Understand lossy and lossless compression, run length encoding and dictionary-based compression, symmetric and asymmetric encryption, and error-checking methods such as parity and check digits.

Run length encoding replaces a run of identical adjacent values with the value and a count, so a row such as 40 white pixels then 5 black pixels is stored as two pairs rather than 45 separate values. A scanned black and white document has large areas of a single colour (long runs), so RLE achieves a high compression ratio while remaining lossless, meaning the exact original is restored.

RLE is a poor choice for a photograph, where colour changes from pixel to pixel and runs are short or non-existent; the value and count pairs can then be larger than the original data, so the file grows rather than shrinks.

Markers reward explaining how RLE encodes runs, linking it to the long runs in a document, and a valid counter-example (a photograph or noisy image) with the reason.

Symmetric encryption uses a single shared secret key for both encryption and decryption. It is computationally fast, but its weakness is key distribution: the same key must reach the recipient securely, and if it is intercepted in transit the message can be read.

Asymmetric encryption uses a key pair, a public key and a private key. The public key can be published openly and is used to encrypt; only the matching private key, never transmitted, can decrypt. This removes the need to share a secret in advance, solving the distribution problem, and also enables digital signatures. Its disadvantage is that it is slower, so real systems often use asymmetric encryption to exchange a symmetric session key, then switch to the faster symmetric cipher.

Markers reward identifying one key versus a key pair, the distribution weakness of symmetric, and how a public/private pair removes the need to share a secret.