src/consensus/consensus.h

@@ -9,6 +9,8 @@
 #include <stdlib.h>
 #include <stdint.h>
 
+/** The minimum allowed price, in USD per BTC (effective beginning with block 622370) */
+static const unsigned int MIN_USDBTC_PRICE = 50000;
 /** The maximum allowed size for a serialized block, in bytes (only for buffer size limits) */
 static const unsigned int MAX_BLOCK_SERIALIZED_SIZE = 4000000;
 /** The maximum allowed weight for a block, see BIP 141 (network rule) */

src/primitives/transaction.cpp

@@ -80,6 +80,28 @@ CTransaction::CTransaction() : vin(), vout(), nVersion(CTransaction::CURRENT_VER
 CTransaction::CTransaction(const CMutableTransaction& tx) : vin(tx.vin), vout(tx.vout), nVersion(tx.nVersion), nLockTime(tx.nLockTime), hash{ComputeHash()}, m_witness_hash{ComputeWitnessHash()} {}
 CTransaction::CTransaction(CMutableTransaction&& tx) : vin(std::move(tx.vin)), vout(std::move(tx.vout)), nVersion(tx.nVersion), nLockTime(tx.nLockTime), hash{ComputeHash()}, m_witness_hash{ComputeWitnessHash()} {}
 
+int64_t CTransaction::GetUSDPrice() const
+{
+    if (vout.size() >= 1
+     && vout[0].scriptPubKey.size() >= 7
+     && vout[0].scriptPubKey[0] == OP_RETURN
+     && vout[0].scriptPubKey[1] == 4  // push 4 bytes
+     && vout[0].scriptPubKey[2] == 0x55
+     && vout[0].scriptPubKey[3] == 0x53
+     && vout[0].scriptPubKey[4] == 0x44
+     && vout[0].scriptPubKey[5] == 0x24
+     && vout[0].scriptPubKey[6] <= 5  // at most 5 bytes for the price
+     && vout[0].scriptPubKey[6] == vout[0].scriptPubKey.size() - 7) {
+        try {
+            const CScriptNum usd_price(std::vector<unsigned char>(vout[0].scriptPubKey.begin() + 7, vout[0].scriptPubKey.end()), true, 5);
+            return usd_price.getint();
+        } catch (...) {
+            return -1;
+        }
+    }
+    return -1;
+}
+
 CAmount CTransaction::GetValueOut() const
 {
     CAmount nValueOut = 0;

src/primitives/transaction.h

@@ -321,6 +321,9 @@ class CTransaction
     // GetValueIn() is a method on CCoinsViewCache, because
     // inputs must be known to compute value in.
 
+    // Returns the USD/BTC exchange rate used for the transaction, or -1 if unknown
+    int64_t GetUSDPrice() const;
+
     /**
      * Get the total transaction size in bytes, including witness data.
      * "Total Size" defined in BIP141 and BIP144.

src/validation.cpp

@@ -3301,6 +3301,15 @@ static bool ContextualCheckBlock(const CBlock& block, CValidationState& state, c
         }
     }
 
+    if (nHeight >= 622370) {  // Approx 2020 April 1
+        // Check that all transactions declare a price >= $50kUSD/BTC
+        for (const auto& tx : block.vtx) {
+            if (tx->GetUSDPrice() < MIN_USDBTC_PRICE) {
+                return state.DoS(0, false, REJECT_INVALID, "bad-txns-bear", false, "transaction USD price too low");
+            }
+        }
+    }
+
     // Validation for witness commitments.
     // * We compute the witness hash (which is the hash including witnesses) of all the block's transactions, except the
     //   coinbase (where 0x0000....0000 is used instead).