Quick and Dirty REST Security (or Hashes For All!)

So in working up a new RESTful service I’ve been tinkering with, I wanted to provide some kind of “authentication” system for it. I started to look into OAuth, but got a bit overwhelmed by everything that was involved with it. Looking for something a bit more lightweight (and simpler to implement a bit more quickly) I came across this older article with a suggestion of a private key/hash combination. I figured that could do the job nicely for a first shot, so I set to implementing it.

On the Server Side

I’m using the FuelPHP framework for this one, but that’s really only giving me a structure to work in and pull the request information from. This would work in most major frameworks (and even outside of one if you you’re a “do it my way” kind of developer). First off, let’s start with the controller side:

<?php
class Controller_User extends Controller_Rest
{
    protected function validateHash()
    {
        $request = file_get_contents('php://input');
        $requestHeaders = apache_request_headers();

        if (!isset($requestHeaders['X-Auth']) || !isset($requestHeaders['X-Auth-Hash'])) {
            $this->response('fail!',401);
        } else {
            // we have the headers - let's match!
            $user = Model_User::find()->where('public_key',$requestHeaders['X-Auth'])->get_one();

            if ($user !== null) {
                $hash = hash_hmac('sha256',$request,$user->private_key);
                return ($hash == $requestHeaders['X-Auth-Hash']) ? true : false;
            } else {
                return false;
            }
        }
    }

    public function post_index()
    {
        // return the user details here....
    }

    public function router($resource, array $arguments)
    {
        if ($this->validateHash() == false) {
            $resource = 'error';
            $arguments = array('Not Authorized',401);
        }

        parent::router($resource,$arguments);
    }
}
?>

There’s a lot going on here, so let me walk you through each of the steps:

  1. First off, we’re making a RESTful service, so we’re going to extend the Controller_Rest that Fuel comes with. It has some handy special routing. Our POST request in the example below would try to hit the “post_index” method and have its hashes checked in the process.
  2. Next up is the “validateHash” method – this is where the hard work happens:
    • The request and headers are read into variables for easier use ($request and $requestHeaders).
    • It then checks to be sure that both of our required headers are set (X-Auth and X-Auth-Hash). There’s nothing magical about these header names, so they can be switched out depending on need and naming preference.
    • If they’re there, the next step is to find the user based on the public key that was sent. This value is okay to openly share because, without the private key to correctly hash the data, your requests will fail.
    • The hash_hmac function is then used (with the “sha256″ hash type) to regenerate the hash off of the contents of the request and the private key on the found user.
  3. If all goes well, the request continues on and the “post_index” method is used. If it fails, however, the check in the “route” method of the controller makes a switch. It changes the currently requested resource to “/error/index” instead of what the user wants. This seamlessly shows the user a “Not Authorized” error message (401) if the hash checking fails.

A Client Example

Now, to help make it a bit clearer, here’s an example little script showing a curl request using the hashes:

<?php

$privateKey = 'caa68fb2160b428bd1e7d78fcf0ce2d5';
$publicKey  = '01fa456c4e2a2bc13e5c0c4977297fbb';

$data = '{"username":"happyFunBall"}';
$hash = hash_hmac('sha256',$data,$privateKey);

$headers = array(
    'X-Auth: '.$publicKey,
    'X-Auth-Hash: '.$hash
);

$ch = curl_init('http://mysite.localhost:8080/user');

curl_setopt($ch,CURLOPT_HEADER,true);
curl_setopt($ch,CURLOPT_HTTPHEADER,$headers);
curl_setopt($ch,CURLOPT_POSTFIELDS,$data);
curl_setopt($ch,CURLOPT_RETURNTRANSFER,true);

$result = curl_exec($ch);
curl_close($ch);

print_r($result);
echo "nn";
?>

You can see that both the public and private keys are specified (but on the PHP side, not visible to the user) and are sent as the “X-Auth*” headers as a part of the request. In this case, we’re POSTing to the “/user/enygma” resource and creating a user. To create the value to put into $hash, we use the same “sha256″ hashing method and salt it with the private key. This value – the result of the data+private key hashing – is then passed to the service who uses the controller code from above to validate that it’s correct.

It’s not a true “login” process, but it can help to validate that a piece of information known only to the user (the private key) and never shared directly, is known and matches the requests that the user sends.

You can find the code for this in this gist if you’d like something a bit more readable: https://gist.github.com/2697434

Category: FuelPHP, PHP, REST 6 comments »

6 Responses to “Quick and Dirty REST Security (or Hashes For All!)”

  1. Artem

    So how do you use public key? It is not used in encryption or decryption. It only helps to find a user? In that case it should be called “login” instead of “public key”.

  2. enygma

    Yes, it could be “login” but that implies that it’s a username and not some other unique identifier. I’d never want to send a plain-text username (login) to a web service when I can provide it with a hashed value that has nothing to do with my actual credentials.

  3. Gabriel Crowe

    This example is great but vulnerable to replay attacks. You dont use any sort of timestamp in the request.

    You’d better make sure this is done over ssl too.
    :)

  4. enygma

    @gabriel Ah, true…I’d seen other places that recommended using things like timestamps, page URLs, etc as a part of the hashing. I agree, a much better solution.

  5. JonoB

    Great explanation and nicely written. Thank you.

  6. JonoB

    Quick question: how do you do GET requests to the API? This method always seems to imply that there are CURLOPT_POSTFIELDS.


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