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XML Encryption

XML Encryption

Published: 15 Dec 2006
By: Derek Smyth
XML Encryption is a W3C standard for encrypting XML elements. The encryption process involves taking an element from an xml document, encrypting it and it's children, and then replacing the original XML content with the generated encrypted XML in such a way as the document remains well formed.

Introduction

There are three approaches to Xml Encryption.

1. Encrypt the xml using symmetric encryption only

Only one session key is used and it’s the same key that encrypts the xml which is used to decrypt it. The key is not stored with the encrypted xml and so the key needs to be loaded during the process and protected when stored.

2. Encrypt the xml using a combination of asymmetric and symmetric encryption

The dual approach requires a symmetric session key to encrypt the data and an asymmetric key to protect the session key. Both the encrypted session key and the encrypted data are stored together in the xml document. The public asymmetric key is used to encrypt the session key while the private asymmetric key is used to decrypt the key.

This article covers this approach. To learn about the other approaches please look at the MSDN help for more information.

3. Encrypt the xml using a X.509 Certificate This approach uses a X.509 certificate as the symmetrical key. X.509 certificates are provided by a third party vendor such as VeriSign.

Approaches

Xml encryption, regardless of how the encryption is performed, can store the encrypted data in one of two ways.

1. After encryption the whole element is replaced with an element named <EncryptedData>
2. After encryption only the data in the element is replaced and its name remains readable in the document.

The difference is very subtle but it’s rather important. For example:

Your xml document contains a root element called <employee> that contains a child element called <WrittenWarning> in which details of disciplinary action is stored. If you were sending this xml and wanted the <WrittenWarning> elements details protected with approach 1 the <WrittenWarning> is replaced with an element called <EncryptedData> and no information can be gathered from the document.

With approach 2 however the <WrittenWarning> element stays and only the data is encrypted. Anyone who intercepted this document might not know the specific details of the discipline action but they will still know that something has happened with that employee. Any attributes on the <WrittenWarning> element are also not encrypted.

So the approach you take depends on what the data is and how much information you want to give away. In .NET v2.0 deciding on which approach to take is specified using a Boolean value and can be easily modified.

Example of XML Encryption

Below is an example of XML encryption using the asymmetric approach where the author element in the xml document is replaced with an <EncryptedData> element.

The XML Document

<?xml version="1.0" standalone="no"?>
<article>
  <articleinfo>
    <title>XPath Queries on XmlDocument objects in .NET 1.1</title>
    <abstract>
      <para>This article covers the basics.</para>
    </abstract>
    <author>
      <honorific>Mr.</honorific>
      <firstname>George</firstname>
      <surname>James</surname>
      <email>gjames@doman.com</email>
    </author>
  </articleinfo>  
</article> 

The encrypted XML Document

<?xml version="1.0" standalone="no"?>
<article>
  <articleinfo>
    <title>XPath Queries on XmlDocument objects in .NET 1.1</title>
    <abstract>
      <para>This article covers the basics.</para>
      <para>This article does not cover.</para>
    </abstract>
    <EncryptedData Type="http://www.w3.org/2001/04/xmlenc#Element" 
		xmlns="http://www.w3.org/2001/04/xmlenc#">
      <EncryptionMethod 
           Algorithm="http://www.w3.org/2001/04/xmlenc#aes256-cbc" />
      <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
        <EncryptedKey xmlns="http://www.w3.org/2001/04/xmlenc#">
          <EncryptionMethod 
               Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5" />
          <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
            <KeyName>session</KeyName>
          </KeyInfo>
          <CipherData>
            <CipherValue>r4f7SI1aZKSvibb…CipherValue>
          </CipherData>
        </EncryptedKey>
      </KeyInfo>
      <CipherData>
        <CipherValue>sGNhKqcSovipJdOFCFKYEEMRFd…</CipherValue>
      </CipherData>
    </EncryptedData>
  </articleinfo>
</article> 

The author element and its children have been replaced with the <EncryptedData> element which contains a number of other elements that are used to describe the encrypted data, i.e. the encryption algorithms used, the session key used, etc.

The <EncryptedData> element

Looking at the tree hierarchy of the <EncryptedData> element you can see the <EncryptedData> element is broken down into a number of child elements. The <KeyInfo> element is the same as the <KeyInfo> element used in XML Digital Signatures.

The EncryptedData element is contained in the "http://www.w3.org/2001/04/xmlenc#” namespace. It is the root of the encrypted data.

The EncryptionMethod element is used to specify the symmetric method used when encrypting the data. It does this by using an Algorithm attribute containing a W3 URL that describes the method used. "http://www.w3.org/2001/04/xmlenc#aes256-cbc" indicates the data was encrypted using AES (Rijndael) with a 256k key size.

The KeyInfo element is borrowed from XML Digital Signatures and is used to store information about the symmetric keys. The KeyInfo element can store information about more than one key.

The EncryptedKey element and its child elements contain information about one key stored in a KeyInfo element.

The EncryptionMethod element of the KeyInfo contains the asymmetric encryption method used to encrypt the session key. It does this using an Algorithm attribute set to a W3 URL. For example: http://www.w3.org/2001/04/xmlenc#rsa-1_5 describes that RSA asymmetric encryption was used to encrypt the session key.

The KeyName element is an identifier used to find the key. You’ll see the importance of this later when it comes to coding XML Encryption.

The CipherData and CipherValue elements that are found as part of the EncryptedKey and EncryptedData elements contain the cipher data. The actual cipher data is stored in the CipherValue element. The EncryptedKey element stores the encrypted key, while in the encrypted data is stored in the CipherValue for the EncryptedData element.

Asymmetric XML Encryption Process

The process of XML encryption can be summarized in five steps:

1. Select an element in an XML document (selecting the root will encrypt the whole document).
2. Encrypt the element using a symmetric encryption key, known as the session key.
3. Encrypt the session key using asymmetric encryption (the public key is used).
4. Create an EncryptedData element which will contain the encrypted data and the encrypted session key.
5. Replace the original element with the EncryptedData element. Most of the steps are performed automatically for you by .NET v2.0 classes.

Asymmetric XML Decryption Process

The process of decrypting the XML can be summarized into four steps,

1. Select the EncryptedData element in an XML document
2. Decrypt the session key using an asymmetric key (the private key is used)
3. Decrypt the cipher data using the unencrypted symmetric encryption.
4. Replace the EncryptedData element with the unencrypted element.

Most of the steps are performed automatically for you but .NET v2.0 classes.

Namespaces

The classes needed to perform XML Encryption can be found in three namespaces.

• System.Xml – contains XML classes that are needed to contain XML data.
• System.Security.Cryptography – contains encryption classes used to generate encryption keys.
• System.Security.Cryptography.Xml – contains XML Encryption classes that are used to perform the encryption.

Encrypting XML with .NET

A working sample application accompanies this article which is similar to the code we’ll examine in this section. The sample application can serve as a base for you to add additional functionality, such as selecting a single node or leading the asymmetric key.

First load the asymmetric public key used to encrypt the session key.

//create asymmetric key for encrypting the session key
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
//load the public keys, to encrypt the XML is the recipients public key
XmlDocument pubKeys = new XmlDocument();
pubKeys.Load(Application.StartupPath + "\\xml.dev.keys.public");
//use the public key to encrypt the session key
rsa.FromXmlString(pubKeys.OuterXml);

Next load the Xml Document and select the node that’s going to be encrypted. The following example selects the node to encrypt using an XPath expression. If no expression is given then the whole document is encrypted.

//xml document
this.xmlEncDoc = new XmlDocument();
//TODO load some XML into the XmlDocument

XmlElement encElement;
//if there is no xpath value then
if (xpath == string.Empty)
{
  //encrypt the document element
  encElement = this.xmlEncDoc.DocumentElement;
}
else
{
  XmlNamespaceManager xmlns = this.xmlCntrlr.xmlnsManager;
  //select only the first matching node (be precise with xpath)
  encElement = this.xmlEncDoc.SelectSingleNode(xpath, xmlns) as XmlElement;
}

Use the EncryptedXml class to encrypt the data and session keys.

//a class to perform encryption on Xml Document
EncryptedXml xmlEnc = new EncryptedXml(this.xmlEncDoc);
//add a "session" key encoded with rsa
xmlEnc.AddKeyNameMapping("session", rsa);

//encrypt the data using the session key
//creates a new session key called "session"
//this is automatically stored in KeyInfo element
EncryptedData encData = xmlEnc.Encrypt(encElement, "session");

Replace the original element with its encrypted equivalent.

//replace the original element with the encrypted element
EncryptedXml.ReplaceElement(encElement, encData, false);

Decrypting XML with .NET

First load the private asymmetric key needed to decrypt the session key.

//create asymmetric key for decrypting the session key
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();

//load the private keys, to decrypt the session keys
XmlDocument privKeys = new XmlDocument();
privKeys.Load(Application.StartupPath + "\\xml.dev.keys.private");
//use the private key to decrypt the session key
rsa.FromXmlString(privKeys.OuterXml);

Add a key name mapping to the encrypted document that specifies which private key should be used to decrypt the named session key.

//decrypt the document <EncryptedData> elements using the named key
EncryptedXml encXml = new EncryptedXml(xmlEncDoc);
encXml.AddKeyNameMapping("session", rsa);

Then decrypt the document, this will decrypt each of the Encrypted Data elements with the specified keys.

//decrypts all the <EncryptedData> items,
//with the "session" key that was encrypted with rsa
encXml.DecryptDocument();

Summary

XML Encryption is a W3 Standard to encrypting XML. It does this in such a way that the encrypted data remains and can be treated as XML. It uses both asymmetric and symmetric encryption algorithms, symmetric to encrypt the data and asymmetric to encrypt the symmetric session key. Both the session key and the cipher data are stored together in an XML element called EncryptedData. The EncryptedData element contains a series of child elements that describe the algorithms used during the encryption process, as well as containing key information and the cipher data.

References

W3C Encryption Standard

Download Source Code

XMLEncryption Sample Project

打印 | 发表于 2007年2月1日 16:03