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Tor Rendezvous Specification - Version 3
This document specifies how the hidden service version 3 protocol works. This
text used to be proposal 224-rend-spec-ng.txt.
Table of contents:
```text
0. Hidden services: overview and preliminaries.
0.1. Improvements over previous versions.
0.2. Notation and vocabulary
0.3. Cryptographic building blocks
0.4. Protocol building blocks [BUILDING-BLOCKS]
0.5. Assigned relay cell types
0.6. Acknowledgments
1. Protocol overview
1.1. View from 10,000 feet
1.2. In more detail: naming hidden services [NAMING]
1.3. In more detail: Access control [IMD:AC]
1.4. In more detail: Distributing hidden service descriptors. [IMD:DIST]
1.5. In more detail: Scaling to multiple hosts
1.6. In more detail: Backward compatibility with older hidden service
1.7. In more detail: Keeping crypto keys offline
1.8. In more detail: Encryption Keys And Replay Resistance
1.9. In more detail: A menagerie of keys
1.9.1. In even more detail: Client authorization [CLIENT-AUTH]
2. Generating and publishing hidden service descriptors [HSDIR]
2.1. Deriving blinded keys and subcredentials [SUBCRED]
2.2. Locating, uploading, and downloading hidden service descriptors
2.2.1. Dividing time into periods [TIME-PERIODS]
2.2.2. When to publish a hidden service descriptor [WHEN-HSDESC]
2.2.3. Where to publish a hidden service descriptor [WHERE-HSDESC]
2.2.4. Using time periods and SRVs to fetch/upload HS descriptors
2.2.5. Expiring hidden service descriptors [EXPIRE-DESC]
2.2.6. URLs for anonymous uploading and downloading
2.3. Publishing shared random values [PUB-SHAREDRANDOM]
2.3.1. Client behavior in the absence of shared random values
2.3.2. Hidden services and changing shared random values
2.4. Hidden service descriptors: outer wrapper [DESC-OUTER]
2.5. Hidden service descriptors: encryption format [HS-DESC-ENC]
2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER]
2.5.1.1. First layer encryption logic
2.5.1.2. First layer plaintext format
2.5.1.3. Client behavior
2.5.1.4. Obfuscating the number of authorized clients
2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER]
2.5.2.1. Second layer encryption keys
2.5.2.2. Second layer plaintext format
2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS]
3. The introduction protocol [INTRO-PROTOCOL]
3.1. Registering an introduction point [REG_INTRO_POINT]
3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO]
3.1.1.1. Denial-of-Server Defense Extension. [EST_INTRO_DOS_EXT]
3.1.2. Registering an introduction point on a legacy Tor node [LEGACY_EST_INTRO]
3.1.3. Acknowledging establishment of introduction point [INTRO_ESTABLISHED]
3.2. Sending an INTRODUCE1 cell to the introduction point. [SEND_INTRO1]
3.2.1. INTRODUCE1 cell format [FMT_INTRO1]
3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK]
3.3. Processing an INTRODUCE2 cell at the hidden service. [PROCESS_INTRO2]
3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS]
3.3.2. Example encryption handshake: ntor with extra data [NTOR-WITH-EXTRA-DATA]
3.4. Authentication during the introduction phase. [INTRO-AUTH]
3.4.1. Ed25519-based authentication.
4. The rendezvous protocol
4.1. Establishing a rendezvous point [EST_REND_POINT]
4.2. Joining to a rendezvous point [JOIN_REND]
4.2.1. Key expansion
4.3. Using legacy hosts as rendezvous points
5. Encrypting data between client and host
6. Encoding onion addresses [ONIONADDRESS]
7. Open Questions:
-1. Draft notes
```
This document describes a proposed design and specification for
hidden services in Tor version 0.2.5.x or later. It's a replacement
for the current rend-spec.txt, rewritten for clarity and for improved
design.
Look for the string "TODO" below: it describes gaps or uncertainties
in the design.
Change history:
2013-11-29: Proposal first numbered. Some TODO and XXX items remain.
2014-01-04: Clarify some unclear sections.
2014-01-21: Fix a typo.
```text
2014-02-20: Move more things to the revised certificate format in the
new updated proposal 220.
2015-05-26: Fix two typos.
```
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