[tor-commits] [torspec/master] Make rend-spec-ng.txt into a proper (pair of) proposals.
nickm at torproject.org
nickm at torproject.org
Sat Nov 30 01:23:51 UTC 2013
commit ac3824c45dde87849e531e75248e7495e0047028
Author: Nick Mathewson <nickm at torproject.org>
Date: Fri Nov 29 20:22:57 2013 -0500
Make rend-spec-ng.txt into a proper (pair of) proposals.
See rendspec-ng branch in my torspec repository for the revision history.
---
proposals/000-index.txt | 4 +
proposals/224-rend-spec-ng.txt | 1709 ++++++++++++++++++++++++++++++++
proposals/225-strawman-shared-rand.txt | 113 +++
3 files changed, 1826 insertions(+)
diff --git a/proposals/000-index.txt b/proposals/000-index.txt
index 99845a4..0d79a82 100644
--- a/proposals/000-index.txt
+++ b/proposals/000-index.txt
@@ -144,6 +144,8 @@ Proposals by number:
221 Stop using CREATE_FAST [CLOSED]
222 Stop sending client timestamps [CLOSED]
223 Ace: Improved circuit-creation key exchange [OPEN]
+224 Next-Generation Hidden Services in Tor [DRAFT]
+225 Strawman proposal: commit-and-reveal shared rng [DRAFT]
Proposals by status:
@@ -160,6 +162,8 @@ Proposals by status:
203 Avoiding censorship by impersonating an HTTPS server
219 Support for full DNS and DNSSEC resolution in Tor [for 0.2.5.x]
220 Migrate server identity keys to Ed25519 [for 0.2.5.x]
+ 224 Next-Generation Hidden Services in Tor
+ 225 Strawman proposal: commit-and-reveal shared rng
NEEDS-REVISION:
131 Help users to verify they are using Tor
190 Bridge Client Authorization Based on a Shared Secret
diff --git a/proposals/224-rend-spec-ng.txt b/proposals/224-rend-spec-ng.txt
new file mode 100644
index 0000000..f88a287
--- /dev/null
+++ b/proposals/224-rend-spec-ng.txt
@@ -0,0 +1,1709 @@
+Filename: 224-rend-spec-ng.txt
+Title: Next-Generation Hidden Services in Tor
+Author: Nick Mathewson
+Created: 2013-11-29
+Status: Draft
+
+
+-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.
+
+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.
+
+ Other features include:
+
+ * [TODO: WRITE ME once there have been some more drafts and we know
+ what the summary should say.]
+
+ 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.
+
+ [TODO write me once there have been more drafts and we know what the
+ summary should say.]
+
+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". These
+ characters in these string 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].
+
+0.3. Cryptographic building blocks
+
+ This specification uses the following cryptographic building blocks:
+
+ * 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_HANDHAKE(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(key data, salt, personalization,
+ n) that outputs n bytes.
+
+ As a first pass, I suggest:
+
+ * Instantiate STREAM with AES128-CTR. [TODO: or ChaCha20?]
+
+ * Instantiate SIGN with Ed25519 and the blinding protocol in
+ [KEYBLIND].
+
+ * Instantiate PK with Curve25519.
+
+ * Instantiate H with SHA256. [TODO: really?]
+
+ * Instantiate MAC with HMAC using H.
+
+ * Instantiate KDF with HKDF using H.
+
+ 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. Except as noted, all RSA keys MUST have exponent
+ values of 65537.
+
+ 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) and [02] (legacy node identity).
+
+ 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 client to introduction point; requests
+ introduction. Same format as INTRODUCE1. Discussed in
+ [FMT_INTRO1] and [PROCESS_INTRO2]
+
+ 36 -- RELAY_COMMAND_RENDEZVOUS1
+
+ Sent from introduction point to rendezvous point;
+ attempts to join introduction point's circuit to
+ client's circuit. Discussed in [JOIN_REND]
+
+ 37 -- RELAY_COMMAND_RENDEZVOUS2
+
+ Sent from introduction point to rendezvous point;
+ reports join of introduction point'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 form introduction point to client; acknowledges
+ receipt of INTRODUCE1 cell and reports success/failure.
+ Discussed in [INTRO_ACK]
+
+0.5. Acknowledgments
+
+ [TODO reformat these once the lists are more complete.]
+
+ 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,
+
+ 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,
+
+
+ [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, and
+ adding the string ".onion" at the end.
+
+ (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:
+
+ a1uik0w1gmfq3i5ievxdm9ceu27e88g6o7pe0rffdw9jmntwkdsd.onion
+
+ Note that since master keys are 32 bytes long, and 52 bytes of base
+ 32 encoding can hold 260 bits of information, we have four unused
+ bits in each of these names.
+
+ [TODO: Alternatively, we could require that the first bit of the
+ master key always be zero, and use a 51-byte encoding. Or we could
+ require that the first two bits be zero, and use a 51-byte encoding
+ and reserve the first bit. Or we could require that the first nine
+ bits, or ten bits be zero, etc.]
+
+1.3. In more detail: Access control [IMD:AC]
+
+ Access control for a hidden service is imposed at multiple points
+ through the process above.
+
+ 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.) This blinded signing key is derived from the
+ service's public key and, optionally, an additional secret that is
+ not part of the hidden service's onion address. The public key and
+ this secret together constitute the service's "credential".
+
+ When the secret is in use, the hidden service gains protections
+ equivalent to the "stealth mode" in previous designs.
+
+ To learn the introduction points, the clients must decrypt the body
+ of the hidden service descriptor. The encryption key for these is
+ derived from the service's credential.
+
+ In order to make an introduction point send a request to the server,
+ the client must know the introduction point and know the service's
+ per-introduction-point authentication key from 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 od the contents of the encrypted portion of the
+ hidden service descriptor. The service may additionally require a
+ user- or group-specific access token before it responds to requests.
+
+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 [TODO: add a reference]) That value, combined with hidden service
+ directories' public identity keys, determines each HSDirs' 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 SECTION IS UNFINISHED]
+
+ In order to allow multiple hosts to provide a single hidden service,
+ I'm considering two options.
+
+ * We can have each server build an introduction circuit to each
+ introduction point, and have the introduction points responsible
+ for round-robining between these circuits. One service host is
+ responsible for picking the introduction points and publishing
+ the descriptors.
+
+ * We can have servers choose their introduction points
+ independently, and build circuits to them. One service host is
+ responsible for combining these introduction points into a
+ single descriptor.
+
+ If we want to avoid having a single "master" host without which the
+ whole service goes down (the "one service host" in the description
+ above), we need a way to fail over from one host to another. We also
+ need a way to coordinate between the hosts. This is as yet
+ undesigned. Maybe it should use a hidden service?
+
+ See [SCALING-REFS] for discussion on this topic.
+
+ [TODO: Finalize this design.]
+
+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: Offline operation
+
+ In this design, a hidden service's secret identity key may be stored
+ offline. It's used only to generate blinded identity keys, which are
+ used to sign descriptor signing keys. In order to operate a hidden
+ service, the operator can generate a number of descriptor signing
+ keys and their certifications (see [DESC-OUTER] and [ENCRYPTED-DATA]
+ below), and their corresponding descriptor encryption keys, and
+ export those to the hidden service hosts.
+
+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 a
+ 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 not used
+ on its own to sign anything; it is only used to generate blinded
+ signing keys as described in [KEYBLIND] and [SUBCRED].
+
+ Blinded signing key -- A keypair derived from the identity key,
+ used to sign descriptor signing keys. 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.
+
+ 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.
+
+ 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.
+
+
+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 HSDir3 flag.
+
+ (Authorities should assign this flag as they currently assign the
+ HSDir flag, except that they should restrict it to Tor versions
+ implementing the HSDir parts of this specification.)
+
+2.1. Deriving blinded keys and subcredentials [SUBCRED]
+
+ In each time period (see [TIME-PERIOD] 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:
+ H("subcredential" |
+ credential |
+ blinded-public-key).
+
+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.
+
+ 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 1500 (one day plus
+ one hour).
+
+ Time periods start with the Unix epoch (Jan 1, 1970), and are
+ computed by taking the number of whole minutes since the epoch and
+ dividing by the time period. So if the current time is 2013-11-12
+ 13:44:32 UTC, making the seconds since the epoch 1384281872, the
+ number of minutes since the epoch is 23071364. If the current time
+ period length is 1500 (the default), then the current time period
+ number is 15380. It began 15380*1500*60 seconds after the epoch at
+ 2013-11-11 20:00:00 UTC, and will end at (15380+1)*1500*60 seconds
+ after the epoch at 2013-11-12 21:00:00 UTC.
+
+2.2.2. Overlapping time periods to avoid thundering herds [TIME-OVERLAP]
+
+ If every hidden service host were to generate a new set of keys and
+ upload a new descriptor at exactly the start of each time period, the
+ directories would be overwhelmed by every host uploading at the same
+ time. Instead, each public key becomes valid at its new location at a
+ deterministic time somewhat _before_ the period begins, depending on
+ the public key and the period.
+
+ The time at which a key might first become valid is determined by the
+ consensus parameter "hsdir-overlap-begins", which is an integer in
+ range [1,100] with default value 80. This parameter denotes a
+ percentage of the interval for which no overlap occurs. So for the
+ default interval (1500 minutes) and default overlap-begins value
+ (80%), new keys do not become valid for the first 1200 minutes of the
+ interval.
+
+ The new shared random value must be published *before* the start of
+ the next overlap interval by at least enough time to ensure that
+ clients all get it. [TODO: how much earlier?]
+
+ The time at which a key from the next interval becomes valid is
+ determined by taking the first two bytes of
+
+ OFFSET = H(Key | INT_8(Next_Period_Num))
+
+ as a big-endian integer, dividing by 65536, and treating that as a
+ fraction of the overlap interval.
+
+ For example, if the period is 1500 minutes long, and overlap interval
+ is 300 minutes long, and OFFSET begins with [90 50], then the next
+ key becomes valid at 1200 + 300 * (0x9050 / 65536) minutes, or
+ approximately 22 hours and 49 minutes after the beginning of the
+ period.
+
+ Hidden service directories should accept descriptors at least [TODO:
+ how much?] minutes before they would become valid, and retain them
+ for at least [TODO: how much?] minutes after the end of the period.
+
+ When a client is looking for a service, it must calculate its key
+ both for the current and for the subsequent period, to decide whether
+ the next period's key is valid yet.
+
+2.2.3. Where to publish a service descriptor
+
+ 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.
+
+ hsdir_spread_accept = an integer in range [1,128]
+ with default value 8.
+
+ 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 calculate
+
+ for replicanum in 1...hsdir_n_replicas:
+ hs_index(replicanum) = H("store-at-idx" |
+ blinded_public_key | replicanum |
+ periodnum)
+
+ where blinded_public_key is specified in section KEYBLIND, and
+ periodnum is defined in section TIME-PERIODS.
+
+ where n_replicas is determined by the consensus parameter
+ "hsdir_n_replicas".
+
+ Then, for each node listed in the current consensus with the HSDir3
+ flag, we compute a directory index for that node as:
+
+ hsdir_index(node) = H(node_identity_digest |
+ shared_random |
+ INT_8(period_num) )
+
+ where shared_random is the shared value generated by the authorities
+ in section PUB-SHAREDRANDOM.
+
+ 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).
+
+ 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.)
+
+ An HSDir should rejects a descriptor if that HSDir is not one of the
+ first hsdir_spread_accept HSDirs for that node.
+
+ [TODO: Incorporate the findings from proposal 143 here. But watch
+ out: proposal 143 did not analyze how much the set of nodes changes
+ over time, or how much client and host knowledge might diverge.]
+
+2.2.4. URLs for anonymous uploading and downloading
+
+ Hidden service descriptors conforming to this specification are
+ uploaded with an HTTP POST request to the URL
+ /tor/rendezvous3/publish relative to the hidden service directory's
+ root, and downloaded with an HTTP GET request for the URL
+ /tor/rendezvous3/<z> where z is a base-64 encoding of the hidden
+ service's blinded public key.
+
+ [TODO: raw base64 is not super-nice for URLs, since it can have
+ slashes. We already use it for microdescriptor URLs, though. Do we
+ care here?]
+
+ These requests must be made anonymously, on circuits not used for
+ anything else.
+
+2.3. Publishing shared random values [PUB-SHAREDRANDOM]
+
+ Our design for limiting the predictability of HSDir upload locations
+ relies on a shared random value 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 one possible (somewhat broken) protocol, see
+ Appendix [SHAREDRANDOM].
+
+ We add a new line in votes and consensus documents:
+
+ "hsdir-shared-random" PERIOD-START VALUE
+ PERIOD-START = YYYY-MM-DD HH:MM:SS
+ VALUE = A base-64 encoded 256-bit value.
+
+ To decide which hsdir-shared-random line to include in a consensus
+ for a given PERIOD-START, we choose whichever line appears verbatim
+ in the most votes, so long as it is listed by at least three
+ authorities. Ties are broken in favor of the lower value. More than
+ one PERIOD-START is allowed per vote, and per consensus. The same
+ PERIOD-START must not appear twice in a vote or in a consensus.
+
+ [TODO: Need to define a more robust algorithm. Need to cover cases
+ where multiple cluster of authorities publish a different value,
+ etc.]
+
+ The hs-dir-shared-random lines appear, sorted by PERIOD-START, in the
+ consensus immediately after the "params" line.
+
+ The authorities should publish the shared random value for the
+ current period, and, at a time at least three voting periods before
+ the overlap interval begins, the shared random value for the next
+ period.
+
+[TODO: find out what weasel doesn't like here.]
+
+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 "3" SP public-key SP certification NL
+
+ [At start, exactly once.]
+
+ public-key is the blinded public key for the service, encoded in
+ base 64. Certification is a certification of a short-term ed25519
+ descriptor signing key using the public key, in the format of
+ proposal 220.
+
+ "time-period" SP YYYY-MM-DD HH:MM:SS NUM NL
+
+ [Exactly once.]
+
+ The time period for which this descriptor is relevant, including
+ its starting time and its period number.
+
+ "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.)
+
+ "encrypted" NL encrypted-string
+
+ [Exactly once.]
+
+ An encrypted blob, whose format is discussed in [ENCRYPTED-DATA]
+ below. The blob is base-64 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
+ hs-descriptor line. We use a separate key for signing, so that
+ the hidden service host does not need to have its private blinded
+ key online.
+
+
+2.5. Hidden service descriptors: encryption format [ENCRYPTED-DATA]
+
+ The encrypted part of the hidden service descriptor is encrypted and
+ authenticated with symmetric keys generated as follows:
+
+ salt = 16 random bytes
+
+ secret_input = nonce | blinded_public_key | subcredential |
+ INT_4(revision_counter)
+ keys = KDF(secret_input, salt, "hsdir-encrypted-data",
+ 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 (random bytes from above) [16 bytes]
+ ENCRYPTED The plaintext encrypted with S [variable]
+ MAC MAC of both above fields [32 bytes]
+
+ The encryption format is ENCRYPTED =
+ STREAM(SECRET_IV,SECRET_KEY) xor Plaintext
+
+ Before encryption, the plaintext must be padded to a multiple of ???
+ bytes with NUL bytes. The plaintext must not be longer than ???
+ bytes. [TODO: how much? Should this be a parameter? What values in
+ practice is needed to hide how many intro points we have, and how
+ many might be legacy ones?]
+
+ The plaintext format is:
+
+ "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 TAP and
+ ntor as described in tor-spec.txt. See tor-spec section 5.1 for a
+ list of recognized handshake types.
+
+ "authentication-required" SP types NL
+
+ [At most once]
+
+ A space-separated list of authentication types. 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'. See [INTRO-AUTH] below.
+
+ At least once:
+
+ "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.
+
+ "auth-key" SP "ed25519" SP key SP certification NL
+
+ [Exactly once per introduction point]
+
+ Base-64 encoded introduction point authentication key that was
+ used to establish introduction point circuit, cross-certifying
+ the blinded public key key using the certification format of
+ proposal 220.
+
+ "enc-key" SP "ntor" SP key NL
+
+ [At most once per introduction point]
+
+ Base64-encoded curve25519 key used to encrypt request to
+ hidden service.
+
+ [TODO: I'd like to have a cross-certification here too.]
+
+ "enc-key" SP "legacy" NL key NL
+
+ [At most once per introduction point]
+
+ Base64-encoded RSA key, wrapped in "----BEGIN RSA PUBLIC
+ KEY-----" armor, for use with a legacy introduction point as
+ described in [LEGACY_EST_INTRO] and [LEGACY-INTRODUCE1] below.
+
+ Exactly one of the "enc-key ntor" and "enc-key legacy"
+ elements must be present for each introduction point.
+
+ [TODO: I'd like to have a cross-certification here too.]
+
+ Other encryption and authentication key formats are allowed; clients
+ should ignore ones they do not recognize.
+
+
+3. The introduction 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.
+
+ [Between these steps, 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 a ESTABLISH_INTRO cell with the following contents:
+
+ AUTH_KEY_TYPE [1 byte]
+ AUTH_KEY_LEN [1 byte]
+ AUTH_KEY [AUTH_KEY_LEN bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+ ZERO [1 byte]
+ HANDSHAKE_AUTH [MAC_LEN bytes]
+ SIGLEN [1 byte]
+ SIG [SIGLEN bytes]
+
+ The AUTH_KEY_TYPE field indicates the type of the introduction point
+ authentication key and the type of the MAC to use in for
+ HANDSHAKE_AUTH. Recognized types are:
+
+ [00, 01] -- Reserved for legacy introduction cells; see
+ [LEGACY_EST_INTRO below]
+ [02] -- Ed25519; HMAC-SHA256.
+ [FF] -- Reserved for maintenance messages on existing
+ circuits; see MAINT_INTRO below.
+
+ [TODO: Should this just be a new relay cell type?
+ Matthew and George think so.]
+
+ 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 ZERO field contains the byte zero; it marks the end of the
+ extension fields.
+
+ 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.
+
+ SIGLEN 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.
+
+ [TODO: The above will work fine with what we do today, but it will do
+ quite badly if we ever freak out and want to go back to RSA2048 or
+ bigger. Do we care?]
+
+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_LENGTH [2 bytes]
+ KEY [KEY_LENGTH bytes]
+ HANDSHAKE_AUTH [20 bytes]
+ SIG [variable, up to end of relay payload]
+
+ The KEY_LENGTH variable determines the length of the KEY field.
+
+ The KEY field is a ASN1-encoded RSA public key.
+
+ 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.
+
+ Note that since the relay payload itself may be no more than 498
+ bytes long, the KEY_LENGTH field can never have a first byte other
+ than [00] or [01]. These values are used to distinguish legacy
+ ESTABLISH_INTRO cells from newer ones.
+
+ Older versions of Tor always use a 1024-bit RSA key for these
+ introduction authentication keys.
+
+ Newer hidden services MAY use RSA keys up 1904 bits. Any more than
+ that will not fit in a RELAY cell payload.
+
+3.1.3. Managing introduction circuits [MAINT_INTRO]
+
+ If the first byte of an ESTABLISH_INTRO cell is [FF], the cell's body
+ contains an administrative command for the circuit. The format of
+ such a command is:
+
+ Any number of times:
+ SUBCOMMAND_TYPE [2 bytes]
+ SUBCOMMAND_LEN [2 bytes]
+ SUBCOMMAND [COMMAND_LEN bytes]
+
+ Recognized SUBCOMMAND_TYPE values are:
+
+ [00 01] -- update encryption keys
+
+ [TODO: Matthew says, "This can be used to fork an intro point to
+ balance traffic over multiple hidden service servers while
+ maintaining the criteria for a valid ESTABLISH_INTRO
+ cell. -MF". Investigate.]
+
+ Unrecognized SUBCOMMAND_TYPE values should be ignored.
+
+3.1.3.1. Updating encryption keys (subcommand 0001) [UPDATE-KEYS-SUBCMD]
+
+ Hidden service hosts send this subcommand to set their initial
+ encryption keys or update the configured public encryption keys
+ associated with this circuit. This message must be sent after
+ establishing an introduction point, before the circuit can be
+ advertised. These keys are given in the form:
+
+ NUMKEYS [1 byte]
+ NUMKEYS times:
+ KEYTYPE [1 byte]
+ KEYLEN [1 byte]
+ KEY [KEYLEN bytes]
+ COUNTER [4 bytes]
+ SIGLEN [1 byte]
+ SIGNATURE [SIGLEN bytes.]
+
+ The KEYTYPE value [01] is for Curve25519 keys.
+
+ The COUNTER field is a monotonically increasing value across a given
+ introduction point authentication key.
+
+ The SIGNATURE must be generated with the introduction point
+ authentication key, and must cover the entire subcommand body,
+ prefixed with the string "Tor hidden service introduction encryption
+ keys v1".
+
+ [TODO: Nothing is done here to prove ownership of the encryption
+ keys. Does that matter?]
+
+ [TODO: The point here is to allow encryption keys to change while
+ maintaining an introduction point and not forcing a client to
+ download a new descriptor. I'm not sure if that's worth it. It makes
+ clients who have seen a key before distinguishable from ones who have
+ not.]
+
+ [Matthew says: "Repeat-client over long periods of time will always
+ be distinguishable. It may be better to simply expire intro points
+ than try to preserve forward-secrecy, though". Must find out what he
+ meant.]
+
+ Setting the encryption keys for a given circuit replaces the previous
+ keys for that circuit. Clients who attempt to connect using the old
+ key receive an INTRO_ACK cell with error code [00 02] as described in
+ section [INTRO_ACK] below.
+
+3.1.4. 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 empty
+ INTRO_ESTABLISHED cell.
+
+ [TODO: make this cell type extensible. It should be able to include
+ data if that turns out to be needed.]
+
+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.
+
+3.2.1. INTRODUCE1 cell format [FMT_INTRO1]
+
+ An INTRODUCE1 cell has the following contents:
+
+ AUTH_KEYID [32 bytes]
+ ENC_KEYID [8 bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+ ZERO [1 byte]
+ ENCRYPTED [Up to end of relay payload]
+
+ [TODO: Should we have a field to determine the type of ENCRYPTED, or
+ should we instead assume that there is exactly one encryption key per
+ encryption method? The latter is probably safer.]
+
+ Upon receiving an INTRODUCE1 cell, the introduction point checks
+ whether AUTH_KEYID and ENC_KEYID match a configured introduction
+ point authentication key and introduction point encryption key. If
+ they do, the cell is relayed; if not, it is not.
+
+ The AUTH_KEYID for an Ed25519 public key is the public key itself.
+ The ENC_KEYID for a Curve25519 public key is the first 8 bytes of the
+ public key. (This key ID is safe to truncate, since all the keys are
+ generated by the hidden service host, and the ID is only valid
+ relative to a single AUTH_KEYID.) The ENCRYPTED field is as
+ described in 3.3 below.
+
+ To relay an INTRODUCE1 cell, the introduction point sends an
+ INTRODUCE2 cell with exactly the same contents.
+
+3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK]
+
+ An INTRODUCE_ACK cell has the following fields:
+ STATUS [2 bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+
+ Recognized status values are:
+ [00 00] -- Success: cell relayed to hidden service host.
+ [00 01] -- Failure: service ID not recognzied
+ [00 02] -- Failure: key ID not recognized
+ [00 03] -- Bad message format
+
+ Recognized extension field types:
+ [00 01] -- signed set of encryption keys
+
+ The extension field type 0001 is a signed set of encryption keys; its
+ body matches the body of the key update command in
+ [UPDATE-KEYS-CMD]. Whenever sending status [00 02], the introduction
+ point MUST send this extension field.
+
+3.2.3. Legacy formats [LEGACY-INTRODUCE1]
+
+ When the ESTABLISH_INTRO cell format of [LEGACY_EST_INTRO] is used,
+ INTRODUCE1 cells are of the form:
+
+ AUTH_KEYID_HASH [20 bytes]
+ ENC_KEYID [8 bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+ ZERO [1 byte]
+ ENCRYPTED [Up to end of relay payload]
+
+ Here, AUTH_KEYID_HASH is the hash of the introduction point
+ authentication key used to establish the introduction.
+
+ Because of limitations in older versions of Tor, the relay payload
+ size for these INTRODUCE1 cells must always be at least 246 bytes, or
+ they will be rejected as invalid.
+
+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_KEYID/AUTH_KEYID_HASH field and the ENC_KEYID fields
+ are as expected, and match the configured authentication and
+ encryption key(s) on that circuit.
+
+ The service host then checks whether it has received a cell with
+ these contents 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.)
+
+ 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:
+
+ REND_TOKEN [20 bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+ ZERO [1 byte]
+ ONION_KEY_TYPE [2 bytes]
+ ONION_KEY [depends on ONION_KEY_TYPE]
+ 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 one of:
+
+ [01] TAP-RSA-1024: ONION_KEY is 128 bytes long.
+ [02] 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.
+
+ 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]
+
+ 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
+ "hidden-service-ntor-curve25519-sha256-1". Define the tweak value
+ t_hsenc, and the tag value m_hsexpand as:
+
+ t_hsenc = PROTOID | ":hs_key_extract"
+ m_hsexpand = PROTOID | ":hs_key_expand"
+
+ To make an INTRODUCE 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:
+ secret_hs_input = EXP(B,x) | AUTH_KEYID | X | B | PROTOID
+ info = m_hsexpand | subcredential
+ hs_keys = HKDF(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 [G_LENGTH 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
+
+ AUTH_KEYID [32 bytes]
+ ENC_KEYID [8 bytes]
+ Any number of times:
+ EXT_FIELD_TYPE [1 byte]
+ EXT_FIELD_LEN [1 byte]
+ EXT_FIELD [EXTRA_FIELD_LEN bytes]
+ ZERO [1 byte]
+ ENCRYPTED:
+ CLIENT_PK [G_LENGTH 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. If the introduction
+ point is running an older Tor that does not support this protocol,
+ the first field is replaced by a 20-byte AUTH_KEYID_HASH field as
+ described in [LEGACY-INTRODUCE1].)
+
+ Here, the encryption key plays the role of B in the regular ntor
+ handshake, and the AUTH_KEYID 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_KEYID 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
+ 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:
+
+ xb = EXP(X,b)
+
+ secret_hs_input = xb | AUTH_KEYID | X | B | PROTOID
+ info = m_hsexpand | subcredential
+ hs_keys = HKDF(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.)
+
+ ntor_secret_input = EXP(X,y) | xb | ID | B | X | Y | PROTOID
+ NTOR_KEY_SEED = H(secret_input, t_key)
+ verify = H(secret_input, t_verify)
+ auth_input = verify | ID | B | Y | X | PROTOID | "Server"
+
+ (The above are used to finish the ntor handshake.)
+
+ The server's handshake reply is:
+ SERVER_PK Y [G_LENGTH bytes]
+ AUTH H(auth_input, t_mac) [H_LENGTH bytes]
+
+ These faileds can be send to the client in a RENDEZVOUS1 cell.
+ (See [JOIN_REND] below.)
+
+ 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. For more
+ fine-grained conntrol, a hidden service can be configured with
+ password-based or public-key-based authentication.
+
+3.4.1. Password-based authentication.
+
+ To authenticate with a password, the user must include an extension
+ field in the encrypted part of the INTRODUCE cell with an
+ EXT_FIELD_TYPE type of [01] and the contents:
+
+ Username [00] Password.
+
+ The username may not include any [00] bytes. The password may.
+
+ On the server side, the password MUST be stored hashed and salted,
+ ideally with scrypt or something better.
+
+3.4.2. Ed25519-based authentication.
+
+ To authenticate with an Ed25519 private key, the user must include an
+ extension field in the encrypted part of the INTRODUCE 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_KEYID (As in the INTRODUCE1 cell)
+ ENC_KEYID (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 message. (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 a 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. [TODO: make this extensible]
+
+ 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.]
+
+ 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 contents of the RENDEZVOUS1 cell.
+
+ Upon receiving the RENDEZVOUS2 cell, the client verifies that the
+ HANDSHAKE_INFO correctly completes a handshake, and uses the
+ handshake output to derive shared keys for use on the circuit.
+
+ [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 [00] bytes.
+
+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
+ encryption. The client encrypts with Kf and decrypts with Kb; the
+ service host does the opposite.
+
+6. 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
+
+[SHAREDRANDOM-REFS]:
+ https://trac.torproject.org/projects/tor/ticket/8244
+ https://lists.torproject.org/pipermail/tor-dev/2013-November/005847.html
+ https://lists.torproject.org/pipermail/tor-talk/2013-November/031230.html
+
+[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
+
+
+Appendix A. Signature scheme with key blinding [KEYBLIND]
+
+ 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 make 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.]
+
+
+ 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 the basepoint be written as B. 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 h as H(A | s, B, N), and let:
+
+ 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. I've transcribed this from a description by
+ Tanja Lange at the end of the thread. [TODO: We'll want a proof for
+ this.]
+
+ (To use this with Tor, set N = INT_8(period-number) | INT_8(Start of
+ period in seconds since epoch).)
+
+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 1 to sk
+ Add the scalar B to pk
+
+ Return sk, pk.
+
+ 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.
+
+Appendix D. Numeric values reserved in this document
+
+ [TODO: collect all the lists of commands and values mentioned above]
diff --git a/proposals/225-strawman-shared-rand.txt b/proposals/225-strawman-shared-rand.txt
new file mode 100644
index 0000000..4062ef1
--- /dev/null
+++ b/proposals/225-strawman-shared-rand.txt
@@ -0,0 +1,113 @@
+Filename: 225-strawman-shared-rand.txt
+Title: Strawman proposal: commit-and-reveal shared rng
+Author: Nick Mathewson
+Created: 2013-11-29
+Status: Draft
+
+1. Introduction
+
+ This is a strawman proposal: I don't think we should actually build
+ it. It's just a simple writeup of the more trivial commit-then-reveal
+ protocol for generating a shared random value. It's insecure to the
+ extent that an adversary who controls b of the authorities gets to
+ choose among 2^b outcomes for the result of the protocol.
+
+ See proposal 224, section HASHRING for some motivation of why we want
+ one of these in Tor.
+
+ Let's do better!
+
+ [TODO: Are we really stuck with Tor's nasty metaformat here?]
+
+2. The protocol
+
+ Here's a protocol for producing a shared random value. It should run
+ less frequently than the directory consensus algorithm. It runs in
+ these phases.
+
+ 1. COMMITMENT
+ 2. REVEAL
+ 3. COMPUTE SHARED RANDOM
+
+ It should be implemented by software other than Tor, which should be
+ okay for authorities.
+
+ Note: This is not a great protocol. It has a number of failure
+ modes. Better protocols seem hard to implement, though, and it ought
+ to be possible to drop in a replacement here, if we do it right.
+
+ At the start of phase 1, each participating authority publishes a
+ statement of the form:
+
+ shared-random 1
+ shared-random-type commit
+ signing-key-certification (certification here; see proposal 220)
+ commitment-key-certification (certification here; see proposal 220)
+ published YYYY-MM-DD HH:MM:SS
+ period-start YYYY-MM-DD HH:MM:SS
+ attempt INT
+ commitment sha512 C
+ signature (made with commitment key; see proposal 220)
+
+ The signing key is the one used for consensus votes, signed by the
+ directory authority identity key. The commitment key is used for this
+ protocol only. The signature is made with the commitment key. The
+ period-start value is the start of the period for which the shared
+ random value should be in use. The attempt value starts at 1, and
+ increments by 1 for each time that the protocol fails.
+
+ The other fields should be self-explanatory.
+
+ The commitment value C is a base64-encoded SHA-512 hash of a 256-bit
+ random value R.
+
+ During the rest of phase 1, every authority collects the commitments
+ from other authorities, and publishes them to other authorities, as
+ they do today with directory votes.
+
+ At the start of phase 2, each participating authority publishes:
+
+ shared-random 1
+ shared-random-type reveal
+ signing-key-certification (certification here; see proposal 220)
+ commitment-key-certification (certification here; see proposal 220)
+ received-commitment ID sig
+ received-commitment ID sig
+ published YYYY-MM-DD HH:MM:SS
+ period-start YYYY-MM-DD HH:MM:SS
+ attempt INT
+ commitment sha512 C
+ reveal R
+ signature (made with commitment key; see proposal 220)
+
+ The R value is the one used to generate C. The received-commitment
+ lines are the signatures on the documents from other authorities in
+ phase 1. All other fields are as in the commitments.
+
+ During the rest of phase 2, every authority collects the
+ reveals from other authorities, as above with commitments.
+
+ At the start of phase 3, each participating authority either has a
+ reveal from every authority that it received a commitment from, or it
+ does not. Each participating authority then says
+
+ shared-random 1
+ shared-random-type finish
+ signing-key-certification (certification here; see proposal 220)
+ commitment-key-certification (certification here; see proposal 220)
+ received-commitment ID sig R
+ received-commitment ID sig R ...
+ published YYYY-MM-DD HH:MM:SS
+ period-start YYYY-MM-DD HH:MM:SS
+ attempt INT
+ consensus C
+ signature (made with commitment key; see proposal 220)
+
+ Where C = SHA256(ID | R | ID | R | ID | R | ...) where the ID
+ values appear in ascending order and the R values appear after
+ their corresponding ID values.
+
+ See [SHAREDRANDOM-REFS] for more discussion here.
+
+ (TODO: should this be its own spec? If so, does it have to use our
+ regular metaformat or can it use something less sucky?)
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