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?

Community · Accepted Answer · 2017-05-23 12:16:59Z

15

+150

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).

edited May 23, 2017 at 12:16

CommunityBot

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answered Sep 1, 2014 at 18:21

i_turo

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8 Comments

jjv360 Over a year ago

If I provide the ArrayBuffer directly to WordArray.create, sigBytes is NaN. If I wrap it in a Uint8Array it's quite a large number. Is the sigBytes field important? Either way, AES.decrypt returns an empty WordArray... Also, when converting back to an ArrayBuffer won't a Base64 string be too big? The files can be up to 30MB that are being decrypted...

i_turo Over a year ago

sigBytes is only important if you are using a signature/MAC, which looking at your working String-encryption-example you don't. If your files are that large, that could really get a problem, I'll try to find a better solution. Could you maybe provide some test-data (encrypted ArrayBuffer (a small one if possible) + key + IV), so I can try out decryption myself. Thanks.

i_turo Over a year ago

Sorry, was completely wrong with sigBytes. They tell how many bytes are saved in the WordArray. The mustn't be NaN. But actually they aren't for me when testing it locally. (when you include lib-typedarrays.js)

jjv360 Over a year ago

I made a simple JSFiddle here, hopefully it helps... The ArrayBuffer created there is just a simple text file (48 bytes) and I showed decrypting it as a Base64 string which works, and as an ArrayBuffer, which doesn't...

i_turo Over a year ago

It seems that CryptoJS for some reason does only decrypt Base64-Strings. (see also this similar problem) Possibly the easiest solution would be ArrayBuffer -> String -> decrypt() -> ArrayBuffer. I'll update my answer with that info later :)

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邹俊辉 · Accepted Answer · 2019-09-17 08:28:57Z

-3

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.

answered Sep 17, 2019 at 8:28

邹俊辉

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