Compression (lossy and lossless, run length encoding and dictionary coding), encryption (symmetric and asymmetric) and hashing, including their characteristics, differences and appropriate uses.

Award up to three marks for each compression type with its use.

Lossless compression reduces file size while allowing the exact original to be reconstructed; no data is permanently discarded (techniques include run length encoding and dictionary coding). Appropriate use: text documents, program code, spreadsheets or a ZIP archive, where losing any data would corrupt the file.

Lossy compression reduces file size by permanently removing data the human eye or ear is unlikely to notice, so the original cannot be perfectly restored, but the ratio is much higher. Appropriate use: streaming photos, music or video (JPEG, MP3), where a small quality loss is acceptable in exchange for a much smaller file.

Markers reward the reversible-versus-irreversible distinction plus a justified use for each (exactness needed -> lossless; bandwidth/size matters and small quality loss acceptable -> lossy).

Hashing versus encryption (up to 4): encryption is a reversible two-way process, ciphertext can be decrypted back to plaintext with the key, whereas hashing is a one-way process that maps input to a fixed-length digest and cannot (feasibly) be reversed. A good hash function is deterministic (same input gives same hash), produces a fixed-length output and makes collisions extremely unlikely.

Why hash passwords (up to 2): the system never needs the original password back; it only needs to check a login by hashing the entered password and comparing digests. Storing hashes means that if the database is stolen, the actual passwords are not directly exposed, whereas reversible encryption could be decrypted if the key were also compromised. Markers reward one-way versus two-way plus the security argument for not needing the original.