Improve Game Sync ID processing

Also fixes an exception that occurs when a negative PID is sent through Memory Link.

Author: kuroppoi

src/main/java/entralinked/model/player/PlayerManager.java

@@ -18,6 +18,7 @@
 import com.fasterxml.jackson.databind.SerializationFeature;
 
 import entralinked.GameVersion;
+import entralinked.utility.GsidUtility;
 
 /**
  * Manager class for managing {@link Player} information (Global Link users)
@@ -117,9 +118,14 @@ private void loadPlayer(File inputFile) {
             Player player = mapper.readValue(inputFile, PlayerDto.class).toPlayer();
             String gameSyncId = player.getGameSyncId();
 
+            // Check if Game Sync ID is valid
+            if(!GsidUtility.isValidGameSyncId(gameSyncId)) {
+                throw new IOException("Invalid Game Sync ID: %s".formatted(gameSyncId));
+            }
+            
             // Check for duplicate Game Sync ID
             if(doesPlayerExist(gameSyncId)) {
-                throw new IOException("Duplicate Game Sync ID %s".formatted(gameSyncId));
+                throw new IOException("Duplicate Game Sync ID: %s".formatted(gameSyncId));
             }
 
             player.setDataDirectory(inputFile.getParentFile());

src/main/java/entralinked/network/http/pgl/PglHandler.java

@@ -321,6 +321,13 @@ private void handleDownloadSaveData(PglRequest request, Context ctx) throws IOEx
      */
     private void handleMemoryLink(PglRequest request, Context ctx) throws IOException {
         LEOutputStream outputStream = new LEOutputStream(ctx.outputStream());
+        
+        // Check if Game Sync ID is valid
+        if(!GsidUtility.isValidGameSyncId(request.gameSyncId())) {
+            writeStatusCode(outputStream, 8); // Invalid Game Sync ID
+            return;
+        }
+        
         Player player = playerManager.getPlayer(request.gameSyncId());
         User user = ctx.attribute("user");
 
@@ -472,9 +479,9 @@ private void handleCreateAccount(PglRequest request, Context ctx) throws IOExcep
         // Prepare response
         LEOutputStream outputStream = new LEOutputStream(ctx.outputStream());
 
-        // Make sure Game Sync ID is present
-        if(request.gameSyncId() == null) {
-            writeStatusCode(outputStream, 1); // Unauthorized
+        // Check if Game Sync ID is valid
+        if(!GsidUtility.isValidGameSyncId(request.gameSyncId())) {
+            writeStatusCode(outputStream, 8); // Invalid Game Sync ID
             return;
         }
 
@@ -511,6 +518,12 @@ private void handleCreateData(PglRequest request, Context ctx) throws IOExceptio
         LEOutputStream outputStream = new LEOutputStream(ctx.outputStream());
         String gameSyncId = GsidUtility.stringifyGameSyncId(Integer.parseInt(ctx.body().replace("\u0000", ""))); // So quirky
 
+        // Check if Game Sync ID is valid
+        if(!GsidUtility.isValidGameSyncId(request.gameSyncId())) {
+            writeStatusCode(outputStream, 8); // Invalid Game Sync ID
+            return;
+        }
+        
         // Check if player doesn't exist already
         if(playerManager.doesPlayerExist(gameSyncId)) {
             writeStatusCode(outputStream, 2); // Duplicate Game Sync ID

src/main/java/entralinked/serialization/GsidDeserializer.java

@@ -25,12 +25,6 @@ protected GsidDeserializer(Class<?> type) {
 
     @Override
     public String deserialize(JsonParser parser, DeserializationContext context) throws IOException {
-        int gsid = parser.getIntValue();
-        
-        if(gsid < 0) {
-            throw new IOException("Game Sync ID cannot be a negative number.");
-        }
-        
-        return GsidUtility.stringifyGameSyncId(gsid);
+        return GsidUtility.stringifyGameSyncId(parser.getValueAsInt(-1));
     }
 }

src/main/java/entralinked/utility/GsidUtility.java

@@ -2,40 +2,87 @@
 
 import java.util.regex.Pattern;
 
+/**
+ * Game Sync ID generation process:
+ *
+ * Let's take example PID "1231499195".
+ * Start by storing both the PID and its checksum (35497) in working variable "ugsid".
+ * We do this by simply shifting the checksum 32 bits to the left: ugsid = pid | (checksum << 32) = 0x8AA949672FBB
+ * 
+ * Calculating each character is pretty straightforward.
+ * We just use the 5 least significant bits of ugsid as the index for the character table.
+ * Character 1: 0x8AA949672FBB & 0x1F = 27 = '5'
+ * 
+ * After each character, we shift ugsid 5 bits to the right. Since ugsid contains 48 bits of data,
+ * taking the 5 least significant bits each time gives us enough indexes for (if we round up) exactly 10 characters.
+ * If we take a look at the full value of ugsid (0x8AA949672FBB) in binary and split it into sections of 5 bits,
+ * we'll actually already be able to see the entire Game Sync ID in reverse:
+ * 
+ * Character:        'E'   'L'   'X'   'F'   'E'   'Y'   'Q'   'M'   '7'   '5'
+ * Chartable index:   4    10    21     5     4    22    14    11    29    27
+ * Binary:          XX100 01010 10101 00101 00100 10110 01110 01011 11101 11011
+ *                                           
+ * Adding all of the characters together gets us the Game Sync ID "57MQYEFXLE".
+ * 
+ * Reversing this process to retrieve the PID and checksum is very straightforward.
+ * Simply go through each character, find the index of it in the character table & left shift the total value 5 bits each time.
+ * If we do this with the Game Sync ID we just created, then it should give us back the value of ugsid: 0x8AA949672FBB
+ * To then retrieve the PID, simply do: ugsid & 0xFFFFFFFF = 1231499195
+ * To retrieve the checksum, simply do: (ugsid >> 32) & 0xFFFF = 35497
+ * We can then validate the Game Sync ID if we want to by comparing the checksums.
+ */
 public class GsidUtility {
 
     public static final String GSID_CHARTABLE = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789";
     public static final Pattern GSID_PATTERN = Pattern.compile("[A-HJ-NP-Z2-9]{10}");
 
     /**
-     * Stringifies the specified numerical Game Sync ID
+     * Stringifies the specified numerical Game Sync ID.
      * 
      * Black 2 - {@code sub_21B480C} (overlay #199)
      */
     public static String stringifyGameSyncId(int gsid) {
         char[] output = new char[10];
-        int index = 0;
+        long checksum = Crc16.calc(gsid);
+        long ugsid = gsid | (checksum << 32);
 
-        // v12 = gsid
-        // v5 = sub_204405C(gsid, 4u)
-        // v8 = v5 + __CFSHR__(v12, 31) + (v12 >> 31)
-        
-        // uses unsigned ints for bitshift operations
-        long ugsid = gsid;
-        long checksum = Crc16.calc(gsid); // + __CFSHR__(v12, 31) + (v12 >> 31); ??
-        
-        // do while v4 < 10
         for(int i = 0; i < output.length; i++) {
-            index = (int)((ugsid & 0x1F) & 0x1FFFF); // chartable string is unicode, so normally multiplies by 2
-            ugsid = (ugsid >> 5) | (checksum << 27);
-            checksum >>= 5;
-            output[i] = GSID_CHARTABLE.charAt(index); // sub_2048734(v4, chartable + index)
+            int index = (int)((ugsid >> (5 * i)) & 0x1F);
+            output[i] = GSID_CHARTABLE.charAt(index);
         }
 
         return new String(output);
     }
 
+    /**
+     * Determines if a Game Sync ID is valid by checking its length, characters & checksum.
+     * 
+     * @return {@code true} if the Game Sync ID is valid, otherwise {@code false}.
+     */
     public static boolean isValidGameSyncId(String gsid) {
-        return GSID_PATTERN.matcher(gsid).matches();
+        if(gsid == null) {
+            return false;
+        }
+        
+        int length = gsid.length();
+        long ugsid = 0;
+        
+        if(length != 10) {
+            return false;
+        }
+                
+        for(int i = 0; i < length; i++) {
+            int index = GSID_CHARTABLE.indexOf(gsid.charAt(i));
+            
+            if(index == -1) {
+                return false;
+            }
+            
+            ugsid |= (long)index << (5 * i);
+        }
+        
+        int output = (int)(ugsid & 0xFFFFFFFF);
+        int checksum = (int)((ugsid >> 32) & 0xFFFF);
+        return output >= 0 && Crc16.calc(output) == checksum;
     }
 }

src/test/java/entralinked/utility/GsidUtilityTest.java

@@ -31,15 +31,22 @@ void testInvalidGameSyncIds() {
         // Illegal length (should be 10)
         assertFalse(GsidUtility.isValidGameSyncId("Y67UEN38K"));
         assertFalse(GsidUtility.isValidGameSyncId("3ER5K8MBN4C"));
+        
+        // Invalid checksum
+        assertFalse(GsidUtility.isValidGameSyncId("VFWM2Q2ADH"));
+        assertFalse(GsidUtility.isValidGameSyncId("44DAWDA4SH"));
+        assertFalse(GsidUtility.isValidGameSyncId("J6F55U7FUE"));
+        assertFalse(GsidUtility.isValidGameSyncId("8FAB4ZF6JF"));
+        assertFalse(GsidUtility.isValidGameSyncId("HWLNS77HWD"));
     }
 
     @Test
     @DisplayName("Test if valid Game Sync IDs are seen as valid")
     void testValidGameSyncIds() {
         assertTrue(GsidUtility.isValidGameSyncId("VFWM2QAXNF"));
         assertTrue(GsidUtility.isValidGameSyncId("44DAWDJKJ8"));
-        assertTrue(GsidUtility.isValidGameSyncId("J6F55UB2X9"));
-        assertTrue(GsidUtility.isValidGameSyncId("8FAB4Z3EN9"));
+        assertTrue(GsidUtility.isValidGameSyncId("J6F55UB2XD"));
+        assertTrue(GsidUtility.isValidGameSyncId("8FAB4Z3END"));
         assertTrue(GsidUtility.isValidGameSyncId("HWLNS7BTNB"));
     }
 }