Authorization code + PKCE grant type

With Hosted Login’s implementation of OpenID Connect (OIDC), the basic workflow for authenticating a user goes something like this:

  1. The  user (via an OpenID Connect client) makes an authentication request and is authenticated.
  2. The server sends the client an authorization code.
  3. 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:

AdleUo9ZVcn0J7HkXOdzeqN6pWrW36K3JgVRwMW8BBQazEPV3kFnHyWIZi2jt9gA

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:

E88B32EBB77AE452C6503E5C8D9987B2025A571A59E2E46032AB61FC8644C7A7

The client then base64-url encodes the hashed string to create a code challenge string. For example:

RTg4QjMyRUJCNzdBRTQ1MkM2NTAzRTVDOEQ5OTg3QjIwMjVBNTcxQTU5RTJFNDYwMzJBQjYxRkM4NjQ0QzdBNw

📘

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:

https://v1.api.us.janrain.com/00000000-0000-0000-0000-000000000000/login/authorize?  
  client_id=55c9604-x457-464f-bgf5-83hj229ju5rf  
  &redirect_uri=https://documentation.akamai.com  
  &scope=openid profile email  
  &response_type=code  
  &state=3bd5262737237ef4a  
  &code_challenge= RTg4QjMyRUJCNzdBRTQ1MkM2NTAzRTVDOEQ5OTg3QjIwMjVBNTcxQTU5RTJFNDYwMzJBQjYxRkM4NjQ0QzdBNw 
  &code_challenge_method=S256

The following sections summarize the parameters used in the request, and also details the optional parameters available for use with Hosted Login:


client_id (required)

Unique identifier of the OIDC login client used to make the authorization request. For example:

cliient_id=22c9604-7b27-464f-bff5-83ba229323af

response_type (required)

Specifies the type of response expected from the authorization server. Hosted Login supports the following responses types:

code. Returns an authorization code following a successful authentication. This code can then be exchanged at the token endpoint for an access token, a refresh token, and an identity token.

token. The authorization endpoint returns an access token following a successful authentication.

id_token. The authorization endpoint returns an identity token following a successful authentication.

Note that the response_type parameter is only used with the authorization_code and the implicit grant types. If you intend to use the client_credentials or the refresh_token grant, you need to omit the response_type parameter and use the grant_type parameter instead. See Supported authorization grant types for more information.


scope (required)

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 Scopes and claims. You can request multiple scopes by separating each scope using a blank space:

scope=openid email profile

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.

Using the scopes Parameter


redirect_uri (required)

Specifies the URL of the page the user is redirected to following a successful authentication and login. For example:

redirect_uri=https://identitydocs.akamai.com/redirect

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.

Adding a Redirect URI


code_challenge (required)

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. For example:

code_challenge= RTg4QjMyRUJCNzdBRTQ1MkM2NTAzRTVDOEQ5OTg3QjIwMjVBNTcxQTU5RTJFNDYwMzJBQjYxRkM4NjQ0QzdBNw

code_challenge_method (required)

Hashing algorithm used to generate the value of the code_challenge parameter. For Hosted Login, this will always be S256:

code_challenge_method=S256

state (optional, but recommended)

Random string that helps guard against cross-site request forgery (CSRF). For example, suppose your authentication includes the following state parameter and parameter value:

state=GA-ISU_6CwFn0tQTFiYD_-Gvy39Nb6iTdugdGIzTUng

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:

https://identitydocs.akamai.com/redirect_uri
  ?code=AH6S2WG_XchALC-p
  &state=GA-ISU_6CwFn0tQTFiYD_-Gvy39Nb6iTdugdGIzTUng

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.

As noted, the state parameter is typically a random value generated by the client and then stored somewhere on that client: for example, in local storage or as a cookie. (Exactly what gets stored and where it gets stored is left to the individual developer.) If you choose to store the state value as a cookie, it's recommended that you sign that cookie. That signature provides assurance that the cookie value hasn't been tampered with.


prompt (optional)

Specifies which screen (if any) is displayed when a user makes an authorization request. Allowed values are:

  • none. When prompt is set to none, Hosted Login first checks to see if the client has a valid session. If a valid session is found the user doesn't need to authenticate; instead, he or she is automatically logged in using the existing session. If a valid session can't be found a "No authenticated session found" error is generated and the user is not given the option of logging in.

    If you set the prompt parameter to none, it's recommended that you write code that: 1) looks for the "No authenticated session found" error ; and, 2) displays the sign-in screen. (If you don't, a user without an existing session would never be able to log in.) You might consider creating a cookie indicating that the user has been denied access because they didin't have a valid session. If you do that then, the next time user accesses your site , you'll know to employ an authorization request where the prompt is set to login (i.e., a request where the sign-in screen is always displayed).

  • login. The sign-in screen is always displayed first, even if a valid session is found. This ensures that users log in each time they access the site.

  • create. The traditional registration screen (used for creating new account) is always displayed first. Note, however, that the Sign In link isn’t found on the traditional registration screen. That means that setting the prompt to create represents a dead-end for existing users: they don’t need to create account, but they can’t log on using their existing account.

If this parameter isn't included then Hosted Login first checks to see if the client has a valid session. If a valid session exists the user doesn't need to authenticate; instead, he or she is automatically logged in using the existing session. If a valid session can't be found then the sign-in screen is displayed and the user can log in.

For example:

prompt=login

Using the prompt Parameter


max_age (optional)

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. |


ui_locales (optional)

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:

ui_locales= fr-FR es-ES

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 thatfails, 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.

Using the ui_locales Parameter


nonce (optional)

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.

Using the nonce Parameter


login_hint (optional)

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:

https://v1.api.us.janrain.com/e0a70b4f-1eef-4856-bcdb-f050fee66aae/login/authorize?  
  &client_id=a123ef65-83dc-4094-a09a-76e1bec424e7  
  &redirect_uri=https://identitydocs.akamai.com/redirect_uri     
  &code_challenge=MJm7VEGLvMtD4Mi1SGUc2QPRPVKqyaoEbBTxYKC4UJk   
  &code_challenge_method=S256  <br/>&response\_type=code  
  &scope=openid  
  &state=5TK5-3LXryr8EIxn6kV4mgqEa3KqRA4-HwHJbyzlgU0   
  &login_hint=gmstemp@hotmail.com

When you submit your authorization request, the email address will be included on the sign-in screen:

Note that Hosted Login cannot determine the email address to be included in the authorization request. Instead, you will need to use an alternate approach to determine the email address (for example, getting the email address when the user logs on to the computer) and then take the steps needed to add that address to the authorization request.

Using the login_hint Parameter


claims (optional)

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 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:

claims={"userinfo":{"birthdate":null}}

Meanwhile, this syntax adds a custom claim named organization to the identity token:

claims={"id_token":{"organization":null}}

And this syntax makes the organization claim accessible from the userinfo endpoint and adds that same claim to the identity token:

claims={"userinfo":{"organization":null},<br/>"id_token":{"organization":null}}

Using the claims Parameter

Note that Hosted Login also supports the use of "push claims," which provide a way to standardize and centralize scopes and claims in your organization. See Push claims for more information.


display (optional)

Specifies where (and how) the sign-in screen is displayed. Allowed values are:

  • page (the default value)., When you submit your authorization request, you’ll be redirected to a separate page that contains the sign-in screen (and nothing but the sign-in screen). After you’ve successfully logged on you'll be redirected to the page specified in the redirectURIs property of your OIDC client.

  • popup. When you submit your authorization request you are not redirected to a standalone login page. Instead, the login page appears in a pop-up window, no redirection required. After you’ve successfully logged on then you’ll be redirected to the page specified in the redirectURIs property of your OIDC client.

For example:

display=page

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.


grant_type

Specifies the type of authorization grant you are requesting and, by extension, specifies what you would like to get back from the authorization server. Note that, while Hosted Login supports 4 different authorization grant types, the grant_type parameter is used only with the client_credentials grant and the refresh_token grant. If you're using the authorization_code or the implicit grant type, leave out the grant_type parameter and use the response_type parameter instead.


response_mode

When used with the authorization_code or the implict grant types, specifies the mechanism used to return tokens and codes to a client. Hosted Login supports the following response modes:

  • query. The server response is sent as query parameters in the redirect URI. Because query is the least-secure response mode, it can only be used to return authorization codes: you can't use this mode to return access tokens or identity tokens.

  • fragment. The server response is sent as a URI fragment. Although more-secure than the query response mode, URI fragments are visible to the user agent.

  • form_post. The most-secure response type. With form_post, parameters and parameter values are encoded as application/x-www-form-urlencoded body parameters, and are transmitted to the client by using the HTTP POST method.

Admittedly, mixing and matching response modes, response types, and grant types can be a little tricky, Because of that, we recommend that you take a look at Supported response modes before you include the response_mode parameter in a request.


resource

Enables you to specify that a token can only be used with a specific protected resource (or set of resources). See Secure audience injection and the resource parameter for details.


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:

https://v1.api.us.janrain.com/00000000-0000-0000-0000-000000000000/login/code
  ?state=security\_token%3bd5262737237ef4a %url%https://documentation.akamai.com
  &code=4/JR27W91a-ofgCe9ur2m6bTghy77

Note that the response includes the authorization code, but it does not include either the code challenge or the code challenge method. You’ll see why in just a moment.


Exchange 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:

curl -X POST \  
https://v1.api.us.janrain.com/00000000-0000-0000-0000-000000000000/login/token \  
  -H 'Content-Type: application/x-www-form-urlencoded' \  
  -d 'grant_type=authorization_code' \  
  -d 'client_id=55c9604-x457-464f-bgf5-83hj229ju5rf  
  -d 'code=tpKqJ7c_g2bOKBpl' \  
  -d 'code_verifier=AdleUo9ZVcn0J7HkXOdzeqN6pWrW36K3JgVRwMW8BBQazEPV3kFnHyWIZi2jt9gA'

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 its version of the code challenger. Let’s see if they match:

Bob                 1234ABCD  
Server              1234ABCD

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:

Bob                 1234ABCD  
Server              EF432KLO1

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. 

Period.

If the authorization code is accepted, the token exchange endpoint returns an API response similar to this:

{
   "access\_token": "03v-eeodppPrrHXXIx56pRLyDBaOldDxqEwI59MFCFGVuSkLRapzgmfwmEHyKWle",
   "refresh\_token": "uHs1rLqRSpSyBpRpfplTI44Oh3gdkjJAa8Gzs3C5uDulN2yOnxU9mg1L6CaUAqz5",
   "expires\_in": 3600,
   "token\_type": "Bearer",
   "scope": "address email openid phone profile",
   "id\_token": "eyJhbGciOiJSUzI1NiIsImtpZCI6ImE5NjRhNjE3YTc0YjZjZWNlMDM4NTdkYWExZThlMTQ0ZDExMTMyYTkiLCJ0e
XAiOiJKV1QifQ.eyJhdF9oYXNoIjoibklpWVRQaG9TaWs4Rmt0ZFl5cktZZyIsImF1ZCI6WyJhMjJjOTYwNC03YjI3LTQ2NGYtYmZm
NS04M2JhMjI5MzIzYWYiLCJodHRwczovL29wZW5pZGNvbm5lY3QubmV0L2NhbGxiYWNrIl0sImF1dGhfdGltZSI6MTU1MjU5
OTgyOCwiYXpwIjoiYTIyYzk2MDQtN2IyNy00NjRmLWJmZjUtODNiYTIyOTMyM2FmIiwiZXhwIjoxNTUyNjAzNDQyLCJnbG9iYW
xfc3ViIjoiY2FwdHVyZS12MTovL2NhcHR1cmUtYWxiLWJvcmRlci5tdWx0aS5kZXYub3IuamFucmFpbi5jb20veDNnbW5uamV5
enlycnQybm01ZHJmNW5rbjgvdXNlci8yZWRkMmYzMi0xZTQ5LTRiZjItYjE2NC03NjM3ODE3NjFiNTIiLCJpYXQiOjE1NTI1OTk4
NDIsImlzcyI6Imh0dHBzOi8vYXBpLm11bHRpLmRldi5vci5qYW5yYWluLmNvbS8wMDAwMDAwMC0wMDAwLTMwMDAtOD
AwMC0wMDAwMDAwMDAwMDAvbG9naW4iLCJzdWIiOiIyZWRkMmYzMi0xZTQ5LTRiZjItYjE2NC03NjM3ODE3NjFiNTIifQ.k
KPbex5j3ADyxZ\_t8B8wiWUoDB7o8tamMjswCxMQKaTEJBpJBiYVATMdLvnd5HpZ5Hj\_I0omt7Zq3svPFLvdy1xHC95KWyJu3
HK65ZP8Hc0tM3oLFjWhLYcRoJZVi5ButzP4RZr6QJgfUyKF3QT-GECFLXgOyRy1DP4j4Xev7F\_MJ\_nX4xdAutNsDvu6PGyI752
nS4cJ13kAbyD0puaoLwg1aAoMSa4wm1limPvv5HcnRAAZcyMQhaC13vHMnvCCRWzuHl94oNl2\_ZblEtDQv\_q\_GfCvhXLrd1
VH7azarkeOtCNrD1aTyQ9owXJDxYJrcs2UTaop9tyA7\_HgctWQ"
}

Here's what the different name-value pairs in that response represent:

PropertyDescription
access_tokenThe newly-issued access token.
refresh_tokenThe refresh token that accompanies the access token.
expires_inAmount 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.
token_typeAccess 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.
scopeThe OIDC scopes that the token has permission to retrieve. Scopes represent different sets of user profile attributes; for example, the profilescope enables you to return such things as the user’s name, his or her gender, his or her birthdate, etc.
id_tokenThe user’s identity token]

If you’re curious about the actual contents of a token, see the article 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 key (JWT) decoders in order to view the contents of an identity token.


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