/commit/02d8c56a18b9a2960888d6ec1209955105bae847
  laanwj:
    ACK 02d8c56a18
  meshcollider:
    utACK 02d8c56a18

Tree-SHA512: 28eb39a201ee2b13393c5c64dbf7c1913f3482f095969ef5141bfe549ce77dd63bb5f14738f6eedb296c686ea36014aa157b9c5e8059710a318590f30e9caa14

src/net.cpp

@@ -16,6 +16,7 @@
 #include <crypto/sha256.h>
 #include <netbase.h>
 #include <net_permissions.h>
+#include <random.h>
 #include <scheduler.h>
 #include <ui_interface.h>
 #include <util/strencodings.h>
@@ -445,6 +446,9 @@ CNode* CConnman::ConnectNode(CAddress addrConnect, const char *pszDest, bool fCo
     CNode* pnode = new CNode(id, nLocalServices, GetBestHeight(), hSocket, addrConnect, CalculateKeyedNetGroup(addrConnect), nonce, addr_bind, pszDest ? pszDest : "", false, block_relay_only);
     pnode->AddRef();
 
+    // We're making a new connection, harvest entropy from the time (and our peer count)
+    RandAddEvent((uint32_t)id);
+
     return pnode;
 }
 
@@ -693,6 +697,9 @@ CNetMessage V1TransportDeserializer::GetMessage(const CMessageHeader::MessageSta
     msg.m_message_size = hdr.nMessageSize;
     msg.m_raw_message_size = hdr.nMessageSize + CMessageHeader::HEADER_SIZE;
 
+    // We just received a message off the wire, harvest entropy from the time (and the message checksum)
+    RandAddEvent(ReadLE32(hash.begin()));
+
     msg.m_valid_checksum = (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) == 0);
     if (!msg.m_valid_checksum) {
         LogPrint(BCLog::NET, "CHECKSUM ERROR (%s, %u bytes), expected %s was %s\n",
@@ -1017,6 +1024,9 @@ void CConnman::AcceptConnection(const ListenSocket& hListenSocket) {
         LOCK(cs_vNodes);
         vNodes.push_back(pnode);
     }
+
+    // We received a new connection, harvest entropy from the time (and our peer count)
+    RandAddEvent((uint32_t)id);
 }
 
 void CConnman::DisconnectNodes()

src/random.cpp

@@ -6,6 +6,7 @@
 #include <random.h>
 
 #include <compat/cpuid.h>
+#include <crypto/sha256.h>
 #include <crypto/sha512.h>
 #include <support/cleanse.h>
 #ifdef WIN32
@@ -449,6 +450,23 @@ static void SeedFast(CSHA512& hasher) noexcept
     SeedTimestamp(hasher);
 }
 
+// We use only SHA256 for the events hashing to get the ASM speedups we have for SHA256,
+// since we want it to be fast as network peers may be able to trigger it repeatedly.
+static Mutex events_mutex;
+static CSHA256 events_hasher;
+static void SeedEvents(CSHA512& hasher)
+{
+    LOCK(events_mutex);
+
+    unsigned char events_hash[32];
+    events_hasher.Finalize(events_hash);
+    hasher.Write(events_hash, 32);
+
+    // Re-initialize the hasher with the finalized state to use later.
+    events_hasher.Reset();
+    events_hasher.Write(events_hash, 32);
+}
+
 static void SeedSlow(CSHA512& hasher) noexcept
 {
     unsigned char buffer[32];
@@ -460,6 +478,9 @@ static void SeedSlow(CSHA512& hasher) noexcept
     GetOSRand(buffer);
     hasher.Write(buffer, sizeof(buffer));
 
+    // Add the events hasher into the mix
+    SeedEvents(hasher);
+
     // High-precision timestamp.
     //
     // Note that we also commit to a timestamp in the Fast seeder, so we indirectly commit to a
@@ -485,6 +506,9 @@ static void SeedPeriodic(CSHA512& hasher, RNGState& rng)
     // High-precision timestamp
     SeedTimestamp(hasher);
 
+    // Add the events hasher into the mix
+    SeedEvents(hasher);
+
     // Dynamic environment data (performance monitoring, ...)
     auto old_size = hasher.Size();
     RandAddDynamicEnv(hasher);
@@ -553,6 +577,15 @@ void GetRandBytes(unsigned char* buf, int num) noexcept { ProcRand(buf, num, RNG
 void GetStrongRandBytes(unsigned char* buf, int num) noexcept { ProcRand(buf, num, RNGLevel::SLOW); }
 void RandAddPeriodic() { ProcRand(nullptr, 0, RNGLevel::PERIODIC); }
 
+void RandAddEvent(const uint32_t event_info) {
+    LOCK(events_mutex);
+    events_hasher.Write((const unsigned char *)&event_info, sizeof(event_info));
+    // Get the low four bytes of the performance counter. This translates to roughly the
+    // subsecond part.
+    uint32_t perfcounter = (GetPerformanceCounter() & 0xffffffff);
+    events_hasher.Write((const unsigned char*)&perfcounter, sizeof(perfcounter));
+}
+
 bool g_mock_deterministic_tests{false};
 
 uint64_t GetRand(uint64_t nMax) noexcept

src/random.h

@@ -89,6 +89,14 @@ void GetStrongRandBytes(unsigned char* buf, int num) noexcept;
  */
 void RandAddPeriodic();
 
+/**
+ * Gathers entropy from the low bits of the time at which events occur. Should
+ * be called with a uint32_t describing the event at the time an event occurs.
+ *
+ * Thread-safe.
+ */
+void RandAddEvent(const uint32_t event_info);
+
 /**
  * Fast randomness source. This is seeded once with secure random data, but
  * is completely deterministic and does not gather more entropy after that.