Session encryption protocol.

Changes from #390 and #101 (service repo)

app/src/main/java/org/thoughtcrime/securesms/ApplicationContext.java

@@ -562,7 +562,16 @@ public class ApplicationContext extends MultiDexApplication implements Dependenc
     });
   }
 
-  public void clearData() {
+  //FIXME AC: Using this method to cleanup app data is unsafe due to potential concurrent
+  // activity that still might be using the data that is being deleted here.
+  // The most reliable and safe way to do this is to use official API call:
+  // https://developer.android.com/reference/android/app/ActivityManager.html#clearApplicationUserData()
+  // The downside is it kills the app in the process and there's no any conventional way to start
+  // another activity when the task is done.
+  // Dev community is in demand for such a feature, so check on it some time in the feature
+  // and replace our implementation with the API call when it's safe to do so.
+  // Here's a feature request related https://issuetracker.google.com/issues/174903931
+  public void clearAllData() {
     String token = TextSecurePreferences.getFCMToken(this);
     if (token != null && !token.isEmpty()) {
       LokiPushNotificationManager.unregister(token, this);

app/src/main/java/org/thoughtcrime/securesms/dependencies/SignalCommunicationModule.java

@@ -31,6 +31,7 @@ import org.thoughtcrime.securesms.jobs.StickerPackDownloadJob;
 import org.thoughtcrime.securesms.jobs.TypingSendJob;
 import org.thoughtcrime.securesms.linkpreview.LinkPreviewRepository;
 import org.thoughtcrime.securesms.logging.Log;
+import org.thoughtcrime.securesms.loki.api.SessionProtocolImpl;
 import org.thoughtcrime.securesms.loki.protocol.SessionResetImplementation;
 import org.thoughtcrime.securesms.preferences.AppProtectionPreferenceFragment;
 import org.thoughtcrime.securesms.push.MessageSenderEventListener;
@@ -100,8 +101,8 @@ public class SignalCommunicationModule {
                                                           DatabaseFactory.getSSKDatabase(context),
                                                           DatabaseFactory.getLokiThreadDatabase(context),
                                                           DatabaseFactory.getLokiMessageDatabase(context),
-//                                                          DatabaseFactory.getLokiPreKeyBundleDatabase(context),
+                                                          null, // DatabaseFactory.getLokiPreKeyBundleDatabase(context)
-                                                          null,
+                                                          new SessionProtocolImpl(context),
                                                           new SessionResetImplementation(context),
                                                           DatabaseFactory.getLokiUserDatabase(context),
                                                           DatabaseFactory.getGroupDatabase(context),

app/src/main/java/org/thoughtcrime/securesms/jobs/PushDecryptJob.java

@@ -63,6 +63,7 @@ import org.thoughtcrime.securesms.linkpreview.LinkPreview;
 import org.thoughtcrime.securesms.linkpreview.LinkPreviewUtil;
 import org.thoughtcrime.securesms.logging.Log;
 import org.thoughtcrime.securesms.loki.activities.HomeActivity;
+import org.thoughtcrime.securesms.loki.api.SessionProtocolImpl;
 import org.thoughtcrime.securesms.loki.database.LokiMessageDatabase;
 import org.thoughtcrime.securesms.loki.database.LokiThreadDatabase;
 import org.thoughtcrime.securesms.loki.protocol.ClosedGroupsProtocol;
@@ -244,7 +245,7 @@ public class PushDecryptJob extends BaseJob implements InjectableType {
       SignalProtocolStore  axolotlStore         = new SignalProtocolStoreImpl(context);
       SessionResetProtocol sessionResetProtocol = new SessionResetImplementation(context);
       SignalServiceAddress localAddress         = new SignalServiceAddress(TextSecurePreferences.getLocalNumber(context));
-      LokiServiceCipher    cipher               = new LokiServiceCipher(localAddress, axolotlStore, DatabaseFactory.getSSKDatabase(context), sessionResetProtocol, UnidentifiedAccessUtil.getCertificateValidator());
+      LokiServiceCipher    cipher               = new LokiServiceCipher(localAddress, axolotlStore, DatabaseFactory.getSSKDatabase(context), new SessionProtocolImpl(context), sessionResetProtocol, UnidentifiedAccessUtil.getCertificateValidator());
 
       SignalServiceContent content = cipher.decrypt(envelope);
 

app/src/main/java/org/thoughtcrime/securesms/loki/activities/HomeActivity.kt

@@ -204,7 +204,7 @@ class HomeActivity : PassphraseRequiredActionBarActivity, ConversationClickListe
         // Clear all data if this is a secondary device
         if (TextSecurePreferences.getMasterHexEncodedPublicKey(this) != null) {
             TextSecurePreferences.setWasUnlinked(this, true)
-            ApplicationContext.getInstance(this).clearData()
+            ApplicationContext.getInstance(this).clearAllData()
         }
 
         // Perform chat sessions reset if requested (usually happens after backup restoration).
@@ -220,36 +220,6 @@ class HomeActivity : PassphraseRequiredActionBarActivity, ConversationClickListe
         if (hasViewedSeed || !isMasterDevice) {
             seedReminderView.visibility = View.GONE
         }
-
-        // Multi device removal sheet
-        if (!TextSecurePreferences.getHasSeenMultiDeviceRemovalSheet(this)) {
-            TextSecurePreferences.setHasSeenMultiDeviceRemovalSheet(this)
-            val userPublicKey = TextSecurePreferences.getLocalNumber(this)
-            val deviceLinks = DatabaseFactory.getLokiAPIDatabase(this).getDeviceLinks(userPublicKey)
-            if (deviceLinks.isNotEmpty()) {
-                val bottomSheet = MultiDeviceRemovalBottomSheet()
-                bottomSheet.onOKTapped = {
-                    bottomSheet.dismiss()
-                }
-                bottomSheet.onLinkTapped = {
-                    bottomSheet.dismiss()
-                    val url = "https://getsession.org/faq"
-                    val intent = Intent(Intent.ACTION_VIEW, Uri.parse(url))
-                    startActivity(intent)
-                }
-                bottomSheet.show(supportFragmentManager, bottomSheet.tag)
-                return
-            }
-        }
-
-        // Light theme introduction sheet
-        if (!TextSecurePreferences.hasSeenLightThemeIntroSheet(this) &&
-                UiModeUtilities.isDayUiMode(this)) {
-            TextSecurePreferences.setHasSeenLightThemeIntroSheet(this)
-            val bottomSheet = LightThemeFeatureIntroBottomSheet()
-            bottomSheet.show(supportFragmentManager, bottomSheet.tag)
-            return
-        }
     }
 
     override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {

app/src/main/java/org/thoughtcrime/securesms/loki/api/SessionProtocolImpl.kt

@@ -0,0 +1,99 @@
+package org.thoughtcrime.securesms.loki.api
+
+import android.content.Context
+import android.util.Log
+import com.goterl.lazycode.lazysodium.LazySodiumAndroid
+import com.goterl.lazycode.lazysodium.SodiumAndroid
+import com.goterl.lazycode.lazysodium.interfaces.Box
+import com.goterl.lazycode.lazysodium.interfaces.Sign
+import org.session.libsignal.libsignal.util.Hex
+import org.session.libsignal.service.api.messages.SignalServiceEnvelope
+import org.session.libsignal.service.internal.push.SignalServiceProtos.Envelope.Type.CLOSED_GROUP_CIPHERTEXT_VALUE
+import org.session.libsignal.service.internal.push.SignalServiceProtos.Envelope.Type.UNIDENTIFIED_SENDER_VALUE
+import org.session.libsignal.service.loki.api.crypto.SessionProtocol
+import org.session.libsignal.service.loki.utilities.removing05PrefixIfNeeded
+import org.session.libsignal.service.loki.utilities.toHexString
+import org.thoughtcrime.securesms.crypto.IdentityKeyUtil
+import org.thoughtcrime.securesms.database.DatabaseFactory
+import org.thoughtcrime.securesms.loki.utilities.KeyPairUtilities
+import org.thoughtcrime.securesms.util.TextSecurePreferences
+
+class SessionProtocolImpl(private val context: Context) : SessionProtocol {
+
+    override fun encrypt(plaintext: ByteArray, recipientHexEncodedX25519PublicKey: String): ByteArray {
+        val userED25519KeyPair = KeyPairUtilities.getUserED25519KeyPair(context) ?: throw SessionProtocol.Exception.NoUserED25519KeyPair
+        val recipientX25519PublicKey = Hex.fromStringCondensed(recipientHexEncodedX25519PublicKey.removing05PrefixIfNeeded())
+        val sodium = LazySodiumAndroid(SodiumAndroid())
+
+        val verificationData = plaintext + userED25519KeyPair.publicKey.asBytes + recipientX25519PublicKey
+        val signature = ByteArray(Sign.BYTES)
+        try {
+            sodium.cryptoSignDetached(signature, verificationData, verificationData.size.toLong(), userED25519KeyPair.secretKey.asBytes)
+        } catch (exception: Exception) {
+            Log.d("Loki", "Couldn't sign message due to error: $exception.")
+            throw SessionProtocol.Exception.SigningFailed
+        }
+        val plaintextWithMetadata = plaintext + userED25519KeyPair.publicKey.asBytes + signature
+        val ciphertext = ByteArray(plaintextWithMetadata.size + Box.SEALBYTES)
+        try {
+            sodium.cryptoBoxSeal(ciphertext, plaintextWithMetadata, plaintextWithMetadata.size.toLong(), recipientX25519PublicKey)
+        } catch (exception: Exception) {
+            Log.d("Loki", "Couldn't encrypt message due to error: $exception.")
+            throw SessionProtocol.Exception.EncryptionFailed
+        }
+
+        return ciphertext
+    }
+
+    override fun decrypt(envelope: SignalServiceEnvelope): Pair<ByteArray, String> {
+        val ciphertext = envelope.content ?: throw SessionProtocol.Exception.NoData
+        val recipientX25519PrivateKey: ByteArray
+        val recipientX25519PublicKey: ByteArray
+        when (envelope.type) {
+            UNIDENTIFIED_SENDER_VALUE -> {
+                recipientX25519PrivateKey = IdentityKeyUtil.getIdentityKeyPair(context).privateKey.serialize()
+                recipientX25519PublicKey = Hex.fromStringCondensed(TextSecurePreferences.getLocalNumber(context).removing05PrefixIfNeeded())
+            }
+            CLOSED_GROUP_CIPHERTEXT_VALUE -> {
+                val hexEncodedGroupPublicKey = envelope.source
+                val sskDB = DatabaseFactory.getSSKDatabase(context)
+                if (!sskDB.isSSKBasedClosedGroup(hexEncodedGroupPublicKey)) { throw SessionProtocol.Exception.InvalidGroupPublicKey }
+                val hexEncodedGroupPrivateKey = sskDB.getClosedGroupPrivateKey(hexEncodedGroupPublicKey) ?: throw SessionProtocol.Exception.NoGroupPrivateKey
+                recipientX25519PrivateKey = Hex.fromStringCondensed(hexEncodedGroupPrivateKey)
+                recipientX25519PublicKey = Hex.fromStringCondensed(hexEncodedGroupPublicKey.removing05PrefixIfNeeded())
+            }
+            else -> throw AssertionError()
+        }
+        val sodium = LazySodiumAndroid(SodiumAndroid())
+        val signatureSize = Sign.BYTES
+        val ed25519PublicKeySize = Sign.PUBLICKEYBYTES
+
+        // 1. ) Decrypt the message
+        val plaintextWithMetadata = ByteArray(ciphertext.size - Box.SEALBYTES)
+        try {
+            sodium.cryptoBoxSealOpen(plaintextWithMetadata, ciphertext, ciphertext.size.toLong(), recipientX25519PublicKey, recipientX25519PrivateKey)
+        } catch (exception: Exception) {
+            Log.d("Loki", "Couldn't decrypt message due to error: $exception.")
+            throw SessionProtocol.Exception.DecryptionFailed
+        }
+        if (plaintextWithMetadata.size <= (signatureSize + ed25519PublicKeySize)) { throw SessionProtocol.Exception.DecryptionFailed }
+        // 2. ) Get the message parts
+        val signature = plaintextWithMetadata.sliceArray(plaintextWithMetadata.size - signatureSize until plaintextWithMetadata.size)
+        val senderED25519PublicKey = plaintextWithMetadata.sliceArray(plaintextWithMetadata.size - (signatureSize + ed25519PublicKeySize) until plaintextWithMetadata.size - signatureSize)
+        val plaintext = plaintextWithMetadata.sliceArray(0 until plaintextWithMetadata.size - (signatureSize + ed25519PublicKeySize))
+        // 3. ) Verify the signature
+        val verificationData = (plaintext + senderED25519PublicKey + recipientX25519PublicKey)
+        try {
+            val isValid = sodium.cryptoSignVerifyDetached(signature, verificationData, verificationData.size, senderED25519PublicKey)
+            if (!isValid) { throw SessionProtocol.Exception.InvalidSignature }
+        } catch (exception: Exception) {
+            Log.d("Loki", "Couldn't verify message signature due to error: $exception.")
+            throw SessionProtocol.Exception.InvalidSignature
+        }
+        // 4. ) Get the sender's X25519 public key
+        val senderX25519PublicKey = ByteArray(Sign.CURVE25519_PUBLICKEYBYTES)
+        sodium.convertPublicKeyEd25519ToCurve25519(senderX25519PublicKey, senderED25519PublicKey)
+
+        return Pair(plaintext, "05" + senderX25519PublicKey.toHexString())
+    }
+}

app/src/main/java/org/thoughtcrime/securesms/loki/dialogs/ClearAllDataDialog.kt

@@ -27,13 +27,13 @@ class ClearAllDataDialog : DialogFragment() {
 
     private fun clearAllData() {
         if (KeyPairUtilities.hasV2KeyPair(requireContext())) {
-            ApplicationContext.getInstance(context).clearData()
+            ApplicationContext.getInstance(context).clearAllData()
         } else {
             val dialog = AlertDialog.Builder(requireContext())
             val message = "We’ve upgraded the way Session IDs are generated, so you will be unable to restore your current Session ID."
             dialog.setMessage(message)
             dialog.setPositiveButton("Yes") { _, _ ->
-                ApplicationContext.getInstance(context).clearData()
+                ApplicationContext.getInstance(context).clearAllData()
             }
             dialog.setNegativeButton("Cancel") { _, _ ->
                 // Do nothing

app/src/main/java/org/thoughtcrime/securesms/loki/utilities/KeyPairUtilities.kt

@@ -3,6 +3,7 @@ package org.thoughtcrime.securesms.loki.utilities
 import android.content.Context
 import com.goterl.lazycode.lazysodium.LazySodiumAndroid
 import com.goterl.lazycode.lazysodium.SodiumAndroid
+import com.goterl.lazycode.lazysodium.utils.Key
 import com.goterl.lazycode.lazysodium.utils.KeyPair
 import org.thoughtcrime.securesms.crypto.IdentityKeyUtil
 import org.thoughtcrime.securesms.util.Base64
@@ -44,6 +45,13 @@ object KeyPairUtilities {
         return (IdentityKeyUtil.retrieve(context, IdentityKeyUtil.ED25519_SECRET_KEY) != null)
     }
 
+    fun getUserED25519KeyPair(context: Context): KeyPair? {
+        val hexEncodedED25519PublicKey = IdentityKeyUtil.retrieve(context, IdentityKeyUtil.ED25519_PUBLIC_KEY) ?: return null
+        val hexEncodedED25519SecretKey = IdentityKeyUtil.retrieve(context, IdentityKeyUtil.ED25519_SECRET_KEY) ?: return null
+        val ed25519PublicKey = Key.fromBase64String(hexEncodedED25519PublicKey)
+        val ed25519SecretKey = Key.fromBase64String(hexEncodedED25519SecretKey)
+        return KeyPair(ed25519PublicKey, ed25519SecretKey)
+    }
 
     data class KeyPairGenerationResult(
             val seed: ByteArray,

libsignal/src/main/java/org/session/libsignal/service/api/SignalServiceMessageSender.java

@@ -77,6 +77,7 @@ import org.session.libsignal.service.loki.api.LokiDotNetAPI;
 import org.session.libsignal.service.loki.api.PushNotificationAPI;
 import org.session.libsignal.service.loki.api.SignalMessageInfo;
 import org.session.libsignal.service.loki.api.SnodeAPI;
+import org.session.libsignal.service.loki.api.crypto.SessionProtocol;
 import org.session.libsignal.service.loki.api.fileserver.FileServerAPI;
 import org.session.libsignal.service.loki.api.opengroups.PublicChat;
 import org.session.libsignal.service.loki.api.opengroups.PublicChatAPI;
@@ -142,6 +143,7 @@ public class SignalServiceMessageSender {
   private final LokiThreadDatabaseProtocol                          threadDatabase;
   private final LokiMessageDatabaseProtocol                         messageDatabase;
   private final LokiPreKeyBundleDatabaseProtocol                    preKeyBundleDatabase;
+  private final SessionProtocol                                     sessionProtocolImpl;
   private final SessionResetProtocol                                sessionResetImpl;
   private final LokiUserDatabaseProtocol                            userDatabase;
   private final LokiOpenGroupDatabaseProtocol                       openGroupDatabase;
@@ -171,12 +173,13 @@ public class SignalServiceMessageSender {
                                     LokiThreadDatabaseProtocol threadDatabase,
                                     LokiMessageDatabaseProtocol messageDatabase,
                                     LokiPreKeyBundleDatabaseProtocol preKeyBundleDatabase,
+                                    SessionProtocol sessionProtocolImpl,
                                     SessionResetProtocol sessionResetImpl,
                                     LokiUserDatabaseProtocol userDatabase,
                                     LokiOpenGroupDatabaseProtocol openGroupDatabase,
                                     Broadcaster broadcaster)
   {
-    this(urls, new StaticCredentialsProvider(user, password, null), store, userAgent, isMultiDevice, pipe, unidentifiedPipe, eventListener, userPublicKey, apiDatabase, sskDatabase, threadDatabase, messageDatabase, preKeyBundleDatabase, sessionResetImpl, userDatabase, openGroupDatabase, broadcaster);
+    this(urls, new StaticCredentialsProvider(user, password, null), store, userAgent, isMultiDevice, pipe, unidentifiedPipe, eventListener, userPublicKey, apiDatabase, sskDatabase, threadDatabase, messageDatabase, preKeyBundleDatabase, sessionProtocolImpl, sessionResetImpl, userDatabase, openGroupDatabase, broadcaster);
   }
 
   public SignalServiceMessageSender(SignalServiceConfiguration urls,
@@ -193,6 +196,7 @@ public class SignalServiceMessageSender {
                                     LokiThreadDatabaseProtocol threadDatabase,
                                     LokiMessageDatabaseProtocol messageDatabase,
                                     LokiPreKeyBundleDatabaseProtocol preKeyBundleDatabase,
+                                    SessionProtocol sessionProtocolImpl,
                                     SessionResetProtocol sessionResetImpl,
                                     LokiUserDatabaseProtocol userDatabase,
                                     LokiOpenGroupDatabaseProtocol openGroupDatabase,
@@ -211,6 +215,7 @@ public class SignalServiceMessageSender {
     this.threadDatabase            = threadDatabase;
     this.messageDatabase           = messageDatabase;
     this.preKeyBundleDatabase      = preKeyBundleDatabase;
+    this.sessionProtocolImpl       = sessionProtocolImpl;
     this.sessionResetImpl          = sessionResetImpl;
     this.userDatabase              = userDatabase;
     this.openGroupDatabase         = openGroupDatabase;
@@ -1383,6 +1388,30 @@ public class SignalServiceMessageSender {
     return new OutgoingPushMessageList(recipient.getNumber(), timestamp, messages, online);
   }
 
+  private OutgoingPushMessageList getSessionProtocolEncryptedMessages(PushServiceSocket            socket,
+                                                                      SignalServiceAddress         recipient,
+                                                                      Optional<UnidentifiedAccess> unidentifiedAccess,
+                                                                      long                         timestamp,
+                                                                      byte[]                       plaintext,
+                                                                      boolean                      online,
+                                                                      boolean                      useFallbackEncryption,
+                                                                      boolean                      isClosedGroup)
+  {
+    List<OutgoingPushMessage> messages = new LinkedList<>();
+
+    PushTransportDetails transportDetails = new PushTransportDetails(3);
+    String publicKey = recipient.getNumber(); // Could be a contact's public key or the public key of a SSK group
+    byte[] ciphertext = sessionProtocolImpl.encrypt(transportDetails.getPaddedMessageBody(plaintext), publicKey);
+    String body = Base64.encodeBytes(ciphertext);
+    boolean isSSKBasedClosedGroup = sskDatabase.isSSKBasedClosedGroup(publicKey);
+    int type = isSSKBasedClosedGroup ? SignalServiceProtos.Envelope.Type.CLOSED_GROUP_CIPHERTEXT_VALUE :
+            SignalServiceProtos.Envelope.Type.UNIDENTIFIED_SENDER_VALUE;
+    OutgoingPushMessage message = new OutgoingPushMessage(type, 1, 0, body);
+    messages.add(message);
+
+    return new OutgoingPushMessageList(publicKey, timestamp, messages, online);
+  }
+
   private OutgoingPushMessage getUnencryptedMessage(byte[] plaintext) {
     Log.d("Loki", "Bypassing cipher and preparing a plaintext message.");
     int deviceID = SignalServiceAddress.DEFAULT_DEVICE_ID;
@@ -1417,7 +1446,7 @@ public class SignalServiceMessageSender {
   {
       int deviceID = SignalServiceAddress.DEFAULT_DEVICE_ID;
       SignalProtocolAddress signalProtocolAddress = new SignalProtocolAddress(groupPublicKey, deviceID);
-      SignalServiceCipher cipher = new SignalServiceCipher(localAddress, store, sskDatabase, sessionResetImpl, null);
+      SignalServiceCipher cipher = new SignalServiceCipher(localAddress, store, sskDatabase, sessionResetImpl, sessionProtocolImpl, null);
       try {
           return cipher.encrypt(signalProtocolAddress, Optional.of(unidentifiedAccess), plaintext);
       } catch (org.session.libsignal.libsignal.UntrustedIdentityException e) {
@@ -1435,7 +1464,7 @@ public class SignalServiceMessageSender {
     Log.d("Loki", "Using Signal cipher.");
     int deviceID = SignalServiceAddress.DEFAULT_DEVICE_ID;
     SignalProtocolAddress signalProtocolAddress = new SignalProtocolAddress(recipient.getNumber(), deviceID);
-    SignalServiceCipher cipher = new SignalServiceCipher(localAddress, store, sskDatabase, sessionResetImpl, null);
+    SignalServiceCipher cipher = new SignalServiceCipher(localAddress, store, sskDatabase, sessionResetImpl, sessionProtocolImpl, null);
 
     try {
       String contactPublicKey = recipient.getNumber();

libsignal/src/main/java/org/session/libsignal/service/api/crypto/SignalServiceCipher.java

@@ -83,6 +83,7 @@ import org.session.libsignal.service.internal.push.SignalServiceProtos.SyncMessa
 import org.session.libsignal.service.internal.push.SignalServiceProtos.TypingMessage;
 import org.session.libsignal.service.internal.push.SignalServiceProtos.Verified;
 import org.session.libsignal.service.internal.util.Base64;
+import org.session.libsignal.service.loki.api.crypto.SessionProtocol;
 import org.session.libsignal.service.loki.api.opengroups.PublicChat;
 import org.session.libsignal.service.loki.protocol.closedgroups.ClosedGroupUtilities;
 import org.session.libsignal.service.loki.protocol.closedgroups.SharedSenderKeysDatabaseProtocol;
@@ -114,18 +115,21 @@ public class SignalServiceCipher {
   private final SessionResetProtocol             sessionResetProtocol;
   private final SharedSenderKeysDatabaseProtocol sskDatabase;
   private final SignalServiceAddress             localAddress;
+  private final SessionProtocol                  sessionProtocolImpl;
   private final CertificateValidator             certificateValidator;
 
   public SignalServiceCipher(SignalServiceAddress localAddress,
                              SignalProtocolStore signalProtocolStore,
                              SharedSenderKeysDatabaseProtocol sskDatabase,
                              SessionResetProtocol sessionResetProtocol,
+                             SessionProtocol sessionProtocolImpl,
                              CertificateValidator certificateValidator)
   {
     this.signalProtocolStore  = signalProtocolStore;
     this.sessionResetProtocol = sessionResetProtocol;
     this.sskDatabase          = sskDatabase;
     this.localAddress         = localAddress;
+    this.sessionProtocolImpl  = sessionProtocolImpl;
     this.certificateValidator = certificateValidator;
   }
 
@@ -317,12 +321,23 @@ public class SignalServiceCipher {
       int sessionVersion;
 
       if (envelope.isClosedGroupCiphertext()) {
+        try {
+          // Try the Session protocol
+          kotlin.Pair<byte[], String> plaintextAndSenderPublicKey = sessionProtocolImpl.decrypt(envelope);
+          paddedMessage = plaintextAndSenderPublicKey.getFirst();
+          String senderPublicKey = plaintextAndSenderPublicKey.getSecond();
+          if (senderPublicKey.equals(localAddress.getNumber())) { throw new SelfSendException(); } // Will be caught and ignored in PushDecryptJob
+          metadata = new Metadata(senderPublicKey, 1, envelope.getTimestamp(), false);
+          sessionVersion = sessionCipher.getSessionVersion();
+        } catch (Exception exception) {
+          // Fall back on shared sender keys
           Pair<byte[], String> plaintextAndSenderPublicKey = ClosedGroupUtilities.decrypt(envelope);
           String senderPublicKey = plaintextAndSenderPublicKey.second();
           if (senderPublicKey.equals(localAddress.getNumber())) { throw new SelfSendException(); } // Will be caught and ignored in PushDecryptJob
           paddedMessage = plaintextAndSenderPublicKey.first();
           metadata = new Metadata(senderPublicKey, envelope.getSourceDevice(), envelope.getTimestamp(), false);
           sessionVersion = sessionCipher.getSessionVersion();
+        }
       } else if (envelope.isPreKeySignalMessage()) {
         paddedMessage  = sessionCipher.decrypt(new PreKeySignalMessage(ciphertext));
         metadata       = new Metadata(envelope.getSource(), envelope.getSourceDevice(), envelope.getTimestamp(), false);
@@ -332,11 +347,21 @@ public class SignalServiceCipher {
         metadata       = new Metadata(envelope.getSource(), envelope.getSourceDevice(), envelope.getTimestamp(), false);
         sessionVersion = sessionCipher.getSessionVersion();
       } else if (envelope.isUnidentifiedSender()) {
+        try {
+          // Try the Session protocol
+          kotlin.Pair<byte[], String> plaintextAndSenderPublicKey = sessionProtocolImpl.decrypt(envelope);
+          paddedMessage = plaintextAndSenderPublicKey.getFirst();
+          String senderPublicKey = plaintextAndSenderPublicKey.getSecond();
+          metadata = new Metadata(senderPublicKey, 1, envelope.getTimestamp(), false);
+          sessionVersion = sealedSessionCipher.getSessionVersion(new SignalProtocolAddress(metadata.getSender(), metadata.getSenderDevice()));
+        } catch (Exception exception) {
+          // Fall back on the Signal protocol
           Pair<SignalProtocolAddress, Pair<Integer, byte[]>> results = sealedSessionCipher.decrypt(certificateValidator, ciphertext, envelope.getServerTimestamp(), envelope.getSource());
           Pair<Integer, byte[]> data = results.second();
           paddedMessage = data.second();
           metadata = new Metadata(results.first().getName(), results.first().getDeviceId(), envelope.getTimestamp(), false);
           sessionVersion = sealedSessionCipher.getSessionVersion(new SignalProtocolAddress(metadata.getSender(), metadata.getSenderDevice()));
+        }
       } else {
         throw new InvalidMetadataMessageException("Unknown type: " + envelope.getType());
       }

libsignal/src/main/java/org/session/libsignal/service/loki/api/crypto/SessionProtocol.kt

@@ -0,0 +1,40 @@
+package org.session.libsignal.service.loki.api.crypto
+
+import org.session.libsignal.service.api.messages.SignalServiceEnvelope
+
+interface SessionProtocol {
+
+    sealed class Exception(val description: String) : kotlin.Exception(description) {
+        // Encryption
+        object NoUserED25519KeyPair : Exception("Couldn't find user ED25519 key pair.")
+        object SigningFailed : Exception("Couldn't sign message.")
+        object EncryptionFailed : Exception("Couldn't encrypt message.")
+        // Decryption
+        object NoData : Exception("Received an empty envelope.")
+        object InvalidGroupPublicKey : Exception("Invalid group public key.")
+        object NoGroupPrivateKey : Exception("Missing group private key.")
+        object DecryptionFailed : Exception("Couldn't decrypt message.")
+        object InvalidSignature : Exception("Invalid message signature.")
+    }
+
+    /**
+     * Encrypts `plaintext` using the Session protocol for `hexEncodedX25519PublicKey`.
+     *
+     * @param plaintext the plaintext to encrypt. Must already be padded.
+     * @param recipientHexEncodedX25519PublicKey the X25519 public key to encrypt for. Could be the Session ID of a user, or the public key of a closed group.
+     *
+     * @return the encrypted message.
+     */
+    fun encrypt(plaintext: ByteArray, recipientHexEncodedX25519PublicKey: String): ByteArray
+
+    /**
+     * Decrypts `envelope.content` using the Session protocol. If the envelope type is `UNIDENTIFIED_SENDER` the message is assumed to be a one-to-one
+     * message. If the envelope type is `CLOSED_GROUP_CIPHERTEXT` the message is assumed to be a closed group message. In the latter case `envelope.source`
+     * must be set to the closed group's public key.
+     *
+     * @param envelope the envelope for which to decrypt the content.
+     *
+     * @return the padded plaintext.
+     */
+    fun decrypt(envelope: SignalServiceEnvelope): Pair<ByteArray, String>
+}

libsignal/src/main/java/org/session/libsignal/service/loki/crypto/LokiServiceCipher.kt

@@ -9,15 +9,23 @@ import org.session.libsignal.service.api.crypto.SignalServiceCipher
 import org.session.libsignal.service.api.messages.SignalServiceEnvelope
 import org.session.libsignal.service.api.push.SignalServiceAddress
 import org.session.libsignal.service.internal.push.PushTransportDetails
+import org.session.libsignal.service.loki.api.crypto.SessionProtocol
 import org.session.libsignal.service.loki.protocol.closedgroups.SharedSenderKeysDatabaseProtocol
 
 class LokiServiceCipher(
         localAddress: SignalServiceAddress,
         private val signalProtocolStore: SignalProtocolStore,
         private val sskDatabase: SharedSenderKeysDatabaseProtocol,
+        sessionProtocolImpl: SessionProtocol,
         sessionResetProtocol: SessionResetProtocol,
         certificateValidator: CertificateValidator?)
-    : SignalServiceCipher(localAddress, signalProtocolStore, sskDatabase, sessionResetProtocol, certificateValidator) {
+    : SignalServiceCipher(
+        localAddress,
+        signalProtocolStore,
+        sskDatabase,
+        sessionResetProtocol,
+        sessionProtocolImpl,
+        certificateValidator) {
 
     private val userPrivateKey get() = signalProtocolStore.identityKeyPair.privateKey.serialize()