diff --git a/src/crypto/algorithms/base.js b/src/crypto/algorithms/base.js
index 10a4ce02f..5d9df319d 100644
--- a/src/crypto/algorithms/base.js
+++ b/src/crypto/algorithms/base.js
@@ -157,6 +157,21 @@ module.exports.DecryptionError = function(msg) {
 };
 utils.inherits(module.exports.DecryptionError, Error);
 
+/**
+ * Exception thrown specifically when we want to warn the user to consider
+ * the security of their conversation before continuing
+ *
+ * @constructor
+ * @param {string} msg message describing the problem
+ * @extends Error
+ */
+module.exports.UnknownDeviceError = function(msg, devices) {
+    this.name = "UnknownDeviceError";
+    this.message = msg;
+    this.devices = devices;
+};
+utils.inherits(module.exports.UnknownDeviceError, Error);
+
 /**
  * Registers an encryption/decryption class for a particular algorithm
  *
diff --git a/src/crypto/algorithms/megolm.js b/src/crypto/algorithms/megolm.js
index c645d5a09..2abf3f945 100644
--- a/src/crypto/algorithms/megolm.js
+++ b/src/crypto/algorithms/megolm.js
@@ -388,6 +388,10 @@ MegolmEncryption.prototype._shareKeyWithDevices = function(session, devicesByUse
 MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     const self = this;
     return this._getDevicesInRoom(room).then(function(devicesInRoom) {
+        // check if any of these devices are not yet known to the user.
+        // if so, warn the user so they can verify or ignore.
+        self._checkForUnknownDevices(devicesInRoom);
+
         return self._ensureOutboundSession(devicesInRoom);
     }).then(function(session) {
         const payloadJson = {
@@ -415,6 +419,38 @@ MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     });
 };
 
+/**
+ * Checks the devices we're about to send to and see if any are entirely
+ * unknown to the user.  If so, warn the user, and mark them as known to
+ * give the user a chance to go verify them before re-sending this message.
+ */
+MegolmEncryption.prototype._checkForUnknownDevices = function(devicesInRoom) {
+    const unknownDevices = {};
+
+    Object.keys(devicesInRoom).forEach(userId=>{
+        Object.keys(devicesInRoom[userId]).forEach(deviceId=>{
+            const device = devicesInRoom[userId][deviceId];
+            if (device.isUnverified() && !device.isKnown()) {
+                // mark the devices as known to the user, given we're about to
+                // yell at them.
+                //this._crypto.setDeviceVerification(userId, device.deviceId,
+                //                                   undefined, undefined, true);
+                if (!unknownDevices[userId]) {
+                    unknownDevices[userId] = {};
+                }
+                unknownDevices[userId][deviceId] = device;
+            }
+        });
+    });
+
+    if (Object.keys(unknownDevices).length) {
+        // it'd be kind to pass unknownDevices up to the user in this error
+        throw new base.UnknownDeviceError(
+            "This room contains unknown devices which have not been verified. " +
+            "We strongly recommend you verify them before continuing.", unknownDevices);
+    }
+};
+
 /**
  * Get the list of unblocked devices for all users in the room
  *
@@ -433,6 +469,9 @@ MegolmEncryption.prototype._getDevicesInRoom = function(room) {
     // have a list of the user's devices, then we already share an e2e room
     // with them, which means that they will have announced any new devices via
     // an m.new_device.
+    //
+    // XXX: what if the cache is stale, and the user left the room we had in common
+    // and then added new devices before joining this one? --Matthew
     return this._crypto.downloadKeys(roomMembers, false).then(function(devices) {
         // remove any blocked devices
         for (const userId in devices) {
diff --git a/src/crypto/deviceinfo.js b/src/crypto/deviceinfo.js
index d3dd682ca..1ddee5d00 100644
--- a/src/crypto/deviceinfo.js
+++ b/src/crypto/deviceinfo.js
@@ -34,7 +34,11 @@ limitations under the License.
   *      <key type>:<id> -> <base64-encoded key>>
   *
   * @property {module:crypto/deviceinfo.DeviceVerification} verified
-  *     whether the device has been verified by the user
+  *     whether the device has been verified/blocked by the user
+  *
+  * @property {boolean} known
+  *     whether the user knows of this device's existence (useful when warning
+  *     the user that a user has added new devices)
   *
   * @property {Object} unsigned  additional data from the homeserver
   *
@@ -50,6 +54,7 @@ function DeviceInfo(deviceId) {
     this.algorithms = [];
     this.keys = {};
     this.verified = DeviceVerification.UNVERIFIED;
+    this.known = false;
     this.unsigned = {};
 }
 
@@ -130,6 +135,24 @@ DeviceInfo.prototype.isVerified = function() {
     return this.verified == DeviceVerification.VERIFIED;
 };
 
+/**
+ * Returns true if this device is unverified
+ *
+ * @return {Boolean} true if unverified
+ */
+DeviceInfo.prototype.isUnverified = function() {
+    return this.verified == DeviceVerification.UNVERIFIED;
+};
+
+/**
+ * Returns true if the user knows about this device's existence
+ *
+ * @return {Boolean} true if known
+ */
+DeviceInfo.prototype.isKnown = function() {
+    return this.known == true;
+};
+
 /**
  * @enum
  */
diff --git a/src/crypto/index.js b/src/crypto/index.js
index ce834d4bf..822184c48 100644
--- a/src/crypto/index.js
+++ b/src/crypto/index.js
@@ -684,8 +684,12 @@ Crypto.prototype.getDeviceByIdentityKey = function(userId, algorithm, sender_key
  *
  * @param {?boolean} blocked whether to mark the device as blocked. Null to
  *      leave unchanged.
+ *
+ * @param {?boolean} known whether to mark that the user has been made aware of
+ *      the existence of this device. Null to leave unchanged
  */
-Crypto.prototype.setDeviceVerification = function(userId, deviceId, verified, blocked) {
+Crypto.prototype.setDeviceVerification = function(userId, deviceId, verified,
+                                                  blocked, known) {
     const devices = this._sessionStore.getEndToEndDevicesForUser(userId);
     if (!devices || !devices[deviceId]) {
         throw new Error("Unknown device " + userId + ":" + deviceId);
@@ -706,10 +710,16 @@ Crypto.prototype.setDeviceVerification = function(userId, deviceId, verified, bl
         verificationStatus = DeviceVerification.UNVERIFIED;
     }
 
-    if (dev.verified === verificationStatus) {
+    let knownStatus = dev.known;
+    if (known !== null) {
+        knownStatus = known;
+    }
+
+    if (dev.verified === verificationStatus && dev.known === knownStatus) {
         return;
     }
     dev.verified = verificationStatus;
+    dev.known = knownStatus;
     this._sessionStore.storeEndToEndDevicesForUser(userId, devices);
 };