2.2 Hidden Services Infrastructure: v2→v3 Transition, Cryptographic Upgrade
Hidden services (now known as Onion Services) are one of Tor’s most important innovations.
Where onion routing hides a user, onion services hide a server.
This allows a website to exist without revealing its IP address, physical location, or hosting environment.
Between 2004 and 2017, Tor used version 2 (v2) hidden services.
Today, all modern onion services use version 3 (v3), which introduced a new design with stronger cryptography, significantly improved security, and modern threat-model resilience.
This chapter explains the architecture, the transition, and why the upgrade was essential.
A. What Is a Tor Hidden Service?
A Tor hidden service (or Onion Service):
hides the server’s network location
communicates only through Tor circuits
uses end-to-end encrypted interactions
is accessed through a .onion address
Unlike normal websites, onion services never expose their IP address or hosting server location to the public.
This is accomplished through rendezvous circuits and introduction points, explained below.
B. Architectural Overview of Onion Services
Onion services rely on these main components:
Service Descriptor — a metadata file that helps clients find introduction points.
Introduction Points (IPs) — relays selected by the onion service to receive connection requests.
Rendezvous Point (RP) — a Tor relay where the client and the service meet anonymously.
.onion Address — the domain-like identifier derived from a public key.
High-Level Workflow
Server selects a few introduction points.
Server publishes a descriptor to a distributed directory system (HSDirs).
Client retrieves the descriptor using the .onion address.
Client chooses a rendezvous point.
Both build circuits to the RP.
Anonymous communication begins.
Neither party learns the other’s IP address.
C. Why Tor Moved from v2 to v3 Hidden Services
Version 2 was functional but had several weaknesses:
1. Weak Cryptography
v2 used:
1024-bit RSA keys
SHA-1 hashing
These were becoming outdated by modern security standards.
2. Short Onion Addresses
v2 addresses were 16 characters, which allowed:
enumeration
impersonation attacks
lower security margins
3. HSDir (hidden service directory) weaknesses
v2 relied on HSDirs that could be “positioned” maliciously to gather descriptors, enabling certain deanonymization research attacks.
4. No Modern Crypto Agility
v2 lacked flexibility for future cryptographic upgrades and post-quantum considerations.
These limitations motivated Tor developers to redesign the system.
D. The v3 Hidden Services Architecture (Modern Standard)
v3 onion services introduced one of the most advanced anonymity architectures deployed at scale.
1. Stronger Cryptography
v3 uses:
ed25519 keys
SHA3 family hashing
56-character onion addresses
modern “crypto-by-design” architecture
This upgrade dramatically increased resistance to attacks.
2. Powerful Address Security
v3 onion addresses are self-authenticating, derived from:
ed25519_public_key + checksum + version byte
This reduces impersonation and makes brute-force enumeration infeasible.
3. Improved Descriptor System
v3 redesigned how service descriptors are stored and retrieved.
Key improvements:
encryption of descriptors
per-day ephemeral keys
more secure placement of descriptors in the directory system
stronger resistance to targeted HSDir attacks
4. Introduction Point Reinforcement
v3 strengthens the interaction between clients and introduction points, reducing risks of exploitation.
5. Post-Quantum Consideration
While not fully post-quantum, v3 was built with:
modular cryptography
ability to upgrade algorithms in the future
This allows future resistance improvements without major architectural rewrites.
E. Structural Differences: v2 vs v3 Onion Services
| Feature | v2 | v3 |
|---|---|---|
| Address Length | 16 chars | 56 chars |
| Crypto Algorithm | RSA-1024 | Ed25519 |
| Hash Function | SHA-1 | SHA3 / SHAKE |
| Descriptor Encryption | Limited | Strong, multi-layered |
| HSDir Vulnerabilities | Present | Highly reduced |
| Service Identity | Less secure | Cryptographically stronger |
| Enumeration Risk | Higher | Nearly impossible |
| Protocol Modernity | Legacy | Current & future-proof |
v3 was designed explicitly to remove the shortcomings of v2.
F. v3 Descriptor System: How It Works (High-Level)
v3 descriptors introduce several important components:
1. Blinded Keys
Each day, the onion service derives a temporary key based on its master key.
This prevents long-term tracking.
2. Replicated Descriptor Placement
Descriptors are stored at multiple directory nodes to ensure:
availability
robustness
protection against targeted enumeration
3. Encrypted Descriptors
Clients need the correct cryptographic material to decrypt descriptors.
This stops passive HSDir snooping.
G. Security Improvements in v3 (Academic Perspective)
Research has shown v3 onion services improve:
1. Resistance to HSDir Attacks
Rotating blinded keys and randomized descriptor placement reduce targeted takeover opportunities.
2. Strong Identity Binding
The longer, crypto-derived domain prevents impersonation and phishing.
3. Metadata Protection
v3 includes additional protections for preventing service enumeration and behavior analysis.
4. Better Code and Protocol Design
Tor developers incorporated lessons from:
traffic correlation studies
descriptor harvesting research
malicious relay investigations
v3 reflects nearly a decade of academic findings.
