How Do Bob and Alice and Carol Add Their Encryption Keys To Data?

When I worked as an electrical engineer, we had a hasp to isolate the electric power on a device we were working on:

With this, each person who was working on the equipment, would put on their own padlock, and where we could not put the power back on, until all the padlocks had been taken off. The padlocks could be put on in any order, and taken off in any order, but there was no way to putting the power back on, until everyone had taken their padlock off.

So how could we do this with data. Let’s say that Bob, Alice and Carol want to apply their “data hasp”, so that the data cannot be revealed until they have all taken off their padlock. Well, with symmetric key block ciphers, such as AES, we cannot do this, as we must decrypt in the reverse order of they keys being applied:

To encrypt: Bob → Alice → Carol … and then to decrypt: Carol → Alice →Bob

There are ways to do it with RSA, such as with SRA [here], but these methods significantly reduce the security of the process. The solution is to use a stream cipher, as we basically just X-OR the data when we are encrypting, and then X-OR again with the same key when we decrypt. We can apply multiple keys to the data, and in any order and it will always decrypt properly once we have applied all the keys.