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Add card mapping description to the README

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Trevor Xiao 2021-04-21 19:34:14 -04:00
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@ -63,6 +63,65 @@ 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).
#### Alternative playing-card-compatible board
The following board can be used so that characters and offsets can be easily
mapped to playing cards, similar to the Solitaire cipher [2]. This could be
more innocuous and easily explainable to the secret police than a set of
peculiar numbered tiles.
```
0 a b c d e f
g h i j k l m
n o p q r s t
u v w x y z .
, - + * / : ?
! ' ( ) _ 1 2
3 4 5 6 7 8 9
```
With this layout, the following mapping to playing cards is used:
| Character | Card | Index | Character | Card | Index | Character | Card | Index | Character | Card | Index |
|-----------|------|-------|-----------|------|-------|-----------|------|-------|-----------|------|-------|
| a | A♦ | 1 | n | A♣ | 14 | . | A♥ | 27 | 1 | A♠ | 40 |
| b | 2♦ | 2 | o | 2♣ | 15 | , | 2♥ | 28 | 2 | 2♠ | 41 |
| c | 3♦ | 3 | p | 3♣ | 16 | - | 3♥ | 29 | 3 | 3♠ | 42 |
| ... | | | ... | | | ... | | | ... | | |
| j | 10♦ | 10 | w | 10♣ | 23 | ' | 10♥ | 39 | 7 | 7♠ | 46 |
| k | J♦ | 11 | x | j♣ | 24 | ( | j♥ | 39 | 8 | 8♠ | 47 |
| l | Q♦ | 12 | y | Q♣ | 25 | ) | Q♥ | 39 | 9 | 9♠ | 48 |
| m | K♦ | 13 | z | K♣ | 26 | _ | K♥ | 39 | 0 | 10♠ | 49 (0) |
The (x, y) offset is calculated from the index with the usual modular
arithmetic: `x = index % 7` and `y = index / 7`.
Note that the offset can be easily calculated from a given card: take the
order of the suit (0-3), multiply by 13, and add the card number (11-13 for
face cards, 1 for the Ace) to get the index. The offset can then be calculated
from the index.
Alphanumeric characters can be easily mapped to cards as well, with letters
mapping to Diamonds or Clubs, and numbers mapping to Spades (and optionally
a Joker). All of the special characters are mapped to the Hearts, though
coming up with a mnemonic for the mapping of each special character to each
Heart is left as an exercise for the reader.
For LC4, the following board could be used:
```
# a b c d e
f g h i j k
l m n o p q
r s t u v w
x y z _ 2 3
4 5 6 7 8 9
```
The "Hearts" suit would be ommitted from the card mappin table. `#` and `_`
would map to the Joker/10♠ and A♠, respectively.
## How-To
You may as well see the paper [1], there are also pictures. This is somewhat more concentrated:
@ -199,3 +258,4 @@ text after several characters.
## References
[1] *Kaminsky, Alan. "ElsieFour: A Low-Tech Authenticated Encryption Algorithm For Human-to-Human Communication." IACR Cryptology ePrint Archive 2017 (2017): 339.*
[2] *Schneier, Bruce. ["The Solitaire Encryption Algorith"](https://www.schneier.com/academic/solitaire/).