For a project I’ve been working on recently we had need to create a module that provides secure redirects from a Drupal site to FormAssembly. Overall the module does a number of things, but handling dynamic parameter signing was the thing that took the most time.
FormAssembly provides a variety of great features for creating flexible forms that integrate with Salesforce. One of the more popular features is its ability to pull data from Salesforce to prefill fields on a form. But the downside is that it is easy to create forms that leak information from Salesforce into those forms, and create privacy risks.
To address this, FormAssembly allows 3rd party tools to securely sign URLs that contain parameters (often Salesforce IDs) that could be used to extract information through an iteration attack and other basic approaches. This secure signing process can be done statically but for most interesting projects you want to sign the URLs dynamically. The dynamic signing process allows you alter the parameters on the fly and set an expiration date to limit the value of a stolen link. Our project required this approach.
But the dynamic signing process has a couple sharp corners. First, it’s rarely done outside of Salesforce so there aren’t a lot of code samples around, and none that I could find in PHP. Second, FormAssembly is very open and honest about the fact that they do not provide support on this feature. So I had to create my own process from the documentation they provide. The docs are good, but very Salesforce centric, with all code samples in APEX.
The process involves preparing the data for signature, generating a HMAC-SHA256 with a form specific pre-shared key (in binary mode), converted to a string using base64, and finally URL encode the result.
Their convention for preparing the data is straightforward. You format all parameters as just their key and value strung together:
The interesting part is the actual HMAC-SHA256, which needs to be generated in binary mode, something that is often the default mode but not in PHP (in fact most PHP devs I’ve talked don’t realize the last parameter to hash_hmac() is useful, if you are doing this in another language check out this collection of examples).
From there you encode the output in base-64 (which results in a 44 character hash), and URL encode the hash to make sure it’s URL safe, and you’ll end up a few characters longer.
Finally you add you hash to the query string, and you’re ready to go.
To help anyone else who needs to do this, I generalized this part of the solution and I created and tossed it into Gist.