Add info to MessageKey HKDF, change order of RootKey derivation.

library/src/org/whispersystems/textsecure/crypto/ratchet/ChainKey.java

@@ -38,7 +38,7 @@ public class ChainKey {
   public MessageKeys getMessageKeys() {
     HKDF           kdf              = new HKDF();
     byte[]         inputKeyMaterial = getBaseMaterial(MESSAGE_KEY_SEED);
-    DerivedSecrets keyMaterial      = kdf.deriveSecrets(inputKeyMaterial, null);
+    DerivedSecrets keyMaterial      = kdf.deriveSecrets(inputKeyMaterial, "WhisperMessageKeys".getBytes());
 
     return new MessageKeys(keyMaterial.getCipherKey(), keyMaterial.getMacKey(), index);
   }

library/src/org/whispersystems/textsecure/crypto/ratchet/RatchetingSession.java

@@ -71,8 +71,8 @@ public class RatchetingSession {
     sessionRecord.setRootKey(sendingChain.first);
   }
 
-  public static Pair<RootKey, ChainKey> calculate3DHE(ECKeyPair ourEphemeral, ECPublicKey theirEphemeral,
-                                                      IdentityKeyPair ourIdentity, IdentityKey theirIdentity)
+  private static Pair<RootKey, ChainKey> calculate3DHE(ECKeyPair ourEphemeral, ECPublicKey theirEphemeral,
+                                                       IdentityKeyPair ourIdentity, IdentityKey theirIdentity)
       throws InvalidKeyException
   {
     try {

library/src/org/whispersystems/textsecure/crypto/ratchet/RootKey.java

@@ -30,8 +30,8 @@ public class RootKey {
     HKDF           kdf          = new HKDF();
     byte[]         sharedSecret = Curve.calculateAgreement(theirEphemeral, ourEphemeral.getPrivateKey());
     DerivedSecrets keys         = kdf.deriveSecrets(sharedSecret, key, "WhisperRatchet".getBytes());
-    RootKey        newRootKey   = new RootKey(keys.getMacKey().getEncoded());
-    ChainKey       newChainKey  = new ChainKey(keys.getCipherKey().getEncoded(), 0);
+    RootKey        newRootKey   = new RootKey(keys.getCipherKey().getEncoded());
+    ChainKey       newChainKey  = new ChainKey(keys.getMacKey().getEncoded(), 0);
 
     return new Pair<RootKey, ChainKey>(newRootKey, newChainKey);
   }