The perfect crime has been made more likely by new computer technology, according to Kent Walker, the district attorney who led the successful hunt for Kevin Mitnick, the world's most wanted hacker.
"When people are free to behave in anonymous ways, you will see a rise in the anti-social behaviour. Anonymity is the urbanisation of cyberspace," Mr Walker, who now works for a pager company, told last week's conference on Computers, Freedom and Privacy (CFP) in San Francisco.
Computers which communicate valuable information over insecure networks, like the telephone system, must have some way to keep their secrets in transit. Banks use largely private networks but if much of the world's commerce is to move on to the Internet, it will need security.
Techniques known as "strong encryption" can provide absolute security for data. The problem for governments and law-enforcement agencies is that absolute security means security from governments too.
Mr Walker said strong encryption combined with growing possibilities of anonymity in cyberspace would make money laundering and tax evasion easier. The same techniques can be used to make digital cash, which differs from bank accounts and credit cards in that it is anonymous. Digital cash would take the form of encrypted numbers passed down phone lines from computer to computer but exchangeable into paper money or bank credits. Such developments could make kidnapping far easier and more profitable, Mr Walker said. A ransom paid in digital cash across the Internet could be untraceable.
At present, the US government uses arms-control legislation to forbid the export of any form of cryptography which it cannot crack with its supercomputers. However, PGP, a free program available almost everywhere in the world, has made strong encryption available to anyone.
Matt Blaze, a mathematician at Bell Labs, who was also at the CFP conference, estimated that governments have about two years to come up with political rules for encryption before events overtake them. The mathematicians and computer scientists who must work out the details cannot start without political guidance, he said.
But the real difficulty was that most governments seem to want only encryption which governments can crack to be legal, which is technically impossible. However, Stewart Baker, a former lawyer for the National Security Agency, said encryption need not be perfect to work: "It just has to cost more to break than it would cost the bad guys to bribe your cleaning lady."