US Federal attorneys have agreed to a plea deal that would let them view encrypted content on the laptop of a convicted murderer and sex offender.

Joseph Edward Duncan, a former computer science student, has confessed to killing three people in northern Idaho and abducting and sexually assaulting two children.

Associated Press reported that Duncan had kept an encrypted journal on his laptop that is believed to contain evidence relevant to the case.

The computer had spent over a year at the FBI's headquarters, but operatives had been unable to decrypt the data.

Under the plea bargain, Duncan has agreed to release the encryption key to his defence attorney and to plead guilty to three counts of murder and two counts of kidnapping. He will be eligible for the death penalty.

Two days before the murders took place, Duncan wrote in his blog: "I figure in 30 years or more we will have the technology to easily crack the encryption (currently very un-crackable PGP) and then the world will know who I really was, and what I really did, and what I really thought."

PGP, or Pretty Good Privacy, is an encryption program first introduced in 1991 and said to be virtually impossible to crack through conventional means.

For several years the program was classified as "munitions" by the US government and was illegal to export.

The use of encryption programs, and the ability of law enforcement officials to obtain decryption keys, has become an issue as more and more criminals use PGP to cover their tracks.

UK officials amended an existing law in May to allow police to seize encryption keys.