# Is there any strong enough pen-and-paper or mind cipher?

Some ciphers are talked about here, but 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?

-
pan $\mapsto$ pen $\:$ –  Ricky Demer May 4 '13 at 21:52
So basically you want to know how strong RC4 or Solitaire is? –  rath May 4 '13 at 22:24
RC4 doesn't have good reputation, though I don't know if it's completely broken yet. According to answer, Solitaire seems to be a weaker version of RC4. –  Smit Johnth May 5 '13 at 7:55
I wonder how long it would take to do AES-256 with pen and paper? :-) –  Roland Smith May 5 '13 at 12:41
I think that the suggestion to use RC4-52 in this answer can be made reasonably secure, and practicable with a deck of card by a trained operator. Devil is in the details, in particular the key and nounce/salt setup. I add that much less than 52 symbols should be used for plain and ciphertext, and keystream outside that range should be discarded (I conjecture it strengthen the keystream generator significantly). –  fgrieu May 6 '13 at 10:54
show 1 more comment

None of them are both strong enough and practical enough to be reasonable to carry out in real life. The strong ones aren't really practical; the practical ones aren't very strong.

-

The only serious cypher you can do with pen and paper is One Time Pad - https://en.wikipedia.org/wiki/One-time_pad

-
IMHO it's not possible to remember the keys. –  Smit Johnth May 5 '13 at 8:18
The OTP is a perfectly strong encryption system, simple enough to be performed with pen and paper, but is neither a cipher by some theoretical definitions thereof; and is not practical, for the pad is hard to generate, impossible to remember, and hard to conceal. –  fgrieu May 5 '13 at 14:24
It can be generated with a computer, but it will be hard to remember it. –  Smit Johnth May 13 '13 at 22:34
+1. As the question didn't mandate you had to remember the key in your head; and 'cipher' terminology excluding one-time-pads is splitting hairs for this question. –  LateralFractal Oct 18 '13 at 0:20

"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.

-