Enforce all hmac/mac to always be lower case.

app/src/main/java/at/bitfire/davdroid/journalmanager/Helpers.java

@@ -120,9 +120,13 @@ public class Helpers {
         try {
             MessageDigest digest = MessageDigest.getInstance("SHA-256");
             byte[] hash = digest.digest(base);
-            return Hex.toHexString(hash);
+            return toHex(hash);
         } catch (Exception ex) {
             throw new RuntimeException(ex);
         }
     }
+
+    public static String toHex(byte[] bytes) {
+        return Hex.toHexString(bytes).toLowerCase();
+    }
 }

app/src/main/java/at/bitfire/davdroid/journalmanager/JournalEntryManager.java

@@ -105,7 +105,7 @@ public class JournalEntryManager extends BaseManager {
                 uuid = previous.getUuid();
             }
 
-            return Hex.toHexString(calculateHmac(keyBase64, uuid));
+            return Helpers.toHex(calculateHmac(keyBase64, uuid));
         }
     }
 

app/src/main/java/at/bitfire/davdroid/journalmanager/JournalManager.java

@@ -19,6 +19,7 @@ import okhttp3.Response;
 import okhttp3.ResponseBody;
 
 import static at.bitfire.davdroid.journalmanager.Helpers.sha256;
+import static at.bitfire.davdroid.journalmanager.Helpers.toHex;
 
 public class JournalManager extends BaseManager {
     final static private Type journalType = new TypeToken<List<Journal>>() {
@@ -104,7 +105,7 @@ public class JournalManager extends BaseManager {
 
             byte[] correctHash = calculateHmac(keyBase64);
             if (!Arrays.areEqual(hmac, correctHash)) {
-                throw new Exceptions.IntegrityException("Bad HMAC. " + Hex.toHexString(hmac) + " != " + Hex.toHexString(correctHash));
+                throw new Exceptions.IntegrityException("Bad HMAC. " + toHex(hmac) + " != " + toHex(correctHash));
             }
         }