Auth from backend perspective pt1: basics

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Category : Auth from backend perspective

Why you may want to read this article

To be honest, I planned to write this set of articles about a year ago, because for a long time the authentication & authorization process was for me not so clear. I didn’t find any book or article which in simple words can show the whole picture of that process, especially digging into details.

So in that article, we are covering:

  • What is auth
  • What types of auth could be
  • What is the general flow of auth
  • In the latter parts we will implement and consider Basic, Digest, JWT-based, OAuth + OpenId Connect authorizations

This article should be useful for people who managed to get the RFC but didn’t get the completely OAuth flow. I am here to help you because I was in the same situation.

Note: I am using official info like RFC or the documentation provided by specific tools, so you can check everything by yourself if you want to.

Git repository with samples


Prior to starting to talk about different approaches to authentication and authorization, we should consider the definitions.

Authorization & Authentication

From Wikipedia: “Authorization is the function of specifying access rights/privileges to resources, which is related to general information security and computer security, and to access control in particular”.

From MSDN: “Authorization is the process of determining whether a user has access to a resource. Authentication is the process of determining a user’s identity”.

From Martin Fowler’s blog: “Authorization defines whether a user is allowed to do something. Authentication confirms that the users are who they claim to be”.

You could pick up whatever definition you like, they are basically the same.

I unite Authorization & Authentication to “auth” because most usually (and in examples, we’ll see that) - the authorization is impossible without authentication. If you don’t know who the User is - you cannot say what the User is allowed to do via Client.

API or Resource Server

In terms of OAuth it is named “Resource Server”, but for simplicity let me put it in the following way: it is the Server (just a piece of software running on some machine) that handles requests from the Client.


The Client is basically a piece of software that talks to the API and needs to authorize requests (when the endpoints are not public). Most usually this software runs on some device (mobile, desktop, and browser). The User is interacting with the API using the Client.


In terms of OAuth the resource owner - is most usually a person who would like to use the Client and the application itself.

Authorization Server

This Server is fully responsible for the identity of the user, authentication, and creation of tokens. This enables you just to use some verification based on signatures (on Resource Server) and not worry about implementation details of authentication.

We will use this term in the next part for OAuth and OpenId Connect. For this part, it is not possible to decouple the Authorization Server from Resource Server. So I will refer to API, Resource Server, Authorization Server, or just Server as to the same thing in this part.


The basic flow looks like that:

1.The User wants to interact with the application somehow, this makes the Client send and receive data from API back and forth. Since the API has protected endpoints - it requires the Client to authorize its requests and responds with 401 Not Authorized.

2.The Client then should first authenticate the user - make the request to the Server so that it confirms the User is who he claims to be. It may be the same Server where the API is serving or a standalone Server.

Most usually authentication is performed by sending the login and password (some identity) of the User to the Server that responds with a ticket (sequence of characters, typically signed or encrypted, the API can validate).

This ticket can be used to access the API. Most usually this ticket is either a token or cookies. Meanwhile, for sure, the Server should have information about this user (to be able to say this login + password does exist and what he has access to).

3.The Client performs a request to the API including the authorization ticket (either token or cookies) - and then it can get the response from it.

The most common approach for implementing auth is to use auth tokens in headers. For sure there are other ways like sessions. But the main advantage of tokens is that they are stateless: you are not required to make other requests prior to the current one to be authorized. You only need to use the appropriate token.

The token is kept via request headers in the following format: Authorization: {auth scheme} {token value}.

Basically, the scheme means the way you can create or get your token and validate it, so let’s consider the most popular auth schemes.

Auth schemes

Although we merged Authentication and Authorization terms together because most usually Auth schemes do not have a clear separation some of them do have. Let me provide examples.

Basic Authentication

Covered in this article

The Basic Authentication Scheme should be considered mostly like an authentication scheme (not authorization). Whenever the Client sends a request to a protected resource it should include the user login and password in the header. Otherwise, it will receive 401.

This scheme should use Resource Server and Authorization Server as the same thing. So each request from the Client goes to Resource Server and it authenticates and returns a response to the Client back.

Basic Authentication has a resource-splitting mechanism called a realm, it is something similar called scope in terms of OAuth. But from the Client, you cannot specify it.

Digest Authentication

Covered in this article

The Digest Authentication Scheme acts both as an authentication scheme and some kind of authorization protocol as well. The idea here is to use hash of user credentials + server secret for authenticating.

This scheme should use Resource Server and Authorization Server as the same thing.

Digest Authentication has a realm as well that is in fact similar to scope in terms of OAuth. It could be specified from the Client side, so it has a better mechanism of scopes compared to the Basic.


Covered in this article

OAuth is an Authorization protocol only, so it does not cover user authentication (how login and password are sent to the server and checked on its side).

However important detail is that it cannot be without authentication still, you should have it implemented to use OAuth, but it is not covered in what way exactly.

OAuth has 4 main flows: Authorization Code, Implicit, Resource Owner Password Credentials, Client Credentials. They varying depending on your use case. You might need server-to-server authorization, client-server authorization, with use of browser or without, more secured, less secured.

This protocol has a clear separation of the Resource Server and Authorization Server. Though, you could implement it in a way that those sit in the same physical server and even port (application)

OAuth operates on cookies for authentication (usually) and access tokens for calling Resource Server.

OpenId Connect

OpenId Connect is an Authentication protocol only. It does not cover authorization.

OpenId Connect uses OAuth. To put it simply, OpenId Connect provides Id Token along with Access Token, and this Id Token is used for user identity providing information about the User.

Since it is built on top of OAuth (that does not cover the flow of authentication itself), OpenId Connect also does not provide authentication flow, but it allows you to work with user identity and verify the user is who he claims to be.

Custom Schemes based on JWT

The flow is typically the following: the Client authenticates the User with login + password and the Authorization Server returns Access Token as the response of Authentication. With that token, the Client can go and make subsequent requests to Resource Server.

This allows to have UI-less authentication, or in other words to make authentication UI implemented on the client, and do authentication and authorization as a Backend concept only.

Custom schemes resemble the simplified Password Grant type from OAuth protocol, but they do not include unnecessary details of OAuth making it less secure and reliable, but stay easier to implement.

Those act with JWT (access tokens) and maybe refresh tokens as OAuth does as well.

Follow up

There are resources where you can find all commonly used schemes and RFC documentation for each of them.

In the next articles, we will consider all auth schemes we have discussed above.

About Andrii Bui

Hi, my name is Andrii. I'm Software Engineer at Microsoft with 5 years of experience.