If you’re interested in that kind of problems, here’s some pointer: https://en.m.wikipedia.org/wiki/Secure_multi-party_computation
This! The prior knowledge is even fairly small, everyone can toss in a random string + key. The only drawback is that all participants need to have synchronized rounds (one for collecting the random values, one for the decryption keys), and the whole protocol fails if someone decides not send timely their decryption key
atool, wraps many archiver in one command
That doesn’t sound like proper bircher muesli to me. Bircher is: soak oats in water, add grated apple, lemon juice, ground hazelnuts, “Kondensmilch” (like tubed, sweetened concentrated milk).
I usually do it as follows (but it’s not “Bircher”): Mix oats with soymilk, sugar and cinnamon. Sometimes I add a little joghurt to get a denser consistency. Then some fruit. Berries, grated apple or banana (in slices) work well. Mix and soak overnight. Eat cold in the morning. I usually also prep 3 portions and eat them over the next 3 days.
Throw in some chia seeds or ground flaxseed if you want to publish the recipe on a foodblog.
A similar case was google reimplementing Sun/Oracle Java APIs. Which has been deemed legal after all.