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InstAccess

An InstAccess token is a signed, encrypted JWT (aka a JWE) that is usable to authenticate to the Canvas API and any other APIs in the Instructure ecosystem that choose to use it. Depending on how this gem is configured, the InstAccess::Token class can (a) both issue encrypted tokens and validate and deserialize decrypted tokens, or (b) just validate and deserialize decrypted tokens. It can therefore be used by the identity provider (today this is Canvas) to produce tokens, but also by any services that want to accept InstAccess tokens for authentication (including Canvas -- it is both an InstAccess token provider and an InstAccess token consumer).

Signing and encryption are each asymmetrical procedures. Signing is done with an RSA private key, encrypting is done with an RSA public key, and these should not be corresponding keys. I.e. there are two RSA keypairs involved.

In addition to the single InstAccess token provider and many InstAccess token consumers, there is a single InstAccess token decrypter. This is an API Gateway tier that sits between the public internet, which always deals with encrypted InstAccess tokens, and the private network of Instructure services, which are only able to deal with decrypted InstAccess tokens.

The InstAccess::Token class can generate encrypted InstAccess tokens when it's configured with both a private signing key and a public encryption key. It can also validate and deserialize decrypted InstAccess tokens with this configuration. The single InstAccess provider should be configured this way.

All InstAccess token consumers should configure the class with just the public signing key, which will allow them only to validate and deserialize decrypted tokens.

The API Gateway needs to know the private encryption key and the public signing key so that it can decrypt tokens and validate them before passing them to InstAccess token consumers.

Configuration Cheat Sheet

Given two keypairs in plaintext:

private_signing_key = "-----BEGIN RSA PRIVATE KEY-----\n123abc..."
public_signing_key = "-----BEGIN PUBLIC KEY-----\n456def..."
private_encryption_key = "-----BEGIN RSA PRIVATE KEY-----\n321qwe..."
public_encryption_key = "-----BEGIN PUBLIC KEY-----\n987asd..."

The InstAccess token provider should be configured like:

InstAccess.configure(
  signing_key: private_signing_key,
  encryption_key: public_encryption_key
)

InstAccess token consumers should be configured like:

InstAccess.configure(
  signing_key: public_signing_key
)

And the private_encryption_key is only known to the API Gateway.

How do I decrypt tokens?

Unless you're the API Gateway, you shouldn't be asking!

As noted earlier, clients only get to handle encrypted tokens, while services only accept decrypted ones. Esablishing this asymmetry along with a single decrypter in between -- the API Gateway -- gives us a few benefits.

  1. It forces clients to go through the API Gateway whenever they use an InstAccess token, even if they only need to interact with one underlying service. This reduces the amount of responsibility individual services have, allowing them to rely on the gateway for things like throttling and token verification.

  2. It reduces the surface area for an attack on the private decryption key. If every service needed to be able to accept encrypted keys, then they'd each need access to the private decryption key to do so.

Since this gem isn't intended to be used by the API Gateway, it provides no functionality for decrypting tokens. But there's no special sauce here -- the tokens are encrypted according to the open JWE standard, for which there exist many libraries that make decryption easy.