diff --git a/qa/rpc-tests/test_framework/mininode.py b/qa/rpc-tests/test_framework/mininode.py
index 5b563c58ae1e..ba7fe98825c5 100755
--- a/qa/rpc-tests/test_framework/mininode.py
+++ b/qa/rpc-tests/test_framework/mininode.py
@@ -73,9 +73,25 @@ def sha256(s):
 def ripemd160(s):
     return hashlib.new('ripemd160', s).digest()
 
+def hash160(s):
+    return ripemd160(sha256(s))
+
 def hash256(s):
     return sha256(sha256(s))
 
+def upgrade_160_hash_to_256(algo):
+    def algowrapper(s):
+        return algo(s) + (b'\0' * 12)
+    return algowrapper
+
+powalgos = (sha256, hash256, upgrade_160_hash_to_256(ripemd160), upgrade_160_hash_to_256(hash160))
+def powhash(s, nTime):
+    if nTime < 1296688603:
+        algo = hash256
+    else:
+        algo = powalgos[int(nTime / 3600) % len(powalgos)]
+    return algo(s)
+
 def ser_compact_size(l):
     r = b""
     if l < 253:
@@ -577,8 +593,9 @@ def calc_sha256(self):
             r += struct.pack("<I", self.nTime)
             r += struct.pack("<I", self.nBits)
             r += struct.pack("<I", self.nNonce)
-            self.sha256 = uint256_from_str(hash256(r))
-            self.hash = encode(hash256(r)[::-1], 'hex_codec').decode('ascii')
+            rawhash = powhash(r, self.nTime)
+            self.sha256 = uint256_from_str(rawhash)
+            self.hash = encode(rawhash[::-1], 'hex_codec').decode('ascii')
 
     def rehash(self):
         self.sha256 = None
diff --git a/src/Makefile.am b/src/Makefile.am
index 76dbbced6cd4..fb2f45311210 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -270,7 +270,6 @@ libbitcoin_consensus_a_SOURCES = \
   hash.cpp \
   hash.h \
   prevector.h \
-  primitives/block.cpp \
   primitives/block.h \
   primitives/transaction.cpp \
   primitives/transaction.h \
@@ -306,6 +305,7 @@ libbitcoin_common_a_SOURCES = \
   keystore.cpp \
   netaddress.cpp \
   netbase.cpp \
+  primitives/block.cpp \
   protocol.cpp \
   scheduler.cpp \
   script/sign.cpp \
diff --git a/src/chain.cpp b/src/chain.cpp
index a5b369c4fc4c..bde337a7f565 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -4,6 +4,8 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "chain.h"
+#include "chainparams.h"
+#include "consensus/params.h"
 
 /**
  * CChain implementation
@@ -111,6 +113,26 @@ const CBlockIndex* CBlockIndex::GetAncestor(int height) const
     return const_cast<CBlockIndex*>(this)->GetAncestor(height);
 }
 
+int64_t CBlockIndex::GetEarliestNextBlockTime(const Consensus::Params& consensusParams) const
+{
+    int64_t nMinTime = GetMedianTimePast() + 1;
+    int64_t nMaxTime = GetBlockTime();
+    const auto current_pow_algo = consensusParams.PowAlgorithmForTime(nMaxTime);
+    if (nMinTime < nMaxTime && current_pow_algo != consensusParams.PowAlgorithmForTime(nMinTime)) {
+        int64_t nTryTime;
+        ++nMinTime;
+        while (nMinTime < nMaxTime) {
+            nTryTime = nMinTime + ((nMaxTime - nMinTime) / 2);
+            if (current_pow_algo != consensusParams.PowAlgorithmForTime(nTryTime)) {
+                nMinTime = nTryTime + 1;
+            } else {
+                nMaxTime = nTryTime - 1;
+            }
+        }
+    }
+    return nMinTime;
+}
+
 void CBlockIndex::BuildSkip()
 {
     if (pprev)
diff --git a/src/chain.h b/src/chain.h
index acb29b667b76..f4b42b97a5e6 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -306,6 +306,8 @@ class CBlockIndex
         return pbegin[(pend - pbegin)/2];
     }
 
+    int64_t GetEarliestNextBlockTime(const Consensus::Params&) const;
+
     std::string ToString() const
     {
         return strprintf("CBlockIndex(pprev=%p, nHeight=%d, merkle=%s, hashBlock=%s)",
diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 383d9849af97..6428e15e9e93 100644
--- a/src/chainparams.cpp
+++ b/src/chainparams.cpp
@@ -99,6 +99,9 @@ class CMainParams : public CChainParams {
         // The best chain should have at least this much work.
         consensus.nMinimumChainWork = uint256S("0x0000000000000000000000000000000000000000003f94d1ad391682fe038bf5");
 
+        consensus.HardforkTime = std::numeric_limits<int64_t>::max();
+        consensus.nPowChangeTargetShift = 20;
+
         // By default assume that the signatures in ancestors of this block are valid.
         consensus.defaultAssumeValid = uint256S("0x00000000000000000013176bf8d7dfeab4e1db31dc93bc311b436e82ab226b90"); //453354
 
@@ -115,7 +118,7 @@ class CMainParams : public CChainParams {
         nPruneAfterHeight = 100000;
 
         genesis = CreateGenesisBlock(1231006505, 2083236893, 0x1d00ffff, 1, 50 * COIN);
-        consensus.hashGenesisBlock = genesis.GetHash();
+        consensus.hashGenesisBlock = genesis.GetHash(consensus);
         assert(consensus.hashGenesisBlock == uint256S("0x000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f"));
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
@@ -203,6 +206,9 @@ class CTestNetParams : public CChainParams {
         // The best chain should have at least this much work.
         consensus.nMinimumChainWork = uint256S("0x00000000000000000000000000000000000000000000001f057509eba81aed91");
 
+        consensus.HardforkTime = std::numeric_limits<int64_t>::max();
+        consensus.nPowChangeTargetShift = 20;
+
         // By default assume that the signatures in ancestors of this block are valid.
         consensus.defaultAssumeValid = uint256S("0x00000000000128796ee387cf110ccb9d2f36cffaf7f73079c995377c65ac0dcc"); //1079274
 
@@ -214,7 +220,7 @@ class CTestNetParams : public CChainParams {
         nPruneAfterHeight = 1000;
 
         genesis = CreateGenesisBlock(1296688602, 414098458, 0x1d00ffff, 1, 50 * COIN);
-        consensus.hashGenesisBlock = genesis.GetHash();
+        consensus.hashGenesisBlock = genesis.GetHash(consensus);
         assert(consensus.hashGenesisBlock == uint256S("0x000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943"));
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
@@ -288,6 +294,10 @@ class CRegTestParams : public CChainParams {
         // The best chain should have at least this much work.
         consensus.nMinimumChainWork = uint256S("0x00");
 
+        consensus.HardforkTime = 1296688603;  // Just past the genesis block
+        consensus.PowChangeAlgo = HashAlgorithm::NUM_HASH_ALGOS;
+        consensus.nPowChangeTargetShift = 20;
+
         // By default assume that the signatures in ancestors of this block are valid.
         consensus.defaultAssumeValid = uint256S("0x00");
 
@@ -299,7 +309,7 @@ class CRegTestParams : public CChainParams {
         nPruneAfterHeight = 1000;
 
         genesis = CreateGenesisBlock(1296688602, 2, 0x207fffff, 1, 50 * COIN);
-        consensus.hashGenesisBlock = genesis.GetHash();
+        consensus.hashGenesisBlock = genesis.GetHash(consensus);
         assert(consensus.hashGenesisBlock == uint256S("0x0f9188f13cb7b2c71f2a335e3a4fc328bf5beb436012afca590b1a11466e2206"));
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
diff --git a/src/consensus/params.h b/src/consensus/params.h
index 6240e82857eb..ad24d1d47183 100644
--- a/src/consensus/params.h
+++ b/src/consensus/params.h
@@ -6,6 +6,7 @@
 #ifndef BITCOIN_CONSENSUS_PARAMS_H
 #define BITCOIN_CONSENSUS_PARAMS_H
 
+#include "hash.h"
 #include "uint256.h"
 #include <map>
 #include <string>
@@ -62,6 +63,23 @@ struct Params {
     int64_t nPowTargetTimespan;
     int64_t DifficultyAdjustmentInterval() const { return nPowTargetTimespan / nPowTargetSpacing; }
     uint256 nMinimumChainWork;
+
+    /** Hardfork parameters */
+    int64_t HardforkTime;
+    HashAlgorithm PowChangeAlgo;
+    int nPowChangeTargetShift;
+    HashAlgorithm PowAlgorithmForTime(int64_t nTime) const {
+        if (nTime >= HardforkTime) {
+            if (PowChangeAlgo == HashAlgorithm::NUM_HASH_ALGOS) {
+                // Indicates a rotating hash algo, for testing
+                return (HashAlgorithm)((nTime / 3600) % (unsigned int)HashAlgorithm::NUM_HASH_ALGOS);
+            }
+            return PowChangeAlgo;
+        } else {
+            return HashAlgorithm::SHA256d;
+        }
+    }
+
     uint256 defaultAssumeValid;
 };
 } // namespace Consensus
diff --git a/src/hash.h b/src/hash.h
index eacb8f04fef3..795d73f82199 100644
--- a/src/hash.h
+++ b/src/hash.h
@@ -15,6 +15,14 @@
 
 #include <vector>
 
+enum class HashAlgorithm: unsigned int {
+    SHA256,
+    SHA256d,
+    RIPEMD160,
+    HASH160,
+    NUM_HASH_ALGOS,
+};
+
 typedef uint256 ChainCode;
 
 /** A hasher class for Bitcoin's 256-bit hash (double SHA-256). */
diff --git a/src/miner.cpp b/src/miner.cpp
index a12dcec2ce7e..72e85998668e 100644
--- a/src/miner.cpp
+++ b/src/miner.cpp
@@ -60,7 +60,7 @@ class ScoreCompare
 int64_t UpdateTime(CBlockHeader* pblock, const Consensus::Params& consensusParams, const CBlockIndex* pindexPrev)
 {
     int64_t nOldTime = pblock->nTime;
-    int64_t nNewTime = std::max(pindexPrev->GetMedianTimePast()+1, GetAdjustedTime());
+    int64_t nNewTime = std::max(pindexPrev->GetEarliestNextBlockTime(consensusParams), GetAdjustedTime());
 
     if (nOldTime < nNewTime)
         pblock->nTime = nNewTime;
diff --git a/src/pow.cpp b/src/pow.cpp
index e57fd866f8a6..36f740537d7b 100644
--- a/src/pow.cpp
+++ b/src/pow.cpp
@@ -18,6 +18,8 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
     if (pindexLast == NULL)
         return nProofOfWorkLimit;
 
+    uint32_t nBits;
+
     // Only change once per difficulty adjustment interval
     if ((pindexLast->nHeight+1) % params.DifficultyAdjustmentInterval() != 0)
     {
@@ -30,23 +32,38 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
                 return nProofOfWorkLimit;
             else
             {
-                // Return the last non-special-min-difficulty-rules-block
+                // Look back to the last non-special-min-difficulty-rules-block
                 const CBlockIndex* pindex = pindexLast;
                 while (pindex->pprev && pindex->nHeight % params.DifficultyAdjustmentInterval() != 0 && pindex->nBits == nProofOfWorkLimit)
                     pindex = pindex->pprev;
-                return pindex->nBits;
+                nBits = pindex->nBits;
             }
+        } else {
+            nBits = pindexLast->nBits;
         }
-        return pindexLast->nBits;
+    } else {
+        // Go back by what we want to be 14 days worth of blocks
+        int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
+        assert(nHeightFirst >= 0);
+        const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
+        assert(pindexFirst);
+
+        nBits = CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime(), params);
     }
 
-    // Go back by what we want to be 14 days worth of blocks
-    int nHeightFirst = pindexLast->nHeight - (params.DifficultyAdjustmentInterval()-1);
-    assert(nHeightFirst >= 0);
-    const CBlockIndex* pindexFirst = pindexLast->GetAncestor(nHeightFirst);
-    assert(pindexFirst);
+    if (params.PowAlgorithmForTime(pblock->nTime) != params.PowAlgorithmForTime(pindexLast->nTime)) {
+        // Adjust target for PoW change
+        arith_uint256 bnNew;
+        bnNew.SetCompact(nBits);
+        bnNew <<= params.nPowChangeTargetShift;
+        const arith_uint256 bnPowLimit = UintToArith256(params.powLimit);
+        if (bnNew > bnPowLimit) {
+            bnNew = bnPowLimit;
+        }
+        nBits = bnNew.GetCompact();
+    }
 
-    return CalculateNextWorkRequired(pindexLast, pindexFirst->GetBlockTime(), params);
+    return nBits;
 }
 
 unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nFirstBlockTime, const Consensus::Params& params)
diff --git a/src/primitives/block.cpp b/src/primitives/block.cpp
index 9a979094ccb1..380e0b6db700 100644
--- a/src/primitives/block.cpp
+++ b/src/primitives/block.cpp
@@ -8,11 +8,47 @@
 #include "hash.h"
 #include "tinyformat.h"
 #include "utilstrencodings.h"
+#include "chainparams.h"
+#include "consensus/params.h"
 #include "crypto/common.h"
+#include "streams.h"
+
+#include <cstdlib>
+
+uint256 CBlockHeader::GetHash(const Consensus::Params& consensusParams) const
+{
+    CDataStream ss(SER_GETHASH, PROTOCOL_VERSION);
+    ss << *this;
+
+    const auto pbegin = (const unsigned char *)&ss.begin()[0];
+    uint256 hash;
+
+    const HashAlgorithm algo = consensusParams.PowAlgorithmForTime(nTime);
+    switch (algo) {
+        case HashAlgorithm::SHA256:
+            CSHA256().Write(pbegin, ss.size()).Finalize((unsigned char*)&hash);
+            break;
+        case HashAlgorithm::SHA256d:
+            CHash256().Write(pbegin, ss.size()).Finalize((unsigned char*)&hash);
+            break;
+        case HashAlgorithm::RIPEMD160:
+            CRIPEMD160().Write(pbegin, ss.size()).Finalize((unsigned char*)&hash);
+            break;
+        case HashAlgorithm::HASH160:
+            CHash160().Write(pbegin, ss.size()).Finalize((unsigned char*)&hash);
+            break;
+        case HashAlgorithm::NUM_HASH_ALGOS:
+            // Should be impossible
+            abort();
+    }
+
+    return hash;
+}
 
 uint256 CBlockHeader::GetHash() const
 {
-    return SerializeHash(*this);
+    const Consensus::Params& consensusParams = Params().GetConsensus();
+    return GetHash(consensusParams);
 }
 
 std::string CBlock::ToString() const
diff --git a/src/primitives/block.h b/src/primitives/block.h
index 4c6eb20ad5e3..aacf4f5abc57 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -10,6 +10,10 @@
 #include "serialize.h"
 #include "uint256.h"
 
+namespace Consensus {
+    struct Params;
+}
+
 /** Nodes collect new transactions into a block, hash them into a hash tree,
  * and scan through nonce values to make the block's hash satisfy proof-of-work
  * requirements.  When they solve the proof-of-work, they broadcast the block
@@ -60,6 +64,7 @@ class CBlockHeader
         return (nBits == 0);
     }
 
+    uint256 GetHash(const Consensus::Params&) const;
     uint256 GetHash() const;
 
     int64_t GetBlockTime() const
diff --git a/src/rpc/mining.cpp b/src/rpc/mining.cpp
index 33e234a95efb..eb26bf0eb72a 100644
--- a/src/rpc/mining.cpp
+++ b/src/rpc/mining.cpp
@@ -678,7 +678,7 @@ UniValue getblocktemplate(const JSONRPCRequest& request)
     result.push_back(Pair("coinbasevalue", (int64_t)pblock->vtx[0]->vout[0].nValue));
     result.push_back(Pair("longpollid", chainActive.Tip()->GetBlockHash().GetHex() + i64tostr(nTransactionsUpdatedLast)));
     result.push_back(Pair("target", hashTarget.GetHex()));
-    result.push_back(Pair("mintime", (int64_t)pindexPrev->GetMedianTimePast()+1));
+    result.push_back(Pair("mintime", (int64_t)pindexPrev->GetEarliestNextBlockTime(consensusParams)));
     result.push_back(Pair("mutable", aMutable));
     result.push_back(Pair("noncerange", "00000000ffffffff"));
     int64_t nSigOpLimit = MAX_BLOCK_SIGOPS_COST;
diff --git a/src/test/miner_tests.cpp b/src/test/miner_tests.cpp
index f856d8a91a70..33e193d21eae 100644
--- a/src/test/miner_tests.cpp
+++ b/src/test/miner_tests.cpp
@@ -6,6 +6,7 @@
 #include "coins.h"
 #include "consensus/consensus.h"
 #include "consensus/merkle.h"
+#include "consensus/params.h"
 #include "consensus/validation.h"
 #include "validation.h"
 #include "miner.h"
@@ -187,6 +188,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
 {
     // Note that by default, these tests run with size accounting enabled.
     const CChainParams& chainparams = Params(CBaseChainParams::MAIN);
+    const Consensus::Params& consensusParams = chainparams.GetConsensus();
     CScript scriptPubKey = CScript() << ParseHex("04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f") << OP_CHECKSIG;
     std::unique_ptr<CBlockTemplate> pblocktemplate;
     CMutableTransaction tx,tx2;
@@ -211,7 +213,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
     {
         CBlock *pblock = &pblocktemplate->block; // pointer for convenience
         pblock->nVersion = 1;
-        pblock->nTime = chainActive.Tip()->GetMedianTimePast()+1;
+        pblock->nTime = chainActive.Tip()->GetEarliestNextBlockTime(consensusParams);
         CMutableTransaction txCoinbase(*pblock->vtx[0]);
         txCoinbase.nVersion = 1;
         txCoinbase.vin[0].scriptSig = CScript();
@@ -393,7 +395,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
     }
 
     // non-final txs in mempool
-    SetMockTime(chainActive.Tip()->GetMedianTimePast()+1);
+    SetMockTime(chainActive.Tip()->GetEarliestNextBlockTime(consensusParams));
     int flags = LOCKTIME_VERIFY_SEQUENCE|LOCKTIME_MEDIAN_TIME_PAST;
     // height map
     std::vector<int> prevheights;
diff --git a/src/test/skiplist_tests.cpp b/src/test/skiplist_tests.cpp
index 0b2fe0ef9db1..db12c5cf791d 100644
--- a/src/test/skiplist_tests.cpp
+++ b/src/test/skiplist_tests.cpp
@@ -3,6 +3,8 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "chain.h"
+#include "chainparams.h"
+#include "consensus/params.h"
 #include "util.h"
 #include "test/test_bitcoin.h"
 #include "test/test_random.h"
@@ -102,6 +104,7 @@ BOOST_AUTO_TEST_CASE(getlocator_test)
 
 BOOST_AUTO_TEST_CASE(findearliestatleast_test)
 {
+    const Consensus::Params& consensusParams = Params(CBaseChainParams::MAIN).GetConsensus();
     std::vector<uint256> vHashMain(100000);
     std::vector<CBlockIndex> vBlocksMain(100000);
     for (unsigned int i=0; i<vBlocksMain.size(); i++) {
@@ -115,7 +118,7 @@ BOOST_AUTO_TEST_CASE(findearliestatleast_test)
             vBlocksMain[i].nTimeMax = i;
         } else {
             // randomly choose something in the range [MTP, MTP*2]
-            int64_t medianTimePast = vBlocksMain[i].GetMedianTimePast();
+            int64_t medianTimePast = vBlocksMain[i].GetEarliestNextBlockTime(consensusParams);
             int r = insecure_rand() % medianTimePast;
             vBlocksMain[i].nTime = r + medianTimePast;
             vBlocksMain[i].nTimeMax = std::max(vBlocksMain[i].nTime, vBlocksMain[i-1].nTimeMax);
diff --git a/src/validation.cpp b/src/validation.cpp
index c9135c442b1a..55ce10215529 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2981,8 +2981,12 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
     if (block.nBits != GetNextWorkRequired(pindexPrev, &block, consensusParams))
         return state.DoS(100, false, REJECT_INVALID, "bad-diffbits", false, "incorrect proof of work");
 
+    if (consensusParams.PowAlgorithmForTime(block.GetBlockTime()) != consensusParams.PowAlgorithmForTime(pindexPrev->GetBlockTime()) && block.GetBlockTime() < pindexPrev->GetBlockTime()) {
+        return state.Invalid(false, REJECT_INVALID, "pow-reversed", "cannot reverse PoW change");
+    }
+
     // Check timestamp against prev
-    if (block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
+    if (block.GetBlockTime() < pindexPrev->GetEarliestNextBlockTime(consensusParams))
         return state.Invalid(false, REJECT_INVALID, "time-too-old", "block's timestamp is too early");
 
     // Check timestamp