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Lesson · Grade 6 · Applied Design, Skills and Technologies

Secret codes: the Caesar shift cipher

  • Grade 6
  • Applied Design, Skills and Technologies
  • 1 min 29 s
  • British Columbia
  • BC6-AD-02

An alphabet strip, with a second alphabet shifted 3 places under it, shows how the Caesar cipher turns PIZZA into SLCCD, one letter at a time, and how the letters wrap around from Z back to A. Shifting back 3 decodes the message, and a code breaker who tries every shift finds the word quickly, because there are only 25 shifts to try.

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In one sentence

The question this lesson answers

How do you send a secret message that only your friend can read?

One very old way is the Caesar shift cipher, named after Julius Caesar.

Transcript

What the lesson says

How do you send a secret message that only your friend can read?

One very old way is the Caesar shift cipher, named after Julius Caesar.

First, write the alphabet in a row.

Under it, write the alphabet again, shifted 3 places, so A sits above D.

Letters that slide off the front wrap around, so X, Y and Z sit above A, B and C.

The number 3 is the secret key, and only you and your friend know it.

To encode PIZZA, find each letter on top, read the letter below it, and repeat for every letter.

P becomes S, I becomes L, each Z becomes C, and A becomes D.

So PIZZA becomes S, L, C, C, D.

Exact steps like these, followed in order, are called an algorithm.

Your friend decodes it by shifting each letter back 3 places.

A code breaker without the key can just try every shift.

With 26 letters, there are only 25 shifts to try, so it is easy to break.

A Caesar cipher hides a message by shifting every letter by the same key.

Curriculum

Where this fits

British Columbia, Grade 6

Applied Design, Skills and Technologies, Computational thinking: simple algorithms that reflect computational thinkingBC6-AD-02

Previous: Algorithms: Steps in Order, Choices and LoopsNext: Sorting and Searching: Selection Sort and Binary Search

British Columbia, Grade 7

Applied Design, Skills and Technologies, Computational thinking: simple algorithms that reflect computational thinkingBC7-AD-02

Previous: Algorithms: Steps in Order, Choices and LoopsNext: Flow Charts: Drawing the Steps of a Plan

This lesson is also listed in Grade 7.

The outcomes and strands follow the British Columbia curriculum.

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