Added utility to extract XML-Enc file.

README.md

@@ -56,7 +56,8 @@ Example output:
 
 `MyTapWater   12345678     6.388 m3     6.377 m3    8°C    23°C   DRY(dry 22-31 days)     2018-03-05 12:02.50`
 
-(Temperature is only sent every 8 message. Until such a full message has been received, the temperature is reported as 127°C.)
+(Temperature is only sent every 8 message. Until such a full message has been received, the temperature is reported as -°C
+in the human readable output, and as 127°C in the robot outputs and json.)
 
 Example robot json output:
 
@@ -90,6 +91,11 @@ If the meter does not use encryption of its meter data, then enter an empty key
 
 `wmbusmeters --robot --meterfiles auto MyTapWater multical21 12345678 ""`
 
+If you have a Multical21 meter and you have received a KEM file and its password,
+from your water municipality, then you can use the XMLExtract.java utility to get
+the meter key from the KEM file. You need to unzip the the KEM file first though,
+if it is zipped.
+
 You can run wmbusmeters with --logtelegrams to get log output that can be placed in a simulation.txt
 file. You can then run wmbusmeter and instead of auto (or an usb device) provide the simulationt.xt
 file as argument. See test.sh for more info.

XMLExtract.java

@@ -0,0 +1,41 @@
+//
+// A utility to extract the contents of an
+// XML containing an XML-Encrypted CipherValue.
+//
+// Compile with javac XMLExtract.java (requires jdk8 or above).
+//
+import javax.crypto.Cipher;
+import javax.crypto.SecretKey;
+import javax.crypto.spec.IvParameterSpec;
+import javax.crypto.spec.SecretKeySpec;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.Base64;
+import java.util.stream.Collectors;
+
+public class XMLExtract
+{
+    static Cipher getCipherDecrypt(String password) throws Exception {
+        byte[] key = Arrays.copyOf(password.toUpperCase().getBytes("UTF-8"), 16);
+        Cipher c = Cipher.getInstance("AES/CBC/NoPadding");
+        SecretKeySpec sks = new SecretKeySpec(key, "AES");
+        IvParameterSpec ips = new IvParameterSpec(key);
+        c.init(Cipher.DECRYPT_MODE, sks, ips);
+        return c;
+    }
+
+    public static void main(String args[]) throws Exception
+    {
+        if (args.length < 2) {
+            System.out.println("java -cp . XMLExtract [password] [encrypted_xml_file] ");
+            return;
+        }
+        Cipher c = getCipherDecrypt(args[0]);
+        String xml = Files.lines(Paths.get(args[1])).collect(Collectors.joining(""));
+        String hex = xml.substring(xml.indexOf("<CipherValue>")+13, xml.indexOf("</CipherValue>"));
+        byte[] raw = Base64.getDecoder().decode(hex);
+        String plain = new String(c.doFinal(raw));
+        System.out.println(plain);
+    }
+}