After millions of hours of processor work and four years of human effort, the RC5 64-bit encryption algorithm has finally been broken.

Using 331,252 volunteer machines, a crypto group called Distributed.net cracked RSA Security's encryption challenge and picked up a cheque for $10,000.

"While it's debatable that the duration of this project does much to devalue the security of a 64-bit RC5 key, we can say with confidence that RC5-64 is not an appropriate algorithm to use for data that will still be sensitive in more than a few years' time," said the successful group.

After 15,769,938,165,961,326,592 keys, a PC in Tokyo returned the winning solution to the Distributed.net key servers.

The key was delivered on 14 July, but a glitch in the reporting software meant that the discovery was not detected until recently.

Burt Kaliski, chief scientist at RSA Laboratories, said: "We appreciate the persistence of Distributed.net and the many individuals involved in completing the search for this one key.

"Their work helps the industry confirm how much work is involved to search exhaustively for a key, and how a huge volume of computing time can be harnessed."

David McNett, Distributed.net co-founder and president, said: "We have once again shown how collective computing power can be applied to security technology with ordinary PCs.

"We look forward to future challenges that will assist in helping the cryptographic community gauge the strength of an algorithm or application against exhaustive key search."

The decrypted message read: "Some things are better left unread."