Android payment apps developer guide

Rouslan is a contributor to WebFundamentals

Introduction

The Payment Request API brought to the web a native browser-based interface that allows users to enter required purchase information easier than ever before. The API can also invoke payment apps that provide various kinds of payment methods such as e-money, cryptocurrency, bank transfers, and more.

You may implement a native Android app or convert an existing Android app in order to process such payment methods. This document describes a basic method for any Android payment app to work with any Android web browser through Android intents.

Requirements

Browser permissions: A payment app must know where the money is going. Therefore, the payment app should be able to control which browsers are allowed to call it.
Website permissions: A payment app should be able to control which websites are allowed to invoke it. Therefore, a browser should pass the origin and certificate of the calling website to the payment app.
Payment app permissions: If a merchant specifies that they accept a certain payment app, then the merchant needs to know that an impersonator app cannot steal a user’s credentials. Therefore, a payment app should be able to control which apps can respond to its payment method identifier. For example, https://bobpay.com may allow only the Bob Pay native Android app to send payments, while http://www.alice.com/web-pay may allow payments from any payment app.
Loose integration: Web browsers should not be compiling against payment app SDKs, and payment apps should not be compiling against web browser SDKs.
Performance: The integration should be fast even in a worst case scenario, such as a device with 512MB of RAM and a cold start for both the browser and the payment app.
Install awareness: If a user installs or uninstalls a payment app, a web browser should be aware of this change. Even if a web browser is installed after a payment app, the browser should be able to use the pre-installed payment app for web payments.

Finding a valid payment app

When a merchant requests payment via https://bobpay.com method, the web browser queries the Package Manager for any app that can respond to https://bobpay.com intent. Checking locally-installed apps first reduces the number of server requests for payment-manifest.json file. If a compatible app is found, then the browser downloads the HEAD of https://bobpay.com, and then downloads the JSON manifest file pointed to by the HTTP header link with the rel="payment-method-manifest" attribute. The HTTP header might look like this:

Link: <payment-manifest.json>; rel="payment-method-manifest"

Next, the browser downloads the manifest, https://bobpay.com/payment-manifest.json, which contains pointers to the default applications of that payment method. The payment method manifest might include:

{"default_applications": ["https://bobpay.com/bobpay-app.json"]}

The browser then downloads https://bobpay.com/bobpay-app.json and verifies the installed app against the version and signatures in it. The requirements for this verification are that all downloads must be over HTTPS, HTTP response codes must be 200, and HTTP redirects are not followed.

Caching payment
apps process — Caching payment apps process

After the browser has been used for one or more web payments, it has a cache of locally-installed payment apps. The cache allows faster display of the payment UI on subsequent visits. When a user taps the merchant website's "Buy" button, the browser shows the cached list of apps along with a "Refreshing..." indicator. When the payment app cache has been refreshed, the user can tap a "Refresh" button to see the updated list of payment apps.

Protecting the cache from malware is outside the scope of this project, but is an important consideration. If the user's device is infected with malware that can read and write other apps' data directories the malware could, for example, read the user's credit card numbers, addresses, and passwords from disk. See https://developer.android.com/guide/topics/data/data-storage.html for more information.

Preloading a payment request

A web browser queries installed payment apps when the JavaScript PaymentRequest object is constructed. To save time, a website can create a PaymentRequest object when showing the "Buy" button, but not call PaymentRequest.show() until the user taps the button. This allows a faster UI response.

Messages

Browsers and payment apps pass data to each other via Intent extras, which are key-value string pairs.

Optional: “Is ready to pay”

"Is ready to
pay" process — "Is ready to pay" process

If the payment app has a service with the IS_READY_TO_PAY Android intent handler, then the browser can check with the payment app before showing it as an option for payment.

“Is ready to pay” parameters

ArrayList<String> methodNames - The names of the methods being queried. The elements are the keys in methodData dictionary.
Bundle[String] methodData - A mapping from each methodName to the output of JSON.stringify(methodData[methodName].data).
String topLevelOrigin - The schemeless origin of the top-level browsing context. For example, https://mystore.com/checkout will be passed as mystore.com.
Parcelable[] topLevelCertificateChain - The certificate chain of the top-level browsing context. Null for localhost and file on disk, which are both secure contexts without SSL certificates. The certificate chain is necessary because a payment app might have different trust requirements for websites.
String paymentRequestOrigin - The schemeless origin of the iframe browsing context that invoked the new PaymentRequest(methodData, details, options) constructor. If the constructor was invoked from the top-level context, then the value of this parameter equals the value of topLevelOrigin parameter.

Not all browsers can determine the values for all parameters, so the payment app should check for existence of the parameters before attempting to access them.

These parameters are sent to the payment app using intent extras.

Bundle extras = new Bundle();
extras.putString("key", "value");
intent.putExtras(extras);

The certificate chain is serialized as follows.

Parcelable[] certificateChain;
Bundle certficate = new Bundle();
certificate.putByteArray("certificate", certificateByteArray[i]);
certificateChain[i] = certificate;
extras.putParcelableArray("certificateChain", certificateChain);

“Is ready to pay” response

boolean readyToPay - Whether the payment app is ready to pay.

The response is sent back via the handleIsReadyToPay(isReadyToPay) method.

callback.handleIsReadyToPay(true);

Payment

A web browser invokes the payment app via an Android intent with payment request information in the intent parameters. The payment app responds with methodName and details, which are payment app specific and are opaque to the browser. The browser does not parse the details; that parameter's value goes directly to the merchant website.

Payment parameters

ArrayList<String> methodNames - The names of the methods being used. The elements are the keys in the methodData dictionary, and indicate the methods that the payment app supports.
Bundle[String] methodData - A mapping from each methodName to the output of JSON.stringify(methodData[methodName].data).
String merchantName - The contents of the <title> HTML tag of the top-level browsing context on the checkout web page.
String topLevelOrigin - The schemeless origin of the top-level browsing context. For example, https://mystore.com/checkout is passed as mystore.com.
Parcelable[] topLevelCertificateChain - The certificate chain of the top-level browsing context. Null for localhost and files on disk, which are both secure contexts without SSL certificates. Each Parcelable is a Bundle with a certificate key and a byte array value.
String paymentRequestOrigin - The schemeless origin of the iframe browsing context that invoked the new PaymentRequest(methodData, details, options) constructor. If the constructor was invoked from top-level context, then the value of this parameter equals the value of topLevelOrigin parameter.
String total - The output of JSON.stringify(details.total.amount).
String modifiers - The output of JSON.stringify(details.modifiers), where details.modifiers contain only supportedMethods and total.
String paymentRequestId - The PaymentRequest.id field that “push-payment” apps should associate with transaction state. Merchant websites will use this field to query the “push-payment” apps for the state of transaction out of band.

Not all browsers can determine the values for all parameters. Therefore, the payment app should check for existence of the parameters before attempting to access them.

These parameters are sent to the payment app using intent extras.

Bundle extras = new Bundle();
extras.putString("key", "value");
intent.putExtras(extras);

If the browser determines that the certificate is invalid, then PaymentRequest.show() should not invoke payment apps. Even if the user bypasses the browser’s interstitial warning about the site, the Payment Request API is still available for manual data entry but not for quick and painless payments. Therefore, only a valid certificate chain will be sent to the payment app, after the browser has resolved it to its root.

Payment response

int success - The activity result of either RESULT_OK or RESULT_CANCELED, depending on whether the payment app was able to complete its part of the transaction successfully. For example, success can be RESULT_CANCELED if the user fails to enter the correct PIN code for their account in the payment app.
String methodName - The name of the method being used.
String details - A JSON string containing information necessary for the merchant website to complete the transaction. If success is true, then details must be constructed in such a way that JSON.parse(details) will succeed.

The response is sent back via Activity.setResult() method.

Intent result = new Intent();
Bundle extras = new Bundle();
extras.putString("key", "value");
result.putExtras(extras);
setResult(RESULT_OK, result); // Change to RESULT_CANCELED on failure.
finish(); // Close the payment activity.

If the payment app returns RESULT_CANCELED, then the browser may let the user choose a different payment app. The merchant website does not observe this, so there’s no need for detailed error codes to be sent from the payment app to the merchant website.

Algorithms

This section describes in detail the steps of algorithms that determine the list of possible Android payment apps on the user device.

Basic process

The merchant website provides a list of payment methods in the PaymentRequest constructor.
The browser finds the locally installed Android payment apps that claim support for the given payment methods.
The browser downloads and validates the payment method manifests and the web app manifests for the default applications of the payment method manifests.
The browser shows the apps that match the information in these web app manifests.
The browser checks for * in the supported_origins of the payment method manifests. If found, the browser shows all matching apps.
The browser downloads and validates the default payment method manifests and the default web app manifests for the installed Android payment apps.
The browser shows the apps that match the information in these web app manifests.

See authoritative specification in Ingesting payment method manifests.

Find payment apps

This algorithm queries locally installed Android apps for possible payment apps. It runs when PaymentRequest.canMakePayment() or PaymentRequest.show() is called.

Let apps be an empty list of payment apps.
If PaymentMethodData.supportedMethods contains the string "basic-card", then query all apps that can respond to org.chromium.intent.action.PAY action and have "basic-card" in <meta-data>. Add these apps to the apps list.
Let urlPaymentMethods be the subset of PaymentMethodData.supportedMethods that are valid, absolute URLs with HTTPS scheme.
Query all apps that can respond to the org.chromium.intent.action.PAY action with any of the urlPaymentMethods in <meta-data>. Add these apps to the apps list.

Remove all apps from the appslist that have an empty label.

ResolveInfo app; // Needs to be assigned.
boolean isLabelEmpty = !TextUtils.isEmpty(app.loadLabel(
getContext().getPackageManager()));

Return the urlPaymentMethods and apps lists.

Download payment method manifests

See the authoritative specification at Fetching payment method manifests.

Validate payment method manifest

See the authoritative specification at Validating and parsing payment method manifests.

Download web app manifest

See the authoritative specification at Fetching web app manifests.

Validate web app manifest

The algorithm operates on the contents downloaded in the Download web app manifest algorithm and returns true for a valid manifest. Here’s an example of the file to be parsed.

{
  "related_applications": [{
    "platform": "play",
    "id": "com.bobpay.app",
    "min_version": "1",
    "fingerprints": [{
      "type": "sha256_cert",
      "value": "92:5A:39:05:C5:B9:EA:BC:71:48:5F:F2"
    }]
  }]
}

Let manifest be the output of JSON-parsing the downloaded manifest data, which was decoded as UTF-8.
If JSON-parsing fails, then return false.
If manifest is not a dictionary, then return false.
If manifest does not have "related_applications" member that is a non-empty list of dictionaries with at least one "platform": "play", then return false.
For every dictionary in "related_applications" with "platform": "play":
1. If "id" is absent, is not a string, or is an empty string, return false.
2. If "min_version" is absent, is not a string, or cannot be parsed into an integer, then return false.
3. If "fingerprints" is absent, or is not a list, or is an empty list, return false.
4. For every item in the "fingerprints":
  1. If the "type" is not "sha256_cert", then return false.
  2. If the "value" is not a string of 32 colon-separated, upper-case hex digits, return false.
Return true.

Validate payment apps against web app manifests

This algorithm returns true if a payment app is allowed to handle payment method, according to a web app manifest.

Let app be an Android payment app for a paymentMethodUrl with a list of downloaded and validated relatedApplications ("related_applications" with "platform": "play" from validate web app manifest algorithm).
For each section in relatedApplications, if each of the following conditions is met, return true:
1. "id" equals app package name.
2. "min_version" is greater than or equal to app version.
3. The sorted list of the values in "fingerprints" equals the sorted list of the SHA256 hash of the certificates of the app.
Return false.

Manifests

See authoritative specification in Manifest format.

The manifests are machine readable files that reside on a server owned by the payment app developer. The locations of these files are derived from the payment method names. For example, if the payment method is called https://bobpay.com, then the payment method manifest may be located at https://bobpay.com/payment-method-manifest.json and a corresponding web app manifest may be located at https://bobpay.com/bobpay-app.json. These files describe the Android apps that are allowed to handle payments for the given payment method.

Here is an example payment method manifest that would be found at https://bobpay.com/payment-method-manifest.json.

{"default_applications": ["https://bobpay.com/bobpay-app.json"]}

An example web app manifest that would be found at https://bobpay.com/bobpay-app.json:

{
  "related_applications": [{
    "platform": "play",
    "id": "com.bobpay.app",
    "min_version": "1",
    "fingerprints": [{
      "type": "sha256_cert",
      "value": "92:5A:39:05:C5:B9:EA:BC:71:48:5F:F2"
    }],
    "url": "https://play.google.com/store/apps/details?id=com.bobpay.app"
  }]
}

This fingerprint format is inspired by Digital Asset Links. The file format is an extension of Web App Manifest, and allows for multiple payment apps, multiple versions of the same app, and multiple operating systems. The Android operating system has had support for Digital Asset Links since Marshmallow, but web browsers also need to support older versions of Android, so the built-in functionality found in the operating system is not useful.

All of the fingerprints in "fingerprints" should match all of the fingerprints in an installed app. To enable multiple versions of the same app with different fingerprints, list each version separately under "related_applications".

The "min_version" parameter is the minimum version of the payment app that can be used.

To allow unrestricted use of a payment method identifier, specify "supported_origins": "*" in the payment method manifest.

The "id", "min_version", and "fingerprints" values are required. The "id" value should be non-empty. The "fingerprints" list must be non-empty, and each dictionary in the list must have both "type" and "value". The order of the items in "fingerprints" is not important. Only the "sha256_cert" fingerprint type is supported.

The values of "fingerprints" can be computed as follows.

PackageInfo packageInfo = ...
MessageDigest md = MessageDigest.getInstance("SHA-256")
md.update(packageInfo.signatures[i].toByteArray();
byte[] digest = md.digest();
StringBuilder builder = new StringBuilder(digest.length * 3);
Formatter formatter = new Formatter(builder);
for (byte b : digest) {
    formatter.format(":%02X", b);
}
// Cut off the first ":".
return builder.substring(1);

AndroidManifest.xml

Add this in AndroidManifest.xml for the payment app.

<manifest  package="com.bobpay.app">
    <service android:name=".IsReadyToPayService"
             android:enabled="true"
             android:exported="true">
        <intent-filter>
            <action android:name="org.chromium.intent.action.IS_READY_TO_PAY"  />
        </intent-filter>
    </service>
    <activity android:name=".PaymentActivity"
              android:exported="true">
    <intent-filter>
        <action android:name="org.chromium.intent.action.PAY" />
    </intent-filter>
        <meta-data android:name="org.chromium.default_payment_method_name"
                   android:value="https://bobpay.com/put/optional/path/here" />
    </activity>
</manifest>

Recall that the "IS_READY_TO_PAY" service is optional. If there’s no such intent handler in the payment app, then the web browser assumes that the app can always make payments.

The activity with the "PAY" intent filter should have a <meta-data> tag that identifies the default payment method name for the app.

There should be at most one activity that handles "org.chromium.intent.action.PAY" and at most one service that handles "org.chromium.intent.action.IS_READY_TO_PAY". These are invoked regardless of the payment method.

To support multiple payment methods, add a <meta-data> tag with a <string-array> resource.

<manifest  package="com.bobpay.app">
    <service  android:name=".IsReadyToPayService"
              android:enabled="true"
              android:exported="true">
        <intent-filter>
            <action  android:name="org.chromium.intent.action.IS_READY_TO_PAY" />
        </intent-filter>
    </service>
    <activity  android:name=".PaymentActivity"
               android:exported="true">
        <intent-filter>
            <action  android:name="org.chromium.intent.action.PAY" />
        </intent-filter>
        <meta-data android:name="org.chromium.default_payment_method_name"
                   android:value="https://bobpay.com/put/optional/path/here" />
        <meta-data android:name="org.chromium.payment_method_names"
                   android:resource="@array/my_payment_method_names" />
    </activity>
</manifest>

All prefixes are "org.chromium", because the W3C is not involved in Chromium’s Android-specific APIs.

The resource must be a list of strings, each of which must be a valid, absolute URL with an HTTPS scheme as shown here.

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string-array  name="my_payment_method_names">
        <item>https://alicepay.com/put/optional/path/here</item>
        <item>https://evepay.com/put/optional/path/here</item>
    </string-array>
</resources>

`“basic-card”`

Any payment app can support "basic-card" payment method. This payment method does not require a payment app manifest. Chrome does not perform signature verification of a payment app that supports only "basic-card". To enable support for this payment method, add the following to the AndroidManifest.xml file of the payment app.

<manifest  package="com.bobpay.app">
    <service android:name=".IsReadyToPayService"
             android:enabled="true"
             android:exported="true">
        <intent-filter>
            <action android:name="org.chromium.intent.action.IS_READY_TO_PAY"  />
        </intent-filter>
    </service>
    <activity android:name=".PaymentActivity"
              android:exported="true">
        <intent-filter>
            <action android:name="org.chromium.intent.action.PAY"  />
        </intent-filter>
        <meta-data android:name="org.chromium.default_payment_method_name"
                   android:value="basic-card" />
    </activity>
</manifest>

Alternatively, "basic-card" can be one of the multiple supported payment methods through the use of a <resources> file.

Services

Querying IS_READY_TO_PAY is a one-time communication that takes place without invoking the payment app’s user interface. Messenger fits this paradigm well, but Messenger.sendingUid is available only in newer versions of Android, and the alternative call Binder.getCallingUid() is not reliable in Messenger. The solution is to use an Android Interface Definition Language (AIDL).

package org.chromium;

interface  IsReadyToPayServiceCallback {
    oneway  void  handleIsReadyToPay(boolean isReadyToPay);
}

Save this in org/chromium/IsReadyToPayServiceCallback.aidl in your project. The callback is used to enable asynchronous querying.

package org.chromium;

import  org.chromium.IsReadyToPayServiceCallback;

interface  IsReadyToPayService {
    oneway  void  isReadyToPay(IsReadyToPayServiceCallback callback);
}

Save this in org/chromium/IsReadyToPayService.aidl in your project. The oneway keyword is necessary to avoid blocking on the call. If querying takes more than 400 ms, the call times out and behaves as if callback.handleIsReadyToPay(false); is called. Responding to the "IS_READY_TO_PAY" intent works as follows.

import  org.chromium.IsReadyToPayService;
import  org.chromium.IsReadyToPayServiceCallback;

public  class  IsReadyToPayServiceImpl  extends Service {
    private final IsReadyToPayService.Stub mBinder =
        new IsReadyToPayService.Stub() {
            @Override
            public void isReadyToPay(IsReadyToPayServiceCallback callback) {
                // Check permission here.
                callback.handleIsReadyToPay(true);
            }
        });

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }
}

Permissions

`IS_READY_TO_PAY` intent

The permission check can be accomplished by checking Binder.getCallingUid(). The onBind() method in a Service is called only once during the lifetime of the Service. If multiple apps connect to the Service while it’s alive, they will all get the same instance. This means, that multiple apps may be talking to same instance of the payment app’s IsReadyToPayService. Therefore, permission check must happen inside of isReadyToPay() call.

PackageManager pm = getPackageManager();
Signature[] callerSignatures = pm.getPackageInfo(
    pm.getNameForUid(Binder.getCallingUid()),
    PackageManager.GET_SIGNATURES).signatures;

PAY intent

Android intents do not receive a Message. Therefore, there’s no sendingUid to get the name of the package. A payment app should use Activity.getCallingActivity().getPackageName() for signature verification in the "PAY" intent .

Signature[] callerSignatures = getPackageManager().getPackageInfo(  
    getCallingActivity().getPackageName(),  
    PackageManager.GET_SIGNATURES).signatures;

Beware that getCallingActivity() is not guaranteed to return an object. Check for null before using its result.

Summary

It makes good sense that native Android payment app support should be available in Android web browsers, because browsers cannot be expected to support every possible payment app SDK. Thus the method described here allows any Android payment app to work with any Android web browser, giving users more flexibility in making payments.

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