The most secure hand cipher in 2020

Question

I found this thread from 2011: Is there a secure cryptosystem that can be performed mentally?

Are there new secure hand or pen-and-paper ciphers in 2020? (I know, that hand ciphers are not as secured as digital ciphers).

Answer 1

Are there new secure hand or pen-and-paper ciphers in 2020?

Not that I know for the modern definition of "cipher", which implies a short key reusable for multiple messages. It's difficult to make such cipher convenient and secure.

The closest match in the open literature may be Bruce Schneier's Solitaire, aka Pontifex. It's a stream cipher (barely) doable by hand with a deck of cards, but theoretically broken in the modern meaning of that: a sizable bias in the keystream was found by Paul Crowley (circa 1999). And there's further Analysis of Solitaire by Daniel Shiu, arXiv (2019). Yet according to Schneier's assessment of Solitaire in light of that:

for the short message lengths you’re likely to use a manual cipher for, it’s still secure and will likely remain secure.

Note: Shiu has outlined proposed modifications to improve Solitaire, but they make it even less practical.

If we relax what a cipher is, we can use the One Time Pad: it is very doable by hand and remains secure as long as the pad remains confidential and not reused. If the encryption is error-free and the plaintext natural language, humans doing decryption can manage to recover from their own errors or small alterations in ciphertext. Tamper-evident opaque envelopes provides some protection of the pad against xerox copy during transfer. Some paper allows dissolving used pages in hot water.

Problem is that carrying a sheet of paper with random characters, such as the pad, could get one a bullet in the head, or worse. So we might want to use an ordinary-looking book as a substitute. Say, the starting page number, line, and offset are chosen at random with the additional condition that no material in the book gets reused, and is coded in the first characters. Alphabetical characters starting from there are grouped by blocks of 5, only the last character in such block is used, and the sum modulo 26 of 6 such characters yields the pad character, near uniformly random and independent of others. Nowadays it's economical to print a single copy of a book, and plausible randomized variants of an existing novel can be AI-generated, so that computer-assisted cryptanalysis will fail even if it tries the original as the pad, and the printing will pass casual scrutiny by an observer. Legal draft have an advantage over novels: adversaries fall asleep.