How to: Encrypt XML Elements with Symmetric Keys

You can use the classes in the System.Security.Cryptography.Xml namespace to encrypt an element within an XML document. XML Encryption allows you to store or transport sensitive XML, without worrying about the data being easily read. This procedure decrypts an XML element using the Advanced Encryption Standard (AES) algorithm, also known as Rijndael.

For information about how to decrypt an XML element that was encrypted using this procedure, see How to: Decrypt XML Elements with Symmetric Keys.

When you use a symmetric algorithm like AES to encrypt XML data, you must use the same key to encrypt and decrypt the XML data. The example in this procedure assumes that the encrypted XML will be decrypted using the same key, and that the encrypting and decrypting parties agree on the algorithm and key to use. This example does not store or encrypt the AES key within the encrypted XML.

This example is appropriate for situations where a single application needs to encrypt data based on a session key stored in memory, or based on a cryptographically strong key derived from a password. For situations where two or more applications need to share encrypted XML data, consider using an encryption scheme based on an asymmetric algorithm or an X.509 certificate.

To encrypt an XML element with a symmetric key

  1. Generate a symmetric key using the RijndaelManaged class. This key will be used to encrypt the XML element.

    Dim key As RijndaelManaged = Nothing 
    
    Try 
        ' Create a new Rijndael key.
        key = New RijndaelManaged()
    
  2. Create an XmlDocument object by loading an XML file from disk. The XmlDocument object contains the XML element to encrypt.

    ' Load an XML document. 
    Dim xmlDoc As New XmlDocument()
    xmlDoc.PreserveWhitespace = True
    xmlDoc.Load("test.xml")
    
  3. 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(ElementName)(0)
    
  4. Create a new instance of the EncryptedXml class and use it to encrypt the XmlElement with the symmetric key. The EncryptData method returns the encrypted element as an array of encrypted bytes.

    Dim eXml As New EncryptedXml()
    
    Dim encryptedElement As Byte() = eXml.EncryptData(elementToEncrypt, Key, False)
    
  5. Construct an EncryptedData object and populate it with the URL identifier of the XML Encryption element. This URL identifier lets a decrypting party know that the XML contains an encrypted element. You can use the XmlEncElementUrl field to specify the URL identifier.

    Dim edElement As New EncryptedData()
    edElement.Type = EncryptedXml.XmlEncElementUrl
    
  6. Create an EncryptionMethod object that is initialized to the URL identifier of the cryptographic algorithm used to generate the key. Pass the EncryptionMethod object to the EncryptionMethod property.

    Dim encryptionMethod As String = Nothing 
    
    If TypeOf Key Is TripleDES Then
        encryptionMethod = EncryptedXml.XmlEncTripleDESUrl
    ElseIf TypeOf Key Is DES Then
        encryptionMethod = EncryptedXml.XmlEncDESUrl
    End If 
    If TypeOf Key Is Rijndael Then 
        Select Case Key.KeySize
            Case 128
                encryptionMethod = EncryptedXml.XmlEncAES128Url
            Case 192
                encryptionMethod = EncryptedXml.XmlEncAES192Url
            Case 256
                encryptionMethod = EncryptedXml.XmlEncAES256Url
        End Select 
    Else 
        ' Throw an exception if the transform is not in the previous categories 
        Throw New CryptographicException("The specified algorithm is not supported for XML Encryption.")
    End If
    
    edElement.EncryptionMethod = New EncryptionMethod(encryptionMethod)
    
  7. Add the encrypted element data to the EncryptedData object.

    edElement.CipherData.CipherValue = encryptedElement
    
  8. Replace the element from the original XmlDocument object with the EncryptedData element.

    EncryptedXml.ReplaceElement(elementToEncrypt, edElement, False)
    

<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




Module Program

    Sub Main(ByVal args() As String)
        Dim key As RijndaelManaged = Nothing 

        Try 
            ' Create a new Rijndael key.
            key = New RijndaelManaged()
            ' Load an XML document. 
            Dim xmlDoc As New XmlDocument()
            xmlDoc.PreserveWhitespace = True
            xmlDoc.Load("test.xml")
            ' Encrypt the "creditcard" element.
            Encrypt(xmlDoc, "creditcard", key)

            Console.WriteLine("The element was encrypted")

            Console.WriteLine(xmlDoc.InnerXml)

            Decrypt(xmlDoc, key)

            Console.WriteLine("The element was decrypted")

            Console.WriteLine(xmlDoc.InnerXml)


        Catch e As Exception
            Console.WriteLine(e.Message)
        Finally 
            ' Clear the key. 
            If Not (key Is Nothing) Then
                key.Clear()
            End If 
        End Try 

    End Sub 


    Sub Encrypt(ByVal Doc As XmlDocument, ByVal ElementName As String, ByVal Key As SymmetricAlgorithm)
        ' Check the arguments.   
        If Doc Is Nothing Then 
            Throw New ArgumentNullException("Doc")
        End If 
        If ElementName Is Nothing Then 
            Throw New ArgumentNullException("ElementToEncrypt")
        End If 
        If Key Is Nothing Then 
            Throw New ArgumentNullException("Alg")
        End If 
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        ' Find the specified element in the XmlDocument 
        ' object and create a new XmlElemnt object. 
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        Dim elementToEncrypt As XmlElement = Doc.GetElementsByTagName(ElementName)(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  
        ' symmetric key. 
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        Dim eXml As New EncryptedXml()

        Dim encryptedElement As Byte() = eXml.EncryptData(elementToEncrypt, Key, False)
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        ' Construct an EncryptedData object and populate 
        ' it with the desired encryption information. 
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        Dim edElement As New EncryptedData()
        edElement.Type = EncryptedXml.XmlEncElementUrl
        ' Create an EncryptionMethod element so that the  
        ' receiver knows which algorithm to use for decryption. 
        ' Determine what kind of algorithm is being used and 
        ' supply the appropriate URL to the EncryptionMethod element. 
        Dim encryptionMethod As String = Nothing 

        If TypeOf Key Is TripleDES Then
            encryptionMethod = EncryptedXml.XmlEncTripleDESUrl
        ElseIf TypeOf Key Is DES Then
            encryptionMethod = EncryptedXml.XmlEncDESUrl
        End If 
        If TypeOf Key Is Rijndael Then 
            Select Case Key.KeySize
                Case 128
                    encryptionMethod = EncryptedXml.XmlEncAES128Url
                Case 192
                    encryptionMethod = EncryptedXml.XmlEncAES192Url
                Case 256
                    encryptionMethod = EncryptedXml.XmlEncAES256Url
            End Select 
        Else 
            ' Throw an exception if the transform is not in the previous categories 
            Throw New CryptographicException("The specified algorithm is not supported for XML Encryption.")
        End If

        edElement.EncryptionMethod = New EncryptionMethod(encryptionMethod)
        ' Add the encrypted element data to the  
        ' EncryptedData object.
        edElement.CipherData.CipherValue = encryptedElement
        '''''''''''''''''''''''''''''''''''''''''''''''''' 
        ' Replace the element from the original XmlDocument 
        ' object with the EncryptedData element. 
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        EncryptedXml.ReplaceElement(elementToEncrypt, edElement, False)

    End Sub 'Encrypt


    Sub Decrypt(ByVal Doc As XmlDocument, ByVal Alg As SymmetricAlgorithm)
        ' Check the arguments.   
        If Doc Is Nothing Then 
            Throw New ArgumentNullException("Doc")
        End If 
        If Alg Is Nothing Then 
            Throw New ArgumentNullException("Alg")
        End If 
        ' Find the EncryptedData element in the XmlDocument. 
        Dim encryptedElement As XmlElement = Doc.GetElementsByTagName("EncryptedData")(0)

        ' If the EncryptedData element was not found, throw an exception. 
        If encryptedElement Is Nothing Then 
            Throw New XmlException("The EncryptedData element was not found.")
        End If 


        ' Create an EncryptedData object and populate it. 
        Dim edElement As New EncryptedData()
        edElement.LoadXml(encryptedElement)
        ' Create a new EncryptedXml object. 
        Dim exml As New EncryptedXml()


        ' Decrypt the element using the symmetric key. 
        Dim rgbOutput As Byte() = exml.DecryptData(edElement, Alg)
        ' Replace the encryptedData element with the plaintext XML element.
        exml.ReplaceData(encryptedElement, rgbOutput)
    End Sub 
End Module

Never store a cryptographic key in plaintext or transfer a key between machines in plaintext. Instead, use a secure key container to store cryptographic keys.

When you are done using a cryptographic key, clear it from memory by setting each byte to zero or by calling the Clear method of the managed cryptography class.

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