#2105 P25 P1/2 Motorola Talker Alias CRC Check (#2106)

Co-authored-by: Dennis Sheirer <[email protected]>

src/main/java/io/github/dsheirer/edac/CRC16.java

@@ -0,0 +1,53 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2024 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ * ****************************************************************************
+ */
+
+package io.github.dsheirer.edac;
+
+import io.github.dsheirer.bits.BinaryMessage;
+import io.github.dsheirer.bits.IntField;
+
+/**
+ * Utility for calculating the CRC checksum for CRC-16 using polynomial 0x1021 and Initial Fill/Residual of 0xFFFF
+ */
+public class CRC16
+{
+    /**
+     * Calculates the 16-bit CRC checksum for the message using polynomial 0x1021 and residual 0xFFFF
+     * @param message with transmitted 16-bit checksum at the end.
+     * @return true if the check is correct or false if it fails the CRC check.
+     */
+    public static boolean check(BinaryMessage message)
+    {
+        BinaryMessage copy = message.copy();
+        BinaryMessage polynomial = BinaryMessage.loadHex("11021");
+        polynomial.rotateLeft(3, 0, 20);
+        polynomial.setSize(message.size());
+
+        int previousX = 0;
+        for(int x = copy.nextSetBit(0); x < copy.size() - 16; x = copy.nextSetBit(x + 1))
+        {
+            polynomial.rotateRight(x - previousX, previousX, 17 + x);
+            previousX = x;
+            copy.xor(polynomial);
+        }
+
+        IntField crc = IntField.length16(copy.size() - 16);
+        return copy.getInt(crc) == 0xFFFF;
+    }
+}

src/main/java/io/github/dsheirer/module/decode/p25/phase1/P25P1DecoderState.java

@@ -317,7 +317,7 @@ public void receive(IMessage iMessage)
                     break;
             }
         }
-        else if(iMessage instanceof MotorolaTalkerAliasComplete tac)
+        else if(iMessage instanceof MotorolaTalkerAliasComplete tac && tac.isValid())
         {
             mTrafficChannelManager.getTalkerAliasManager().update(tac.getRadio(), tac.getAlias());
         }

src/main/java/io/github/dsheirer/module/decode/p25/phase1/message/lc/motorola/LCMotorolaTalkerAliasAssembler.java

@@ -19,7 +19,9 @@
 
 package io.github.dsheirer.module.decode.p25.phase1.message.lc.motorola;
 
+import io.github.dsheirer.bits.BinaryMessage;
 import io.github.dsheirer.bits.CorrectedBinaryMessage;
+import io.github.dsheirer.edac.CRC16;
 import io.github.dsheirer.message.TimeslotMessage;
 import io.github.dsheirer.module.decode.p25.phase1.message.lc.LinkControlWord;
 import io.github.dsheirer.protocol.Protocol;
@@ -137,11 +139,37 @@ public MotorolaTalkerAliasComplete assemble() throws IllegalStateException
             offset += DATA_BLOCK_FRAGMENT_LENGTH;
         }
 
+        trimTalkerAliasLength(reassembled);
         MotorolaTalkerAliasComplete complete = new MotorolaTalkerAliasComplete(reassembled, mHeader.getTalkgroup(),
                 mHeader.getSequence(), TimeslotMessage.TIMESLOT_0, mMostRecentTimestamp, Protocol.APCO25);
 
+        //   Data:  wwwww sss iiiiii aaaa...aaaa cccc
+        //
+        //   - w = WACN
+        //   - s = system
+        //   - i = id
+        //   - a = encoded alias
+        //   - c = CRC-16/GSM of the previous bytes
+        complete.setValid(CRC16.check(reassembled));
+
         mHeader = null;
         mDataBlocks.clear();
         return complete;
     }
+
+
+    /**
+     * Trims the message length to exclude pad zeros so that the CRC calculation knows where to finish.
+     * @param message containing an encoded talker alias.
+     */
+    public static void trimTalkerAliasLength(BinaryMessage message)
+    {
+        int x = 72; //Minimum bit size WACN + SYS + RADIO + 1 CHARACTER = 18 Hex Characters * 4 Bits Each
+        while(message.nextSetBit(x) > 0 && x < message.size())
+        {
+            x += 8;
+        }
+
+        message.setSize(x);
+    }
 }

src/main/java/io/github/dsheirer/module/decode/p25/phase1/message/lc/motorola/MotorolaTalkerAliasComplete.java

@@ -1,6 +1,6 @@
 /*
  * *****************************************************************************
- * Copyright (C) 2014-2024 Dennis Sheirer, 2024 Ilya Smirnov
+ * Copyright (C) 2014-2024 Dennis Sheirer
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -99,10 +99,15 @@ public String toString()
         {
             sb.append("TS").append(getTimeslot()).append(" ");
         }
+
+        if(!isValid())
+        {
+            sb.append("(CRC FAILED) ");
+        }
+
         sb.append("MOTOROLA TALKER ALIAS COMPLETE");
         sb.append(" RADIO:").append(getRadio());
         sb.append(" TG:").append(getTalkgroup());
-        sb.append(" ENCODED:").append(getEncodedAlias().toHexString());
         sb.append(" ALIAS:").append(getAlias());
         sb.append(" SEQUENCE:").append(mSequence);
         sb.append(" MSG:").append(getMessage().toHexString());
@@ -135,19 +140,17 @@ public P25TalkerAliasIdentifier getAlias()
         {
             byte[] encoded = getEncodedAlias().toByteArray();
 
-            // TODO: check CRC in last two bytes to validate alias has no errors
-            //
+            // Note: CRC check is performed by assembler.
             //   Data:  wwwww sss iiiiii aaaa...aaaa cccc
             //
             //   - w = WACN
             //   - s = system
             //   - i = id
             //   - a = encoded alias
             //   - c = CRC-16/GSM of the previous bytes
-
             // Get number of bytes and characters excluding checksum
-            char bytes = (char)(encoded.length - 2);
-            char chars = (char)(bytes / 2);
+            char bytes = (char) (encoded.length - 2);
+            char chars = (char) (bytes / 2);
 
             // Create array for decoded computed bytes (big-endian)
             byte[] decoded = new byte[encoded.length];
@@ -160,35 +163,37 @@ public P25TalkerAliasIdentifier getAlias()
             do
             {
                 // Multiplication step 1
-                char accumMult = (char)(((accumulator + 65536) % 65536) * 293 + 0x72E9);
+                char accumMult = (char) (((accumulator + 65536) % 65536) * 293 + 0x72E9);
 
                 // Lookup table step
                 byte lut = LOOKUP_TABLE[encoded[byteIndex] + 128];
-                byte mult1 = (byte)(lut - (byte)(accumMult >> 8));
+                byte mult1 = (byte) (lut - (byte) (accumMult >> 8));
 
                 // Incrementing step
                 byte mult2 = 1;
-                byte shortstop = (byte)(accumMult | 0x1);
-                byte increment = (byte)(shortstop << 1);
+                byte shortstop = (byte) (accumMult | 0x1);
+                byte increment = (byte) (shortstop << 1);
 
-                while(mult2 != -1 && shortstop != 1) {
-                    shortstop = (byte)(shortstop + increment);
+                while(mult2 != -1 && shortstop != 1)
+                {
+                    shortstop = (byte) (shortstop + increment);
                     mult2 += 2;
                 }
 
                 // Multiplication step 2
-                decoded[byteIndex] = (byte)(mult1 * mult2);
+                decoded[byteIndex] = (byte) (mult1 * mult2);
 
                 // Update the accumulator
-                accumulator += (char)(((encoded[byteIndex] + 256) % 256) + 1);
+                accumulator += (char) (((encoded[byteIndex] + 256) % 256) + 1);
                 byteIndex += 1;
             }
             while(byteIndex < bytes);
 
             // Copy decoded bytes (as chars) to our alias string
             String alias = "";
-            for (char i = 0; i < chars; i++) {
-                alias += (char)((decoded[i * 2] << 8) | decoded[i * 2 + 1]);
+            for(char i = 0; i < chars; i++)
+            {
+                alias += (char) ((decoded[i * 2] << 8) | decoded[i * 2 + 1]);
             }
 
             mAlias = P25TalkerAliasIdentifier.create(alias);
@@ -267,7 +272,12 @@ public List<Identifier> getIdentifiers()
             mIdentifiers = new ArrayList<>();
             mIdentifiers.add(getTalkgroup());
             mIdentifiers.add(getRadio());
-            mIdentifiers.add(getAlias());
+
+            //Only add the alias if it passes the CRC check.
+            if(isValid())
+            {
+                mIdentifiers.add(getAlias());
+            }
         }
 
         return mIdentifiers;

src/main/java/io/github/dsheirer/module/decode/p25/phase2/P25P2DecoderState.java

@@ -262,7 +262,7 @@ else if(message instanceof EncryptionSynchronizationSequence ess)
                     continueState(State.CALL);
                 }
             }
-            else if(message instanceof MotorolaTalkerAliasComplete tac)
+            else if(message instanceof MotorolaTalkerAliasComplete tac && tac.isValid())
             {
                 mTrafficChannelManager.getTalkerAliasManager().update(tac.getRadio(), tac.getAlias());
             }

src/main/java/io/github/dsheirer/module/decode/p25/phase2/message/mac/structure/motorola/MotorolaTalkerAliasAssembler.java

@@ -19,7 +19,9 @@
 
 package io.github.dsheirer.module.decode.p25.phase2.message.mac.structure.motorola;
 
+import io.github.dsheirer.bits.BinaryMessage;
 import io.github.dsheirer.bits.CorrectedBinaryMessage;
+import io.github.dsheirer.edac.CRC16;
 import io.github.dsheirer.module.decode.p25.phase1.message.lc.motorola.MotorolaTalkerAliasComplete;
 import io.github.dsheirer.module.decode.p25.phase2.message.mac.structure.MacStructure;
 import io.github.dsheirer.protocol.Protocol;
@@ -142,11 +144,36 @@ public MotorolaTalkerAliasComplete assemble() throws IllegalStateException
             offset += DATA_BLOCK_FRAGMENT_LENGTH;
         }
 
+        trimTalkerAliasLength(reassembled);
         MotorolaTalkerAliasComplete complete = new MotorolaTalkerAliasComplete(reassembled, mHeader.getTalkgroup(),
                 mHeader.getSequence(), mTimeslot, mMostRecentTimestamp, Protocol.APCO25_PHASE2);
 
+        //   Data:  wwwww sss iiiiii aaaa...aaaa cccc
+        //
+        //   - w = WACN
+        //   - s = system
+        //   - i = id
+        //   - a = encoded alias
+        //   - c = CRC-16/GSM of the previous bytes
+        complete.setValid(CRC16.check(reassembled));
+
         mHeader = null;
         mDataBlocks.clear();
         return complete;
     }
+
+    /**
+     * Trims the message length to exclude pad zeros so that the CRC calculation knows where to finish.
+     * @param message containing an encoded talker alias.
+     */
+    public static void trimTalkerAliasLength(BinaryMessage message)
+    {
+        int x = 72; //Minimum bit size WACN + SYS + RADIO + 1 CHARACTER = 18 Hex Characters * 4 Bits Each
+        while(message.nextSetBit(x) > 0 && x < message.size())
+        {
+            x += 8;
+        }
+
+        message.setSize(x);
+    }
 }