Pad a short RSA puzzle answer instead of failing the connect

An answer with a leading zero byte comes out of toByteArray() as 63 bytes,
and copying it started one byte too far, failing about one connect in 256.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main/java/com/github/manevolent/ts3j/protocol/packet/handler/client/LocalClientHandlerConnecting.java

@@ -159,16 +159,7 @@ public class LocalClientHandlerConnecting extends LocalClientHandler {
                     BigInteger x = new BigInteger(1, serverReplyStep3.getX());
                     BigInteger n = new BigInteger(1, serverReplyStep3.getN());
 
-                    byte[] y = new byte[64];
+                    byte[] y = unsigned64(x.modPow(BigInteger.valueOf(2L).pow(serverReplyStep3.getLevel()), n));
-
-                    byte[] solution =
-                            x.modPow(BigInteger.valueOf(2L).pow(serverReplyStep3.getLevel()), n).toByteArray();
-
-                    System.arraycopy(
-                            solution, Math.abs(solution.length - 64),
-                            y, solution.length < 64 ? 64 - solution.length : 0,
-                            solution.length < 64 ? solution.length : 64
-                    );
 
                     // Build response
                     PacketBody8Init1.Step4 step4 = new PacketBody8Init1.Step4();
@@ -360,4 +351,17 @@ public class LocalClientHandlerConnecting extends LocalClientHandler {
 
         getClient().writePacket(new PacketBody2Command(ProtocolRole.CLIENT, clientinit));
     }
+
+    /**
+     * The puzzle's answer as 64 unsigned big-endian bytes. {@link BigInteger#toByteArray()}
+     * adds a sign byte when the top bit is set and drops leading zero bytes, so it can be
+     * 65 bytes or fewer than 64.
+     */
+    static byte[] unsigned64(BigInteger value) {
+        byte[] bytes = value.toByteArray();
+        byte[] y = new byte[64];
+        int length = Math.min(bytes.length, 64);
+        System.arraycopy(bytes, bytes.length - length, y, 64 - length, length);
+        return y;
+    }
 }

src/test/java/com/github/manevolent/ts3j/protocol/packet/handler/client/PuzzleAnswerTest.java

@@ -0,0 +1,33 @@
+package com.github.manevolent.ts3j.protocol.packet.handler.client;
+
+import junit.framework.TestCase;
+
+import java.math.BigInteger;
+import java.util.Arrays;
+
+public class PuzzleAnswerTest extends TestCase {
+    private static byte[] bytes(int length, int first) {
+        byte[] b = new byte[length];
+        Arrays.fill(b, (byte) 0x11);
+        b[0] = (byte) first;
+        return b;
+    }
+
+    /** Top bit set: toByteArray() gives 65 bytes, the first a sign byte. */
+    public void testDropsTheSignByte() {
+        byte[] answer = bytes(64, 0x80);
+        assertTrue(Arrays.equals(answer, LocalClientHandlerConnecting.unsigned64(new BigInteger(1, answer))));
+    }
+
+    public void testKeepsSixtyFourBytes() {
+        byte[] answer = bytes(64, 0x7f);
+        assertTrue(Arrays.equals(answer, LocalClientHandlerConnecting.unsigned64(new BigInteger(1, answer))));
+    }
+
+    /** A leading zero byte: toByteArray() gives 63 bytes, which used to fail the connect. */
+    public void testPadsAShortAnswerWithZeros() {
+        byte[] answer = bytes(64, 0x00);
+        answer[1] = 0x42;
+        assertTrue(Arrays.equals(answer, LocalClientHandlerConnecting.unsigned64(new BigInteger(1, answer))));
+    }
+}