Imagine a document that has legal value. Such a document may contain important information about rights and obligations, in which case you need to ensure its authenticity. You don’t want people to deny the commitments they’ve written down. Furthermore, this document probably has to be mailed to, viewed and stored by different parties. On different places in the workflow, at different moments in time, the document can be altered, be it voluntary, for instance to add an extra signature, involuntary, for example due to a transmission error, or deliberately, if somebody wants to create a forgery from the original document.For centuries, we’ve tried to solve this problem by putting a so-called ‘wet ink signature’ on paper. Nowadays, we can use digital signatures to ensure:

• the integrity of the document— we want assurance that the document hasn’t been changed somewhere in the workflow,
• the authenticity of the document— we want assurance that the author of the document is who we think it is (and not somebody else),
• non-repudiation— we want assurance that the author can’t deny his or her authorship.

In this book, we’ll focus on documents in the portable document format (PDF).