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improve the how-to a bit with pics

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README.md
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@ -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