7.2. Trithemius Cipher

The Trithemius Cipher attempts to formalize the polyalphabetic cipher process by using very predictable keys. The first letter in your plaintext will be enciphered by key letter/value of A and each subsequent letter in the plaintext is enciphered using the subsequent letters that follow A. For example, to encipher the plaintext message do not press this button you would start by writing:

keystream: ABCDEFGHIJKLMNOPQRST
plaintext: donotpressthisbutton

Then you would use the tabula rectra or mathematical approach to encipher the message as seen in the previous section.

Variations of the Trithemius Cipher

Since the Trithemius cipher as described above would be used the same way every time, it’s not a secure cipher. Once you know a message was enciphered with the Trithemius method, you know exactly how to decipher it. This goes against one of the components that makes an encryption scheme valid, The method need not be secret to guarantee security. To make this cipher valid, there needs to be some variation possible.

Offset Values

Instead of starting at A, the keystream could start at any of the 26 characters of the alphabet

keystream: HIJKLMNOPQRSTUVWXYZA
plaintext: donotpressthisbutton

Step Value

Instead of stepping by one letter in the alphabet when creating the keystream, you could step by 2, 3, 4, etc.

keystream: ADGJMPSVYBEHKMQTWZCF
plaintext: donotpressthisbutton