Define a custom app permission

This document describes how app developers can use the security features provided by Android to define their own permissions. By defining custom permissions, an app can share its resources and capabilities with other apps. For more information about permissions, see the permissions overview.

Background

Android is a privilege-separated operating system, in which each app runs with a distinct system identity (Linux user ID and group ID). Parts of the system are also separated into distinct identities. Linux thereby isolates apps from each other and from the system.

Apps can expose their functionality to other apps by defining permissions that other apps can request. They can also define permissions that are automatically made available to any other apps that are signed with the same certificate.

App signing

All APKs must be signed with a certificate whose private key is held by their developer. The certificate does not need to be signed by a certificate authority. It's allowable, and typical, for Android apps to use self-signed certificates. The purpose of certificates in Android is to distinguish app authors. This lets the system grant or deny apps access to signature-level permissions and grant or deny an app's request to be given the same Linux identity as another app.

Grant signature permissions after device manufacturing time

Starting in Android 12 (API level 31), the knownCerts attribute for signature-level permissions lets you refer to the digests of known signing certificates at declaration time.

You can declare the knownCerts attribute and use the knownSigner flag in your app's protectionLevel attribute for a particular signature-level permission. Then, the system grants that permission to a requesting app if any signer in the requesting app's signing lineage, including the current signer, matches one of the digests that's declared with the permission in the knownCerts attribute.

The knownSigner flag lets devices and apps grant signature permissions to other apps without having to sign the apps at the time of device manufacturing and shipment.

User IDs and file access

At install time, Android gives each package a distinct Linux user ID. The identity remains constant for the duration of the package's life on that device. On a different device, the same package might have a different UID—what matters is that each package has a distinct UID on a given device.

Because security enforcement happens at the process level, the code of any two packages can't normally run in the same process, since they need to run as different Linux users.

Any data stored by an app is assigned that app's user ID and isn't normally accessible to other packages.

For more information about Android's security model, see Android Security Overview.

Define and enforce permissions

To enforce your own permissions, you must first declare them in your AndroidManifest.xml using one or more <permission> elements.

Naming convention

The system doesn't allow multiple packages to declare a permission with the same name unless all the packages are signed with the same certificate. If a package declares a permission, the system also doesn't permit the user to install other packages with the same permission name, unless those packages are signed with the same certificate as the first package.

We recommend prefixing permissions with an app's package name, using reverse-domain-style naming, followed by .permission. and then a description of the capability that the permission represents, in upper SNAKE_CASE. For example, com.example.myapp.permission.ENGAGE_HYPERSPACE.

Following this recommendation avoids naming collisions and helps clearly identify the owner and intention of a custom permission.

Example

For example, an app that needs to control which other apps can start one of its activities can declare a permission for this operation as follows:

<manifest
  xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.example.myapp" >
    
    <permission
      android:name="com.example.myapp.permission.DEADLY_ACTIVITY"
      android:label="@string/permlab_deadlyActivity"
      android:description="@string/permdesc_deadlyActivity"
      android:permissionGroup="android.permission-group.COST_MONEY"
      android:protectionLevel="dangerous" />
    ...
</manifest>

The protectionLevel attribute is required and tells the system how to inform users of apps requiring the permission or what apps can hold the permission, as described in the linked documentation.

The android:permissionGroup attribute is optional and only used to help the system display permissions to the user. In most cases, you set this to a standard system group (listed in android.Manifest.permission_group), although you can define a group yourself, as described in the following section. We recommend using an existing group, because this simplifies the permission UI shown to the user.

You need to supply both a label and description for the permission. These are string resources that the user can see when they are viewing a list of permissions (android:label) or details on a single permission (android:description). The label is short: a few words describing the key piece of functionality the permission is protecting. The description is a couple of sentences describing what the permission lets a holder do. Our convention is a two-sentence description where the first sentence describes the permission and the second sentence warns the user of the type of things that can go wrong if an app is granted the permission.

Here is an example of a label and description for the CALL_PHONE permission:

<string name="permlab_callPhone">directly call phone numbers</string>
<string name="permdesc_callPhone">Allows the app to call non-emergency
phone numbers without your intervention. Malicious apps may cause unexpected
calls on your phone bill.</string>

Create a permission group

As shown in the previous section, you can use the android:permissionGroup attribute to help the system describe permissions to the user. In most cases, you set this to a standard system group (listed in android.Manifest.permission_group), but you can also define your own group with <permission-group>.

The <permission-group> element defines a label for a set of permissions—both those declared in the manifest with <permission> elements and those declared elsewhere. This affects only how the permissions are grouped when presented to the user. The <permission-group> element doesn't specify the permissions that belong to the group, but it gives the group a name.

You can place a permission in the group by assigning the group name to the <permission> element's permissionGroup attribute.

The <permission-tree> element declares a namespace for a group of permissions that are defined in code.

Custom permission recommendations

You can define custom permissions for your apps and request custom permissions from other apps by defining <uses-permission> elements. However, carefully assess whether it is necessary to do so.

• If you are designing a suite of apps that expose functionality to one another, try to design the apps so that each permission is defined only once. You must do this if the apps aren't all signed with the same certificate. Even if the apps are all signed with the same certificate, it's a best practice to define each permission only once.
• If the functionality is only available to apps signed with the same signature as the providing app, you might be able to avoid defining custom permissions by using signature checks. When one of your apps makes a request of another of your apps, the second app can verify that both apps are signed with the same certificate before complying with the request.

If a custom permission is necessary, consider whether only applications signed by the same developer as the application performing the permission check need to access it—such as when implementing secure interprocess communications between two applications from the same developer. If so, we recommend using signature permissions. Signature permissions are transparent to the user and avoid user-confirmed permissions, which can be confusing to users.