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diff --git a/spec/rend-spec/client-authorization.md b/spec/rend-spec/client-authorization.md new file mode 100644 index 0000000..1cffce4 --- /dev/null +++ b/spec/rend-spec/client-authorization.md @@ -0,0 +1,105 @@ +<a id="rend-spec-v3.txt-G"></a> + +# 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 + +```text + 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 + <HiddenServiceDir>/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: + + <auth-type>:<key-type>:<base32-encoded-public-key> + + 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 <DIR> + + The <DIR> 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: + + <onion-address>:descriptor:x25519:<base32-encoded-privkey> + + 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?] +``` diff --git a/spec/rend-spec/deriving-keys.md b/spec/rend-spec/deriving-keys.md new file mode 100644 index 0000000..69eeee6 --- /dev/null +++ b/spec/rend-spec/deriving-keys.md @@ -0,0 +1,425 @@ +<a id="rend-spec-v3.txt-2.1"></a> + +# 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: + +```text +N_hs_subcred = H("subcredential" | N_hs_cred | blinded-public-key). +``` + +In the above formula, credential corresponds to: + +```text +N_hs_cred = H("credential" | public-identity-key) +``` + +where `public-identity-key` is the public identity master key of the hidden +service. + +# 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: + +```text + * 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. +``` + +<a id="rend-spec-v3.txt-2.2.1"></a> + +## 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 a regular offset +from the SRV voting schedule, so we subtract a "rotation time offset" +of 12 voting periods 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 when the voting period is 1 hour.) + +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. + +<a id="rend-spec-v3.txt-2.2.2"></a> + +## 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. +\[TODO: Control republish period using a consensus parameter?\] + +<a id="rend-spec-v3.txt-2.2.2.1"></a> + +### Overlapping descriptors {#OVERLAPPING-DESCS} + +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. + +\[TODO: What to do when we run multiple hidden services in a single host?\] + +<a id="rend-spec-v3.txt-2.2.3"></a> + +## 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; + +```text + 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: + +```text + for replicanum in 1...hsdir_n_replicas: + hs_service_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: + +```text + hs_relay_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_service_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. + +<a id="rend-spec-v3.txt-2.2.4"></a> + +## 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: + +```text + +------------------------------------------------------------------+ + | | + | 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] +``` + +<a id="rend-spec-v3.txt-2.2.4.1"></a> + +### 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: + +```text ++------------------------------------------------------------------+ +| | +| 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. + +<a id="rend-spec-v3.txt-2.2.4.2"></a> + +### 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: + +<a id="rend-spec-v3.txt-2.2.4.2.1"></a> + +#### 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: + +```text ++------------------------------------------------------------------+ +| | +| 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. + +<a id="rend-spec-v3.txt-2.2.4.2.2"></a> + +#### 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: + +```text ++------------------------------------------------------------------+ +| | +| 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. + +<a id="rend-spec-v3.txt-2.2.4.3"></a> + +### Directory behavior for handling descriptor uploads \[DIRUPLOAD\] + +Upon receiving a hidden service descriptor publish request, directories MUST +check the following: + +```text + * The outer wrapper of the descriptor can be parsed according to + [DESC-OUTER] + * The version-number of the descriptor is "3" + * If the directory has already cached a descriptor for this hidden service, + the revision-counter of the uploaded descriptor must be greater than the + revision-counter of the cached one + * The descriptor signature is valid +``` + +If any of these basic validity checks fails, the directory MUST reject the +descriptor upload. + +NOTE: Even if the descriptor passes the checks above, its first and second +layers could still be invalid: directories cannot validate the encrypted +layers of the descriptor, as they do not have access to the public key of the +service (required for decrypting the first layer of encryption), or the +necessary client credentials (for decrypting the second layer). + +<a id="rend-spec-v3.txt-2.2.5"></a> + +## 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). + +<a id="rend-spec-v3.txt-2.2.6"></a> + +## URLs for anonymous uploading and downloading {#urls} + +Hidden service descriptors conforming to this specification are uploaded +with an HTTP POST request to the URL `/tor/hs/<version>/publish` relative to +the hidden service directory's root, and downloaded with an HTTP GET +request for the URL `/tor/hs/<version>/<z>` where `<z>` is a base64 encoding of +the hidden service's blinded public key and `<version>` is the protocol +version which is "3" in this case. + +These requests must be made anonymously, on circuits not used for +anything else. + +<a id="rend-spec-v3.txt-2.2.7"></a> + +## Client-side validation of onion addresses {#addr-validation} + +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 \[TORSION-REFS\]. diff --git a/spec/rend-spec/encoding-onion-addresses.md b/spec/rend-spec/encoding-onion-addresses.md new file mode 100644 index 0000000..be8daf7 --- /dev/null +++ b/spec/rend-spec/encoding-onion-addresses.md @@ -0,0 +1,28 @@ +<a id="rend-spec-v3.txt-6"></a> + +# 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: + +```text + 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 historical notes and rationales about this encoding, +> see [this discussion thread](https://lists.torproject.org/pipermail/tor-dev/2017-January/011816.html). + diff --git a/spec/rend-spec/encrypting-user-data.md b/spec/rend-spec/encrypting-user-data.md new file mode 100644 index 0000000..fdf1a30 --- /dev/null +++ b/spec/rend-spec/encrypting-user-data.md @@ -0,0 +1,19 @@ +<a id="rend-spec-v3.txt-5"></a> + +# Encrypting data between client and host + +A successfully completed handshake, as embedded in the +INTRODUCE/RENDEZVOUS messages, 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. + +As mentioned +[previously](./introduction-protocol.md#INTRO-HANDSHAKE-REQS), +these keys are used the same as for +[regular relay cell encryption](../tor-spec/routing-relay-cells.md), +except that instead of using AES-128 and SHA1, +both parties use AES-256 and SHA3-256. diff --git a/spec/rend-spec/fs-contents.md b/spec/rend-spec/fs-contents.md new file mode 100644 index 0000000..f5c06f4 --- /dev/null +++ b/spec/rend-spec/fs-contents.md @@ -0,0 +1,30 @@ +<a id="rend-spec-v3.txt-F"></a> + +# 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. +\[TODO: Offline keys\] + +```text + - "./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.) diff --git a/spec/rend-spec/hsdesc-encrypt.md b/spec/rend-spec/hsdesc-encrypt.md new file mode 100644 index 0000000..3d2117e --- /dev/null +++ b/spec/rend-spec/hsdesc-encrypt.md @@ -0,0 +1,521 @@ +<a id="rend-spec-v3.txt-2.5"></a> + +# 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. + +<a id="rend-spec-v3.txt-2.5.1"></a> + +## 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. + +<a id="rend-spec-v3.txt-2.5.1.1"></a> + +### First layer encryption logic {#first-layer-logic} + +The encryption keys and format for the first layer of encryption are +generated as specified in \[HS-DESC-ENCRYPTION-KEYS\] with customization +parameters: + +```text + 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. + +<a id="rend-spec-v3.txt-2.5.1.2"></a> + +### First layer plaintext format {#first-layer-plaintext} + +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 (`N_hs_desc_enc`, 32 random bytes) and encrypts +it using each authorized client's identity x25519 key. Authorized +clients can use the descriptor cookie (`N_hs_desc_enc`) to decrypt +the second (inner) 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 + +\[Exactly once\] + +```text + 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 KP_hs_desc_ephem NL + + [Exactly once] + + This field contains `KP_hss_desc_enc`, 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 `N_hs_desc_enc` encrypted to each + individual client. We assume that each authorized client possesses + a pre-shared x25519 keypair (`KP_hsc_desc_enc`) which is used to + decrypt the descriptor cookie. + + We now describe the descriptor cookie encryption scheme. Here is what + the hidden service computes: + + SECRET_SEED = x25519(KS_hs_desc_ephem, KP_hsc_desc_enc) + 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 N_hs_desc_enc. + + 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. + + Compatibility note: The C Tor implementation does not include a final + newline when generating this first-layer-plaintext section; other + implementations MUST accept this section even if it is missing its final + newline. Other implementations MAY generate this section without a final + newline themselves, to avoid being distinguishable from C tor. +``` + +<a id="rend-spec-v3.txt-2.5.1.3"></a> + +### Client behavior {#FIRST-LAYER-CLIENT-BEHAVIOR} + +```text + 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]. +``` + +<a id="rend-spec-v3.txt-2.5.1.4"></a> + +### Hiding client authorization data {#hiding-client-auth} + +```text + 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). +``` + +<a id="rend-spec-v3.txt-2.5.2"></a> + +## 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. + +<a id="rend-spec-v3.txt-2.5.2.1"></a> + +### Second layer encryption keys {#second-layer-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: + +```text + 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. +``` + +<a id="rend-spec-v3.txt-2.5.2.2"></a> + +### Second layer plaintext format {#second-layer-plaintext} + +After decrypting the second layer ciphertext, clients can finally learn the +list of intro points etc. The plaintext has the following format: + +```text +"create2-formats" SP formats NL + +\[Exactly once\] + + A space-separated list of integers denoting CREATE2 cell HTYPEs + (handshake types) 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. +``` + +```text + "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: 'ed25519'. +``` + +```text + "single-onion-service" + + [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. +``` + +```text + "pow-params" SP scheme SP seed-b64 SP suggested-effort + SP expiration-time NL + + If present, this line provides parameters for an optional proof-of-work + client puzzle. A client that supports an offered scheme can include a + corresponding solution in its introduction request to improve priority + in the service's processing queue. + + Only scheme `v1` is currently defined. + It may appear only once. + + Unknown schemes found in a descriptor must be completely ignored: + future schemes might have a different format (in the parts of the + Item after the "scheme"; this could even include an Object); and + future schemes might allow repetition, and might appear in any order. + + Introduced in tor-0.4.8.1-alpha. + + scheme: The PoW system used. We call the one specified here "v1". + + seed-b64: A random seed that should be used as the input to the PoW + hash function. Should be 32 random bytes encoded in base64 + without trailing padding. + + suggested-effort: An unsigned integer specifying an effort value that + clients should aim for when contacting the service. Can be + zero to mean that PoW is available but not currently + suggested for a first connection attempt. + + expiration-time: A timestamp in "YYYY-MM-DDTHH:MM:SS" format (iso time + with no space) after which the above seed expires and + is no longer valid as the input for PoW. +``` + +Followed by zero or more introduction points as follows (see section +\[NUM_INTRO_POINT\] below for accepted values): + +```text + "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] above. + + 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 SHOULD perform the basic validity checks on the link + specifiers in the descriptor, described in `tor-spec.txt` + section 5.1.2. These checks SHOULD NOT leak + detailed information about the client's version, configuration, + or consensus. (See 3.3 for service link specifier handling.) + + When connecting to the introduction point, the client SHOULD send + this list of link specifiers verbatim, in the same order as given + here. + + The client MAY reject the list of link specifiers if it is + inconsistent with relay information from the directory, but SHOULD + NOT modify it. +``` + +```text + "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. +``` + +```text + "onion-key" SP KeyType SP key.. NL + + [Any number of times] + + Implementations should accept other types of onion keys using this + syntax (where "KeyType" is some string other than "ntor"); + unrecognized key types should be ignored. +``` + +<a id="auth-key"></a> +```text + "auth-key" NL certificate NL + + [Exactly once per introduction point] + + The certificate is a proposal 220 certificate wrapped in + "-----BEGIN ED25519 CERT-----". It contains the introduction + point authentication key (`KP_hs_ipt_sid`), signed by + the descriptor signing key (`KP_hs_desc_sign`). The + certificate type must be [09], and the signing key extension + is mandatory. + + NOTE: This certificate was originally intended to be + constructed the other way around: the signing and signed keys + are meant to be reversed. However, C tor implemented it + backwards, and other implementations now need to do the same + in order to conform. (Since this section is inside the + descriptor, which is _already_ signed by `KP_hs_desc_sign`, + the verification aspect of this certificate serves no point in + its current form.) +``` + +```text + "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. (`KP_hss_ntor`) +``` + +```text + "enc-key" SP KeyType SP key.. NL + + [Any number of times] + + Implementations should accept other types of onion keys using this + syntax (where "KeyType" is some string other than "ntor"); + unrecognized key types should be ignored. +``` + +<a id="enc-key-cert"></a> +```text + "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. + + The subject + key is the the ed25519 equivalent of a curve25519 public + encryption key (`KP_hss_ntor`), with the ed25519 key + derived using the process in proposal 228 appendix A, + and its sign bit set to zero. + + The + signing key is the descriptor signing key (`KP_hs_desc_sign`). + The certificate type must be [0B], and the signing-key + extension is mandatory. + + NOTE: As with "auth-key", this certificate was intended to be + constructed the other way around. However, for compatibility + with C tor, implementations need to construct it this way. It + serves even less point than "auth-key", however, since the + encryption key `KP_hss_ntor` is already available from + the `enc-key` entry. + + ALSO NOTE: Setting the sign bit of the subject key + to zero makes the subjected unusable for verification; + this is also a mistake preserved for compatiblility with + C tor. + + "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 \[INTRO-PROTOCOL\]. + +Compatibility note: At least some versions of OnionBalance do not include +a final newline when generating this inner plaintext section; other +implementations MUST accept this section even if it is missing its final +newline. + +<a id="rend-spec-v3.txt-2.5.3"></a> + +## 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: + +```text + 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)). +``` + +<a id="rend-spec-v3.txt-2.5.4"></a> + +## 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. diff --git a/spec/rend-spec/hsdesc-outer.md b/spec/rend-spec/hsdesc-outer.md new file mode 100644 index 0000000..f67fc6c --- /dev/null +++ b/spec/rend-spec/hsdesc-outer.md @@ -0,0 +1,83 @@ +<a id="rend-spec-v3.txt-2.4"></a> + +# 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 + +\[At start, exactly once.\] + +```text + 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). +``` + +<a id="descriptor-signing-key-cert"></a> +```text + "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. +``` + +```text + "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. +``` + +```text + "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.) +``` + +```text + "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). diff --git a/spec/rend-spec/hsdesc.md b/spec/rend-spec/hsdesc.md new file mode 100644 index 0000000..8b1d75a --- /dev/null +++ b/spec/rend-spec/hsdesc.md @@ -0,0 +1,8 @@ +<a id="rend-spec-v3.txt-2"></a> + +# 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). diff --git a/spec/rend-spec/index.md b/spec/rend-spec/index.md new file mode 100644 index 0000000..47e65aa --- /dev/null +++ b/spec/rend-spec/index.md @@ -0,0 +1,9 @@ +# 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. + +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. diff --git a/spec/rend-spec/introduction-protocol.md b/spec/rend-spec/introduction-protocol.md new file mode 100644 index 0000000..0181dd2 --- /dev/null +++ b/spec/rend-spec/introduction-protocol.md @@ -0,0 +1,755 @@ +<a id="rend-spec-v3.txt-3"></a> + +# 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: + +```text + * 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. + +<a id="rend-spec-v3.txt-3.1"></a> + +## Registering an introduction point {#REG_INTRO_POINT} + +<a id="rend-spec-v3.txt-3.1.1"></a> + +### Extensible ESTABLISH_INTRO protocol {#EST_INTRO} + +When a hidden service is establishing a new introduction point, it +sends an ESTABLISH_INTRO message with the following contents: + +```text + 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: + +```text + [00, 01] -- Reserved for legacy introduction messages; 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, KP_hs_ipt_sid. + +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.) + +```text + 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 following extensions are currently defined: + +| `EXT_FIELD_TYPE` | Name | +| ---------------- | -------------- | +| `[01]` | [`DOS_PARAMS`] | + +[`DOS_PARAMS`]: #DOS_PARAMS + +The HANDSHAKE_AUTH field contains the MAC of all earlier fields in +the message 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 messages. + +SIG_LEN is the length of the signature. + +SIG is a signature, using AUTH_KEY, of all contents of the message, up +to but not including SIG_LEN and SIG. These contents are prefixed +with the string "Tor establish-intro cell v1". + +> (Note that this string is _sic_; +> it predates our efforts to distinguish cells from relay messages.) + +Upon receiving an ESTABLISH_INTRO message, a Tor node first decodes the +key and the signature, and checks the signature. The node must reject +the ESTABLISH_INTRO message and destroy the circuit in these cases: + +```text + * 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 messages. + + + +<a id="rend-spec-v3.txt-3.1.1.1"></a> + +#### Denial-of-Service defense extension (DOS\_PARAMS) {#EST_INTRO_DOS_EXT} + +<a id="DOS_PARAMS"></a> +The `DOS_PARAMS` extension +in ESTABLISH_INTRO +is used to send Denial-of-Service (DoS) parameters to +the introduction point in order for it to apply them for the introduction +circuit. + +This is for the [rate limiting DoS mitigation](../dos-spec/overview.md#hs-intro-rate) specifically. + +The `EXT_FIELD_TYPE` value for the `DOS_PARAMS` extension is `[01]`. + +The content is defined as follows: + +| Field | Size | Description | +| ----------------- | ---- | -------------------- | +| `N_PARAMS` | 1 | Number of parameters | +| `N_PARAMS` times: | | | +| - PARAM_TYPE | 1 | Identifier for a parameter | +| - PARAM_VALUE | 8 | Integer value | + +Recognized values for `PARAM_TYPE` in this extension are: + +| `PARAM_TYPE` | Name | Min | Max | +| ----------- | -------------------------------- | --- | ---------- | +| `[01]` | [`DOS_INTRODUCE2_RATE_PER_SEC`] | 0 | 0x7fffffff | +| `[02]` | [`DOS_INTRODUCE2_BURST_PER_SEC`] | 0 | 0x7fffffff | + +[`DOS_INTRODUCE2_RATE_PER_SEC`]: #DOS_INTRODUCE2_RATE_PER_SEC +[`DOS_INTRODUCE2_BURST_PER_SEC`]: #DOS_INTRODUCE2_BURST_PER_SEC + +Together, these parameters configure a token bucket +that determines how many INTRODUCE2 messages +the introduction point may send to the service. + +<span id="DOS_INTRODUCE2_RATE_PER_SEC"> +The `DOS_INTRODUCE2_RATE_PER_SEC` parameter defines the maximum +average rate of messages; +</span> +<span id="DOS_INTRODUCE2_BURST_PER_SEC"> +The `DOS_INTRODUCE2_BURST_PER_SEC` parameter defines the largest +allowable burst of messages +(that is, the size of the token bucket). +</span> + +> Technically speaking, the `BURST` parameter is misnamed +> in that it is not actually "per second": +> only a _rate_ has an associated time. + +If either of these parameters is set to 0, +the defense is disabled, +and the introduction point should ignore the other parameter. + +If the burst is lower than the rate, +the introduction point SHOULD ignore the extension. + +> Using this extension extends the body of the ESTABLISH_INTRO message by 19 +> bytes bringing it from 134 bytes to 155 bytes. + +When this extension is not _sent_, +introduction points use default settings +taken from taken from the consensus parameters +[HiddenServiceEnableIntroDoSDefense](../param-spec.md#HiddenServiceEnableIntroDoSDefense), +[HiddenServiceEnableIntroDoSRatePerSec](../param-spec.md#HiddenServiceEnableIntroDoSRatePerSec), +and +[HiddenServiceEnableIntroDoSBurstPerSec](../param-spec.md#HiddenServiceEnableIntroDoSBurstPerSec). + +This extension can only be used with relays supporting the protocol version +["HSIntro=5"](../tor-spec/subprotocol-versioning.md#HSIntro). + +Introduced in tor-0.4.2.1-alpha. + +<a id="rend-spec-v3.txt-3.1.2"></a> + +## 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 +message, 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 message contains: + +```text + KEY_LEN [2 bytes] + KEY [KEY_LEN bytes] + HANDSHAKE_AUTH [20 bytes] + SIG [variable, up to end of relay message body] + + 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. + +<a id="rend-spec-v3.txt-3.1.3"></a> + +### 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 message. + +The INTRO_ESTABLISHED message has the following contents: + +```text + 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 message instead. +Services must accept an empty INTRO_ESTABLISHED message 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. + +<a id="rend-spec-v3.txt-3.2"></a> + +## Sending an INTRODUCE1 message to the introduction point {#SEND_INTRO1} + +In order to participate in the introduction protocol, a client must +know the following: + +```text + * 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 message 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 message back to +the client, either informing it that its request has been delivered, +or that its request will not succeed. + +If the INTRODUCE_ACK message indicates success, +the client SHOULD close the circuit to the introduction point, +and not use it for anything else. +If the INTRODUCE_ACK message indicates failure, +the client MAY try a different introduction point. +It MAY reach the different introduction point +either by extending its introduction circuit an additional hop, +or by building a new introduction circuit. + +```text + [TODO: specify what tor should do when receiving a malformed message. Drop it? + Kill circuit? This goes for all possible messages.] +``` + +<a id="rend-spec-v3.txt-3.2.1"></a> + +### Extensible INTRODUCE1 message format {#FMT_INTRO1} + +When a client is connecting to an introduction point, INTRODUCE1 messages +should be of the form: + +```text + 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 message body] +``` + +The `ENCRYPTED` field is described in the \[PROCESS_INTRO2\] section. + +AUTH_KEY_TYPE is defined as in \[EST_INTRO\]. Currently, the only value of +AUTH_KEY_TYPE for this message is an Ed25519 public key \[02\]. + +The LEGACY_KEY_ID field is used to distinguish between legacy and new style +INTRODUCE1 messages. In new style INTRODUCE1 messages, LEGACY_KEY_ID is 20 zero +bytes. Upon receiving an INTRODUCE1 messages, the introduction point checks the +LEGACY_KEY_ID field. If LEGACY_KEY_ID is non-zero, the INTRODUCE1 message +should be handled as a legacy INTRODUCE1 message by the intro point. + +Upon receiving a INTRODUCE1 message, 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 message with exactly the same contents to the service, and sends an +INTRODUCE_ACK response to the client. + +(Note that the introduction point does not "clean up" the +INTRODUCE1 message that it retransmits. Specifically, it does not +change the order or multiplicity of the extensions sent by the +client.) + +The same rules for multiplicity, ordering, and handling unknown types +apply to the extension fields here as described \[EST_INTRO\] above. + +<a id="rend-spec-v3.txt-3.2.2"></a> + +### INTRODUCE_ACK message format. {#INTRO_ACK} + +An INTRODUCE_ACK message has the following fields: + +```text + 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: message relayed to hidden service host. + [00 01] -- Failure: service ID not recognized + [00 02] -- Bad message format + [00 03] -- Can't relay message to service +``` + +The same rules for multiplicity, ordering, and handling unknown types +apply to the extension fields here as described \[EST_INTRO\] above. + +<a id="rend-spec-v3.txt-3.3"></a> + +## Processing an INTRODUCE2 message at the hidden service. {#PROCESS_INTRO2} + +Upon receiving an INTRODUCE2 message, 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 message 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 messages +with the same encryption key. Note that the encryption format below should +be non-malleable.) + +If the message is not a replay, it decrypts the ENCRYPTED field, +establishes a shared key with the client, and authenticates the whole +contents of the message 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: + +```text + 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 messages. All available addresses SHOULD be included in the +message, 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 +SHOULD perform basic validity checks on link specifiers, and SHOULD NOT +reject unrecognised link specifiers, to avoid information leaks. +The list of link specifiers received here SHOULD either be rejected, or +sent verbatim when extending to the rendezvous point, in the same order +received. + +The service MAY reject the list of link specifiers if it is +inconsistent with relay information from the directory, but SHOULD +NOT modify it. + +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. + +The PAD field should be filled with zeros; its size should be chosen +so that the INTRODUCE2 message occupies a fixed maximum size, in +order to hide the length of the encrypted data. (This maximum size is +490, since we assume that a future Tor implementations will implement +proposal 340 and thus lower the number of bytes that can be contained +in a single relay message.) Note also that current versions of Tor +only pad the INTRODUCE2 message up to 246 bytes. + +Upon receiving a well-formed INTRODUCE2 message, the hidden service host +will have: + +```text + * 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. + +### INTRODUCE1/INTRODUCE2 Extensions + +The following sections details the currently supported or reserved extensions +of an `INTRODUCE1`/`INTRODUCE2` message. + +Note that there are two sets of extensions in `INTRODUCE1`/`INTRODUCE2`: +one in the top-level, unencrypted portion, +and one within the plaintext of ENCRYPTED +(ie, after RENDEZVOUS_COOKIE and before ONION_KEY_TYPE. + +The sets of extensions allowed in each part of the message are disjoint: +each extension is valid in only *one* of the two places. + +Nevertheless, for historical reasons, +both kinds of extension are listed in this section, +and they use nonoverlapping values of `EXT_FIELD_TYPE`. + +#### Congestion Control + +This is used to request that the rendezvous circuit with the service be +configured with congestion control. + + EXT_FIELD_TYPE: + + \[01\] -- Congestion Control Request. + +This field is has zero body length. Its presence signifies that the client +wants to use congestion control. The client MUST NOT set this field, or use +ntorv3, if the service did not list "2" in the `FlowCtrl` line in the +descriptor. The client SHOULD NOT provide this field if the consensus parameter +'cc_alg' is 0. + +This appears in the ENCRYPTED section of the INTRODUCE1/INTRODUCE2 message. + +#### Proof-of-Work (PoW) {#INTRO1_POW_EXT} + +This extension can be used to optionally attach a proof of work to the introduction request. +The proof must be calculated using unique parameters appropriate for this specific service. +An acceptable proof will raise the priority of this introduction request according to the proof's verified computational effort. + +This is for the [proof-of-work DoS mitigation](../dos-spec/overview.md#hs-intro-pow), described in depth by the [Proof of Work for onion service introduction](../hspow-spec/index.md) specification. + +This appears in the ENCRYPTED section of the INTRODUCE1/INTRODUCE2 message. + +The content is defined as follows: + +EXT_FIELD_TYPE: + +\[02\] -- `PROOF_OF_WORK` + +```text +The EXT_FIELD content format is: + + POW_SCHEME [1 byte] + POW_NONCE [16 bytes] + POW_EFFORT [4 bytes] + POW_SEED [4 bytes] + POW_SOLUTION [16 bytes] + +where: + +POW_SCHEME is 1 for the `v1` protocol specified here +POW_NONCE is the nonce value chosen by the client's solver +POW_EFFORT is the effort value chosen by the client, + as a 32-bit integer in network byte order +POW_SEED identifies which seed was in use, by its first 4 bytes +POW_SOLUTION is a matching proof computed by the client's solver +``` + +Only SCHEME 1, `v1`, is currently defined. +Other schemes may have a different format, +after the POW_SCHEME byte. +A correctly functioning client only submits solutions with a scheme and seed which were advertised by the server +(using a "pow-params" Item in the +[HS descriptor](hsdesc-encrypt.md#second-layer-plaintext)) +and have not yet expired. +An extension with an unknown scheme or expired seed is suspicious and SHOULD result in introduction failure. + +Introduced in tor-0.4.8.1-alpha. + +#### Subprotocol Request + +\[RESERVED\] + + EXT_FIELD_TYPE: + + \[03\] -- Subprotocol Request + +<a id="rend-spec-v3.txt-3.3.1"></a> + +### Introduction handshake encryption requirements {#INTRO-HANDSHAKE-REQS} + +When decoding the encrypted information in an INTRODUCE2 message, a +hidden service host must be able to: + +```text + * Decrypt additional information included in the INTRODUCE2 message, + 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 message 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. + +```text +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: + +```text + 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 message, 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: + +```text + 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 message: + + CLIENT_PK [PK_PUBKEY_LEN bytes] + ENCRYPTED_DATA [Padded to length of plaintext] + MAC [MAC_LEN bytes] +``` + +Substituting those fields into the INTRODUCE1 message body format +described in \[FMT_INTRO1\] above, we have + +```text + 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 message 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 message. 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: + +```text + 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 message 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 with the +[ntor handshake](../tor-spec/create-created-cells.html#ntor), +except that instead of using +AES-128 and SHA1 for this hop, we use AES-256 and SHA3-256. + +<a id="rend-spec-v3.txt-3.4"></a> + +## 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. + +There is one defined authentication type: `ed25519`. + +<a id="rend-spec-v3.txt-3.4.1"></a> + +### Ed25519-based authentication `ed25519` {#ed25519-auth} + +(NOTE: This section is not implemented by Tor. It is likely +that we would want to change its design substantially before +deploying any implementation. At the very least, we would +want to bind these extensions to a single onion service, to +prevent replays. We might also want to look for ways to limit +the number of keys a user needs to have.) + +To authenticate with an Ed25519 private key, the user must include an +extension field in the encrypted part of the INTRODUCE1 message with an +EXT_FIELD_TYPE type of \[02\] and the contents: + +```text + 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: + +```text + "hidserv-userauth-ed25519" + Nonce (same as above) + Pubkey (same as above) + AUTH_KEY (As in the INTRODUCE1 message) +``` + +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 message is sufficient to prevent replays +on the authentication. + +Users SHOULD NOT use the same public key with multiple hidden +services. diff --git a/spec/rend-spec/keyblinding-scheme.md b/spec/rend-spec/keyblinding-scheme.md new file mode 100644 index 0000000..b69a578 --- /dev/null +++ b/spec/rend-spec/keyblinding-scheme.md @@ -0,0 +1,104 @@ +<a id="rend-spec-v3.txt-A"></a> + +# Appendix A: Signature scheme with key blinding {#KEYBLIND} + +<a id="rend-spec-v3.txt-A.1"></a> + +## Key derivation overview {#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). + +```text + 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 id="rend-spec-v3.txt-A.2"></a> + +## Tor's key derivation scheme {#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 \[ED25519-B-REF\]: + +```text + 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: + +```text + 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'. + +```text + (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. diff --git a/spec/rend-spec/managing-streams.md b/spec/rend-spec/managing-streams.md new file mode 100644 index 0000000..9eb1e82 --- /dev/null +++ b/spec/rend-spec/managing-streams.md @@ -0,0 +1,33 @@ + +# Managing streams + +## Sending BEGIN messages { #send-begin } + +In order to open a new stream to an onion service, +the client sends a BEGIN message on an established rendezvous circuit. + +When sending a BEGIN message to an onion service, +a client should use an empty string as the target address, +and not set any flags on the begin message. + +> For example, to open a connection to `<some_addr>.onion` +> on port 443, a client would send a BEGIN message with +> the address:port string of `":443"`, and a `FLAGS` value of 0. +> The 0-values `FLAGS` would not be encoded, according to +> the instructions for [encoding BEGIN messages](../tor-spec/opening-streams.md#opening). + +## Receiving BEGIN messages { #receive-begin } + +When a service receives a BEGIN message, it should check its port, +_and ignore all other fields in the begin message_, including its +address and flags. + +If a service chooses to reject a BEGIN message, it should typically +destroy the circuit entirely to prevent port scanning, +resource exhaustion, and other undesirable behaviors. +But if it rejects the BEGIN without destroy the circuit, +it should send back an `END` message with the `DONE` reason, +to avoid leaking any further information. + +If the service chooses to accept the BEGIN message, +it should send back a CONNECTED message with an empty body. diff --git a/spec/rend-spec/overview.md b/spec/rend-spec/overview.md new file mode 100644 index 0000000..5ccddd4 --- /dev/null +++ b/spec/rend-spec/overview.md @@ -0,0 +1,315 @@ +<a id="rend-spec-v3.txt-0"></a> + +# 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 + +```text + 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. +``` + +<a id="rend-spec-v3.txt-0.1"></a> + +## Improvements over previous versions {#improvements} + +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 + +<a id="rend-spec-v3.txt-0.2"></a> + +## Notation and vocabulary {#notation} + +Unless specified otherwise, all multi-octet integers are big-endian. + +We write sequences of bytes in two ways: + +```text + 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). + +<a id="rend-spec-v3.txt-0.3"></a> + +## Cryptographic building blocks {#cryptography} + +This specification uses the following cryptographic building blocks: + +```text + * 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)). +``` + +When we need a particular MAC key length below, we choose +MAC_KEY_LEN=32 (256 bits). + +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. + +<a id="rend-spec-v3.txt-0.4"></a> + +## 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 messages in the Tor protocol. + +```text + 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 SHOULD include at minimum specifiers of type \[00\] +(TLS-over-TCP, IPv4), \[02\] (legacy node identity) and \[03\] (ed25519 +identity key). Sets of link specifiers without these three types +SHOULD be rejected. + +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. + +<a id="rend-spec-v3.txt-0.5"></a> + +## Assigned relay message types {#relay-cell-types} + +These relay message types are reserved for use in the hidden service +protocol. + +32 -- RELAY_COMMAND_ESTABLISH_INTRO + +```text + 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 message. Discussed in + [EST_REND_POINT] + + 40 -- RELAY_COMMAND_INTRODUCE_ACK + + Sent from introduction point to client; acknowledges + receipt of INTRODUCE1 message and reports success/failure. + Discussed in [INTRO_ACK] +``` + +<a id="rend-spec-v3.txt-0.6"></a> + +## Acknowledgments + +This design includes ideas from many people, including + +```text + 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 + +```text + 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 + +```text + 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 + +```text + 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. diff --git a/spec/rend-spec/protocol-overview.md b/spec/rend-spec/protocol-overview.md new file mode 100644 index 0000000..9f7c96f --- /dev/null +++ b/spec/rend-spec/protocol-overview.md @@ -0,0 +1,355 @@ +<a id="rend-spec-v3.txt-1"></a> + +# 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. + +<a id="rend-spec-v3.txt-1.1"></a> + +## View from 10,000 feet {#10000-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 use Tor relay cells +to deliver relay messages to the server: +Whenever the rendezvous point receives as relay cell from one of +the circuits, it transmits it to the other. +(It accepts both RELAY and RELAY_EARLY cells, +and retransmits them all as RELAY cells.) + +The two parties use these relay messages to implement Tor's +usual application stream protocol: +RELAY_BEGIN messages open streams to an external process +or processes configured by the server; RELAY_DATA messages are used to +communicate data on those streams, and so forth. + +<a id="rend-spec-v3.txt-1.2"></a> + +## 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: + +```text + unlikelynamefora.onion + yyhws9optuwiwsns.onion + + And a new name following this specification might look like: + + l5satjgud6gucryazcyvyvhuxhr74u6ygigiuyixe3a6ysis67ororad.onion + + Please see section [ONIONADDRESS] for the encoding specification. +``` + +<a id="rend-spec-v3.txt-1.3"></a> + +## 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. + +<a id="rend-spec-v3.txt-1.4"></a> + +## 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: + +```text + * 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. + +<a id="rend-spec-v3.txt-1.5"></a> + +## In more detail: Scaling to multiple hosts {#imd-scaling} + +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. + +```text +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. + +<a id="rend-spec-v3.txt-1.7"></a> + +## In more detail: Keeping crypto keys offline {#imd-offline-keys} + +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.) + +<a id="rend-spec-v3.txt-1.8"></a> + +## In more detail: Encryption Keys And Replay Resistance {#imd-encryption-keys} + +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.) + +<a id="rend-spec-v3.txt-1.9"></a> + +## In more detail: A menagerie of keys {#imd-key-menagerie} + +\[In the text below, an "encryption keypair" is roughly "a keypair you +can do Diffie-Hellman with" and a "signing keypair" is roughly "a +keypair you can do ECDSA with."\] + +Public/private keypairs defined in this document: + +<a id="hs_id"></a> +```text + 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. +``` + +<a id="hs_blind_id"></a> +```text + 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_hs_blind_id, KS_hs_blind_id. + +``` + +<a id="hs_desc_sign"></a> +```text + 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. +``` + +<a id="hs_ipt_sid"></a> +```text + Introduction point authentication key -- A short-term signing + keypair used to identify a hidden service's session at a given + introduction point. The service makes a fresh keypair 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_ipt_sid, KS_hs_ipt_sid + ("hidden service introduction point session id"). +``` + +<a id="hss_ntor"></a> +```text + 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. The service + makes a fresh keypair for each introduction point. + KP_hss_ntor, KS_hss_ntor. +``` + +<a id="hss_desc_enc"></a> +```text + Ephemeral descriptor encryption key -- A short-lived encryption + keypair made by the service, and used to encrypt the inner layer + of hidden service descriptors when client authentication is in + use. + KP_hss_desc_enc, KS_hss_desc_enc +``` + +```text + Nonces defined in this document: + + N_hs_desc_enc -- a nonce used to derive keys to decrypt the inner + encryption layer of hidden service descriptors. This is + sometimes also called a "descriptor cookie". + + Public/private keypairs defined elsewhere: + + Onion key -- Short-term encryption keypair (KS_ntor, KP_ntor). + + (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. +``` + +<a id="rend-spec-v3.txt-1.9.1"></a> + +### 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: + +```text + - An x25519 keypair used to compute decryption keys that allow the client to + decrypt the hidden service descriptor. See [HS-DESC-ENC]. This is + the client's counterpart to KP_hss_desc_enc. + KP_hsc_desc_enc, KS_hsd_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 message, and without them the + introduction to the hidden service cannot be completed. See [INTRO-AUTH]. + KP_hsc_intro_auth, KS_hsc_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.) diff --git a/spec/rend-spec/references.md b/spec/rend-spec/references.md new file mode 100644 index 0000000..6488ebe --- /dev/null +++ b/spec/rend-spec/references.md @@ -0,0 +1,89 @@ +<a id="rend-spec-v3.txt-7"></a> + +# References + +How can we improve the HSDir unpredictability design proposed in +\[SHAREDRANDOM\]? See these references for discussion. + +```text +[SHAREDRANDOM-REFS]: + https://gitweb.torproject.org/torspec.git/tree/proposals/250-commit-reveal-consensus.txt + https://trac.torproject.org/projects/tor/ticket/8244 +``` + + +Scaling hidden services is hard. There are on-going discussions that +you might be able to help with: + +```text +[SCALING-REFS]: + https://lists.torproject.org/pipermail/tor-dev/2013-October/005556.html +``` + +How can hidden service addresses become memorable while retaining +their self-authenticating and decentralized nature? See +these references for some proposals; many more are possible. + +```text +[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 +``` + +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 these references for some +suggestions. + +```text +[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? :( ] +``` + +Other references: + +```text +[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 + +[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 +``` diff --git a/spec/rend-spec/rendezvous-protocol.md b/spec/rend-spec/rendezvous-protocol.md new file mode 100644 index 0000000..0f6d4e9 --- /dev/null +++ b/spec/rend-spec/rendezvous-protocol.md @@ -0,0 +1,138 @@ +<a id="rend-spec-v3.txt-4"></a> + +# 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 message. The hidden service +later connects to the same node and sends a RENDEZVOUS message. Once +this has occurred, the relay forwards the contents of the RENDEZVOUS +message 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: + +```text + * 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. +``` + +<a id="rend-spec-v3.txt-4.1"></a> + +## Establishing a rendezvous point {#EST_REND_POINT} + +The client sends the rendezvous point a RELAY_COMMAND_ESTABLISH_RENDEZVOUS +message containing a 20-byte value. + +RENDEZVOUS_COOKIE \[20 bytes\] + +Rendezvous points MUST ignore any extra bytes in an +ESTABLISH_RENDEZVOUS message. (Older versions of Tor did not.) + +The rendezvous cookie is an arbitrary 20-byte value, chosen randomly +by the client. The client SHOULD choose a new rendezvous cookie for +each new connection attempt. If the rendezvous cookie is already in +use on an existing circuit, the rendezvous point should reject it and +destroy the circuit. + +Upon receiving an ESTABLISH_RENDEZVOUS message, the rendezvous point associates +the cookie with the circuit on which it was sent. It replies to the client +with an empty RENDEZVOUS_ESTABLISHED message to indicate success. Clients MUST +ignore any extra bytes in a RENDEZVOUS_ESTABLISHED message. + +The client MUST NOT use the circuit which sent the message 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. + +<a id="rend-spec-v3.txt-4.2"></a> + +## 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 message containing: + +```text + 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 message to the +client containing the HANDSHAKE_INFO field of the RENDEZVOUS1 message. + +Upon receiving the RENDEZVOUS2 message, 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: + +```text + rend_secret_hs_input = EXP(Y,x) | EXP(B,x) | 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) +``` + +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: + +<a id="rend-spec-v3.txt-4.2.1"></a> + +### 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. + +\[TODO: Should we encrypt HANDSHAKE_INFO as we did INTRODUCE2 +contents? It's not necessary, but it could be wise. Similarly, we +should make it extensible.\] + +<a id="rend-spec-v3.txt-4.3"></a> + +## Using legacy hosts as rendezvous points {#legacy-rend-hosts} + +\[This section is obsolete and refers to a workaround for now-obsolete Tor +relay versions. It is included for historical reasons.\] + +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 message bodies be exactly +168 bytes long. All shorter rendezvous bodies 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 messages. Hence the "HSRend" protocol from proposal#264 should be +used to select relays for rendezvous points. diff --git a/spec/rend-spec/reserved-numbers.md b/spec/rend-spec/reserved-numbers.md new file mode 100644 index 0000000..70c9e36 --- /dev/null +++ b/spec/rend-spec/reserved-numbers.md @@ -0,0 +1,17 @@ +<a id="rend-spec-v3.txt-E"></a> + +# Appendix E: Reserved numbers + +We reserve these certificate type values for Ed25519 certificates: + +```text + [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. +``` diff --git a/spec/rend-spec/revision-counter-mgt.md b/spec/rend-spec/revision-counter-mgt.md new file mode 100644 index 0000000..f5c9a9c --- /dev/null +++ b/spec/rend-spec/revision-counter-mgt.md @@ -0,0 +1,103 @@ +<a id="rend-spec-v3.txt-F"></a> + +# 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 {#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 {#encrypted-time} + +This scheme is what we recommend for situations when multiple +service instances need to +[coordinate their revision counters](#use-case), +without an actual coordination mechanism. + +Let T be the number of seconds that have elapsed +since the start time of the SRV protocol run +that corresponds to this descriptor, +plus 1. (T must be at least 1.) + +Let S be a per-time-period secret that all the service providers share. +(C tor and arti use `S = KS_hs_blind_id`; +when `KS_hs_blind_id` is not available, +implementations may use `S = KS_hs_desc_sign`.) + +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, plus T, be the revision counter. + +> (We include T in the sum so that every increment in T adds at least +> one to the output.) + +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 +that can generate test vectors, +see `src/test/ope_ref.py` in the +Tor source repository. + +> <h6 id="use-case">Note:</h6> +> +> Some onion service operators have historically relied upon +> the behavior of this OPE scheme to provide +> a kind of ersatz load-balancing: +> they run multiple onion services, +> each with the the same `KP_hs_id`. +> The onion services choose different introduction points, +> and race each other to publish their HSDescs. +> +> It's probably better to use +> [Onionbalance](https://onionbalance.readthedocs.io/en/latest/) +> or a similar tool for this purpose. +> Nonetheless, onion services implemenentations +> MAY choose to implement this particular OPE scheme exactly +> in order to provide interoperability with C tor +> in this use case. + +## F.X. Some revision-counter strategies to avoid {#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. diff --git a/spec/rend-spec/selecting-nodes-picknodes.md b/spec/rend-spec/selecting-nodes-picknodes.md new file mode 100644 index 0000000..8b0973c --- /dev/null +++ b/spec/rend-spec/selecting-nodes-picknodes.md @@ -0,0 +1,10 @@ +<a id="rend-spec-v3.txt-B"></a> + +# Appendix B: Selecting nodes \[PICKNODES\] + +Picking introduction points +Picking rendezvous points +Building paths +Reusing circuits + +(TODO: This needs a writeup) diff --git a/spec/rend-spec/shared-random.md b/spec/rend-spec/shared-random.md new file mode 100644 index 0000000..e6b2452 --- /dev/null +++ b/spec/rend-spec/shared-random.md @@ -0,0 +1,49 @@ +<a id="rend-spec-v3.txt-2.3"></a> + +# 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: + +```text + "shared-rand-previous-value" SP NUM_REVEALS SP VALUE NL + "shared-rand-current-value" SP NUM_REVEALS SP VALUE NL +``` + +<a id="rend-spec-v3.txt-2.3.1"></a> + +## Client behavior in the absence of shared random values {#client-disaster} + +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, +rounded down; period_num is calculated as specified in +\[TIME-PERIODS\] for the wanted shared random value that could not be +found originally. + +<a id="rend-spec-v3.txt-2.3.2"></a> + +## Hidden services and changing shared random values {#service-problems} + +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? diff --git a/spec/rend-spec/test-vectors.md b/spec/rend-spec/test-vectors.md new file mode 100644 index 0000000..d77049e --- /dev/null +++ b/spec/rend-spec/test-vectors.md @@ -0,0 +1,101 @@ +<a id="rend-spec-v3.txt-G"></a> + +# Appendix G: Test vectors + +G.1. Test vectors for hs-ntor / NTOR-WITH-EXTRA-DATA + +```text + Here is a set of test values for the hs-ntor handshake, called + [NTOR-WITH-EXTRA-DATA] in this document. They were generated by + instrumenting Tor's code to dump the values for an INTRODUCE/RENDEZVOUS + handshake, and then by running that code on a Chutney network. + + We assume an onion service with: + + KP_hs_ipd_sid = 34E171E4358E501BFF21ED907E96AC6B + FEF697C779D040BBAF49ACC30FC5D21F + KP_hss_ntor = 8E5127A40E83AABF6493E41F142B6EE3 + 604B85A3961CD7E38D247239AFF71979 + KS_hss_ntor = A0ED5DBF94EEB2EDB3B514E4CF6ABFF6 + 022051CC5F103391F1970A3FCD15296A + N_hs_subcred = 0085D26A9DEBA252263BF0231AEAC59B + 17CA11BAD8A218238AD6487CBAD68B57 + + The client wants to make in INTRODUCE request. It generates + the following header (everything before the ENCRYPTED portion) + of its INTRODUCE1 message: + + H = 000000000000000000000000000000000000000002002034E171E4358E501BFF + 21ED907E96AC6BFEF697C779D040BBAF49ACC30FC5D21F00 + + It generates the following plaintext body to encrypt. (This + is the "decrypted plaintext body" from [PROCESS_INTRO2]. + + P = 6BD364C12638DD5C3BE23D76ACA05B04E6CE932C0101000100200DE6130E4FCA + C4EDDA24E21220CC3EADAE403EF6B7D11C8273AC71908DE565450300067F0000 + 0113890214F823C4F8CC085C792E0AEE0283FE00AD7520B37D0320728D5DF39B + 7B7077A0118A900FF4456C382F0041300ACF9C58E51C392795EF870000000000 + 0000000000000000000000000000000000000000000000000000000000000000 + 000000000000000000000000000000000000000000000000000000000000 + + (Note! This should in fact be padded to be longer; when these + test vectors were generated, the target INTRODUCE1 length in C + Tor was needlessly short.) + + The client now begins the hs-ntor handshake. It generates + a curve25519 keypair: + + x = 60B4D6BF5234DCF87A4E9D7487BDF3F4 + A69B6729835E825CA29089CFDDA1E341 + X = BF04348B46D09AED726F1D66C618FDEA + 1DE58E8CB8B89738D7356A0C59111D5D + + Then it calculates: + + ENC_KEY = 9B8917BA3D05F3130DACCE5300C3DC27 + F6D012912F1C733036F822D0ED238706 + MAC_KEY = FC4058DA59D4DF61E7B40985D122F502 + FD59336BC21C30CAF5E7F0D4A2C38FD5 + + With these, it encrypts the plaintext body P with ENC_KEY, getting + an encrypted value C. It computes MAC(MAC_KEY, H | X | C), + getting a MAC value M. It then assembles the final INTRODUCE1 + body as H | X | C | M: + + 000000000000000000000000000000000000000002002034E171E4358E501BFF + 21ED907E96AC6BFEF697C779D040BBAF49ACC30FC5D21F00BF04348B46D09AED + 726F1D66C618FDEA1DE58E8CB8B89738D7356A0C59111D5DADBECCCB38E37830 + 4DCC179D3D9E437B452AF5702CED2CCFEC085BC02C4C175FA446525C1B9D5530 + 563C362FDFFB802DAB8CD9EBC7A5EE17DA62E37DEEB0EB187FBB48C63298B0E8 + 3F391B7566F42ADC97C46BA7588278273A44CE96BC68FFDAE31EF5F0913B9A9C + 7E0F173DBC0BDDCD4ACB4C4600980A7DDD9EAEC6E7F3FA3FC37CD95E5B8BFB3E + 35717012B78B4930569F895CB349A07538E42309C993223AEA77EF8AEA64F25D + DEE97DA623F1AEC0A47F150002150455845C385E5606E41A9A199E7111D54EF2 + D1A51B7554D8B3692D85AC587FB9E69DF990EFB776D8 +``` + +Later the service receives that body in an INTRODUCE2 message. It +processes it according to the hs-ntor handshake, and recovers +the client's plaintext P. To continue the hs-ntor handshake, +the service chooses a curve25519 keypair: + +```text + y = 68CB5188CA0CD7924250404FAB54EE13 + 92D3D2B9C049A2E446513875952F8F55 + Y = 8FBE0DB4D4A9C7FF46701E3E0EE7FD05 + CD28BE4F302460ADDEEC9E93354EE700 + + From this and the client's input, it computes: + + AUTH_INPUT_MAC = 4A92E8437B8424D5E5EC279245D5C72B + 25A0327ACF6DAF902079FCB643D8B208 + NTOR_KEY_SEED = 4D0C72FE8AFF35559D95ECC18EB5A368 + 83402B28CDFD48C8A530A5A3D7D578DB +``` + +The service sends back Y | AUTH_INPUT_MAC in its RENDEZVOUS1 message +body. From these, the client finishes the handshake, validates +AUTH_INPUT_MAC, and computes the same NTOR_KEY_SEED. + +Now that both parties have the same NTOR_KEY_SEED, they can derive +the shared key material they will use for their circuit. diff --git a/spec/rend-spec/vanity-onions.md b/spec/rend-spec/vanity-onions.md new file mode 100644 index 0000000..f07d108 --- /dev/null +++ b/spec/rend-spec/vanity-onions.md @@ -0,0 +1,45 @@ +<a id="rend-spec-v3.txt-C"></a> + +# 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: + +```text + 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. |