I have posted code here before for a key system to protect an app etc.
I had to choose the static letters though.
What is true randomness and how would you reverse the process to un-encrypt stuff if it was random ?
I might be missing something but if you chose a random chunk of letters for a caeser cipher then i don't know how you'd unencrypt it again.
Google search Hitler Enigma machine too that is another example..
Like you do with one time pad. XOR the random data with the plaintext to encrypt XOR again to decrypt.[1] Since the key is (truely) random, the encrypted data is indistinguishable from randomness. IIRC its the only method that is provably uncrackable. It comes with a lot of downsides though, since you need to securly store a key the same length as the data.
[1] or using ceasar: shift by mod 13