diff --git a/src/crypto/OlmDevice.js b/src/crypto/OlmDevice.js
index 894d958e8..8d1817998 100644
--- a/src/crypto/OlmDevice.js
+++ b/src/crypto/OlmDevice.js
@@ -435,9 +435,9 @@ OlmDevice.prototype.getSessionInfoForDevice = async function(deviceIdentityKey)
  * @param {string} sessionId  the id of the active session
  * @param {string} payloadString  payload to be encrypted and sent
  *
- * @return {string} ciphertext
+ * @return {Promise<string>} ciphertext
  */
-OlmDevice.prototype.encryptMessage = function(
+OlmDevice.prototype.encryptMessage = async function(
     theirDeviceIdentityKey, sessionId, payloadString,
 ) {
     const self = this;
@@ -460,9 +460,9 @@ OlmDevice.prototype.encryptMessage = function(
  * @param {number} message_type  message_type field from the received message
  * @param {string} ciphertext base64-encoded body from the received message
  *
- * @return {string} decrypted payload.
+ * @return {Promise<string>} decrypted payload.
  */
-OlmDevice.prototype.decryptMessage = function(
+OlmDevice.prototype.decryptMessage = async function(
     theirDeviceIdentityKey, sessionId, message_type, ciphertext,
 ) {
     const self = this;
@@ -484,10 +484,10 @@ OlmDevice.prototype.decryptMessage = function(
  * @param {number} message_type  message_type field from the received message
  * @param {string} ciphertext base64-encoded body from the received message
  *
- * @return {boolean} true if the received message is a prekey message which matches
+ * @return {Promise<boolean>} true if the received message is a prekey message which matches
  *    the given session.
  */
-OlmDevice.prototype.matchesSession = function(
+OlmDevice.prototype.matchesSession = async function(
     theirDeviceIdentityKey, sessionId, message_type, ciphertext,
 ) {
     if (message_type !== 0) {
diff --git a/src/crypto/algorithms/olm.js b/src/crypto/algorithms/olm.js
index 36ba16384..495b125e5 100644
--- a/src/crypto/algorithms/olm.js
+++ b/src/crypto/algorithms/olm.js
@@ -251,7 +251,7 @@ OlmDecryption.prototype._decryptMessage = async function(
     for (let i = 0; i < sessionIds.length; i++) {
         const sessionId = sessionIds[i];
         try {
-            const payload = this._olmDevice.decryptMessage(
+            const payload = await this._olmDevice.decryptMessage(
                 theirDeviceIdentityKey, sessionId, message.type, message.body,
             );
             console.log(
@@ -260,7 +260,7 @@ OlmDecryption.prototype._decryptMessage = async function(
             );
             return payload;
         } catch (e) {
-            const foundSession = this._olmDevice.matchesSession(
+            const foundSession = await this._olmDevice.matchesSession(
                 theirDeviceIdentityKey, sessionId, message.type, message.body,
             );
 
diff --git a/src/crypto/olmlib.js b/src/crypto/olmlib.js
index 317f27955..161d762f9 100644
--- a/src/crypto/olmlib.js
+++ b/src/crypto/olmlib.js
@@ -102,7 +102,7 @@ module.exports.encryptMessageForDevice = async function(
 
     utils.extend(payload, payloadFields);
 
-    resultsObject[deviceKey] = olmDevice.encryptMessage(
+    resultsObject[deviceKey] = await olmDevice.encryptMessage(
         deviceKey, sessionId, JSON.stringify(payload),
     );
 };