Document Type Definitions (DTDs) are important in data exchange. Parties exchanging data must agree on a format, and a DTD allows the specification of that format.
DTDs are used to specify the allowed syntax of an XML application [XML], including the values of entities and special properties of attributes—for example, that an attribute is a unique element identifier (ID). Familiarity with DTDs is useful because they are a fundamental part of XML parsing. In this book, we use DTDs to specify the syntax for XML signatures and some other XML security structures.