Add HKDF support for new sessions.

library/src/org/whispersystems/textsecure/crypto/SessionCipher.java

@@ -20,6 +20,7 @@ import android.content.Context;
 import android.util.Log;
 
 import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.whispersystems.textsecure.crypto.kdf.DerivedSecrets;
 import org.whispersystems.textsecure.crypto.protocol.CiphertextMessage;
 import org.whispersystems.textsecure.storage.CanonicalRecipientAddress;
 import org.whispersystems.textsecure.storage.InvalidKeyIdException;
@@ -38,11 +39,8 @@ import javax.crypto.spec.SecretKeySpec;
 import java.math.BigInteger;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.InvalidKeyException;
-import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.Arrays;
-import java.util.LinkedList;
-import java.util.List;
 
 /**
  * This is where the session encryption magic happens.  Implements a compressed version of the OTR protocol.
@@ -148,23 +146,16 @@ public class SessionCipher {
     }
   }
 
-  private SecretKeySpec deriveMacSecret(SecretKeySpec key) {
+  private byte[] getPlaintext(byte[] cipherText, SecretKeySpec key, int counter)
-    try {
+      throws IllegalBlockSizeException, BadPaddingException
-      MessageDigest md = MessageDigest.getInstance("SHA-1");
+  {
-      byte[] secret    = md.digest(key.getEncoded());
-		
-      return new SecretKeySpec(secret, "HmacSHA1");
-    } catch (NoSuchAlgorithmException e) {
-      throw new IllegalArgumentException("SHA-1 Not Supported!",e);
-    }
-  }
- 	
-  private byte[] getPlaintext(byte[] cipherText, SecretKeySpec key, int counter) throws IllegalBlockSizeException, BadPaddingException {
     Cipher cipher = getCipher(Cipher.DECRYPT_MODE, key, counter);
     return cipher.doFinal(cipherText);
   }
 	
-  private byte[] getCiphertext(byte[] message, SecretKeySpec key, int counter) throws IllegalBlockSizeException, BadPaddingException {
+  private byte[] getCiphertext(byte[] message, SecretKeySpec key, int counter)
+      throws IllegalBlockSizeException, BadPaddingException
+  {
     Cipher cipher = getCipher(Cipher.ENCRYPT_MODE, key, counter);
     return cipher.doFinal(message);
   }
@@ -193,46 +184,50 @@ public class SessionCipher {
     }
   }
 
-  private SecretKeySpec deriveCipherSecret(int mode, List<BigInteger> sharedSecret,
+  private SessionKey getSessionKey(MasterSecret masterSecret, int mode,
-                                           KeyRecords records, int localKeyId,
+                                   int messageVersion,
-                                           int remoteKeyId)
+                                   IdentityKeyPair localIdentityKey,
+                                   KeyRecords records,
+                                   int localKeyId, int remoteKeyId)
       throws InvalidKeyIdException
   {
-    byte[] sharedSecretBytes = concatenateSharedSecrets(sharedSecret);
+    Log.w("SessionCipher", "Getting session key for local: " + localKeyId + " remote: " + remoteKeyId);
-    byte[] derivedBytes      = deriveBytes(sharedSecretBytes, 16 * 2);
+    SessionKey sessionKey = records.getSessionRecord().getSessionKey(mode, localKeyId, remoteKeyId);
-    byte[] cipherSecret      = new byte[16];
 
+    if (sessionKey != null)
+      return sessionKey;
+
+    DerivedSecrets derivedSecrets = calculateSharedSecret(messageVersion, mode, localIdentityKey,
+                                                          records, localKeyId, remoteKeyId);
+
+    return new SessionKey(mode, localKeyId, remoteKeyId, derivedSecrets.getCipherKey(),
+                          derivedSecrets.getMacKey(), masterSecret);
+  }
+	
+  private DerivedSecrets calculateSharedSecret(int messageVersion, int mode,
+                                               IdentityKeyPair localIdentityKey,
+                                               KeyRecords records,
+                                               int localKeyId, int remoteKeyId)
+      throws InvalidKeyIdException
+  {
+    KeyPair               localKeyPair      = records.getLocalKeyRecord().getKeyPairForId(localKeyId);
+    ECPublicKeyParameters remoteKey         = records.getRemoteKeyRecord().getKeyForId(remoteKeyId).getKey();
+    IdentityKey           remoteIdentityKey = records.getSessionRecord().getIdentityKey();
     boolean               isLowEnd          = isLowEnd(records, localKeyId, remoteKeyId);
+
     isLowEnd = (mode == Cipher.ENCRYPT_MODE ? isLowEnd : !isLowEnd);
 
-    if (isLowEnd)  {
+    if (isInitiallyExchangedKeys(records, localKeyId, remoteKeyId) &&
-      System.arraycopy(derivedBytes, 16, cipherSecret, 0, 16);
+        messageVersion >= CiphertextMessage.DHE3_INTRODUCED_VERSION)
+    {
+      return SharedSecretCalculator.calculateSharedSecret(isLowEnd,
+                                                          localKeyPair, localKeyId, localIdentityKey,
+                                                          remoteKey, remoteKeyId, remoteIdentityKey);
     } else {
-      System.arraycopy(derivedBytes, 0, cipherSecret, 0, 16);
+      return SharedSecretCalculator.calculateSharedSecret(messageVersion, isLowEnd,
+                                                          localKeyPair, localKeyId,
+                                                          remoteKey, remoteKeyId);
     }
-		
-    return new SecretKeySpec(cipherSecret, "AES");
-  }
-
-  private byte[] concatenateSharedSecrets(List<BigInteger> sharedSecrets) {
-    int          totalByteSize = 0;
-    List<byte[]> byteValues    = new LinkedList<byte[]>();
-
-    for (BigInteger sharedSecret : sharedSecrets) {
-      byte[] byteValue = sharedSecret.toByteArray();
-      totalByteSize += byteValue.length;
-      byteValues.add(byteValue);
-    }
-
-    byte[] combined = new byte[totalByteSize];
-    int offset      = 0;
-
-    for (byte[] byteValue : byteValues) {
-      System.arraycopy(byteValue, 0, combined, offset, byteValue.length);
-      offset += byteValue.length;
-    }
-
-    return combined;
   }
 
   private boolean isLowEnd(KeyRecords records, int localKeyId, int remoteKeyId)
@@ -247,67 +242,6 @@ public class SessionCipher {
     return local.compareTo(remote) < 0;
   }
 
-  private byte[] deriveBytes(byte[] seed, int bytesNeeded) {
-    MessageDigest md;
-
-    try {
-      md = MessageDigest.getInstance("SHA-256");
-    } catch (NoSuchAlgorithmException e) {
-      Log.w("SessionCipher",e);
-      throw new IllegalArgumentException("SHA-256 Not Supported!");
-    }
-		
-    int rounds = bytesNeeded / md.getDigestLength();
-		
-    for (int i=1;i<=rounds;i++) {
-      byte[] roundBytes = Conversions.intToByteArray(i);
-      md.update(roundBytes);
-      md.update(seed);
-    }
-		
-    return md.digest();
-  }
-	
-  private SessionKey getSessionKey(MasterSecret masterSecret, int mode,
-                                   int messageVersion,
-                                   IdentityKeyPair localIdentityKey,
-                                   KeyRecords records,
-                                   int localKeyId, int remoteKeyId)
-      throws InvalidKeyIdException
-  {
-    Log.w("SessionCipher", "Getting session key for local: " + localKeyId + " remote: " + remoteKeyId);
-    SessionKey sessionKey = records.getSessionRecord().getSessionKey(localKeyId, remoteKeyId);
-
-    if (sessionKey != null)
-      return sessionKey;
-		
-    List<BigInteger> sharedSecret = calculateSharedSecret(messageVersion, localIdentityKey, records, localKeyId, remoteKeyId);
-    SecretKeySpec    cipherKey    = deriveCipherSecret(mode, sharedSecret, records, localKeyId, remoteKeyId);
-    SecretKeySpec    macKey       = deriveMacSecret(cipherKey);
-
-    return new SessionKey(localKeyId, remoteKeyId, cipherKey, macKey, masterSecret);
-  }
-	
-  private List<BigInteger> calculateSharedSecret(int messageVersion,
-                                                 IdentityKeyPair localIdentityKey,
-                                                 KeyRecords records,
-                                                 int localKeyId, int remoteKeyId)
-      throws InvalidKeyIdException
-  {
-    KeyPair               localKeyPair      = records.getLocalKeyRecord().getKeyPairForId(localKeyId);
-    ECPublicKeyParameters remoteKey         = records.getRemoteKeyRecord().getKeyForId(remoteKeyId).getKey();
-    IdentityKey           remoteIdentityKey = records.getSessionRecord().getIdentityKey();
-
-    if (isInitiallyExchangedKeys(records, localKeyId, remoteKeyId) &&
-        messageVersion >= CiphertextMessage.CRADLE_AGREEMENT_VERSION)
-    {
-      return SharedSecretCalculator.calculateSharedSecret(localKeyPair, localIdentityKey,
-                                                          remoteKey, remoteIdentityKey);
-    } else {
-      return SharedSecretCalculator.calculateSharedSecret(localKeyPair, remoteKey);
-    }
-  }
-
   private boolean isInitiallyExchangedKeys(KeyRecords records, int localKeyId, int remoteKeyId)
       throws InvalidKeyIdException
   {

library/src/org/whispersystems/textsecure/crypto/SharedSecretCalculator.java

@@ -5,6 +5,12 @@ import android.util.Log;
 import org.spongycastle.crypto.CipherParameters;
 import org.spongycastle.crypto.agreement.ECDHBasicAgreement;
 import org.spongycastle.crypto.params.ECPublicKeyParameters;
+import org.whispersystems.textsecure.crypto.kdf.DerivedSecrets;
+import org.whispersystems.textsecure.crypto.kdf.HKDF;
+import org.whispersystems.textsecure.crypto.kdf.KDF;
+import org.whispersystems.textsecure.crypto.kdf.NKDF;
+import org.whispersystems.textsecure.crypto.protocol.CiphertextMessage;
+import org.whispersystems.textsecure.util.Conversions;
 
 import java.math.BigInteger;
 import java.util.LinkedList;
@@ -12,15 +18,18 @@ import java.util.List;
 
 public class SharedSecretCalculator {
 
-  public static List<BigInteger> calculateSharedSecret(KeyPair localKeyPair,
+  public static DerivedSecrets calculateSharedSecret(boolean isLowEnd, KeyPair localKeyPair,
+                                                     int localKeyId,
                                                      IdentityKeyPair localIdentityKeyPair,
                                                      ECPublicKeyParameters remoteKey,
+                                                     int remoteKeyId,
                                                      IdentityKey remoteIdentityKey)
   {
     Log.w("SharedSecretCalculator", "Calculating shared secret with cradle agreement...");
+    KDF              kdf     = new HKDF();
     List<BigInteger> results = new LinkedList<BigInteger>();
 
-    if (isLowEnd(localKeyPair.getPublicKey().getKey(), remoteKey)) {
+    if (isSmaller(localKeyPair.getPublicKey().getKey(), remoteKey)) {
       results.add(calculateAgreement(localIdentityKeyPair.getPrivateKey(), remoteKey));
 
       results.add(calculateAgreement(localKeyPair.getKeyPair().getPrivate(),
@@ -33,17 +42,40 @@ public class SharedSecretCalculator {
     }
 
     results.add(calculateAgreement(localKeyPair.getKeyPair().getPrivate(), remoteKey));
-    return results;
+
+    return kdf.deriveSecrets(results, isLowEnd, getInfo(localKeyId,remoteKeyId));
   }
 
-  public static List<BigInteger> calculateSharedSecret(KeyPair localKeyPair,
+  public static DerivedSecrets calculateSharedSecret(int messageVersion, boolean isLowEnd,
-                                                       ECPublicKeyParameters remoteKey)
+                                                     KeyPair localKeyPair, int localKeyId,
+                                                     ECPublicKeyParameters remoteKey, int remoteKeyId)
   {
     Log.w("SharedSecretCalculator", "Calculating shared secret with standard agreement...");
+    KDF kdf;
+
+    if (messageVersion >= CiphertextMessage.DHE3_INTRODUCED_VERSION) kdf = new HKDF();
+    else                                                             kdf = new NKDF();
+
+    Log.w("SharedSecretCalculator", "Using kdf:  " + kdf);
+
     List<BigInteger> results = new LinkedList<BigInteger>();
     results.add(calculateAgreement(localKeyPair.getKeyPair().getPrivate(), remoteKey));
 
-    return results;
+    return kdf.deriveSecrets(results, isLowEnd, getInfo(localKeyId, remoteKeyId));
+  }
+
+  private static byte[] getInfo(int localKeyId, int remoteKeyId) {
+    byte[] info = new byte[3 * 2];
+
+    if (localKeyId < remoteKeyId) {
+      Conversions.mediumToByteArray(info, 0, localKeyId);
+      Conversions.mediumToByteArray(info, 3, remoteKeyId);
+    } else {
+      Conversions.mediumToByteArray(info, 0, remoteKeyId);
+      Conversions.mediumToByteArray(info, 3, localKeyId);
+    }
+
+    return info;
   }
 
   private static BigInteger calculateAgreement(CipherParameters privateKey,
@@ -56,7 +88,7 @@ public class SharedSecretCalculator {
   }
 
 
-  private static boolean isLowEnd(ECPublicKeyParameters localPublic,
+  private static boolean isSmaller(ECPublicKeyParameters localPublic,
                                    ECPublicKeyParameters remotePublic)
   {
     BigInteger local  = localPublic.getQ().getX().toBigInteger();

library/src/org/whispersystems/textsecure/crypto/kdf/DerivedSecrets.java

@@ -0,0 +1,22 @@
+package org.whispersystems.textsecure.crypto.kdf;
+
+import javax.crypto.spec.SecretKeySpec;
+
+public class DerivedSecrets {
+
+  private final SecretKeySpec cipherKey;
+  private final SecretKeySpec macKey;
+
+  public DerivedSecrets(SecretKeySpec cipherKey, SecretKeySpec macKey) {
+    this.cipherKey = cipherKey;
+    this.macKey    = macKey;
+  }
+
+  public SecretKeySpec getCipherKey() {
+    return cipherKey;
+  }
+
+  public SecretKeySpec getMacKey() {
+    return macKey;
+  }
+}

library/src/org/whispersystems/textsecure/crypto/kdf/HKDF.java

@@ -0,0 +1,101 @@
+package org.whispersystems.textsecure.crypto.kdf;
+
+import java.io.ByteArrayOutputStream;
+import java.math.BigInteger;
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+import java.util.List;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+
+public class HKDF extends KDF {
+
+  private static final int HASH_OUTPUT_SIZE  = 32;
+  private static final int KEY_MATERIAL_SIZE = 72;
+
+  private static final int CIPHER_KEYS_OFFSET = 0;
+  private static final int MAC_KEYS_OFFSET    = 32;
+
+  @Override
+  public DerivedSecrets deriveSecrets(List<BigInteger> sharedSecret,
+                                      boolean isLowEnd, byte[] info)
+  {
+    byte[] inputKeyMaterial = concatenateSharedSecrets(sharedSecret);
+    byte[] salt             = new byte[HASH_OUTPUT_SIZE];
+    byte[] prk              = extract(salt, inputKeyMaterial);
+    byte[] okm              = expand(prk, info, KEY_MATERIAL_SIZE);
+
+    SecretKeySpec cipherKey = deriveCipherKey(okm, isLowEnd);
+    SecretKeySpec macKey    = deriveMacKey(okm, isLowEnd);
+
+    return new DerivedSecrets(cipherKey, macKey);
+  }
+
+  private SecretKeySpec deriveCipherKey(byte[] okm, boolean isLowEnd) {
+    byte[] cipherKey = new byte[16];
+
+    if (isLowEnd) {
+      System.arraycopy(okm, CIPHER_KEYS_OFFSET + 0, cipherKey, 0, cipherKey.length);
+    } else {
+      System.arraycopy(okm, CIPHER_KEYS_OFFSET + 16, cipherKey, 0, cipherKey.length);
+    }
+
+    return new SecretKeySpec(cipherKey, "AES");
+  }
+
+  private SecretKeySpec deriveMacKey(byte[] okm, boolean isLowEnd) {
+    byte[] macKey = new byte[20];
+
+    if (isLowEnd) {
+      System.arraycopy(okm, MAC_KEYS_OFFSET + 0, macKey, 0, macKey.length);
+    } else {
+      System.arraycopy(okm, MAC_KEYS_OFFSET + 20, macKey, 0, macKey.length);
+    }
+
+    return new SecretKeySpec(macKey, "HmacSHA1");
+  }
+
+  private byte[] extract(byte[] salt, byte[] inputKeyMaterial) {
+    try {
+      Mac mac = Mac.getInstance("HmacSHA256");
+      mac.init(new SecretKeySpec(salt, "HmacSHA256"));
+      return mac.doFinal(inputKeyMaterial);
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    } catch (InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  private byte[] expand(byte[] prk, byte[] info, int outputSize) {
+    try {
+      int                   iterations = (int)Math.ceil((double)outputSize/(double)HASH_OUTPUT_SIZE);
+      byte[]                mixin      = new byte[0];
+      ByteArrayOutputStream results    = new ByteArrayOutputStream();
+
+      for (int i=0;i<iterations;i++) {
+        Mac mac = Mac.getInstance("HmacSHA256");
+        mac.init(new SecretKeySpec(prk, "HmacSHA256"));
+
+        mac.update(mixin);
+        mac.update(info);
+        mac.update((byte)i);
+
+        byte[] stepResult = mac.doFinal();
+        results.write(stepResult, 0, stepResult.length);
+
+        mixin = stepResult;
+      }
+
+      return results.toByteArray();
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    } catch (InvalidKeyException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+
+
+}

library/src/org/whispersystems/textsecure/crypto/kdf/KDF.java

@@ -0,0 +1,33 @@
+package org.whispersystems.textsecure.crypto.kdf;
+
+import java.math.BigInteger;
+import java.util.LinkedList;
+import java.util.List;
+
+public abstract class KDF {
+
+  public abstract DerivedSecrets deriveSecrets(List<BigInteger> sharedSecret,
+                                               boolean isLowEnd, byte[] info);
+
+  protected byte[] concatenateSharedSecrets(List<BigInteger> sharedSecrets) {
+    int          totalByteSize = 0;
+    List<byte[]> byteValues    = new LinkedList<byte[]>();
+
+    for (BigInteger sharedSecret : sharedSecrets) {
+      byte[] byteValue = sharedSecret.toByteArray();
+      totalByteSize += byteValue.length;
+      byteValues.add(byteValue);
+    }
+
+    byte[] combined = new byte[totalByteSize];
+    int offset      = 0;
+
+    for (byte[] byteValue : byteValues) {
+      System.arraycopy(byteValue, 0, combined, offset, byteValue.length);
+      offset += byteValue.length;
+    }
+
+    return combined;
+  }
+
+}

library/src/org/whispersystems/textsecure/crypto/kdf/NKDF.java

@@ -0,0 +1,73 @@
+package org.whispersystems.textsecure.crypto.kdf;
+
+import android.util.Log;
+
+import org.whispersystems.textsecure.util.Conversions;
+
+import java.math.BigInteger;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.List;
+
+import javax.crypto.spec.SecretKeySpec;
+
+public class NKDF extends KDF {
+
+  @Override
+  public DerivedSecrets deriveSecrets(List<BigInteger> sharedSecret,
+                                      boolean isLowEnd, byte[] info)
+  {
+    SecretKeySpec cipherKey = deriveCipherSecret(isLowEnd, sharedSecret);
+    SecretKeySpec macKey    = deriveMacSecret(cipherKey);
+
+    return new DerivedSecrets(cipherKey, macKey);
+  }
+
+  private SecretKeySpec deriveCipherSecret(boolean isLowEnd, List<BigInteger> sharedSecret) {
+    byte[] sharedSecretBytes = concatenateSharedSecrets(sharedSecret);
+    byte[] derivedBytes      = deriveBytes(sharedSecretBytes, 16 * 2);
+    byte[] cipherSecret      = new byte[16];
+
+    if (isLowEnd)  {
+      System.arraycopy(derivedBytes, 16, cipherSecret, 0, 16);
+    } else {
+      System.arraycopy(derivedBytes, 0, cipherSecret, 0, 16);
+    }
+
+    return new SecretKeySpec(cipherSecret, "AES");
+  }
+
+  private SecretKeySpec deriveMacSecret(SecretKeySpec key) {
+    try {
+      MessageDigest md = MessageDigest.getInstance("SHA-1");
+      byte[] secret    = md.digest(key.getEncoded());
+
+      return new SecretKeySpec(secret, "HmacSHA1");
+    } catch (NoSuchAlgorithmException e) {
+      throw new IllegalArgumentException("SHA-1 Not Supported!",e);
+    }
+  }
+
+  private byte[] deriveBytes(byte[] seed, int bytesNeeded) {
+    MessageDigest md;
+
+    try {
+      md = MessageDigest.getInstance("SHA-256");
+    } catch (NoSuchAlgorithmException e) {
+      Log.w("NKDF", e);
+      throw new IllegalArgumentException("SHA-256 Not Supported!");
+    }
+
+    int rounds = bytesNeeded / md.getDigestLength();
+
+    for (int i=1;i<=rounds;i++) {
+      byte[] roundBytes = Conversions.intToByteArray(i);
+      md.update(roundBytes);
+      md.update(seed);
+    }
+
+    return md.digest();
+  }
+
+
+}

library/src/org/whispersystems/textsecure/crypto/protocol/CiphertextMessage.java

@@ -10,7 +10,7 @@ import org.whispersystems.textsecure.util.Conversions;
 public class CiphertextMessage {
 
   public static final int SUPPORTED_VERSION       = 2;
-  public static final int CRADLE_AGREEMENT_VERSION = 2;
+  public static final int DHE3_INTRODUCED_VERSION = 2;
 
           static final int VERSION_LENGTH         = 1;
   private static final int SENDER_KEY_ID_LENGTH   = 3;

library/src/org/whispersystems/textsecure/storage/SessionKey.java

@@ -16,6 +16,7 @@
  */
 package org.whispersystems.textsecure.storage;
 
+import org.whispersystems.textsecure.crypto.InvalidMessageException;
 import org.whispersystems.textsecure.crypto.MasterCipher;
 import org.whispersystems.textsecure.crypto.MasterSecret;
 import org.whispersystems.textsecure.crypto.SessionCipher;
@@ -34,13 +35,18 @@ import javax.crypto.spec.SecretKeySpec;
 
 public class SessionKey {
 
+  private int           mode;
   private int           localKeyId;
   private int           remoteKeyId;
   private SecretKeySpec cipherKey;
   private SecretKeySpec macKey;
   private MasterCipher  masterCipher;
 
-  public SessionKey(int localKeyId, int remoteKeyId, SecretKeySpec cipherKey, SecretKeySpec macKey, MasterSecret masterSecret) {
+  public SessionKey(int mode, int localKeyId, int remoteKeyId,
+                    SecretKeySpec cipherKey, SecretKeySpec macKey,
+                    MasterSecret masterSecret)
+  {
+    this.mode         = mode;
     this.localKeyId   = localKeyId;
     this.remoteKeyId  = remoteKeyId;
     this.cipherKey    = cipherKey;
@@ -48,7 +54,7 @@ public class SessionKey {
     this.masterCipher = new MasterCipher(masterSecret);
   }
 
-  public SessionKey(byte[] bytes, MasterSecret masterSecret) {
+  public SessionKey(byte[] bytes, MasterSecret masterSecret) throws InvalidMessageException {
     this.masterCipher = new MasterCipher(masterSecret);
     deserialize(bytes);
   }
@@ -58,13 +64,16 @@ public class SessionKey {
     byte[] remoteKeyIdBytes = Conversions.mediumToByteArray(remoteKeyId);
     byte[] cipherKeyBytes   = cipherKey.getEncoded();
     byte[] macKeyBytes      = macKey.getEncoded();
-    byte[] combined         = Util.combine(localKeyIdBytes, remoteKeyIdBytes, cipherKeyBytes, macKeyBytes);
+    byte[] modeBytes        = {(byte)mode};
+    byte[] combined         = Util.combine(localKeyIdBytes, remoteKeyIdBytes,
+                                           cipherKeyBytes, macKeyBytes, modeBytes);
 
     return masterCipher.encryptBytes(combined);
   }
 
-  private void deserialize(byte[] bytes) {
+  private void deserialize(byte[] bytes) throws InvalidMessageException {
-    byte[] decrypted = masterCipher.encryptBytes(bytes);
+    byte[] decrypted = masterCipher.decryptBytes(bytes);
+    
     this.localKeyId  = Conversions.byteArrayToMedium(decrypted, 0);
     this.remoteKeyId = Conversions.byteArrayToMedium(decrypted, 3);
 
@@ -74,6 +83,12 @@ public class SessionKey {
     byte[] macBytes  = new byte[SessionCipher.MAC_KEY_LENGTH];
     System.arraycopy(decrypted, 6 + keyBytes.length, macBytes, 0, macBytes.length);
     
+    if (decrypted.length < 6 + SessionCipher.CIPHER_KEY_LENGTH + SessionCipher.MAC_KEY_LENGTH + 1) {
+      throw new InvalidMessageException("No mode included");
+    }
+    
+    this.mode = decrypted[6 + keyBytes.length + macBytes.length];
+
     this.cipherKey         = new SecretKeySpec(keyBytes, "AES");
     this.macKey            = new SecretKeySpec(macBytes, "HmacSHA1");
   }
@@ -94,4 +109,7 @@ public class SessionKey {
     return this.macKey;
   }
 
+  public int getMode() {
+    return mode;
+  }
 }

library/src/org/whispersystems/textsecure/storage/SessionRecord.java

@@ -21,6 +21,7 @@ import android.util.Log;
 
 import org.whispersystems.textsecure.crypto.IdentityKey;
 import org.whispersystems.textsecure.crypto.InvalidKeyException;
+import org.whispersystems.textsecure.crypto.InvalidMessageException;
 import org.whispersystems.textsecure.crypto.MasterSecret;
 
 import java.io.FileInputStream;
@@ -208,8 +209,14 @@ public class SessionRecord extends Record {
           this.remoteFingerprint     = readBlob(in);
           this.currentSessionVersion = 31337;
 
-          if (in.available() != 0)
+          if (in.available() != 0) {
+            try {
               this.sessionKeyRecord = new SessionKey(readBlob(in), masterSecret);
+            } catch (InvalidMessageException e) {
+              Log.w("SessionRecord", e);
+              this.sessionKeyRecord = null;
+            }
+          }
 
           in.close();
         } else {
@@ -230,8 +237,14 @@ public class SessionRecord extends Record {
             this.negotiatedSessionVersion = currentSessionVersion;
           }
 
-          if (in.available() != 0)
+          if (in.available() != 0) {
+            try {
               this.sessionKeyRecord = new SessionKey(readBlob(in), masterSecret);
+            } catch (InvalidMessageException e) {
+              Log.w("SessionRecord", e);
+              this.sessionKeyRecord = null;
+            }
+          }
 
           in.close();
         }
@@ -245,12 +258,15 @@ public class SessionRecord extends Record {
     }
   }
 
-  public SessionKey getSessionKey(int localKeyId, int remoteKeyId) {
+  public SessionKey getSessionKey(int mode, int localKeyId, int remoteKeyId) {
     if (this.sessionKeyRecord == null) return null;
 
     if ((this.sessionKeyRecord.getLocalKeyId() == localKeyId)   &&
-        (this.sessionKeyRecord.getRemoteKeyId() == remoteKeyId))
+        (this.sessionKeyRecord.getRemoteKeyId() == remoteKeyId) &&
+        (this.sessionKeyRecord.getMode() == mode))
+    {
         return this.sessionKeyRecord;
+    }
 
     return null;
   }

library/src/org/whispersystems/textsecure/util/Util.java

@@ -18,32 +18,18 @@ import java.util.List;
 
 public class Util {
 
-  public static byte[] combine(byte[] one, byte[] two) {
+  public static byte[] combine(byte[]... elements) {
-    byte[] combined = new byte[one.length + two.length];
+    try {
-    System.arraycopy(one, 0, combined, 0, one.length);
+      ByteArrayOutputStream baos = new ByteArrayOutputStream();
-    System.arraycopy(two, 0, combined, one.length, two.length);
 
-    return combined;
+      for (byte[] element : elements) {
+        baos.write(element);
       }
 
-  public static byte[] combine(byte[] one, byte[] two, byte[] three) {
+      return baos.toByteArray();
-    byte[] combined = new byte[one.length + two.length + three.length];
+    } catch (IOException e) {
-    System.arraycopy(one, 0, combined, 0, one.length);
+      throw new AssertionError(e);
-    System.arraycopy(two, 0, combined, one.length, two.length);
-    System.arraycopy(three, 0, combined, one.length + two.length, three.length);
-
-    return combined;
     }
-
-  public static byte[] combine(byte[] one, byte[] two, byte[] three, byte[] four) {
-    byte[] combined = new byte[one.length + two.length + three.length + four.length];
-    System.arraycopy(one, 0, combined, 0, one.length);
-    System.arraycopy(two, 0, combined, one.length, two.length);
-    System.arraycopy(three, 0, combined, one.length + two.length, three.length);
-    System.arraycopy(four, 0, combined, one.length + two.length + three.length, four.length);
-
-    return combined;
-
   }
 
   public static byte[][] split(byte[] input, int firstLength, int secondLength) {