With Hosted Login’s implementation of OpenID Connect (OIDC), the basic workflow for authenticating a user goes something like this:
- The user (via an OpenID Connect client) makes an authentication request and is authenticated.
- The server sends the client an authorization code.
- The client exchanges the authorization code for an access token.
As a general rule, this is a very secure process, especially when carried out by web applications running over a TLS network connection. However, there is at least one potential problem here, a problem that can be exacerbated if users are connecting by using mobile devices. Let’s take a moment to explain what that problem is, and then we'll detail how Hosted Login deals with that problem.
By default, a client can exchange the authorization code (and get back an access token) without having to prove that they are the rightful owner of that authorization code. For example, suppose Bob successfully logs on to a website and, as a result, the server sends him an authorization code. That’s good: that’s the way things are supposed to work.
However, suppose Toni somehow manages to hijack that authorization code and present it to the token endpoint (something known as an Authorization Code Interception Attack). The token endpoint won’t question this request: as long as the authorization code is valid the server exchanges that code for an access token. That means that Toni now has access to all the resources that Bob has access to. And she has that access for as long as the token remains valid (typically one hour).
Fortunately there’s a way to to help avoid code interceptions attacks: the Proof Key for Code Exchange (PKCE, pronounced “pixie”) extension. This extension enables clients to assure the token exchange server that the authorization code they want to exchange really does belong to them. Best of all, the client can do that without having to exchange a client secret with the server. That's the value of using PKCE, and that's why Akamai recommends that you use PKCE, a public clients (OIDC clients that don't even have client secrets) for user authentication and authorization.
But what exactly does PKCE do and how does it differ from the standard OIDC authorization flow? Here’s a brief explanation of how the process works:
PKCE Setup and Configuration
Before a PKCE client makes an authentication request it creates a code verifier, a random string of 43 to 128 characters. For example:
After the client creates the code verifier, if takes that value and “hashes” it using the S256 hashing function. That turns the code verifier into a value similar to this:
The client then base64-url encodes the hashed string to create a code challenge string. For example:
Note. Hosted login requires the use of S256 to hash code verifiers. Although the PKCE standard allows for the use of plain-text code challenges, plain text is not supported by hosted login. You can verify this by looking at the discovery document to see which code challenge methods are supported.
As soon as you have the code verifier and code challenge string you're right to make an authorization request.
The Initial Authorization Request
To make an authorization request using PKCE, two pieces of information must be included in that request: the code challenge string and the hashing method (S256) used to generate that string. This information is specified in the request as the code_challenge and code_challenge_method parameters, respectively. For example:
The following table summarizes the parameters used in the request, and also details the optional parameters available for use with Hosted Login:
Unique identifier of the OIDC login client used to make the authorization request. For example:
Specifies the type of response expected from the authorization server; at this point in time the Identity Cloud only supports the code response type. Note that this parameter is required even though there’s only one supported response type:
The code response indicates that the client expects to get an authorization code back following a successful authentication. In turn, the client exchanges that code for a set of tokens.
Specifies the OpenID Connect scopes to be accessible from the userinfo endpoint following a successful authentication and login. Note that you must include the scope parameter and, at a minimum, request the openid scope; this tells the authorization server that you want to authenticate by using OpenID Connect.
Other scopes supported by the Identity Cloud are detailed in the article OpenID Connect Scopes and Claims. You can request multiple scopes by separating each scope using a blank space:
You can include any (or all) the supported scopes in your authentication request. However, that doesn’t mean that you’ll get back all of those scopes. Instead, the scopes made accessible from the userinfo endpoint depend on the value of the allowedScopes property found in the token policy applied during a user login.
For example, suppose the allowedScopes property only specifies the openid and email scopes. In that case you can only get back those two scopes; any other scopes mentioned in your authorization request (such as profile or address) are ignored and are not returned.
Specifies the URL of the page the user is redirected to following a successful authentication and login. For example:
Note that the specified URL must be exactly match one of the URLs listed in the OIDC login client’s redirectURIs property. If the URL isn’t included in the redirectURIs property then the authorization request fails with an Invalid client error and the user will not be authenticated.
Hashed and encoded value generated by the client. This value should be verified before the client is allowed to exchange an authorization code for a set of tokens.
Hashing algorithm used to generate the value of the code_challenge parameter. For Hosted Login, this will always be S256:
No (but reccommended)
A random string that helps guard against cross-site request forgery (CSRF). For example, suppose your authentication includes the following state parameter and parameter value:
After a successful authentication, you’ll be redirected to the URL specified by the redirect_uri parameter. If you were redirected by the authorization server then the state parameter and value will be included in the URI:
If the state parameter in the redirect URI doesn’t match your original parameter value then you might be the victim of CSFR attack (defined as an attack in which malware tries to trick you into carrying out some sort of action you never intended to carry out). In that case, you should restart the authentication process.
Specifies which screen (if any) is displayed when a user makes an authorization request. Allowed values are:
Specifies the amount of time, in seconds, that can elapse before a user is required to reauthenticate. For example, suppose the max_age parameter is set to 3600 seconds (one hour). A user logs on, leaves the website, then returns 30 minutes later. Because the max_age limit of 1 hour has not been reached, the user will automatically be authenticated and resume their previous session.
Now, suppose a second user logs on, leaves the website, then comes back 2 hours later. because the max_age value has been exceeded, this user will be forced to reauthenticate.
Note that the max_age parameter applies only to logins. Suppose a third user logs on and stays on the site for 2 hours. That user will not be forced to reauthenticate halfway through their session. As noted, max_age only applies to logins.
Specifies the language/locale used when displaying Hosted Login login, registration, and user profile screens. Language preferences are passed as a space-delimited set of RFC 5646 language codes. For example:
In the preceding example, Hosted Login first tries to render screens by using French (fr-FR); if that fails, Hosted Login tries to render the screens by using Spanish (es-ES). If that fails, then Hosted Login defaults to displaying all screens in English.
Why would an attempt to render screens fail? This is almost always because you specified a language/locale that can’t be found in your flow: you can specify any language or locale that you want, but to actually display screens using that language/locale requires you to have the locale (and the accompanying translations) in your flow. See this article for more information.
Yes (with PKCE)
Hashed and encoded value generated by the client. This value will need to be verified before the client will be allowed to exchange an authorization code for a set of tokens.
Yes (with PKCE)
Hashing algorithm used to generate code challenge:
No (but recommended)
Helps ensure that the identity token you receive is the same identity token that you requested (in other words, you got back a token sent in direct response to your authentication request).
To use the nonce parameter, simply enter a random string in the Nonce field and then make your authentication request, When you decode the returned identity token, you should see a nonce property. The value in the identity token should be the same as the value included in your authentication request.
Provides a way to prepopulate the email address field on the Hosted Login sign-in screen. In your authorization request, include the login_hint parameter followed by the email address of the user who needs to be authenticated. For example:
When you submit your authorization request, the email address will be included on the sign-in screen:
Specifies the claims (i.e., user profile attributes) to be included in the identity token or to be made accessible from the userinfo endpoint (or both). These claims can either be standard OpenID Connect claims (see OpenID Connect Scopes and Claims for more information) or custom claims created by your organization and defined in your login policies.
For example, this syntax makes the birthdate claim accessible from the userinfo endpoint:
Meanwhile, this syntax adds a custom claim named organization to the identity token:
And this syntax makes the organization claim accessible from the userinfo endpoint and adds that same claim to the identity token:
Specifies where (and how) the sign-in screen is displayed. Allowed values are:
As noted, for a Hosted Login end user, the preceding activities are carried out by clicking a Login button that takes them to the login page. Once there, the user is asked to log on to their existing account, either by logging on to a social login identity provider (social login) or by supplying a username and password (traditional login):
After supplying their email address and password (in the case of a traditional login) the user clicks Sign In and authentication takes place. To the end user, nothing has changed: they still log on to your website the way they log on to most websites.
Meanwhile, the authorization server uses the supplied credentials (or the social login token received from the social identity provider) and attempts to log the user on.
The Redirect URI and Authorization Code
When the authorization server receives the PKCE request, the server saves a copy of the code challenge and the code challenge method before authenticating the user. If authentication is successful, the server returns the standard authorization response:
Note that the response includes the authorization code (highlighted in red), but it does notinclude either the code challenge or the code challenge method. You’ll see why in just a moment.
Exchanging the Authorization Code for an Access Token
Let’s assume that Bob made the authorization request and that, after successfully logging on, he received his authorization code. It’s now time for the Open ID Connect client to exchange that code for an access token. When he presents the code to the token exchange server, he must also present the code verifier (the original string value AdleUo9ZVcn0J7HkXOdzeqN6pWrW36K3JgVRwMW8BBQazEPV3kFnHyWIZi2jt9gA).
For example, a curl command for exchanging an authorization code for a set of tokens might look similar to this:
Two things to note here. First, no authentication is required. That's because this is a PKCE flow. Instead of configuring, say, Basic authentication, just be sure and include the code_verifier parameter. Second, remember that code_verifier, and all the other parameters, must be passed as an xxx-www-urlencoded body parameter.
As you no doubt recall, when Bob made his initial authorization request the server took note of the code challenge and the hashing method associated with that request. Because of that, the server can now take the code verifier, hash the verifier using SHA 256, then base64url-encode the hashed string. If the value derived by the server matches the code challenge included in the original request, then the exchange will be approved and Bob will be sent his tokens. Why? That’s right: if Bob’s original code challenge and the code challenge calculated by the server match, the authorization server can be confident that it is communicating with the correct client.
To use a simple (and, admittedly, unrealistic) example, suppose Bob’s original code challenge was 1234ABCD. Bob submits his token exchange request, and the server calculates it’s version of the code challenger. Let’s see if they match:
Looks like we have a winner!
But suppose that, somewhere along the way, Toni intercepted Bob’s authorization code in the hopes of also snagging Bob’s access token. That’s going to be tough: after all, the authorization response does not include the code verifier, the code challenge, or the code challenge method. Toni can try including a code verifier but if it’s not the right code verifier (and the right algorithm) then the server won’t be able to recreate the code challenge. For example:
Those two values don’t match. And that’s because, even though Toni was able to hijack the authorization code, she does not have possession of the code verifier. In turn, that means that server will not honor her exchange request.
If the authorization code is accepted, the token exchange endpoint returns an API response similar to this:
Here's what the different name-value pairs in that response represent:
The newly-issued access token.
The refresh token that accompanies the access token.
Amount of time (in seconds) before the access token expires. In this case, that's 1 hour (60 seconds x 60 minutes = 3,600 seconds).
Incidentally, identity tokens also expire after 1 hour (although that doesn’t matter too much because identity tokens are rarely used after they have been issued). Refresh tokens have a default lifespan of 90 days.
Access token type. The token type will always be set to bearer, meaning that whoever has possession of the token is considered the rightful owner of that token. To gain access to resources, you only have to present the access token: you do not have to do anything to “prove” that the token belongs to you.
The OIDC scopes that the token has permission to retrieve. Scopes represent different sets of user profile attributes; for example, the profile scope enables you to return such things as the user’s name, his or her gender, his or her birthdate, etc.
The user’s identity token.
If you’re curious about the actual contents of a token, see the article Hosted Login Token Reference. In addition to that, you can decode an access token or a refresh token by using the introspection endpoint, and you can use any of a number of different JSON Web Token (JWT) decoders in order to view the contents of an identity token.