Data compression and encryption

Lossless and lossy

Lossless compression reconstructs the original exactly — required for text, program code, spreadsheets and anything where a changed byte matters. Lossy compression discards information the human eye or ear is unlikely to notice, achieving much smaller files, and is used for photographs, audio and video. The choice is decided by one question: does the original have to come back exactly?

Run length encoding

Replaces runs of identical values with a count and the value: AAAAABBB becomes 5A3B. It works well on images with large flat areas and badly on data with few repeats — on random data it can make the file larger, and saying that shows real understanding.

Dictionary compression

Builds a dictionary of repeated sequences and stores an index instead of the sequence. Effective on text, where words repeat. The dictionary itself must be stored or reconstructible, which is part of the cost.

Symmetric and asymmetric encryption

Symmetric uses one key for both encryption and decryption: fast, but the key must reach the other party securely, and that is the hard problem. Asymmetric uses a public key to encrypt and a private key to decrypt, solving key distribution at the cost of speed. In practice asymmetric is used to exchange a symmetric session key, then symmetric does the bulk work.

Digital signatures and certificates

A digital signature reverses the roles: the sender encrypts a hash of the message with their private key, and anyone can decrypt it with the public key — which proves it came from that sender and that the message has not changed. A digital certificate from a certification authority binds a public key to an identity, which is what stops an attacker publishing their own key under someone else's name.

Hashing is not encryption

A hash is one-way: it cannot be reversed to recover the input. That is exactly why passwords are stored hashed — a stolen database does not reveal the passwords themselves.