Tor V2 vs V3 Onion Addresses: Key Differences Explained

Tor onion services provide anonymous routing across the Tor network, and their underlying architecture has undergone a major evolution from version 2 (V2) to version 3 (V3). While V2 addresses were the standard for years, they relied on outdated cryptography and were completely retired by the Tor Project in late 2021. V3 onion addresses replace them with significantly stronger encryption, longer addresses, enhanced privacy protocols to prevent address discovery, and a more robust directory system.

Address Length and Appearance

• V2 Onion Addresses: Consist of 16 characters (base32) followed by .onion (e.g., expyuzz5wqqfdgah.onion). The 16 characters are derived from a truncated hash of the public key.
• V3 Onion Addresses: Consist of 56 characters (base32) followed by .onion (e.g., vww6ybal4bd7szmgncyruucpgfkqahzddi37ktceo3ah7ngmcopnpyyd.onion). The address includes the full public key, a checksum, and a version byte.

Cryptographic Security

• V2 Cryptography: Utilized RSA-1024 keys and the SHA-1 hashing algorithm. Modern computing capabilities made RSA-1024 susceptible to factorization and SHA-1 prone to collision attacks, creating critical security risks.
• V3 Cryptography: Implements Ed25519 (elliptic curve cryptography) for signatures, Curve25519 for key exchange, and SHA3-256 for hashing. These modern standards provide significantly higher security margins with smaller key footprints.

Directory Privacy and Blinded Keys

• V2 Weakness: When a V2 onion service published its descriptor to the hidden service directory (HSDir), directory operators could inspect the descriptor, discover the full .onion address, and enumerate private services without authorization.
• V3 Blinded Key Mechanism: V3 uses “blinded keys” derived from the service’s master key and the current date. Directory servers only see this blinded key, meaning they cannot discover or access the .onion address unless the owner explicitly shares the original address with them.

Network Protocol and Performance

• V2 Protocol: Suffered from slower connection handshakes, high overhead, and vulnerabilities to denial-of-service (DoS) attacks at the rendezvous point.
• V3 Protocol: Optimized the rendezvous protocol to reduce latency, streamlined the descriptor publication process, and introduced better resistance against DoS attacks and traffic analysis.

Current Status and Compatibility

• V2 Status: Fully deprecated and disabled. Tor client versions 0.4.6.x and newer refuse to connect to or host V2 onion addresses.
• V3 Status: The current, active standard for all Tor hidden services. All modern Tor browsers and relays exclusively support V3 addresses.