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OCR H446 1.3.1 Symmetric and asymmetric encryption

Part 4 of 7 · H446 1.3.1 · Compression, encryption and hashing

Key ownership is what encryption questions at H446 1.3.1 really test, and answers fail when the wrong person holds the wrong key. This worksheet traces symmetric and asymmetric journeys by owner and direction, then asks for choices justified from key distribution, speed and trust.

Students will:

  • follow data through plaintext, ciphertext and decryption as separate states
  • name which key encrypts and which decrypts in an asymmetric confidentiality journey
  • explain why sending a shared secret key alongside the ciphertext is unsafe
  • recommend symmetric or asymmetric encryption from the constraints of a described situation
  • correct common claims about what encryption does and does not protect against

Inside: 6 explanation cells, 2 multiple-choice questions, 2 fill-in-the-blanks cells and 3 written answers. 38 marks, about 45 to 55 minutes.

Series: H446 1.3.1 · Compression, encryption and hashing, part 4 of 7.

Shared by Coding PathwayVerified teacher

  • 13 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

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Symmetric and asymmetric encryption

Encryption makes protected data difficult to interpret without the required key. It supports confidentiality; it does not stop an attacker intercepting ciphertext, deleting a file or using an already authorised account.

Read every journey by owner and direction

Owner-labelled symmetric and asymmetric key journeys Symmetric: same shared secret Ksender plaintextencrypt with Kciphertextmay be interceptedrecipientdecrypts with KAsymmetric confidentialitysender plaintextencrypt with recipientpublic keyciphertextnot secret itselfrecipientdecrypts withprivate key Key ownership is part of the answerrecipient public → encryptmatching recipient private → decryptConfidentiality does not prevent interception or deletion.

Symmetric encryption uses the same secret key to encrypt and decrypt. It is efficient for stored data or repeated transfer, but authorised parties need the key and distributing that secret safely is a problem.

For asymmetric confidentiality, a sender encrypts using the recipient's public key. Only the matching recipient private key should be able to decrypt. The public key can be distributed; the private key remains controlled by its owner. This Essential route does not require digital signatures, certificates, hybrid systems or cryptographic mathematics.

Keep data state separate from access route

  1. Plaintext is the readable original.
  2. An encryption method combines the plaintext with a key to produce ciphertext.
  3. Ciphertext can be stored or transmitted and may still be copied or intercepted.
  4. A matching decryption operation and correct key reconstruct the plaintext.

Reversible with the correct key is what separates encryption from one-way password hashing. “Scrambled” is weaker than naming plaintext, key, ciphertext and reconstruction.

Worked symmetric choice: field-record archive

Two authorised tablets repeatedly read and update a protected local archive. Both can be provisioned securely before use with shared secret key K.

Symmetric encryption is defensible because the same secret key can encrypt and decrypt many records efficiently. The deployment process must protect K: anyone who obtains the shared secret may be able to decrypt the archive, and replacing a compromised shared key affects every authorised holder. Encryption alone does not stop an unlocked authorised tablet exposing plaintext.

Multiple choice1 mark

Ravi wants to send confidential data that only Mei should decrypt. Which asymmetric key journey is correct?

  • AEncrypt with Mei's public key; decrypt with Mei's matching private key
  • BEncrypt with Ravi's private key; decrypt with any public key
  • CEncrypt with Mei's private key; decrypt with Ravi's public key
  • DHash with Mei's public key; decrypt the digest with her private key
Multiple choice1 mark

Two devices use symmetric encryption. Why is sending the secret key beside the ciphertext an unsafe solution to key distribution?

  • AThe ciphertext automatically becomes a hash digest
  • BAn interceptor who obtains both may be able to decrypt the ciphertext
  • CSymmetric encryption requires a public key to travel beside every message
  • DThe recipient must publish the secret key before decryption
Fill in the blanks4 marks
Symmetric encryption uses the same gap 1 key for encryption and decryption. In asymmetric confidentiality, encrypt with the recipient's gap 2 key and decrypt with the matching recipient gap 3 key. The protected transmitted form is gap 4.
  • secret
  • public
  • private
  • ciphertext
  • digest
Written answer10 marks

Trace these two journeys with owners and data states: (A) Noor encrypts a backup with symmetric key K and later restores it; (B) Jo sends a confidential report to Amina using Amina's asymmetric key pair. State one key-management risk for each.

Name plaintext, encryption key owner/access, ciphertext, decryption key and reconstructed plaintext in order.

Students type their answer here.

Apply the model independently

The remaining tasks change the context or reduce the support. Complete them without copying the worked model, then check that each explanation connects a mechanism to its consequence.

Written answer12 marks

Recommend an encryption approach for each: (1) one organisation encrypting a large offline backup under tightly controlled key storage, (2) a member of the public sending a confidential form to a council without first sharing a secret, and (3) two pre-provisioned sensors repeatedly exchanging protected readings. Explain the mechanism, benefit and limitation in each context.

Expected symmetric, asymmetric confidentiality, then symmetric; credit a different design only when its key-distribution assumption is explicit.

Students type their answer here.

Written answer6 marks

Correct each claim: (a) ‘encrypted data cannot be intercepted’, (b) ‘a public key decrypts anything encrypted with any private key’, and (c) ‘asymmetric encryption is always more secure than symmetric encryption’.

Replace each absolute claim with an accurate, qualified statement.

Students type their answer here.

Fill in the blanks4 marks
Encryption changes plaintext into checkpoint gap 1 using a key and is checkpoint gap 2 with the correct key. Symmetric encryption has a secret-key checkpoint gap 3 problem. Asymmetric confidentiality ends with the recipient's checkpoint gap 4 key.

Review your understanding

Before submitting, check that you can explain the main distinction in your own words, apply it in an unfamiliar context and justify each consequence rather than only naming a feature.