How to: Encrypt XML Elements with X.509 Certificates

You can use the classes in the System.Security.Cryptography.Xml namespace to encrypt an element within an XML document. XML Encryption is a standard way to exchange or store encrypted XML data, without worrying about the data being easily read. For more information about the XML Encryption standard, see the World Wide Web Consortium (W3C) specification for XML Encryption located at http://www.w3.org/TR/xmldsig-core/.

You can use XML Encryption to replace any XML element or document with an <EncryptedData> element that contains the encrypted XML data. The <EncryptedData> element can contain sub elements that include information about the keys and processes used during encryption. XML Encryption allows a document to contain multiple encrypted elements and allows an element to be encrypted multiple times. The code example in this procedure shows you how to create an <EncryptedData> element along with several other sub elements that you can use later during decryption.

This example encrypts an XML element using two keys. It generates a test X.509 certificate using the Makecert.exe (Certificate Creation Tool) and saves the certificate to a certificate store. The example then programmatically retrieves the certificate and uses it to encrypt an XML element using the Encrypt method. Internally, the Encrypt method creates a separate session key and uses it to encrypt the XML document. This method encrypts the session key and saves it along with the encrypted XML within a new <EncryptedData> element.

To decrypt the XML element, simply call the DecryptDocument method, which automatically retrieves the X.509 certificate from the store and performs the necessary decryption. For more information about how to decrypt an XML element that was encrypted using this procedure, see How to: Decrypt XML Elements with X.509 Certificates.

This example is appropriate for situations where multiple applications need to share encrypted data or where an application needs to save encrypted data between the times that it runs.

To encrypt an XML element with an X.509 certificate

  1. Use the Makecert.exe (Certificate Creation Tool) to generate a test X.509 certificate and place it in the local user store. You must generate an exchange key and you must make the key exportable. Run the following command:

    makecert -r -pe -n "CN=XML_ENC_TEST_CERT" -b 01/01/2005 -e 01/01/2010 -sky exchange -ss my
    
  2. Create an X509Store object and initialize it to open the current user store.

    X509Store store = new X509Store(StoreLocation.CurrentUser);
    
  3. Open the store in read-only mode.

    store.Open(OpenFlags.ReadOnly);
    
  4. Initialize an X509Certificate2Collection with all of the certificates in the store.

    X509Certificate2Collection certCollection = store.Certificates;
    
  5. Enumerate through the certificates in the store and find the certificate with the appropriate name. In this example, the certificate is named "CN=XML_ENC_TEST_CERT".

    X509Certificate2 cert = null;
    
    // Loop through each certificate and find the certificate  
    // with the appropriate name. 
    foreach (X509Certificate2 c in certCollection)
    {
        if (c.Subject == "CN=XML_ENC_TEST_CERT")
        {
            cert = c;
    
            break;
        }
    }
    
  6. Close the store after the certificate is located.

    store.Close();
    
  7. Create an XmlDocument object by loading an XML file from disk. The XmlDocument object contains the XML element to encrypt.

    XmlDocument xmlDoc = new XmlDocument();
    
  8. Find the specified element in the XmlDocument object and create a new XmlElement object to represent the element you want to encrypt. In this example, the "creditcard" element is encrypted.

    XmlElement elementToEncrypt = Doc.GetElementsByTagName(ElementToEncrypt)[0] as XmlElement;
    
  9. Create a new instance of the EncryptedXml class and use it to encrypt the specified element using the X.509 certificate. The Encrypt method returns the encrypted element as an EncryptedData object.

    EncryptedXml eXml = new EncryptedXml();
    
    // Encrypt the element.
    EncryptedData edElement = eXml.Encrypt(elementToEncrypt, Cert);
    
  10. Replace the element from the original XmlDocument object with the EncryptedData element.

    EncryptedXml.ReplaceElement(elementToEncrypt, edElement, false);
    
  11. Save the XmlDocument object.

    xmlDoc.Save("test.xml");
    

This example assumes that a file named "test.xml" exists in the same directory as the compiled program. It also assumes that "test.xml" contains a "creditcard" element. You can place the following XML into a file called test.xml and use it with this example.

<root>
    <creditcard>
        <number>19834209</number>
        <expiry>02/02/2002</expiry>
    </creditcard>
</root>
using System;
using System.Xml;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;
using System.Security.Cryptography.X509Certificates;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            // Create an XmlDocument object.
            XmlDocument xmlDoc = new XmlDocument();

            // Load an XML file into the XmlDocument object.
            xmlDoc.PreserveWhitespace = true;
            xmlDoc.Load("test.xml");

            // Open the X.509 "Current User" store in read only mode.
            X509Store store = new X509Store(StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadOnly);

            // Place all certificates in an X509Certificate2Collection object.
            X509Certificate2Collection certCollection = store.Certificates;

            X509Certificate2 cert = null;

            // Loop through each certificate and find the certificate  
            // with the appropriate name. 
            foreach (X509Certificate2 c in certCollection)
            {
                if (c.Subject == "CN=XML_ENC_TEST_CERT")
                {
                    cert = c;

                    break;
                }
            }

            if (cert == null)
            {
                throw new CryptographicException("The X.509 certificate could not be found.");
            }

            // Close the store.
            store.Close();

            // Encrypt the "creditcard" element.
            Encrypt(xmlDoc, "creditcard", cert);

            // Save the XML document.
            xmlDoc.Save("test.xml");

            // Display the encrypted XML to the console.
            Console.WriteLine("Encrypted XML:");
            Console.WriteLine();
            Console.WriteLine(xmlDoc.OuterXml);

        }
        catch (Exception e)
        {
            Console.WriteLine(e.Message);
        }

    }

    public static void Encrypt(XmlDocument Doc, string ElementToEncrypt, X509Certificate2 Cert)
    {
        // Check the arguments.   
        if (Doc == null)
            throw new ArgumentNullException("Doc");
        if (ElementToEncrypt == null)
            throw new ArgumentNullException("ElementToEncrypt");
        if (Cert == null)
            throw new ArgumentNullException("Cert");

        //////////////////////////////////////////////// 
        // Find the specified element in the XmlDocument 
        // object and create a new XmlElemnt object. 
        ////////////////////////////////////////////////

        XmlElement elementToEncrypt = Doc.GetElementsByTagName(ElementToEncrypt)[0] as XmlElement;
        // Throw an XmlException if the element was not found. 
        if (elementToEncrypt == null)
        {
            throw new XmlException("The specified element was not found");

        }

        ////////////////////////////////////////////////// 
        // Create a new instance of the EncryptedXml class  
        // and use it to encrypt the XmlElement with the  
        // X.509 Certificate. 
        //////////////////////////////////////////////////

        EncryptedXml eXml = new EncryptedXml();

        // Encrypt the element.
        EncryptedData edElement = eXml.Encrypt(elementToEncrypt, Cert);

        //////////////////////////////////////////////////// 
        // Replace the element from the original XmlDocument 
        // object with the EncryptedData element. 
        ////////////////////////////////////////////////////
        EncryptedXml.ReplaceElement(elementToEncrypt, edElement, false);
    }
}

The X.509 certificate used in this example is for test purposes only. Applications should use an X.509 certificate generated by a trusted certificate authority or use a certificate generated by the Microsoft Windows Certificate Server.

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