I previously wrote an article on how to use Proof-Key for Code Exchange (PKCE) in a server-side ASP.NET Core application. In the IdentityServer world authorization code with PKCE now replaces OpenID Connect's (OIDC) hybrid flow as our most secure authorization method; however, not all client libraries or even OpenID Providers support PKCE yet. An alternative approach that gives a comparatively high level of assurance is to use the OIDC hybrid flow in combination with encrypted identity tokens via JSON Web Encryption (JWE).

Using the hybrid flow with encrypted identity tokens allows us to validate the authorization response (via identity token validation), ensure that the authorization code was intended for us (via c_hash validation), and prevent PII passing via the browser (thanks to JWE).

To help a 10% project at work, the Rock Solid Knowledge IdentityServer team has been creating a basic OpenID Connect library for a Flutter application. After poking around in Dart over the weekend, I found that Dart did not have a straightforward way to create a cryptographically random string suitable for OAuth/OpenID Connect values such as state, nonce, or PKCE’s code challenge. So, in this article, I’m going to share a straightforward way to generate one.

Dart’s Random Number Generator

Luckily, Dart does have a cryptographically secure random number generator that we can use, found in the dart:math library.

I’ve recently started the cryptopals crypto challenges, and, frankly, even the basics are kicking my ass. However, I seem to be enjoying them, and I’m finally starting to understand some of the Computer Science topics I really should have listened to at University. If you are like me and prefer learning by getting your hands dirty and hacking some code together, then I highly recommend working through some of these challenges.

If you have not heard about cryptopals before, then I'll leave it to the creators to explain:

“We've built a collection of 48 exercises that demonstrate attacks on real-world crypto.

This is a different way to learn about crypto than taking a class or reading a book. We give you problems to solve. They're derived from weaknesses in real-world systems and modern cryptographic constructions. We give you enough info to learn about the underlying crypto concepts yourself. When you're finished, you'll not only have learned a good deal about how cryptosystems are built, but you'll also understand how they're attacked.”

We’ve all used Convert.ToBase64String() but what is it actually happening under the covers? Sure, it’s taking a value and representing it using only characters from a range of 64 characters, but how exactly does it do that? Up until now, I probably couldn’t have told you.

My favorite example for understanding how Base64 encoding works is actually from Wikipedia...

The next few challenges cover implementing and then breaking the Caesar and Vigenère ciphers. These ciphers usually serve as the introduction to most cryptography books, as a history lesson of what we used to use and how easy they are to break. However, with cryptopals, we take this academic knowledge and turn it into practice.