Add the CRC and callsign encoding.

M17CRC.cpp

@@ -0,0 +1,69 @@
+/*
+ *   Copyright (C) 2018,2020 by Jonathan Naylor G4KLX
+ *
+ *   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 2 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, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "M17CRC.h"
+
+#include <cstdio>
+#include <cassert>
+
+const uint8_t  BIT_MASK_TABLE1[] = { 0x80U, 0x40U, 0x20U, 0x10U, 0x08U, 0x04U, 0x02U, 0x01U };
+
+#define READ_BIT(p,i)    (p[(i)>>3] & BIT_MASK_TABLE1[(i)&7])
+
+bool CM17CRC::checkCRC(const unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+	assert(nBytes > 2U);
+
+	uint16_t crc = createCRC(in, nBytes);
+
+	uint8_t temp[2U];
+	temp[0U] = (crc >> 8) & 0xFFU;
+	temp[1U] = (crc >> 0) & 0xFFU;
+
+	return temp[0U] == in[nBytes - 2U] && temp[1U] == in[nBytes - 1U];
+}
+
+void CM17CRC::encodeCRC(unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+	assert(nBytes > 2U);
+
+	uint16_t crc = createCRC(in, nBytes);
+
+	in[nBytes - 2U] = (crc >> 8) & 0xFFU;
+	in[nBytes - 1U] = (crc >> 0) & 0xFFU;
+}
+uint16_t CM17CRC::createCRC(const unsigned char* in, unsigned int nBytes)
+{
+	assert(in != NULL);
+
+	uint16_t crc = 0xFFFFU;
+
+	for (unsigned int i = 0U; i < nBytes; i++) {
+		bool bit1 = READ_BIT(in, i) != 0x00U;
+		bool bit2 = (crc & 0x8000U) == 0x8000U;
+
+		crc <<= 1;
+
+		if (bit1 ^ bit2)
+			crc ^= 0x5935U;
+	}
+
+	return crc;
+}

M17CRC.h

@@ -24,11 +24,11 @@
 class CM17CRC
 {
 public:
-	static bool checkCRC(const unsigned char* in, unsigned int length);
+	static bool checkCRC(const unsigned char* in, unsigned int nBytes);
-	static void encodeCRC(unsigned char* in, unsigned int length);
+	static void encodeCRC(unsigned char* in, unsigned int nBytes);
 
 private:
-	static uint16_t createCRC(const unsigned char* in, unsigned int length);
+	static uint16_t createCRC(const unsigned char* in, unsigned int nBytes);
 };
 
 #endif

M17Control.cpp

@@ -169,7 +169,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		unsigned char frame[M17_LICH_LENGTH_BYTES];
 		conv.chainback(frame, M17_LICH_LENGTH_BITS);
 
-		bool valid = CM17CRC::checkCRC(frame, M17_LICH_LENGTH_BITS);
+		bool valid = CM17CRC::checkCRC(frame, M17_LICH_LENGTH_BYTES);
 		if (valid) {
 			m_rfFrames = 0U;
 			m_rfErrs = 0U;
@@ -225,7 +225,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 				m_rfLICH.getLinkSetup(data + 2U);
 
 				// Add the CRC
-				CM17CRC::encodeCRC(data + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+				CM17CRC::encodeCRC(data + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 
 				// Add the convolution FEC
 				CM17Convolution conv;
@@ -257,7 +257,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		unsigned char frame[M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES];
 		conv.chainback(frame, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
 
-		bool valid = CM17CRC::checkCRC(frame, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+		bool valid = CM17CRC::checkCRC(frame, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 		if (valid) {
 			m_rfFN = (frame[0U] << 8) + (frame[1U] << 0);
 
@@ -323,7 +323,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 					m_rfLICH.getLinkSetup(data + 2U);
 
 					// Add the CRC
-					CM17CRC::encodeCRC(data + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+					CM17CRC::encodeCRC(data + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 
 					// Add the convolution FEC
 					CM17Convolution conv;
@@ -357,7 +357,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		unsigned char frame[M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES];
 		conv.chainback(frame, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
 
-		bool valid = CM17CRC::checkCRC(frame, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+		bool valid = CM17CRC::checkCRC(frame, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 		if (valid) {
 			m_rfFN = (frame[0U] << 8) + (frame[1U] << 0);
 		} else {
@@ -383,7 +383,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 			}
 
 			// Add the CRC
-			CM17CRC::encodeCRC(frame, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+			CM17CRC::encodeCRC(frame, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 		}
 
 		unsigned char rfData[2U + M17_FRAME_LENGTH_BYTES];
@@ -422,7 +422,7 @@ bool CM17Control::writeModem(unsigned char* data, unsigned int len)
 		// Copy the FN and payload from the frame
 		::memcpy(netData + M17_LICH_LENGTH_BYTES, frame, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES);
 
-		CM17CRC::encodeCRC(netData, M17_LICH_LENGTH_BITS + M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+		// The CRC is added in the networking code
 
 		writeNetwork(netData);
 
@@ -552,7 +552,7 @@ void CM17Control::writeNetwork()
 		m_netLICH.getLinkSetup(start + 2U);
 
 		// Add the CRC
-		CM17CRC::encodeCRC(start + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+		CM17CRC::encodeCRC(start + 2U + M17_SYNC_LENGTH_BYTES, M17_LICH_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 
 		// Add the convolution FEC
 		CM17Convolution conv;
@@ -612,7 +612,7 @@ void CM17Control::writeNetwork()
 	::memcpy(payload, netData + 38U, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES);
 
 	// Add the CRC
-	CM17CRC::encodeCRC(payload, M17_FN_LENGTH_BITS + M17_PAYLOAD_LENGTH_BITS + M17_CRC_LENGTH_BITS);
+	CM17CRC::encodeCRC(payload, M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 
 	// Add the Convolution FEC
 	CM17Convolution conv;
@@ -726,7 +726,7 @@ void CM17Control::decorrelator(const unsigned char* in, unsigned char* out) cons
 	assert(out != NULL);
 
 	for (unsigned int i = M17_SYNC_LENGTH_BYTES; i < M17_FRAME_LENGTH_BYTES; i++)
-		out[i] = in[i] ^ SCRAMBLER[i];
+		out[i] = in[i] ^ SCRAMBLER[i - M17_SYNC_LENGTH_BYTES];
 }
 
 bool CM17Control::openFile()

M17Network.cpp

@@ -20,6 +20,7 @@
 #include "M17Defines.h"
 #include "M17Utils.h"
 #include "Defines.h"
+#include "M17CRC.h"
 #include "Utils.h"
 #include "Log.h"
 
@@ -119,7 +120,10 @@ bool CM17Network::write(const unsigned char* data)
 	buffer[4U] = m_outId / 256U;	// Unique session id
 	buffer[5U] = m_outId % 256U;
 
-	::memcpy(buffer + 6U, data, 48U);
+	::memcpy(buffer + 6U, data, 46U);
+
+	// Add the CRC
+	CM17CRC::encodeCRC(buffer + 6U, M17_LICH_LENGTH_BYTES + M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
 
 	if (m_debug)
 		CUtils::dump(1U, "M17 data transmitted", buffer, 54U);
@@ -205,6 +209,13 @@ void CM17Network::clock(unsigned int ms)
 			return;
 	}
 
+	// Check the CRC
+	bool valid = CM17CRC::checkCRC(buffer + 6U, M17_LICH_LENGTH_BYTES + M17_FN_LENGTH_BYTES + M17_PAYLOAD_LENGTH_BYTES + M17_CRC_LENGTH_BYTES);
+	if (!valid) {
+		LogMessage("M17, network packet received with an invalid CRC");
+		return;
+	}
+
 	// EOT?
 	uint16_t fn = (buffer[38U] << 8) + (buffer[39U] << 0);
 	if ((fn & 0x8000U) == 0x8000U)

M17Utils.cpp

@@ -0,0 +1,83 @@
+/*
+ *   Copyright (C) 2020 by Jonathan Naylor G4KLX
+ *
+ *   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 2 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, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "M17Utils.h"
+
+#include <cassert>
+
+void CM17Utils::encodeCallsign(const std::string& callsign, unsigned char* encoded)
+{
+	assert(encoded != NULL);
+
+    std::string call = callsign;
+    call.resize(8U, ' ');
+
+    uint64_t enc = 0ULL;
+    for (int i = 7U; i >= 0; i--) {
+        char c = call.at(i);
+
+        enc *= 40ULL;
+
+        // If speed is more important than code space, you can replace this with a lookup into a 256 byte array.
+        if (c >= 'A' && c <= 'Z') // 1-26
+            enc += c - 'A' + 1ULL;
+        else if (c >= '0' && c <= '9') // 27-36
+            enc += c - '0' + 27ULL;
+        else if (c == '-') // 37
+            enc += 37ULL;
+
+        // These are just place holders. If other characters make more sense, change these.
+        // Be sure to change them in the decode array below too.
+        else if (c == '/') // 38
+            enc += 38ULL;
+        else if (c == '.') // 39
+            enc += 39ULL;
+        else
+            // Invalid character or space, represented by 0, decoded as a space.
+            enc += 0ULL;
+    }
+
+    encoded[0U] = enc >> 40;
+    encoded[1U] = enc >> 32;
+    encoded[2U] = enc >> 24;
+    encoded[3U] = enc >> 16;
+    encoded[4U] = enc >> 8;
+    encoded[5U] = enc >> 0;
+}
+
+void CM17Utils::decodeCallsign(const unsigned char* encoded, std::string& callsign)
+{
+    assert(encoded != NULL);
+
+    callsign.empty();
+
+    uint64_t enc = (uint64_t(encoded[5U]) << 40) +
+                   (uint64_t(encoded[4U]) << 32) +
+                   (uint64_t(encoded[3U]) << 24) +
+                   (uint64_t(encoded[2U]) << 16) +
+                   (uint64_t(encoded[1U]) << 8)  +
+                   (uint64_t(encoded[0U]) << 0);
+
+    if (enc >= 262144000000000ULL) // 40^9
+        return;
+
+    while (enc > 0ULL) {
+        callsign += " ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-/."[enc % 40ULL];
+        enc /= 40ULL;
+    }
+}

M17Utils.h

@@ -26,9 +26,9 @@ public:
 	CM17Utils();
 	~CM17Utils();
 
-	static void encodeCallsign(const std::string& callsign, unsigned char* data);
+	static void encodeCallsign(const std::string& callsign, unsigned char* encoded);
 
-	static void decodeCallsign(const unsigned char* data, std::string& callsign);
+	static void decodeCallsign(const unsigned char* encoded, std::string& callsign);
 
 private:
 };

MMDVMHost.vcxproj

@@ -291,6 +291,7 @@
     <ClCompile Include="LCDproc.cpp" />
     <ClCompile Include="Log.cpp" />
     <ClCompile Include="M17Control.cpp" />
+    <ClCompile Include="M17CRC.cpp" />
     <ClCompile Include="M17Network.cpp" />
     <ClCompile Include="MMDVMHost.cpp" />
     <ClCompile Include="Modem.cpp" />
@@ -330,6 +331,7 @@
     <ClCompile Include="RSSIInterpolator.cpp" />
     <ClCompile Include="SerialController.cpp" />
     <ClCompile Include="SerialPort.cpp" />
+    <ClCompile Include="M17Utils.cpp" />
     <ClCompile Include="StopWatch.cpp" />
     <ClCompile Include="Sync.cpp" />
     <ClCompile Include="TFTSerial.cpp" />

MMDVMHost.vcxproj.filters

@@ -589,5 +589,11 @@
     <ClCompile Include="M17Network.cpp">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="M17CRC.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
+    <ClCompile Include="M17Utils.cpp">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
 </Project>