[tor-commits] [torspec/master] Create rend-spec-v3.txt based on proposal 224
nickm at torproject.org
nickm at torproject.org
Tue Sep 19 18:33:37 UTC 2017
commit 471af27b55ff3894551109b45848f2ce1002441b
Author: Nick Mathewson <nickm at torproject.org>
Date: Tue Sep 19 14:33:33 2017 -0400
Create rend-spec-v3.txt based on proposal 224
---
proposals/000-index.txt | 4 +-
proposals/224-rend-spec-ng.txt | 3 +-
rend-spec.txt => rend-spec-v2.txt | 0
rend-spec-v3.txt | 2369 +++++++++++++++++++++++++++++++++++++
4 files changed, 2373 insertions(+), 3 deletions(-)
diff --git a/proposals/000-index.txt b/proposals/000-index.txt
index e184aab..6f6fca0 100644
--- a/proposals/000-index.txt
+++ b/proposals/000-index.txt
@@ -144,7 +144,7 @@ Proposals by number:
221 Stop using CREATE_FAST [CLOSED]
222 Stop sending client timestamps [CLOSED]
223 Ace: Improved circuit-creation key exchange [RESERVE]
-224 Next-Generation Hidden Services in Tor [ACCEPTED]
+224 Next-Generation Hidden Services in Tor [CLOSED]
225 Strawman proposal: commit-and-reveal shared rng [SUPERSEDED]
226 "Scalability and Stability Improvements to BridgeDB: Switching to a Distributed Database System and RDBMS" [OPEN]
227 Include package fingerprints in consensus documents [CLOSED]
@@ -263,7 +263,6 @@ Proposals by status:
172 GETINFO controller option for circuit information
173 GETINFO Option Expansion
188 Bridge Guards and other anti-enumeration defenses
- 224 Next-Generation Hidden Services in Tor
258 Denial-of-service resistance for directory authorities
265 Load Balancing with Overhead Parameters [for 0.2.9.x]
META:
@@ -345,6 +344,7 @@ Proposals by status:
218 Controller events to better understand connection/circuit usage [in 0.2.5.2-alpha]
221 Stop using CREATE_FAST [for 0.2.5.x]
222 Stop sending client timestamps [in 0.2.4.18]
+ 224 Next-Generation Hidden Services in Tor [in 0.3.2.1-alpha]
227 Include package fingerprints in consensus documents [in 0.2.6.3-alpha]
228 Cross-certifying identity keys with onion keys
235 Stop assigning (and eventually supporting) the Named flag [in 0.2.6, 0.2.7]
diff --git a/proposals/224-rend-spec-ng.txt b/proposals/224-rend-spec-ng.txt
index 73f8936..b2c0fea 100644
--- a/proposals/224-rend-spec-ng.txt
+++ b/proposals/224-rend-spec-ng.txt
@@ -2,7 +2,8 @@ Filename: 224-rend-spec-ng.txt
Title: Next-Generation Hidden Services in Tor
Author: David Goulet, George Kadianakis, Nick Mathewson
Created: 2013-11-29
-Status: Accepted
+Status: Closed
+Implemented-In: 0.3.2.1-alpha
Table of contents:
diff --git a/rend-spec.txt b/rend-spec-v2.txt
similarity index 100%
rename from rend-spec.txt
rename to rend-spec-v2.txt
diff --git a/rend-spec-v3.txt b/rend-spec-v3.txt
new file mode 100644
index 0000000..5540f2e
--- /dev/null
+++ b/rend-spec-v3.txt
@@ -0,0 +1,2369 @@
+ 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:
+
+ 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 absense 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]
+ 3.1.2. 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.
+
+ 2014-02-20: Move more things to the revised certificate format in the
+ new updated proposal 220.
+
+ 2015-05-26: Fix two typos.
+
+
+0. Hidden services: overview and preliminaries.
+
+ Hidden services aim to provide responder anonymity for bidirectional
+ stream-based communication on the Tor network. Unlike regular Tor
+ connections, where the connection initiator receives anonymity but
+ the responder does not, hidden services attempt to provide
+ bidirectional anonymity.
+
+ Participants:
+
+ Operator -- A person running a hidden service
+
+ Host, "Server" -- The Tor software run by the operator to provide
+ a hidden service.
+
+ User -- A person contacting a hidden service.
+
+ Client -- The Tor software running on the User's computer
+
+ Hidden Service Directory (HSDir) -- A Tor node that hosts signed
+ statements from hidden service hosts so that users can make
+ contact with them.
+
+ Introduction Point -- A Tor node that accepts connection requests
+ for hidden services and anonymously relays those requests to the
+ hidden service.
+
+ Rendezvous Point -- A Tor node to which clients and servers
+ connect and which relays traffic between them.
+
+0.1. Improvements over previous versions.
+
+ Here is a list of improvements of this proposal over the legacy hidden
+ services:
+
+ a) Better crypto (replaced SHA1/DH/RSA1024 with SHA3/ed25519/curve25519)
+ b) Improved directory protocol leaking less to directory servers.
+ c) Improved directory protocol with smaller surface for targeted attacks.
+ d) Better onion address security against impersonation.
+ e) More extensible introduction/rendezvous protocol.
+ f) Offline keys for onion services
+ g) Advanced client authorization
+
+0.2. Notation and vocabulary
+
+ Unless specified otherwise, all multi-octet integers are big-endian.
+
+ We write sequences of bytes in two ways:
+
+ 1. A sequence of two-digit hexadecimal values in square brackets,
+ as in [AB AD 1D EA].
+
+ 2. A string of characters enclosed in quotes, as in "Hello". The
+ characters in these strings are encoded in their ascii
+ representations; strings are NOT nul-terminated unless
+ explicitly described as NUL terminated.
+
+ We use the words "byte" and "octet" interchangeably.
+
+ We use the vertical bar | to denote concatenation.
+
+ We use INT_N(val) to denote the network (big-endian) encoding of the
+ unsigned integer "val" in N bytes. For example, INT_4(1337) is [00 00
+ 05 39]. Values are truncated like so: val % (2 ^ (N * 8)). For example,
+ INT_4(42) is 42 % 4294967296 (32 bit).
+
+0.3. Cryptographic building blocks
+
+ This specification uses the following cryptographic building blocks:
+
+ * A pseudorandom number generator backed by a strong entropy source.
+ The output of the PRNG should always be hashed before being posted on
+ the network to avoid leaking raw PRNG bytes to the network
+ (see [PRNG-REFS]).
+
+ * A stream cipher STREAM(iv, k) where iv is a nonce of length
+ S_IV_LEN bytes and k is a key of length S_KEY_LEN bytes.
+
+ * A public key signature system SIGN_KEYGEN()->seckey, pubkey;
+ SIGN_SIGN(seckey,msg)->sig; and SIGN_CHECK(pubkey, sig, msg) ->
+ { "OK", "BAD" }; where secret keys are of length SIGN_SECKEY_LEN
+ bytes, public keys are of length SIGN_PUBKEY_LEN bytes, and
+ signatures are of length SIGN_SIG_LEN bytes.
+
+ This signature system must also support key blinding operations
+ as discussed in appendix [KEYBLIND] and in section [SUBCRED]:
+ SIGN_BLIND_SECKEY(seckey, blind)->seckey2 and
+ SIGN_BLIND_PUBKEY(pubkey, blind)->pubkey2 .
+
+ * A public key agreement system "PK", providing
+ PK_KEYGEN()->seckey, pubkey; PK_VALID(pubkey) -> {"OK", "BAD"};
+ and PK_HANDSHAKE(seckey, pubkey)->output; where secret keys are
+ of length PK_SECKEY_LEN bytes, public keys are of length
+ PK_PUBKEY_LEN bytes, and the handshake produces outputs of
+ length PK_OUTPUT_LEN bytes.
+
+ * A cryptographic hash function H(d), which should be preimage and
+ collision resistant. It produces hashes of length HASH_LEN
+ bytes.
+
+ * A cryptographic message authentication code MAC(key,msg) that
+ produces outputs of length MAC_LEN bytes.
+
+ * A key derivation function KDF(message, n) that outputs n bytes.
+
+ As a first pass, I suggest:
+
+ * Instantiate STREAM with AES256-CTR.
+
+ * Instantiate SIGN with Ed25519 and the blinding protocol in
+ [KEYBLIND].
+
+ * Instantiate PK with Curve25519.
+
+ * Instantiate H with SHA3-256.
+
+ * Instantiate KDF with SHAKE-256.
+
+ * Instantiate MAC(key=k, message=m) with H(k_len | k | m),
+ where k_len is htonll(len(k)).
+
+ For legacy purposes, we specify compatibility with older versions of
+ the Tor introduction point and rendezvous point protocols. These used
+ RSA1024, DH1024, AES128, and SHA1, as discussed in
+ rend-spec.txt.
+
+ As in [proposal 220], all signatures are generated not over strings
+ themselves, but over those strings prefixed with a distinguishing
+ value.
+
+0.4. Protocol building blocks [BUILDING-BLOCKS]
+
+ In sections below, we need to transmit the locations and identities
+ of Tor nodes. We do so in the link identification format used by
+ EXTEND2 cells in the Tor protocol.
+
+ NSPEC (Number of link specifiers) [1 byte]
+ NSPEC times:
+ LSTYPE (Link specifier type) [1 byte]
+ LSLEN (Link specifier length) [1 byte]
+ LSPEC (Link specifier) [LSLEN bytes]
+
+ Link specifier types are as described in tor-spec.txt. Every set of
+ link specifiers MUST include at minimum specifiers of type [00]
+ (TLS-over-TCP, IPv4), [02] (legacy node identity) and [03] (ed25519
+ identity key).
+
+ We also incorporate Tor's circuit extension handshakes, as used in
+ the CREATE2 and CREATED2 cells described in tor-spec.txt. In these
+ handshakes, a client who knows a public key for a server sends a
+ message and receives a message from that server. Once the exchange is
+ done, the two parties have a shared set of forward-secure key
+ material, and the client knows that nobody else shares that key
+ material unless they control the secret key corresponding to the
+ server's public key.
+
+0.5. Assigned relay cell types
+
+ These relay cell types are reserved for use in the hidden service
+ protocol.
+
+ 32 -- RELAY_COMMAND_ESTABLISH_INTRO
+
+ Sent from hidden service host to introduction point;
+ establishes introduction point. Discussed in
+ [REG_INTRO_POINT].
+
+ 33 -- RELAY_COMMAND_ESTABLISH_RENDEZVOUS
+
+ Sent from client to rendezvous point; creates rendezvous
+ point. Discussed in [EST_REND_POINT].
+
+ 34 -- RELAY_COMMAND_INTRODUCE1
+
+ Sent from client to introduction point; requests
+ introduction. Discussed in [SEND_INTRO1]
+
+ 35 -- RELAY_COMMAND_INTRODUCE2
+
+ Sent from introduction point to hidden service host; requests
+ introduction. Same format as INTRODUCE1. Discussed in
+ [FMT_INTRO1] and [PROCESS_INTRO2]
+
+ 36 -- RELAY_COMMAND_RENDEZVOUS1
+
+ Sent from hidden service host to rendezvous point;
+ attempts to join host's circuit to
+ client's circuit. Discussed in [JOIN_REND]
+
+ 37 -- RELAY_COMMAND_RENDEZVOUS2
+
+ Sent from rendezvous point to client;
+ reports join of host's circuit to
+ client's circuit. Discussed in [JOIN_REND]
+
+ 38 -- RELAY_COMMAND_INTRO_ESTABLISHED
+
+ Sent from introduction point to hidden service host;
+ reports status of attempt to establish introduction
+ point. Discussed in [INTRO_ESTABLISHED]
+
+ 39 -- RELAY_COMMAND_RENDEZVOUS_ESTABLISHED
+
+ Sent from rendezvous point to client; acknowledges
+ receipt of ESTABLISH_RENDEZVOUS cell. Discussed in
+ [EST_REND_POINT]
+
+ 40 -- RELAY_COMMAND_INTRODUCE_ACK
+
+ Sent from introduction point to client; acknowledges
+ receipt of INTRODUCE1 cell and reports success/failure.
+ Discussed in [INTRO_ACK]
+
+0.6. Acknowledgments
+
+ This design includes ideas from many people, including
+ Christopher Baines,
+ Daniel J. Bernstein,
+ Matthew Finkel,
+ Ian Goldberg,
+ George Kadianakis,
+ Aniket Kate,
+ Tanja Lange,
+ Robert Ransom,
+ Roger Dingledine,
+ Aaron Johnson,
+ Tim Wilson-Brown ("teor"),
+ special (John Brooks),
+ s7r
+
+ It's based on Tor's original hidden service design by Roger
+ Dingledine, Nick Mathewson, and Paul Syverson, and on improvements to
+ that design over the years by people including
+ Tobias Kamm,
+ Thomas Lauterbach,
+ Karsten Loesing,
+ Alessandro Preite Martinez,
+ Robert Ransom,
+ Ferdinand Rieger,
+ Christoph Weingarten,
+ Christian Wilms,
+
+ We wouldn't be able to do any of this work without good attack
+ designs from researchers including
+ Alex Biryukov,
+ Lasse Øverlier,
+ Ivan Pustogarov,
+ Paul Syverson
+ Ralf-Philipp Weinmann,
+ See [ATTACK-REFS] for their papers.
+
+ Several of these ideas have come from conversations with
+ Christian Grothoff,
+ Brian Warner,
+ Zooko Wilcox-O'Hearn,
+
+ And if this document makes any sense at all, it's thanks to
+ editing help from
+ Matthew Finkel
+ George Kadianakis,
+ Peter Palfrader,
+ Tim Wilson-Brown ("teor"),
+
+
+ [XXX Acknowledge the huge bunch of people working on 8106.]
+ [XXX Acknowledge the huge bunch of people working on 8244.]
+
+
+ Please forgive me if I've missed you; please forgive me if I've
+ misunderstood your best ideas here too.
+
+
+1. Protocol overview
+
+ In this section, we outline the hidden service protocol. This section
+ omits some details in the name of simplicity; those are given more
+ fully below, when we specify the protocol in more detail.
+
+1.1. View from 10,000 feet
+
+ A hidden service host prepares to offer a hidden service by choosing
+ several Tor nodes to serve as its introduction points. It builds
+ circuits to those nodes, and tells them to forward introduction
+ requests to it using those circuits.
+
+ Once introduction points have been picked, the host builds a set of
+ documents called "hidden service descriptors" (or just "descriptors"
+ for short) and uploads them to a set of HSDir nodes. These documents
+ list the hidden service's current introduction points and describe
+ how to make contact with the hidden service.
+
+ When a client wants to connect to a hidden service, it first chooses
+ a Tor node at random to be its "rendezvous point" and builds a
+ circuit to that rendezvous point. If the client does not have an
+ up-to-date descriptor for the service, it contacts an appropriate
+ HSDir and requests such a descriptor.
+
+ The client then builds an anonymous circuit to one of the hidden
+ service's introduction points listed in its descriptor, and gives the
+ introduction point an introduction request to pass to the hidden
+ service. This introduction request includes the target rendezvous
+ point and the first part of a cryptographic handshake.
+
+ Upon receiving the introduction request, the hidden service host
+ makes an anonymous circuit to the rendezvous point and completes the
+ cryptographic handshake. The rendezvous point connects the two
+ circuits, and the cryptographic handshake gives the two parties a
+ shared key and proves to the client that it is indeed talking to the
+ hidden service.
+
+ Once the two circuits are joined, the client can send Tor RELAY cells
+ to the server. RELAY_BEGIN cells open streams to an external process
+ or processes configured by the server; RELAY_DATA cells are used to
+ communicate data on those streams, and so forth.
+
+1.2. In more detail: naming hidden services [NAMING]
+
+ A hidden service's name is its long term master identity key. This is
+ encoded as a hostname by encoding the entire key in Base 32, including a
+ version byte and a checksum, and then appending the string ".onion" at the
+ end. The result is a 56-character domain name.
+
+ (This is a change from older versions of the hidden service protocol,
+ where we used an 80-bit truncated SHA1 hash of a 1024 bit RSA key.)
+
+ The names in this format are distinct from earlier names because of
+ their length. An older name might look like:
+
+ unlikelynamefora.onion
+ yyhws9optuwiwsns.onion
+
+ And a new name following this specification might look like:
+
+ l5satjgud6gucryazcyvyvhuxhr74u6ygigiuyixe3a6ysis67ororad.onion
+
+ Please see section [ONIONADDRESS] for the encoding specification.
+
+1.3. In more detail: Access control [IMD:AC]
+
+ Access control for a hidden service is imposed at multiple points through
+ the process above. Furthermore, there is also the option to impose
+ additional client authorization access control using pre-shared secrets
+ exchanged out-of-band between the hidden service and its clients.
+
+ The first stage of access control happens when downloading HS descriptors.
+ Specifically, in order to download a descriptor, clients must know which
+ blinded signing key was used to sign it. (See the next section for more info
+ on key blinding.)
+
+ To learn the introduction points, clients must decrypt the body of the
+ hidden service descriptor. To do so, clients must know the _unblinded_
+ public key of the service, which makes the descriptor unuseable by entities
+ without that knowledge (e.g. HSDirs that don't know the onion address).
+
+ Also, if optional client authorization is enabled, hidden service
+ descriptors are superencrypted using each authorized user's identity x25519
+ key, to further ensure that unauthorized entities cannot decrypt it.
+
+ In order to make the introduction point send a rendezvous request to the
+ service, the client needs to use the per-introduction-point authentication
+ key found in the hidden service descriptor.
+
+ The final level of access control happens at the server itself, which may
+ decide to respond or not respond to the client's request depending on the
+ contents of the request. The protocol is extensible at this point: at a
+ minimum, the server requires that the client demonstrate knowledge of the
+ contents of the encrypted portion of the hidden service descriptor. If
+ optional client authorization is enabled, the service may additionally
+ require the client to prove knowledge of a pre-shared private key.
+
+1.4. In more detail: Distributing hidden service descriptors. [IMD:DIST]
+
+ Periodically, hidden service descriptors become stored at different
+ locations to prevent a single directory or small set of directories
+ from becoming a good DoS target for removing a hidden service.
+
+ For each period, the Tor directory authorities agree upon a
+ collaboratively generated random value. (See section 2.3 for a
+ description of how to incorporate this value into the voting
+ practice; generating the value is described in other proposals,
+ including [SHAREDRANDOM-REFS].) That value, combined with hidden service
+ directories' public identity keys, determines each HSDir's position
+ in the hash ring for descriptors made in that period.
+
+ Each hidden service's descriptors are placed into the ring in
+ positions based on the key that was used to sign them. Note that
+ hidden service descriptors are not signed with the services' public
+ keys directly. Instead, we use a key-blinding system [KEYBLIND] to
+ create a new key-of-the-day for each hidden service. Any client that
+ knows the hidden service's credential can derive these blinded
+ signing keys for a given period. It should be impossible to derive
+ the blinded signing key lacking that credential.
+
+ The body of each descriptor is also encrypted with a key derived from
+ the credential.
+
+ To avoid a "thundering herd" problem where every service generates
+ and uploads a new descriptor at the start of each period, each
+ descriptor comes online at a time during the period that depends on
+ its blinded signing key. The keys for the last period remain valid
+ until the new keys come online.
+
+1.5. In more detail: Scaling to multiple hosts
+
+ This design is compatible with our current approaches for scaling hidden
+ services. Specifically, hidden service operators can use onionbalance to
+ achieve high availability between multiple nodes on the HSDir
+ layer. Furthermore, operators can use proposal 255 to load balance their
+ hidden services on the introduction layer. See [SCALING-REFS] for further
+ discussions on this topic and alternative designs.
+
+1.6. In more detail: Backward compatibility with older hidden service
+ protocols
+
+ This design is incompatible with the clients, server, and hsdir node
+ protocols from older versions of the hidden service protocol as
+ described in rend-spec.txt. On the other hand, it is designed to
+ enable the use of older Tor nodes as rendezvous points and
+ introduction points.
+
+1.7. In more detail: Keeping crypto keys offline
+
+ In this design, a hidden service's secret identity key may be
+ stored offline. It's used only to generate blinded signing keys,
+ which are used to sign descriptor signing keys.
+
+ In order to operate a hidden service, the operator can generate in
+ advance a number of blinded signing keys and descriptor signing
+ keys (and their credentials; see [DESC-OUTER] and [HS-DESC-ENC]
+ below), and their corresponding descriptor encryption keys, and
+ export those to the hidden service hosts.
+
+ As a result, in the scenario where the Hidden Service gets
+ compromised, the adversary can only impersonate it for a limited
+ period of time (depending on how many signing keys were generated
+ in advance).
+
+ It's important to not send the private part of the blinded signing
+ key to the Hidden Service since an attacker can derive from it the
+ secret master identity key. The secret blinded signing key should
+ only be used to create credentials for the descriptor signing keys.
+
+1.8. In more detail: Encryption Keys And Replay Resistance
+
+ To avoid replays of an introduction request by an introduction point,
+ a hidden service host must never accept the same request
+ twice. Earlier versions of the hidden service design used an
+ authenticated timestamp here, but including a view of the current
+ time can create a problematic fingerprint. (See proposal 222 for more
+ discussion.)
+
+1.9. In more detail: A menagerie of keys
+
+ [In the text below, an "encryption keypair" is roughly "a keypair you
+ can do Diffie-Hellman with" and a "signing keypair" is roughly "a
+ keypair you can do ECDSA with."]
+
+ Public/private keypairs defined in this document:
+
+ Master (hidden service) identity key -- A master signing keypair
+ used as the identity for a hidden service. This key is long
+ term and not used on its own to sign anything; it is only used
+ to generate blinded signing keys as described in [KEYBLIND]
+ and [SUBCRED]. The public key is encoded in the ".onion"
+ address according to [NAMING].
+
+ Blinded signing key -- A keypair derived from the identity key,
+ used to sign descriptor signing keys. It changes periodically for
+ each service. Clients who know a 'credential' consisting of the
+ service's public identity key and an optional secret can derive
+ the public blinded identity key for a service. This key is used
+ as an index in the DHT-like structure of the directory system
+ (see [SUBCRED]).
+
+ Descriptor signing key -- A key used to sign hidden service
+ descriptors. This is signed by blinded signing keys. Unlike
+ blinded signing keys and master identity keys, the secret part
+ of this key must be stored online by hidden service hosts. The
+ public part of this key is included in the unencrypted section
+ of HS descriptors (see [DESC-OUTER]).
+
+ Introduction point authentication key -- A short-term signing
+ keypair used to identify a hidden service to a given
+ introduction point. A fresh keypair is made for each
+ introduction point; these are used to sign the request that a
+ hidden service host makes when establishing an introduction
+ point, so that clients who know the public component of this key
+ can get their introduction requests sent to the right
+ service. No keypair is ever used with more than one introduction
+ point. (previously called a "service key" in rend-spec.txt)
+
+ Introduction point encryption key -- A short-term encryption
+ keypair used when establishing connections via an introduction
+ point. Plays a role analogous to Tor nodes' onion keys. A fresh
+ keypair is made for each introduction point.
+
+ Symmetric keys defined in this document:
+
+ Descriptor encryption keys -- A symmetric encryption key used to
+ encrypt the body of hidden service descriptors. Derived from the
+ current period and the hidden service credential.
+
+ Public/private keypairs defined elsewhere:
+
+ Onion key -- Short-term encryption keypair
+
+ (Node) identity key
+
+ Symmetric key-like things defined elsewhere:
+
+ KH from circuit handshake -- An unpredictable value derived as
+ part of the Tor circuit extension handshake, used to tie a request
+ to a particular circuit.
+
+1.9.1. In even more detail: Client authorization keys [CLIENT-AUTH]
+
+ When client authorization is enabled, each authorized client of a hidden
+ service has two more assymetric keypairs which are shared with the hidden
+ service. An entity without those keys is not able to use the hidden
+ service. Throughout this document, we assume that these pre-shared keys are
+ exchanged between the hidden service and its clients in a secure out-of-band
+ fashion.
+
+ Specifically, each authorized client possesses:
+
+ - An x25519 keypair used to compute decryption keys that allow the client to
+ decrypt the hidden service descriptor. See [HS-DESC-ENC].
+
+ - An ed25519 keypair which allows the client to compute signatures which
+ prove to the hidden service that the client is authorized. These
+ signatures are inserted into the INTRODUCE1 cell, and without them the
+ introduction to the hidden service cannot be completed. See [INTRO-AUTH].
+
+ The right way to exchange these keys is to have the client generate keys and
+ send the corresponding public keys to the hidden service out-of-band. An
+ easier but less secure way of doing this exchange would be to have the
+ hidden service generate the keypairs and pass the corresponding private keys
+ to its clients. See section [CLIENT-AUTH-MGMT] for more details on how these
+ keys should be managed.
+
+ [TODO: Also specify stealth client authorization.]
+
+2. Generating and publishing hidden service descriptors [HSDIR]
+
+ Hidden service descriptors follow the same metaformat as other Tor
+ directory objects. They are published anonymously to Tor servers with the
+ HSDir flag, HSDir=2 protocol version and tor version >= 0.3.0.8 (because a
+ bug was fixed in this version).
+
+2.1. Deriving blinded keys and subcredentials [SUBCRED]
+
+ In each time period (see [TIME-PERIODS] for a definition of time
+ periods), a hidden service host uses a different blinded private key
+ to sign its directory information, and clients use a different
+ blinded public key as the index for fetching that information.
+
+ For a candidate for a key derivation method, see Appendix [KEYBLIND].
+
+ Additionally, clients and hosts derive a subcredential for each
+ period. Knowledge of the subcredential is needed to decrypt hidden
+ service descriptors for each period and to authenticate with the
+ hidden service host in the introduction process. Unlike the
+ credential, it changes each period. Knowing the subcredential, even
+ in combination with the blinded private key, does not enable the
+ hidden service host to derive the main credential--therefore, it is
+ safe to put the subcredential on the hidden service host while
+ leaving the hidden service's private key offline.
+
+ The subcredential for a period is derived as:
+
+ subcredential = H("subcredential" | credential | blinded-public-key).
+
+ In the above formula, credential corresponds to:
+
+ credential = H("credential" | public-identity-key)
+
+ where public-identity-key is the public identity master key of the hidden
+ service.
+
+2.2. Locating, uploading, and downloading hidden service descriptors
+ [HASHRING]
+
+ To avoid attacks where a hidden service's descriptor is easily
+ targeted for censorship, we store them at different directories over
+ time, and use shared random values to prevent those directories from
+ being predictable far in advance.
+
+ Which Tor servers hosts a hidden service depends on:
+
+ * the current time period,
+ * the daily subcredential,
+ * the hidden service directories' public keys,
+ * a shared random value that changes in each time period,
+ * a set of network-wide networkstatus consensus parameters.
+ (Consensus parameters are integer values voted on by authorities
+ and published in the consensus documents, described in
+ dir-spec.txt, section 3.3.)
+
+ Below we explain in more detail.
+
+2.2.1. Dividing time into periods [TIME-PERIODS]
+
+ To prevent a single set of hidden service directory from becoming a
+ target by adversaries looking to permanently censor a hidden service,
+ hidden service descriptors are uploaded to different locations that
+ change over time.
+
+ The length of a "time period" is controlled by the consensus
+ parameter 'hsdir-interval', and is a number of minutes between 30 and
+ 14400 (10 days). The default time period length is 1440 (one day).
+
+ Time periods start at the Unix epoch (Jan 1, 1970), and are computed by
+ taking the number of minutes since the epoch and dividing by the time
+ period. However, we want our time periods to start at 12:00UTC every day, so
+ we subtract a "rotation time offset" of 12*60 minutes from the number of
+ minutes since the epoch, before dividing by the time period (effectively
+ making "our" epoch start at Jan 1, 1970 12:00UTC).
+
+ Example: If the current time is 2016-04-13 11:15:01 UTC, making the seconds
+ since the epoch 1460546101, and the number of minutes since the epoch
+ 24342435. We then subtract the "rotation time offset" of 12*60 minutes from
+ the minutes since the epoch, to get 24341715. If the current time period
+ length is 1440 minutes, by doing the division we see that we are currently
+ in time period number 16903.
+
+ Specifically, time period #16903 began 16903*1440*60 + (12*60*60) seconds
+ after the epoch, at 2016-04-12 12:00 UTC, and ended at 16904*1440*60 +
+ (12*60*60) seconds after the epoch, at 2016-04-13 12:00 UTC.
+
+2.2.2. When to publish a hidden service descriptor [WHEN-HSDESC]
+
+ Hidden services periodically publish their descriptor to the responsible
+ HSDirs. The set of responsible HSDirs is determined as specified in
+ [WHERE-HSDESC].
+
+ Specifically, everytime a hidden service publishes its descriptor, it also
+ sets up a timer for a random time between 60 minutes and 120 minutes in the
+ future. When the timer triggers, the hidden service needs to publish its
+ descriptor again to the responsible HSDirs for that time period.
+ [TODO: Control republish period using a consensus parameter?]
+
+2.2.2.1. Overlapping descriptors
+
+ Hidden services need to upload multiple descriptors so that they can be
+ reachable to clients with older or newer consensuses than them. Services
+ need to upload their descriptors to the HSDirs _before_ the beginning of
+ each upcoming time period, so that they are readily available for clients to
+ fetch them. Furthermore, services should keep uploading their old descriptor
+ even after the end of a time period, so that they can be reachable by
+ clients that still have consensuses from the previous time period.
+
+ Hence, services maintain two active descriptors at every point. Clients on
+ the other hand, don't have a notion of overlapping descriptors, and instead
+ always download the descriptor for the current time period and shared random
+ value. It's the job of the service to ensure that descriptors will be
+ available for all clients. See section [FETCHUPLOADDESC] for how this is
+ achieved.
+
+ [TODO: What to do when we run multiple hidden services in a single host?]
+
+2.2.3. Where to publish a hidden service descriptor [WHERE-HSDESC]
+
+ This section specifies how the HSDir hash ring is formed at any given
+ time. Whenever a time value is needed (e.g. to get the current time period
+ number), we assume that clients and services use the valid-after time from
+ their latest live consensus.
+
+ The following consensus parameters control where a hidden service
+ descriptor is stored;
+
+ hsdir_n_replicas = an integer in range [1,16] with default value 2.
+ hsdir_spread_fetch = an integer in range [1,128] with default value 3.
+ hsdir_spread_store = an integer in range [1,128] with default value 3.
+
+ To determine where a given hidden service descriptor will be stored
+ in a given period, after the blinded public key for that period is
+ derived, the uploading or downloading party calculates:
+
+ for replicanum in 1...hsdir_n_replicas:
+ hs_index(replicanum) = H("store-at-idx" |
+ blinded_public_key |
+ INT_8(replicanum) |
+ INT_8(period_length) |
+ INT_8(period_num) )
+
+ where blinded_public_key is specified in section [KEYBLIND], period_length
+ is the length of the time period in minutes, and period_num is calculated
+ using the current consensus "valid-after" as specified in section
+ [TIME-PERIODS].
+
+ Then, for each node listed in the current consensus with the HSDirV3 flag,
+ we compute a directory index for that node as:
+
+ hsdir_index(node) = H("node-idx" | node_identity |
+ shared_random_value |
+ INT_8(period_num) |
+ INT_8(period_length) )
+
+ where shared_random_value is the shared value generated by the authorities
+ in section [PUB-SHAREDRANDOM], and node_identity is the ed25519 identity
+ key of the node.
+
+ Finally, for replicanum in 1...hsdir_n_replicas, the hidden service
+ host uploads descriptors to the first hsdir_spread_store nodes whose
+ indices immediately follow hs_index(replicanum). If any of those
+ nodes have already been selected for a lower-numbered replica of the
+ service, any nodes already chosen are disregarded (i.e. skipped over)
+ when choosing a replica's hsdir_spread_store nodes.
+
+ When choosing an HSDir to download from, clients choose randomly from
+ among the first hsdir_spread_fetch nodes after the indices. (Note
+ that, in order to make the system better tolerate disappearing
+ HSDirs, hsdir_spread_fetch may be less than hsdir_spread_store.)
+ Again, nodes from lower-numbered replicas are disregarded when
+ choosing the spread for a replica.
+
+2.2.4. Using time periods and SRVs to fetch/upload HS descriptors [FETCHUPLOADDESC]
+
+ Hidden services and clients need to make correct use of time periods (TP)
+ and shared random values (SRVs) to successfuly fetch and upload
+ descriptors. Furthermore, to avoid problems with skewed clocks, both clients
+ and services use the 'valid-after' time of a live consensus as a way to take
+ decisions with regards to uploading and fetching descriptors. By using the
+ consensus times as the ground truth here, we minimize the desynchronization
+ of clients and services due to system clock. Whenever time-based decisions
+ are taken in this section, assume that they are consensus times and not
+ system times.
+
+ As [PUB-SHAREDRANDOM] specifies, consensuses contain two shared random
+ values (the current one and the previous one). Hidden services and clients
+ are asked to match these shared random values with descriptor time periods
+ and use the right SRV when fetching/uploading descriptors. This section
+ attempts to precisely specify how this works.
+
+ Let's start with an illustration of the system:
+
+ +------------------------------------------------------------------+
+ | |
+ | 00:00 12:00 00:00 12:00 00:00 12:00 |
+ | SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
+ | |
+ | $==========|-----------$===========|-----------$===========| |
+ | |
+ | |
+ +------------------------------------------------------------------+
+
+ Legend: [TP#1 = Time Period #1]
+ [SRV#1 = Shared Random Value #1]
+ ["$" = descriptor rotation moment]
+
+2.2.4.1. Client behavior for fetching descriptors [CLIENTFETCH]
+
+ And here is how clients use TPs and SRVs to fetch descriptors:
+
+ Clients always aim to synchronize their TP with SRV, so they always want to
+ use TP#N with SRV#N: To achieve this wrt time periods, clients always use
+ the current time period when fetching descriptors. Now wrt SRVs, if a client
+ is in the time segment between a new time period and a new SRV (i.e. the
+ segments drawn with "-") it uses the current SRV, else if the client is in a
+ time segment between a new SRV and a new time period (i.e. the segments
+ drawn with "="), it uses the previous SRV.
+
+ Example:
+
+ +------------------------------------------------------------------+
+ | |
+ | 00:00 12:00 00:00 12:00 00:00 12:00 |
+ | SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
+ | |
+ | $==========|-----------$===========|-----------$===========| |
+ | ^ ^ |
+ | C1 C2 |
+ +------------------------------------------------------------------+
+
+ If a client (C1) is at 13:00 right after TP#1, then it will use TP#1 and
+ SRV#1 for fetching descriptors. Also, if a client (C2) is at 01:00 right
+ after SRV#2, it will still use TP#1 and SRV#1.
+
+2.2.4.2. Service behavior for uploading descriptors [SERVICEUPLOAD]
+
+ As discussed above, services maintain two active descriptors at any time. We
+ call these the "first" and "second" service descriptors. Services rotate
+ their descriptor everytime they receive a consensus with a valid_after time
+ past the next SRV calculation time. They rotate their descriptors by
+ discarding their first descriptor, pushing the second descriptor to the
+ first, and rebuilding their second descriptor with the latest data.
+
+ Services like clients also employ a different logic for picking SRV and TP
+ values based on their position in the graph above. Here is the logic:
+
+2.2.4.2.1. First descriptor upload logic [FIRSTDESCUPLOAD]
+
+ Here is the service logic for uploading its first descriptor:
+
+ When a service is in the time segment between a new time period a new SRV
+ (i.e. the segments drawn with "-"), it uses the previous time period and
+ previous SRV for uploading its first descriptor: that's meant to cover
+ for clients that have a consensus that is still in the previous time period.
+
+ Example: Consider in the above illustration that the service is at 13:00
+ right after TP#1. It will upload its first descriptor using TP#0 and SRV#0.
+ So if a client still has a 11:00 consensus it will be able to access it
+ based on the client logic above.
+
+ Now if a service is in the time segment between a new SRV and a new time
+ period (i.e. the segments drawn with "=") it uses the current time period
+ and the previous SRV for its first descriptor: that's meant to cover clients
+ with an up-to-date consensus in the same time period as the service.
+
+ Example:
+
+ +------------------------------------------------------------------+
+ | |
+ | 00:00 12:00 00:00 12:00 00:00 12:00 |
+ | SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
+ | |
+ | $==========|-----------$===========|-----------$===========| |
+ | ^ |
+ | S |
+ +------------------------------------------------------------------+
+
+ Consider that the service is at 01:00 right after SRV#2: it will upload its
+ first descriptor using TP#1 and SRV#1.
+
+2.2.4.2.2. Second descriptor upload logic [SECONDDESCUPLOAD]
+
+ Here is the service logic for uploading its second descriptor:
+
+ When a service is in the time segment between a new time period a new SRV
+ (i.e. the segments drawn with "-"), it uses the current time period and
+ current SRV for uploading its second descriptor: that's meant to cover for
+ clients that have an up-to-date consensus on the same TP as the service.
+
+ Example: Consider in the above illustration that the service is at 13:00
+ right after TP#1: it will upload its second descriptor using TP#1 and SRV#1.
+
+ Now if a service is in the time segment between a new SRV and a new time
+ period (i.e. the segments drawn with "=") it uses the next time period and
+ the current SRV for its second descriptor: that's meant to cover clients
+ with a newer consensus than the service (in the next time period).
+
+ Example:
+
+ +------------------------------------------------------------------+
+ | |
+ | 00:00 12:00 00:00 12:00 00:00 12:00 |
+ | SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
+ | |
+ | $==========|-----------$===========|-----------$===========| |
+ | ^ |
+ | S |
+ +------------------------------------------------------------------+
+
+ Consider that the service is at 01:00 right after SRV#2: it will upload its
+ second descriptor using TP#2 and SRV#2.
+
+2.2.5. Expiring hidden service descriptors [EXPIRE-DESC]
+
+ Hidden services set their descriptor's "descriptor-lifetime" field to 180
+ minutes (3 hours). Hidden services ensure that their descriptor will remain
+ valid in the HSDir caches, by republishing their descriptors periodically as
+ specified in [WHEN-HSDESC].
+
+ Hidden services MUST also keep their introduction circuits alive for as long
+ as descriptors including those intro points are valid (even if that's after
+ the time period has changed).
+
+2.2.6. URLs for anonymous uploading and downloading
+
+ Hidden service descriptors conforming to this specification are uploaded
+ with an HTTP POST request to the URL /tor/hs/<version>/publish relative to
+ the hidden service directory's root, and downloaded with an HTTP GET
+ request for the URL /tor/hs/<version>/<z> where <z> is a base64 encoding of
+ the hidden service's blinded public key and <version> is the protocol
+ version which is "3" in this case.
+
+ These requests must be made anonymously, on circuits not used for
+ anything else.
+
+2.2.7. Client-side validation of onion addresses
+
+ When a Tor client receives a prop224 onion address from the user, it
+ MUST first validate the onion address before attempting to connect or
+ fetch its descriptor. If the validation fails, the client MUST
+ refuse to connect.
+
+ As part of the address validation, Tor clients should check that the
+ underlying ed25519 key does not have a torsion component. If Tor accepted
+ ed25519 keys with torsion components, attackers could create multiple
+ equivalent onion addresses for a single ed25519 key, which would map to the
+ same service. We want to avoid that because it could lead to phishing
+ attacks and surprising behaviors (e.g. imagine a browser plugin that blocks
+ onion addresses, but could be bypassed using an equivalent onion address
+ with a torsion component).
+
+ The right way for clients to detect such fraudulent addresses (which should
+ only occur malevolently and never natutally) is to extract the ed25519
+ public key from the onion address and multiply it by the ed25519 group order
+ and ensure that the result is the ed25519 identity element. For more
+ details, please see [TORSION-REFS].
+
+2.3. Publishing shared random values [PUB-SHAREDRANDOM]
+
+ Our design for limiting the predictability of HSDir upload locations
+ relies on a shared random value (SRV) that isn't predictable in advance or
+ too influenceable by an attacker. The authorities must run a protocol
+ to generate such a value at least once per hsdir period. Here we
+ describe how they publish these values; the procedure they use to
+ generate them can change independently of the rest of this
+ specification. For more information see [SHAREDRANDOM-REFS].
+
+ According to proposal 250, we add two new lines in consensuses:
+
+ "shared-rand-previous-value" SP NUM_REVEALS SP VALUE NL
+ "shared-rand-current-value" SP NUM_REVEALS SP VALUE NL
+
+2.3.1. Client behavior in the absense of shared random values
+
+ If the previous or current shared random value cannot be found in a
+ consensus, then Tor clients and services need to generate their own random
+ value for use when choosing HSDirs.
+
+ To do so, Tor clients and services use:
+
+ SRV = H("shared-random-disaster" | INT_8(period_length) | INT_8(period_num))
+
+ where period_length is the length of a time period in minutes, period_num is
+ calculated as specified in [TIME-PERIODS] for the wanted shared random value
+ that could not be found originally.
+
+2.3.2. Hidden services and changing shared random values
+
+ It's theoretically possible that the consensus shared random values will
+ change or disappear in the middle of a time period because of directory
+ authorities dropping offline or misbehaving.
+
+ To avoid client reachability issues in this rare event, hidden services
+ should use the new shared random values to find the new responsible HSDirs
+ and upload their descriptors there.
+
+ XXX How long should they upload descriptors there for?
+
+2.4. Hidden service descriptors: outer wrapper [DESC-OUTER]
+
+ The format for a hidden service descriptor is as follows, using the
+ meta-format from dir-spec.txt.
+
+ "hs-descriptor" SP version-number NL
+
+ [At start, exactly once.]
+
+ The version-number is a 32 bit unsigned integer indicating the version
+ of the descriptor. Current version is "3".
+
+ "descriptor-lifetime" SP LifetimeMinutes NL
+
+ [Exactly once]
+
+ The lifetime of a descriptor in minutes. An HSDir SHOULD expire the
+ hidden service descriptor at least LifetimeMinutes after it was
+ uploaded.
+
+ The LifetimeMinutes field can take values between 30 and 3000 (50 hours).
+
+ "descriptor-signing-key-cert" NL certificate NL
+
+ [Exactly once.]
+
+ The 'certificate' field contains a certificate in the format from
+ proposal 220, wrapped with "-----BEGIN ED25519 CERT-----". The
+ certificate cross-certifies the short-term descriptor signing key with
+ the blinded public key. The certificate type must be [08], and the
+ blinded public key must be present as the signing-key extension.
+
+ "revision-counter" SP Integer NL
+
+ [Exactly once.]
+
+ The revision number of the descriptor. If an HSDir receives a
+ second descriptor for a key that it already has a descriptor for,
+ it should retain and serve the descriptor with the higher
+ revision-counter.
+
+ (Checking for monotonically increasing revision-counter values
+ prevents an attacker from replacing a newer descriptor signed by
+ a given key with a copy of an older version.)
+
+ "superencrypted" NL encrypted-string
+
+ [Exactly once.]
+
+ An encrypted blob, whose format is discussed in [HS-DESC-ENC] below. The
+ blob is base64 encoded and enclosed in -----BEGIN MESSAGE---- and
+ ----END MESSAGE---- wrappers.
+
+ "signature" SP signature NL
+
+ [exactly once, at end.]
+
+ A signature of all previous fields, using the signing key in the
+ descriptor-signing-key-cert line, prefixed by the string "Tor onion
+ service descriptor sig v3". We use a separate key for signing, so that
+ the hidden service host does not need to have its private blinded key
+ online.
+
+ HSDirs accept hidden service descriptors of up to 50k bytes (a consensus
+ parameter should also be introduced to control this value).
+
+2.5. Hidden service descriptors: encryption format [HS-DESC-ENC]
+
+ Hidden service descriptors are protected by two layers of encryption.
+ Clients need to decrypt both layers to connect to the hidden service.
+
+ The first layer of encryption provides confidentiality against entities who
+ don't know the public key of the hidden service (e.g. HSDirs), while the
+ second layer of encryption is only useful when client authorization is enabled
+ and protects against entities that do not possess valid client credentials.
+
+2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER]
+
+ The first layer of HS descriptor encryption is designed to protect
+ descriptor confidentiality against entities who don't know the blinded
+ public key of the hidden service.
+
+2.5.1.1. First layer encryption logic
+
+ The encryption keys and format for the first layer of encryption are
+ generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization
+ parameters:
+
+ SECRET_DATA = blinded-public-key
+ STRING_CONSTANT = "hsdir-superencrypted-data"
+
+ The ciphertext is placed on the "superencrypted" field of the descriptor.
+
+ Before encryption the plaintext is padded with NUL bytes to the nearest
+ multiple of 10k bytes.
+
+2.5.1.2. First layer plaintext format
+
+ After clients decrypt the first layer of encryption, they need to parse the
+ plaintext to get to the second layer ciphertext which is contained in the
+ "encrypted" field.
+
+ If client auth is enabled, the hidden service generates a fresh
+ descriptor_cookie key (32 random bytes) and encrypts it using each
+ authorized client's identity x25519 key. Authorized clients can use the
+ descriptor cookie to decrypt the second layer of encryption. Our encryption
+ scheme requires the hidden service to also generate an ephemeral x25519
+ keypair for each new descriptor.
+
+ If client auth is disabled, fake data is placed in each of the fields below
+ to obfuscate whether client authorization is enabled.
+
+ Here are all the supported fields:
+
+ "desc-auth-type" SP type NL
+
+ [Exactly once]
+
+ This field contains the type of authorization used to protect the
+ descriptor. The only recognized type is "x25519" and specifies the
+ encryption scheme described in this section.
+
+ If client authorization is disabled, the value here should be "x25519".
+
+ "desc-auth-ephemeral-key" SP key NL
+
+ [Exactly once]
+
+ This field contains an ephemeral x25519 public key generated by the
+ hidden service and encoded in base64. The key is used by the encryption
+ scheme below.
+
+ If client authorization is disabled, the value here should be a fresh
+ x25519 pubkey that will remain unused.
+
+ "auth-client" SP client-id SP iv SP encrypted-cookie
+
+ [Any number]
+
+ When client authorization is enabled, the hidden service inserts an
+ "auth-client" line for each of its authorized clients. If client
+ authorization is disabled, the fields here can be populated with random
+ data of the right size (that's 8 bytes for 'client-id', 16 bytes for 'iv'
+ and 16 bytes for 'encrypted-cookie' all encoded with base64).
+
+ When client authorization is enabled, each "auth-client" line contains
+ the descriptor cookie encrypted to each individual client. We assume that
+ each authorized client possesses a pre-shared x25519 keypair which is
+ used to decrypt the descriptor cookie.
+
+ We now describe the descriptor cookie encryption scheme. Here are the
+ relevant keys:
+
+ client_x = private x25519 key of authorized client
+ client_X = public x25519 key of authorized client
+ hs_y = private key of ephemeral x25519 keypair of hidden service
+ hs_Y = public key of ephemeral x25519 keypair of hidden service
+ descriptor_cookie = descriptor cookie used to encrypt the descriptor
+
+ And here is what the hidden service computes:
+
+ SECRET_SEED = x25519(hs_y, client_X)
+ KEYS = KDF(SECRET_SEED, 40)
+ CLIENT-ID = fist 8 bytes of KEYS
+ COOKIE-KEY = last 32 bytes of KEYS
+
+ Here is a description of the fields in the "auth-client" line:
+
+ - The "client-id" field is CLIENT-ID from above encoded in base64.
+
+ - The "iv" field is 16 random bytes encoded in base64.
+
+ - The "encrypted-cookie" field contains the descriptor cookie ciphertext
+ as follows and is encoded in base64:
+ encrypted-cookie = STREAM(iv, COOKIE-KEY) XOR descriptor_cookie
+
+ See section [FIRST-LAYER-CLIENT-BEHAVIOR] for the client-side logic of
+ how to decrypt the descriptor cookie.
+
+ "encrypted" NL encrypted-string
+
+ [Exactly once]
+
+ An encrypted blob containing the second layer ciphertext, whose format is
+ discussed in [HS-DESC-SECOND-LAYER] below. The blob is base64 encoded
+ and enclosed in -----BEGIN MESSAGE---- and ----END MESSAGE---- wrappers.
+
+2.5.1.3. Client behavior [FIRST-LAYER-CLIENT-BEHAVIOR]
+
+ The goal of clients at this stage is to decrypt the "encrypted" field as
+ described in [HS-DESC-SECOND-LAYER].
+
+ If client authorization is enabled, authorized clients need to extract the
+ descriptor cookie to proceed with decryption of the second layer as
+ follows:
+
+ An authorized client parsing the first layer of an encrypted descriptor,
+ extracts the ephemeral key from "desc-auth-ephemeral-key" and calculates
+ CLIENT-ID and COOKIE-KEY as described in the section above using their
+ x25519 private key. The client then uses CLIENT-ID to find the right
+ "auth-client" field which contains the ciphertext of the descriptor
+ cookie. The client then uses COOKIE-KEY and the iv to decrypt the
+ descriptor_cookie, which is used to decrypt the second layer of descriptor
+ encryption as described in [HS-DESC-SECOND-LAYER].
+
+2.5.1.4. Hiding client authorization data
+
+ Hidden services should avoid leaking whether client authorization is
+ enabled or how many authorized clients there are.
+
+ Hence even when client authorization is disabled, the hidden service adds
+ fake "desc-auth-type", "desc-auth-ephemeral-key" and "auth-client" lines to
+ the descriptor, as described in [HS-DESC-FIRST-LAYER].
+
+ The hidden service also avoids leaking the number of authorized clients by
+ adding fake "auth-client" entries to its descriptor. Specifically,
+ descriptors always contain a number of authorized clients that is a
+ multiple of 16 by adding fake "auth-client" entries if needed.
+ [XXX consider randomization of the value 16]
+
+ Clients MUST accept descriptors with any number of "auth-client" lines as
+ long as the total descriptor size is within the max limit of 50k (also
+ controlled with a consensus parameter).
+
+2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER]
+
+ The second layer of descriptor encryption is designed to protect descriptor
+ confidentiality against unauthorized clients. If client authorization is
+ enabled, it's encrypted using the descriptor_cookie, and contains needed
+ information for connecting to the hidden service, like the list of its
+ introduction points.
+
+ If client authorization is disabled, then the second layer of HS encryption
+ does not offer any additional security, but is still used.
+
+2.5.2.1. Second layer encryption keys
+
+ The encryption keys and format for the second layer of encryption are
+ generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization
+ parameters as follows:
+
+ SECRET_DATA = blinded-public-key | descriptor_cookie
+ STRING_CONSTANT = "hsdir-encrypted-data"
+
+ If client authorization is disabled the 'descriptor_cookie' field is left blank.
+
+ The ciphertext is placed on the "encrypted" field of the descriptor.
+
+2.5.2.2. Second layer plaintext format
+
+ After decrypting the second layer ciphertext, clients can finally learn the
+ list of intro points etc. The plaintext has the following format:
+
+ "create2-formats" SP formats NL
+
+ [Exactly once]
+
+ A space-separated list of integers denoting CREATE2 cell format numbers
+ that the server recognizes. Must include at least ntor as described in
+ tor-spec.txt. See tor-spec section 5.1 for a list of recognized
+ handshake types.
+
+ "intro-auth-required" SP types NL
+
+ [At most once]
+
+ A space-separated list of introduction-layer authentication types; see
+ section [INTRO-AUTH] for more info. A client that does not support at
+ least one of these authentication types will not be able to contact the
+ host. Recognized types are: 'password' and 'ed25519'.
+
+ "single-onion-service"
+
+ [None or at most once]
+
+ If present, this line indicates that the service is a Single Onion
+ Service (see prop260 for more details about that type of service). This
+ field has been introduced in 0.3.0 meaning 0.2.9 service don't include
+ this.
+
+ Followed by zero or more introduction points as follows (see section
+ [NUM_INTRO_POINT] below for accepted values):
+
+ "introduction-point" SP link-specifiers NL
+
+ [Exactly once per introduction point at start of introduction
+ point section]
+
+ The link-specifiers is a base64 encoding of a link specifier
+ block in the format described in BUILDING-BLOCKS.
+
+ "onion-key" SP "ntor" SP key NL
+
+ [Exactly once per introduction point]
+
+ The key is a base64 encoded curve25519 public key which is the onion
+ key of the introduction point Tor node used for the ntor handshake
+ when a client extends to it.
+
+ "auth-key" NL certificate NL
+
+ [Exactly once per introduction point]
+
+ The certificate is a proposal 220 certificate wrapped in
+ "-----BEGIN ED25519 CERT-----", cross-certifying the descriptor
+ signing key with the introduction point authentication key, which
+ is included in the mandatory signing-key extension. The certificate
+ type must be [09].
+
+ "enc-key" SP "ntor" SP key NL
+
+ [Exactly once per introduction point]
+
+ The key is a base64 encoded curve25519 public key used to encrypt
+ the introduction request to service.
+
+ "enc-key-cert" NL certificate NL
+
+ [Exactly once per introduction point]
+
+ Cross-certification of the descriptor signing key by the encryption
+ key.
+
+ For "ntor" keys, certificate is a proposal 220 certificate wrapped
+ in "-----BEGIN ED25519 CERT-----" armor, cross-certifying the
+ descriptor signing key with the ed25519 equivalent of a curve25519
+ public encryption key derived using the process in proposal 228
+ appendix A. The certificate type must be [0B], and the signing-key
+ extension is mandatory.
+
+ "legacy-key" NL key NL
+
+ [None or at most once per introduction point]
+
+ The key is an ASN.1 encoded RSA public key in PEM format used for a
+ legacy introduction point as described in [LEGACY_EST_INTRO].
+
+ This field is only present if the introduction point only supports
+ legacy protocol (v2) that is <= 0.2.9 or the protocol version value
+ "HSIntro 3".
+
+ "legacy-key-cert NL certificate NL
+
+ [None or at most once per introduction point]
+
+ MUST be present if "legacy-key" is present.
+
+ The certificate is a proposal 220 RSA->Ed cross-certificate wrapped
+ in "-----BEGIN CROSSCERT-----" armor, cross-certifying the
+ descriptor signing key with the RSA public key found in
+ "legacy-key".
+
+ To remain compatible with future revisions to the descriptor format,
+ clients should ignore unrecognized lines in the descriptor.
+ Other encryption and authentication key formats are allowed; clients
+ should ignore ones they do not recognize.
+
+ Clients who manage to extract the introduction points of the hidden service
+ can prroceed with the introduction protocol as specified in [INTRO-PROTOCOL].
+
+2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS]
+
+ In this section we present the generic encryption format for hidden service
+ descriptors. We use the same encryption format in both encryption layers,
+ hence we introduce two customization parameters SECRET_DATA and
+ STRING_CONSTANT which vary between the layers.
+
+ The SECRET_DATA parameter specifies the secret data that are used during
+ encryption key generation, while STRING_CONSTANT is merely a string constant
+ that is used as part of the KDF.
+
+ Here is the key generation logic:
+
+ SALT = 16 bytes from H(random), changes each time we rebuld the
+ descriptor even if the content of the descriptor hasn't changed.
+ (So that we don't leak whether the intro point list etc. changed)
+
+ secret_input = SECRET_DATA | subcredential | INT_8(revision_counter)
+
+ keys = KDF(secret_input | salt | STRING_CONSTANT, S_KEY_LEN + S_IV_LEN + MAC_KEY_LEN)
+
+ SECRET_KEY = first S_KEY_LEN bytes of keys
+ SECRET_IV = next S_IV_LEN bytes of keys
+ MAC_KEY = last MAC_KEY_LEN bytes of keys
+
+ The encrypted data has the format:
+
+ SALT hashed random bytes from above [16 bytes]
+ ENCRYPTED The ciphertext [variable]
+ MAC MAC of both above fields [32 bytes]
+
+ The final encryption format is ENCRYPTED = STREAM(SECRET_IV,SECRET_KEY) XOR Plaintext
+
+2.5.4. Number of introduction points [NUM_INTRO_POINT]
+
+ This section defines how many introduction points an hidden service
+ descriptor can have at minimum, by default and the maximum:
+
+ Minimum: 0 - Default: 3 - Maximum: 20
+
+ A value of 0 would means that the service is still alive but doesn't want
+ to be reached by any client at the moment. Note that the descriptor size
+ increases considerably as more introduction points are added.
+
+ The reason for a maximum value of 20 is to give enough scalability to tools
+ like OnionBalance to be able to load balance up to 120 servers (20 x 6
+ HSDirs) but also in order for the descriptor size to not overwhelmed hidden
+ service directories with user defined values that could be gigantic.
+
+3. The introduction protocol [INTRO-PROTOCOL]
+
+ The introduction protocol proceeds in three steps.
+
+ First, a hidden service host builds an anonymous circuit to a Tor
+ node and registers that circuit as an introduction point.
+
+ [After 'First' and before 'Second', the hidden service publishes its
+ introduction points and associated keys, and the client fetches
+ them as described in section [HSDIR] above.]
+
+ Second, a client builds an anonymous circuit to the introduction
+ point, and sends an introduction request.
+
+ Third, the introduction point relays the introduction request along
+ the introduction circuit to the hidden service host, and acknowledges
+ the introduction request to the client.
+
+3.1. Registering an introduction point [REG_INTRO_POINT]
+
+3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO]
+
+ When a hidden service is establishing a new introduction point, it
+ sends an ESTABLISH_INTRO cell with the following contents:
+
+ AUTH_KEY_TYPE [1 byte]
+ AUTH_KEY_LEN [2 bytes]
+ AUTH_KEY [AUTH_KEY_LEN bytes]
+ N_EXTENSIONS [1 byte]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+ HANDSHAKE_AUTH [MAC_LEN bytes]
+ SIG_LEN [2 bytes]
+ SIG [SIG_LEN bytes]
+
+ The AUTH_KEY_TYPE field indicates the type of the introduction point
+ authentication key and the type of the MAC to use in
+ HANDSHAKE_AUTH. Recognized types are:
+
+ [00, 01] -- Reserved for legacy introduction cells; see
+ [LEGACY_EST_INTRO below]
+ [02] -- Ed25519; SHA3-256.
+
+ The AUTH_KEY_LEN field determines the length of the AUTH_KEY
+ field. The AUTH_KEY field contains the public introduction point
+ authentication key.
+
+ The EXT_FIELD_TYPE, EXT_FIELD_LEN, EXT_FIELD entries are reserved for
+ future extensions to the introduction protocol. Extensions with
+ unrecognized EXT_FIELD_TYPE values must be ignored.
+
+ The HANDSHAKE_AUTH field contains the MAC of all earlier fields in
+ the cell using as its key the shared per-circuit material ("KH")
+ generated during the circuit extension protocol; see tor-spec.txt
+ section 5.2, "Setting circuit keys". It prevents replays of
+ ESTABLISH_INTRO cells.
+
+ SIG_LEN is the length of the signature.
+
+ SIG is a signature, using AUTH_KEY, of all contents of the cell, up
+ to but not including SIG. These contents are prefixed with the string
+ "Tor establish-intro cell v1".
+
+ Upon receiving an ESTABLISH_INTRO cell, a Tor node first decodes the
+ key and the signature, and checks the signature. The node must reject
+ the ESTABLISH_INTRO cell and destroy the circuit in these cases:
+
+ * If the key type is unrecognized
+ * If the key is ill-formatted
+ * If the signature is incorrect
+ * If the HANDSHAKE_AUTH value is incorrect
+
+ * If the circuit is already a rendezvous circuit.
+ * If the circuit is already an introduction circuit.
+ [TODO: some scalability designs fail there.]
+ * If the key is already in use by another circuit.
+
+ Otherwise, the node must associate the key with the circuit, for use
+ later in INTRODUCE1 cells.
+
+3.1.2. Registering an introduction point on a legacy Tor node
+ [LEGACY_EST_INTRO]
+
+ Tor nodes should also support an older version of the ESTABLISH_INTRO
+ cell, first documented in rend-spec.txt. New hidden service hosts
+ must use this format when establishing introduction points at older
+ Tor nodes that do not support the format above in [EST_INTRO].
+
+ In this older protocol, an ESTABLISH_INTRO cell contains:
+
+ KEY_LEN [2 bytes]
+ KEY [KEY_LEN bytes]
+ HANDSHAKE_AUTH [20 bytes]
+ SIG [variable, up to end of relay payload]
+
+ The KEY_LEN variable determines the length of the KEY field.
+
+ The KEY field is the ASN1-encoded legacy RSA public key that was also
+ included in the hidden service descriptor.
+
+ The HANDSHAKE_AUTH field contains the SHA1 digest of (KH | "INTRODUCE").
+
+ The SIG field contains an RSA signature, using PKCS1 padding, of all
+ earlier fields.
+
+ Older versions of Tor always use a 1024-bit RSA key for these introduction
+ authentication keys.
+
+3.1.3. Acknowledging establishment of introduction point [INTRO_ESTABLISHED]
+
+ After setting up an introduction circuit, the introduction point reports its
+ status back to the hidden service host with an INTRO_ESTABLISHED cell.
+
+ The INTRO_ESTABLISHED cell has the following contents:
+
+ N_EXTENSIONS [1 byte]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+
+ Older versions of Tor send back an empty INTRO_ESTABLISHED cell instead.
+ Services must accept an empty INTRO_ESTABLISHED cell from a legacy relay.
+
+3.2. Sending an INTRODUCE1 cell to the introduction point. [SEND_INTRO1]
+
+ In order to participate in the introduction protocol, a client must
+ know the following:
+
+ * An introduction point for a service.
+ * The introduction authentication key for that introduction point.
+ * The introduction encryption key for that introduction point.
+
+ The client sends an INTRODUCE1 cell to the introduction point,
+ containing an identifier for the service, an identifier for the
+ encryption key that the client intends to use, and an opaque blob to
+ be relayed to the hidden service host.
+
+ In reply, the introduction point sends an INTRODUCE_ACK cell back to
+ the client, either informing it that its request has been delivered,
+ or that its request will not succeed.
+
+ [TODO: specify what tor should do when receiving a malformed cell. Drop it?
+ Kill circuit? This goes for all possible cells.]
+
+3.2.1. INTRODUCE1 cell format [FMT_INTRO1]
+
+ When a client is connecting to an introduction point, INTRODUCE1 cells
+ should be of the form:
+
+ LEGACY_KEY_ID [20 bytes]
+ AUTH_KEY_TYPE [1 byte]
+ AUTH_KEY_LEN [2 bytes]
+ AUTH_KEY [AUTH_KEY_LEN bytes]
+ N_EXTENSIONS [1 byte]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+ ENCRYPTED [Up to end of relay payload]
+
+ AUTH_KEY_TYPE is defined as in [EST_INTRO]. Currently, the only value of
+ AUTH_KEY_TYPE for this cell is an Ed25519 public key [02].
+
+ The LEGACY_KEY_ID field is used to distinguish between legacy and new style
+ INTRODUCE1 cells. In new style INTRODUCE1 cells, LEGACY_KEY_ID is 20 zero
+ bytes. Upon receiving an INTRODUCE1 cell, the introduction point checks the
+ LEGACY_KEY_ID field. If LEGACY_KEY_ID is non-zero, the INTRODUCE1 cell
+ should be handled as a legacy INTRODUCE1 cell by the intro point.
+
+ Upon receiving a INTRODUCE1 cell, the introduction point checks
+ whether AUTH_KEY matches the introduction point authentication key for an
+ active introduction circuit. If so, the introduction point sends an
+ INTRODUCE2 cell with exactly the same contents to the service, and sends an
+ INTRODUCE_ACK response to the client.
+
+3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK]
+
+ An INTRODUCE_ACK cell has the following fields:
+
+ STATUS [2 bytes]
+ N_EXTENSIONS [1 bytes]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+
+ Recognized status values are:
+ [00 00] -- Success: cell relayed to hidden service host.
+ [00 01] -- Failure: service ID not recognized
+ [00 02] -- Bad message format
+ [00 03] -- Can't relay cell to service
+
+3.3. Processing an INTRODUCE2 cell at the hidden service. [PROCESS_INTRO2]
+
+ Upon receiving an INTRODUCE2 cell, the hidden service host checks whether
+ the AUTH_KEY or LEGACY_KEY_ID field matches the keys for this
+ introduction circuit.
+
+ The service host then checks whether it has received a cell with these
+ contents or rendezvous cookie before. If it has, it silently drops it as a
+ replay. (It must maintain a replay cache for as long as it accepts cells
+ with the same encryption key. Note that the encryption format below should
+ be non-malleable.)
+
+ If the cell is not a replay, it decrypts the ENCRYPTED field,
+ establishes a shared key with the client, and authenticates the whole
+ contents of the cell as having been unmodified since they left the
+ client. There may be multiple ways of decrypting the ENCRYTPED field,
+ depending on the chosen type of the encryption key. Requirements for
+ an introduction handshake protocol are described in
+ [INTRO-HANDSHAKE-REQS]. We specify one below in section
+ [NTOR-WITH-EXTRA-DATA].
+
+ The decrypted plaintext must have the form:
+
+ RENDEZVOUS_COOKIE [20 bytes]
+ N_EXTENSIONS [1 byte]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+ ONION_KEY_TYPE [1 bytes]
+ ONION_KEY_LEN [2 bytes]
+ ONION_KEY [ONION_KEY_LEN bytes]
+ NSPEC (Number of link specifiers) [1 byte]
+ NSPEC times:
+ LSTYPE (Link specifier type) [1 byte]
+ LSLEN (Link specifier length) [1 byte]
+ LSPEC (Link specifier) [LSLEN bytes]
+ PAD (optional padding) [up to end of plaintext]
+
+ Upon processing this plaintext, the hidden service makes sure that
+ any required authentication is present in the extension fields, and
+ then extends a rendezvous circuit to the node described in the LSPEC
+ fields, using the ONION_KEY to complete the extension. As mentioned
+ in [BUILDING-BLOCKS], the "TLS-over-TCP, IPv4" and "Legacy node
+ identity" specifiers must be present.
+
+ The hidden service SHOULD NOT reject any LSTYPE fields which it
+ doesn't recognize; instead, it should use them verbatim in its EXTEND
+ request to the rendezvous point.
+
+ The ONION_KEY_TYPE field is:
+
+ [01] NTOR: ONION_KEY is 32 bytes long.
+
+ The ONION_KEY field describes the onion key that must be used when
+ extending to the rendezvous point. It must be of a type listed as
+ supported in the hidden service descriptor.
+
+ When using a legacy introduction point, the INTRODUCE cells must be padded
+ to a certain length using the PAD field in the encrypted portion.
+
+ Upon receiving a well-formed INTRODUCE2 cell, the hidden service host
+ will have:
+
+ * The information needed to connect to the client's chosen
+ rendezvous point.
+ * The second half of a handshake to authenticate and establish a
+ shared key with the hidden service client.
+ * A set of shared keys to use for end-to-end encryption.
+
+3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS]
+
+ When decoding the encrypted information in an INTRODUCE2 cell, a
+ hidden service host must be able to:
+
+ * Decrypt additional information included in the INTRODUCE2 cell,
+ to include the rendezvous token and the information needed to
+ extend to the rendezvous point.
+
+ * Establish a set of shared keys for use with the client.
+
+ * Authenticate that the cell has not been modified since the client
+ generated it.
+
+ Note that the old TAP-derived protocol of the previous hidden service
+ design achieved the first two requirements, but not the third.
+
+3.3.2. Example encryption handshake: ntor with extra data
+ [NTOR-WITH-EXTRA-DATA]
+
+ [TODO: relocate this]
+
+ This is a variant of the ntor handshake (see tor-spec.txt, section
+ 5.1.4; see proposal 216; and see "Anonymity and one-way
+ authentication in key-exchange protocols" by Goldberg, Stebila, and
+ Ustaoglu).
+
+ It behaves the same as the ntor handshake, except that, in addition
+ to negotiating forward secure keys, it also provides a means for
+ encrypting non-forward-secure data to the server (in this case, to
+ the hidden service host) as part of the handshake.
+
+ Notation here is as in section 5.1.4 of tor-spec.txt, which defines
+ the ntor handshake.
+
+ The PROTOID for this variant is "tor-hs-ntor-curve25519-sha3-256-1".
+ We also use the following tweak values:
+
+ t_hsenc = PROTOID | ":hs_key_extract"
+ t_hsverify = PROTOID | ":hs_verify"
+ t_hsmac = PROTOID | ":hs_mac"
+ m_hsexpand = PROTOID | ":hs_key_expand"
+
+ To make an INTRODUCE1 cell, the client must know a public encryption
+ key B for the hidden service on this introduction circuit. The client
+ generates a single-use keypair:
+ x,X = KEYGEN()
+ and computes:
+ intro_secret_hs_input = EXP(B,x) | AUTH_KEY | X | B | PROTOID
+ info = m_hsexpand | subcredential
+ hs_keys = KDF(intro_secret_hs_input | t_hsenc | info, S_KEY_LEN+MAC_LEN)
+ ENC_KEY = hs_keys[0:S_KEY_LEN]
+ MAC_KEY = hs_keys[S_KEY_LEN:S_KEY_LEN+MAC_KEY_LEN]
+
+ and sends, as the ENCRYPTED part of the INTRODUCE1 cell:
+
+ CLIENT_PK [PK_PUBKEY_LEN bytes]
+ ENCRYPTED_DATA [Padded to length of plaintext]
+ MAC [MAC_LEN bytes]
+
+
+ Substituting those fields into the INTRODUCE1 cell body format
+ described in [FMT_INTRO1] above, we have
+
+ LEGACY_KEY_ID [20 bytes]
+ AUTH_KEY_TYPE [1 byte]
+ AUTH_KEY_LEN [2 bytes]
+ AUTH_KEY [AUTH_KEY_LEN bytes]
+ N_EXTENSIONS [1 bytes]
+ N_EXTENSIONS times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXT_FIELD_LEN bytes]
+ ENCRYPTED:
+ CLIENT_PK [PK_PUBKEY_LEN bytes]
+ ENCRYPTED_DATA [Padded to length of plaintext]
+ MAC [MAC_LEN bytes]
+
+
+ (This format is as documented in [FMT_INTRO1] above, except that here
+ we describe how to build the ENCRYPTED portion.)
+
+ Here, the encryption key plays the role of B in the regular ntor
+ handshake, and the AUTH_KEY field plays the role of the node ID.
+ The CLIENT_PK field is the public key X. The ENCRYPTED_DATA field is
+ the message plaintext, encrypted with the symmetric key ENC_KEY. The
+ MAC field is a MAC of all of the cell from the AUTH_KEY through the
+ end of ENCRYPTED_DATA, using the MAC_KEY value as its key.
+
+ To process this format, the hidden service checks PK_VALID(CLIENT_PK)
+ as necessary, and then computes ENC_KEY and MAC_KEY as the client did
+ above, except using EXP(CLIENT_PK,b) in the calculation of
+ intro_secret_hs_input. The service host then checks whether the MAC is
+ correct. If it is invalid, it drops the cell. Otherwise, it computes
+ the plaintext by decrypting ENCRYPTED_DATA.
+
+ The hidden service host now completes the service side of the
+ extended ntor handshake, as described in tor-spec.txt section 5.1.4,
+ with the modified PROTOID as given above. To be explicit, the hidden
+ service host generates a keypair of y,Y = KEYGEN(), and uses its
+ introduction point encryption key 'b' to computes:
+
+ intro_secret_hs_input = EXP(X,b) | AUTH_KEY | X | B | PROTOID
+ info = m_hsexpand | subcredential
+ hs_keys = KDF(intro_secret_hs_input | t_hsenc | info, S_KEY_LEN+MAC_LEN)
+ HS_DEC_KEY = hs_keys[0:S_KEY_LEN]
+ HS_MAC_KEY = hs_keys[S_KEY_LEN:S_KEY_LEN+MAC_KEY_LEN]
+
+ (The above are used to check the MAC and then decrypt the
+ encrypted data.)
+
+ rend_secret_hs_input = EXP(X,y) | EXP(X,b) | AUTH_KEY | B | X | Y | PROTOID
+ NTOR_KEY_SEED = MAC(rend_secret_hs_input, t_hsenc)
+ verify = MAC(rend_secret_hs_input, t_hsverify)
+ auth_input = verify | AUTH_KEY | B | Y | X | PROTOID | "Server"
+ AUTH_INPUT_MAC = MAC(auth_input, t_hsmac)
+
+ (The above are used to finish the ntor handshake.)
+
+ The server's handshake reply is:
+ SERVER_PK Y [PK_PUBKEY_LEN bytes]
+ AUTH AUTH_INPUT_MAC [MAC_LEN bytes]
+
+ These fields will be sent to the client in a RENDEZVOUS1 cell using the
+ HANDSHAKE_INFO element (see [JOIN_REND]).
+
+ The hidden service host now also knows the keys generated by the
+ handshake, which it will use to encrypt and authenticate data
+ end-to-end between the client and the server. These keys are as
+ computed in tor-spec.txt section 5.1.4.
+
+3.4. Authentication during the introduction phase. [INTRO-AUTH]
+
+ Hidden services may restrict access only to authorized users.
+ One mechanism to do so is the credential mechanism, where only users who
+ know the credential for a hidden service may connect at all.
+
+3.4.1. Ed25519-based authentication.
+
+ To authenticate with an Ed25519 private key, the user must include an
+ extension field in the encrypted part of the INTRODUCE1 cell with an
+ EXT_FIELD_TYPE type of [02] and the contents:
+
+ Nonce [16 bytes]
+ Pubkey [32 bytes]
+ Signature [64 bytes]
+
+ Nonce is a random value. Pubkey is the public key that will be used
+ to authenticate. [TODO: should this be an identifier for the public
+ key instead?] Signature is the signature, using Ed25519, of:
+
+ "hidserv-userauth-ed25519"
+ Nonce (same as above)
+ Pubkey (same as above)
+ AUTH_KEY (As in the INTRODUCE1 cell)
+
+ The hidden service host checks this by seeing whether it recognizes
+ and would accept a signature from the provided public key. If it
+ would, then it checks whether the signature is correct. If it is,
+ then the correct user has authenticated.
+
+ Replay prevention on the whole cell is sufficient to prevent replays
+ on the authentication.
+
+ Users SHOULD NOT use the same public key with multiple hidden
+ services.
+
+4. The rendezvous protocol
+
+ Before connecting to a hidden service, the client first builds a
+ circuit to an arbitrarily chosen Tor node (known as the rendezvous
+ point), and sends an ESTABLISH_RENDEZVOUS cell. The hidden service
+ later connects to the same node and sends a RENDEZVOUS cell. Once
+ this has occurred, the relay forwards the contents of the RENDEZVOUS
+ cell to the client, and joins the two circuits together.
+
+4.1. Establishing a rendezvous point [EST_REND_POINT]
+
+ The client sends the rendezvous point a RELAY_COMMAND_ESTABLISH_RENDEZVOUS
+ cell containing a 20-byte value.
+
+ RENDEZVOUS_COOKIE [20 bytes]
+
+ Rendezvous points MUST ignore any extra bytes in an
+ ESTABLISH_RENDEZVOUS cell. (Older versions of Tor did not.)
+
+ The rendezvous cookie is an arbitrary 20-byte value, chosen randomly
+ by the client. The client SHOULD choose a new rendezvous cookie for
+ each new connection attempt. If the rendezvous cookie is already in
+ use on an existing circuit, the rendezvous point should reject it and
+ destroy the circuit.
+
+ Upon receiving an ESTABLISH_RENDEZVOUS cell, the rendezvous point associates
+ the cookie with the circuit on which it was sent. It replies to the client
+ with an empty RENDEZVOUS_ESTABLISHED cell to indicate success. Clients MUST
+ ignore any extra bytes in a RENDEZVOUS_ESTABLISHED cell.
+
+ The client MUST NOT use the circuit which sent the cell for any
+ purpose other than rendezvous with the given location-hidden service.
+
+ The client should establish a rendezvous point BEFORE trying to
+ connect to a hidden service.
+
+4.2. Joining to a rendezvous point [JOIN_REND]
+
+ To complete a rendezvous, the hidden service host builds a circuit to
+ the rendezvous point and sends a RENDEZVOUS1 cell containing:
+
+ RENDEZVOUS_COOKIE [20 bytes]
+ HANDSHAKE_INFO [variable; depends on handshake type
+ used.]
+
+ where RENDEZVOUS_COOKIE is the cookie suggested by the client during the
+ introduction (see [PROCESS_INTRO2]) and HANDSHAKE_INFO is defined in
+ [NTOR-WITH-EXTRA-DATA].
+
+ If the cookie matches the rendezvous cookie set on any
+ not-yet-connected circuit on the rendezvous point, the rendezvous
+ point connects the two circuits, and sends a RENDEZVOUS2 cell to the
+ client containing the HANDSHAKE_INFO field of the RENDEZVOUS1 cell.
+
+ Upon receiving the RENDEZVOUS2 cell, the client verifies that HANDSHAKE_INFO
+ correctly completes a handshake. To do so, the client parses SERVER_PK from
+ HANDSHAKE_INFO and reverses the final operations of section
+ [NTOR-WITH-EXTRA-DATA] as shown here:
+
+ rend_secret_hs_input = EXP(Y,x) | EXP(B,x) | AUTH_KEY | B | X | Y | PROTOID
+ NTOR_KEY_SEED = MAC(ntor_secret_input, t_hsenc)
+ verify = MAC(ntor_secret_input, t_hsverify)
+ auth_input = verify | AUTH_KEY | B | Y | X | PROTOID | "Server"
+ AUTH_INPUT_MAC = MAC(auth_input, t_hsmac)
+
+ Finally the client verifies that the received AUTH field of HANDSHAKE_INFO
+ is equal to the computed AUTH_INPUT_MAC.
+
+ Now both parties use the handshake output to derive shared keys for use on
+ the circuit as specified in the section below:
+
+4.2.1. Key expansion
+
+ The hidden service and its client need to derive crypto keys from the
+ NTOR_KEY_SEED part of the handshake output. To do so, they use the KDF
+ construction as follows:
+
+ K = KDF(NTOR_KEY_SEED | m_hsexpand, HASH_LEN * 2 + S_KEY_LEN * 2)
+
+ The first HASH_LEN bytes of K form the forward digest Df; the next HASH_LEN
+ bytes form the backward digest Db; the next S_KEY_LEN bytes form Kf, and the
+ final S_KEY_LEN bytes form Kb. Excess bytes from K are discarded.
+
+ Subsequently, the rendezvous point passes relay cells, unchanged, from each
+ of the two circuits to the other. When Alice's OP sends RELAY cells along
+ the circuit, it authenticates with Df, and encrypts them with the Kf, then
+ with all of the keys for the ORs in Alice's side of the circuit; and when
+ Alice's OP receives RELAY cells from the circuit, it decrypts them with the
+ keys for the ORs in Alice's side of the circuit, then decrypts them with Kb,
+ and checks integrity with Db. Bob's OP does the same, with Kf and Kb
+ interchanged.
+
+ [TODO: Should we encrypt HANDSHAKE_INFO as we did INTRODUCE2
+ contents? It's not necessary, but it could be wise. Similarly, we
+ should make it extensible.]
+
+4.3. Using legacy hosts as rendezvous points
+
+ The behavior of ESTABLISH_RENDEZVOUS is unchanged from older versions
+ of this protocol, except that relays should now ignore unexpected
+ bytes at the end.
+
+ Old versions of Tor required that RENDEZVOUS cell payloads be exactly
+ 168 bytes long. All shorter rendezvous payloads should be padded to
+ this length with random bytes, to make them difficult to distinguish from
+ older protocols at the rendezvous point.
+
+ Relays older than 0.2.9.1 should not be used for rendezvous points by next
+ generation onion services because they enforce too-strict length checks to
+ rendezvous cells. Hence the "HSRend" protocol from proposal#264 should be
+ used to select relays for rendezvous points.
+
+5. Encrypting data between client and host
+
+ A successfully completed handshake, as embedded in the
+ INTRODUCE/RENDEZVOUS cells, gives the client and hidden service host
+ a shared set of keys Kf, Kb, Df, Db, which they use for sending
+ end-to-end traffic encryption and authentication as in the regular
+ Tor relay encryption protocol, applying encryption with these keys
+ before other encryption, and decrypting with these keys before other
+ decryption. The client encrypts with Kf and decrypts with Kb; the
+ service host does the opposite.
+
+6. Encoding onion addresses [ONIONADDRESS]
+
+ The onion address of a hidden service includes its identity public key, a
+ version field and a basic checksum. All this information is then base32
+ encoded as shown below:
+
+ onion_address = base32(PUBKEY | CHECKSUM | VERSION) + ".onion"
+ CHECKSUM = H(".onion checksum" | PUBKEY | VERSION)[:2]
+
+ where:
+ - PUBKEY is the 32 bytes ed25519 master pubkey of the hidden service.
+ - VERSION is an one byte version field (default value '\x03')
+ - ".onion checksum" is a constant string
+ - CHECKSUM is truncated to two bytes before inserting it in onion_address
+
+ Here are a few example addresses:
+
+ pg6mmjiyjmcrsslvykfwnntlaru7p5svn6y2ymmju6nubxndf4pscryd.onion
+ sp3k262uwy4r2k3ycr5awluarykdpag6a7y33jxop4cs2lu5uz5sseqd.onion
+ xa4r2iadxm55fbnqgwwi5mymqdcofiu3w6rpbtqn7b2dyn7mgwj64jyd.onion
+
+ For more information about this encoding, please see our discussion thread
+ at [ONIONADDRESS-REFS].
+
+7. Open Questions:
+
+ Scaling hidden services is hard. There are on-going discussions that
+ you might be able to help with. See [SCALING-REFS].
+
+ How can we improve the HSDir unpredictability design proposed in
+ [SHAREDRANDOM]? See [SHAREDRANDOM-REFS] for discussion.
+
+ How can hidden service addresses become memorable while retaining
+ their self-authenticating and decentralized nature? See
+ [HUMANE-HSADDRESSES-REFS] for some proposals; many more are possible.
+
+ Hidden Services are pretty slow. Both because of the lengthy setup
+ procedure and because the final circuit has 6 hops. How can we make
+ the Hidden Service protocol faster? See [PERFORMANCE-REFS] for some
+ suggestions.
+
+References:
+
+[KEYBLIND-REFS]:
+ https://trac.torproject.org/projects/tor/ticket/8106
+ https://lists.torproject.org/pipermail/tor-dev/2012-September/004026.html
+
+[KEYBLIND-PROOF]:
+ https://lists.torproject.org/pipermail/tor-dev/2013-December/005943.html
+
+[SHAREDRANDOM-REFS]:
+ https://gitweb.torproject.org/torspec.git/tree/proposals/250-commit-reveal-consensus.txt
+ https://trac.torproject.org/projects/tor/ticket/8244
+
+[SCALING-REFS]:
+ https://lists.torproject.org/pipermail/tor-dev/2013-October/005556.html
+
+[HUMANE-HSADDRESSES-REFS]:
+ https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/ideas/xxx-onion-nyms.txt
+ http://archives.seul.org/or/dev/Dec-2011/msg00034.html
+
+[PERFORMANCE-REFS]:
+ "Improving Efficiency and Simplicity of Tor circuit
+ establishment and hidden services" by Overlier, L., and
+ P. Syverson
+
+ [TODO: Need more here! Do we have any? :( ]
+
+[ATTACK-REFS]:
+ "Trawling for Tor Hidden Services: Detection, Measurement,
+ Deanonymization" by Alex Biryukov, Ivan Pustogarov,
+ Ralf-Philipp Weinmann
+
+ "Locating Hidden Servers" by Lasse Øverlier and Paul
+ Syverson
+
+[ED25519-REFS]:
+ "High-speed high-security signatures" by Daniel
+ J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, and
+ Bo-Yin Yang. http://cr.yp.to/papers.html#ed25519
+
+[ED25519-B-REF]:
+ https://tools.ietf.org/html/draft-josefsson-eddsa-ed25519-03#section-5:
+
+[PRNG-REFS]:
+ http://projectbullrun.org/dual-ec/ext-rand.html
+ https://lists.torproject.org/pipermail/tor-dev/2015-November/009954.html
+
+[SRV-TP-REFS]:
+ https://lists.torproject.org/pipermail/tor-dev/2016-April/010759.html
+
+[VANITY-REFS]:
+ https://github.com/Yawning/horse25519
+
+[ONIONADDRESS-REFS]:
+ https://lists.torproject.org/pipermail/tor-dev/2017-January/011816.html
+
+[TORSION-REFS]:
+ https://lists.torproject.org/pipermail/tor-dev/2017-April/012164.html
+ https://getmonero.org/2017/05/17/disclosure-of-a-major-bug-in-cryptonote-based-currencies.html
+
+Appendix A. Signature scheme with key blinding [KEYBLIND]
+
+A.1. Key derivation overview
+
+ As described in [IMD:DIST] and [SUBCRED] above, we require a "key
+ blinding" system that works (roughly) as follows:
+
+ There is a master keypair (sk, pk).
+
+ Given the keypair and a nonce n, there is a derivation function
+ that gives a new blinded keypair (sk_n, pk_n). This keypair can
+ be used for signing.
+
+ Given only the public key and the nonce, there is a function
+ that gives pk_n.
+
+ Without knowing pk, it is not possible to derive pk_n; without
+ knowing sk, it is not possible to derive sk_n.
+
+ It's possible to check that a signature was made with sk_n while
+ knowing only pk_n.
+
+ Someone who sees a large number of blinded public keys and
+ signatures made using those public keys can't tell which
+ signatures and which blinded keys were derived from the same
+ master keypair.
+
+ You can't forge signatures.
+
+ [TODO: Insert a more rigorous definition and better references.]
+
+A.2. Tor's key derivation scheme
+
+ We propose the following scheme for key blinding, based on Ed25519.
+
+ (This is an ECC group, so remember that scalar multiplication is the
+ trapdoor function, and it's defined in terms of iterated point
+ addition. See the Ed25519 paper [Reference ED25519-REFS] for a fairly
+ clear writeup.)
+
+ Let B be the ed25519 basepoint as found in section 5 of [ED25519-B-REF]:
+ B = (15112221349535400772501151409588531511454012693041857206046113283949847762202,
+ 46316835694926478169428394003475163141307993866256225615783033603165251855960)
+
+ Assume B has prime order l, so lB=0. Let a master keypair be written as
+ (a,A), where a is the private key and A is the public key (A=aB).
+
+ To derive the key for a nonce N and an optional secret s, compute the
+ blinding factor like this:
+
+ h = H(BLIND_STRING | A | s | B | N)
+ BLIND_STRING = "Derive temporary signing key"
+ N = "key-blind" | INT_8(period-number) | INT_8(period_length)
+
+ then clamp the blinding factor 'h' according to the ed25519 spec:
+
+ h[0] &= 248;
+ h[31] &= 127;
+ h[31] |= 64;
+
+ and do the key derivation as follows:
+
+ private key for the period: a' = h a
+ public key for the period: A' = h A = (ha)B
+
+ Generating a signature of M: given a deterministic random-looking r
+ (see EdDSA paper), take R=rB, S=r+hash(R,A',M)ah mod l. Send signature
+ (R,S) and public key A'.
+
+ Verifying the signature: Check whether SB = R+hash(R,A',M)A'.
+
+ (If the signature is valid,
+ SB = (r + hash(R,A',M)ah)B
+ = rB + (hash(R,A',M)ah)B
+ = R + hash(R,A',M)A' )
+
+ See [KEYBLIND-REFS] for an extensive discussion on this scheme and
+ possible alternatives. Also, see [KEYBLIND-PROOF] for a security
+ proof of this scheme.
+
+Appendix B. Selecting nodes [PICKNODES]
+
+ Picking introduction points
+ Picking rendezvous points
+ Building paths
+ Reusing circuits
+
+ (TODO: This needs a writeup)
+
+Appendix C. Recommendations for searching for vanity .onions [VANITY]
+
+ EDITORIAL NOTE: The author thinks that it's silly to brute-force the
+ keyspace for a key that, when base-32 encoded, spells out the name of
+ your website. It also feels a bit dangerous to me. If you train your
+ users to connect to
+
+ llamanymityx4fi3l6x2gyzmtmgxjyqyorj9qsb5r543izcwymle.onion
+
+ I worry that you're making it easier for somebody to trick them into
+ connecting to
+
+ llamanymityb4sqi0ta0tsw6uovyhwlezkcrmczeuzdvfauuemle.onion
+
+ Nevertheless, people are probably going to try to do this, so here's a
+ decent algorithm to use.
+
+ To search for a public key with some criterion X:
+
+ Generate a random (sk,pk) pair.
+
+ While pk does not satisfy X:
+
+ Add the number 8 to sk
+ Add the scalar 8*B to pk
+
+ Return sk, pk.
+
+ We add 8 and 8*B, rather than 1 and B, so that sk is always a valid
+ Curve25519 private key, with the lowest 3 bits equal to 0.
+
+ This algorithm is safe [source: djb, personal communication] [TODO:
+ Make sure I understood correctly!] so long as only the final (sk,pk)
+ pair is used, and all previous values are discarded.
+
+ To parallelize this algorithm, start with an independent (sk,pk) pair
+ generated for each independent thread, and let each search proceed
+ independently.
+
+ See [VANITY-REFS] for a reference implementation of this vanity .onion
+ search scheme.
+
+Appendix D. Numeric values reserved in this document
+
+ [TODO: collect all the lists of commands and values mentioned above]
+
+Appendix E. Reserved numbers
+
+ We reserve these certificate type values for Ed25519 certificates:
+
+ [08] short-term descriptor signing key, signed with blinded
+ public key. (Section 2.4)
+ [09] intro point authentication key, cross-certifying the descriptor
+ signing key. (Section 2.5)
+ [0B] ed25519 key derived from the curve25519 intro point encryption key,
+ cross-certifying the descriptor signing key. (Section 2.5)
+
+ Note: The value "0A" is skipped because it's reserved for the onion key
+ cross-certifying ntor identity key from proposal 228.
+
+Appendix F. Hidden service directory format [HIDSERVDIR-FORMAT]
+
+ This appendix section specifies the contents of the HiddenServiceDir directory:
+
+ - "hostname" [FILE]
+
+ This file contains the onion address of the onion service.
+
+ - "private_key_ed25519" [FILE]
+
+ This file contains the private master ed25519 key of the onion service.
+ [TODO: Offline keys]
+
+ - "client_authorized_pubkeys" [FILE]
+
+ If client authorization is _enabled_, this is a newline-separated file of
+ "<client name> <pubkeys> entries for authorized clients. You can think of it
+ as the ~/.ssh/authorized_keys of onion services. See [CLIENT-AUTH-MGMT] for
+ more details.
+
+ - "./client_authorized_privkeys/" [DIRECTORY]
+ "./client_authorized_privkeys/alice.privkey" [FILE]
+ "./client_authorized_privkeys/bob.privkey" [FILE]
+ "./client_authorized_privkeys/charlie.privkey" [FILE]
+
+ If client authorization is _enabled_ _AND_ if the hidden service is
+ responsible for generating and distributing private keys for its clients,
+ then this directory contains files with client's private keys. See
+ [CLIENT-AUTH-MGMT] for more details.
+
+Appendix E. Managing authorized client data [CLIENT-AUTH-MGMT]
+
+ Hidden services and clients can configure their authorized client data either
+ using the torrc, or using the control port. This section presents a suggested
+ scheme for configuring client authorization. Please see appendix
+ [HIDSERVDIR-FORMAT] for more information about relevant hidden service files.
+
+ E.1. Configuring client authorization using torrc
+
+ E.1.1. Hidden Service side
+
+ A hidden service that wants to perform client authorization, adds a new
+ option HiddenServiceAuthorizeClient to its torrc file:
+
+ HiddenServiceAuthorizeClient auth-type client-name,client-name,...
+
+ The only recognized auth-type value is "basic" which describes the scheme in
+ section [CLIENT-AUTH]. The rest of the line is a comma-separated list of
+ human-readable authorized client names.
+
+ Let's consider that one of the listed client names is "alice". In this
+ case, Tor checks the "client_authorized_pubkeys" file for any entries
+ with client_name being "alice". If an "alice" entry is found, we use the
+ relevant pubkeys to authenticate Alice.
+
+ If no "alice" entry is found in the "client_authorized_pubkeys" file, Tor
+ is tasked with generating public/private keys for Alice. To do so, Tor
+ generates x25519 and ed25519 keypairs for Alice, then makes a
+ "client_authorized_privkeys/alice.privkey" file and writes the private
+ keys inside; it also adds an entry for alice to the
+ "client_authorized_pubkeys" file.
+
+ In this last case, the hidden service operator has the responsibility to
+ pass the .key file to Alice in a secure out-of-band way. After the file
+ is passed to Alice, it can be shredded from the filesystem, as only the
+ public keys are required for the hidden service to function.
+
+ E.1.2. Client side
+
+ A client who wants to register client authorization data for a hidden service
+ needs to add the following line to their torrc:
+
+ HidServAuth onion-address x25519-private-key ed25519-private-key
+
+ The keys above are either generated by Alice using a key generation utility,
+ or they are extracted from a .key file provided by the hidden service.
+
+ In the former case, the client is also tasked with transfering the public
+ keys to the hidden service in a secure out-of-band way.
+
+ E.2. Configuring client authorization using the control port
+
+ E.2.1. Service side
+
+ A hidden service also has the option to configure authorized clients
+ using the control port. The idea is that hidden service operators can use
+ controller utilities that manage their access control instead of using
+ the filesystem to register client keys.
+
+ Specifically, we require a new control port command ADD_ONION_CLIENT_AUTH
+ which is able to register x25519/ed25519 public keys tied to a specific
+ authorized client.
+ [XXX figure out control port command format]
+
+ Hidden services who use the control port interface for client auth need
+ to perform their own key management.
+
+ E.2.2. Client side
+
+ There should also be a control port interface for clients to register
+ authorization data for hidden services without having to use the
+ torrc. It should allow both generation of client authorization private
+ keys, and also to import client authorization data provided by a hidden
+ service
+
+ This way, Tor Browser can present "Generate client auth keys" and "Import
+ client auth keys" dialogs to users when they try to visit a hidden service
+ that is protected by client authorization.
+
+ Specifically, we require two new control port commands:
+ IMPORT_ONION_CLIENT_AUTH_DATA
+ GENERATE_ONION_CLIENT_AUTH_DATA
+ which import and generate client authorization data respectively.
+
+ [XXX how does key management work here?]
+ [XXX what happens when people use both the control port interface and the
+ filesystem interface?]
+
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