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MOTAT
Boy thinking with coloured light and code background

Operation Decode

Mission Two: Shift Cipher

Shift ciphers are among the oldest methods of encryption that we know of. Initially used as a means of encrypting military orders, they are sometimes known as Caesar Ciphers, after Julius Caesar who used them to securely communicate with his generals while in the field.

 Shift ciphers work by taking the letters in a plaintext message and ‘shifting’ them by a set number of places. This results in a jumbled collection of letters that won’t make any sense to anybody who stumbles across it. At the same time, the message remains easy for the intended recipient to decrypt as they only need to shift the letters back by the same value to reveal the plaintext again.

Encrypting a Message
  • Say we wanted to encrypt the message “COME AT ONCE” with a shift value of “2”
  • This means that all the letters will be replaced with the letter two places in front of them in the alphabet. A becomes C, B becomes D, etc.
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Once encrypted our message would therefore read “EQOG CV QPEG”

Decrypting a Message
  • To decrypt the message, we simply do the opposite. This time we would shift the letters back two places. A becomes Y, B becomes Z, C becomes A, etc.
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Doing this means that the text “K YKNN DG VJGTG” would become “I WILL BE THERE”

Activity 2

Using the Caesar Cipher wheel below, see if you can decrypt the given secret message.

Secret Message: Ftq zadftqdz iuzpe efuxx nxai

Shift Value: 12

You can check your answer here



Strengths & Weaknesses

Strengths

  • Relatively easy to use
  • Very little special equipment needed
  • It is possible to work out the code in your head if you know the shift value.

Weaknesses

  • Being easy to use, means it is also easy to crack
  • Vowels and commonly occurring letters can give away the shift value easily
  • Only 26 possible shift values means little trials before the eventual correct answer reveals it self

Caesar Ciphers are somewhat effective at protecting information against casual observation