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MOTAT
Boy thinking with coloured light and code background

Operation Decode

Mission Three: Poly-Alphabetic Ciphers

Poly-alphabetic ciphers are a more complex form of shift cipher. Unlike Caesar ciphers that “shift” all the characters by a single, constant value, poly-alphabetic ciphers, instead, use a number of different shift values in the same message. This makes them much harder to crack.  

While there are many different forms of poly-alphabetic cipher, one of the best known is the Vigenère cipher. Named after 16th century cryptographer Blaise de Vigenère, this cipher uses a repeating keyword, alongside a special table called a “Tabula Recta” in order to encrypt and decrypt messages.

Activity 3

ENCRYPTING A MESSAGE

Say we wanted to encrypt the message “Meet Me Later” using the keyword “Pancake”

The sender would first write the message, then write the keyword beneath it. The keyword is repeated until there are no more letters remaining in the Message.

The letters are then enccrypted by finding the row of the Tabula Recta that begins with the current letter from the keyword. The sender then finds the table column that matches the letter from the message. The letter at the position where the row and column meet is the letter used in the cipher text. 

MESSAGEMEETMELATER
KEYWORDPANCAKEPANC
CIPHER TEXTBERVMOPPTRT

(Note: Spaces between words are ignored for simplicity)

DECRYPTING A MESSAGE

Say we received the text “AGI AFY SAWP” and the keyword “Ice cream”. To decrypt the cipher text and reveal the message, we would follow these steps.

First we would write the keyword beneath the cipher text. The keyword is repeated until there are no more letters remaining in the message.

The letters are then decrypted by finding the row of the Tabula Recta that begins with the current letter from the keyword. We then move along the row until we find the matching letter from the cipher text. The letter at the head of the corresponding column will be the letter from the plain text message.  

CIPHER TEXTAGIAFYSAWP
KEYWORDICECREAM IC
MESSAGESEEYOUSOON

By following these steps until we have decrypted all the letters, we can see that the plain text message is “SEE YOU SOON”.

(Note: Spaces between words are ignored for simplicity)

With the provided code word and secret message, see if you can crack the code by using the code word repeated along the secret message to shift the each character until the message is revealed. There is an interactive tool below that you can use to help you.

Key Word: STEAM

Cipher Text: KNFJQUM VAHWG WPALMID

KEY WORD LETTER
CIPHER TEXT LETTER
SHIFT VALUE
SHIFT VALUE
ADDED VALUES
PLAINTEXT LETTER


Strengths and Weaknesses

Strengths

  • Stronger than basic shift ciphers
  • Keyword can be sent separately to the main cipher for added security
  • Difficult to crack if keyword is sufficiently long and complicated

Weaknesses

  • Security depends on length and complexity of keyword. Short or obvious keywords can make the cipher very easy to crack
  • Still somewhat vulnerable to frequency analysis
  • Can be difficult to use without tools