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One of the distinguishing features of a good encryption algorithm, is that it is easy to encrypt, and hard to crack. Are there any that are easy enough for average folk to remember, and calculate by hand, and still stand up to brute force attacks on a computer.

Imagine, a prisoner (with pen and paper) sending a message to another inmate, and the guards seize the handwritten message - and put their prison-crypto-cracking department on it.

Currently, I am thinking TEA is the best candidate, but pretty hard to remember I think.

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marked as duplicate by owlstead, Hendrik Brummermann Mar 23 '13 at 23:26

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A paper by Hopper and Blum discusses the possibility of carrying out by hand cryptography based on the hardness of learning parities with noise. Since their paper several other works have explored carrying out other cryptographic tasks, including symmetric-key encryption, with similar efficiency.

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The short answer is that there are no algorithms that are really feasible to do by hand and are really secure (unless maybe you are really patient and really proficient).

See What is the most secure hand cipher? and Is there a secure cryptosystem that can be performed mentally? for more details.

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