[rayiner]

This is the UK not the US. They don't have a 5th amendment.

In the US courts are split on the issue. Some say giving an encryption key is testifying, an act of the mind, and you can't force someone to testify against themselves under the 5th. Others say its like handing over a regular key, which you can be forced to do, because the 5th covers testimony, not everything incriminating. It was intended to prevent forced confessions.

Once you're in front of a court you don't get to keep secrets, with the exception of some narrow protections. This has always been the case in the Anglo-American system.

[IsThisObvious]

The debate is partly this: forcing you to produce an encryption key /also/ testifies that the drive belongs to you (or you had access to it).

You are not necessarily obligated to testify to that fact for the police, and unless they can demonstrate that the drive belonged to you (or you had access) through some other means, the production of an encryption key is tantamount to forcing that confession.

It's much like if there were a lock on a gun they found on the street: if they can't link the gun to you already, they can't demand you turn over the combination for the lock, because knowing such a combination would be a tacit admission to knowing about the gun.

[just2n]

I'd think it's also an issue of the 4th. Consider sharing a machine with different user accounts. Asking me to decrypt a drive just might be an unreasonable search, because outside of logical partitioning, there's no concept of "a user's files" when the police just use a disk copy tool to pull all files off the disk.