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|Tutorial Series||KAuth Tutorial|
|What's Next||Using the caller-helper model to perform actions as root|
What is KAuth
KAuth (part of kdelibs/kdecore) is an authentication framework for KDE. Just like any other components in the KDE Development Platform, it is a wrapper around lower-level tools. If you're planning to use KAuth, however, you won't have to care about what authentication system is the system you're targeting using: KAuth will take care of that on its own.
In addition, KAuth is also able to perform privilege elevation on restricted portions of code (helpers), giving the developer an efficient and easy to use pipe to communicate with them, and making them secure throughout authorization.
There are a few concepts to understand when using KAuth. Much of those are carried from underlying APIs such as polkit, so if you are familiar with one of them you might as well skip this section.
- An action is a single task that needs authorization to be performed. Each action has an unique action identifier, which is a string in reverse domain name syntax, like org.kde.this.is.an.action. For example, if the date/time control center module needs to change the date, it would need an action like "org.kde.datatime.change". Please note that each action has to refer to a single task: this allows system administrators to fine tune the policies that allow users to perform your actions, and also a more secure way of locking down the privileged actions in your application.
- Authorization is a particular phase where the underlying authorization framework performs the needed checks (and eventually asks the user its credentials in order to authorize him). Before any action is executed, the Authorization phase takes place. This is handled internally by KAuth: even if you are able to trigger this phase manually, most of the times you don't need to: KAuth will still execute an action only if the underlying authorization framework allows its execution.
- Authentication is an optional phase that takes place during authorization, if the policy for the action requests the user to input a credential to give him an explicit authorization. This phase is external and not handled by KAuth, but entirely by the underlying authorization framework. It is, however, important for you to know something about it even if KAuth has no way to hijack the Authentication phase by design.
Execution happens only if the Authorization was successful: the execution might consist in a simple confirmation of the successful authorization, or eventually in the execution of a function in an helper.
- An Helper
Supposing that you want to use KAuth to perform a privileged operation and the action you are considering requires the user to authenticate (which is the most common use case of KAuth), the break down of phases would be:
- The user want to perform some privileged task
- The application creates an action for the task in question.
- The action is requested to be executed.
- The following steps are handed internally, either by KAuth or the underlying authorization system.
- Authorization phase begins
- The system detects that the user needs to authenticate to authorize the action.
- Authentication phase begins
The user is requested to input his password, swipe his finger, press a button...
- Authentication phase ends
- Otherwise, the system grants an explicit authorization to the user
- Authorization phase ends
If authorization was unsuccessful, the action is rejected.
- Otherwise, the action is executed
- Execution phase begins
- A separate application is spawned by root, and the requested portion of code is executed.
- The helper code, immediately after starting, checks the authorization again to improve security, and also because some authorization systems delay the authorization phase in the beginning of the execution phase. If the helper is not authorized, the execution is aborted.
- If the caller is authorized, the helper executes the task
- Execution phase ends
- The application receives the result of the execution from the helper
This is how, concept-proof, KAuth works. However, please note that in your implementation you will have to deal with the pre-authorization phase only, since everything else is handled internally.