Add card mapping description to the README

1 files changed, 60 insertions, 0 deletions

diff --git a/README.md b/README.md
index 94e33e4..d3a2a5c 100644
--- a/README.md
+++ b/README.md
@@ -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/).