Salesforce Canvas Apps

A Salesforce Canvas app is an often overlooked easy way to integrate existing apps into Salesforce. A Canvas app is inlined into the Salesforce user interface and it requires only a very small change to your app to have it play nice with Salesfore. In theory you could get away without any change but usually you’d like to know who the calling user is. What’s really great about a Canvas App is that this information is POST’ed to the application at invocation together with an OAuth access_token to allow authenticated callbacks to Salesforce. To implement this you need to:

  1. Support POST at a URL you specify
  2. Render the application from here or redirect the user after the POST has been received
  3. Receive and handle the signed request

The signed request is a base64 encoded blob in two parts separated by a period. It looks very much like a JSON Web Token (jwt). To verify it you compute keyed hash (hmac) using the sha-256 algorithm with the client secret of the Connected App from Salesforce being the secret. Doing this in node.js is done like so:

const ourSignature = Buffer.from(crypto.createHmac(algorithm, clientSecret).update(objPart).digest()).toString('base64')
The algorithm is “sha-256”, the client secret is a string and objPart of the object part of the signed request.
To make it even easier I’ve created a repo showing how it’s done in node.js in an Express app. The source including an example app is available at https://github.com/lekkimworld/salesforce-oauth-express-middleware. The repo also contains a test app (canvas-test-app) that is easily deployable to Heroku.