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).

  • $\begingroup$ What is the aim? Do you want to use one? Note that the linked question asked mentally not by hand? There is a lot difference in these two. $\endgroup$ – kelalaka Nov 1 '20 at 14:12
  • $\begingroup$ yes, the linked question asked mentally. But I look for a pen and paper cipher $\endgroup$ – TheDummy Nov 2 '20 at 8:05
  • $\begingroup$ crypto.stackexchange.com/q/63259/18298 $\endgroup$ – kelalaka Nov 2 '20 at 8:08

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.

  • 1
    $\begingroup$ I think the OTP with a book as pad is an good idea. The Beale-cipher shows how hard it is to break this system. I think the Solitaire-Cipher and the OTP are the the most secure hand ciphers. $\endgroup$ – TheDummy Nov 2 '20 at 8:13
  • $\begingroup$ @TheDummy: A semantic caveat is that the OTP is not a cipher by the modern definition of that, which requires a key re-usable for many messages. $\endgroup$ – fgrieu Nov 2 '20 at 8:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.