fix: read the TAR of 03.48 response packets at the right offset

pom.xml

@@ -4,7 +4,7 @@
    <groupId>com.mobiera.libs</groupId>
    <artifactId>common-sim</artifactId>
    <packaging>jar</packaging>
-   <version>1.0.22</version>
+   <version>1.0.23</version>
    <name>Common Sim</name>
    <description>Common Sim</description>
    <url>https://github.com/mobiera/common-sim</url>

src/main/java/com/mobiera/java/sim/tools/PacketUtil.java

@@ -1,43 +1,116 @@
 package com.mobiera.java.sim.tools;
 
+/**
+ * Helpers for GSM 03.48 / ETSI TS 102 225 secured packets carried in SMS user data.
+ *
+ * Both packet kinds start with a 3 byte user data header: UDHL (02), IEI, IEDL (00).
+ * IEI 0x70 is a command packet, IEI 0x71 a response packet (Proof of Receipt).
+ *
+ * <pre>
+ * command packet  : UDH(3) CPL(2) CHL(1) SPI(2) KIc(1) KID(1) TAR(3) CNTR(5) PCNTR(1) ...
+ * response packet : UDH(3) RPL(2) RHL(1) TAR(3) CNTR(5) PCNTR(1) STATUS(1) ...
+ * </pre>
+ */
 public class PacketUtil {
 
 	private static int MIN_PACKET_SIZE = 10;
-
+
+	public static final byte IEI_COMMAND_PACKET = (byte) 0x70;
+	public static final byte IEI_RESPONSE_PACKET = (byte) 0x71;
+
+	public static final int UDH_LENGTH = 3;
+	public static final int COMMAND_PACKET_TAR_OFFSET = UDH_LENGTH + 2 + 1 + 2 + 1 + 1; // 10
+	public static final int RESPONSE_PACKET_TAR_OFFSET = UDH_LENGTH + 2 + 1;             // 6
+	public static final int TAR_LENGTH = 3;
+
 	// check if looks a command packet
 	// 027100000E0A00000000000000000000016101
 	// 027100000B0A0000000000000000000A
-
+
+	/**
+	 * True when the message starts with a 03.48 user data header, either command (0x70)
+	 * or response (0x71) packet. Kept for backward compatibility: despite its name it
+	 * accepts both kinds.
+	 */
 	public static boolean isCommandPacket(byte[] msg) {
-		
-		if (msg.length >= MIN_PACKET_SIZE) {
+
+		if (msg != null && msg.length >= MIN_PACKET_SIZE) {
 			// binary msg
-			if ( (msg[0] == (byte)0x02) && ( (msg[1] == (byte)0x71) || (msg[1] == (byte)0x70) ) && (msg[2] == (byte)0x00)) {
+			if ( (msg[0] == (byte)0x02) && ( (msg[1] == IEI_RESPONSE_PACKET) || (msg[1] == IEI_COMMAND_PACKET) ) && (msg[2] == (byte)0x00)) {
 					// looks like a packet
-				
+
 				return true;
-		
+
 			}
 		}
-		
+
 		return false;
 	}
-
+
+	/**
+	 * True when the message is a 03.48 response packet (Proof of Receipt), IEI 0x71.
+	 */
+	public static boolean isResponsePacket(byte[] msg) {
+		return isCommandPacket(msg) && msg[1] == IEI_RESPONSE_PACKET;
+	}
+
+	/**
+	 * TAR of a command packet (offset 10, after the security parameters). Do not use on a
+	 * response packet, see {@link #getPacketTar(byte[])}.
+	 */
 	public static byte[] getCommandPacketTar(byte[] msg) {
-
-		byte[] binTar = new byte[3];
-		System.arraycopy(msg, 10, binTar, 0, 3);
+
+		byte[] binTar = new byte[TAR_LENGTH];
+		System.arraycopy(msg, COMMAND_PACKET_TAR_OFFSET, binTar, 0, TAR_LENGTH);
+		return binTar;
+
+	}
+
+	/**
+	 * TAR of a response packet (offset 6, right after RPL and RHL).
+	 */
+	public static byte[] getResponsePacketTar(byte[] msg) {
+
+		byte[] binTar = new byte[TAR_LENGTH];
+		System.arraycopy(msg, RESPONSE_PACKET_TAR_OFFSET, binTar, 0, TAR_LENGTH);
 		return binTar;
-		
+
 	}
-
+
+	/**
+	 * TAR of either packet kind, chosen from the UDH IEI. Returns null when the message is
+	 * not a packet or is too short to hold a TAR.
+	 */
+	public static byte[] getPacketTar(byte[] msg) {
+
+		if (!isCommandPacket(msg)) return null;
+
+		int offset = (msg[1] == IEI_RESPONSE_PACKET) ? RESPONSE_PACKET_TAR_OFFSET : COMMAND_PACKET_TAR_OFFSET;
+		if (msg.length < offset + TAR_LENGTH) return null;
+
+		byte[] binTar = new byte[TAR_LENGTH];
+		System.arraycopy(msg, offset, binTar, 0, TAR_LENGTH);
+		return binTar;
+	}
+
+	/**
+	 * Packet without its user data header, i.e. starting at CPL or RPL. Returns the
+	 * message itself when it does not carry a 03.48 UDH.
+	 */
+	public static byte[] stripUdh(byte[] msg) {
+		if (!isCommandPacket(msg)) return msg;
+		byte[] out = new byte[msg.length - UDH_LENGTH];
+		System.arraycopy(msg, UDH_LENGTH, out, 0, out.length);
+		return out;
+	}
+
 	public static boolean isEncrypted(byte[] msg) {
-		
+
 		if (((msg[6]) & (msg[7])) != 0x00) {
 			return true;
 		}
 		return false;
 	}
-	
-	
+
+
 }