About the Ciphers
The last two weeks we have been working on substitution ciphers (monoalphabetic and polyalphabetic). Recall that substitution ciphers are ones in which each letter is replaced by another letter (or symbol) in some systematic way. However, the order in which the letters appear stays the same. This week, we're going to work on a few transposition ciphers; ones for which the letters stay the same, but the order is all mixed up!
One of the oldest ways to do this was created by the ancient
Egyptians and Greeks. It uses a stick called scytale
They would have used wooden sticks and parchment,
but we're going to use poster tubes and adding machine tape!
How the scytale cipher works
How the scytale cipher works
This technique was very useful in ancient battles; the Spartans are known to have used this rather extensively. Each general was given a stick of uniform diameter so that he could quickly encipher and decipher any message sent from other generals. Notice how quick and easy this is to use!
However, it is also rather easy to crack. In a battle situation, the most likely way to crack this would be to steal a general's scytale. Then, each message could be read easily. However, it can be cracked even without stooping to theivery. As it ends up, the scytale is just a very old (and rather simple) version of a greater class of ciphers called
matrix transposition ciphers.
The way the simplest of these works is by picking a matrix of a
fixed size (say, 6x10) and then writing your message across the rows.
The encipherment step consists of writing down the letters in the matrix by following the columns. Here's a simple 6x10 example:
Where we've written the message:
row by row into the matrix. Then, to encipher this, we simply read off the columns to get:
The scytale cipher is just like one of these. Note that the number of "rows" in your message is determined by the diameter of your stick and the size of your writing. Cracking them, as you may guess, is just a matter of systematic guess-and-check.
How to crack the simple matrix transposition ciphers:
A harder version of the matrix transposition cipher is the column-scrambled matrix transposition cipher. Just like the ones above, you find a matrix of suitable dimensions and write your text in row-by-row. If there are blank cells left, fill them in with a dummy character (sometimes an 'X'). However, before writing down the ciphertext from the columns, you first scramble the columns. This generates a new matrix of the same size. Now read off the text down the columns, as before. This is a harder cipher, but there is a systematic way to crack it.
How to crack the column-scrambled matrix transposition ciphers:
NNRTA NNFTO IONEL IEKSD MRTLE LRTNE EGRTA NTAEI HXOIO
EMOIO DMRTI HLDHR SNEEG UHLEG IHNNB GMAND NBTX
After you have tried the examples above, try the ciphers on the challenge sheet.