Charles Engelke's Blog

I’m very interested in doing cryptography in the browser for things like end-to-end sealing of data and digital signatures. It’s been possible to do some of these things, but not practical. The W3C’s new Web Cryptography API should change that. I’ve been following its progress with interest, and just discovered that it has been partially implemented in Internet Explorer 11 and Google Chrome 35 (behind an experimental flag). So I’ve started fiddling with it, and I’m going to put notes here in my blog using the webcrypto tag.

Before I could really do anything with the API, I needed to get familiar with a couple of new JavaScript features: Typed Arrays and Promises. You can follow those links for more background, but here’s what I needed to understand:

• Typed arrays are similar to arrays in more traditional programming languages like C, where every element of the array must be of the same type. It seems they’re always implemented as contiguous blocks of memory, making them much faster for low-level operations than regular JavaScript arrays. The only kind of typed array we seem to need for cryptography is Uint8Array, arrays of unsigned 8-bit integers. That is, arrays of bytes.
• Promises are JavaScript objects to help work with asynchronous operations. Instead of giving a callback function when you request an asynchronous operation, you’d just have the operation return a Promise. You invoke the Promise’s then method to provide a function to call when the operation is done. If the operation is finished before you call then, that’s fine. The result will be held until then is eventually invoked. You can pass two functions to then if you’d like: the first is called with the result of the asynchronous operation if it succeeds, and the second is called with the error if it fails. Or you can handle errors by passing the error handler to the Promise’s catch method instead. The result returned by calling then is another Promise, making it easy to chain Promises to perform asynchronous operations serially.

Examples:

The statement var b = new Uint8Array(1024); creates a data structure that can hold one kilobyte. You can work with each byte individually, as in b[25] = 72; b[26] = b[25] + 1; and so on.

Suppose the asynchronous operation doSomething returns a promise. Then you can do the following:

var p = doSomething(param1, param2);
var p1 = p.then(handleResult, handleError);
p1.then(doSomethingNext, handleNextError);

A popular way to write this seems to be:

doSomething(param1, param2
).then(handleResult, handleError
).then(doSomethingNext, handleNextError);

Although I’m considering writing it like this (at least until I know a reason not to):

doSomething(param1, param2).
then(handleResult, handleError).
then(doSomethingNext, handleNextError);

I think that’s all the special background needed to use the API. In my next post I’ll work up to encrypting and decrypting files using AES in CBC mode symmetric encryption.