Columnar Transposition Cipher

I have previously written about using the Search and Rescue Numerical Encryption Grid (SARNEG) to encrypt numerical portions of a message that must be sent over non-secure channels. But, how do you encrypt the words of a message, not just the numbers? 

There are many ciphers that can be used to encrypt messages. Pencil and paper ciphers -  encryption/decryption done by hand - are not particularly secure and are certainly no challenge for government agencies such as the Spetssvyaz (Служба специальной связи и информации). However, this does not mean that a simple field cipher is without use. Casual listeners, and even many local government agencies, lack the capability to break even simple ciphers. With a short message, and a low volume of message traffic, a simple cipher may be secure against all but dedicate cryptanalysis. 

Here we will look at a simple field cipher called the columnar transposition cipher.

Using a 10-letter key word, as we did with the SARNEG, we number the letters in the order in which they appear in the alphabet, beginning in the case of the key word AFTERSHOCK, with A=0 and ending with T=9. 

The next step in encrypting our message is to write out the plaintext of our message in rows beneath the key word. If a row is left unfilled at the end of the message, add random letters to that row so that it is complete.

Now extract the message by columns in numerical order, beginning with column zero (0) and ending with column nine (9): mryrogf - eshfnfx - tseeaoo … eshhnth. 

When transmitting a message by voice (such as over a radio) the ciphertext is often read in five-letter groups: mryro, gfesh, fnfxt… and so forth. However, any method of transmission that allows the receiver to copy the message accurately will work. 

To decrypt a message, you write the cipher under the key word by columns in numerical order and read the message from the grid row by row. The number of letters in your cipher message divided by the number of letters in your key word tells you how many rows you need (i.e. the length of your columns). In the case above, seventy (70) letters in the message, divided by ten (10) letters in the key word, gives us seven (7) rows. 

The columnar transposition cipher was once used as a low-level military field cipher. Although pencil and paper ciphers are no longer used by most modern military units, as they lack sufficient security for military operations, this does not however mean that pencil and paper ciphers are not secure enough to meet your personal encryption needs. 

To learn more about ciphers and different methods of encryption, I recommend the following web-sites:

Rumkin.Com Cipher Tools

Dirk Rijmenants' Cipher Machines & Cryptology  

   

In the case where you need a very secure cipher, consider the One-Time Pad.