OAuth2 demystified


I was trying to pair on writing a simple app that uses Hacker School's OAuth2 API, and hit a roadblock on the first step of requesting an authorization from the user. Once the user authorized my app, I would see an error that said, "The authorization server does not support this response type". I was using a client library that I had used before, and the server was using a what seemed like a popular implementation for ruby on rails. Getting weird errors is not done!

I have used OAuth2 based authentication before, but the thought of using it always makes me a little nervous, just because

  1. I don't understand it very well.
  2. Like almost everything else, there seem to be so many libraries for doing this in Python, and I'm never sure which one to use, or which one I used the last time around. Not understanding the protocol also doesn't let me debug anything that comes up.

To fix this, I set about to read and understand the OAuth2 protocol. This blog post is an attempt to record it for future reference, and possibly act as a reference for others.

Why OAuth

OAuth is simply a way for an end-user to allow third parties to use protected data, without sharing the user's credentials with the third-party.

For example, an end-user (Jane) can grant a printing service (Printo) access to her protected photos stored at a photo-sharing service (Picasa), without sharing her username and password with the printing service. Instead, she authenticates directly with a server trusted by the photo-sharing service, which issues the printing service delegation-specific credentials. (example from the OAuth 2.0 spec)

Protocol Flow

The flow occurs through a sequence of user actions, client requests and user-agent (browser) redirects.

  • (A) Printo asks Jane to allow using Picasa Data. The request can be sent directly to Jane, but is usually routed via Picasa/Google.
  • (B) Printo gets back an authorization grant, which is a credential representing Jane's authorization or approval. The type of the actual grant credential depends on the type of request that Printo used.
  • (C, D) Printo gets back to Google with the credentials it obtained in the previous step and obtains a token that it can use to talk with Picasa.
  • (E, F) Printo asks for the desired photo with the token it obtained previously, and Picasa gives back the photo to print. Jane gets her framed photo!

But before any of this happens, the client needs to register with the authorization server and obtain a client_id and client_secret, that will be used to identify the client making the requests.

Pythonized "authorization code" work-flow.

The OAuth2 spec allows the authorization request/grant to be of 4 different types. It also allows some flexibility in the token type.

In my experience, the most common work-flow seems to be using an authorization code as an authorization grant, and using a Bearer type token. This work-flow is explained in the diagram below (taken from the spec document). This diagram zooms in, onto the steps A-D in the diagram above.

This python code snippet is a simple implementation of this workflow, using the Hacker School API.


I think, I understand the OAuth2 spec a lot better now, and hope that this will help others understand it, too. And more importantly, I won't get nervous when I have to add it to my projects.

Also, oauthlib for Python seems to be a pretty thorough implementation of the spec, and requests-oauthlib seems to wrap it for use with requests. I think I'm going to use this in my future projects.

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