Early encryption algorithms like these had to be applied manually by the sender and receiver. They typically consisted of simple letter rearrangement, such a transposition or substitution.

To decipher the message, recipients had to know both the algorithm and the ... Let's look at a simple example, called a columnar transposition cipher. Here, we take a message, and fill the letters ...

In the United States, information about cryptanalysis of the cipher remained classified until a few years ago. Double Transposition consists of two applications of columnar transposition to a message.