Home

Awesome

OpenPGP API library

The OpenPGP API provides methods to execute OpenPGP operations, such as sign, encrypt, decrypt, verify, and more without user interaction from background threads. This is done by connecting your client application to a remote service provided by OpenKeychain or other OpenPGP providers.

News

Version 12

Full changelog here…

License

While OpenKeychain itself is GPLv3+, the API library is licensed under Apache License v2. Thus, you are allowed to also use it in closed source applications as long as you respect the Apache License v2.

Add the API library to your project

Add this to your build.gradle:

repositories {
    maven { url 'https://jitpack.io' }
}

dependencies {
    implementation 'com.github.open-keychain.open-keychain:openpgp-api:v5.7.1'
}

Full example

A full working example is available in the example project. The OpenPgpApiActivity.java contains most relevant sourcecode.

API

OpenPgpApi contains all possible Intents and available extras.

Short tutorial

This tutorial only covers the basics, please consult the full example for a complete overview over all methods

The API is not designed around Intents which are started via startActivityForResult. These Intent actions typically start an activity for user interaction, so they are not suitable for background tasks. Most API design decisions are explained at the bottom of this wiki page.

We will go through the basic steps to understand how this API works, following this (greatly simplified) sequence diagram:

In this diagram the client app is depicted on the left side, the OpenPGP provider (in this case OpenKeychain) is depicted on the right. The remote service is defined via the AIDL file IOpenPgpService. It contains only one exposed method which can be invoked remotely:

interface IOpenPgpService {
    Intent execute(in Intent data, in ParcelFileDescriptor input, in ParcelFileDescriptor output);
}

The interaction between the apps is done by binding from your client app to the remote service of OpenKeychain. OpenPgpServiceConnection is a helper class from the library to ease this step:

OpenPgpServiceConnection mServiceConnection;

public void onCreate(Bundle savedInstance) {
    [...]
    mServiceConnection = new OpenPgpServiceConnection(this, "org.sufficientlysecure.keychain");
    mServiceConnection.bindToService();
}

public void onDestroy() {
    [...]
    if (mServiceConnection != null) {
        mServiceConnection.unbindFromService();
    }
}

Following the sequence diagram, these steps are executed:

  1. Define an Intent containing the actual PGP instructions which should be done, e.g.

    Intent data = new Intent();
data.setAction(OpenPgpApi.ACTION_ENCRYPT);
data.putExtra(OpenPgpApi.EXTRA_USER_IDS, new String[]{"dominik@dominikschuermann.de"});
data.putExtra(OpenPgpApi.EXTRA_REQUEST_ASCII_ARMOR, true);

    Define an InputStream currently holding the plaintext, and an OutputStream where you want the ciphertext to be written by OpenKeychain's remote service:

    InputStream is = new ByteArrayInputStream("Hello world!".getBytes("UTF-8"));
ByteArrayOutputStream os = new ByteArrayOutputStream();

    Using a helper class from the library, is and os are passed via ParcelFileDescriptors as input and output together with Intent data, as depicted in the sequence diagram, from the client to the remote service. Programmatically, this can be done with:

    OpenPgpApi api = new OpenPgpApi(this, mServiceConnection.getService());
Intent result = api.executeApi(data, is, os);

  2. The PGP operation is executed by OpenKeychain and the produced ciphertext is written into os which can then be accessed by the client app.

  3. A result Intent is returned containing one of these result codes:

    • OpenPgpApi.RESULT_CODE_ERROR
    • OpenPgpApi.RESULT_CODE_SUCCESS
    • OpenPgpApi.RESULT_CODE_USER_INTERACTION_REQUIRED

    If RESULT_CODE_USER_INTERACTION_REQUIRED is returned, an additional PendingIntent is returned to the client, which must be used to get user input required to process the request. A PendingIntent is executed with startIntentSenderForResult, which starts an activity, originally belonging to OpenKeychain, on the task stack of the client. Only if RESULT_CODE_SUCCESS is returned, os actually contains data. A nearly complete example looks like this:

    switch (result.getIntExtra(OpenPgpApi.RESULT_CODE, OpenPgpApi.RESULT_CODE_ERROR)) {
    case OpenPgpApi.RESULT_CODE_SUCCESS: {
        try {
            Log.d(OpenPgpApi.TAG, "output: " + os.toString("UTF-8"));
        } catch (UnsupportedEncodingException e) {
            Log.e(Constants.TAG, "UnsupportedEncodingException", e);
        }

        if (result.hasExtra(OpenPgpApi.RESULT_SIGNATURE)) {
            OpenPgpSignatureResult sigResult
                    = result.getParcelableExtra(OpenPgpApi.RESULT_SIGNATURE);
            [...]
        }
        break;
    }
    case OpenPgpApi.RESULT_CODE_USER_INTERACTION_REQUIRED: {
        PendingIntent pi = result.getParcelableExtra(OpenPgpApi.RESULT_INTENT);
        try {
            startIntentSenderForResult(pi.getIntentSender(), 42, null, 0, 0, 0);
        } catch (IntentSender.SendIntentException e) {
            Log.e(Constants.TAG, "SendIntentException", e);
        }
        break;
    }
    case OpenPgpApi.RESULT_CODE_ERROR: {
        OpenPgpError error = result.getParcelableExtra(OpenPgpApi.RESULT_ERROR);
        [...]
        break;
    }
}

  4. Results from a PendingIntent are returned in onActivityResult of the activity, which executed startIntentSenderForResult. The returned Intent data in onActivityResult contains the original PGP operation definition and new values acquired from the user interaction. Thus, you can now execute the Intent again, like done in step 1. This time it should return with RESULT_CODE_SUCCESS because all required information has been obtained by the previous user interaction stored in this Intent.

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    [...]
    // try again after user interaction
    if (resultCode == RESULT_OK) {
        switch (requestCode) {
            case 42: {
                encrypt(data); // defined like in step 1
                break;
            }
        }
    }
}

Tipps