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
Target: 0.2.1.x

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
  19-Jul-2008  Added INTRODUCE1V cell type (1.2), improved replay
               protection for INTRODUCE2 cells (1.3), described limitations
               for auth protocols (1.6), improved hidden service protocol
               without client authorization (2.1), added second, more
               scalable authorization protocol (2.2), rewrote existing
               authorization protocol (2.3); changes based on discussion
               with Nick

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
  specific authorization protocols as instances that implement the
  presented authorization 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 specific authorization protocols 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. 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.
  Unfortunately, unversioned INTRODUCE1 cells consist only of a fixed-size,
  seemingly random PK_ID, followed by the encrypted INTRODUCE2 cell. This
  makes it impossible to distinguish unversioned 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.

  We propose that new versioned INTRODUCE1 cells use the new cell type 41
  RELAY_INTRODUCE1V (where V stands for versioned):

  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.

  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 used            [1 octet]
     AUTHL  Length of auth data                  [2 octets]
     AUTHD  Auth data                            [variable]
     TS     Timestamp (seconds since 1-1-1970)   [4 octets]
     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 INTRODUCE1V 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 INTRODUCE1V 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 also requires that the
  timestamp is no more than 30 minutes in the past or future and that the
  rendezvous cookie has not been used in the past 60 minutes to prevent
  replay attacks by rogue introduction points. If all checks pass, Bob
  builds a circuit to the provided rendezvous point and otherwise drops the
  cell.

  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 INTRODUCE1V 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.

  1.6. Limitations for authorization protocols

  There are two limitations of the current hidden service protocol for
  authorization protocols that shall be identified here.

    1. The three cell types ESTABLISH_INTRO, INTRODUCE1V, and INTRODUCE2
       restricts the amount of data that can be used for authorization.
       This forces authorization protocols that require per-user
       authorization data at the introduction point to restrict the number
       of authorized clients artifically. A possible solution could be to
       split contents among multiple cells and reassemble them at the
       introduction points.

    2. The current hidden service protocol does not specify cell types to
       perform interactive authorization between client and introduction
       point or hidden service. If there should be an authorization
       protocol that requires interaction, new cell types would have to be
       defined and integrated into the hidden service protocol.


  2. Specific authorization protocol instances

  In the following we present three specific authorization protocols that
  make use of (parts of) the new authorization infrastructure:

    1. The first protocol does not really perform client authorization, but
       requires clients to have downloaded a service descriptor before
       establishing a connection in order to prevent introduction points
       from accessing a service.

    2. The second protocol allows a service provider to restrict access
       to clients with a previously received secret key only, but does not
       attempt to hide service activity from others.

    3. The third protocol, albeit being feasible for a limited set of about
       16 clients, performs client authorization and hides service activity
       from everyone but the authorized clients.

  These three protocol instances together are intended to replace the
  existing hidden service protocol versions 0 and 2 in the long run and
  shall therefore be considered hidden service protocol version 3. All
  changes in this version 3 are designed to be fully backward-compatible to
  version 2 and can be run in parallel to version 0.

  2.1. Services without client authorization

  Although hidden services without client authorization could be run as
  before, this proposal allows us to add a new security property at almost
  no costs: Denying the introduction points to access the hidden service.
  While this constitutes a defense against rogue introduction points, it
  also reduces responsibility of a Tor node operator for the doings of a
  hidden service offering illegal or unethical contents.

  The original hidden service design used the service's permanent key to
  establish introduction points. If an introduction point wanted to access
  the service, it could easily download the service's descriptor using its
  permanent key ID and establish a connection or generate an INTRODUCE2
  cell itself and forward it directly to the service.

  Hidden service protocol version 2 made it more difficult for introduction
  points to find out which service they are serving. Here, the hidden
  service created a fresh introduction key for each introduction point
  which 1) did not reveal the hidden service's identity and 2) did not
  allow downloading the service's descriptor. However, the introduction
  point could still generate an INTRODUCE2 cell itself and establish a
  connection to the service to find out what it is serving.

  Beginning with this proposal can include a so-called "introduction
  cookie" in v2 hidden service descriptors and v3 INTRODUCE2 cells. If
  both, service and client implement this proposal, a service receiving a
  v3 INTRODUCE2 cell with an introduction cookie in it can be sure that the
  client has downloaded its descriptor before. As long as hidden services
  also permit v2 INTRODUCE2 cells, introduction points can work around this
  safeguard. But the earlier this protocol is introduced, the earlier the
  services can stop supporting version 2 introductions.

  A hidden service generates a unique introduction cookie for each
  established introduction point and puts it in the "intro-authentication"
  field in its descriptor for auth-type "1". Further, the service sets the
  "protocol-versions" field to "2,3" to announce that it understands both,
  requests with and without introduction cookie. Clients that understand
  protocol version 3 include the introduction cookie in the v3 INTRODUCE2
  cell as auth-type "1" that they send to the service. (Clients that don't
  understand the protocol v3 do not recognize the authorization data and
  send a v2 INTRODUCE2 cell as usual.) The hidden service can compare a
  received introduction cookie with the value that it expects and grant or
  deny service correspondingly.

  2.2. Service with large-scale client authorization

  The first client authorization protocol aims at performing access control
  while consuming as little additional resources as possible. A service
  provider should be able to permit access to a large number of clients
  while denying access for everyone else. However, the price for
  scalability is that the service won't be able to hide its activity from
  unauthorized or formerly authorized clients.

  The main idea of this protocol is to encrypt the introduction-point part
  in hidden service descriptors to authorized clients using symmetric keys.
  This ensures that nobody else but authorized clients can learn which
  introduction points a service currently uses, nor can someone send a
  valid INTRODUCE1 message without knowing the introduction key. Therefore,
  a subsequent authorization at the introduction point is not required.

  A service provider generates symmetric "descriptor cookies" for his
  clients and distributes them outside of Tor. The suggested key size is
  128 bits, so that descriptor cookies can be encoded in 22 base64 chars
  (which can hold up to 22 * 5 = 132 bits, leaving 4 bits to encode the
  authorization type "2" and allow a client to distinguish this
  authorization protocol from others like the one proposed below).
  Typically, the contact information for a hidden service using this
  authorization protocol looks like this:

    v2cbb2l4lsnpio4q.onion Ll3X7Xgz9eHGKCCnlFH0uz

  When generating a hidden service descriptor, the service encrypts the
  introduction-point part with a single randomly generated symmetric
  128-bit session key using AES-CTR as described for v2 hidden service
  descriptors in rend-spec. Afterwards, the service encrypts the session
  key to all descriptor cookies using AES.

  ### What would be a simple solution to include n encrypted session keys
  ### in the descriptor? The format may be binary and has no strict upper
  ### size limit. An authorized client should be able to efficiently find
  ### the session key that is encrypted for him/her. It should be
  ### impossible to track certain authorized clients over time by finding
  ### that the session key was encrypted for them in different descriptors.
  ### It should be hard to determine the exact number of authorized
  ### clients.
  ###
  ### Here comes the voodoo I've conceived:
  ###
  ###   ATYPE  Authorization type: set to 2.                      [1 octet]
  ###   ALEN   Number of authorized clients div 16                [1 octet]
  ### for each symmetric descriptor cookie:
  ###   ID     Client ID: H(descriptor cookie | IV)[:4]          [4 octets]
  ###   SKEY   Session key encrypted with descriptor cookie     [16 octets]
  ### (end of client-specific part)
  ###   RND    Random data  [(16 - (number-of-clients mod 16)) * 20 octets]
  ###   IV     AES initialization vector                        [16 octets]
  ###   IPOS   Intro points, encrypted with session key  [remaining octets]

  An authorized client needs to configure Tor to use the descriptor cookie
  when accessing the hidden service. Therefore, a user adds the contact
  information that she received from the service provider to her torrc
  file. Upon downloading a hidden service descriptor, Tor finds the
  encrypted introduction-point part and attempts to decrypt it using the
  configured descriptor cookie. (In the rare event of two or more client
  IDs being equal a client tries to decrypt all of them.)

  Upon sending the introduction, the client includes her descriptor cookie
  as auth type "2" in the INTRODUCE2 cell that she sends to the service.
  The hidden service checks whether the included descriptor cookie is
  authorized to access the service and either responds to the introduction
  request, or not.

  2.3. Authorization for limited number of clients

  A second, more sophisticated client authorization protocol goes the extra
  mile of hiding service activity from unauthorized clients. With all else
  being equal to the preceding authorization protocol, the second protocol
  publishes hidden service descriptors for each user separately and gets
  along with encrypting the introduction-point part of descriptors to a
  single client. This allows the service to stop publishing descriptors for
  removed clients. As long as a removed client cannot link descriptors
  issued for other clients to the service, it cannot derive service
  activity any more. The downside of this approach is limited scalability.
  Even though the distributed storage of descriptors (cf. proposal 114)
  tackles the problem of limited scalability to a certain extent, this
  protocol should not be used for services with more than 16 clients. (In
  fact, Tor should refuse to advertise services for more than this number
  of clients.)

  A hidden service generates an asymmetric "client key" and a symmetric
  "descriptor cookie" for each client. The client key is used as
  replacement for the service's permanent key, so that the service uses a
  different identity for each of his clients. The descriptor cookie is used
  to store descriptors at changing directory nodes that are unpredictable
  for anyone but service and client, to encrypt the introduction-point
  part, and to be included in INTRODUCE2 cells. Once the service has
  created client key and descriptor cookie, he tells them to the client
  outside of Tor. The contact information string looks similar to the one
  used by the preceding authorization protocol (with the only difference
  that it has "3" encoded as auth-type in the remaining 4 of 132 bits
  instead of "2" as before).

  When creating a hidden service descriptor for an authorized client, the
  hidden service uses the client key and descriptor cookie to compute
  secret ID part and descriptor ID:

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

    descriptor-id = H(client-key[:10] | secret-id-part)

  The hidden service also replaces permanent-key in the descriptor with
  client-key and encrypts introduction-points with the descriptor cookie.

     ATYPE  Authorization type: set to 3.                         [1 octet]
     IV     AES initialization vector                           [16 octets]
     IPOS   Intro points, encr. with descriptor cookie   [remaining octets]

  When uploading descriptors, the hidden service needs to make sure that
  descriptors for different clients are not uploaded at the same time (cf.
  Section 1.1) which is also a limiting factor for the number of clients.

  When a client is requested to establish a connection to a hidden service
  it looks up whether it has any authorization data configured for that
  service. If the user has configured authorization data for authorization
  protocol "3", the descriptor ID is determined as described in the last
  paragraph. Upon receiving a descriptor, the client decrypts the
  introduction-point part using its descriptor cookie. Further, the client
  includes its descriptor cookie as auth-type "3" in INTRODUCE2 cells that
  it sends to the service.

  2.4. Hidden service configuration

  A hidden service that implements this proposal and that is meant to use
  the new protocols (including the protocol without client authorization as
  described in 2.1) adds version 3 to the list of supported hidden service
  protocols:

    HiddenServiceVersion version,version,... (Default: 0, 2, 3)

  If the service shall perform client authorization, another config option
  is set to either "1" for the protocol described in 2.2 or "2" for the
  protocol in 2.3 (auth type numbers differ from the internally used
  numbers primarily to avoid user questions about the whereabouts of auth
  type 1). This config option also includes a comma-separated list of
  human-readable client names, so that Tor can create authorization data
  for these clients:

    HiddenServiceAuthorizeClient auth-type client-name,client-name,...

  If this option is configured, HiddenServiceVersion is automatically
  reconfigured to contain only version numbers of 3 or higher. If this
  config option is not set but the configured hidden service version
  includes 3, the protocol without client authorization as described in 2.1
  is offered to clients (possibly in parallel to versions 0 and 2).

  Tor stores all generated authorization data for the authorization
  protocols described in Sections 2.2 and 2.3 in a new file using the
  following file format:

     "client-name" human-readable client identifier NL
     "descriptor-cookie" 128-bit key ^= 22 base64 chars NL

  If the authorization protocol of Section 2.3 is used, Tor also generates
  and stores the following data:

     "service-address" client-specific-onion-address NL
     "client-key" NL a public key in PEM format

  2.5. Client configuration

  Clients need to make their authorization data known to Tor using another
  configuration option that contains a service name (mainly for the sake of
  convenience), the service address, and the descriptor cookie that is
  required to access a hidden service (the authorization protocol number is
  encoded in the descriptor cookie):

    HidServAuth service-name service-address descriptor-cookie

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.