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XML Encryption Elements
Let's look at a typical XML-encrypted document. Figure 1 shows the XML data structure before encryption.
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Figure 1 XML data structure without encryption.
Now, if we want to treat this entire data set as confidential information, we can encrypt the whole document, as shown in Figure 2.
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Figure 2 Fully Encrypted XML Data-structure
Don't worry so much about the process of encrypting for now. Focus on the elements and syntax that make up an encrypted XML document.
<EncryptedData> Element
As shown in Figure 2, the <EncryptedData> element begins the XML-encrypted section within the document. In this case, because we decided to encrypt the entire document, we see the <EncryptedData> element in the location of the original data elements.
The following rules govern the <EncryptedData> element:
It is the core element that should enclose all encrypted XML data.
It cannot be a parent or child of another <EncryptedData> element.
It may become a root of the document (in case the whole document is encrypted, as in this example).
Its standard namespace is xmlns:xenc='http://www.w3c.org/2001/04/xmlenc#'.
<CipherData> Element
The second element in the encrypted XML document that draws our attention is the <CipherData> element. Being a child of the <EncryptedData> element, it encloses the actual cipher string that results from the encryption process.
In Figure 2, the <CipherData> element makes use of the <CipherValue> tag to enclose the encrypted string. This is called enveloping the raw encrypted data.
In case the encrypted string is available elsewhere in the network, it is referred within the <CipherReference> tag. This is called referencing the encrypted data.
Let's now look at a slightly different version of Figure 2. Suppose we consider only <Salary> as data to be protected and apply encryption to the sameβleaving the rest as plain XML. Figure 3 illustrates this scenario.
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Figure 3 XML data structure with the <Salary> element
encrypted.
In Figure 3, we encrypted the <Salary> element all together. Now, what if we need to encrypt only the data contained in this element? Have a look at Figure 4.
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Figure 4 XML data structure with the contents of the <Salary>
element encrypted.
The subtle difference is that the Encrypted Data type has changed from Element to Content because encryption has been applied only to the data within the element!