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Code Issues Packages Projects Releases 2 Wiki Activity

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

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Co-authored-by: Dennis Sheirer <dsheirer@github.com>
This commit is contained in:
Denny Sheirer
2024-12-06 04:33:14 -05:00
committed by GitHub
parent 4cf51405ea
commit 49cf92c7ab
6 changed files with 138 additions and 20 deletions
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53
src/main/java/io/github/dsheirer/edac/CRC16.java Normal file
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@@ -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;
}
}

2
src/main/java/io/github/dsheirer/module/decode/p25/phase1/P25P1DecoderState.java
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@@ -317,7 +317,7 @@ public class P25P1DecoderState extends DecoderState implements IChannelEventList
break;
}
}
else if(iMessage instanceof MotorolaTalkerAliasComplete tac)
else if(iMessage instanceof MotorolaTalkerAliasComplete tac && tac.isValid())
{
mTrafficChannelManager.getTalkerAliasManager().update(tac.getRadio(), tac.getAlias());
}

28
src/main/java/io/github/dsheirer/module/decode/p25/phase1/message/lc/motorola/LCMotorolaTalkerAliasAssembler.java
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@@ -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 class LCMotorolaTalkerAliasAssembler
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);
}
}

46
src/main/java/io/github/dsheirer/module/decode/p25/phase1/message/lc/motorola/MotorolaTalkerAliasComplete.java
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@@ -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 class MotorolaTalkerAliasComplete extends TimeslotMessage implements IMes
{
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());
@@ -135,8 +140,7 @@ public class MotorolaTalkerAliasComplete extends TimeslotMessage implements IMes
{
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
@@ -144,10 +148,9 @@ public class MotorolaTalkerAliasComplete extends TimeslotMessage implements IMes
// - 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 class MotorolaTalkerAliasComplete extends TimeslotMessage implements IMes
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 class MotorolaTalkerAliasComplete extends TimeslotMessage implements IMes
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;

2
src/main/java/io/github/dsheirer/module/decode/p25/phase2/P25P2DecoderState.java
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@@ -262,7 +262,7 @@ public class P25P2DecoderState extends TimeslotDecoderState implements Identifie
continueState(State.CALL);
}
}
else if(message instanceof MotorolaTalkerAliasComplete tac)
else if(message instanceof MotorolaTalkerAliasComplete tac && tac.isValid())
{
mTrafficChannelManager.getTalkerAliasManager().update(tac.getRadio(), tac.getAlias());
}

27
src/main/java/io/github/dsheirer/module/decode/p25/phase2/message/mac/structure/motorola/MotorolaTalkerAliasAssembler.java
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@@ -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 class MotorolaTalkerAliasAssembler
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);
}
}