OCR A Level CS 1.3.1 Compression, Encryption and Hashing

What is compression?

The process of reducing file size to save storage and speed up data transfer.

Why is compression important?

It reduces storage needs, speeds up file transfers, and improves efficiency for networks and the internet.

What is lossy compression?

A method that removes some data permanently to reduce file size (e.g., JPEG, MP3)

What are the advantages of lossy compression?

Smaller file sizes, faster downloads, and efficient for media files.

What are the disadvantages of lossy compression?

Data is lost permanently, reducing quality (e.g., lower image/audio resolution).

What is lossless compression?

A method that retains all original data while reducing file size (e.g., PNG, FLAC).

What are the advantages of lossless compression?

No data loss, retains full quality, and allows file reconstruction.

What are the disadvantages of lossless compression?

Larger file sizes compared to lossy compression.

What is Run-Length Encoding (RLE)?

A lossless compression technique that replaces repeated data with a count (e.g., AAAAAABBBB → A6B4).

When does RLE work best?

When data contains large repeating patterns.

When is RLE inefficient?

When data has little repetition, increasing the file size instead.

What is dictionary encoding?

A lossless compression technique that replaces frequently used words or symbols with shorter references.

How does dictionary encoding work?

Builds a dictionary of common phrases

• Replaces phrases with an index

• The dictionary is sent with the compressed data for reconstruction

What is a drawback of dictionary encoding?

The dictionary must be stored or transmitted with the file, increasing storage requirements.

What is encryption?

A method of scrambling data so it can only be read by authorized parties.

Why is encryption used?

To secure sensitive data from unauthorized access during storage or transmission.

What is symmetric encryption?

A method where the same key is used for both encryption and decryption.

What is a key exchange?

The process of sharing a secret key between sender and receiver.

What is a weakness of symmetric encryption?

If the key is intercepted, the data can be decrypted easily.

What is asymmetric encryption?

A method that uses a public key for encryption and a private key for decryption.

What are the benefits of asymmetric encryption?

More secure than symmetric encryption because the private key is never shared.

How do public and private keys work?

The public key encrypts data

• Only the private key can decrypt it

What is a real-world use of asymmetric encryption?

Digital signatures to verify authenticity and secure web communication (SSL/TLS).

What is hashing?

A process that converts input data into a fixed-length hash value.

How is hashing different from encryption?

Hashing is one-way and cannot be reversed.

What is hashing used for?

Password storage

• Data integrity checks

• Efficient data retrieval (hash tables)

What makes a good hash function?

Fast computation

• Low chance of collisions

• Produces a unique hash for different inputs

What is a hash collision?

When two different inputs produce the same hash value, reducing security