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User authentication system

Your task is now to create a user authentication system.

This document will guide you through all the features and implication of such system, so that you don't have to search them yourself.

We will focus on web/browser-technologies, however similar concept can be widely applied. This guide, is a work in progress, feel free to comment and provide feedbacks.

Expected Workflows

The following are the actions that users expect to take in your platform, some are very simple, others are more complex and requires sophisticated implementations.

The workflows that any user expect in any web based application are:

  1. Signup
  2. Login
  3. Logout
  4. Recover password
  5. Change contact information
  6. Delete account

More complex applications may also implement:

  1. Two-Factor authentication
  2. Change Second Contact information

Common Authentication Systems

The common authentication systems on which the above workflow are implemented are:

  1. Email & Password
  2. Email
  3. Social Login (Login with: Facebook, Twitter, Instagram, LinkedIn, etc...)
  4. SSO (Single Sign On)

Not all the workflow methods make sense with all the authentication systems. As an example, in general if you use Social Login, you don't need to implement yourself the Recover Password workflow, since it would be done by the Social platform itself for you.

If you implement more than a single authentication system, you will need to harmonize them behind a single interface. Mostly this means having a way to unequivocally identify users that use different authentication systems, it is usually a bad idea to use the email address for this and you most likely want to associate an identifier to each user.

How to keep users logged in

Cookies! Cookies are used to keep user logged in your application. When a user login in your application, you send them a cookie that usually stores a session identifier. Each request that the user will now do to your webservice will now contain this cookie, so you can identify the session and with that the user. When the user logs out, you simply remove the cookie.

Email & Password

Email & Password is the most common way to implement an authentication system. In this section we will explore how to implement each workflow for this authentication system. We will always refer to emails, however this is not strictly true, whichever communication system can work, another common option is to use phone number.


To signup the user needs to provide an email address and a password, only this information are strictly necessary. Some platforms may decide to ask for more fields, like "username/nickname" or to type the password twice for confirmation. None of that is necessary, you can ask the user a username after, if they want to or you can just assign a random username and let the user change it inside the application. Also typing the password twice is not necessary, since anyway you will provide a password recovery workflow. For paid product, it might be wise to ask upfront for the credit card information even if the product adopt a freemium model, this will immediately weeds out the freeloaders that have no intention to ever pay for the product, of course it will result in less overall users.

On the frontend of your application a simple HTML form will capture the email and the password, and this information will be sent to your backend. The connection should be a secure (hence HTTPS) connection, otherwise the browser will inform the user that your application is not following the basic security guidelines. A simple form to capture this information will look like the following:

<form method="POST" action="/signup">
    <input type="email" name="email" required="true">
    <input type="password" name="password" required="true">

The method for the form must be POST otherwise the email and password will appears in the URL.

Once your application receives the data it must first verify that the email is not already registered, if it is, you should not give away this information, an attacker could be trying to understand if the target signed-up for a website. If the email is not already registered, now you can store those information. The email address can be stored in plain text, indeed you may want to send emails to your users. The password must not be stored in plain text, but you need to store its cryptographic secure hash, ideally with salt. Do not make the mistake of storing the password as plain text.

In order to store password securely the standard algorithm is:

  1. Generate a random cryptographic salt
  2. Hash the original, user provided, password with a fast hash function, usually something like HASH-512
  3. Concatenate the salt (from point 1) and the insecure hash of the password (from point 2)
  4. Hash the concatenation of the salt and password (from point 3) using a slow cryptographic secure function.
  5. Store the salt (from point 1) and the cryptographic secure hash (from point 4) in the database, one next to the other.

The cryptographic secure functions to use are, in order:

  1. Argon2
  2. PBKDF2
  3. Scrypt
  4. Bcrypt

For more details please visit the owasp cheatsheet.

Concerning security, the correct approach would be to first verify that the email is a valid email at which the user has access. So, immediately after you receive email and password, you mark the account as "not verified", store the password as mentioned above, and send a verification email with an activation link to the user. If the user visits the link, then you mark the account as verified and now, only now, you let the user login.

Business reasons will invite you to immediately log in the user if the email they provide is not already registered or to return an error if the email is already registered. This will provide valuable information to a possible attacker.

There is not a definitive answer here, and it depends on the nature of the business.


In order to login, you can provide a form very similar to the one used for signing up.

<form method="POST" action="/login">
    <input type="email" name="email" required="true">
    <input type="password" name="password" required="true">

Again, as all forms that require a password, this form should be served using a secure connection.

Once the backend receives the authentication data, it must verify that the information is correct. In your database you will query for the email and extract the salt and the hash of the password. Then you redo exactly the same operation you have done for generating the hash:

  1. Get the salt from the database
  2. Hash the original, user provided, password with a fast hash function, usually something like HASH-512
  3. Concatenate the salt (from point 1 extracted from the database) and the insecure hash of the password (from point 2)
  4. Hash the concatenation of the salt and password (from point 3) using a slow cryptographic secure function.
  5. Verify that the result (from point 4) is exactly the same that is stored in the database

If the two results match, then the user successfully authenticates in your application, if they don't match you should not give away any information to the user, again it could be an attacker.

Do not say things like "The email does not exist" or "We found the email, but the password does not match". Just say things like, "Email and Password don't match" even if the email is not in the database.

After the authentication is successfully completed, you can login the user. The simplest, reliable, and most robust approach to login a user is to simply send them a cookie. The cookie should be a Secure Cookie.

Cookies are just a key value storage, the key can be anything and a good key can be something like id or session or sessionId. The value should be a cryptographic secure random string. In your backend, you map the cookie value to the specific user, this can be done in another table of your database or also in some in-memory data structure like Redis.

The cookie should expires in a reasonable amount of time, not as short to force the user to login again every time, not too long that allows attacker to access it. Some website use Session Cookie that are reset as soon as the browser quits.

When you receive a request from a client you can immediately understand if the request is from an authenticated user, or from somebody else. This will allow you to create different response for each user and for not authenticated users.

In order to check if the user is authenticated you can simply check if the cookie you would set is present, and if the value of the cookie actually matches one of the saved one. If there is a match, the user is authenticated, if there is not a match the user is not authenticated.


As mentioned above, the user is logged in, if it sends -through cookies- a session id that you have previously recorded. In order to logout a user you can simply delete its session id from the list of logged in sessions, in this way, the next time the user sends a new request with an old cookie value, it won't match and you will know that the user logged out.

Another approach would be to also delete the cookie from the user’s browser, you can do this by setting the cookie value to the empty string. In this way, the next time the user sends you a request it will not contain the cookie used for authentication.

Recover/Change password

When the user asks to recover or change the password you simply send them (through email) a token that will allow them to reset and input a new password. Of course the token must be valid only for a limited amount of time and for a single use.

There are two main implementations at this stage.

  1. You could send a deep-link that will land the user on a form where it is prompted to insert the new password
  2. You can send a random few-digit token, that the user needs to insert into a form (along with the new password) in order to upgrade its credentials

Both approaches work fine and are similarly difficult to implement.

As always, it is important not to leak any information regarding the presence or absence of a set of credentials, if the user inputs an email that does not exist, never say something like "Email not found".
Instead say something like: "If we recognize the email we are sending the appropriate steps in order to change password".

Of course, the new password must be safely saved following the procedure mentioned above of hashing and salting.

Change contact information

If the user wishes to change its contact information, since the contact information is used to recover and change the password, you should verify that the user actually knows the password.

After the user was able to input the correct password, before actually changing the contact information, you must verify that the user has access to the new contact information. The procedure of the verification could be the same of the procedure used during the signup process.

In order to be safe, it is wise to send an email also to the old contact information, in this way, an intrusion can be detected promptly.

Delete account

Different philosophies apply when deleting accounts.

The most straightforward and honest approach would be to actually delete all the data connected to the account, along with email, hashed password and other application data. This is what is generally expected from users and what I believe should be implemented for not breaking the trust of the users.

The second, more subtle, approach is to mark the user as deleted while keeping all its information (this is usually known as shallow-delete). During Login, a user with an account marked as deleted would not be authenticated, just like if it provided the wrong credentials.

Shallow-deletes provide the benefits that, if a user changes its mind, he can always come back.

A reasonable middle ground is to shallow-delete users for a short amount of time (few days) and then completely delete their information.

For account deletion you should ask confirmation, especially if it means to loose important data, moreover I would suggest to ask the user to provide its password again.

Social Logins

Social logins allow you to identify people using another (generally big and reliable) service. Examples of services that provide this Social Login are Facebook, Twitter, Google, Github, Pinterest, etc... we will call these services Identity Providers.

The identity provider takes care of managing the direct contact with the users, so you don't have to manage emails, phone numbers, reset passwords, etc... And the user does not have to manage and remember, yet another set of credentials.

This advantage comes at some costs. First of all, it means that all your contact with the users are gatekeeped by the identity provider, generally they are benevolent, but it is always a risk to consider. Second, you may not have access to a direct contact for your user, you may not have access to the user's email address for example. Finally, some users are reluctant to login using social logins.

Another advantage is that Social Login is very personal, so different people will most likely not share the same account.

All the services that provide Social Login features use very similar implementation of OAuth (a protocol to share identities online), which means that what works for Twitter will need only minor modification to work for Facebook or Google. Moreover the underlying principles are very similar.

Finally, while you don't keep direct information of your user, you still need to maintain indirect information about the user, and to create and manage the cookies.

Let's now dive deep into what is necessary to create a User Management system that uses Social Logins.

General structure

The first step is to register an application with the Identity Provider. You will need to provide a callback url, which is the URL that users will visit after the Login in your application through the identity provider.

After you register your application, the identity provider will return a couple of strings, the ClientID and the ClientSecret, despite the name, both of them should be kept private.

At this point, you registered your application with the Identity Provide, your provide a callback URL and they provided you two strings.

Those strings should be saved somewhere, not in the code, since the code will be checked-in git (or another Version Control System). The usual approach is to have them available to your code using Environment Variables, but those credentials could be passed as inputs to your application, or could be read from a database.


Using Social Login the users, technically, don't signup to your service. They just Login. The process is quite complex, but its final goal is to let you have another set of credentials that are associated with the user. Hence, at the end of the process you will have two more strings, one that identifies the user (that we will call UserID) and a secret one (the UserSecret). The one that identifies the user can be shared (for example it can be used as value in the cookie) while the secret one should be kept secret.

The very first step of the process is to render the Social Login button in your application. In theory you could use whichever button, but usually it is a good idea to use the buttons provided by the Identity Provider. When the user clicks on the button, the Identity Provider receives a request, but it does not know that it is from you. To obviate, you send also your ClientID as GET parameter. Often, you can send also the scope parameter that indicates what kind of information from the user you will like to access, depending on your application this may change, but usually you always want at least the email.

When the user finally clicks on the Social Login button it gets redirected to a page from the Identity Provider, there it will Login in the Identity Provider service, and will analyze what information you are requesting from them. At this point the user can either authorize or dismiss your Login.

If the user authorizes the Login, the Identity Provider invokes the callback that we provided at the very beginning passing as parameter the UserID.

As a last step, you need to exchange the UserID with the UserSecret. Finally, with the UserSecret you can invoke the API at which you required access through the scope.

The UserSecret expires rather quickly, and you will need to continuously refresh it. Usually this can be done with the UserID.

Please note that while writing this article I researched implementations from different Identity Providers (Twitter, Github and Google) and none of them is equal to the other, nor they work exactly as I mentioned. However they follow the spirit of this flow.

To recap:

  1. The user wants to Login
  2. You redirect the user to the Identity Provider service, passing as parameter your identifier
  3. The user logins, and it gets redirected to your callback with some sort of tokens
  4. You exchange the token from the callback for a (set of) secret tokens
  5. You store the token and the secret token to make requests to the identity provider
  6. Eventually you refresh the secret token against the Identity Provider
  7. Store the session cookie in the client browser

Again, please note that each Identity Provider implements these steps a little bit differently.

Also remember that you most likely want to store a session cookie in the client browser, just like we did in the Email & Password workflow.


As we mentioned before there is not a real Signup, the user just logins. However, the first login is different from the others. As soon as the user logins, you want to store its information in your database. Most likely you will require some basic information to the Identity Provider (usually the email address) and other information that could be specific to your app and the integration with the Identity Provider. Also, it is important to store the credentials provided by the Identity Provider.

Some identity providers allows you to refresh the tokens for quite a bit of time (in the order of 2 or 3 months) but eventually the refreshing of the token fails, and the user will need to signup again. In such case you will need to identify the user only from the tokens provided by the Identity Provider.


To Logout you simply remove the cookie from the client browser, just like in the Email & Password case. Eventually the user will Login again, and in such case you need to retrieve its account only from the tokens provided by the Identity Provider.

Change Contact information

Eventually the user might decide to change its contact information. Maybe it stopped using Facebook but want to keep using your service, or maybe it used Twitter to Login only to try your service, but now he likes it and want to switch to using normal Email & Password. This case is analogous to the one in the Email & Password workflow, you verify the email and store the password in a secure way. It may also happen that users decide to link multiple social accounts to its profile, in general this should be allowed. Even further, a user that has linked multiple accounts to its profile should be able to Login with any of those accounts.

Delete account

To delete the account the same considerations of the Email & Password applies. The most straightforward and honest approach would be to actually delete all the data connected to the account, all the token that are associated with the user and all the data fetched from the Identity Provider.

Shallow delete is possible, but if the user removes the authorization to your application the Identity Provider will stop providing you any information.

The reasonable middle ground of shallow delete for a few days, and then completely deleting the information must be considered also in this case.


In this article we explore how authentication systems are implemented in modern web applications. User management seems a very simple thing to implement, but it is actually quite hard to get right and it definitely takes more time and efforts than a developer's initial estimate. If you decide to implement Social Logins, start immediately to create the application in the Identity Provider platform, there might be hidden surprises. Moreover, beware that the documentation is usually lacking and not so reliable. Also, consider that it is very complex to test reliably. If you are about to start a project that needs a user management system, feel free to contact me to work together:

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