Another "paper and pen" cipher that was published a few years ago is ElsieFour (a modified version of RC4)[1]. One of the nice things is that it even offers a

Another "paper and pen" cipher that was published a few years ago is ElsieFour (a modified version of RC4)[1]. One of the nice things is that it even offers a form of authenticated encryption, though it is a very string-and-staples approach. The idea is that you would append every message with some secret text that the recipient would know to look for (if the ciphertext is modified with ElsieFour/RC4 all subsequent characters become garbage). In theory you would include the hash of the plaintext but humans aren't great at computing hash algorithms in their head. Obviously these kinds of ciphers have lots of serious problems (awful timing attacks for instance :P) but I think they are a key example we can point to when arguing that banning (or restricting) encryption software is a fruitless measure to stop "the bad guys" from using encryption. [1]: https://eprint.iacr.org/2017/339.pdf https://eprint.iacr.org/2017/339.pdf