Using Adobe's PDF Merchant for secure ebook distribution

By Karl De Abrew

Introduction

This column will focus specifically on Adobe’s PDF Merchant for the secure distribution of books "published" as PDF files. Note (at least) the following companies, SoftLock, FileOpen, PageVault and Authentica, also implement third party solutions for secure distribution of PDF, although coverage of these is out of the scope of this column.

Technical Background

The primary viewer for PDF documents, Adobe Acrobat, provides a plug-in architecture to extend its functionality. Plug-ins can provide a rich and powerful addition to the core product, and give the capability to implement many new commands, and functions. Of specific interest to us, the Adobe Acrobat Software Development Kit (SDK) provides an Application Programmer Interface (API) for the creation of a security handler for Adobe Acrobat.

Security Handlers provide an event driven mechanism and are called when a:

  1. Document is opened
  2. Document is saved
  3. User selects the "Security" button in the "Save-as" dialog
  4. User tries to alter the security settings

Security Handlers are responsible for a number of different functions. They set the access permissions for a file during a save. They authorize access to a document when opening. They setup and establish the key used for encryption and decryption and finally they are responsible for maintaining the values within the Encryption dictionary of a PDF file. A dictionary is a fundamental objects used in the PDF language. This construct "is an associative table containing pairs of objects".

The PDF Reference Manual Version 1.3 states, "every protected document must have an Encryption dictionary, which specifies the security handler to be used to authorize access to the document."

Note that this discussion applies to both the free Reader, and the full product, Adobe Acrobat. That is, both of these products provide the capability for a security handler.

PDF Merchant

Adobe Systems has implemented its own secure distribution system for Adobe Acrobat that allows authors, publishers and retailers to distribute PDF files with a relatively higher level of security than an unprotected PDF. This system is called PDF Merchant, and is a Software Development Kit (SDK) consisting of three components.

  1. WebLock
  2. WebSell
  3. WebBuy

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