While writing this book, I've been haunted by the possibility that there may be something inherently evil or wrong in these algorithms. If criminals are able to hide information so effectively, justice becomes more elusive. There is less the police can do before a crime is committed, and there is less evidence after the fact. All of the ideas in this book, no matter how philosophical or embellished with allegory or cute jokes, carry this implicit threat.

This threat is a bit more obvious after the destruction of the World Trade Center on September 11, 2001. Some news reports offered the supposition that the attackers may have coordinated their efforts by hiding information in images. While there is no evidence that this occurred as I rewrite this chapter, there's no doubt that it could have happened.

The U.S. Federal Bureau of Investigation or at least its senior officers are clearly of the opinion that they need ready access to all communications. If someone is saying it, writing it, mailing it, or faxing it, the FBI would like to be able to listen in so they can solve crimes. This is a sensible attitude. More information can only help make sure that justice is fair and honest. People are convicted on the basis of their own words not the testimony of stool pigeons who often point the finger in order to receive a lighter sentence.

The arguments against giving the FBI and the police such power are more abstract and anecdotal. Certainly,...