How to decrypt an ArrayBuffer?

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.

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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` */

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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).