For The Love of Ciphers: Fractionating and Transposition Ciphers — The Delastelle Cipher

Didn’t you just love sending secret messages as a child … if not perhaps don’t read on? If so, let’s look at an interesting cipher created by Felix Delastelle.

In Morse Code we have the problem of coding with varying lengths of encoded characters (the dots and dashes), where we must put a space (or pause) in-between. To overcome this we can use a fractionating cipher, where we convert our encoded characters into three character sequences. For example “Hello World” in Morse Code becomes:

.... . .-.. .-.. --- /      .-- --- .-. .-.. -..
H    E   L    L   O  SPACE    W  O   R   L    D

We can then make this into a string with an ‘x’ between characters:

Plain text:    H    e l    l    o    w   o   r   l    d
Morse string:  ....x.x.-..x.-..x---xx.--x---x.-.x.-..x-..

We can now use three-character mappings to convert them back to text:

['...', '..-', '..x', '.-.', '.--', '.-x', '.x.', '.x-', '.xx', 
'-..', '-.-', '-.x', '--.', '---', '--x', '-x.', '-x-','-xx', 
'x..', 'x.-', 'x.x', 'x-.', 'x--', 'x-x', 'xx.', 'xx-']

This mapping is:

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
. . . . . . . . . - - - - - - - - - x x x…