Add challenge-response key derivation interfaces

src/main/java/com/sparrowwallet/drongo/crypto/ChallengeResponseException.java

@@ -0,0 +1,11 @@
+package com.sparrowwallet.drongo.crypto;
+
+public class ChallengeResponseException extends Exception {
+    public ChallengeResponseException(String message) {
+        super(message);
+    }
+
+    public ChallengeResponseException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}

src/main/java/com/sparrowwallet/drongo/crypto/ChallengeResponseKeyDeriver.java

@@ -0,0 +1,51 @@
+package com.sparrowwallet.drongo.crypto;
+
+import com.sparrowwallet.drongo.protocol.Sha256Hash;
+
+import java.nio.ByteBuffer;
+import java.util.Arrays;
+
+public class ChallengeResponseKeyDeriver implements AsymmetricKeyDeriver {
+    private final AsymmetricKeyDeriver innerDeriver;
+    private final ChallengeResponseProvider provider;
+
+    public ChallengeResponseKeyDeriver(AsymmetricKeyDeriver innerDeriver, ChallengeResponseProvider provider) {
+        this.innerDeriver = innerDeriver;
+        this.provider = provider;
+    }
+
+    @Override
+    public ECKey deriveECKey(CharSequence password) throws KeyCrypterException {
+        ECKey innerKey = innerDeriver.deriveECKey(password);
+        byte[] innerKeyBytes = null;
+        byte[] hmacResponse = null;
+        byte[] bufferArray = null;
+        byte[] combined = null;
+        try {
+            byte[] challenge = innerDeriver.getSalt();
+            hmacResponse = provider.getResponse(challenge);
+            innerKeyBytes = innerKey.getPrivKeyBytes();
+
+            ByteBuffer buffer = ByteBuffer.allocate(innerKeyBytes.length + hmacResponse.length);
+            buffer.put(innerKeyBytes);
+            buffer.put(hmacResponse);
+            bufferArray = buffer.array();
+
+            combined = Sha256Hash.hash(bufferArray);
+            return ECKey.fromPrivate(combined);
+        } catch(ChallengeResponseException e) {
+            throw new KeyCrypterException(e.getMessage(), e);
+        } finally {
+            if(innerKeyBytes != null) Arrays.fill(innerKeyBytes, (byte) 0);
+            if(hmacResponse != null) Arrays.fill(hmacResponse, (byte) 0);
+            if(bufferArray != null) Arrays.fill(bufferArray, (byte) 0);
+            if(combined != null) Arrays.fill(combined, (byte) 0);
+            innerKey.clear();
+        }
+    }
+
+    @Override
+    public byte[] getSalt() {
+        return innerDeriver.getSalt();
+    }
+}

src/main/java/com/sparrowwallet/drongo/crypto/ChallengeResponseProvider.java

@@ -0,0 +1,7 @@
+package com.sparrowwallet.drongo.crypto;
+
+public interface ChallengeResponseProvider {
+    byte[] getResponse(byte[] challenge) throws ChallengeResponseException;
+
+    String getName();
+}

src/main/java/com/sparrowwallet/drongo/crypto/ECKey.java

@@ -25,6 +25,7 @@
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.lang.reflect.Field;
 import java.math.BigInteger;
 import java.nio.charset.StandardCharsets;
 import java.security.SecureRandom;
@@ -837,8 +838,21 @@ public byte[] getPrivKeyBytes() {
     }
 
     public void clear() {
-        for(int i = 0; i < priv.bitLength(); i++) {
-            priv.clearBit(i);
+        if(priv == null) {
+            return;
+        }
+        try {
+            Field magField = BigInteger.class.getDeclaredField("mag");
+            magField.setAccessible(true);
+            int[] mag = (int[]) magField.get(priv);
+            if(mag != null) {
+                Arrays.fill(mag, 0);
+            }
+            Field sigField = BigInteger.class.getDeclaredField("signum");
+            sigField.setAccessible(true);
+            sigField.setInt(priv, 0);
+        } catch(NoSuchFieldException | IllegalAccessException e) {
+            log.warn("Could not zero private key via reflection; add --add-opens java.base/java.math=ALL-UNNAMED to JVM args");
         }
     }