Stephen Flatley told the International Conference on Cyber Security in New York on Wednesday that one example of the way that Apple had made it harder for him and his colleagues to break into the iPhone was by recently making the password guesses slower, with a change in hash iterations from 10,000 to 10,000,000.

A report on the Motherboard website said Flatley explained that this change meant that the speed at which one could brute-force passwords went from 45 attempts a second to one every 18 seconds.

"Your crack time just went from two days to two months," he was quoted as saying.

In 2016, the FBI clashed with Apple in court after the spy agency ordered the company to give it access to an iPhone belonging to one of two people involved in killing 14 people in California in December the previous year.

The FBI finally withdrew its case, having got a third party — which many suspect is the Israeli firm Cellebrite — to break into the iPhone and obtain the data sought by the FBI.

There have been differing reports about what the FBI paid for gaining access to the device, with amounts ranging from US$15,000 to US$90,000 being cited.

Flatley praised Cellebrite, which sells cracking devices and like technologies to authorities around the world.

“If you have another evil genius, Cellebrite,then maybe we can get into that front," he said.