Make ICipher.decrypt async

In preparation for using SubtleCrypto.decrypt, which returns a promise.

Resolves #1293.

src/common/lib/types/message.ts

@@ -247,7 +247,7 @@ class Message {
                 if (xformAlgorithm != cipher.algorithm) {
                   throw new Error('Unable to decrypt message with given cipher; incompatible cipher params');
                 }
-                data = cipher.decrypt(data);
+                data = await cipher.decrypt(data);
                 continue;
               } else {
                 throw new Error('Unable to decrypt message; not an encrypted channel');

src/common/types/ICipher.ts

@@ -1,5 +1,5 @@
 export default interface ICipher<InputPlaintext, OutputCiphertext, InputCiphertext, OutputPlaintext> {
   algorithm: string;
   encrypt: (plaintext: InputPlaintext, callback: (error: Error | null, data: OutputCiphertext | null) => void) => void;
-  decrypt: (ciphertext: InputCiphertext) => OutputPlaintext;
+  decrypt: (ciphertext: InputCiphertext) => Promise<OutputPlaintext>;
 }

src/platform/nodejs/lib/util/crypto.ts

@@ -249,7 +249,7 @@ var CryptoFactory = function (bufferUtils: typeof BufferUtils) {
       return callback(null, ciphertext);
     }
 
-    decrypt(ciphertext: InputCiphertext) {
+    async decrypt(ciphertext: InputCiphertext): Promise<OutputPlaintext> {
       var blockLength = this.blockLength,
         decryptCipher = crypto.createDecipheriv(this.algorithm, this.key, ciphertext.slice(0, blockLength)),
         plaintext = toBuffer(decryptCipher.update(ciphertext.slice(blockLength))),

src/platform/web/lib/util/crypto.ts

@@ -315,7 +315,7 @@ var CryptoFactory = function (config: IPlatformConfig, bufferUtils: typeof Buffe
       }
     }
 
-    decrypt(ciphertext: InputCiphertext) {
+    async decrypt(ciphertext: InputCiphertext): Promise<OutputPlaintext> {
       Logger.logAction(Logger.LOG_MICRO, 'CBCCipher.decrypt()', '');
       ciphertext = bufferUtils.toWordArray(ciphertext);
       var blockLengthWords = this.blockLengthWords,