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Year 11
OCR

Common ciphers

I can describe how common ciphers work, and use them to encrypt and decrypt information.

Download all resources
Share activities with pupils
New
New
Year 11
OCR

Common ciphers

I can describe how common ciphers work, and use them to encrypt and decrypt information.

Download all resources
Share activities with pupils

These resources will be removed by end of Summer Term 2025.

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These resources were created for remote use during the pandemic and are not designed for classroom teaching.

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Lesson details

Key learning points

  1. Ciphers are used to encrypt and decrypt messages, turning readable text into unreadable ciphertext.
  2. Common ciphers like the Caesar and substitution ciphers use patterns or keys to shift or replace letters in a message.
  3. More complex ciphers use multiple shifts or a keyword, making them harder to break.

Keywords

  • Encrypt - to take readable information and convert it into a coded unreadable form

  • Decrypt - to take unreadable information and convert it back into a readable form

  • Ciphertext - a scrambled, unreadable version of a message

Common misconception

Ciphers and encryption are the same thing.

Ciphers are one type of encryption method. Encryption includes ciphers but also involves algorithms, keys, and protocols used in modern digital security systems.


To help you plan your year 11 computer science lesson on: Common ciphers, download all teaching resources for free and adapt to suit your pupils' needs...

Use fun, hands-on activities like decoding secret messages with the Caesar cipher or creating simple substitution ciphers. These activities help students understand how encryption works by letting them see and experience the process of turning plaintext into ciphertext and back.
Teacher tip

Equipment

Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Prior knowledge starter quiz

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6 Questions

Q1.
What is the main purpose of encryption?
to delete data
Correct answer: to make data unreadable to unauthorised users
to compress files
to organise files alphabetically
Q2.
Match the term to its definition:
Correct Answer:encryption,the process of converting data into an unreadable format

the process of converting data into an unreadable format

Correct Answer:cipher,a method or algorithm used to encrypt or decrypt information

a method or algorithm used to encrypt or decrypt information

Correct Answer:key,information used by an algorithm to lock or unlock data

information used by an algorithm to lock or unlock data

Correct Answer:ciphertext,the unreadable output of encryption

the unreadable output of encryption

Q3.
Why might someone use encryption on their messages?
to make messages longer
to delete messages automatically
Correct answer: to keep messages unreadable by unauthorised users
to translate messages into another language
Q4.
Arrange these steps in the process of encrypting a message:
1 - write the original message
2 - choose an encryption algorithm (cipher)
3 - use a key to encrypt the message
4 - send the encrypted message (ciphertext)
Q5.
What is the secure protocol used by websites to protect data during online transactions?
Correct Answer: HTTPS
Q6.
Which of these is an example of where encryption is used?
Correct answer: messaging apps
sending a postcard
drawing software
word processors

Assessment exit quiz

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6 Questions

Q1.
Which of the following is a common example of a simple cipher?
Correct answer: Caesar cipher
binary code
Morse code
HTML
Q2.
What is the process of turning ciphertext back into readable text called?
Correct Answer: decrypt
Q3.
Match the term with its definition:
Correct Answer:encrypt,to turn readable text into ciphertext

to turn readable text into ciphertext

Correct Answer:decrypt,to turn ciphertext back into readable text

to turn ciphertext back into readable text

Correct Answer:cipher,a method for encrypting or decrypting messages

a method for encrypting or decrypting messages

Correct Answer:ciphertext,the unreadable output of a cipher

the unreadable output of a cipher

Q4.
Arrange the steps for brute force decryption of a Caesar cipher:
1 - obtain the ciphertext
2 - set the shift value to 1
3 - apply the current shift to the ciphertext to generate possible plaintext
4 - increase the shift value by 1 and repeat until all shifts have been tried
5 - identify the correct readable message from the possible plaintexts
Q5.
What is used in a cipher to set the pattern for encryption and decryption?
Correct Answer: key
Q6.
Which of these statements is correct about ciphers and encryption?
Ciphers and encryption are exactly the same.
Correct answer: All encryption uses ciphers, but not all encryption is just ciphers.
Encryption is a type of cipher.
Ciphers are only used in ancient times.