Digital Rights Management (DRM) systems are great for enforcing the rights associated with digital resources, such as the ability to copy, print, or view a file. But DRM systems are lousy at interoperating, partly because there's no single method for defining just what a "right" is. The Extensible Rights Markup Language (XrML) wants to change that.

XrML is a rights expression language. It provides a standard grammar for stating the rights associated with digital resources such as media files, electronic documents, and even software code. XrML allows content creators, distributors, and consumers to define and understand who's allowed to use a specific piece of digital content, what rights are available to those users, and the terms and conditions that affect those rights.

A standard rights language isn't really necessary in the closed environs of a corporate network where all end users will have the same DRM software. However, a standard language is essential for anyone who wants to share or sell content to business partners, contractors, distributors, or customers while retaining the ability to enforce rights associated with that content.

Created in 1996 at Xerox's renowned Palo Alto Research Center (PARC), XrML is now being promoted by ContentGuard, a company owned by Xerox. ContentGuard, which controls numerous copyrights associated with the language, hopes it will be adopted as a standard. The company will then charge fees to commercial organizations that use XrML.

ContentGuard has also submitted XrML to the Organization for the Advancement of Structured Information Standards (OASIS), which oversees numerous XML standards.

The essential construct in XrML is a license. Within the license are grants that are handed out to recipients. These grants express the rights and conditions associated with a piece of digital content for a particular recipient. The license itself identifies the entity that issued the grants. The issuer can also digitally sign the license.

Grants are made up of four basic elements: the principal, the right, the resource, and the condition. The principal is the recipient of a grant. The right is the action or actions associated with a resource, such as the ability to copy or print. The resource is some form of digital content, a service, or even a piece of information. The condition describes the terms, conditions, and obligations that affect how the rights associated with a resource are exercised. For example, a condition may impose a time limit on a principal's right to use a resource, or specify a particular authentication mechanism to identify the principal.

Originally written in a Lisp-style programming language, XrML was rewritten in XML in 1999. One reason for the switch was so that the rights language could be used within the framework of a Web service, facilitating interorganizational access to information about the rights and conditions associated with digital resources. XrML supports the Simple Object Access Protocol (SOAP), a Web services communications protocol, and the Web Services Description Language (WSDL). XrML also supports XML standards for digital signatures to authenticate participants, authorize transactions, and secure content in transit.

XrML enjoys support from two industry heavyweights: Microsoft and the Moving Picture Experts Group (MPEG), the organization that establishes international standards for coding audio and video into digital formats. MPEG is close to ratifying XrML as the basis for an international rights expression language.

As for Microsoft, it has licensed XrML for Windows Rights Management, a DRM service available in Windows Server 2003. Windows Rights Management enables persistent policy enforcement for content created within Office 2003 applications. Microsoft also owns a minority stake in ContentGuard.

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Contributing editor Andrew Conry-Murray can be reached at aconrymurray@yahoo.com.