From f7303f97933be33e34d482cf8348d180c8da2a26 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Tue, 17 Mar 2015 06:35:59 -0700
Subject: Use equivalent PoW for non-main-chain requests

---
 src/main.cpp           |  9 +++++----
 src/pow.cpp            | 17 +++++++++++++++++
 src/pow.h              |  3 +++
 src/test/pow_tests.cpp | 24 ++++++++++++++++++++++++
 4 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 4352c719a1..c5168ec8c4 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -3459,7 +3459,6 @@ bool static AlreadyHave(const CInv& inv)
     return true;
 }
 
-
 void static ProcessGetData(CNode* pfrom)
 {
     std::deque<CInv>::iterator it = pfrom->vRecvGetData.begin();
@@ -3487,11 +3486,13 @@ void static ProcessGetData(CNode* pfrom)
                     if (chainActive.Contains(mi->second)) {
                         send = true;
                     } else {
+                        static const int nOneMonth = 30 * 24 * 60 * 60;
                         // To prevent fingerprinting attacks, only send blocks outside of the active
-                        // chain if they are valid, and no more than a month older than the best header
-                        // chain we know about.
+                        // chain if they are valid, and no more than a month older (both in time, and in
+                        // best equivalent proof of work) than the best header chain we know about.
                         send = mi->second->IsValid(BLOCK_VALID_SCRIPTS) && (pindexBestHeader != NULL) &&
-                            (mi->second->GetBlockTime() > pindexBestHeader->GetBlockTime() - 30 * 24 * 60 * 60);
+                            (pindexBestHeader->GetBlockTime() - mi->second->GetBlockTime() < nOneMonth) &&
+                            (GetBlockProofEquivalentTime(*pindexBestHeader, *mi->second, *pindexBestHeader, Params().GetConsensus()) < nOneMonth);
                         if (!send) {
                             LogPrintf("%s: ignoring request from peer=%i for old block that isn't in the main chain\n", __func__, pfrom->GetId());
                         }
diff --git a/src/pow.cpp b/src/pow.cpp
index fc6ed4f3d1..bb53ad204b 100644
--- a/src/pow.cpp
+++ b/src/pow.cpp
@@ -114,3 +114,20 @@ arith_uint256 GetBlockProof(const CBlockIndex& block)
     // or ~bnTarget / (nTarget+1) + 1.
     return (~bnTarget / (bnTarget + 1)) + 1;
 }
+
+int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& from, const CBlockIndex& tip, const Consensus::Params& params)
+{
+    arith_uint256 r;
+    int sign = 1;
+    if (to.nChainWork > from.nChainWork) {
+        r = to.nChainWork - from.nChainWork;
+    } else {
+        r = from.nChainWork - to.nChainWork;
+        sign = -1;
+    }
+    r = r * arith_uint256(params.nPowTargetSpacing) / GetBlockProof(tip);
+    if (r.bits() > 63) {
+        return sign * std::numeric_limits<int64_t>::max();
+    }
+    return sign * r.GetLow64();
+}
diff --git a/src/pow.h b/src/pow.h
index a5d32db178..e864a474cc 100644
--- a/src/pow.h
+++ b/src/pow.h
@@ -22,4 +22,7 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
 bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params&);
 arith_uint256 GetBlockProof(const CBlockIndex& block);
 
+/** Return the time it would take to redo the work difference between from and to, assuming the current hashrate corresponds to the difficulty at tip, in seconds. */
+int64_t GetBlockProofEquivalentTime(const CBlockIndex& to, const CBlockIndex& from, const CBlockIndex& tip, const Consensus::Params&);
+
 #endif // BITCOIN_POW_H
diff --git a/src/test/pow_tests.cpp b/src/test/pow_tests.cpp
index 4ce1591c35..a436749287 100644
--- a/src/test/pow_tests.cpp
+++ b/src/test/pow_tests.cpp
@@ -69,4 +69,28 @@ BOOST_AUTO_TEST_CASE(get_next_work_upper_limit_actual)
     BOOST_CHECK_EQUAL(CalculateNextWorkRequired(&pindexLast, nLastRetargetTime, params), 0x1d00e1fd);
 }
 
+BOOST_AUTO_TEST_CASE(GetBlockProofEquivalentTime_test)
+{
+    SelectParams(CBaseChainParams::MAIN);
+    const Consensus::Params& params = Params().GetConsensus();
+
+    std::vector<CBlockIndex> blocks(10000);
+    for (int i = 0; i < 10000; i++) {
+        blocks[i].pprev = i ? &blocks[i - 1] : NULL;
+        blocks[i].nHeight = i;
+        blocks[i].nTime = 1269211443 + i * params.nPowTargetSpacing;
+        blocks[i].nBits = 0x207fffff; /* target 0x7fffff000... */
+        blocks[i].nChainWork = i ? blocks[i - 1].nChainWork + GetBlockProof(blocks[i - 1]) : arith_uint256(0);
+    }
+
+    for (int j = 0; j < 1000; j++) {
+        CBlockIndex *p1 = &blocks[GetRand(10000)];
+        CBlockIndex *p2 = &blocks[GetRand(10000)];
+        CBlockIndex *p3 = &blocks[GetRand(10000)];
+
+        int64_t tdiff = GetBlockProofEquivalentTime(*p1, *p2, *p3, params);
+        BOOST_CHECK_EQUAL(tdiff, p1->GetBlockTime() - p2->GetBlockTime());
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()
-- 
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