Hackers are working to unlock Apple's iPhone, but the job appears to be more difficult than initially expected.

Hackers had hoped that modifying the iPhone's bootloader -- the program that runs before the operating system is loaded -- would unlock the handset. But that turned out to be a dead end, as the bootloader code must be signed using a 1,024-bit RSA private key employed by Apple, according to an update released last week by hackers working together on the #iphone IRC (Internet Relay Chat) channel.

Now, the hackers on #iphone are working to create an assembler utility for the iPhone's processor. "This is our last major hurdle to overcome in order to write programs for the phone," they said last week.

Once hackers have the ability to write programs for the iPhone, they can write one to unlock the handset, they said.

The iPhone is locked for use with AT&T's EDGE network. Unlocking the iPhone will allow the handsets to be used with other operators, including carriers in Asia and Europe, where the phone is not yet available.

Efforts to unlock the iPhone began almost as soon as the phone hit store shelves on June 29. Within days, hackers managed to circumvent the phone's activation function, allowing users to access many of the phone's features, except its cellular phone function.

The first to release an iPhone activation tool was Jon Lech Johansen -- better known as DVD Jon, a hacker who helped develop a tool for decrypting the content on DVDs -- who released a tool on July 3. Within 20 minutes of that release, a second activation tool was released by the hackers on #iphone.

While hackers have yet to unlock the iPhone, they have made other breakthroughs since overcoming the activation hurdle. For instance, hackers last week released a tool that allows users to install custom ringtones on the iPhone.