Make CRollingBloomFilter set nTweak for you

While CBloomFilter is usually used with an explicitly set nTweak,
CRollingBloomFilter is only used internally. Requiring every caller to
set nTweak is error-prone and redundant; better to have the class handle
that for you with a high-quality randomness source.

Additionally when clearing the filter it makes sense to change nTweak as
well to recover from a bad setting, e.g. due to insufficient randomness
at initialization, so the clear() method is replaced by a reset() method
that sets a new, random, nTweak value.

src/bloom.cpp

@@ -8,6 +8,7 @@
 #include "hash.h"
 #include "script/script.h"
 #include "script/standard.h"
+#include "random.h"
 #include "streams.h"
 
 #include <math.h>
@@ -121,6 +122,12 @@ void CBloomFilter::clear()
     isEmpty = true;
 }
 
+void CBloomFilter::reset(unsigned int nNewTweak)
+{
+    clear();
+    nTweak = nNewTweak;
+}
+
 bool CBloomFilter::IsWithinSizeConstraints() const
 {
     return vData.size() <= MAX_BLOOM_FILTER_SIZE && nHashFuncs <= MAX_HASH_FUNCS;
@@ -217,7 +224,8 @@ CRollingBloomFilter::CRollingBloomFilter(unsigned int nElements, double fpRate,
     // inserted, so at least one always contains the last nElements
     // inserted.
     nBloomSize = nElements * 2;
-    nInsertions = 0;
+
+    reset(nTweak);
 }
 
 void CRollingBloomFilter::insert(const std::vector<unsigned char>& vKey)
@@ -254,9 +262,12 @@ bool CRollingBloomFilter::contains(const uint256& hash) const
     return contains(data);
 }
 
-void CRollingBloomFilter::clear()
+void CRollingBloomFilter::reset(unsigned int nNewTweak)
 {
-    b1.clear();
-    b2.clear();
+    if (!nNewTweak)
+        nNewTweak = GetRand(std::numeric_limits<unsigned int>::max());
+
+    b1.reset(nNewTweak);
+    b2.reset(nNewTweak);
     nInsertions = 0;
 }

src/bloom.h

@@ -89,6 +89,7 @@ public:
     bool contains(const uint256& hash) const;
 
     void clear();
+    void reset(unsigned int nNewTweak);
 
     //! True if the size is <= MAX_BLOOM_FILTER_SIZE and the number of hash functions is <= MAX_HASH_FUNCS
     //! (catch a filter which was just deserialized which was too big)
@@ -103,7 +104,11 @@ public:
 
 /**
  * RollingBloomFilter is a probabilistic "keep track of most recently inserted" set.
- * Construct it with the number of items to keep track of, and a false-positive rate.
+ * Construct it with the number of items to keep track of, and a false-positive
+ * rate. Unlike CBloomFilter, by default nTweak is set to a cryptographically
+ * secure random value for you. Similarly rather than clear() the method
+ * reset() is provided, which also changes nTweak to decrease the impact of
+ * false-positives.
  *
  * contains(item) will always return true if item was one of the last N things
  * insert()'ed ... but may also return true for items that were not inserted.
@@ -111,14 +116,15 @@ public:
 class CRollingBloomFilter
 {
 public:
-    CRollingBloomFilter(unsigned int nElements, double nFPRate, unsigned int nTweak);
+    CRollingBloomFilter(unsigned int nElements, double nFPRate,
+                        unsigned int nTweak = 0);
 
     void insert(const std::vector<unsigned char>& vKey);
     void insert(const uint256& hash);
     bool contains(const std::vector<unsigned char>& vKey) const;
     bool contains(const uint256& hash) const;
 
-    void clear();
+    void reset(unsigned int nNewTweak = 0);
 
 private:
     unsigned int nBloomSize;

src/main.cpp

@@ -4812,7 +4812,7 @@ bool SendMessages(CNode* pto, bool fSendTrickle)
             {
                 // Periodically clear addrKnown to allow refresh broadcasts
                 if (nLastRebroadcast)
-                    pnode->addrKnown.clear();
+                    pnode->addrKnown.reset();
 
                 // Rebroadcast our address
                 AdvertizeLocal(pnode);

src/net.cpp

@@ -2060,7 +2060,7 @@ unsigned int SendBufferSize() { return 1000*GetArg("-maxsendbuffer", 1*1000); }
 
 CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNameIn, bool fInboundIn) :
     ssSend(SER_NETWORK, INIT_PROTO_VERSION),
-    addrKnown(5000, 0.001, insecure_rand()),
+    addrKnown(5000, 0.001),
     setInventoryKnown(SendBufferSize() / 1000)
 {
     nServices = 0;

src/test/bloom_tests.cpp

@@ -469,7 +469,7 @@ static std::vector<unsigned char> RandomData()
 BOOST_AUTO_TEST_CASE(rolling_bloom)
 {
     // last-100-entry, 1% false positive:
-    CRollingBloomFilter rb1(100, 0.01, 0);
+    CRollingBloomFilter rb1(100, 0.01, 1);
 
     // Overfill:
     static const int DATASIZE=399;
@@ -500,7 +500,7 @@ BOOST_AUTO_TEST_CASE(rolling_bloom)
     BOOST_CHECK(nHits < 175);
 
     BOOST_CHECK(rb1.contains(data[DATASIZE-1]));
-    rb1.clear();
+    rb1.reset(1);
     BOOST_CHECK(!rb1.contains(data[DATASIZE-1]));
 
     // Now roll through data, make sure last 100 entries
@@ -527,7 +527,7 @@ BOOST_AUTO_TEST_CASE(rolling_bloom)
     BOOST_CHECK(nHits < 100);
 
     // last-1000-entry, 0.01% false positive:
-    CRollingBloomFilter rb2(1000, 0.001, 0);
+    CRollingBloomFilter rb2(1000, 0.001, 1);
     for (int i = 0; i < DATASIZE; i++) {
         rb2.insert(data[i]);
     }