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Free A-Level Computer Science guide
A compressed file, encrypted message and hash value solve different problems. Begin with the required outcome: smaller data, confidential data or a useful fingerprint.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Compression reduces the representation's size. Lossless compression allows exact reconstruction; lossy compression permanently discards selected information.
Encryption produces ciphertext that an authorised recipient can decrypt with the appropriate key. Hashing produces a fingerprint, without a general recovery operation.
The operations can work together: compress a document, encrypt it for delivery and authenticate its integrity. A hash from an untrusted sender alone does not identify who sent the file.
OCR H446 · 1.3.1(a–d). Original teaching examples; no exam-board questions are reproduced.
Useful foundations
By the end, you should be able to
Choose the permitted loss before choosing the format. Source code and database backups need exact reconstruction: one changed character or record can change their meaning.
A preview photograph can often tolerate lost detail. A diagnostic image may have stricter requirements than a casual thumbnail.
Encoding and decoding cost processing time. Smaller data can save storage and transmission time, but headers can make already compressed or poorly matching data larger. A file extension alone does not establish whether its use is lossless or lossy.
Run-length encoding records consecutive runs. Our count-first notation makes 5A mean five A characters. Long areas of one colour suit this method; alternating symbols can expand the data.
A real RLE format must distinguish counts from symbols and define how large counts are stored. Non-adjacent runs remain separate because order must be recovered.
Dictionary coding replaces repeated strings with references. The receiver needs the dictionary and agreed separators to reconstruct the tokens.
Three entries need two bits per reference. Six references therefore use twelve bits, before dictionary, boundary and format metadata. Include that overhead when claiming a saving.
Agree the dictionary
0=cat, 1=dog, 2=bird. Both encoder and decoder must use this mapping.
Encode
cat dog cat cat bird dog becomes 0,1,0,0,2,1.
Decode
Look up each code in order and restore the separators to recover the original token sequence.
Symmetric encryption uses the same secret key for encryption and decryption. It suits bulk data, but the parties must establish and protect that secret. Sending it openly alongside the ciphertext defeats the purpose.
Asymmetric encryption uses complementary public/private key operations. The public key must be authenticated: an attacker could substitute their own key.
A hybrid system uses asymmetric techniques to establish or protect a symmetric key, then encrypts the larger message symmetrically.
Verify Noor’s public key
Establish that this encryption key really belongs to Noor.
Sender encrypts
Use Noor’s authenticated public encryption key to produce ciphertext.
Noor decrypts
Use the matching private key. Noor keeps this key private.
A cryptographic hash supports integrity checking and digital signatures. Recalculate a file's digest and compare it with one obtained through a trusted channel.
Collisions are mathematically possible. Good cryptographic hash algorithms make useful collisions hard to find; they do not make them impossible.
Password verification compares a new password derivation with a stored, salted value. Use a dedicated, costly password-hashing scheme. An ordinary fast file hash alone is inadequate; a salt stops equal passwords automatically sharing one stored value.
A hash table uses a fast bucket function and compares actual keys to resolve collisions. Its function need not provide cryptographic security.
Worked example
AAAAABBBCCDAAAA from left to right. Start a new pair when the symbol changes; keep the separated A runs distinct.(5,A), (3,B), (2,C), (1,D), (4,A). Repeat each symbol by its count, then concatenate the runs to recover all fifteen symbols.| Run | Count | Symbol |
|---|---|---|
| 1 | 5 | A |
| 2 | 3 | B |
| 3 | 2 | C |
| 4 | 1 | D |
| 5 | 4 | A |
5 original questions total 15 marks. Attempt each before opening the independently written indicative marking guidance.
Encode WWWXXW using count-first RLE, then explain why the two W runs cannot be combined. [3 marks]
Indicative answer
3W2X1W (1). The W runs are separated by X values (1); combining them would change the order and fail to reconstruct the input (1).A text archive and a preview photograph need smaller downloads. Recommend a compression type for each and justify both. [4 marks]
Indicative answer
With 0=cat, 1=dog and 2=bird, decode 2,0,1,0. State one extra item needed when sending only these codes to a new recipient. [3 marks]
Indicative answer
bird cat dog cat in that order (2: one for the first two tokens, one for the last two). The recipient needs the dictionary or an agreed equivalent mapping (1).A sender wants only Imani to decrypt a message. Name the asymmetric encryption and decryption keys, then identify a key-distribution risk. [3 marks]
Indicative answer
Why does a matching file hash obtained from the same untrusted download site not prove the download is authentic? [2 marks]
Indicative answer
This guide addresses OCR H446 1.3.1(a–d). Original teaching examples; no exam-board questions are reproduced.. Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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