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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?

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pan $\mapsto$ pen $\:$ – Ricky Demer May 4 at 21:52
So basically you want to know how strong RC4 or Solitaire is? – rath May 4 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 at 7:55
I wonder how long it would take to do AES-256 with pen and paper? :-) – Roland Smith May 5 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 at 10:54

2 Answers

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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.

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

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IMHO it's not possible to remember the keys. – Smit Johnth May 5 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 at 14:24
It can be generated with a computer, but it will be hard to remember it. – Smit Johnth May 13 at 22:34

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