European police chiefs condemn end-to-end encryption [pdf]
europol.europa.eu
europol.europa.eu
Or they could encode a PNG under a H264 frame almost invisible to the eye (1/60 frames per second) but extractable with precise time related commands with ffmpeg. No encryption, no nothing. But it's there.
Also, GPG existed since the 90's, and there are symetric encrypting tools (even simple ones) everywhere, such as age. Again, they could just use sneakernet and some USB media.
Then they could just share it in the clear, over any protocol they wanted. Once you can shift bits with ease (in a reversible way), it's game over. Split, dd, perl, cat. You get junk with ease and recover that junk with the same ease.
Xor as a pipe: https://unix.stackexchange.com/questions/398481/xor-a-file-a...
Then with split/cat you get a dumb xorg encryption in pieces but the data could be put under a sorted list in a txt file with sha512sums of each part so anyone concat(1)enates the pieces in the right order. See? Is not even an issue on having strong encryption.
Xor the text file bytes, key made from /dev/urandom and base64. Use split to get 20 pieces.. Run sha512sum on all the pieces, pipe it to a text file.
Rename all the files to a random pattern with vidir+vim or dired under Emacs.
Now you have total junk files and nothing illegal. Still, reversing that up it's trivial by renaming the files matching the sha512sum (easy with AWK), and with cat you will get the illegal file back.
If any, you can add (with dd) a fake GB ROM or NES ROM header and rename them as "test ROMs". With dd you recover the files back like nothing, it has options to skip an amount of bytes while copying data.
And, technically, we didn't use dedicated encryption tools, at all.
[0]: https://news.ycombinator.com/item?id=40137783 [1]: https://news.ycombinator.com/item?id=39609846