The French cryptographer Félix-Marie Delastelle was born in 1850 and died in 1902. For most of his life, he worked as a bonded warehouseman at a local port. After his retirement in 1900, he published a 50-page book named Traité Élémentaire de Cryptographie. His love of creating ciphers was basically his hobby, and he was unusual as most of the people who worked in the field at the time were either academics, diplomats, or were in the military. The Delastelle cipher (or Trifid cipher) uses substitution with transposition and fractionation.
Delastelle cipher |
Example
An example key is:
EPSDUCVWYM.ZLKXNBTFGORIJHAQ
We then make three squares from this:
square 1 square 2 square 3 1 2 3 1 2 3 1 2 3 1 E P S 1 M . Z 1 F G O 2 D U C 2 L K X 2 R I J 3 V W Y 3 N B T 3 H A Q
If we take a plain text message of "THIS IS A TEST", we locate the text in the squares defined above:
THIS IS A TEST -------------- T - 233 H - 331 I - 322 S - 113 I - 322 S - 113 A - 332 T - 233 E - 111 S - 113 T - 233
Next we would order as:
THISISATEST ----------- 23333132211 33221133322 33111113233
And we would read the code in a horizontal way to give:
233 333 321 321 311 111 331 233 232 123 123
And then substitute back the letters on the grid:
233 333 321 321 311 111 331 233 232 123 123 T Q R R F E H T B C C
Try this example: [here]