TonyStill, that is the problem!
The law is essentially guilty until proven innocent. (This is why it isn't a good law.) The file may not even be encrypted - it may be random garbage, such as you might get, say, after a disk recovery.
They can pick a file and demand that you decrypt it - if you can't, you are guilty, unless you can prove that it doesn't contain encrypted data. Good luck with that.
Regarding 'undetectability' - OK, some may be detectable. If you have two pictures, one with and one without your message, that is a giveaway, for example. If you've ever edited a JPEG in some non-obvious manner they might pick this up as a false-positive.
IIRC a Register article described an analysis which suggested that 25% of pictures on the internet may contain steganographic data. Doesn't reassure me.
Maths teacher wanted for top RISC OS-powered school for the deaf RiscPC-using Knightsfield School, in Welwyn Garden City, Herts, has been described as "very well resourced and well equipped" for teaching IT to deaf youngsters, according to standards inspectors. The school is also on the look out for a new maths teacher and has invited RISC OS-aware tutors to apply for the position. Discuss this. Published: 22 Mar 2009