Using receipts to verify product purchases (HTML)
[ This article is for Windows 8.x and Windows Phone 8.x developers writing Windows Runtime apps. If you’re developing for Windows 10, see the latest documentation ]
Each Windows Store transaction that results in a successful product purchase can optionally return a transaction receipt. This receipts provides information about the listed product and monetary cost to the customer.
Having access to this information supports scenarios where your app needs to verify that a user purchased your app, or has made in-app product purchases from the Windows Store. For example, imagine a game that offers downloaded content. If the user who purchased the game content wants to play it on a different device, you need to verify that the user already owns the content. Here's how.
Requesting a receipt
The Windows.ApplicationModel.Store namespace supports two ways of getting a receipt: by using the CurrentApp.RequestProductPurchaseAsync | requestProductPurchaseAsync or CurrentApp.RequestAppPurchaseAsync | requestAppPurchaseAsync method and using the includeReceipt parameter, or by calling the CurrentApp.GetAppReceiptAsync | getAppReceiptAsync method. An app receipt looks something like this.
<Receipt Version="1.0" ReceiptDate="2012-08-30T23:10:05Z" CertificateId="b809e47cd0110a4db043b3f73e83acd917fe1336" ReceiptDeviceId="4e362949-acc3-fe3a-e71b-89893eb4f528">
<AppReceipt Id="8ffa256d-eca8-712a-7cf8-cbf5522df24b" AppId="55428GreenlakeApps.CurrentAppSimulatorEventTest_z7q3q7z11crfr" PurchaseDate="2012-06-04T23:07:24Z" LicenseType="Full" />
<ProductReceipt Id="6bbf4366-6fb2-8be8-7947-92fd5f683530" ProductId="Product1" PurchaseDate="2012-08-30T23:08:52Z" ExpirationDate="2012-09-02T23:08:49Z" ProductType="Durable" AppId="55428GreenlakeApps.CurrentAppSimulatorEventTest_z7q3q7z11crfr" />
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
<SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
<Reference URI="">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" />
<DigestValue>cdiU06eD8X/w1aGCHeaGCG9w/kWZ8I099rw4mmPpvdU=</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>SjRIxS/2r2P6ZdgaR9bwUSa6ZItYYFpKLJZrnAa3zkMylbiWjh9oZGGng2p6/gtBHC2dSTZlLbqnysJjl7mQp/A3wKaIkzjyRXv3kxoVaSV0pkqiPt04cIfFTP0JZkE5QD/vYxiWjeyGp1dThEM2RV811sRWvmEs/hHhVxb32e8xCLtpALYx3a9lW51zRJJN0eNdPAvNoiCJlnogAoTToUQLHs72I1dECnSbeNPXiG7klpy5boKKMCZfnVXXkneWvVFtAA1h2sB7ll40LEHO4oYN6VzD+uKd76QOgGmsu9iGVyRvvmMtahvtL1/pxoxsTRedhKq6zrzCfT8qfh3C1w==</SignatureValue>
</Signature>
</Receipt>
A product receipt looks like this.
<Receipt Version="1.0" ReceiptDate="2012-08-30T23:08:52Z" CertificateId="b809e47cd0110a4db043b3f73e83acd917fe1336" ReceiptDeviceId="4e362949-acc3-fe3a-e71b-89893eb4f528">
<ProductReceipt Id="6bbf4366-6fb2-8be8-7947-92fd5f683530" ProductId="Product1" PurchaseDate="2012-08-30T23:08:52Z" ExpirationDate="2012-09-02T23:08:49Z" ProductType="Durable" AppId="55428GreenlakeApps.CurrentAppSimulatorEventTest_z7q3q7z11crfr" />
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
<SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
<Reference URI="">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" />
<DigestValue>Uvi8jkTYd3HtpMmAMpOm94fLeqmcQ2KCrV1XmSuY1xI=</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>TT5fDET1X9nBk9/yKEJAjVASKjall3gw8u9N5Uizx4/Le9RtJtv+E9XSMjrOXK/TDicidIPLBjTbcZylYZdGPkMvAIc3/1mdLMZYJc+EXG9IsE9L74LmJ0OqGH5WjGK/UexAXxVBWDtBbDI2JLOaBevYsyy+4hLOcTXDSUA4tXwPa2Bi+BRoUTdYE2mFW7ytOJNEs3jTiHrCK6JRvTyU9lGkNDMNx9loIr+mRks+BSf70KxPtE9XCpCvXyWa/Q1JaIyZI7llCH45Dn4SKFn6L/JBw8G8xSTrZ3sBYBKOnUDbSCfc8ucQX97EyivSPURvTyImmjpsXDm2LBaEgAMADg==</SignatureValue>
</Signature>
</Receipt>
You can use either of these receipt examples to test your validation code.
Validating a receipt
After you get a receipt, you need your back-end system (a web service or something similar) to validate it. Here's a .NET Framework example of that validation process.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Xml;
using System.IO;
using System.Security.Cryptography.Xml;
using System.Net;
namespace ReceiptVerificationSample
{
public sealed class RSAPKCS1SHA256SignatureDescription : SignatureDescription
{
public RSAPKCS1SHA256SignatureDescription()
{
base.KeyAlgorithm = typeof(RSACryptoServiceProvider).FullName;
base.DigestAlgorithm = typeof(SHA256Managed).FullName;
base.FormatterAlgorithm = typeof(RSAPKCS1SignatureFormatter).FullName;
base.DeformatterAlgorithm = typeof(RSAPKCS1SignatureDeformatter).FullName;
}
public override AsymmetricSignatureDeformatter CreateDeformatter(AsymmetricAlgorithm key)
{
if (key == null)
{
throw new ArgumentNullException("key");
}
RSAPKCS1SignatureDeformatter deformatter = new RSAPKCS1SignatureDeformatter(key);
deformatter.SetHashAlgorithm("SHA256");
return deformatter;
}
public override AsymmetricSignatureFormatter CreateFormatter(AsymmetricAlgorithm key)
{
if (key == null)
{
throw new ArgumentNullException("key");
}
RSAPKCS1SignatureFormatter formatter = new RSAPKCS1SignatureFormatter(key);
formatter.SetHashAlgorithm("SHA256");
return formatter;
}
}
class Program
{
// Utility function to read the bytes from an HTTP response
private static int ReadResponseBytes(byte[] responseBuffer, Stream resStream)
{
int count = 0;
int numBytesRead = 0;
int numBytesToRead = responseBuffer.Length;
do
{
count = resStream.Read(responseBuffer, numBytesRead, numBytesToRead);
numBytesRead += count;
numBytesToRead -= count;
} while (count > 0);
return numBytesRead;
}
public static X509Certificate2 RetrieveCertificate(string certificateId)
{
const int MaxCertificateSize = 10000;
// We are attempting to retrieve the following url. The getAppReceiptAsync website at
// https://msdn.microsoft.com/en-us/library/windows/apps/windows.applicationmodel.store.currentapp.getappreceiptasync.aspx
// lists the following format for the certificate url.
String certificateUrl = String.Format("https://go.microsoft.com/fwlink/?LinkId=246509&cid={0}", certificateId);
// Make an HTTP GET request for the certificate
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(certificateUrl);
request.Method = "GET";
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
// Retrieve the certificate out of the response stream
byte[] responseBuffer = new byte[MaxCertificateSize];
Stream resStream = response.GetResponseStream();
int bytesRead = ReadResponseBytes(responseBuffer, resStream);
if (bytesRead < 1)
{
//TODO: Handle error here
}
return new X509Certificate2(responseBuffer);
}
static bool ValidateXml(XmlDocument receipt, X509Certificate2 certificate)
{
// Create the signed XML object.
SignedXml sxml = new SignedXml(receipt);
// Get the XML Signature node and load it into the signed XML object.
XmlNode dsig = receipt.GetElementsByTagName("Signature", SignedXml.XmlDsigNamespaceUrl)[0];
if (dsig == null)
{
// If signature is not found return false
System.Console.WriteLine("Signature not found.");
return false;
}
sxml.LoadXml((XmlElement)dsig);
// Check the signature
bool isValid = sxml.CheckSignature(certificate, true);
return isValid;
}
static void Main(string[] args)
{
// .NET does not support SHA256-RSA2048 signature verification by default, so register this algorithm for verification
CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA256SignatureDescription), "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256");
// Load the receipt that needs to be verified as an XML document
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load("..\\..\\receipt.xml");
// The certificateId attribute is present in the document root, retrieve it
XmlNode node = xmlDoc.DocumentElement;
string certificateId = node.Attributes["CertificateId"].Value;
// Retrieve the certificate from the official site.
// NOTE: For sake of performance, you would want to cache this certificate locally.
// Otherwise, every single call will incur the delay of certificate retrieval.
X509Certificate2 verificationCertificate = RetrieveCertificate(certificateId);
try
{
// Validate the receipt with the certificate retrieved earlier
bool isValid = ValidateXml(xmlDoc, verificationCertificate);
System.Console.WriteLine("Certificate valid: " + isValid);
}
catch (Exception ex)
{
System.Console.WriteLine(ex.ToString());
}
}
}
}