From 8dec3b6817fa1c7558fd425d07215f6502b21956 Mon Sep 17 00:00:00 2001 From: Mirek Kratochvil Date: Tue, 26 Sep 2017 22:48:15 +0200 Subject: [PATCH] improve the how-to a bit with pics --- README.md | 110 +++++++++++++++++++++++++++++++++++++++++------------- 1 file changed, 85 insertions(+), 25 deletions(-) diff --git a/README.md b/README.md index 9333fb4..53f96c9 100644 --- a/README.md +++ b/README.md @@ -1,15 +1,18 @@ # LS47 hand-computable cipher -This is a slight improvement of the ElsieFour cipher as described by Alan Kaminsky [1]. We use 7x7 characters instead of previous (barely fitting) 6x6 to be able to encrypt some structured information and add a simple key-expansion algorithm. Similar security considerations hold. +This is a slight improvement of the ElsieFour cipher as described by Alan +Kaminsky [1]. We use 7x7 characters instead of original (barely fitting) 6x6, +to be able to encrypt some structured information. We also describe a simple +key-expansion algorithm, because remembering passwords is popular. Similar +security considerations as with ElsieFour hold. -There's 3D-printable SCAD model of the whole thing. +There's also a 3D-printable SCAD model of the whole thing. Yay! ### Character board -We have added some real punctuation, basic stuff for writing a bit of markdown, -and quotes&parens for writing structured information. The letters of the board -now look like this: +We have added some real punctuation, basic stuff for writing expressions, +punctuation and quotes. The letters of the board now look like this: ``` _ a b c d e f @@ -21,26 +24,29 @@ u v w x y z . / : ? ! ' ( ) ``` -Zoomed in, it's nice to have extra position information written on the tiles: +Zoomed in, it's very practical to have extra position information written on +the tiles: ``` -/-----\ /-----\ /-----\ /-----\ /-----\ -| | | | | | | | | | +/-----\ /-----\ /-----\ /-----\ /-----\ +| | | | | | | | | | | _ 0| | a 1| | b 2| | c 3| | d 4| ... -| 0 | | 0 | | 0 | | 0 | | 0 | -\-----/ \-----/ \-----/ \-----/ \-----/ +| 0 | | 0 | | 0 | | 0 | | 0 | +\-----/ \-----/ \-----/ \-----/ \-----/ -/-----\ /-----\ -| | | | +/-----\ /-----\ +| | | | | g 0| | h 1| ... -| 1 | | 1 | -\-----/ \-----/ +| 1 | | 1 | +\-----/ \-----/ . . . . . . ``` -You also need some kind of a marker (e.g. a small shiny stone, bolt nut or similar kinds of well-shaped trash). +To run (hand-run) the encryption/decryption, you will also need some kind of a +marker (e.g. a small shiny stone, bolt nut or similar kind of well-shaped +trash). ## How-To @@ -58,6 +64,58 @@ You may as well see the paper [1], there are also pictures. This is somewhat mor 8. Update the position of the marker: `M := M + C' mod (7,7)` where `C'` are the numbers written on the ciphertext tile. 9. Repeat from 3 as many times as needed to encrypt the whole plaintext. +#### Encryption example with ascii images! + +``` +1,2. Symmetric key with 3,4. we want to encrypt 'y'. + marker put on 'e' Look at the marker: + + [e]f _ a b c d /-----\ + l m g h i j k | | + ( ) / : ? ! ' | e 5| + s t n o p q r | 0 | + z . u v w x y \-----/ + 5 6 0 1 2 3 4 + + * 7 8 9 , - + +5. Ciphertext is 'w' 6. Rotate the plaintext 1 position + (='y' moved by (5,0)) right, possibly carrying the marker. + + [e]f _ a b c d [e]f _ a b c d + Output 'w'! l m g h i j k l m g h i j k + ( ) / : ? ! ' ( ) / : ? ! ' + s t n o p q r s t n o p q r + z . u v w x y >> y z . u v w x + 5 6 0 1 2 3 4 5 6 0 1 2 3 4 + + * 7 8 9 , - + * 7 8 9 , - + + +7. Rotate the ciphertext 1 Now look at the ciphertext tile. + position down, also carry. + + [e]f _ a b , d /-----\ + l m g h i c k | | + ( ) / : ? j ' | w 2| + s t n o p ! r | 3 | + y z . u v q x \-----/ + 5 6 0 1 2 w 4 + + * 7 8 9 3 - + +8. Update the marker position 9. GOTO 3. + by ciphertext offset (2,3). + + + e f _ a i c d + l m g h ? j k + ( ) / : p ! ' + s t[n]o v q r + y z . u 2 w x + 5 6 0 1 9 3 4 + + * 7 8 b , - + + +``` + ### Decryption Decryption procedure is basically the same, except that in step 5 you know `C` @@ -99,13 +157,13 @@ The actual expansion can be as simple as this: ### Undistinguishable ciphertexts -To get a different ciphertext even if the same plaintext is encrypted; prepend -it with a nonce. A nonce is a completely random sequence of letters of a -pre-negotiated length (at least 10 tiles drawn randomly from a bag is -adviseable). +To get a different ciphertext even if the same plaintext is encrypted +repeatedly; prepend it with a nonce. A nonce is a completely random sequence of +letters of a pre-negotiated length (e.g. N tiles drawn randomly from a bag, +adviseable value of N is at least 10). You may also want to add a random number of spaces to the end of the ciphertext --- it prevents the enemy from seeing the difference between ciphertexts for +-- it prevents the enemy from seeing the difference between ciphertexts of 'yes please' and 'no', which would otherwise encrypt to easily measurable gibberish like `qwc3w_cs'(` and `+v`. @@ -113,13 +171,15 @@ gibberish like `qwc3w_cs'(` and `+v`. Because ciphertext may be altered in the transfer or during the error-prone human processing, it is advised to append a simple "signature" to the end of -the message; e.g. a simple string `__YourHonorableNameHere`. If the signature -doesn't match expectations (which happens with overwhelming probability if -there was any error in the process), you discard the message and ask the sender -to re-transmit. +the message; which may look as simple as `__YourHonorableNameHere`. If the +signature doesn't match expectations (which happens with overwhelming +probability if there was any error in the process), either try again to see if +you didn't make a mistake, or discard the message and ask the sender to +re-transmit. This works because the cipher output is message-dependent: Having a wrong bit -somewhere in the middle causes avalanche effect and breaks the signature. +somewhere in the middle causes avalanche effect and erases any meaning from the +text after several characters. ## References