the prosecutors must prove to a very high standard that you do actually know the key, and haven't really forgotten. For instance if you typed in the password the day before you were arrested, that's probably a good sign you know it.
I've had a few occasions where, after 2+ months of using a password at least 5 times a day, I roll into work and just can't login. I still know the password I used when I was 9 and dialing into AOL, but my work password rotation policy is so strict I just don't have long term storage for them anymore. I mostly rely on muscle memory the current password too.
So, all that was to say I don't think that standard is "very high" at all.