What is a mix network (mix net), and how does Tor build on the same idea?

A mix net routes messages through a sequence of intermediaries that batch, shuffle, and re-encrypt traffic so the link between sender and receiver is broken. Tor uses a simplified low-latency variant where each hop only sees its predecessor and successor.

How a classic Chaum mix net works:

Alice ─┐
Bob   ─┼─▶ Mix 1 ─▶ Mix 2 ─▶ Mix 3 ─▶ Carol
Dave  ─┘                                Eve

Each mix:

1. Receives a batch of messages from many senders.
2. Decrypts the outermost encryption layer with its private key (onion-routing style).
3. Shuffles the batch into a random order.
4. Adds dummy traffic if needed.
5. Forwards to the next mix.

If at least one mix is honest, the link Alice → Carol is unrecoverable.

How Tor differs:

• Low latency — no batching/shuffling (would add seconds of delay), just onion routing through 3 hops.
• Three relays by default: Guard → Middle → Exit.
• Anonymity vs latency trade-off: Tor sacrifices some anonymity for usable web browsing; pure mix nets (Mixmaster, Nym) give stronger anonymity but with delays of minutes to hours.

Limitations:

• A global passive adversary that observes all network traffic can de-anonymise Tor users via traffic-correlation attacks. Mix nets resist this better because they batch + shuffle.
• Compromising a majority of mixes breaks anonymity in both systems.

Tip: Mix nets are seeing renewed academic interest because of post-quantum concerns — onion encryption can be made post-quantum, but traffic-analysis resistance is unaffected by quantum computers (it's information-theoretic, not computational).