[api] REST API Authentication

Here's a guided approach.

Your authentication service issues a JWT token that is signed using a secret that is also available in your API service. The reason they need to be there too is that you will need to verify the tokens received to make sure you created them. The nice thing about JWTs is that their payload can hold claims as to what the user is authorised to access should different users have different access control levels.

That architecture renders authentication stateless: No need to store any tokens in a database unless you would like to handle token blacklisting (think banning users). Being stateless is crucial if you ever need to scale. That also frees up your API service from having to call the authentication server at all as the information they need for both authentication and authorisation are in the issued token.

Flow (no refresh tokens):

1. User authenticates with the authentication server (eg: POST /auth/login) and receives a JWT token generated and signed by the auth server.
2. User uses that token to talk to your API and assuming user is authorised), gets and posts the necessary resources.

There are a couple of issues here. Namely, that auth token in the wrong hands provides unlimited access to a malicious user to pretend they are the affected user and call your APIs indefinitely. To handle that, tokens have an expiry date and clients are forced to request new tokens whenever expiry happens. That expiry is part of the token's payload. But if tokens are short-lived, do we require users to authenticate with their usernames and password every time? No. We do not want to ask a user for their password every 30min to an hour, and we do not want to persist that password anywhere in the client. To get around that issue, we introduce the concept of refresh tokens. They are longer lived tokens that serve one purpose: act as a user's password, authenticate them to get a new token. Downside is that with this architecture your authentication server needs to persist these refresh token in a database.

New flow (with refresh tokens):

1. User authenticates with the authentication server (eg: POST /auth/login) and receives a JWT token generated and signed by the auth server, alongside a long lived (eg: 6 months) refresh token that they store securely
2. Whenever the user needs to make an API request, the token's expiry is checked. Assuming it has not yet expired, user uses that token to talk to your API and assuming user is authorised), gets and posts the necessary resources.
3. If the token has indeed expired, there is a need to refresh your token, user calls authentication server (EG: POST / auth/token) and passes the securely stored refresh token. Response is a new access token issued.
4. Use that new token to talk to your API image servers.

OPTIONAL (banning users)

How do we ban users? Using that model there is no easy way to do so. Enhancement: Every persisted refresh token includes a blacklisted field and only issue new tokens if the refresh token isn't black listed.

Things to consider:

• You may want to rotate refresh token. To do so, blacklist the refresh token each time your user needs a new access token. That way refresh tokens can only be used once. Downside you will end up with a lot more refresh tokens but that can easily be solved with a job that clears blacklisted refresh tokens (eg: once a day)
• You may want to consider setting a maximum number of allowed refresh tokens issued per user (say 10 or 20) as you issue a new one every time they login (with username and password). This number depends on your flow, how many clients a user may use (web, mobile, etc) and other factors.
• Logout endpoint in your authentication service may or may not blacklist refresh tokens. Something to think about.