ensure grinding for low R is done in the same way as libsecp256k1 so signatures match

src/main/java/com/sparrowwallet/drongo/Utils.java

@@ -12,10 +12,6 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.math.BigInteger;
-import java.nio.Buffer;
-import java.nio.ByteBuffer;
-import java.nio.CharBuffer;
-import java.nio.charset.StandardCharsets;
 import java.util.*;
 
 public class Utils {
@@ -115,6 +111,14 @@ public class Utils {
         return dest;
     }
 
+    public static void reverse(byte[] array) {
+        for (int i = 0; i < array.length / 2; i++) {
+            byte temp = array[i];
+            array[i] = array[array.length - i - 1];
+            array[array.length - i - 1] = temp;
+        }
+    }
+
     /** Parse 4 bytes from the byte array (starting at the offset) as unsigned 32-bit integer in little endian format. */
     public static long readUint32(byte[] bytes, int offset) {
         return (bytes[offset] & 0xffl) |

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

@@ -470,7 +470,8 @@ public class ECKey implements EncryptableItem {
         }
 
         protected boolean hasLowR() {
-            return r.toByteArray().length <= 32;
+            //A low R signature will have less than 71 bytes when encoded to DER
+            return toCanonicalised().encodeToDER().length < 71;
         }
 
         @Override
@@ -530,14 +531,14 @@ public class ECKey implements EncryptableItem {
         }
 
         ECDSASignature signature;
-        int counter = 0;
+        Integer counter = null;
         do {
             ECDSASigner signer = new ECDSASigner(new HMacDSANonceKCalculator(new SHA256Digest(), counter));
             ECPrivateKeyParameters privKey = new ECPrivateKeyParameters(privateKeyForSigning, CURVE);
             signer.init(true, privKey);
             BigInteger[] components = signer.generateSignature(input.getBytes());
             signature = new ECDSASignature(components[0], components[1]).toCanonicalised();
-            counter++;
+            counter = (counter == null ? 1 : counter+1);
         } while(!signature.hasLowR());
 
         return signature;

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

@@ -21,7 +21,7 @@ public class HMacDSANonceKCalculator implements DSAKCalculator {
     private final HMac hMac;
     private final byte[] K;
     private final byte[] V;
-    private final long counter;
+    private final Long counter;
 
     private BigInteger n;
 
@@ -31,11 +31,11 @@ public class HMacDSANonceKCalculator implements DSAKCalculator {
      * @param digest digest to build the HMAC on.
      * @param counter additional data as per RFC 6979 3.6
      */
-    public HMacDSANonceKCalculator(Digest digest, int counter) {
+    public HMacDSANonceKCalculator(Digest digest, Integer counter) {
         this.hMac = new HMac(digest);
         this.V = new byte[hMac.getMacSize()];
         this.K = new byte[hMac.getMacSize()];
-        this.counter = Integer.toUnsignedLong(counter);
+        this.counter = (counter == null ? null : Integer.toUnsignedLong(counter));
     }
 
     public boolean isDeterministic()
@@ -74,8 +74,12 @@ public class HMacDSANonceKCalculator implements DSAKCalculator {
 
         System.arraycopy(mVal, 0, m, m.length - mVal.length, mVal.length);
 
+        byte[] c = null;
+        if(counter != null) {
             BigInteger additional = BigInteger.valueOf(counter);
-        byte[] aData = Utils.bigIntegerToBytes(additional, size);
+            c = Utils.bigIntegerToBytes(additional, size);
+            Utils.reverse(c);
+        }
 
         hMac.init(new KeyParameter(K));
 
@@ -83,7 +87,9 @@ public class HMacDSANonceKCalculator implements DSAKCalculator {
         hMac.update((byte)0x00);
         hMac.update(x, 0, x.length);
         hMac.update(m, 0, m.length);
-        hMac.update(aData, 0, aData.length);
+        if(c != null) {
+            hMac.update(c, 0, c.length);
+        }
 
         hMac.doFinal(K, 0);
 
@@ -97,6 +103,9 @@ public class HMacDSANonceKCalculator implements DSAKCalculator {
         hMac.update((byte)0x01);
         hMac.update(x, 0, x.length);
         hMac.update(m, 0, m.length);
+        if(counter != null) {
+            hMac.update(c, 0, c.length);
+        }
 
         hMac.doFinal(K, 0);
 

src/main/java/com/sparrowwallet/drongo/psbt/PSBTEntry.java

@@ -83,7 +83,7 @@ public class PSBTEntry {
 
         do {
             bb.get(buf);
-            reverse(buf);
+            Utils.reverse(buf);
             ByteBuffer pbuf = ByteBuffer.wrap(buf);
             path.add(new ChildNumber(pbuf.getInt()));
         } while(bb.hasRemaining());
@@ -202,14 +202,6 @@ public class PSBTEntry {
         return bb.array();
     }
 
-    private static void reverse(byte[] array) {
-        for (int i = 0; i < array.length / 2; i++) {
-            byte temp = array[i];
-            array[i] = array[array.length - i - 1];
-            array[array.length - i - 1] = temp;
-        }
-    }
-
     public void checkOneByteKey() throws PSBTParseException {
         if(this.getKey().length != 1) {
             throw new PSBTParseException("PSBT key type must be one byte");