xref: /frameworks/base/docs/html/guide/google/play/billing/billing_overview.jd

page.title=In-app Billing Overview
parent.title=In-app Billing
parent.link=index.html
@jd:body

<div id="qv-wrapper">
<div id="qv">
  <h2>In this document</h2>
  <ol>
    <li><a href="#billing-types">Product and Purchase Types</a></li>
    <li><a href="#billing-arch">In-app Billing Architecture</a></li>
    <li><a href="#billing-msgs">In-app Billing Messages</a></li>
    <ol>
      <li><a href="#billing-request">Request messages</a></li>
      <li><a href="#billing-response">Broadcast intents</a></li>
      <li><a href="#billing-message-sequence">Messaging sequence</a></li>
      <li><a href="#billing-action-notify">Handling IN_APP_NOTIFY messages</a></li>
    </ol>
    <li><a href="#billing-security">Security Controls</a></li>
    <li><a href="#billing-limitations">Requirements and Limitations</a></li>
  </ol>
  <h2>Downloads</h2>
  <ol>
    <li><a href="{@docRoot}guide/google/play/billing/billing_integrate.html#billing-download">Sample
    Application</a></li>
  </ol>
  <h2>See also</h2>
  <ol>
    <li><a href="{@docRoot}guide/google/play/billing/billing_integrate.html">Implementing In-app
    Billing</a></li>
    <li><a href="{@docRoot}guide/google/play/billing/billing_best_practices.html">Security and
    Design</a></li>
    <li><a href="{@docRoot}guide/google/play/billing/billing_testing.html">Testing In-app Billing</a></li>
    <li><a href="{@docRoot}guide/google/play/billing/billing_admin.html">Administering In-app
    Billing</a></li>
    <li><a href="{@docRoot}guide/google/play/billing/billing_reference.html">In-app Billing
    Reference</a></li>
  </ol>
</div>
</div>

<p>In-app Billing is a Google Play service that provides checkout processing for
in-app purchases. To use the service, your application sends a billing request for a specific in-app
product. The service then handles all of the checkout details for the transaction, including
requesting and validating the form of payment and processing the financial transaction. When the
checkout process is complete, the service sends your application the purchase details, such as the
order number, the order date and time, and the price paid. At no point does your application have to
handle any financial transactions; that role is provided by Google Play's in-app billing
service.</p>

<h2 id="billing-types">Product and Purchase Types</h2>

<p>In-app Billing supports different product types and purchase types to give you flexibility in how you monetize your app. In all cases, you define your products using the Google Play Developer Console, including product type, purchase type, SKU, price, description, and so on. For more information, see <a href="billing_admin.html">Administering In-app Billing</a>.</p>

<h3 id="producttypes">Product Types</h3>

<p>With In-app Billing, you can sell two types of products &mdash; <em>in-app products</em> and <em>subscriptions</em>. The billing characteristics of the two types are very different, but the In-app Billing API lets you handle the two product types in your app using the same communication model, data structures, and user interactions, as described later in this document.</p>

<ul>
<li><em>In-app products</em> &mdash; Items that a user would purchase one-at-a-time. For example, typical in-app products would let users purchase digital content, unlock functionality in an app, pay for one-time charges, or add almost anything to the application experience. Unlike with priced applications, once the user has purchased an in-app product there is no refund window. Users desiring refunds must contact the developer directly.

<p>In-app products can be sold using either the "managed per user account" or "unmanaged" purchase type. In-app products are always explicitly associated with one and only one app. That is, one app cannot purchase an in-app product published for another app, even if they are from the same developer. In-app products are supported in all versions of In-app Billing.</p></li>

<li><em>Subscriptions</em> &mdash; Items that are sold with a developer-specified, recurring billing interval. When a user purchases a subscription, Google Play and its payment processor automatically bill the user's account at the specified interval and price, charging the amount to the original payment method. Once the user purchases a subscription, Google Play continues billing the account indefinitely, without requiring approval or action from the user. The user can cancel the subscription at any time. 

<p>Subscriptions can only be sold using the "managed per user account" purchase type. As with in-app products, once the user has purchased an in-app product there is no refund window. Users desiring refunds must contact the developer directly. For more information about subscriptions and how to sell them in your apps, see the <a href="billing_subscriptions.html">Subscriptions</a> document.</p></li>
</ul>

<h3 id="purchasetypes">Purchase Types</h3>

<p>In-app Billing offers two purchase types that you can use when selling in-app products, "managed per user account" and "unmanaged". The purchase type controls how Google Play handles and tracks purchases for the products. </p>

<ul>
<li><em>Managed per user account</em> &mdash; Items that can be purchased only once per user account on Google Play. When a user purchases an item that uses the "managed per user account" purchase type, Google Play permanently stores the transaction information for each item on a per-user basis. This enables you to later query Google Play to restore the state of the items a specific user has purchased. If a user attempts to purchase a managed item that has already been purchased, Google Play prevents the user from purchasing the item again and displays an "Item already purchased" error.

<p>The "managed per user account" purchase type is useful if you are selling items such as game levels or application features. These items are not transient and usually need to be restored whenever a user reinstalls your application, wipes the data on their device, or installs your application on a new device.</p>

<li><em>Unmanaged</em> &mdash; Items that do not have their transaction information stored on Google Play. This means that you cannot later query Google Play to retrieve transaction information for those items. For "unmanaged" purchases, you are responsible for managing the transaction information. Also, Google Play does not attempt to prevent the user from purchasing an item multiple times if it uses the "unmanaged" purchase type. It's up to you to control how many times an unmanaged item can be purchased.</p>

<p>The "unmanaged" purchase type is useful if you are selling consumable items, such as fuel or magic spells. These items are consumed within your application and are usually purchased multiple times.</p></li>
</ul>

<h2 id="billing-arch">In-app Billing Architecture</h2>

<p>Your app accesses the In-app Billing service using an API that is exposed by
the Google Play app installed on the device. The Google Play app then uses an
asynchronous message loop to convey billing requests and responses between your
application and the Google Play server. In practice, your application never
directly communicates with the Google Play server (see figure 1). Instead, your
application sends billing requests to the Google Play application over
interprocess communication (IPC) and receives purchase responses from the Google
Play application in the form of asynchronous broadcast intents. Your application
does not manage any network connections between itself and the Google Play
server or use any special APIs from the Android platform.</p>

<div class="figure" style="width:440px">
<img src="{@docRoot}images/billing_arch.png" alt="" height="582" />
<p class="img-caption">
  <strong>Figure 1.</strong> Your application sends and receives billing messages through the
  Google Play application, which handles all communication with the Google Play server.</p>
</div>

<p>Some in-app billing implementations may also use a private remote server to deliver content or
validate transactions, but a remote server is not required to implement in-app billing. A remote
server can be useful if you are selling digital content that needs to be delivered to a user's
device, such as media files or photos. You might also use a remote server to store users'
transaction history or perform various in-app billing security tasks, such as signature
verification. Although you can handle all security-related tasks in your application, performing
those tasks on a remote server is recommended because it helps make your application less vulnerable
to security attacks.</p>

<p>A typical in-app billing implementation relies on three components:</p>
<ul>
  <li>A {@link android.app.Service} (named <code>BillingService</code> in the sample application),
  which processes purchase messages from the application and sends billing requests to the Google
  Play in-app billing service.</li>
  <li>A {@link android.content.BroadcastReceiver} (named <code>BillingReceiver</code> in the sample
  application), which receives all asynchronous billing responses from the Google Play
  application.</li>
  <li>A security component (named <code>Security</code> in the sample application), which performs
  security-related tasks, such as signature verification and nonce generation. For more information
  about in-app billing security, see <a href="#billing-security">Security controls</a> later in this
  document.</li>
</ul>

<p>You may also want to incorporate two other components to support in-app billing:</p>
<ul>
  <li>A response {@link android.os.Handler} (named <code>ResponseHandler</code> in the sample
  application), which provides application-specific processing of purchase notifications, errors,
  and other status messages.</li>
  <li>An observer (named <code>PurchaseObserver</code> in the sample application), which is
  responsible for sending callbacks to your application so you can update your user interface with
  purchase information and status.</li>
</ul>

<p>In addition to these components, your application must provide a way to store information about
users' purchases and some sort of user interface that lets users select items to purchase. You do
not need to provide a checkout user interface. When a user initiates an in-app purchase, the Google
Play application presents the checkout user interface to your user. When the user completes the
checkout process, your application resumes.</p>

<h2 id="billing-msgs">In-app Billing Messages</h2>

<p>When the user initiates a purchase, your application sends billing messages to Google Play's
in-app billing service (named <code>MarketBillingService</code>) using simple IPC method calls. The
Google Play application responds to all billing requests synchronously, providing your
application with status notifications and other information. The Google Play application also
responds to some billing requests asynchronously, providing your application with error messages and
detailed transaction information. The following section describes the basic request-response
messaging that takes place between your application and the Google Play application.</p>

<h3 id="billing-request">In-app billing requests</h3>

<p>Your application sends in-app billing requests by invoking a single IPC method
(<code>sendBillingRequest()</code>), which is exposed by the <code>MarketBillingService</code>
interface. This interface is defined in an <a
href="{@docRoot}guide/components/aidl.html">Android Interface Definition Language</a> file
(<code>IMarketBillingService.aidl</code>). You can <a
href="{@docRoot}guide/google/play/billing/billing_integrate.html#billing-download">download</a> this AIDL
file with the in-app billing sample application.</p>

<p>The <code>sendBillingRequest()</code> method has a single {@link android.os.Bundle} parameter.
The Bundle that you deliver must include several key-value pairs that specify various parameters for
the request, such as the type of billing request you are making, the item that is being purchased and
its type, and the application that is making the request. For more information about the Bundle keys
that are sent with a request, see <a
href="{@docRoot}guide/google/play/billing/billing_reference.html#billing-interface">In-app Billing
Service Interface</a>.

<p>One of the most important keys that every request Bundle must have is the
<code>BILLING_REQUEST</code> key. This key lets you specify the type of billing request you are
making. Google Play's in-app billing service supports the following five types of billing
requests:</p>

<ul>
  <li><code>CHECK_BILLING_SUPPORTED</code>
    <p>This request verifies that the Google Play application supports in-app billing. You
    usually send this request when your application first starts up. This request is useful if you
    want to enable or disable certain UI features that are relevant only to in-app billing.</p>
  </li>
  <li><code>REQUEST_PURCHASE</code>
    <p>This request sends a purchase message to the Google Play application and is the foundation
    of in-app billing. You send this request when a user indicates that he or she wants to purchase
    an item in your application. Google Play then handles the financial transaction by displaying
    the checkout user interface.</p>
  </li>
  <li><code>GET_PURCHASE_INFORMATION</code>
    <p>This request retrieves the details of a purchase state change. A purchase changes state when
    a requested purchase is billed successfully or when a user cancels a transaction during
    checkout. It can also occur when a previous purchase is refunded. Google Play notifies your
    application when a purchase changes state, so you only need to send this request when there is
    transaction information to retrieve.</p>
  </li>
  <li><code>CONFIRM_NOTIFICATIONS</code>
    <p>This request acknowledges that your application received the details of a purchase state
    change. Google Play sends purchase state change notifications to your application until you
    confirm that you received them.</p>
  </li>
  <li><code>RESTORE_TRANSACTIONS</code>
    <p>This request retrieves a user's transaction status for <a
    href="{@docRoot}guide/google/play/billing/billing_admin.html#billing-purchase-type">managed
    purchases</a> and <a
    href="{@docRoot}guide/google/play/billing/billing_admin.html#billing-purchase-type">subscriptions</a>.
    You should send this request only when you need to retrieve a user's transaction
    status, which is usually only when your application is reinstalled or installed for the first
    time on a device.</p>
  </li>
</ul>

<h3 id="billing-response">In-app Billing Responses</h3>

<p>The Google Play application responds to in-app billing requests with both synchronous and
asynchronous responses. The synchronous response is a {@link android.os.Bundle} with the following
three keys:</p>

<ul>
  <li><code>RESPONSE_CODE</code>
    <p>This key provides status information and error information about a request.</p>
  </li>
  <li><code>PURCHASE_INTENT</code>
    <p>This key provides a {@link android.app.PendingIntent}, which you use to launch the checkout
    activity.</p>
  </li>
  <li><code>REQUEST_ID</code>
    <p>This key provides you with a request identifier, which you can use to match asynchronous
    responses with requests.</p>
  </li>
</ul>
<p>Some of these keys are not relevant to every request. For more information, see <a
href="#billing-message-sequence">Messaging sequence</a> later in this document.</p>

<p>The asynchronous response messages are sent in the form of individual broadcast intents and
include the following:</p>

<ul>
    <li><code>com.android.vending.billing.RESPONSE_CODE</code>
    <p>This response contains a Google Play server response code, and is sent after you make an
    in-app billing request. A server response code can indicate that a billing request was
    successfully sent to Google Play or it can indicate that some error occurred during a billing
    request. This response is <em>not</em> used to report any purchase state changes (such as refund
    or purchase information). For more information about the response codes that are sent with this
    response, see <a
    href="{@docRoot}guide/google/play/billing/billing_reference.html#billing-codes">Server Response Codes
    for In-app Billing</a>.</p>
  </li>
  <li><code>com.android.vending.billing.IN_APP_NOTIFY</code>
    <p>This response indicates that a purchase has changed state, which means a purchase succeeded,
    was canceled, or was refunded. This response contains one or more notification IDs. Each
    notification ID corresponds to a specific server-side message, and each messages contains
    information about one or more transactions. After your application receives an
    <code>IN_APP_NOTIFY</code> broadcast intent, you send a <code>GET_PURCHASE_INFORMATION</code>
    request with the notification IDs to retrieve message details.</p>
  </li>
  <li><code>com.android.vending.billing.PURCHASE_STATE_CHANGED</code>
    <p>This response contains detailed information about one or more transactions. The transaction
    information is contained in a JSON string. The JSON string is signed and the signature is sent
    to your application along with the JSON string (unencrypted). To help ensure the security of
    your in-app billing messages, your application can verify the signature of this JSON string.</p>
  </li>
</ul>

<p>The JSON string that is returned with the <code>PURCHASE_STATE_CHANGED</code> intent provides
your application with the details of one or more billing transactions. An example of this JSON
string for a subscription item is shown below:</p>
<pre class="no-pretty-print" style="color:black">{ "nonce" : 1836535032137741465,
  "orders" :
    [{ "notificationId" : "android.test.purchased",
       "orderId" : "transactionId.android.test.purchased",
       "packageName" : "com.example.dungeons",
       "productId" : "android.test.purchased",
       "developerPayload" : "bGoa+V7g/yqDXvKRqq+JTFn4uQZbPiQJo4pf9RzJ",
       "purchaseTime" : 1290114783411,
       "purchaseState" : 0,
       "purchaseToken" : "rojeslcdyyiapnqcynkjyyjh" }]
}
</pre>

<p>For more information about the fields in this JSON string, see <a
href="{@docRoot}guide/google/play/billing/billing_reference.html#billing-intents">In-app Billing
Broadcast Intents</a>.</p>

<h3 id="billing-message-sequence">Messaging sequence</h3>

<p>The messaging sequence for a typical purchase request is shown in figure 2. Request types for
each <code>sendBillingRequest()</code> method are shown in <strong>bold</strong>, broadcast intents
are shown in <em>italic</em>. For clarity, figure 2 does not show the <code>RESPONSE_CODE</code>
broadcast intents that are sent for every request.</p>

<p>The basic message sequence for an in-app purchase request is as follows:</p>

<ol>
  <li>Your application sends a purchase request (<code>REQUEST_PURCHASE</code> type), specifying a
  product ID and other parameters.</li>
  <li>The Google Play application sends your application a Bundle with the following keys:
  <code>RESPONSE_CODE</code>, <code>PURCHASE_INTENT</code>, and <code>REQUEST_ID</code>. The
  <code>PURCHASE_INTENT</code> key provides a {@link android.app.PendingIntent}, which your
  application uses to start the checkout UI for the given product ID.</li>
  <li>Your application launches the pending intent, which launches the checkout UI.
    <p class="note"><strong>Note:</strong> You must launch the pending intent from an activity
    context and not an application context.</p>
  </li>
  <li>When the checkout flow finishes (that is, the user successfully purchases the item or cancels
  the purchase), Google Play sends your application a notification message (an
  <code>IN_APP_NOTIFY</code> broadcast intent). The notification message includes a notification ID,
  which references the transaction.</li>
  <li>Your application requests the transaction information by sending a
  <code>GET_PURCHASE_STATE_CHANGED</code> request, specifying the notification ID for the
  transaction.</li>
  <li>The Google Play application sends a Bundle with a <code>RESPONSE_CODE</code> key and a 
  <code>REQUEST_ID</code> key.
  <li>Google Play sends the transaction information to your application in a
  <code>PURCHASE_STATE_CHANGED</code> broadcast intent.</li>
  <li>Your application confirms that you received the transaction information for the given
  notification ID by sending a confirmation message (<code>CONFIRM_NOTIFICATIONS</code> type),
  specifying the notification ID for which you received transaction information.</li>
  <li>The Google Play application sends your application a Bundle with a
  <code>RESPONSE_CODE</code> key and a <code>REQUEST_ID</code> key.</li>
</ol>

<img src="{@docRoot}images/billing_request_purchase.png" height="231" id="figure2" />
<p class="img-caption">
  <strong>Figure 2.</strong> Message sequence for a purchase request.
</p>

<p>Keep in mind, you must send a confirmation when you receive transaction information from Google
Play (step 8 in figure 2). If you don't send a confirmation message, Google Play will
continue sending <code>IN_APP_NOTIFY</code> messages for the transactions you have not
confirmed. As a best practice, you should not send a <code>CONFIRM_NOTIFICATIONS</code> request for
a purchased item until you have delivered the item to the user. This way, if your application
crashes or something else prevents your application from delivering the product, your application
will still receive an <code>IN_APP_NOTIFY</code> broadcast intent from Google Play indicating
that you need to deliver the product. Also, as a best practice, your application must be able to
handle <code>IN_APP_NOTIFY</code> messages that contain multiple orders.</p>

<p>The messaging sequence for a restore transaction request is shown in figure 3. Request types for
each <code>sendBillingRequest()</code> method are shown in <strong>bold</strong>, broadcast intents
are shown in <em>italic</em>. For clarity, figure 3 does not show the <code>RESPONSE_CODE</code>
broadcast intents that are sent for every request.</p>

<div class="figure" style="width:490px">
<img src="{@docRoot}images/billing_restore_transactions.png" alt="" height="168" />
<p class="img-caption">
  <strong>Figure 3.</strong> Message sequence for a restore transactions request.
</p>
</div>

<p>The request triggers three responses. The first is a {@link android.os.Bundle} with a
<code>RESPONSE_CODE</code> key and a <code>REQUEST_ID</code> key. Next, the Google Play
application sends a <code>RESPONSE_CODE</code> broadcast intent, which provides status information
or error information about the request. As always, the <code>RESPONSE_CODE</code> message references
a specific request ID, so you can determine which request a <code>RESPONSE_CODE</code> message
pertains to.</p>

<p>The <code>RESTORE_TRANSACTIONS</code> request type also triggers a
<code>PURCHASE_STATE_CHANGED</code> broadcast intent, which contains the same type of transaction
information that is sent during a purchase request. Unlike with a purchase request, however, the transactions
are given without any associated notification IDs, so you do not need to respond to this
intent with a <code>CONFIRM_NOTIFICATIONS</code> message. </p>

<p class="note"><strong>Note:</strong> You should use the <code>RESTORE_TRANSACTIONS</code> request
type only when your application is installed for the first time on a device or when your
application has been removed from a device and reinstalled.</p>

<p>The messaging sequence for checking whether in-app billing is supported is shown in figure 4. The
request type for the <code>sendBillingRequest()</code> method is shown in <strong>bold</strong>.</p>

<div class="figure" style="width:454px">
<img src="{@docRoot}images/billing_check_supported.png" alt="" height="168" />
<p class="img-caption">
  <strong>Figure 4.</strong> Message sequence for checking whether in-app billing is supported.
</p>
</div>

<p>The synchronous response for a <code>CHECK_BILLING_SUPPORTED</code> request provides a Bundle
with a server response code.  A <code>RESULT_OK</code> response code indicates that in-app billing
is supported; a <code>RESULT_BILLING_UNAVAILABLE</code> response code indicates that in-app billing
is unavailable because the API version you specified is unrecognized or the user is not eligible to
make in-app purchases (for example, the user resides in a country that does not allow in-app
billing). A <code>SERVER_ERROR</code> can also be returned, indicating that there was a problem with
the Google Play server.</p>

<h3 id="billing-action-notify">Handling IN_APP_NOTIFY messages</h3>

<p>Usually, your application receives an <code>IN_APP_NOTIFY</code> broadcast intent from Google
Play in response to a <code>REQUEST_PURCHASE</code> message (see figure 2). The
<code>IN_APP_NOTIFY</code> broadcast intent informs your application that the state of a requested
purchase has changed. To retrieve the details of that purchase, your application sends a
<code>GET_PURCHASE_INFORMATION</code> request. Google Play responds with a
<code>PURCHASE_STATE_CHANGED</code> broadcast intent, which contains the details of the purchase
state change. Your application then sends a <code>CONFIRM_NOTIFICATIONS</code> message, informing
Google Play that you have received the purchase state change information.</p>

<p>In some special cases, you may receive multiple <code>IN_APP_NOTIFY</code> messages even though
you have confirmed receipt of the purchase information, or you may receive
<code>IN_APP_NOTIFY</code> messages for a purchase change even though you never initiated the
purchase. Your application must handle both of these special cases.</p>

<h4>Handling multiple IN_APP_NOTIFY messages</h4>

<p>When Google Play receives a <code>CONFIRM_NOTIFICATIONS</code> message for a given
<code>PURCHASE_STATE_CHANGED</code> message, it usually stops sending <code>IN_APP_NOTIFY</code>
intents for that <code>PURCHASE_STATE_CHANGED</code> message. Sometimes, however, Google
Play may send repeated <code>IN_APP_NOTIFY</code> intents for a
<code>PURCHASE_STATE_CHANGED</code> message even though your application has sent a
<code>CONFIRM_NOTIFICATIONS</code> message. This can occur if a device loses network connectivity
while you are sending the <code>CONFIRM_NOTIFICATIONS</code> message. In this case, Google Play
might not receive your <code>CONFIRM_NOTIFICATIONS</code> message and it could send multiple
<code>IN_APP_NOTIFY</code> messages until it receives acknowledgement that you received the
transaction message. Therefore, your application must be able to recognize that the subsequent
<code>IN_APP_NOTIFY</code> messages are for a previously processed transaction. You can do this by
checking the <code>orderID</code> that's contained in the JSON string because every transaction has
a unique <code>orderId</code>.</p>

<h4>Handling refunds and other unsolicited IN_APP_NOTIFY messages</h4>

<p>There are two cases where your application may receive <code>IN_APP_NOTIFY</code> broadcast
intents even though your application has not sent a <code>REQUEST_PURCHASE</code> message. Figure 5
shows the messaging sequence for both of these cases. Request types for each
<code>sendBillingRequest()</code> method are shown in <strong>bold</strong>, broadcast intents are
shown in <em>italic</em>. For clarity, figure 5 does not show the <code>RESPONSE_CODE</code>
broadcast intents that are sent for every request.</p>

<div class="figure" style="width:481px">
<img src="{@docRoot}images/billing_refund.png" alt="" height="189" />
<p class="img-caption">
  <strong>Figure 5.</strong> Message sequence for refunds and other unsolicited
IN_APP_NOTIFY messages.</p>
</div>

<p>In the first case, your application may receive an <code>IN_APP_NOTIFY</code> broadcast intent
when a user has your application installed on two (or more) devices and the user makes an in-app
purchase from one of the devices. In this case, Google Play sends an <code>IN_APP_NOTIFY</code>
message to the second device, informing the application that there is a purchase state change. Your
application can handle this message the same way it handles the response from an
application-initiated <code>REQUEST_PURCHASE</code> message, so that ultimately your application
receives a <code>PURCHASE_STATE_CHANGED</code> broadcast intent message that includes information
about the item that has been purchased. This applies only to items that have their <a
href="{@docRoot}guide/google/play/billing/billing_admin.html#billing-purchase-type">purchase type</a> set
to "managed per user account."</p>

<p>In the second case, your application can receive an <code>IN_APP_NOTIFY</code> broadcast intent
when Google Play receives a refund notification from Google Wallet. In this case, Google
Play sends an <code>IN_APP_NOTIFY</code> message to your application. Your application can handle
this message the same way it handles responses from an application-initiated
<code>REQUEST_PURCHASE</code> message so that ultimately your application receives a
<code>PURCHASE_STATE_CHANGED</code> message that includes information about the item that has been
refunded. The refund information is included in the JSON string that accompanies the
<code>PURCHASE_STATE_CHANGED</code> broadcast intent. Also, the <code>purchaseState</code> field in
the JSON string is set to 2.</p>

<p class="caution"><strong>Important:</strong> You cannot use the Google Wallet API to
issue refunds or cancel in-app billing transactions. You must do this manually through your
Google Wallet merchant account. However, you can use the Google Wallet API to retrieve order
information.</p>

<h2 id="billing-security">Security Controls</h2>

<p>To help ensure the integrity of the transaction information that is sent to your application,
Google Play signs the JSON string that is contained in the <code>PURCHASE_STATE_CHANGED</code>
broadcast intent. Google Play uses the private key that is associated with your publisher account
to create this signature. The publisher site generates an RSA key pair for each publisher account.
You can find the public key portion of this key pair on your account's profile page. It is the same
public key that is used with Google Play licensing.</p>

<p>When Google Play signs a billing response, it includes the signed JSON string (unencrypted)
and the signature. When your application receives this signed response you can use the public key
portion of your RSA key pair to verify the signature. By performing signature verification you can
help detect responses that have been tampered with or that have been spoofed. You can perform this
signature verification step in your application; however, if your application connects to a secure
remote server then we recommend that you perform the signature verification on that server.</p>

<p>In-app billing also uses nonces (a random number used once) to help verify the integrity of the
purchase information that's returned from Google Play. Your application must generate a nonce and
send it with a <code>GET_PURCHASE_INFORMATION</code> request and a <code>RESTORE_TRANSACTIONS</code>
request. When Google Play receives the request, it adds the nonce to the JSON string that
contains the transaction information. The JSON string is then signed and returned to your
application. When your application receives the JSON string, you need to verify the nonce as well as
the signature of the JSON string.</p>

<p>For more information about best practices for security and design, see <a
href="{@docRoot}guide/google/play/billing/billing_best_practices.html">Security and Design</a>.</p>

<h2 id="billing-limitations">In-app Billing Requirements and Limitations</h2>

<p>Before you get started with in-app billing, be sure to review the following requirements and
limitations.</p>

<ul>
  <li>In-app billing can be implemented only in applications that you publish through Google
  Play.</li>
  <li>You must have a Google Wallet Merchant account to use Google Play In-app Billing.</li>
  <li>In-app billing requires version 2.3.4 (or higher) of the Android Market application.
  To support subscriptions, version 3.5 or higher of the Google Play app is required. On devices
  running Android 3.0, version 5.0.12 (or higher) of the MyApps application is required.</li>
  <li>An application can use in-app billing only if the device is running Android 1.6 (API level 4)
  or higher.</li>
  <li>You can use in-app billing to sell only digital content. You cannot use in-app billing to sell
  physical goods, personal services, or anything that requires physical delivery.</li>
  <li>Google Play does not provide any form of content delivery. You are responsible for
  delivering the digital content that you sell in your applications.</li>
  <li>You cannot implement in-app billing on a device that never connects to the network. To
  complete in-app purchase requests, a device must be able to access the Google Play server over
  the network. </li>
</ul>

<p>For more information about in-app billing requirements, see <a
href="https://support.google.com/googleplay/android-developer/bin/answer.py?hl=en&answer=1153481">In-App
Billing Availability and Policies</a>.</p>