From 378aa4b2629ec18e945dd7d0ed12b5598a68de3b Mon Sep 17 00:00:00 2001
From: Paul Schaub <vanitasvitae@fsfe.org>
Date: Wed, 10 Oct 2018 14:05:45 +0200
Subject: [PATCH] Business logic for handling read-only devices

---
 .../smackx/omemo/OmemoConfiguration.java      | 57 ++++++++++----
 .../smackx/omemo/OmemoService.java            | 76 ++++++++-----------
 .../exceptions/ReadOnlyDeviceException.java   | 52 +++++++++++++
 .../smackx/omemo/OmemoConfigurationTest.java  | 14 ----
 .../smackx/omemo/OmemoServiceTest.java        |  7 --
 5 files changed, 123 insertions(+), 83 deletions(-)
 create mode 100644 smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/exceptions/ReadOnlyDeviceException.java

diff --git a/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoConfiguration.java b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoConfiguration.java
index a529162fd..8756ef37e 100644
--- a/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoConfiguration.java
+++ b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoConfiguration.java
@@ -23,31 +23,56 @@ package org.jivesoftware.smackx.omemo;
  */
 public final class OmemoConfiguration {
 
+    private static boolean IGNORE_READ_ONLY_DEVICES = true;
+    private static int MAX_READ_ONLY_MESSAGE_COUNT = 400;
+
     /**
-     * Ignore own other stale devices that we did not receive a message from for a period of time.
-     * Ignoring means do not encrypt messages for them. This helps to mitigate stale devices that threaten
-     * forward secrecy by never advancing ratchets.
+     * Set to true, in order to ignore read-only devices.
+     *
+     * @param ignore ignore read-only devices
+     * @see <a href="TODO: Add URL">Blog Post explaining the danger of read-only devices.</a>
      */
-    private static boolean IGNORE_STALE_DEVICES = true;
-    private static int IGNORE_STALE_DEVICE_AFTER_HOURS = 24 * 7;         //One week
-
-    public static void setIgnoreStaleDevices(boolean ignore) {
-        IGNORE_STALE_DEVICES = ignore;
+    public static void setIgnoreReadOnlyDevices(boolean ignore) {
+        IGNORE_READ_ONLY_DEVICES = ignore;
     }
 
-    public static boolean getIgnoreStaleDevices() {
-        return IGNORE_STALE_DEVICES;
+    /**
+     * Return true, if the client should stop encrypting messages to a read-only device.
+     *
+     * @return true if read-only devices should get ignored after a certain amount of unanswered messages.
+     * @see <a href="TODO: Add URL">Blog Post explaining the danger of read-only devices.</a>
+     */
+    public static boolean getIgnoreReadOnlyDevices() {
+        return IGNORE_READ_ONLY_DEVICES;
     }
 
-    public static void setIgnoreStaleDevicesAfterHours(int hours) {
-        if (hours <= 0) {
-            throw new IllegalArgumentException("Hours must be greater than 0.");
+    /**
+     * Set the maximum amount of messages that the client is allowed to send to a read-only device without getting a
+     * response. Once the message counter of a device reaches that value, the client will stop encrypting messages for
+     * the device (given that {@link #getIgnoreReadOnlyDevices()} is true).
+     * This threshold is used to prevent read-only devices from weakening forward secrecy.
+     *
+     * @param maxReadOnlyMessageCount maximum number of allowed messages to a read-only device.
+     * @see <a href="TODO: Add URL">Blog Post explaining the danger of read-only devices.</a>
+     */
+    public static void setMaxReadOnlyMessageCount(int maxReadOnlyMessageCount) {
+        if (maxReadOnlyMessageCount <= 0) {
+            throw new IllegalArgumentException("maxReadOnlyMessageCount MUST be greater than 0.");
         }
-        IGNORE_STALE_DEVICE_AFTER_HOURS = hours;
+        MAX_READ_ONLY_MESSAGE_COUNT = maxReadOnlyMessageCount;
     }
 
-    public static int getIgnoreStaleDevicesAfterHours() {
-        return IGNORE_STALE_DEVICE_AFTER_HOURS;
+    /**
+     * Get the maximum amount of messages that the client is allowed to send to a read-only device without getting a
+     * response. Once the message counter of a device reaches that value, the client will stop encrypting messages for
+     * the device (given that {@link #getIgnoreReadOnlyDevices()} is true).
+     * This threshold is used to prevent read-only devices from weakening forward secrecy.
+     *
+     * @return maximum number of allowed messages to a read-only device.
+     * @see <a href="TODO: Add URL">Blog Post explaining the danger of read-only devices.</a>
+     */
+    public static int getMaxReadOnlyMessageCount() {
+        return MAX_READ_ONLY_MESSAGE_COUNT;
     }
 
     /**
diff --git a/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoService.java b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoService.java
index d29d3339c..f8d38706d 100644
--- a/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoService.java
+++ b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/OmemoService.java
@@ -19,6 +19,9 @@ package org.jivesoftware.smackx.omemo;
 import static org.jivesoftware.smackx.omemo.util.OmemoConstants.Crypto.KEYLENGTH;
 import static org.jivesoftware.smackx.omemo.util.OmemoConstants.Crypto.KEYTYPE;
 
+import javax.crypto.BadPaddingException;
+import javax.crypto.IllegalBlockSizeException;
+import javax.crypto.NoSuchPaddingException;
 import java.io.UnsupportedEncodingException;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.InvalidKeyException;
@@ -36,14 +39,10 @@ import java.util.Random;
 import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
-import javax.crypto.BadPaddingException;
-import javax.crypto.IllegalBlockSizeException;
-import javax.crypto.NoSuchPaddingException;
 
 import org.jivesoftware.smack.SmackException;
 import org.jivesoftware.smack.XMPPConnection;
 import org.jivesoftware.smack.XMPPException;
-
 import org.jivesoftware.smack.packet.Message;
 import org.jivesoftware.smack.packet.Stanza;
 import org.jivesoftware.smack.packet.StanzaError;
@@ -62,7 +61,7 @@ import org.jivesoftware.smackx.omemo.exceptions.CorruptedOmemoKeyException;
 import org.jivesoftware.smackx.omemo.exceptions.CryptoFailedException;
 import org.jivesoftware.smackx.omemo.exceptions.NoIdentityKeyException;
 import org.jivesoftware.smackx.omemo.exceptions.NoRawSessionException;
-import org.jivesoftware.smackx.omemo.exceptions.StaleDeviceException;
+import org.jivesoftware.smackx.omemo.exceptions.ReadOnlyDeviceException;
 import org.jivesoftware.smackx.omemo.exceptions.UndecidedOmemoIdentityException;
 import org.jivesoftware.smackx.omemo.exceptions.UntrustedOmemoIdentityException;
 import org.jivesoftware.smackx.omemo.internal.CipherAndAuthTag;
@@ -323,6 +322,8 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
             throw new AssertionError("We MUST have an identityKey for " + contactsDevice + " since we built a session." + e);
         }
 
+        // Note: We don't need to update our message counter for a ratchet update message.
+
         return builder.finish();
     }
 
@@ -386,28 +387,20 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
                 }
             }
 
-            // Ignore own stale devices
-            if (contactsDevice.getJid().equals(userDevice.getJid()) && OmemoConfiguration.getIgnoreStaleDevices()) {
+            int messageCounter = omemoStore.loadOmemoMessageCounter(userDevice, contactsDevice);
 
-                Date lastMessageDate = getOmemoStoreBackend().getDateOfLastReceivedMessage(userDevice, contactsDevice);
-                if (lastMessageDate == null) {
-                    lastMessageDate = new Date();
-                    getOmemoStoreBackend().setDateOfLastReceivedMessage(userDevice, contactsDevice, lastMessageDate);
-                }
+            // Ignore read-only devices
+            if (OmemoConfiguration.getIgnoreReadOnlyDevices()) {
 
-                Date lastPublicationDate = getOmemoStoreBackend().getDateOfLastDeviceIdPublication(userDevice, contactsDevice);
-                if (lastPublicationDate == null) {
-                    lastPublicationDate = new Date();
-                    getOmemoStoreBackend().setDateOfLastDeviceIdPublication(userDevice, contactsDevice, lastPublicationDate);
-                }
+                boolean readOnly = messageCounter >= OmemoConfiguration.getMaxReadOnlyMessageCount();
 
-                boolean stale = isStale(userDevice, contactsDevice, lastPublicationDate, OmemoConfiguration.getIgnoreStaleDevicesAfterHours());
-                stale &= isStale(userDevice, contactsDevice, lastMessageDate, OmemoConfiguration.getIgnoreStaleDevicesAfterHours());
+                if (readOnly) {
+                    LOGGER.log(Level.FINE, "Device " + contactsDevice + " seems to be read-only (We sent "
+                            + messageCounter + " messages without getting a reply back (max allowed is " +
+                            OmemoConfiguration.getMaxReadOnlyMessageCount() + "). Ignoring the device.");
+                    skippedRecipients.put(contactsDevice, new ReadOnlyDeviceException(contactsDevice));
 
-                if (stale) {
-                    LOGGER.log(Level.FINE, "Device " + contactsDevice + " seems to be stale (last message received "
-                            + lastMessageDate + ", last publication of deviceId: " + lastPublicationDate + "). Ignore it.");
-                    skippedRecipients.put(contactsDevice, new StaleDeviceException(contactsDevice, lastMessageDate, lastPublicationDate));
+                    // Skip this device and handle next device
                     continue;
                 }
             }
@@ -428,6 +421,10 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
                 LOGGER.log(Level.WARNING, "Device " + contactsDevice + " is untrusted. Message is not encrypted for it.");
                 skippedRecipients.put(contactsDevice, e);
             }
+
+            // Increment the message counter of the device
+            omemoStore.storeOmemoMessageCounter(userDevice, contactsDevice,
+                    messageCounter + 1);
         }
 
         OmemoElement element = builder.finish();
@@ -466,6 +463,9 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
             throw new AssertionError("Cannot retrieve OmemoFingerprint of sender although decryption was successful: " + e);
         }
 
+        // Reset the message counter.
+        omemoStore.storeOmemoMessageCounter(manager.getOwnDevice(), senderDevice, 0);
+
         if (omemoElement.isMessageElement()) {
             // Use symmetric message key to decrypt message payload.
             String plaintext = OmemoRatchet.decryptMessageElement(omemoElement, cipherAndAuthTag);
@@ -1009,24 +1009,6 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
         return removeStaleDevicesFromDeviceList(userDevice, userDevice.getJid(), deviceList, maxAgeHours);
     }
 
-    /**
-     * Return a copy of the contactsDeviceList, but with stale devices marked as inactive.
-     * Never mark our own device as stale.
-     * This method ignores {@link OmemoConfiguration#getIgnoreStaleDevices()}!
-     *
-     * In this case, a stale device is one of our devices, from which we haven't received an OMEMO message from
-     * for more than {@link OmemoConfiguration#IGNORE_STALE_DEVICE_AFTER_HOURS} hours.
-     *
-     * @param userDevice our OmemoDevice
-     * @param contact subjects BareJid
-     * @param contactsDeviceList subjects deviceList
-     * @return
-     */
-    private OmemoCachedDeviceList ignoreStaleDevices(OmemoDevice userDevice, BareJid contact, OmemoCachedDeviceList contactsDeviceList) {
-        int maxAgeHours = OmemoConfiguration.getIgnoreStaleDevicesAfterHours();
-        return removeStaleDevicesFromDeviceList(userDevice, contact, contactsDeviceList, maxAgeHours);
-    }
-
     /**
      * Return a copy of the given deviceList of user contact, but with stale devices marked as inactive.
      * Never mark our own device as stale. If we haven't yet received a message from a device, store the current date
@@ -1086,12 +1068,14 @@ public abstract class OmemoService<T_IdKeyPair, T_IdKey, T_PreKey, T_SigPreKey,
     }
 
     /**
+     * Determine, whether another one of *our* devices is stale or not.
      *
-     * @param userDevice
-     * @param subject
-     * @param lastReceipt
-     * @param maxAgeHours
-     * @return
+     * @param userDevice our omemoDevice
+     * @param subject another one of our devices
+     * @param lastReceipt date of last received message from that device
+     * @param maxAgeHours threshold
+     *
+     * @return staleness
      */
     static boolean isStale(OmemoDevice userDevice, OmemoDevice subject, Date lastReceipt, int maxAgeHours) {
         if (userDevice.equals(subject)) {
diff --git a/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/exceptions/ReadOnlyDeviceException.java b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/exceptions/ReadOnlyDeviceException.java
new file mode 100644
index 000000000..6a0832f95
--- /dev/null
+++ b/smack-omemo/src/main/java/org/jivesoftware/smackx/omemo/exceptions/ReadOnlyDeviceException.java
@@ -0,0 +1,52 @@
+/**
+ *
+ * Copyright 2018 Paul Schaub
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smackx.omemo.exceptions;
+
+import org.jivesoftware.smackx.omemo.OmemoConfiguration;
+import org.jivesoftware.smackx.omemo.internal.OmemoDevice;
+
+/**
+ * Exception that signals, that a device is considered read-only.
+ * Read-only devices are devices that receive OMEMO messages, but do not send any.
+ * Those devices are weakening forward secrecy. For that reason, read-only devices are ignored after n messages have
+ * been sent without getting a reply back.
+ */
+public class ReadOnlyDeviceException extends Exception {
+    private static final long serialVersionUID = 1L;
+
+    private final OmemoDevice device;
+
+    /**
+     * Constructor.
+     * We do not need to hand over the current value of the message counter, as that value will always be equal to
+     * {@link OmemoConfiguration#getMaxReadOnlyMessageCount()}. Therefore providing the {@link OmemoDevice} should be
+     * enough.
+     *
+     * @param device device which is considered read-only.
+     */
+    public ReadOnlyDeviceException(OmemoDevice device) {
+        this.device = device;
+    }
+
+    /**
+     * Return the device in question.
+     * @return device
+     */
+    public OmemoDevice getDevice() {
+        return device;
+    }
+}
diff --git a/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoConfigurationTest.java b/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoConfigurationTest.java
index 15bb7a236..bfa5debb2 100644
--- a/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoConfigurationTest.java
+++ b/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoConfigurationTest.java
@@ -52,20 +52,6 @@ public class OmemoConfigurationTest {
             // Expected.
         }
 
-        // Ignore stale device
-        OmemoConfiguration.setIgnoreStaleDevices(false);
-        assertEquals(false, OmemoConfiguration.getIgnoreStaleDevices());
-        OmemoConfiguration.setIgnoreStaleDevices(true);
-        assertEquals(true, OmemoConfiguration.getIgnoreStaleDevices());
-        OmemoConfiguration.setIgnoreStaleDevicesAfterHours(44);
-        assertEquals(44, OmemoConfiguration.getIgnoreStaleDevicesAfterHours());
-        try {
-            OmemoConfiguration.setIgnoreStaleDevicesAfterHours(-5);
-            TestCase.fail("OmemoConfiguration.setIgnoreStaleDevicesAfterHours should not accept values <= 0.");
-        } catch (IllegalArgumentException e) {
-            // Expected
-        }
-
         // Renew signedPreKeys
         OmemoConfiguration.setRenewOldSignedPreKeys(false);
         assertEquals(false, OmemoConfiguration.getRenewOldSignedPreKeys());
diff --git a/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoServiceTest.java b/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoServiceTest.java
index c4aefb815..73c891604 100644
--- a/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoServiceTest.java
+++ b/smack-omemo/src/test/java/org/jivesoftware/smackx/omemo/OmemoServiceTest.java
@@ -32,7 +32,6 @@ import org.jxmpp.stringprep.XmppStringprepException;
 public class OmemoServiceTest extends SmackTestSuite {
 
     private static final long ONE_HOUR = 1000L * 60 * 60;
-    private static final int IGNORE_STALE = OmemoConfiguration.getIgnoreStaleDevicesAfterHours();
     private static final int DELETE_STALE = OmemoConfiguration.getDeleteStaleDevicesAfterHours();
 
     @Test(expected = IllegalStateException.class)
@@ -55,15 +54,9 @@ public class OmemoServiceTest extends SmackTestSuite {
         OmemoDevice other = new OmemoDevice(JidCreate.bareFrom("bob@builder.tv"), 444);
 
         Date now = new Date();
-        Date ignoreMe = new Date(now.getTime() - ((IGNORE_STALE + 1) * ONE_HOUR));
         Date deleteMe = new Date(now.getTime() - ((DELETE_STALE + 1) * ONE_HOUR));
-        Date imFine = new Date(now.getTime() - ONE_HOUR);
-
-        // One hour "old" devices are (probably) not not stale
-        assertFalse(OmemoService.isStale(user, other, imFine, IGNORE_STALE));
 
         // Devices one hour "older" than max ages are stale
-        assertTrue(OmemoService.isStale(user, other, ignoreMe, IGNORE_STALE));
         assertTrue(OmemoService.isStale(user, other, deleteMe, DELETE_STALE));
 
         // Own device is never stale, no matter how old