Factor out a function for doing olm encryption

Make a library file with some constants and a function to pack olm-encrypted
events (which we are going to use from megolm)

lib/olmlib.js

@@ -0,0 +1,78 @@
+/*
+Copyright 2016 OpenMarket Ltd
+
+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.
+*/
+
+/**
+ * @module olmlib
+ *
+ * Utilities common to olm encryption algorithms
+ */
+
+var utils = require("./utils");
+
+/**
+ * matrix algorithm tag for olm
+ */
+module.exports.OLM_ALGORITHM = "m.olm.v1.curve25519-aes-sha2";
+
+/**
+ * matrix algorithm tag for megolm
+ */
+module.exports.MEGOLM_ALGORITHM = "m.megolm.v1.aes-sha2";
+
+
+/**
+ * Encrypt an event payload for a list of devices
+ *
+ * @param {string} ourDeviceId
+ * @param {module:OlmDevice} olmDevice olm.js wrapper
+ * @param {string[]} participantKeys list of curve25519 keys to encrypt for
+ * @param {object} payloadFields fields to include in the encrypted payload
+ *
+ * @return {object} content for an m.room.encrypted event
+ */
+module.exports.encryptMessageForDevices = function(
+    ourDeviceId, olmDevice, participantKeys, payloadFields
+) {
+    participantKeys.sort();
+    var participantHash = ""; // Olm.sha256(participantKeys.join());
+    var payloadJson = {
+        fingerprint: participantHash,
+        sender_device: ourDeviceId,
+    };
+    utils.extend(payloadJson, payloadFields);
+
+    var ciphertext = {};
+    var payloadString = JSON.stringify(payloadJson);
+    for (var i = 0; i < participantKeys.length; ++i) {
+        var deviceKey = participantKeys[i];
+        var sessionId = olmDevice.getSessionIdForDevice(deviceKey);
+        if (sessionId === null) {
+            // If we don't have a session for a device then
+            // we can't encrypt a message for it.
+            continue;
+        }
+        console.log("Using sessionid " + sessionId + " for device " + deviceKey);
+        ciphertext[deviceKey] = olmDevice.encryptMessage(
+            deviceKey, sessionId, payloadString
+        );
+    }
+    var encryptedContent = {
+        algorithm: module.exports.OLM_ALGORITHM,
+        sender_key: olmDevice.deviceCurve25519Key,
+        ciphertext: ciphertext
+    };
+    return encryptedContent;
+};