Detect and block connections to online-mode servers

src/main/java/com/velocitypowered/proxy/connection/backend/LoginSessionHandler.java

@@ -3,6 +3,7 @@ package com.velocitypowered.proxy.connection.backend;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.packets.Disconnect;
+import com.velocitypowered.proxy.protocol.packets.EncryptionRequest;
 import com.velocitypowered.proxy.protocol.packets.ServerLoginSuccess;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.packets.SetCompression;
@@ -16,6 +17,10 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handle(MinecraftPacket packet) {
+        if (packet instanceof EncryptionRequest) {
+            throw new IllegalStateException("Backend server is online-mode!");
+        }
+
         if (packet instanceof Disconnect) {
             Disconnect disconnect = (Disconnect) packet;
             connection.disconnect();
@@ -35,4 +40,9 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
             connection.getChannel().setSessionHandler(new PlaySessionHandler(connection));
         }
     }
+
+    @Override
+    public void exception(Throwable throwable) {
+        connection.getProxyPlayer().handleConnectionException(connection.getServerInfo(), throwable);
+    }
 }

src/main/java/com/velocitypowered/proxy/protocol/ProtocolUtils.java

@@ -49,4 +49,21 @@ public enum ProtocolUtils { ;
         writeVarInt(buf, asUtf8.length);
         buf.writeBytes(asUtf8);
     }
+
+    public static byte[] readByteArray(ByteBuf buf) {
+        return readByteArray(buf, DEFAULT_MAX_STRING_SIZE);
+    }
+
+    public static byte[] readByteArray(ByteBuf buf, int cap) {
+        int length = readVarInt(buf);
+        Preconditions.checkArgument(length <= cap, "Bad string size (got %s, maximum is %s)", length, cap);
+        byte[] array = new byte[length];
+        buf.readBytes(array);
+        return array;
+    }
+
+    public static void writeByteArray(ByteBuf buf, byte[] array) {
+        writeVarInt(buf, array.length);
+        buf.writeBytes(array);
+    }
 }

src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -36,7 +36,7 @@ public enum StateRegistry {
             TO_SERVER.register(0x00, ServerLogin.class, ServerLogin::new);
 
             TO_CLIENT.register(0x00, Disconnect.class, Disconnect::new);
-            // Encryption Success will follow once Mojang auth/encryption is done
+            TO_CLIENT.register(0x01, EncryptionRequest.class, EncryptionRequest::new);
             TO_CLIENT.register(0x02, ServerLoginSuccess.class, ServerLoginSuccess::new);
             TO_CLIENT.register(0x03, SetCompression.class, SetCompression::new);
         }

src/main/java/com/velocitypowered/proxy/protocol/packets/EncryptionRequest.java

@@ -0,0 +1,51 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+
+import java.util.Arrays;
+
+public class EncryptionRequest implements MinecraftPacket {
+    private byte[] publicKey;
+    private byte[] verifyToken;
+
+    public byte[] getPublicKey() {
+        return publicKey;
+    }
+
+    public void setPublicKey(byte[] publicKey) {
+        this.publicKey = publicKey;
+    }
+
+    public byte[] getVerifyToken() {
+        return verifyToken;
+    }
+
+    public void setVerifyToken(byte[] verifyToken) {
+        this.verifyToken = verifyToken;
+    }
+
+    @Override
+    public String toString() {
+        return "EncryptionRequest{" +
+                "publicKey=" + Arrays.toString(publicKey) +
+                ", verifyToken=" + Arrays.toString(verifyToken) +
+                '}';
+    }
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        ProtocolUtils.readString(buf); // Server ID, can be ignored since it is an empty string
+        publicKey = ProtocolUtils.readByteArray(buf, 256);
+        verifyToken = ProtocolUtils.readByteArray(buf, 16);
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        ProtocolUtils.writeString(buf, ""); // Server ID
+        ProtocolUtils.writeByteArray(buf, publicKey);
+        ProtocolUtils.writeByteArray(buf, verifyToken);
+    }
+}