Add+enable service to mint/verify Equihash proofs of work

Add an abstract base class, 'ProofOfWorkService', for the existing PoW
implementation 'HashCashService' and a new 'EquihashProofOfWorkService'
PoW implementation based on Equihash-90-5 (which has 72 byte solutions &
5-10 MB peak memory usage). Since the current 'ProofOfWork' protobuf
object only provides a 64-bit counter field to hold the puzzle solution
(as that is all Hashcash requires), repurpose the 'payload' field to
hold the Equihash puzzle solution bytes, with the 'challenge' field
equal to the puzzle seed: the SHA256 hash of the offerId & makerAddress.

Use a difficulty scale factor of 3e-5 (derived from benchmarking) to try
to make the average Hashcash & Equihash puzzle solution times roughly
equal for any given log-difficulty/numLeadingZeros integer chosen in the
filter.

NOTE: An empty enabled-version-list in the filter defaults to Hashcash
(= version 0) only. The new Equihash-90-5 PoW scheme is version 1.

common/src/main/java/bisq/common/crypto/EquihashProofOfWorkService.java

@@ -0,0 +1,76 @@
+/*
+ * This file is part of Bisq.
+ *
+ * Bisq is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Bisq is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Bisq. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package bisq.common.crypto;
+
+import com.google.common.primitives.Longs;
+
+import org.apache.commons.lang3.StringUtils;
+
+import java.util.Arrays;
+import java.util.concurrent.CompletableFuture;
+
+import lombok.extern.slf4j.Slf4j;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+@Slf4j
+public class EquihashProofOfWorkService extends ProofOfWorkService {
+    /** Rough cost of one Hashcash iteration compared to solving an Equihash-90-5 puzzle of unit difficulty. */
+    private static final double DIFFICULTY_SCALE_FACTOR = 3.0e-5;
+
+    EquihashProofOfWorkService(int version) {
+        super(version);
+    }
+
+    @Override
+    public CompletableFuture<ProofOfWork> mint(String itemId, byte[] challenge, int log2Difficulty) {
+        double difficulty = adjustedDifficulty(log2Difficulty);
+        log.info("Got adjusted difficulty: {}", difficulty);
+
+        return CompletableFuture.supplyAsync(() -> {
+            long ts = System.currentTimeMillis();
+            byte[] solution = new Equihash(90, 5, difficulty).puzzle(challenge).findSolution().serialize();
+            long counter = Longs.fromByteArray(Arrays.copyOf(solution, 8));
+            var proofOfWork = new ProofOfWork(solution, counter, challenge, log2Difficulty,
+                    System.currentTimeMillis() - ts, getVersion());
+            log.info("Completed minting proofOfWork: {}", proofOfWork);
+            return proofOfWork;
+        });
+    }
+
+    @Override
+    public byte[] getChallenge(String itemId, String ownerId) {
+        checkArgument(!StringUtils.contains(itemId, '\0'));
+        checkArgument(!StringUtils.contains(ownerId, '\0'));
+        return Hash.getSha256Hash(checkNotNull(itemId) + "\0" + checkNotNull(ownerId));
+    }
+
+    @Override
+    boolean verify(ProofOfWork proofOfWork) {
+        double difficulty = adjustedDifficulty(proofOfWork.getNumLeadingZeros());
+
+        var puzzle = new Equihash(90, 5, difficulty).puzzle(proofOfWork.getChallenge());
+        return puzzle.deserializeSolution(proofOfWork.getPayload()).verify();
+    }
+
+    private static double adjustedDifficulty(int log2Difficulty) {
+        return Equihash.adjustDifficulty(Math.scalb(DIFFICULTY_SCALE_FACTOR, log2Difficulty),
+                Equihash.EQUIHASH_n_5_MEAN_SOLUTION_COUNT_PER_NONCE);
+    }
+}

common/src/main/java/bisq/common/crypto/HashCashService.java

@@ -35,40 +35,56 @@ import lombok.extern.slf4j.Slf4j;
  * See https://www.hashcash.org/papers/hashcash.pdf
  */
 @Slf4j
-public class HashCashService {
+public class HashCashService extends ProofOfWorkService {
     // Default validations. Custom implementations might use tolerance.
     private static final BiPredicate<byte[], byte[]> isChallengeValid = Arrays::equals;
     private static final BiPredicate<Integer, Integer> isDifficultyValid = Integer::equals;
 
-    public static CompletableFuture<ProofOfWork> mint(byte[] payload,
-                                                      byte[] challenge,
-                                                      int difficulty) {
+    HashCashService() {
+        super(0);
+    }
+
+    @Override
+    public CompletableFuture<ProofOfWork> mint(String itemId, byte[] challenge, int log2Difficulty) {
+        byte[] payload = getBytes(itemId);
+        return mint(payload, challenge, log2Difficulty);
+    }
+
+    @Override
+    public byte[] getChallenge(String itemId, String ownerId) {
+        return getBytes(itemId + ownerId);
+    }
+
+    static CompletableFuture<ProofOfWork> mint(byte[] payload,
+                                               byte[] challenge,
+                                               int difficulty) {
         return HashCashService.mint(payload,
                 challenge,
                 difficulty,
                 HashCashService::testDifficulty);
     }
 
-    public static boolean verify(ProofOfWork proofOfWork) {
+    @Override
+    boolean verify(ProofOfWork proofOfWork) {
         return verify(proofOfWork,
                 proofOfWork.getChallenge(),
                 proofOfWork.getNumLeadingZeros());
     }
 
-    public static boolean verify(ProofOfWork proofOfWork,
-                                 byte[] controlChallenge,
-                                 int controlDifficulty) {
+    static boolean verify(ProofOfWork proofOfWork,
+                          byte[] controlChallenge,
+                          int controlDifficulty) {
         return HashCashService.verify(proofOfWork,
                 controlChallenge,
                 controlDifficulty,
                 HashCashService::testDifficulty);
     }
 
-    public static boolean verify(ProofOfWork proofOfWork,
-                                 byte[] controlChallenge,
-                                 int controlDifficulty,
-                                 BiPredicate<byte[], byte[]> challengeValidation,
-                                 BiPredicate<Integer, Integer> difficultyValidation) {
+    static boolean verify(ProofOfWork proofOfWork,
+                          byte[] controlChallenge,
+                          int controlDifficulty,
+                          BiPredicate<byte[], byte[]> challengeValidation,
+                          BiPredicate<Integer, Integer> difficultyValidation) {
         return HashCashService.verify(proofOfWork,
                 controlChallenge,
                 controlDifficulty,
@@ -139,7 +155,7 @@ public class HashCashService {
     // Utils
     ///////////////////////////////////////////////////////////////////////////////////////////
 
-    public static byte[] getBytes(String value) {
+    private static byte[] getBytes(String value) {
         return value.getBytes(StandardCharsets.UTF_8);
     }
 

common/src/main/java/bisq/common/crypto/ProofOfWorkService.java

@@ -0,0 +1,72 @@
+/*
+ * This file is part of Bisq.
+ *
+ * Bisq is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Bisq is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Bisq. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package bisq.common.crypto;
+
+import com.google.common.base.Preconditions;
+
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.function.BiPredicate;
+
+import lombok.Getter;
+
+public abstract class ProofOfWorkService {
+    private static class InstanceHolder {
+        private static final ProofOfWorkService[] INSTANCES = {
+                new HashCashService(),
+                new EquihashProofOfWorkService(1)
+        };
+    }
+
+    public static Optional<ProofOfWorkService> forVersion(int version) {
+        return version >= 0 && version < InstanceHolder.INSTANCES.length ?
+                Optional.of(InstanceHolder.INSTANCES[version]) : Optional.empty();
+    }
+
+    @Getter
+    private final int version;
+
+    ProofOfWorkService(int version) {
+        this.version = version;
+    }
+
+    public abstract CompletableFuture<ProofOfWork> mint(String itemId, byte[] challenge, int log2Difficulty);
+
+    public abstract byte[] getChallenge(String itemId, String ownerId);
+
+    abstract boolean verify(ProofOfWork proofOfWork);
+
+    public CompletableFuture<ProofOfWork> mint(String itemId, String ownerId, int log2Difficulty) {
+        return mint(itemId, getChallenge(itemId, ownerId), log2Difficulty);
+    }
+
+    public boolean verify(ProofOfWork proofOfWork,
+                          String itemId,
+                          String ownerId,
+                          int controlLog2Difficulty,
+                          BiPredicate<byte[], byte[]> challengeValidation,
+                          BiPredicate<Integer, Integer> difficultyValidation) {
+
+        Preconditions.checkArgument(proofOfWork.getVersion() == version);
+
+        byte[] controlChallenge = getChallenge(itemId, ownerId);
+        return challengeValidation.test(proofOfWork.getChallenge(), controlChallenge) &&
+                difficultyValidation.test(proofOfWork.getNumLeadingZeros(), controlLog2Difficulty) &&
+                verify(proofOfWork);
+    }
+}

common/src/test/java/bisq/common/crypto/HashCashServiceTest.java

@@ -36,7 +36,7 @@ public class HashCashServiceTest {
     @Test
     public void testDiffIncrease() throws ExecutionException, InterruptedException {
         StringBuilder stringBuilder = new StringBuilder();
-        for (int i = 0; i < 12; i++) {
+        for (int i = 0; i < 9; i++) {
             run(i, stringBuilder);
         }
         log.info(stringBuilder.toString());
@@ -70,7 +70,7 @@ public class HashCashServiceTest {
         double size = tokens.size();
         long ts2 = System.currentTimeMillis();
         long averageCounter = Math.round(tokens.stream().mapToLong(ProofOfWork::getCounter).average().orElse(0));
-        boolean allValid = tokens.stream().allMatch(HashCashService::verify);
+        boolean allValid = tokens.stream().allMatch(new HashCashService()::verify);
         assertTrue(allValid);
         double time1 = (System.currentTimeMillis() - ts) / size;
         double time2 = (System.currentTimeMillis() - ts2) / size;

core/src/main/java/bisq/core/filter/FilterManager.java

@@ -37,8 +37,9 @@ import bisq.common.app.DevEnv;
 import bisq.common.app.Version;
 import bisq.common.config.Config;
 import bisq.common.config.ConfigFileEditor;
-import bisq.common.crypto.HashCashService;
+import bisq.common.crypto.ProofOfWorkService;
 import bisq.common.crypto.KeyRing;
+import bisq.common.crypto.ProofOfWork;
 
 import org.bitcoinj.core.ECKey;
 import org.bitcoinj.core.Sha256Hash;
@@ -498,13 +499,23 @@ public class FilterManager {
         }
         checkArgument(offer.getBsqSwapOfferPayload().isPresent(),
                 "Offer payload must be BsqSwapOfferPayload");
-        return HashCashService.verify(offer.getBsqSwapOfferPayload().get().getProofOfWork(),
-                HashCashService.getBytes(offer.getId() + offer.getOwnerNodeAddress().toString()),
+        ProofOfWork pow = offer.getBsqSwapOfferPayload().get().getProofOfWork();
+        var service = ProofOfWorkService.forVersion(pow.getVersion());
+        if (!service.isPresent() || !getEnabledPowVersions().contains(pow.getVersion())) {
+            return false;
+        }
+        return service.get().verify(offer.getBsqSwapOfferPayload().get().getProofOfWork(),
+                offer.getId(), offer.getOwnerNodeAddress().toString(),
                 filter.getPowDifficulty(),
                 challengeValidation,
                 difficultyValidation);
     }
 
+    public List<Integer> getEnabledPowVersions() {
+        Filter filter = getFilter();
+        return filter != null && !filter.getEnabledPowVersions().isEmpty() ? filter.getEnabledPowVersions() : List.of(0);
+    }
+
 
     ///////////////////////////////////////////////////////////////////////////////////////////
     // Private

core/src/main/java/bisq/core/offer/bsq_swap/OpenBsqSwapOfferService.java

@@ -43,7 +43,7 @@ import bisq.network.p2p.P2PService;
 
 import bisq.common.UserThread;
 import bisq.common.app.Version;
-import bisq.common.crypto.HashCashService;
+import bisq.common.crypto.ProofOfWorkService;
 import bisq.common.crypto.PubKeyRing;
 import bisq.common.handlers.ErrorMessageHandler;
 import bisq.common.handlers.ResultHandler;
@@ -61,6 +61,7 @@ import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.Objects;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Consumer;
@@ -195,13 +196,11 @@ public class OpenBsqSwapOfferService {
                 amount.value,
                 minAmount.value);
 
-        NodeAddress makerAddress = p2PService.getAddress();
+        NodeAddress makerAddress = Objects.requireNonNull(p2PService.getAddress());
         offerUtil.validateBasicOfferData(PaymentMethod.BSQ_SWAP, "BSQ");
 
-        byte[] payload = HashCashService.getBytes(offerId);
-        byte[] challenge = HashCashService.getBytes(offerId + Objects.requireNonNull(makerAddress));
-        int difficulty = getPowDifficulty();
-        HashCashService.mint(payload, challenge, difficulty)
+        int log2Difficulty = getPowDifficulty();
+        getPowService().mint(offerId, makerAddress.getFullAddress(), log2Difficulty)
                 .whenComplete((proofOfWork, throwable) -> {
                     // We got called from a non user thread...
                     UserThread.execute(() -> {
@@ -347,11 +346,9 @@ public class OpenBsqSwapOfferService {
         openOfferManager.removeOpenOffer(openOffer);
 
         String newOfferId = OfferUtil.getOfferIdWithMutationCounter(openOffer.getId());
-        byte[] payload = HashCashService.getBytes(newOfferId);
         NodeAddress nodeAddress = Objects.requireNonNull(openOffer.getOffer().getMakerNodeAddress());
-        byte[] challenge = HashCashService.getBytes(newOfferId + nodeAddress);
-        int difficulty = getPowDifficulty();
-        HashCashService.mint(payload, challenge, difficulty)
+        int log2Difficulty = getPowDifficulty();
+        getPowService().mint(newOfferId, nodeAddress.getFullAddress(), log2Difficulty)
                 .whenComplete((proofOfWork, throwable) -> {
                     // We got called from a non user thread...
                     UserThread.execute(() -> {
@@ -393,4 +390,16 @@ public class OpenBsqSwapOfferService {
     private int getPowDifficulty() {
         return filterManager.getFilter() != null ? filterManager.getFilter().getPowDifficulty() : 0;
     }
+
+    private ProofOfWorkService getPowService() {
+        var service = filterManager.getEnabledPowVersions().stream()
+                .flatMap(v -> ProofOfWorkService.forVersion(v).stream())
+                .findFirst();
+        if (!service.isPresent()) {
+            // We cannot exit normally, else we get caught in an infinite loop generating invalid PoWs.
+            throw new NoSuchElementException("Could not find a suitable PoW version to use.");
+        }
+        log.info("Selected PoW version {}, service instance {}", service.get().getVersion(), service.get());
+        return service.get();
+    }
 }