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 absence of shared random values 2.3.2. Hidden services and changing shared random values 2.4. Hidden service descriptors: outer wrapper [DESC-OUTER] 2.5. Hidden service descriptors: encryption format [HS-DESC-ENC] 2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER] 2.5.1.1. First layer encryption logic 2.5.1.2. First layer plaintext format 2.5.1.3. Client behavior 2.5.1.4. Obfuscating the number of authorized clients 2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER] 2.5.2.1. Second layer encryption keys 2.5.2.2. Second layer plaintext format 2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS] 3. The introduction protocol [INTRO-PROTOCOL] 3.1. Registering an introduction point [REG_INTRO_POINT] 3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO] 3.1.1.1. Denial-of-Server Defense Extension. [EST_INTRO_DOS_EXT] 3.1.2. Registering an introduction point on a legacy Tor node [LEGACY_EST_INTRO] 3.1.3. Acknowledging establishment of introduction point [INTRO_ESTABLISHED] 3.2. Sending an INTRODUCE1 cell to the introduction point. [SEND_INTRO1] 3.2.1. INTRODUCE1 cell format [FMT_INTRO1] 3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK] 3.3. Processing an INTRODUCE2 cell at the hidden service. [PROCESS_INTRO2] 3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS] 3.3.2. Example encryption handshake: ntor with extra data [NTOR-WITH-EXTRA-DATA] 3.4. Authentication during the introduction phase. [INTRO-AUTH] 3.4.1. Ed25519-based authentication. 4. The rendezvous protocol 4.1. Establishing a rendezvous point [EST_REND_POINT] 4.2. Joining to a rendezvous point [JOIN_REND] 4.2.1. Key expansion 4.3. Using legacy hosts as rendezvous points 5. Encrypting data between client and host 6. Encoding onion addresses [ONIONADDRESS] 7. Open Questions: -1. Draft notes This document describes a proposed design and specification for hidden services in Tor version 0.2.5.x or later. It's a replacement for the current rend-spec.txt, rewritten for clarity and for improved design. Look for the string "TODO" below: it describes gaps or uncertainties in the design. Change history: 2013-11-29: Proposal first numbered. Some TODO and XXX items remain. 2014-01-04: Clarify some unclear sections. 2014-01-21: Fix a typo. 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). As of 0.4.1.1-alpha, Tor includes both IPv4 and IPv6 link specifiers in v3 onion service protocol link specifier lists. All available addresses SHOULD be included as link specifiers, regardless of the address that Tor actually used to connect/extend to the remote relay. 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 unusable 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 public identity key can derive these blinded signing keys for a given period. It should be impossible to derive the blinded signing key lacking that knowledge. This is achieved using two nonces: * A "credential", derived from the public identity key KP_hs_id. N_hs_cred. * A "subcredential", derived from the credential N_hs_cred and information which various with the current time period. N_hs_subcred. The body of each descriptor is also encrypted with a key derived from the public signing key. 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. (NOTE: although the protocol allows them, offline keys are not implemented as of 0.3.2.1-alpha.) 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]. KP_hs_id, KS_hs_id. 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]). KP_blind_id, KS_blind_id. 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]). KP_hs_desc_sign, KS_hs_desc_sign. 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) KP_hs_intro_auth, KS_hs_intro_auth. 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. KP_hs_intro_ntor, KS_hs_intro_ntor. 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. K_desc_enc. Public/private keypairs defined elsewhere: Onion key -- Short-term encryption keypair (KS_onion_ed, KP_onion_ed). (Node) identity key (KP_relayid). 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 asymmetric 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.] (NOTE: client authorization is implemented as of 0.3.5.1-alpha.) 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: N_hs_subcred = H("subcredential" | N_hs_cred | blinded-public-key). In the above formula, credential corresponds to: N_hs_cred = 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, shared_random_value. * 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, every time 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 4. (Until 0.3.2.8-rc, the default was 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 HSDir 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 successfully 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 every time 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//publish relative to the hidden service directory's root, and downloaded with an HTTP GET request for the URL /tor/hs// where is a base64 encoding of the hidden service's blinded public key and 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 naturally) 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 absence 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 720 (12 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.) Implementations MUST be able to parse 64-bit values for these counters. "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. (The resulting document does not end with a newline character.) "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 public identity 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 encryption scheme in [HS-DESC-ENCRYPTION-KEYS] uses the service credential which is derived from the public identity key (see [SUBCRED]) to ensure that only entities who know the public identity key can decrypt the first descriptor layer. 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 [At least once] 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(N_hs_subcred | 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. As of 0.4.1.1-alpha, services include both IPv4 and IPv6 link specifiers in descriptors. All available addresses SHOULD be included in the descriptor, regardless of the address that the onion service actually used to connect/extend to the intro point. The client SHOULD NOT reject any LSTYPE fields which it doesn't recognize; instead, it should use them verbatim in its EXTEND request to the introduction point. The client MAY perform basic validity checks on the link specifiers in the descriptor. These checks SHOULD NOT leak detailed information about the client's version, configuration, or consensus. (See 3.3 for service link specifier handling.) "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 introduction point authentication key using the descriptor signing key. The introduction point authentication key 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 encryption key using the descriptor signing 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] [This field is obsolete and should never be generated; it is included for historical reasons only.] 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] [This field is obsolete and should never be generated; it is included for historical reasons only.] 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 RSA public key found in "legacy-key" using the descriptor signing 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 proceed 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 rebuild 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 | N_hs_subcred | 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 D_MAC of both above fields [32 bytes] The final encryption format is ENCRYPTED = STREAM(SECRET_IV,SECRET_KEY) XOR Plaintext . Where D_MAC = H(mac_key_len | MAC_KEY | salt_len | SALT | ENCRYPTED) and mac_key_len = htonll(len(MAC_KEY)) and salt_len = htonll(len(SALT)). 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. Single Onion Services attempt to build a non-anonymous single-hop circuit, but use an anonymous 3-hop circuit if: * the intro point is on an address that is configured as unreachable via a direct connection, or * the initial attempt to connect to the intro point over a single-hop circuit fails, and they are retrying the intro point connection. [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 extensions to the introduction protocol. Extensions with unrecognized EXT_FIELD_TYPE values must be ignored. (`EXT_FIELD_LEN` may be zero, in which case EXT_FIELD is absent.) Unless otherwise specified in the documentation for an extension type: * Each extension type SHOULD be sent only once in a message. * Parties MUST ignore any occurrences all occurrences of an extension with a given type after the first such occurrence. * Extensions SHOULD be sent in numerically ascending order by type. (The above extension sorting and multiplicity rules are only defaults; they may be overridden in the descriptions of individual extensions.) 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. 3.1.1.1. Denial-of-Service Defense Extension. [EST_INTRO_DOS_EXT] This extension can be used to send Denial-of-Service (DoS) parameters to the introduction point in order for it to apply them for the introduction circuit. If used, it needs to be encoded within the N_EXTENSIONS field of the ESTABLISH_INTRO cell defined in the previous section. The content is defined as follow: EXT_FIELD_TYPE: [01] -- Denial-of-Service Parameters. If this flag is set, the extension should be used by the introduction point to learn what values the denial of service subsystem should be using. EXT_FIELD content format is: N_PARAMS [1 byte] N_PARAMS times: PARAM_TYPE [1 byte] PARAM_VALUE [8 byte] The PARAM_TYPE possible values are: [01] -- DOS_INTRODUCE2_RATE_PER_SEC The rate per second of INTRODUCE2 cell relayed to the service. [02] -- DOS_INTRODUCE2_BURST_PER_SEC The burst per second of INTRODUCE2 cell relayed to the service. The PARAM_VALUE size is 8 bytes in order to accommodate 64bit values. It MUST match the specified limit for the following PARAM_TYPE: [01] -- Min: 0, Max: 2147483647 [02] -- Min: 0, Max: 2147483647 A value of 0 means the defense is disabled. If the rate per second is set to 0 (param 0x01) then the burst value should be ignored. And vice-versa, if the burst value is 0 (param 0x02), then the rate value should be ignored. In other words, setting one single parameter to 0 disables the defense. The burst can NOT be smaller than the rate. If so, the parameters should be ignored by the introduction point. Any valid value does have precedence over the network wide consensus parameter. Using this extension extends the payload of the ESTABLISH_INTRO cell by 19 bytes bringing it from 134 bytes to 155 bytes. This extension can only be used with relays supporting the protocol version "HSIntro=5". Introduced in tor-0.4.2.1-alpha. 3.1.2. Registering an introduction point on a legacy Tor node [LEGACY_EST_INTRO] [This section is obsolete and refers to a workaround for now-obsolete Tor relay versions. It is included for historical reasons.] 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. [The above paragraph is obsolete and refers to a workaround for now-obsolete Tor relay versions. It is included for historical reasons.] The same rules for multiplicity, ordering, and handling unknown types apply to the extension fields here as described [EST_INTRO] above. 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. The same rules for multiplicity, ordering, and handling unknown types apply to the extension fields here as described [EST_INTRO] above. 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 The same rules for multiplicity, ordering, and handling unknown types apply to the extension fields here as described [EST_INTRO] above. 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 ENCRYPTED 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. As of 0.4.1.1-alpha, clients include both IPv4 and IPv6 link specifiers in INTRODUCE1 cells. All available addresses SHOULD be included in the cell, regardless of the address that the client actually used to extend to the rendezvous point. The hidden service should handle invalid or unrecognised link specifiers the same way as clients do in section 2.5.2.2. In particular, services MAY perform basic validity checks on link specifiers, and SHOULD NOT reject unrecognised link specifiers, to avoid information leaks. 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. The same rules for multiplicity, ordering, and handling unknown types apply to the extension fields here as described [EST_INTRO] above. 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 | N_hs_subcred 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 compute: intro_secret_hs_input = EXP(X,b) | AUTH_KEY | X | B | PROTOID info = m_hsexpand | N_hs_subcred 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. Single Onion Services attempt to build a non-anonymous single-hop circuit, but use an anonymous 3-hop circuit if: * the rend point is on an address that is configured as unreachable via a direct connection, or * the initial attempt to connect to the rend point over a single-hop circuit fails, and they are retrying the rend point connection. 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 [This section is obsolete and refers to a workaround for now-obsolete Tor relay versions. It is included for historical reasons.] 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 a 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" | INT_1(0) N = "key-blind" | INT_8(period-number) | INT_8(period_length) B = "(1511[...]2202, 4631[...]5960)" then clamp the blinding factor 'h' according to the ed25519 spec: h[0] &= 248; h[31] &= 63; h[31] |= 64; and do the key derivation as follows: private key for the period: a' = h a mod l RH' = SHA-512(RH_BLIND_STRING | RH)[:32] RH_BLIND_STRING = "Derive temporary signing key hash input" 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' ) This boils down to regular Ed25519 with key pair (a', 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 point 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] - "./authorized_clients/" [DIRECTORY] "./authorized_clients/alice.auth" [FILE] "./authorized_clients/bob.auth" [FILE] "./authorized_clients/charlie.auth" [FILE] If client authorization is enabled, this directory MUST contain a ".auth" file for each authorized client. Each such file contains the public key of the respective client. The files are transmitted to the service operator by the client. See section [CLIENT-AUTH-MGMT] for more details and the format of the client file. (NOTE: client authorization is implemented as of 0.3.5.1-alpha.) Appendix G. 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. (NOTE: client authorization is implemented as of 0.3.5.1-alpha.) G.1. Configuring client authorization using torrc G.1.1. Hidden Service side configuration A hidden service that wants to enable client authorization, needs to populate the "authorized_clients/" directory of its HiddenServiceDir directory with the ".auth" files of its authorized clients. When Tor starts up with a configured onion service, Tor checks its /authorized_clients/ directory for ".auth" files, and if any recognized and parseable such files are found, then client authorization becomes activated for that service. G.1.2. Service-side bookkeeping This section contains more details on how onion services should be keeping track of their client ".auth" files. For the "descriptor" authentication type, the ".auth" file MUST contain the x25519 public key of that client. Here is a suggested file format: :: Here is an an example: descriptor:x25519:OM7TGIVRYMY6PFX6GAC6ATRTA5U6WW6U7A4ZNHQDI6OVL52XVV2Q Tor SHOULD ignore lines it does not recognize. Tor SHOULD ignore files that don't use the ".auth" suffix. G.1.3. Client side configuration A client who wants to register client authorization data for onion services needs to add the following line to their torrc to indicate the directory which hosts ".auth_private" files containing client-side credentials for onion services: ClientOnionAuthDir The contains a file with the suffix ".auth_private" for each onion service the client is authorized with. Tor should scan the directory for ".auth_private" files to find which onion services require client authorization from this client. For the "descriptor" auth-type, a ".auth_private" file contains the private x25519 key: :descriptor:x25519: The keypair used for client authorization is created by a third party tool for which the public key needs to be transferred to the service operator in a secure out-of-band way. The third party tool SHOULD add appropriate headers to the private key file to ensure that users won't accidentally give out their private key. G.2. Configuring client authorization using the control port G.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. G.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?] Appendix F. Two methods for managing revision counters. Implementations MAY generate revision counters in any way they please, so long as they are monotonically increasing over the lifetime of each blinded public key. But to avoid fingerprinting, implementors SHOULD choose a strategy also used by other Tor implementations. Here we describe two, and additionally list some strategies that implementors should NOT use. F.1. Increment-on-generation This is the simplest strategy, and the one used by Tor through at least version 0.3.4.0-alpha. Whenever using a new blinded key, the service records the highest revision counter it has used with that key. When generating a descriptor, the service uses the smallest non-negative number higher than any number it has already used. In other words, the revision counters under this system start fresh with each blinded key as 0, 1, 2, 3, and so on. F.2. Encrypted time in period This scheme is what we recommend for situations when multiple service instances need to coordinate their revision counters, without an actual coordination mechanism. Let T be the number of seconds that have elapsed since the descriptor became valid, plus 1. (T must be at least 1.) Implementations can use the number of seconds since the start time of the shared random protocol run that corresponds to this descriptor. Let S be a secret that all the service providers share. For example, it could be the private signing key corresponding to the current blinded key. Let K be an AES-256 key, generated as K = H("rev-counter-generation" | S) Use K, and AES in counter mode with IV=0, to generate a stream of T * 2 bytes. Consider these bytes as a sequence of T 16-bit little-endian words. Add these words. Let the sum of these words be the revision counter. Cryptowiki attributes roughly this scheme to G. Bebek in: G. Bebek. Anti-tamper database research: Inference control techniques. Technical Report EECS 433 Final Report, Case Western Reserve University, November 2002. Although we believe it is suitable for use in this application, it is not a perfect order-preserving encryption algorithm (and all order-preserving encryption has weaknesses). Please think twice before using it for anything else. (This scheme can be optimized pretty easily by caching the encryption of X*1, X*2, X*3, etc for some well chosen X.) For a slow reference implementation, see src/test/ope_ref.py in the Tor source repository. [XXXX for now, see the same file in Nick's "ope_hax" branch -- it isn't merged yet.] This scheme is not currently implemented in Tor. F.X. Some revision-counter strategies to avoid Though it might be tempting, implementations SHOULD NOT use the current time or the current time within the period directly as their revision counter -- doing so leaks their view of the current time, which can be used to link the onion service to other services run on the same host. Similarly, implementations SHOULD NOT let the revision counter increase forever without resetting it -- doing so links the service across changes in the blinded public key.