Signature generation and validation for REST API query authentication

4.0.0-alpha 2013-08-19 13:58 UTC


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Signature generation and validation for REST API query authentication

API Query Authentication

Most APIs require some sort of query authentication, frequently a method of signing API requests with an API key and signature. The signature is usually generated using a shared secret. When you're consuming an API, there are (hopefully) easy to follow steps to create signatures. When you're writing your own API, you have to whip up both a server-side signature validation strategy and a client-side signature creation strategy. This library endeavors to handle both of those tasks for you.

Sample Implementation

A sample implementation of the Query Auth library is available in order to better demonstrate how one might employ the library.


There are three components to this library:

  • Request signing
  • Request validation
  • API key and secret generation

Request signing and validation are made possible by the use of request adapters.

Request Adapters

Query Auth request adapters wrap outgoing and incoming requests and adapt them to the request interface that Query Auth expects.


Outgoing request adapters are used to facilitate request signing. There are currently two available in the QueryAuth\Request\Adapter\Outgoing namespace:

  • GuzzleRequestAdapter for use with Guzzle v3
  • GuzzleHttpRequestAdapter for use with Guzzle v4


Incoming request adapters are used to facilitate request validation. There is currently one available in the QueryAuth\Request\Adapter\Incoming namespace:

  • SlimRequestAdapter for use with Slim PHP v2


If you would prefer to use an HTTP library other than Guzzle, or if you prefer to use an application framework other than Slim, you will need to write your own request adapter(s). Please refer to the existing request adapters for examples.

Request Signing

use GuzzleHttp\Client as GuzzleHttpClient;
use QueryAuth\Credentials\Credentials;
use QueryAuth\Factory;
use QueryAuth\Request\Adapter\Outgoing\GuzzleHttpRequestAdapter;

$factory = new Factory();
$requestSigner = $factory->newRequestSigner();
$credentials = new Credentials('key', 'secret');

// Create a GET request and set an endpoint
$guzzle = new GuzzleHttpClient(['base_url' => '']);
$request = $guzzle->createRequest('GET', '/endpoint');

// Sign the request
$requestSigner->signRequest(new GuzzleHttpRequestAdapter($request), $credentials);

// Send signed request
$response = $guzzle->send($request);

Request Validation

use QueryAuth\Credentials\Credentials;
use QueryAuth\Factory;
use QueryAuth\Request\Adapter\Incoming\SlimRequestAdapter;

$factory = new Factory();
$requestValidator = $factory->newRequestValidator();
$credentials = new Credentials('key', 'secret');

// Get the Slim request (in the context of a Slim route, hook, or middleware)
$request = $app->request;

// $isValid is a boolean
$isValid = $requestValidator->isValid(new SlimRequestAdapter($request), $credentials);

RequestValidator::isValid() will return either true or false. It might also throw one of three exceptions:

  • DriftExceededException: It timestamp is beyond +- RequestValidator::$drift
  • SignatureMissingException: If signature is missing from request params
  • TimestampMissingException: If timestamp is missing from request params

Drift defaults to 15 seconds, meaning there is a 30 second window during which the request is valid. The default value can be modified using RequestValidator::setDrift().

Replay Attack Prevention

There are a number of strategies available to prevent replay attacks. The strategy in place here follows this general outline:

  • Validate incoming signature
  • If the signature is valid, check the storage layer to see if that combination of API key and signature have been used before
  • If they have, the request is likely a replay attack and should be denied
  • If they have not, persist the API key, signature, and an expiration timestamp
  • Routinely purge records with a timestamp beyond the expiration time

IMPORTANT: The signature expiration timestamp should be greater than maximum allowable drift. Deleting a signature too soon can leave you vulnerable to a replay attack.

NOTE: Implementing a replay prevention strategy is optional. It is not a requirement for using this library. It is, however, highly recommended.

The QueryAuth\Storage\SignatureStorage interface is provided to aid in implementing replay attack prevention.


namespace QueryAuth\Storage;

interface SignatureStorage
    public function exists($key, $signature);

    public function save($key, $signature, $expires);

    public function purge();

NOTE: Implementing the SignatureStorage interface is not required to prevent replay attacks, it's simply present to assist you in implementing the attack prevention strategy outlined above.

Key Generation

You can generate API keys and secrets in the following manner.

$factory = new QueryAuth\Factory();
$keyGenerator = $factory->newKeyGenerator();

// 40 character random alphanumeric string
$key = $keyGenerator->generateKey();

// 60 character random string containing the characters
// 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ./
$secret = $keyGenerator->generateSecret();

Both key and secret are generated using Anthony Ferrara's RandomLib random string generator.

Versions Less Than 3.0+ Deprecated, Not Obsolete

While I'd advise upgrading to v3 as soon as possible, a happy side effect of refactoring the API without changing the signature creation and validation logic is that Query Auth 3.0+ is compatible with prior versions of Query Auth. This means that you'll be able to upgrade Query Auth on the server-side (validation) without needing to immediately upgrade all client-side (creation) applications. BONUS!


Package installation is handled by Composer.

  • If you haven't already, please install Composer
  • Create composer.json in the root of your project and add query-auth as a dependency:
    "require": {
        "jeremykendall/query-auth": "*"
  • Run composer install
  • Require Composer's vendor/autoload.php script in your bootstrap/init script

Feedback and Contributions

  • Feedback is welcome in the form of pull requests and/or issues.
  • Contributions should generally follow the strategy outlined in "Contributing to a project"
  • Please submit pull requests against the develop branch


  • Query Auth is my own implementation of the Signature Version 2 implementation from the AWS SDK for PHP 2. As such, a version of the Apache License Version 2.0 is included with this distribution, and the applicable portion of the AWS SDK for PHP 2 NOTICE file is included.

  • API key and API secret generation is handled by Anthony Ferrara's RandomLib random string generator.