# How OAuth Works

## Definitions

OAuth uses a series of specially-crafted HTTP views and redirects to allow websites to share information with each other securely, and with the user’s consent [1]. There are four roles in an OAuth interaction:

provider

  A website that has information about a user. Well-known OAuth providers include Google, Facebook, GitHub etc.

consumer

  A website that wants to obtain some information about a user from the provider.

user

  An actual person who controls information stored with the provider.

client

  A program (usually a web browser) that interacts with the provider and consumer on behalf of the user.

In order to securely interact with each other, the provider and consumer must exchange secrets ahead of time, before any OAuth communication actually happens. Generally, this happens when someone who runs the consumer website goes to the provider website and registers an application with the provider, putting in information about the name and URL of the consumer website. The provider then gives the consumer a “client secret”, which is a random string of letters and numbers. By presenting this client secret in future OAuth communication, the provider website can verify that the consumer is who they say they are, and not some other website trying to intercept the communication.

Note

Even though it is called a “client secret”, the secret represents the consumer website, not the client (the user’s web browser).

After the consumer has registered an application with the provider and gotten a client secret, the consumer can do the “OAuth dance” to get consent from a user to share information with the consumer. There are two different versions of the dance: OAuth 1, which is the original version; and OAuth 2, the successor to OAuth 1 which is more flexible and more widely used today.

## OAuth 2

1. The client visits the consumer at a special URL, indicating that they want to connect to the provider with OAuth. Typically, there is a button on the consumer’s website labeled “Log In with Google” or similar, which takes the user to this special URL.

2. The consumer decides how much of the user’s data they want to access, using specific keywords called “scopes”. The consumer also makes up a random string of letters and numbers, called a “state” token. The consumer crafts a special URL that points to the provider, but has the client secret, the scopes, the state token embedded in it. The consumer asks the client to visit the provider using this special URL.

3. When the client visits the provider at that URL, the provider notices the client secret, and looks up the consumer that it belongs to. The provider also notices the scopes that the consumer is requesting. The provider displays a page informing the user what information the consumer wants access to – it may be all of the user’s information, or just some of the user’s information. The user gets to decide if this is OK or not. If the user decides that this is not OK, the dance is over.

4. If the user grants consent, the provider makes up a new secret, called the “authorization code”. The provider crafts a special URL that points to the consumer, but has the authorization code and the state token embedded in it. The provider asks the client to visit the consumer using this special URL.

5. When the client visits the consumer at that URL, the consumer first checks the state token to be sure that it hasn’t changed, just to verify that no one has tampered with the request. Then, the consumer makes a separate request to the provider, passing along the client secret and the authorization code. If everything looks good to the provider, the provider makes up one final secret, called the “access token”, and sends it back to the consumer. This completes the dance.

## OAuth 1

1. The client visits the consumer at a special URL, indicating that they want to connect to the provider with OAuth. Typically, there is a button on the consumer’s website labeled “Log In with Google” or similar, which takes the user to this special URL.

2. The consumer tells the provider that they’re about to do the OAuth dance. The consumer gives the provider the client secret, to verify that everything’s cool. The provider checks the OAuth secret, and if it looks good, the provider makes up a new secret called a “request token”, and gives it to the consumer.

3. The consumer crafts a special URL that points to the provider, but has the client secret and request token embedded in it. The consumer asks the client to visit the provider using this special URL.

4. When the client visits the provider at that URL, the provider notices the request token, and looks up the consumer that it belongs to. The provider tells the user that this consumer wants to access some or all of the user’s information. The user gets to decide if this is OK or not. If the user decides that this is not OK, the dance is over.

5. If the user grants consent, the provider makes up another new secret, called the “authorization code”. The provider crafts a special URL that points to the consumer, but has the authorization code embedded in it. The provider asks the client to go visit the consumer at that special URL.

6. When the client visits the consumer at that URL, the consumer notices the authorization code. The consumer makes another request to the provider, passing along the client secret and the authorization code. If everything looks good to the provider, the provider makes up one final secret, called the “access token”, and sends it back to the consumer. This completes the dance.

## Dance Complete

Phew, that was complicated! But the end result is, the consumer has an access token, which proves that the user has given consent for the provider to give the consumer information about that user. Now, whenever the consumer needs information from the provider about the user, the consumer simply makes an API request to the provider and passes the access token along with the request. The provider sees the access token, looks up the user that granted consent, and determines whether the requested information falls within what the user authorized. If so, the provider returns that information to the consumer. In effect, the consumer is now the user’s client!

Warning

Keep your access tokens secure! Treat a user’s access token like you would treat their password.

Note

The OAuth dance normally only needs to be performed once per user. Once the consumer has an access token, that access token can be used to make many API requests on behalf of the user. Some OAuth implementations put a lifespan on the access token, after which it must be refreshed, but refreshing an access token does not require any interaction from the user.

[1]

Not all OAuth interactions share information about specific users. When no user-specific information is involved, then the consumer is able to get information from the provider without getting a user’s consent, since there is no one to get consent from. In practice, however, most OAuth interactions are about sharing information about users, so this documentation assumes that use-case.