The government faces criticism over plans to give police powers to make suspects produce readable copies of encrypted computer evidence.
Police say more suspects are encrypting files
The police say the powers are needed because criminals are increasingly using encryption to hide evidence.
They estimate that currently there are 30 cases in which encrypted evidence had stumped investigators.
But some peers, academics and cryptographers say the plans are flawed and risk being abused.
The plans to let police demand decryption are part of the controversial Regulation of Investigatory Powers Act (RIPA) that came into force in 2000.
Part III of RIPA gives law enforcement agencies the decryption powers and, provided some conditions are met, makes it a serious offence to refuse to turn scrambled files into an "intelligible" form. Those refusing could see their sentence increased as a result.
The government is holding a consultation exercise on the code of conduct that those using these powers will have to abide by.
The code was debated at a public meeting organised by digital rights group the Foundation for Information Policy Research (FIPR).
At the meeting a government spokesman, Simon Watkin of the Home Office, defended the plans saying there were limits on when powers could be invoked.
He said they would only be used where there was other evidence or intelligence against a suspect.
He said the police were increasingly encountering encrypted material in serious cases involving paedophiles and terrorists.
DCI Matt Sarti, a senior officer in the Metropolitan Police's child abuse investigation team, said he knew of cases where suspected paedophiles had walked free because encrypted files on computers seized during raids could not be unscrambled.
"We have to balance the right to private life with the right to private life of victims and the right to life of victims," he said.
"But the draft code of conduct has no guidance on weighing privacy against the demands of law enforcement," said Caspar Bowden, former head of FIPR.
He questioned how police could balance the rights of victims, suspects and the general public if this was not made explicit.
Mr Bowden also questioned the wisdom of making it an offence to refuse to unscramble evidence. He said there were many scenarios that made it possible for a suspect to deny they ever had the key that unlocked encrypted data.
Already, he said, there had been one court case in which a suspect was acquitted after claiming a computer virus under someone else's control had caused the offences for which he faced trial. Mr Bowden speculated that other suspects could use the same tactic or would fake a virus infection to get themselves off the hook.
He also asked how someone would prove they had genuinely lost or forgotten a password and wondered if the threat of a jail sentence would hamper efforts to make users take more care of personal data.
Some fear the powers will stop people taking care with data
"Will it deter the mass of honest users from properly securing their data?" said Mr Bowden.
Veteran investigative journalist Duncan Campbell said there were broader questions about how the police investigate high profile cases that threw into question the effectiveness of the decryption powers.
He said his work as an expert witness in cases involving charges against suspected terrorists and paedophiles led him to question what use the powers would be or imagine any circumstances in which it would prove useful.
The Earl of Erroll, a cross-bench member of the House of Lords, said there was a real danger of "scope creep" in which the powers given for use in specific circumstances were turned to other purposes they were never intended to tackle.
Professor Douwe Korff, said there was a real question as to whether the powers undermined the presumption of innocence that human rights legislation enshrines. The code of conduct had to be beefed up, he said, to ensure high standards protected fundamental rights.
Lord Phillips of Sudbury described RIPA as a "hair-raising" piece of legislation and expressed reservations about the effect the powers being given to police would have.
"You do not secure the liberty of our country and value of our democracy by undermining them," he said. "That's the road to hell."