Implements the http://www.w3.org/2000/09/xmldsig#base64 decoding transform.

The normative specification for base64 decoding transforms is [MIME]. The base64 Transform element has no content. The input is decoded by the algorithms. This transform is useful if an application needs to sign the raw data associated with the encoded content of an element.

This transform requires an octet stream for input. If an XPath node-set (or sufficiently functional alternative) is given as input, then it is converted to an octet stream by performing operations LOGically equivalent to 1) applying an XPath transform with expression self::text(), then 2) taking the string-value of the node-set. Thus, if an XML element is identified by a barename XPointer in the Reference URI, and its content consists solely of base64 encoded character data, then this transform automatically strips away the start and end tags of the identified element and any of its descendant elements as well as any descendant comments and processing instructions. The output of this transform is an octet stream.