Updated Hackers from the US and Israel say they have broken copyright protections built in to Amazon's Kindle for PC, a feat that allows ebooks stored on the application to work with other devices.

The hack began as an open challenge in this (translated) forum for participants to come up with a way to make ebooks published in Amazon's proprietary format display on competing readers. Eight days later, users going by the handles Labba and i♥cabbages had a working program that did just that.

The hack is the latest to show the futility of digital rights management schemes, which more often than not inconvenience paying customers more than they prevent unauthorized copying.

Once upon a time, Apple laced its iTunes-purchased offerings with similar DRM restrictions that evoked major headaches when trying to do something as simple as transferring songs to a new PC. When reverse engineering specialist DVD Jon neutered the mechanism, that was the beginning of the end to the draconian regimen, which Apple called, ironically enough, Fairplay.

But most vendors don't bow so gracefully or quickly out of the reverse-engineering arms race. Witness, well, Apple, which regularly issues iPhone updates to thwart users who have the audacity to jailbreak the devices they own. Texas Instruments has also been known to take action against customers who reverse engineer calculators.

Amazon representatives have yet to indicate how they plan to respond. Queries put to a spokesman on Tuesday weren't returned.

According to a writeup of the Kindle hack here, Amazon engineers went to considerable lengths to prevent their DRM from being tampered with. The Kindle for PC uses a separate session key to encrypt and decrypt each book "and they seem to have done a reasonable job on the obfuscation," the author, i♥cabbages, says.

Labba and the US-based i♥cabbages, who declined to give his real name, discovered the attack vector independently around the same time. The resulting crack is a piece of software called unswindle, which was written by the latter hacker and is available here. It relies on reverse engineering from a hacker called darkreverser to discover the encryption algorithm used by the universal Mobipocket reader and most Kindle books.

Once unswindle is installed, proprietary Amazon ebooks can be converted into the open Mobi format. And from there, you can enjoy the content any way you like. ®

This article was updated to add details about collaboration between Labba and i♥cabbages.