XML - Managing Data Exchange/XML Encryption

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XML - Managing Data Exchange Chapters Appendices Exercises

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Author: Shayla S. Lee 02:38, 15 November 2005 (UTC)

Introduction

XML encryption was developed to address two common areas not addressed by the Transport Layer Security and Secure Socket Layer protocol (TLS/SSL). TLS/SSL is a very secure and reliable protocol that provides end-to-end security sessions between two parties. XML adds an extra layer of security to TLS/SSL by encrypting part or all of the data being exchanged and by allowing for secure sessions between more than two parties. In other words, each party can maintain secure or insecure sessions with any of the communicating parties, and both secure and non-secure data can be exchanged in the same document. Furthermore, XML encryption can handle both XML and non-XML (e.g. binary) data.

Encryption Syntax

All XML encrypted files must start with the following XML preamble, declaration, internal entity, and import.

Schema Definition:
 <?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE schema  PUBLIC "-//W3C//DTD XMLSchema 200102//EN"
  "http://www.w3.org/2001/XMLSchema.dtd"
  [
    <!ATTLIST schema
      xmlns:xenc CDATA #FIXED 'http://www.w3.org/2001/04/xmlenc#'
      xmlns:ds CDATA #FIXED 'http://www.w3.org/2000/09/xmldsig#'>
    <!ENTITY xenc 'http://www.w3.org/2001/04/xmlenc#'>
    <!ENTITY % p >
    <!ENTITY % s >
   ]>
 
 <schema xmlns='http://www.w3.org/2001/XMLSchema' version='1.0'
         xmlns:ds='http://www.w3.org/2000/09/xmldsig#'
         xmlns:xenc='http://www.w3.org/2001/04/xmlenc#'
         targetNamespace='http://www.w3.org/2001/04/xmlenc#'
         elementFormDefault='qualified'>
   <import namespace='http://www.w3.org/2000/09/xmldsig#'
           schemaLocation='http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-   schema.xsd'/>

EncryptedType Element

EncryptedType is the abstract type from which EncryptedData and EncryptedKey are derived.

 Schema Definition:
  <complexType name='EncryptedType' abstract='true'>
   <sequence>
     <element name='EncryptionMethod' type='xenc:EncryptionMethodType' 
              minOccurs='0'/>
     <element ref='ds:KeyInfo' minOccurs='0'/>
     <element ref='xenc:CipherData'/>
     <element ref='xenc:EncryptionProperties' minOccurs='0'/>
   </sequence>
   <attribute name='Id' type='ID' use='optional'/>
   <attribute name='Type' type='anyURI' use='optional'/>
   <attribute name='MimeType' type='string' use='optional'/>
   <attribute name='Encoding' type='anyURI' use='optional'/> 
  </complexType>

Syntax Explanation

EncryptionMethod is an optional element that describes the encryption algorithm applied to the cipher data. If the element is absent, the encryption algorithm must be known by the recipient or the decryption will fail.

<element name='EncryptionMethod' type='xenc:EncryptionMethodType' 
              minOccurs='0'/>

ds:KeyInfo is an optional element that carries information about the key used to encrypt the data. Subsequent sections of this specification define new elements that may appear as children of ds:KeyInfo.

<element ref='ds:KeyInfo' minOccurs='0'/>

CipherData is a mandatory element that contains the CipherValue or CipherReference with the encrypted data.

<element ref='xenc:CipherData'/>

EncryptionProperties can contain additional information concerning the generation of the EncryptedType (e.g., date/time stamp).

<element ref='xenc:EncryptionProperties' minOccurs='0'/>

Id is an optional attribute providing for the standard method of assigning a string id to the element within the document context.

<attribute name='Id' type='ID' use='optional'/>

Type is an optional attribute identifying type information about the plaintext form of the encrypted content. While optional, this specification takes advantage of it for mandatory processing in dycryption. If the EncryptedData element contains data of Type 'element' or element 'content', and replaces that data in an XML document context, it is strongly recommended the Type attribute be provided. Without this information, the decryptor will be unable to automatically restore the XML document to its original cleartext form.

<attribute name='Type' type='anyURI' use='optional'/>

MimeType is an optional (advisory) attribute which describes the media type of the data which has been encrypted. The value of this attribute is a string with values defined by [MIME]. For example, if the data that is encrypted is a base64 encoded PNG, the transfer Encoding may be specified as 'http://www.w3.org/2000/09/xmldsig#base64' and the MimeType as 'image/png'. This attribute is purely advisory; no validation of the MimeType information is required and it does not indicate the encryption application must do any additional processing. Note, this information may not be necessary if it is already bound to the identifier in the Type attribute. For example, the Element and Content types defined in this specification are always UTF-8 encoded text.

<attribute name='MimeType' type='string' use='optional'/>

EncryptionMethod Element

EncryptionMethod is an optional element that describes the encryption algorithm applied to the cipher data. If the element is absent, the encryption algorithm must be known by the recipient or the decryption will fail. The permitted child elements of the EncryptionMethod are determined by the specific value of the Algorithm attribute URI.

Schema Definition:
 <complexType name='EncryptionMethodType' mixed='true'>
   <sequence>
     <element name='KeySize' minOccurs='0' type='xenc:KeySizeType'/>
     <element name='OAEPparams' minOccurs='0' type='base64Binary'/>
     <any namespace='##other' minOccurs='0' maxOccurs='unbounded'/>
   </sequence>
   <attribute name='Algorithm' type='anyURI' use='required'/>
 </complexType>

CipherData Element

CipherData is a mandatory element that provides the encrypted data. It must either contain the encrypted octet sequence as base64 encoded text of the CipherValue element, or provide a reference to an external location containing the encrypted octet sequence via the CipherReference element.

Schema Definition:
 <element name='CipherData' type='xenc:CipherDataType'/>
 <complexType name='CipherDataType'>
    <choice>
      <element name='CipherValue' type='base64Binary'/>
      <element ref='xenc:CipherReference'/>
    </choice>
  </complexType>

CipherReference Element

CipherReference identifies a source which, when processed, yields the encrypted octet sequence CipherReference is used when CipherValue is not supplied directly. The actual value is obtained as follows. The CipherReference URI contains an identifier that is dereferenced. Should the CipherReference element contain an OPTIONAL sequence of Transforms, the data resulting from dereferencing the URI is transformed as specified so as to yield the intended cipher value. For example, if the value is base64 encoded within an XML document; the transforms could specify an XPath expression followed by a base64 decoding so as to extract the octets.

Schema Definition:
 <element name='CipherReference' type='xenc:CipherReferenceType'/>
  <complexType name='CipherReferenceType'>
      <sequence>
        <element name='Transforms' type='xenc:TransformsType' minOccurs='0'/>
      </sequence>
      <attribute name='URI' type='anyURI' use='required'/>
  </complexType>

   <complexType name='TransformsType'>
      <sequence>
        <element ref='ds:Transform' maxOccurs='unbounded'/> 
      </sequence>
    </complexType>

Cipher Reference with Optional Tranform feature and Tranform Algorithm:

<CipherReference URI="http://www.example.com/CipherValues.xml">
   <Transforms>
     <ds:Transform 
      Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
         <ds:XPath xmlns:rep="http://www.example.org/repository">
           self::text()[parent::rep:CipherValue[@Id="example1"]]
         </ds:XPath>
     </ds:Transform>
     <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#base64"/>
   </Transforms>
 </CipherReference>

EncryptedData Element

EncryptedData is the core element in the syntax. Not only does its CipherData child contain the encrypted data, but it's also the element that replaces the encrypted element, or serves as the new document root.

Schema Definition:
 <element name='EncryptedData' type='xenc:EncryptedDataType'/>
 <complexType name='EncryptedDataType'>
   <complexContent>
    <extension base='xenc:EncryptedType'>
    </extension>
   </complexContent>
 </complexType>

Resources

The information above was obtained from W3C and IBM. For more information, please visit the following links:

http://www.w3.org/TR/2002/CR-xmlenc-core-20020802/#sec-Encryption-Syntax
http://www-128.ibm.com/developerworks/xml/library/x-encrypt/