Tor vs. I2P: Deep Dive Into Architecture and Security Models

Tor uses bidirectional circuits: a single circuit with three relays handles both outbound and inbound data for a connection. The circuit is identified by a circuit ID, and all traffic for that connection (both directions) flows through the same path. I2P uses unidirectional tunnels: outbound traffic goes through an outbound tunnel and inbound traffic arrives through a separate inbound tunnel, each with independently selected nodes. This means I2P uses six nodes per connection (three for each direction) versus Tor's three. The unidirectional design prevents certain correlation attacks possible with bidirectional circuits.

Tor's onion routing wraps each message in successive encryption layers, one per relay. Each relay removes its layer, forwarding the inner package. I2P's garlic routing bundles multiple encrypted messages ('cloves') together, forwarding them as a single 'garlic message.' This bundling obscures traffic volume analysis (multiple messages leave together, preventing per-message timing observation) and improves efficiency for high-volume traffic between the same endpoints. Garlic routing provides additional protection against traffic analysis that onion routing is vulnerable to, at the cost of some latency for message bundling.

Tor uses a small set of directory authorities (approximately 10) to maintain the consensus - the trusted list of all Tor relays. This creates a small set of centralized trust points. I2P uses a fully distributed network database (NetDB) based on a Kademlia DHT, where every I2P node stores information about some subset of the network and queries propagate through the DHT. This eliminates the directory authority single points of failure but creates different attack surfaces (Sybil attacks on the DHT are possible, though mitigated by I2P's design). The DHT design aligns with I2P's design philosophy as a fully decentralized network.

Tor hidden services and I2P eepsites both hide server IP addresses. The cryptographic approaches differ: Tor uses introduction and rendezvous points with encryption to the service's public key. I2P services publish their Lease Set (a set of inbound tunnel entry points) to the NetDB, encrypted such that only the service's private key can read responses. I2P's integrated design means every I2P node can host eepsites without additional configuration complexity - services are native to the network. For operator security, I2P eepsites may have slightly better protection against guard discovery attacks because of the unidirectional tunnel design, but both require careful operational security.

Tor is well-studied for attacks against its design: guard discovery attacks, traffic fingerprinting, Sybil attacks on the relay pool. Mitigations exist for most known attacks (vanguards for guard discovery, circuit isolation for fingerprinting). I2P has been studied less extensively due to smaller user base and research community. The distributed NetDB is potentially more resistant to large-scale Sybil attacks than Tor's fixed relay pool, but also less well-verified against unknown attack patterns. For users whose adversary might attempt active attacks against the anonymity network itself, Tor's extensive research record and mitigations provide more confidence than I2P's less-audited design, despite I2P's architectural advantages in certain areas.