Chapter 4. XML Document Type Definitions

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.

Recently, the W3C devised a new method of syntax specification, called XML Schema [Schema], which is described in detail in Chapter 5. Schemas are ...

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