diff --git a/src/init.cpp b/src/init.cpp
index 76adca769..5843b7d79 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -25,6 +25,7 @@
 #include "policy/policy.h"
 #include "rpcserver.h"
 #include "script/standard.h"
+#include "script/sigcache.h"
 #include "scheduler.h"
 #include "txdb.h"
 #include "txmempool.h"
@@ -434,7 +435,7 @@ std::string HelpMessage(HelpMessageMode mode)
         strUsage += HelpMessageOpt("-logtimemicros", strprintf("Add microsecond precision to debug timestamps (default: %u)", DEFAULT_LOGTIMEMICROS));
         strUsage += HelpMessageOpt("-limitfreerelay=<n>", strprintf("Continuously rate-limit free transactions to <n>*1000 bytes per minute (default: %u)", 15));
         strUsage += HelpMessageOpt("-relaypriority", strprintf("Require high priority for relaying free or low-fee transactions (default: %u)", 1));
-        strUsage += HelpMessageOpt("-maxsigcachesize=<n>", strprintf("Limit size of signature cache to <n> entries (default: %u)", 50000));
+        strUsage += HelpMessageOpt("-maxsigcachesize=<n>", strprintf("Limit size of signature cache to <n> MiB (default: %u)", DEFAULT_MAX_SIG_CACHE_SIZE));
     }
     strUsage += HelpMessageOpt("-minrelaytxfee=<amt>", strprintf(_("Fees (in %s/kB) smaller than this are considered zero fee for relaying (default: %s)"),
         CURRENCY_UNIT, FormatMoney(::minRelayTxFee.GetFeePerK())));
diff --git a/src/script/sigcache.cpp b/src/script/sigcache.cpp
index 099b4ad0e..9dc7f0fcd 100644
--- a/src/script/sigcache.cpp
+++ b/src/script/sigcache.cpp
@@ -5,16 +5,29 @@
 
 #include "sigcache.h"
 
+#include "memusage.h"
 #include "pubkey.h"
 #include "random.h"
 #include "uint256.h"
 #include "util.h"
 
 #include <boost/thread.hpp>
-#include <boost/tuple/tuple_comparison.hpp>
+#include <boost/unordered_set.hpp>
 
 namespace {
 
+/**
+ * We're hashing a nonce into the entries themselves, so we don't need extra
+ * blinding in the set hash computation.
+ */
+class CSignatureCacheHasher
+{
+public:
+    size_t operator()(const uint256& key) const {
+        return key.GetCheapHash();
+    }
+};
+
 /**
  * Valid signature cache, to avoid doing expensive ECDSA signature checking
  * twice for every transaction (once when accepted into memory pool, and
@@ -23,52 +36,48 @@ namespace {
 class CSignatureCache
 {
 private:
-     //! sigdata_type is (signature hash, signature, public key):
-    typedef boost::tuple<uint256, std::vector<unsigned char>, CPubKey> sigdata_type;
-    std::set< sigdata_type> setValid;
+     //! Entries are SHA256(nonce || signature hash || public key || signature):
+    uint256 nonce;
+    typedef boost::unordered_set<uint256, CSignatureCacheHasher> map_type;
+    map_type setValid;
     boost::shared_mutex cs_sigcache;
 
-public:
-    bool
-    Get(const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubKey)
-    {
-        boost::shared_lock<boost::shared_mutex> lock(cs_sigcache);
 
-        sigdata_type k(hash, vchSig, pubKey);
-        std::set<sigdata_type>::iterator mi = setValid.find(k);
-        if (mi != setValid.end())
-            return true;
-        return false;
+public:
+    CSignatureCache()
+    {
+        GetRandBytes(nonce.begin(), 32);
     }
 
-    void Set(const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubKey)
+    void
+    ComputeEntry(uint256& entry, const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubkey)
     {
-        // DoS prevention: limit cache size to less than 10MB
-        // (~200 bytes per cache entry times 50,000 entries)
-        // Since there are a maximum of 20,000 signature operations per block
-        // 50,000 is a reasonable default.
-        int64_t nMaxCacheSize = GetArg("-maxsigcachesize", 50000);
+        CSHA256().Write(nonce.begin(), 32).Write(hash.begin(), 32).Write(&pubkey[0], pubkey.size()).Write(&vchSig[0], vchSig.size()).Finalize(entry.begin());
+    }
+
+    bool
+    Get(const uint256& entry)
+    {
+        boost::shared_lock<boost::shared_mutex> lock(cs_sigcache);
+        return setValid.count(entry);
+    }
+
+    void Set(const uint256& entry)
+    {
+        size_t nMaxCacheSize = GetArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
         if (nMaxCacheSize <= 0) return;
 
         boost::unique_lock<boost::shared_mutex> lock(cs_sigcache);
-
-        while (static_cast<int64_t>(setValid.size()) > nMaxCacheSize)
+        while (memusage::DynamicUsage(setValid) > nMaxCacheSize)
         {
-            // Evict a random entry. Random because that helps
-            // foil would-be DoS attackers who might try to pre-generate
-            // and re-use a set of valid signatures just-slightly-greater
-            // than our cache size.
-            uint256 randomHash = GetRandHash();
-            std::vector<unsigned char> unused;
-            std::set<sigdata_type>::iterator it =
-                setValid.lower_bound(sigdata_type(randomHash, unused, unused));
-            if (it == setValid.end())
-                it = setValid.begin();
-            setValid.erase(*it);
+            map_type::size_type s = GetRand(setValid.bucket_count());
+            map_type::local_iterator it = setValid.begin(s);
+            if (it != setValid.end(s)) {
+                setValid.erase(*it);
+            }
         }
 
-        sigdata_type k(hash, vchSig, pubKey);
-        setValid.insert(k);
+        setValid.insert(entry);
     }
 };
 
@@ -78,13 +87,16 @@ bool CachingTransactionSignatureChecker::VerifySignature(const std::vector<unsig
 {
     static CSignatureCache signatureCache;
 
-    if (signatureCache.Get(sighash, vchSig, pubkey))
+    uint256 entry;
+    signatureCache.ComputeEntry(entry, sighash, vchSig, pubkey);
+
+    if (signatureCache.Get(entry))
         return true;
 
     if (!TransactionSignatureChecker::VerifySignature(vchSig, pubkey, sighash))
         return false;
 
     if (store)
-        signatureCache.Set(sighash, vchSig, pubkey);
+        signatureCache.Set(entry);
     return true;
 }
diff --git a/src/script/sigcache.h b/src/script/sigcache.h
index b299038da..226997256 100644
--- a/src/script/sigcache.h
+++ b/src/script/sigcache.h
@@ -10,6 +10,10 @@
 
 #include <vector>
 
+// DoS prevention: limit cache size to less than 40MB (over 500000
+// entries on 64-bit systems).
+static const unsigned int DEFAULT_MAX_SIG_CACHE_SIZE = 40;
+
 class CPubKey;
 
 class CachingTransactionSignatureChecker : public TransactionSignatureChecker