improve the how-to a bit with pics

1 files 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