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I am writing a login for a forum, and need to hash the password client side in javascript before sending it on to the server. I'm having trouble figuring out which SHA-256 implementation I can actually trust. I was expecting there to be some kind of authoritative script that everyone used, but I'm finding loads of different projects all with their own implementations.

I realize using other people's crypto is always a leap of faith unless you're qualified to review it yourself, and that there is no universal definition of "trustworthy", but this seems like something common and important enough that there ought to be some kind of consensus on what to use. Am I just naive?

Edit since it comes up a lot in the comments: Yes, we do a more stringent hash again on the server side. The client side hashing is not the final result that we save in the database. The client side hashing is because the human client requests it. They have not given a specific reason why, probably they just like overkill.

The Stanford JS Crypto Library contains an implementation of SHA-256. While crypto in JS isn't really as well-vetted an endeavor as other implementation platforms, this one is at least partially developed by, and to a certain extent sponsored by, Dan Boneh, who is a well-established and trusted name in cryptography, and means that the project has some oversight by someone who actually knows what he's doing. The project is also supported by the NSF.

It's worth pointing out, however...

... that if you hash the password client-side before submitting it, then the hash is the password, and the original password becomes irrelevant. An attacker needs only to intercept the hash in order to impersonate the user, and if that hash is stored unmodified on the server, then the server is storing the true password (the hash) in plain-text.

So your security is now worse because you decided add your own improvements to what was previously a trusted scheme.

On https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/digest I found this snippet that uses internal js module:

async function sha256(message) {

    // encode as UTF-8

    const msgBuffer = new TextEncoder('utf-8').encode(message);                    



    // hash the message

    const hashBuffer = await crypto.subtle.digest('SHA-256', msgBuffer);



    // convert ArrayBuffer to Array

    const hashArray = Array.from(new Uint8Array(hashBuffer));



    // convert bytes to hex string                  

    const hashHex = hashArray.map(b => ('00' + b.toString(16)).slice(-2)).join('');

    return hashHex;

}

Note that crypto.subtle in only available on https or localhost - for example for your local development with python3 -m http.server you need to add this line to your /etc/hosts:
0.0.0.0 localhost

Reboot - and you can open localhost:8000 with working crypto.subtle.

Forge's SHA-256 implementation is fast and reliable.

To run tests on several SHA-256 JavaScript implementations, go to http://brillout.github.io/test-javascript-hash-implementations/.

The results on my machine suggests forge to be the fastest implementation and also considerably faster than the Stanford Javascript Crypto Library (sjcl) mentioned in the accepted answer.

Forge is 256 KB big, but extracting the SHA-256 related code reduces the size to 4.5 KB, see https://github.com/brillout/forge-sha256

No, there's no way to use browser JavaScript to improve password security. I highly recommend you read this article. In your case, the biggest problem is the chicken-egg problem:

What's the "chicken-egg problem" with delivering Javascript cryptography?

If you don't trust the network to deliver a password, or, worse, don't trust the server not to keep user secrets, you can't trust them to deliver security code. The same attacker who was sniffing passwords or reading diaries before you introduce crypto is simply hijacking crypto code after you do.

[...]

Why can't I use TLS/SSL to deliver the Javascript crypto code?

You can. It's harder than it sounds, but you safely transmit Javascript crypto to a browser using SSL. The problem is, having established a secure channel with SSL, you no longer need Javascript cryptography; you have "real" cryptography.

Which leads to this:

The problem with running crypto code in Javascript is that practically any function that the crypto depends on could be overridden silently by any piece of content used to build the hosting page. Crypto security could be undone early in the process (by generating bogus random numbers, or by tampering with constants and parameters used by algorithms), or later (by spiriting key material back to an attacker), or --- in the most likely scenario --- by bypassing the crypto entirely.

There is no reliable way for any piece of Javascript code to verify its execution environment. Javascript crypto code can't ask, "am I really dealing with a random number generator, or with some facsimile of one provided by an attacker?" And it certainly can't assert "nobody is allowed to do anything with this crypto secret except in ways that I, the author, approve of". These are two properties that often are provided in other environments that use crypto, and they're impossible in Javascript.

Basically the problem is this:

• Your clients don't trust your servers, so they want to add extra security code.


• That security code is delivered by your servers (the ones they don't trust).

Or alternatively,

• Your clients don't trust SSL, so they want you use extra security code.


• That security code is delivered via SSL.

Note: Also, SHA-256 isn't suitable for this, since it's so easy to brute force unsalted non-iterated passwords. If you decide to do this anyway, look for an implementation of bcrypt, scrypt or PBKDF2.