Question 1

What is a checksum, and why do downloads have hashes?

Accepted Answer

A **checksum** or **hash** is a fixed-length string calculated from a file's contents. Recalculating it after downloading and comparing against a published value confirms the file is **exactly** what was intended.

**How hash functions work.** A cryptographic hash takes input of any size and produces a fixed-length output — 256 bits for SHA-256. The properties that make it useful:

**Deterministic** — the same input always gives the same output.

**Avalanche effect** — changing a **single bit** of input changes roughly half the output bits. There is no partial match; the hash is either identical or completely different.

**One-way** — you cannot derive the file from the hash.

**Collision resistant** — finding two different files with the same hash should be infeasible.

**What checking a hash detects:**

**Corruption** during download — a truncated file, a network error, a failing disk.

**Tampering.** This is the security purpose. If a download mirror is compromised and a modified installer substituted, the hash will not match. Software distributed through many mirrors relies on this, and Linux distributions publish hashes for exactly this reason.

**How to check.** Every major platform has a built-in command — `shasum -a 256` on macOS and Linux, `Get-FileHash` in PowerShell — producing a string to compare against the published one.

**The important limitation.** A hash published **on the same page as the download** protects against corruption but not against a determined attacker, who could alter both. That is why serious projects publish **signed** hashes — a **GPG signature** proving the hash came from the project's key, which the attacker does not have. Checking a signature is stronger than checking a hash.

**Which algorithms.** **MD5** and **SHA-1** are **cryptographically broken** — deliberate collisions have been demonstrated — so they remain adequate for detecting accidental corruption and should not be relied on against tampering. **SHA-256** is the current standard.

**Hashes are also used** for password storage, deduplication, blockchain and version control.

Question 2

What metadata does a file contain, and what can it reveal?

Accepted Answer

Metadata is data about the file rather than its visible content. Almost every file carries some, and it frequently reveals considerably more than people intend to share. Photographs — EXIF data: GPS

Question 3

What is the difference between a file format and a file extension?

Accepted Answer

A file format is the actual structure of the data inside the file. A file extension is the few characters after the dot in the filename — a label, and nothing more. The crucial point is that the

Question 4

What is the difference between lossy and lossless compression?

Accepted Answer

Lossless compression can reconstruct the original data exactly. Lossy compression discards information permanently to achieve much smaller files. Lossless works by finding and encoding redundancy

Question 5

What is content provenance, and what is C2PA?

Accepted Answer

A way of recording where a piece of media came from and what was done to it, cryptographically attached to the file — an approach to the synthetic media problem that works in the opposite direction

Question 6

Why do old files become unreadable, and what is digital preservation?

Accepted Answer

Because a file is only meaningful to software that understands its format, and both the software and the medium it sits on decay — on timescales far shorter than people assume. The three separate

Question 7

How do QR codes encode data?

Accepted Answer

As a two-dimensional pattern of black and white squares read as binary, with substantial built-in error correction — which is why a code still scans when damaged, partially covered or printed over a

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