Simple ciphers: Transposition ciphers

What is a transposition cipher?

Earlier in the activity, we saw the scytale cipher. This is an example of a transposition cipher. In this type of cipher, the characters in the plaintext are rearranged into a different order.

To transpose means to switch or exchange the positions of two things. In the context of a transposition cipher, transposing means rearranging the letters in a message or a word by switching their positions according to a specific pattern or rule. This makes the message harder to read and keeps it secret from anyone who doesn’t know the pattern or rule for rearranging the letters.

For example, if we have the word “CAT”, we can transpose its letters by switching the positions of the first and third letters to get “TAC”. We can also transpose the letters by reversing the order of the letters to get “TAC” again.

In a transposition cipher the rule of how we transpose the characters can be as complicated as we like.

Another example of a transposition cipher

Another example of a transposition which could be used is the following:

• Take the plaintext which you want to encrypt and write it out into a table with two rows, such that you start from the first row of the left hand side of the table and fill in the characters down the column to the second row. .
• With the first two letters now in the first cells of the two rows, you then start at the top of the next column in row 1. And so on.
• To finish the encryption, the ciphertext is produced by reading the symbols left to right across the first row, then the second row of the table.

So, to encrypt the string “university of leeds”, we would fill the table in as follows:

The ciphertext would be “uiest flesnvriyo ed “.

Try it yourself

Your turn – try and encrypt the plaintext “Transposition ciphers are fun” using the same technique as we used in the previous example.