tor/doc/spec/proposals/121-hidden-service-authentication.txt

Filename: 121-hidden-service-authentication.txt
Title: Hidden Service Authentication
Version: $Revision$
Last-Modified: $Date$
Author: Tobias Kamm, Thomas Lauterbach, Karsten Loesing, Ferdinand Rieger,
        Christoph Weingarten
Created: 10-Sep-2007
Status: Open

Change history:

  26-Sep-2007  Initial proposal for or-dev
  08-Dec-2007  Incorporated comments by Nick posted to or-dev on 10-Oct-2007
  15-Dec-2007  Rewrote complete proposal for better readability, modified
               authentication protocol, merged in personal notes
  24-Dec-2007  Replaced misleading term "authentication" by "authorization"
               and added some clarifications (comments by Sven Kaffille)
  28-Apr-2008  Updated most parts of the concrete authorization protocol
  04-Jul-2008  Add a simple algorithm to delay descriptor publication for
               different clients of a hidden service

Overview:

  This proposal deals with a general infrastructure for performing
  authorization (not necessarily implying authentication) of requests to
  hidden services at three points: (1) when downloading and decrypting
  parts of the hidden service descriptor, (2) at the introduction point,
  and (3) at Bob's Tor client before contacting the rendezvous point. A
  service provider will be able to restrict access to his service at these
  three points to authorized clients only. Further, the proposal contains a
  first instance of an authorization protocol for the presented
  infrastructure.

  This proposal is based on v2 hidden service descriptors as described in
  proposal 114 and introduced in version 0.2.0.10-alpha.

  The proposal is structured as follows: The next section motivates the
  integration of authorization mechanisms in the hidden service protocol.
  Then we describe a general infrastructure for authorization in hidden
  services, followed by a specific authorization protocol for this
  infrastructure. At the end we discuss a number of attacks and non-attacks
  as well as compatibility issues.

Motivation:

  The major part of hidden services does not require client authorization
  now and won't do so in the future. To the contrary, many clients would
  not want to be (pseudonymously) identifiable by the service (though this
  is unavoidable to some extent), but rather use the service
  anonymously. These services are not addressed by this proposal.

  However, there may be certain services which are intended to be accessed
  by a limited set of clients only. A possible application might be a
  wiki or forum that should only be accessible for a closed user group.
  Another, less intuitive example might be a real-time communication
  service, where someone provides a presence and messaging service only to
  his buddies. Finally, a possible application would be a personal home
  server that should be remotely accessed by its owner.

  Performing authorization for a hidden service within the Tor network, as
  proposed here, offers a range of advantages compared to allowing all
  client connections in the first instance and deferring authorization to
  the transported protocol:

  (1) Reduced traffic: Unauthorized requests would be rejected as early as
  possible, thereby reducing the overall traffic in the network generated
  by establishing circuits and sending cells.

  (2) Better protection of service location: Unauthorized clients could not
  force Bob to create circuits to their rendezvous points, thus preventing
  the attack described by Øverlier and Syverson in their paper "Locating
  Hidden Servers" even without the need for guards.

  (3) Hiding activity: Apart from performing the actual authorization, a
  service provider could also hide the mere presence of his service from
  unauthorized clients when not providing hidden service descriptors to
  them, rejecting unauthorized requests already at the introduction
  point (ideally without leaking presence information at any of these
  points), or not answering unauthorized introduction requests.

  (4) Better protection of introduction points: When providing hidden
  service descriptors to authorized clients only and encrypting the
  introduction points as described in proposal 114, the introduction points
  would be unknown to unauthorized clients and thereby protected from DoS
  attacks.

  (5) Protocol independence: Authorization could be performed for all
  transported protocols, regardless of their own capabilities to do so.

  (6) Ease of administration: A service provider running multiple hidden
  services would be able to configure access at a single place uniformly
  instead of doing so for all services separately.

  (7) Optional QoS support: Bob could adapt his node selection algorithm
  for building the circuit to Alice's rendezvous point depending on a
  previously guaranteed QoS level, thus providing better latency or
  bandwidth for selected clients.

  A disadvantage of performing authorization within the Tor network is
  that a hidden service cannot make use of authorization data in
  the transported protocol. Tor hidden services were designed to be
  independent of the transported protocol. Therefore it's only possible to
  either grant or deny access to the whole service, but not to specific
  resources of the service.

  Authorization often implies authentication, i.e. proving one's identity.
  However, when performing authorization within the Tor network, untrusted
  points should not gain any useful information about the identities of
  communicating parties, neither server nor client. A crucial challenge is
  to remain anonymous towards directory servers and introduction points.
  However, trying to hide identity from the hidden service is a futile
  task, because a client would never know if he is the only authorized
  client and therefore perfectly identifiable. Therefore, hiding client
  identity from the hidden service is not aimed by this proposal.

  The current implementation of hidden services does not provide any kind
  of authorization. The hidden service descriptor version 2, introduced by
  proposal 114, was designed to use a descriptor cookie for downloading and
  decrypting parts of the descriptor content, but this feature is not yet
  in use. Further, most relevant cell formats specified in rend-spec
  contain fields for authorization data, but those fields are neither
  implemented nor do they suffice entirely.

Details:

  1. General infrastructure for authorization to hidden services 

  We spotted three possible authorization points in the hidden service
  protocol:

    (1) when downloading and decrypting parts of the hidden service
        descriptor,
    (2) at the introduction point, and
    (3) at Bob's Tor client before contacting the rendezvous point.

  The general idea of this proposal is to allow service providers to
  restrict access to some or all of these points to authorized clients
  only.

  1.1. Client authorization at directory

  Since the implementation of proposal 114 it is possible to combine a
  hidden service descriptor with a so-called descriptor cookie. If done so,
  the descriptor cookie becomes part of the descriptor ID, thus having an
  effect on the storage location of the descriptor. Someone who has learned
  about a service, but is not aware of the descriptor cookie, won't be able
  to determine the descriptor ID and download the current hidden service
  descriptor; he won't even know whether the service has uploaded a
  descriptor recently. Descriptor IDs are calculated as follows (see
  section 1.2 of rend-spec for the complete specification of v2 hidden
  service descriptors):

      descriptor-id =
          H(service-id | H(time-period | descriptor-cookie | replica))

  Currently, service-id is equivalent to permanent-id which is calculated
  as in the following formula. But in principle it could be any public
  key.

      permanent-id = H(permanent-key)[:10]

  The second purpose of the descriptor cookie is to encrypt the list of
  introduction points, including optional authorization data. Hence, the
  hidden service directories won't learn any introduction information from
  storing a hidden service descriptor. This feature is implemented but
  unused at the moment, so that this proposal will harness the advantages
  of proposal 114.

  The descriptor cookie can be used for authorization by keeping it secret
  from everyone but authorized clients. A service could then decide whether
  to publish hidden service descriptors using that descriptor cookie later
  on. An authorized client being aware of the descriptor cookie would be
  able to download and decrypt the hidden service descriptor.

  The number of concurrently used descriptor cookies for one hidden service
  is not restricted. A service could use a single descriptor cookie for all
  users, a distinct cookie per user, or something in between, like one
  cookie per group of users. It is up to the specific protocol and how it
  is applied by a service provider.

  Two or more hidden service descriptors for different groups or users
  should not be uploaded at the same time. A directory node could conclude
  easily that the descriptors, were issued by the same hidden service, thus
  being able to link the two groups or users. Therefore, descriptors for
  different users or clients that ought to be stored on the same directory
  are delayed, so that only one descriptor is uploaded to a directory at a
  time. The remaining descriptors are uploaded with a delay of 30 seconds.
  Further, descriptors for different groups or users that are to be stored
  on different directories are delayed for a random time of up to 30
  seconds to hide relations from colluding directories. Certainly, this
  does not prevent linking entirely, but it makes it somewhat harder.
  There is a conflict between hiding links between clients and making a
  service available in a timely manner.

  Although this part of the proposal is meant to describe a general
  infrastructure for authorization, changing the way of using the
  descriptor cookie to look up hidden service descriptors, e.g. applying
  some sort of asymmetric crypto system, would require in-depth changes
  that would be incompatible to v2 hidden service descriptors. On the
  contrary, using another key for en-/decrypting the introduction point
  part of a hidden service descriptor, e.g. a different symmetric key or
  asymmetric encryption, would be easy to implement and compatible to v2
  hidden service descriptors as understood by hidden service directories
  (clients and servers would have to be upgraded anyway for using the new
  features).

  1.2. Client authorization at introduction point

  The next possible authorization point after downloading and decrypting
  a hidden service descriptor is the introduction point. It may be important
  for authorization, because it bears the last chance of hiding presence
  of a hidden service from unauthorized clients. Further, performing
  authorization at the introduction point might reduce traffic in the
  network, because unauthorized requests would not be passed to the
  hidden service. This applies to those clients who are aware of a
  descriptor cookie and thereby of the hidden service descriptor, but do
  not have authorization data to pass the introduction point or access the
  service (such a situation might occur when authorization data for
  authorization at the directory is not issued on a per-user base as
  opposed to authorization data for authorization at the introduction
  point).

  It is important to note that the introduction point must be considered
  untrustworthy, and therefore cannot replace authorization at the hidden
  service itself. Nor should the introduction point learn any sensitive
  identifiable information from either server or client.

  In order to perform authorization at the introduction point, three
  message formats need to be modified: (1) v2 hidden service descriptors,
  (2) ESTABLISH_INTRO cells, and (3) INTRODUCE1 cells.

  A v2 hidden service descriptor needs to contain authorization data that
  is introduction-point-specific and sometimes also authorization data
  that is introduction-point-independent. Therefore, v2 hidden service
  descriptors as specified in section 1.2 of rend-spec already contain two
  reserved fields "intro-authorization" and "service-authorization"
  (originally, the names of these fields were "...-authentication")
  containing an authorization type number and arbitrary authorization
  data. We propose that authorization data consists of base64 encoded
  objects of arbitrary length, surrounded by "-----BEGIN MESSAGE-----" and
  "-----END MESSAGE-----". This will increase the size of hidden service
  descriptors, which however is possible, as there is no strict upper
  limit.

  The current ESTABLISH_INTRO cells as described in section 1.3 of
  rend-spec do not contain either authorization data or version
  information. Therefore, we propose a new version 1 of the ESTABLISH_INTRO
  cells adding these two issues as follows:

     V      Format byte: set to 255               [1 octet]
     V      Version byte: set to 1                [1 octet]
     KL     Key length                           [2 octets]
     PK     Bob's public key                    [KL octets]
     HS     Hash of session info                [20 octets]
     AUTHT  The auth type that is supported       [1 octet]
     AUTHL  Length of auth data                  [2 octets]
     AUTHD  Auth data                            [variable]
     SIG    Signature of above information       [variable]

  From the format it is possible to determine the maximum allowed size for
  authorization data: given the fact that cells are 512 octets long, of
  which 498 octets are usable (see section 6.1 of tor-spec), and assuming
  1024 bit = 128 octet long keys, there are 215 octets left for
  authorization data. Hence, authorization protocols are bound to use no
  more than these 215 octets, regardless of the number of clients that
  shall be authenticated at the introduction point. Otherwise, one would
  need to send multiple ESTABLISH_INTRO cells or split them up, what we do
  not specify here.

  In order to understand a v1 ESTABLISH_INTRO cell, the implementation of
  a relay must have a certain Tor version, which would probably be some
  0.2.1.x. Hidden services need to be able to distinguish relays being
  capable of understanding the new v1 cell formats and perform
  authorization. We propose to use the version number that is contained in
  networkstatus documents to find capable introduction points.

  The current INTRODUCE1 cells as described in section 1.8 of rend-spec is
  not designed to carry authorization data and has no version number, too.
  We propose the following version 1 of INTRODUCE1 cells:

  Cleartext
     V      Version byte: set to 1                [1 octet]
     PK_ID  Identifier for Bob's PK             [20 octets]
     AUTHT  The auth type that is supported       [1 octet]
     AUTHL  Length of auth data                  [2 octets]
     AUTHD  Auth data                            [variable]
  Encrypted to Bob's PK:
     (RELAY_INTRODUCE2 cell)

  The maximum length of contained authorization data depends on the length
  of the contained INTRODUCE2 cell. A calculation follows below when
  describing the INTRODUCE2 cell format we propose to use.

  Unfortunately, v0 INTRODUCE1 cells consist only of a fixed-size,
  seemingly random PK_ID, followed by the encrypted INTRODUCE2 cell. This
  makes it impossible to distinguish v0 INTRODUCE1 cells from any later
  format. In particular, it is not possible to introduce some kind of
  format and version byte for newer versions of this cell. That's probably
  where the comment "[XXX011 want to put intro-level auth info here, but no
  version. crap. -RD]" that was part of rend-spec some time ago comes from.

  Processing of v1 INTRODUCE1 cells therefore requires knowledge about the
  context in which they are used. As a result, we propose that when
  receiving a v1 ESTABLISH_INTRO cell, an introduction point only accepts
  v1 INTRODUCE1 cells later on. Hence, the same introduction point cannot
  be used to accept both v0 and v1 INTRODUCE1 cells for the same service.
  (Another solution would be to distinguish v0 and v1 INTRODUCE1 cells by
  their size, as v0 INTRODUCE1 cells can only have specific cell sizes,
  depending on the version of the contained INTRODUCE2 cell; however, this
  approach does not appear very clean.)

  1.3. Client authorization at hidden service

  The time when a hidden service receives an INTRODUCE2 cell constitutes
  the last possible authorization point during the hidden service
  protocol. Performing authorization here is easier than at the other two
  authorization points, because there are no possibly untrusted entities
  involved.

  In general, a client that is successfully authorized at the introduction
  point should be granted access at the hidden service, too. Otherwise, the
  client would receive a positive INTRODUCE_ACK cell from the introduction
  point and conclude that it may connect to the service, but the request
  will be dropped without notice. This would appear as a failure to
  clients. Therefore, the number of cases in which a client successfully
  passes the introduction point, but fails at the hidden service should be
  zero. However, this does not lead to the conclusion, that the
  authorization data used at the introduction point and the hidden service
  must be the same, but only that both authorization data should lead to
  the same authorization result.

  Authorization data is transmitted from client to server via an
  INTRODUCE2 cell that is forwarded by the introduction point. There are
  versions 0 to 2 specified in section 1.8 of rend-spec, but none of these
  contains fields for carrying authorization data. We propose a slightly
  modified version of v3 INTRODUCE2 cells that is specified in section
  1.8.1 and which is not implemented as of December 2007. In contrast to
  the specified v3 we avoid specifying (and implementing) IPv6 capabilities,
  because Tor relays will be required to support IPv4 addresses for a long
  time in the future, so that this seems unnecessary at the moment. The
  proposed format of v3 INTRODUCE2 cells is as follows:

     VER    Version byte: set to 3.               [1 octet]
     AUTHT  The auth type that is supported       [1 octet]
     AUTHL  Length of auth data                  [2 octets]
     AUTHD  Auth data                            [variable]
     IP     Rendezvous point's address           [4 octets]
     PORT   Rendezvous point's OR port           [2 octets]
     ID     Rendezvous point identity ID        [20 octets]
     KLEN   Length of onion key                  [2 octets]
     KEY    Rendezvous point onion key        [KLEN octets]
     RC     Rendezvous cookie                   [20 octets]
     g^x    Diffie-Hellman data, part 1        [128 octets]

  The maximum possible length of authorization data is related to the
  enclosing INTRODUCE1 cell. A v3 INTRODUCE2 cell with
  1024 bit = 128 octets long public keys without any authorization data
  occupies 306 octets (AUTHL is only used when AUTHT has a value != 0),
  plus 58 octets for hybrid public key encryption (see
  section 5.1 of tor-spec on hybrid encryption of CREATE cells). The
  surrounding v1 INTRODUCE1 cell requires 24 octets. This leaves only 110
  of the 498 available octets free, which must be shared between
  authorization data to the introduction point _and_ to the hidden
  service.

  When receiving a v3 INTRODUCE2 cell, Bob checks whether a client has
  provided valid authorization data to him. He will only then build a
  circuit to the provided rendezvous point and otherwise will drop the
  cell.

  There might be several attacks based on the idea of replaying existing
  cells to the hidden service. In particular, someone (the introduction
  point or an evil authenticated client) might replay valid INTRODUCE2
  cells to make the hidden service build an arbitrary number of circuits to
  (maybe long gone) rendezvous points. Therefore, we propose that hidden
  services maintain a history of received INTRODUCE2 cells within the last
  hour and only accept INTRODUCE2 cells matching the following rules:

    (1) no duplicate requests coming from the same client and containing
        the same rendezvous cookie, and
    (2) a maximum of 10 cells coming from the same client with different
        rendezvous cookies.

  This allows a client to retry connection establishment using the same
  rendezvous point for 3 times and a total number of 10 connection
  establishments (not requests in the transported protocol) per hour.

  1.4. Summary of authorization data fields

  In summary, the proposed descriptor format and cell formats provide the
  following fields for carrying authorization data:

  (1) The v2 hidden service descriptor contains:
      - a descriptor cookie that is used for the lookup process, and
      - an arbitrary encryption schema to ensure authorization to access
        introduction information (currently symmetric encryption with the
        descriptor cookie).

  (2) For performing authorization at the introduction point we can use:
      - the fields intro-authorization and service-authorization in
        hidden service descriptors,
      - a maximum of 215 octets in the ESTABLISH_INTRO cell, and
      - one part of 110 octets in the INTRODUCE1 cell.

  (3) For performing authorization at the hidden service we can use:
      - the fields intro-authorization and service-authorization in
        hidden service descriptors,
      - the other part of 110 octets in the INTRODUCE2 cell.

  It will also still be possible to access a hidden service without any
  authorization or only use a part of the authorization infrastructure.
  However, this requires to consider all parts of the infrastructure. For
  example, authorization at the introduction point relying on confidential
  intro-authorization data transported in the hidden service descriptor
  cannot be performed without using an encryption schema for introduction
  information.

  1.5. Managing authorization data at servers and clients

  In order to provide authorization data at the hidden server and the
  authenticated clients, we propose to use files---either the tor
  configuration file or separate files. The exact format of these special
  files depends on the authorization protocol used.

  Currently, rend-spec contains the proposition to encode client-side
  authorization data in the URL, like in x.y.z.onion. This was never used
  and is also a bad idea, because in case of HTTP the requested URL may be
  contained in the Host and Referer fields.

  2. An authorization protocol based on group and user passwords

  In the following we discuss an authorization protocol for the proposed
  authorization architecture that performs authorization at the directory
  and the hidden service, but not at the introduction point.
  The protocol relies on a distinct asymmetric (client-key) and a
  symmetric key (descriptor-cookie) for
  each client. The asymmetric key replaces the service's permanent key and
  the symmetric key is used as descriptor cookie as described above.

  2.1. Client authorization at directory

  The symmetric key of 128 bits length is used as descriptor cookie for
  publishing/fetching
  hidden service descriptors and for encrypting/decrypting the contained
  introduction points. Further, the asymmetric key replaces the service's
  permanent key that is used to encode and sign a v2 hidden service descriptor.
  The result is a v2 hidden service descriptor with the following format:
  
      descriptor-id =
          H(H(client-key)[:10] | H(time-period | descriptor-cookie | replica))
      descriptor-content = {
        descriptor-id,
        version,
        client-key,
        H(time-period | descriptor-cookie | replica),
        timestamp,
        protocol-versions,
        { introduction-points } encrypted with descriptor-cookie
      } signed with private-key

  Whenever a
  server decides to remove authorization for a client, he can simply stop
  publishing hidden service descriptors using the descriptor cookie.
  The fact that there needs to be a separate
  hidden service descriptor for each user leads to a large number of
  such descriptors. However, this is the only way for a service
  provider to remove a client's authorization without remains. We assume
  that distributing the directory of hidden service descriptors as
  implemented by proposal 114 provides the necessary scalability to do so.

  2.2. Client authorization at introduction point

  There is no need to perform authorization at the introduction point in
  this protocol. Only authorized clients can decrypt the introduction
  point part of a hidden service descriptor. This contains the
  introduction key that was introduced by proposal 114 and that is required
  to get an INTRODUCE1 cell passed at the introduction point.

  2.3. Client authorization at hidden service

  Authorization at the hidden service also makes use of the
  descriptor cookie. The client include this descriptor cookie,
  in INTRODUCE2 cells that it sends to the server.
  The server compares authorization data of incoming INTRODUCE2 cells with
  the locally stored value that it would expect. The authorization type
  number of this protocol for INTRODUCE2 cells is "2".

  2.4. Providing authorization data

  The Tor client of a hidden service needs to know the client keys
  and descriptor cookies of all authorized clients. We decided to
  create a new configuration option that specifies a comma-separated list
  of human-readable client names:

  HiddenServiceAuthorizeClient client-name,client-name,...

  When a hidden service is configured, the client keys and descriptor
  cookies for all configured client names are either read from a file
  or generated and appended to that file. The file format is:

     "client-name" human-readable client identifier NL
     "service-address" onion-address NL
     "descriptor-cookie" 128-bit key ^= 22 base64 chars NL
     "client-key" NL a public key in PEM format

  On client side, we propose to add a new configuration option that
  contains a service name, the service identifier (H(client-key)[:10]),
  and the descriptor cookie that are required to access a hidden service.
  The configuration option has the following syntax:
  
  HidServAuth service-name service-address descriptor-cookie
  
  Whenever the user tries to access the given onion address, the given
  descriptor cookie is used for authorization.

Security implications:

  In the following we want to discuss possible attacks by dishonest
  entities in the presented infrastructure and specific protocol. These
  security implications would have to be verified once more when adding
  another protocol. The dishonest entities (theoretically) include the
  hidden server itself, the authenticated clients, hidden service directory
  nodes, introduction points, and rendezvous points. The relays that are
  part of circuits used during protocol execution, but never learn about
  the exchanged descriptors or cells by design, are not considered.
  Obviously, this list makes no claim to be complete. The discussed attacks
  are sorted by the difficulty to perform them, in ascending order,
  starting with roles that everyone could attempt to take and ending with
  partially trusted entities abusing the trust put in them.

  (1) A hidden service directory could attempt to conclude presence of a
  server from the existence of a locally stored hidden service descriptor:
  This passive attack is possible only for a single client-service
  relation, because descriptors need to contain a
  publicly visible signature of the server using the client key
  A possible protection
  would be to increase the number of hidden service directories in the
  network.

  (2) A hidden service directory could try to break the descriptor cookies
  of locally stored descriptors: This attack can be performed offline. The
  only useful countermeasure against it might be using safe passwords that
  are generated by Tor.

  (3) An introduction point could try to identify the pseudonym of the
  hidden service on behalf of which it operates: This is impossible by
  design, because the service uses a fresh public key for every
  establishment of an introduction point (see proposal 114) and the
  introduction point receives a fresh introduction cookie, so that there is
  no identifiable information about the service that the introduction point
  could learn. The introduction point cannot even tell if client accesses
  belong to the same client or not, nor can it know the total number of
  authorized clients. The only information might be the pattern of
  anonymous client accesses, but that is hardly enough to reliably identify
  a specific service.

  (4) An introduction point could want to learn the identities of accessing
  clients: This is also impossible by design, because all clients use the
  same introduction cookie for authorization at the introduction point.

  (5) An introduction point could try to replay a correct INTRODUCE1 cell
  to other introduction points of the same service, e.g. in order to force
  the service to create a huge number of useless circuits: This attack is
  not possible by design, because INTRODUCE1 cells are encrypted using a
  freshly created introduction key that is only known to authorized
  clients.

  (6) An introduction point could attempt to replay a correct INTRODUCE2
  cell to the hidden service, e.g. for the same reason as in the last
  attack: This attack is very limited by the fact that a server will only
  accept 3 INTRODUCE2 cells containing the same rendezvous cookie and drop
  all further replayed cells.

  (7) An introduction point could block client requests by sending either
  positive or negative INTRODUCE_ACK cells back to the client, but without
  forwarding INTRODUCE2 cells to the server: This attack is an annoyance
  for clients, because they might wait for a timeout to elapse until trying
  another introduction point. However, this attack is not introduced by
  performing authorization and it cannot be targeted towards a specific
  client. A countermeasure might be for the server to periodically perform
  introduction requests to his own service to see if introduction points
  are working correctly.

  (8) The rendezvous point could attempt to identify either server or
  client: This remains impossible as it was before, because the
  rendezvous cookie does not contain any identifiable information.

  (9) An authenticated client could swamp the server with valid INTRODUCE1
  and INTRODUCE2 cells, e.g. in order to force the service to create
  useless circuits to rendezvous points; as opposed to an introduction
  point replaying the same INTRODUCE2 cell, a client could include a new
  rendezvous cookie for every request: The countermeasure for this attack
  is the restriction to 10 connection establishments per client and hour.

Compatibility:

  An implementation of this proposal would require changes to hidden
  servers and clients to process authorization data and encode and
  understand the new formats. However, both servers and clients would
  remain compatible to regular hidden services without authorization.

Implementation:

  The implementation of this proposal can be divided into a number of
  changes to hidden service and client side. There are no
  changes necessary on directory, introduction, or rendezvous nodes. All
  changes are marked with either [service] or [client] do denote on which
  side they need to be made.

  /1/ Configure client authorization [service]

  - Parse configuration option HiddenServiceAuthorizeClient containing
    authorized client names.
  - Load previously created client keys and descriptor cookies.
  - Generate missing client keys and descriptor cookies, add them to
    client_keys file.
  - Rewrite the hostname file.
  - Keep client keys and descriptor cookies of authorized clients in
    memory.
 [- In case of reconfiguration, mark which client authorizations were
    added and whether any were removed. This can be used later when
    deciding whether to rebuild introduction points and publish new
    hidden service descriptors. Not implemented yet.]

  /2/ Publish hidden service descriptors [service]

  - Create and upload hidden service descriptors for all authorized
    clients.
 [- See /1/ for the case of reconfiguration.]

  /3/ Configure permission for hidden services [client]

  - Parse configuration option HidServAuth containing service
    authorization, store authorization data in memory.

  /5/ Fetch hidden service descriptors [client]

  - Look up client authorization upon receiving a hidden service request.
  - Request hidden service descriptor ID including client key and
    descriptor cookie. Only request v2 descriptors, no v0.

  /6/ Process hidden service descriptor [client]

  - Decrypt introduction points with descriptor cookie.

  /7/ Create introduction request [client]

  - Include descriptor cookie in INTRODUCE2 cell to introduction point.
  - Pass descriptor cookie around between involved connections and
    circuits.

  /8/ Process introduction request [service]

  - Read descriptor cookie from INTRODUCE2 cell.
  - Check whether descriptor cookie is authorized for access, including
    checking access counters.
  - Log access for accountability.