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

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.