Cryptography Part 4 - Transposition Ciphers - Security Series #16.3

In my last few posts we talked a lot about substitution ciphers, but I hinted at transposition ciphers and promised to follow up with a post, so here it is.

Transposition Ciphers

In a substitution cipher the characters in the message maintain their original position but are replaced (substituted) with another character. With transposition ciphers the position of the characters in the message actually change in their relation to other characters.

In my first post on Crypto I offered 5 ciphertext messages for you to decrypt. This one one of them.

OMTHTU UMYECN RUIBOI CNSEMT OITSMY

This is actually a message transposed to create the ciphertext. The cipher I used is called a Route Cipher, and actually, I really didn't use it to its full potential. It takes the original plaintext and reorders the words into a grid. So in this case, the original message can be read if you view this 5 blocks of characters reading down and to the left.

O M T H T U
U M Y E C N
R U I B O I
C N S E M T
O I T S M Y

Hopefully you can see that the original message is:

OUR COMMUNITY IS THE BEST COMMUNITY

Now, I said that I did not use this cipher to its fullest potential, and that is true. I laid out the cipher text in the proper order for stacking the blocks of text. I could instead have transposed again by saying that the grid is recreated from this text by reading in a spiral, counter-clockwise, starting from the upper-right and returned this ciphertext:

UTHTM OURCO ITSMY TINCE YMUNS EMOBI

This message would be harder to decrypt than the way I did it. But I was not trying to make it too difficult. I was wanted you to be able to figure it out.

There are many ways to use transposition to encrypt a message, and transposition can be used in combination with substitution to create an even stronger encryption scheme.

To see other examples of transposition ciphers, check out the Wikipedia article on the subject.

Note

Learning about substitution and transposition ciphers is fun, but we need to remember that these ciphers are weak. Modern cryptography may use elements of these things that we are discussing, which is why it is important to learn about them, but modern cryptography also uses advanced mathematics that are well beyond my ability to explain, to achieve amazing results.

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