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 Certificate Creation Tool (Makecert.exe) 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 Certificate Creation Tool (Makecert.exe) 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.

                Dim store As 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.

                Dim certCollection As X509Certificate2Collection = 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".

                Dim cert As X509Certificate2 = Nothing
    
                ' Loop through each certificate and find the certificate 
                ' with the appropriate name.
                Dim c As X509Certificate2
                For Each c In certCollection
                    If c.Subject = "CN=XML_ENC_TEST_CERT" Then
                        cert = c
    
                        Exit For
                    End If
                Next c
    

  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.

                Dim xmlDoc As 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.

            Dim elementToEncrypt As XmlElement = Doc.GetElementsByTagName(ElementToEncryptName)(0)
    
    

  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.

            Dim eXml As New EncryptedXml()
    
            ' Encrypt the element.
            Dim edElement As EncryptedData = 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>  

Imports System
Imports System.Xml
Imports System.Security.Cryptography
Imports System.Security.Cryptography.Xml
Imports System.Security.Cryptography.X509Certificates



Module Program

    Sub Main(ByVal args() As String)
        Try
            ' Create an XmlDocument object.
            Dim xmlDoc As 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.
            Dim store As New X509Store(StoreLocation.CurrentUser)
            store.Open(OpenFlags.ReadOnly)
            ' Place all certificates in an X509Certificate2Collection object.
            Dim certCollection As X509Certificate2Collection = store.Certificates
            Dim cert As X509Certificate2 = Nothing

            ' Loop through each certificate and find the certificate 
            ' with the appropriate name.
            Dim c As X509Certificate2
            For Each c In certCollection
                If c.Subject = "CN=XML_ENC_TEST_CERT" Then
                    cert = c

                    Exit For
                End If
            Next c
            If cert Is Nothing Then
                Throw New CryptographicException("The X.509 certificate could not be found.")
            End If

            ' 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 e As Exception
            Console.WriteLine(e.Message)
        End Try

    End Sub 'Main


    Sub Encrypt(ByVal Doc As XmlDocument, ByVal ElementToEncryptName As String, ByVal Cert As X509Certificate2)
        ' Check the arguments.  
        If Doc Is Nothing Then
            Throw New ArgumentNullException("Doc")
        End If
        If ElementToEncryptName Is Nothing Then
            Throw New ArgumentNullException("ElementToEncrypt")
        End If
        If Cert Is Nothing Then
            Throw New ArgumentNullException("Cert")
        End If
        ''''''''''''''''''''''''''''''''''''''''''''''''
        ' Find the specified element in the XmlDocument
        ' object and create a new XmlElemnt object.
        ''''''''''''''''''''''''''''''''''''''''''''''''
        Dim elementToEncrypt As XmlElement = Doc.GetElementsByTagName(ElementToEncryptName)(0)

        ' Throw an XmlException if the element was not found.
        If elementToEncrypt Is Nothing Then
            Throw New XmlException("The specified element was not found")
        End If

        ''''''''''''''''''''''''''''''''''''''''''''''''
        ' Create a new instance of the EncryptedXml class 
        ' and use it to encrypt the XmlElement with the 
        ' X.509 Certificate.
        ''''''''''''''''''''''''''''''''''''''''''''''''
        Dim eXml As New EncryptedXml()

        ' Encrypt the element.
        Dim edElement As EncryptedData = eXml.Encrypt(elementToEncrypt, Cert)
        ''''''''''''''''''''''''''''''''''''''''''''''''
        ' Replace the element from the original XmlDocument
        ' object with the EncryptedData element.
        ''''''''''''''''''''''''''''''''''''''''''''''''
        EncryptedXml.ReplaceElement(elementToEncrypt, edElement, False)
    End Sub
End Module

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.

System.Security.Cryptography.Xml
How to: Decrypt XML Elements with X.509 Certificates

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