Onion routing

Onion routing is the cryptographic technique behind Tor. The sender encrypts the payload in nested layers, one per relay in the circuit. Each relay decrypts only its own layer, revealing the next-hop address but not the original sender or final destination. The metaphor: peeling an onion layer by layer. The result is sender anonymity: no single relay knows both the source and destination.

Onion routing predates Tor by several years (originating at the U.S. Naval Research Laboratory in the mid-1990s) and has inspired related designs including I2P and mixnets. The cryptographic primitives are well-understood; the practical security depends on the diversity and trustworthiness of relays. Tor's threat model assumes that no global adversary can observe the entire path simultaneously.