How to decrypt an ArrayBuffer?

Question

I've been trying to decrypt an ArrayBuffer object using CryptoJS, but so far it always returns a blank WordArray. The files (images) are encrypted in an iOS and Android app, sent to a server, and downloaded in this web app to be decrypted and displayed. The iOS and Android apps are able to decrypt the files without problems, so there's nothing wrong with the encryption process.

The files are downloaded with an XMLHttpRequest with responseType set to arraybuffer. Here's my code so far:

// Decrypt a Base64 encrypted string (this works perfectly)
String.prototype.aesDecrypt = function(key) {

    var nkey = CryptoJS.enc.Hex.parse(key.sha256());
    return CryptoJS.AES.decrypt(this.toString(), nkey, {
        iv: CryptoJS.enc.Hex.parse('00000000000000000000000000000000'),
        mode: CryptoJS.mode.CBC,
        padding: CryptoJS.pad.Pkcs7
    }).toString(CryptoJS.enc.Utf8);

}

// Decrypt a plain encrypted ArrayBuffer (this is the problem, it always outputs an empty WordArray)
ArrayBuffer.prototype.aesDecrypt = function(key) {

    // Get key
    if (!key) return null;
    var nkey = CryptoJS.enc.Hex.parse(key.sha256());

    // Get input (if I pass the ArrayBuffer directly to the create function, it returns
    // a WordList with sigBytes set to NaN)
    //var input = CryptoJS.lib.WordArray.create(this);
    var input = CryptoJS.lib.WordArray.create(new Uint8Array(this));

    // Decrypt
    var output = CryptoJS.AES.decrypt(input, nkey, {
        iv: CryptoJS.enc.Hex.parse('00000000000000000000000000000000'),
        mode: CryptoJS.mode.CBC,
        padding: CryptoJS.pad.Pkcs7
    });

    // Output is an empty WordList
    console.log("Output: ", output);

}

Another question I have is how do you convert a WordArray to an ArrayBuffer?

Answer 1

The conversion of ArrayBuffer -> WordArray has been discussed in CryptoJS's issue 46. For that reason a TypedWordArraywhere you can also pass an ArrayBuffer has been added.

---

To use that additionally include the following script:

<script src="http://crypto-js.googlecode.com/svn/tags/3.1/build/components/lib-typedarrays.js"></script>

Then you can simply do:

var wordArray = CryptoJS.lib.WordArray.create(arrayBuffer);

/* perform decryption of `wordArray` */

---

To reconvert the resulting decryptedWordArray to an ArrayBuffer, the simplest approach would probably be, to first convert it to a Base64-String (as discussed here) and then decode that String to the desired ArrayBuffer (see here). The whole procedure would look something like this:

dcWordArray = ... // your decrypted WordArray
dcBase64String = dcWordArray.toString(CryptoJS.enc.Base64); // to Base64-String
dcArrayBuffer = base64DecToArr(dcBase64String).buffer; // to ArrayBuffer

Edit:

For a more efficient conversion (no intermediate Base64String necessary) check out Aletheios answer to that question (the function wordToByteArray(wordArray) and then do .buffer).

Answer 2

you can use the web crypto API to directly decrypt arrayBuffer. No need to convert binary data to a string, which is so inefficient, and it could cause memory and CPU rises.