Book Cipher Encryption πŸ”

Materials

  • Paper and pen
  • Sample text below (the Key)

Sample Text (Use for encryption/decryption)

Page 1:

Line Number Text
1 Alice went to the market to buy some apples.
2 Bob stayed home and read a book about coding.
3 They agreed to meet at the park after school.
4 Secret messages should never be sent in plain sight.
5 Always practice encryption to keep messages safe.

Process: Encrypt -> Send -> Decrypt

Step 1: Agree which key to use (only needs to be done once or when changing the key)

For our key, we are using the sample text above.
Rules:
- Use page number, line number, word number to reference a word.

Example: Page 1, Line 1, Word 1 β†’ β€œAlice”

Sample Text (Use for encryption/decryption)

Page 1:

Line Number Text
1 Alice went to the market to buy some apples.
2 Bob stayed home and read a book about coding.
3 They agreed to meet at the park after school.
4 Secret messages should never be sent in plain sight.
5 Always practice encryption to keep messages safe.

Step 2: Encrypt a message

  1. Write a short message (3–5 words) using words found in the sample text.
  2. For each word, write down the page–line–word numbers instead of the word.

Example Message:
Alice meets Bob

Encryption using the sample text:
- Alice β†’ 1-1-1
- meets β†’ 1-3-4 (β€œmeet”)
- Bob β†’ 1-2-1

Encrypted message:
1-1-1 / 1-3-4 / 1-2-1

Everyone should use the same sample text so messages can be swapped and decrypted in class.

Sample Text (Use for encryption/decryption)

Page 1:

Line Number Text
1 Alice went to the market to buy some apples.
2 Bob stayed home and read a book about coding.
3 They agreed to meet at the park after school.
4 Secret messages should never be sent in plain sight.
5 Always practice encryption to keep messages safe.

Send the encrypted message

Swap encrypted messages with a partner.

Step 3: Decrypt the message

  1. Use the same key (sample text) to decode.
  2. Read the decrypted message.

Step 4: Discussion

  • Why is the sample text called the key?
  • What happens if you don’t have the correct key?
  • How is this different from a hash? (once you have completed the lesson on hashing!)

Optional Stretch

  • Create your own short β€œAlice & Bob” message for a partner to encrypt and decrypt.
  • Change the book lines or words β€” what happens if the key is different?
  • Compare to hashing: why could this not be reversed in the same way? (once you have completed the lesson on hashing!)