Language: HTML | XAML

Using receipts to verify product purchases (HTML)

Applies to Windows and Windows Phone

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 
                // http://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());
                }
            }
        }
}

 

 

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