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OCR H446 1.3.1 Compression, encryption and hashing exam transition

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

Sitting at the end of H446 1.3.1, this is an assessment to be completed independently and without checking answers along the way. It covers coding, encryption and hashing at examination weight and is written almost entirely as extended responses.

Students will:

  • judge what is necessarily true of a lossless method
  • encode and decode run-length streams and reason about a sequence that would not benefit
  • construct and decode a dictionary-coded stream and explain why a decoder needs both parts
  • explain an asymmetric confidentiality route for a sender with no pre-shared secret, naming each key owner
  • decide between encryption and hashing for confidential messages, stored passwords and release-file checking

Inside: 2 explanation cells, 1 multiple-choice question, 1 fill-in-the-blanks cell and 8 written answers. 50 marks, about 55 to 70 minutes.

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

Shared by Coding PathwayVerified teacher

  • 12 cells
  • About 60 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

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The whole resource, exactly as a class sees it. Answers and marking are held back.

Compression, encryption and hashing examination transition

Complete this assessment independently and without checking answers as you go. Use the command words, supplied evidence and context carefully; correctness is held where the platform supports it.

Multiple choice1 mark

Which statement about a lossless method is necessarily true?

  • AThe exact original data can be reconstructed
  • BIt always produces a smaller file than every lossy method
  • CIt is used only for text
  • DIt removes selected detail permanently
Written answer6 marks

An online rehearsal service offers a reduced-data live preview and later supplies an archival source file. Compare lossy and lossless compression and recommend a method for each delivery. Refer to start/buffering time, quality and exact reconstruction.

Develop both sides before each conclusion.

Students type their answer here.

Written answer6 marks

A display row is HHHHJJKKKKKL. Using symbol-count RLE: encode it, decode P3 Q1 R4, and explain whether HJHJHJ is likely to benefit when symbol and count fields have equal width.

Show runs and reconstructed lengths.

Students type their answer here.

Fill in the blanks3 marks
A manual defines count-symbol pairs. Under that supplied convention, HHHHJJKKKKKL is gap 1 and 2A3B1C decodes to gap 2. The pair order is a gap 3, not a universal RLE rule.
  • 4H 2J 5K 1L
  • AABBBC
  • convention
  • lossy
Written answer6 marks

Using first appearance for code assignment, construct a dictionary and coded stream for ALPHA BETA ALPHA GAMMA BETA ALPHA. Decode the stream and explain why a decoder needs both parts.

Keep the dictionary cost separate from the stream.

Students type their answer here.

Written answer6 marks

A charity receives confidential applications from members of the public who have not exchanged a secret key with it. Explain an asymmetric confidentiality journey. Name the owner of each key, the data states and one limitation.

Start with applicant plaintext and finish with charity plaintext.

Students type their answer here.

Written answer5 marks

A production system protects a large local archive after authorised devices have received a secret key through a secure provisioning process. Explain why symmetric encryption may suit this use, and give two residual risks or limitations.

Connect same-key operation to the stated provisioning assumption.

Students type their answer here.

Written answer5 marks

Describe account creation and login verification using secure password hashing. Include the entered candidate, digest comparison and the risk from weak guessed passwords.

The stored digest is not decrypted.

Students type their answer here.

Written answer9 marks

A collaboration platform considers encryption and hashing for confidential project messages, stored passwords and release-file checking. Discuss which process should be used for each purpose and why. Compare reversibility, keys, confidentiality, verification and limitations, then reach a supported design judgement.

Apply each mechanism to the named data rather than listing definitions.

Students type their answer here.

Written answer3 marks

Distinguish a password hash from a hash-table hash in a data structure. State what each output is used for and what remains retrievable.

Hash-table implementation belongs to 1.4.2; answer only the conceptual boundary.

Students type their answer here.

Review your responses

Check every response against its command word and the supplied constraints. Strengthen unsupported assertions with an accurate mechanism and a contextual consequence before submitting.