Encryption by Hand?

Arachn1d writes "A question for all those slashdot math-geeks out there: What's the simplest, but most secure encryption algorithm you can devise or provide a link to that can be carried out with nothing but a pen, some paper and a calculator? Bonus points for any public-key cryptography solutions!" Bruce Schneier developed an encryption algorithm designed to be performed with a deck of cards, but it's rather slow to do for fun. Well, you did say "a calculator", and if we assume a programmable calculator your options probably expand quite a bit...

Comments

Knapsack

[heliocentric]

I think Knapsack is a simple system to do by "hand" if you know modular arith / number theory (to calculate the inverse modular operation). Plus it meets your public key "bonus."

It's not the most unbreakable code in the world, but better than ROT13 or even poly alphebetic cyphers (think Kasiski for breaking ploy's).

do-it-yourself one time pad

[chongo]

For a non-public key stream cipher:

If you allow the addition of dice, say a d20 ...

Setup by the sender:

1. Generate a one-time pad by rolling the d20 and writing down the 1's digits (d20 face value mod 10).
2. Transmit the one-time pad in a secure fashion (use somebody's public key suggestion, hand carry, etc.)

Setup by the receiver:

1. Receive the one-time pad from the sender.
2. Store in a secure place.

To encrypt:

1. Convert each plaintext symbol into an alphabet of 100 values (00 thru 99).
2. For each plaintext digit, remove a digit from the one-time pad and transmit the sum mod 10.
3. Destroy the used digits of the one-time pad.

To decrypt:

1. Receive the cipher text from the sender.
2. For each digit in the cipher, remove the next digit from the one-time pad and subtract mod 10, from the cipher digit.
3. Convert the result, pairwise, (00 thru 99 alphabet) into plaintext symbols.
4. Destroy the used digits of the one-time pad.

Encrypt example:

1. Plaintext: Hello
2. One-time: 9690367034
3. Alphabet: 0730373740
4. Transmit: 9320630774

Decrypt example:

1. Receive Ciphertext: 9320630774
2. Receive One-time: 9690367034
3. Subtract mod 10: 0730373740
4. Convert to text: Hello

And yes, the devil in the details is in the secure transmission of the one-time pad (step 2 of sender setup). Key management is never easy. Storage and transmission of one-time pads is one of the reason why this form of crypto is not a realistic choice for most applications. However if you have some way to transmit the one-time pad ahead of time, say visiting the sender ahead of time and dropping off the one-time pad it is not a bad choice.

Do it the Russian way...

[Kirruth]

To avoid the embarassment of being caught with code books, an old method was to take an obscure out of print book, then refer to a letter by page, paragraph, word, then position in the word. The trick is not to repeat a reference, and to change the book you use frequently. Russian spies in England in the 60s (for example the "Lonsdales") used this trick. If you control the channel the message is sent by (for example, dead-letter drops), and if you use other codes in the source (for example, code names for contacts), you can make your own cold-war communications system.