Currently music purchased from Apple's iTunes online music store will only play on Apple's iPod portable media player - a fact causing a growing amount of concern amongst European legislators. In January, the Norwegian government declared iTunes' DRM technology illegal and gave Apple until October 1 to let competing devices play songs downloaded from the iTunes. Consumer rights groups in several European countries have also attacked Apple over digital rights management.

"Perhaps those unhappy with the current situation should redirect their energies towards persuading the music companies to sell their music DRM-free," Jobs wrote.

"Convincing them to license their music to Apple and others DRM-free will create a truly interoperable music marketplace. Apple will embrace this wholeheartedly."

The four big music companies control rights to more than 70 per cent of the world's music, and Jobs blames their insistence on copy protection for the current fragmented online music download industry. He also blames their licensing conditions for Apple's refusal to licence its FairPlay digital rights management to competitors, due to the risk of details about the format leaking.

"A DRM system employs secrets... The problem, of course, is that there are many smart people in the world, some with a lot of time on their hands, who love to discover such secrets and publish a way for everyone to get free (and stolen) music," Jobs wrote.

"A key provision of our agreements with the music companies is that if our DRM system is compromised and their music becomes playable on unauthorized devices, we have only a small number of weeks to fix the problem or they can withdraw their entire music catalog from our iTunes store... It is near impossible if multiple companies control separate pieces of the puzzle, and all of them must quickly act in concert to repair the damage from a leak."

Such smart people include Norwegian reverse engineering specialist Jon Lech Johansen - aka DVD Jon - best known for cracking the encryption on DVDs at the tender age of 15. Last year his company DoubleTwist Ventures managed to reverse-engineered FairPlay. The result was not to strip copy protection from files, but to let people play content from sources other than iTunes on an iPod.{moscomment}