Some ciphers are talked about at “Is there a secure cryptosystem that can be performed mentally?”, but (at the time of writing) I don't see an answer.
Are they strong enough, or are non-computer ciphers more or less just a toy and one should abandon using them for practical purposes?
"Strong enough" is a broad term. Some things that you need to keep in mind are entropy size and cryptanalysis. "Strong ciphers" are ciphers that have shown to have enough entropy to withstand practical attacks over time from public scrutiny.
With that said, the Solitaire cipher has a keyspace of roughly 238 bits. By comparison, many SSL keys on the internet are 128 bit AES. Distributed.net is currently working on cracking a 72 bit key, via brute force, at a pace of about 300 billion keys per second, and they have well over 100 years before the keyspace is fully exhausted.
So, for the Solitaire cipher to not be taken seriously, it needs to show practical weaknesses outside of brute force searching. So far, the only weakness that has been demonstrated is that the output has a bias of 22.5:1 rather than 26:1 pure random output would have. This isn't severe enough to mount a practical attack.
As such, until other attacks are made known, to Solitaire cipher is a "strong" hand cipher, that doesn't have NSA influence, can be used without incriminating tools and is easy to learn and remember.
One Time Pad. It has some serious drawbacks of practicality, but most of these are only really drawbacks in a modern digital world. If your use case is short handwritten messages between two people, the drawbacks are likely not a concern.
Update: this topic is a bit of an obsession of mine, and I recently published a tool called Crosshairs Cryptography to make pen-and-paper One-Time Pad as practical and ergonomic as possible. You can download and print the PDF via one of the following links.
Google Drive link: https://drive.google.com/file/d/1jr2lv9s6ZoUfCvJ5UsORjwdNfvOi2bJL/view?usp=sharing
You can implement a linear feedback shift register using nothing but a series of coins. Define 1 to be heads, 0 for tails. Line them up in to a series 128 coins in length. Then follow the algorithm exactly as you would on a computer.
From this basic generator, you can construct a self-shrunk generator. Self-shrinking generators have somewhat suspect security. You'd probably be fine for a pencil and paper scheme because you can't generate enough ciphertext to run many of the attacks.
The problems are that it is very laborious to encrypt even a small message. Worse, a mistake anywhere in the process will probably completely destroy the security of the scheme.
I have to agree with @D.W. here. This is probably among the better suggestions for a pencil and paper cipher and yet it still poor.
Secure cryptography just requires too much (tedious!) work to be reliably done by hand.
Try to solve :-
9iAbmOW SK6ba4Q zLmWa KrĪZLqb 3mNpR 9axFTOIu 6x b# KrNq41 bkB4A 4SbkQ cXGq5aB3Hab9u 4K9OQJnG- !K2 nNmRJW8nM 418]K2RLs ZG477siG Hr{ĪuĪ MJcR88n k4 X7bAJi6P4bB 9JKi2 Q RMB€4@J∅ mm9Zd∅O4h, 6HLz48 *ĪSFqS1 3mL.. OiKzHV2 8w3ss {#agSQR41ZnL
Ī:- capital i,
∅:- Zero
This is my first cipher, My first time on this website. I am no expert, just a student who has very simple knowledge about ciphers. It took me 30 minutes using a '*pen and paper * to come up with the "algorithm" for the above cipher, I think it is very, VERY strong, but is not practical enough, as it will take you 7-8 minutes to write just a 15-20 word message. But if we program the "Algorithm", i think it can be a great, as computer will take no time to encrypt- decrypt messages. And according to me the cipher is very tough to crack, well i dont really know how tough it is compared to others, as i dont know how difficult ciphers can be.
To answer your question Practically speaking, you cannot solve my cipher, if you put all your energy to it, a guy like me would take atleast a month to crack it. And i consider this cipher as not my best work, i can make it atleast 10 times more difficult.
*Do not try to solve it.
