The Columnar cipher is a transposition cipher that rearranges the characters of a message by writing them in a grid format and then reading them column by column in a specific order. It does not change the actual characters but alters their order to create the ciphertext.
Here's how the Columnar cipher works:
- Choose a keyword or passphrase that will determine the order in which the columns are read.
- Write the plaintext message in rows underneath the keyword, filling up the grid.
- Read the columns of the grid in the order specified by the keyword.
- Concatenate the characters from each column to form the ciphertext.
Let's illustrate this process with an example:
Plaintext: HELLO COLUMNAR CIPHER Keyword: CODE
Step 1: Prepare the grid based on the keyword CODE.
C O D E
H E L L
O C O
L U M N
A R C I
P H E R
Step 2: Read the columns in the order determined by the keyword CODE.
C O D E
L O C A
E U M P
L R H E
L I R
O P
Step 3: Concatenate the characters column-wise to obtain the ciphertext.
CIPHERCOLURMNELO
So, the plaintext HELLO COLUMNAR CIPHER encrypted with the Columnar cipher using the keyword CODE yields the ciphertext CIPHERCOLURMNELO.
To decrypt the ciphertext, you follow the same steps in reverse:
- Use the keyword to determine the column order.
- Write the ciphertext characters in the grid column by column.
- Read the rows of the grid in the original order to obtain the plaintext.
The Columnar cipher relies on the secrecy of the keyword for its security. It is a form of transposition cipher and does not substitute or change the characters themselves. Instead, it rearranges the order of the characters to achieve encryption.