tor/doc/spec/proposals/114-distributed-storage.txt

Filename: 114-distributed-storage.txt
Title: Distributed Storage for Tor Hidden Service Descriptors
Version: $Revision$
Last-Modified: $Date$
Author: Karsten Loesing
Created: 13-May-2007
Status: Open

Change history:

  13-May-2007  Initial proposal
  14-May-2007  Added changes suggested by Lasse Overlier
  30-May-2007  Changed descriptor format, key length discussion, typos

Overview:

  The basic idea of this proposal is to distribute the tasks of storing and
  serving hidden service descriptors from currently three authoritative
  directory nodes among a large subset of all onion routers. The two reasons
  to do this are better scalability and improved security properties. Further,
  this proposal suggests changes to the hidden service descriptor format to
  prevent new security threats coming from decentralization and to gain
  even better security properties.

Motivation:

  The current design of hidden services exhibits the following performance and
  security problems:

  First, the three hidden service authoritative directories constitute a
  performance bottleneck in the system. The directory nodes are responsible
  for storing and serving all hidden service descriptors. At the moment there
  are about 1000 descriptors at a time, but this number is assumed to increase
  in the future. Further, there is no replication protocol for descriptors
  between the three directory nodes, so that hidden services must ensure the
  availability of their descriptors by manually publishing them on all
  directory nodes. Whenever a fourth or fifth hidden service authoritative
  directory is added, hidden services will need to maintain an equally
  increasing number of replicas. These scalability issues have an impact on
  the current usage of hidden services and put an even higher burden on the
  development of new kinds of applications for hidden services that might
  require storing even bigger numbers of descriptors.

  Second, besides posing a limitation to scalability, storing all hidden
  service descriptors on three directory nodes also constitutes a security
  risk. The directory node operators could easily analyze the publish and fetch
  requests to derive information on service activity and usage and read the
  descriptor contents to determine which onion routers work as introduction
  points for a given hidden service and need to be attacked or threatened to
  shut it down. Furthermore, the contents of a hidden service descriptor offer
  only minimal security properties to the hidden service. Whoever gets aware
  of the service ID can easily find out whether the service is active at the
  moment and which introduction points it has. This applies to (former)
  clients, (former) introduction points, and of course to the directory nodes.
  It requires only to request the descriptor for the given service ID --- which
  can be performed by anyone anonymously.

  This proposal suggests two major changes to approach the described
  performance and security problems:

  The first change affects the storage location for hidden service
  descriptors. Descriptors are distributed among a large subset of all onion
  routers instead of three fixed directory nodes. Each storing node is
  responsible for a subset of descriptors for a limited time only. It is not
  able to choose which descriptors it stores at a certain time, because this
  is determined by its onion ID which is hard to change frequently and in time
  (only routers which are stable for a given time are accepted as storing
  nodes). In order to resist single node failures and untrustworthy nodes,
  descriptors are replicated among a certain number of storing nodes. A simple
  replication protocol makes sure that descriptors don't get lost when the
  node population changes. Therefore, a storing node periodically requests the
  descriptors from its siblings. Connections to storing nodes are established
  by extending existing circuits by one hop to the storing node. This also
  ensures that contents are encrypted. The effect of this first change is that
  the probability that a single node operator learns about a certain hidden
  service is very small and that it is very hard to track a service over time,
  even when it collaborates with other node operators.

  The second change concerns the content of hidden service descriptors.
  Obviously, security problems cannot be solved only by decentralizing
  storage; in fact, they could also get worse if done without caution. At
  first, a descriptor ID needs to change periodically in order to be stored on
  changing nodes over time. Next, the descriptor ID needs to be computable only
  for the service's clients, but should be unpredictable for all other nodes.
  Further, the storing node needs to be able to verify that the hidden service
  is the true originator of the descriptor with the given ID even though it is
  not a client. Finally, a storing node should learn as little information as
  necessary by storing a descriptor, because it might not be as trustworthy as
  a directory node; for example it does not need to know the list of
  introduction points. Therefore, a second key is applied that is only known
  to the hidden service provider and its clients and that is not included in
  the descriptor. It is used to calculate descriptor IDs and to encrypt the
  introduction points. This second key can either be given to all clients
  together with the hidden service ID, or to a group or a single client as
  authentication token. In the future this second key could be the result of
  some key agreement protocol between the hidden service and one or more
  clients. A new text-based format is proposed for descriptors instead of an
  extension of the existing binary format for reasons of future extensibility.

Design:

  The proposed design is described by the changes that are necessary to the
  current design. Changes are grouped by content, rather than by affected
  specification documents.

  Tor clients and servers:

    All participants can combine the network status lists received from
    all directory authorities to one routing list containing only those
    servers that store and serve hidden service descriptors and which
    are contained in the majority of network status lists. A participant
    only trusts its own routing list and never learns about routing
    information from other parties. This list should only be created
    on demand by Tor clients and servers that are involved in the new
    hidden service protocol, i.e. hidden service directory node, hidden
    service provider, and hidden service client.

    All parties that are involved in the new hidden service protocol calculate
    the clock skew between their local time and the times of directory
    authorities. If the clock skew exceeds 1 minute (as opposed to 30 minutes
    as in the current implementation), the user is warned upon performing the
    first operation that is related to hidden services. However, the local
    time is not adjusted automatically, because then they would be open
    to attacks based on false times from directory authorities.

  Hidden service directory nodes:

    Every onion router can decide whether it wants to store and serve hidden
    service descriptors by setting a new config option HiddenServiceDirectory
    0|1 to 1. This option should be 1 by default for those onion routers that
    have their directory port open, because the smaller the group of storing
    nodes is, the poorer the security properties are.

    HS directory nodes include the fact that they store and serve hidden
    service descriptors in router descriptors that they send to directory
    authorities.

    HS directory nodes accept publish and fetch requests for hidden service
    descriptors and store/retrieve them to/from their local memory. (It is not
    necessary to make descriptors persistent, because after disconnecting, the
    onion router would not be accepted as storing node anyway, because it is
    not stable.) All requests and replies are formatted as HTTP messages.
    Requests are directed to the router's directory port and are contained
    within BEGIN_DIR cells. A HS directory node stores a descriptor only when
    it thinks that it is responsible for storing that descriptor based on its
    own routing table. Every HS directory node is responsible for the
    descriptor IDs in the interval of its n-th predecessor in the ID circle up
    to its own ID (n denotes the number of replicas).

    A HS directory node replicates descriptors for which it is responsible by
    downloading them from other HS directory nodes. Therefore, it checks its
    routing table periodically every 10 minutes for changes. Whenever it
    realizes that a predecessor has left the network, it establishes a
    connection to the new n-th predecessor and requests its stored descriptors
    in the interval of its (n+1)-th predecessor and the requested n-th
    predecessor. Whenever it realizes that a new onion router has joined with
    an ID higher than its former n-th predecessor, it adds it to its
    predecessors and discards all descriptors in the interval of its (n+1)-th
    and its n-th predecessor.

  Authoritative directory nodes:

    Directory nodes include a new flag for routers that decided to provide
    storage for hidden service descriptors and that are stable for a given
    time. The requirement to be stable prevents a node from frequently
    changing its onion key to become responsible for an identifier it wants
    to target.

  Hidden service provider:

    When setting up the hidden service at introduction points, a hidden service
    provider does not pass its own public key, but the public key of a freshly
    generated key pair. It also includes this public key in the hidden service
    descriptor together with the other introduction point information. The
    reason is that the introduction point does not need to know for which
    hidden service it works, and should not know it to prevent it from
    tracking the hidden service's activity.

    Each hidden service provider publishes a new descriptor whenever
    its content
    changes or a new publication period starts for this descriptor. If the
    current publication period would only last for less than 60 minutes, the
    hidden service provider publishes both a current descriptor and one for
    the next period. Publication is performed by sending the descriptor to all
    hidden service directories that are responsible for keeping replicas for
    the descriptor ID.

  Hidden service client:

    Instead of downloading descriptors from a hidden service authoritative
    directory, a hidden service client downloads it from a randomly chosen
    hidden service directory that is responsible for keeping replica for the
    descriptor ID.

    When contacting an introduction point, the client does not use the
    public key of the hidden service provider, but the freshly-generated public
    key that is included in the hidden service descriptor.

  Hidden service descriptor:

    The descriptor ID needs to change periodically in order for the descriptor
    to be stored on changing nodes over time. It further may only be computable
    by a hidden service provider and all of his clients to prevent unauthorized
    nodes from tracking the service activity by periodically checking whether
    there is a descriptor for this service. Finally, the hidden service
    directory needs to be able to verify that the hidden service provider is
    the true originator of the descriptor with the given ID. Therefore, the
    ID is derived from the public key of the hidden service provider, the
    current time period, and a shared secret between hidden service provider
    and clients. Only the hidden service provider and the clients are able to
    generate future IDs, but together with the descriptor content the hidden
    service directory is able to verify its origin. The formula for calculating
    a descriptor ID is as follows:

      descriptor-id = h(permanent-id + h(time-period + cookie))

    "permanent-id" is the hashed value of the public key of the hidden service
    provider, "time-period" is a periodically changing value, e.g. the current
    date, and "cookie" is a shared secret between the hidden service provider
    and its clients. (The "time-period" should be constructed in a way that
    periods do not change at the same moment for all descriptors by including
    the "permanent-id" in the construction.) Amongst other things, the
    descriptor contains the public key of the hidden service provider, the
    value of h(time-period + cookie), and the signature of the descriptor
    content with the private key of the hidden service provider.

    The introduction points that are included in the descriptor are encrypted
    using a key that is derived from the same shared key that is used to
    generate the descriptor ID. [correction to use another key than
    h(time-period + cookie) as encryption key for introduction points made by
    LO]

    A new text-based format is proposed for descriptors instead of an
    extension of the existing binary format for reasons of future
    extensibility.

    The complete hidden service descriptor format looks like this:

      {
        descriptor-id = h(permanent-id + h(time-period + cookie))
        permanent-public-key   (with permanent-id = h(permanent-public-key))
        h(time-period + cookie)
        timestamp
        {
          list of intro points (ID, IP, onion port, onion key, service key)
        } encrypted with cookie
      } signed with permanent-private-key

    A hidden service directory can verify that a descriptor was created by the
    hidden service provider by checking if the descriptor-id corresponds to
    the permanent-public-key and if the signature can be verified with the
    permanent-public-key.

    A client can download the descriptor by creating the same descriptor-id
    and verify its origin by performing the same operations as the hidden
    service directory.

Security implications:

  The security implications of the proposed changes are grouped by the roles
  of nodes that could perform attacks or on which attacks could be performed.

  Attacks by authoritative directory nodes

    Authoritative directory nodes are not anymore the single places in the
    network that know about a hidden service's activity and introduction
    points. Thus, they cannot perform attacks using this information, e.g.
    track a hidden service's activity or usage pattern or attack its
    introduction points. Formerly, it would only require a single corrupted
    authoritative directory operator to perform such an attack.

  Attacks by hidden service directory nodes

    A hidden service directory node could misuse a stored descriptor to track
    a hidden service's activity and usage pattern by clients. Though there is
    no countermeasure against this kind of attack, it is very expensive to
    track a certain hidden service over time. An attacker would need to run a
    large number of stable onion routers that work as hidden service directory
    nodes to have a good probability to become responsible for its changing
    descriptor IDs. For each period, the probability is:

      1-(N-c choose r)/(N choose r) for N-c>=r and 1 else with N as total
      number of hidden service directories, c as compromised nodes, and r as
      number of replicas

    The hidden service directory nodes could try to make a certain hidden
    service unavailable to its clients. Therefore, they could discard all
    stored descriptors for that hidden service and reply to clients that there
    is no descriptor for the given ID or return an old or false descriptor
    content. The client would detect a false descriptor, because it could not
    contain a correct signature. But an old content or an empty reply could
    confuse the client. Therefore, the countermeasure is to replicate
    descriptors among a small number of hidden service directories, e.g. 5. 
    The probability of a group of collaborating nodes to make a hidden service
    completely unavailable is in each period:

      (c choose r)/(N choose r) for c>=r and N>=r, and 0 else with N as total
      number of hidden service directories, c as compromised nodes, and r as
      number of replicas

    A hidden service directory could try to find out which introduction points
    are working on behalf of a hidden service. In contrast to the previous
    design, this is not possible anymore, because this information is encrypted
    to the clients of a hidden service.

  Attacks on hidden service directory nodes

    An anonymous attacker could try to swamp a hidden service directory with
    false descriptors for a given descriptor ID. This is prevented by requiring
    that descriptors are signed.

    Anonymous attackers could swamp a hidden service directory with correct
    descriptors for non-existing hidden services. There is no countermeasure
    against this attack. However, the creation of valid descriptors is more
    expensive than verification and storage in local memory. This should make
    this kind of attack unattractive.

  Attacks by introduction points

    Current or former introduction points could try to gain information on the
    hidden service they serve. But due to the fresh key pair that is used by
    the hidden service, this attack is not possible anymore.

  Attacks by clients

    Current or former clients could track a hidden service's activity, attack
    its introduction points, or determine the responsible hidden service
    directory nodes and attack them. There is nothing that could prevent them
    from doing so, because honest clients need the full descriptor content to
    establish a connection to the hidden service. At the moment, the only
    countermeasure against dishonest clients is to change the secret cookie
    and pass it only to the honest clients.

Specification:

  The proposed changes affect multiple sections in several specification
  documents that are only mentioned in the following. The detailed
  specification will follow as soon as the design decisions above are final.

  dir-spec-v2.txt

    2.1  The router descriptor format needs to include an additional flag to
    denote that a router is a hidden service directory.

    3  The network status format needs to be extended by a new status flag to
    denote that a router is a hidden service directory.

    4  The sections on directory caches need to be extended by new sections for
    the operation of hidden service directories, including replication of
    descriptors.

  rend-spec.txt

    1.2  The new descriptor format needs to be added.

    1.3  Instead of Bob's public key, the hidden service provider uses a
    freshly generated public key for every introduction point.

    1.4  Bob's OP does not upload his service descriptor to the authoritative
    directories, but to the hidden service directories.

    1.6  Alice's OP downloads the service descriptors similarly as Bob
    published them in 1.4.

    1.8  Alice uses the public key that is included in the descriptor instead
    of Bob's permanent service key.

  tor-spec.txt

    6.2.1  Directory streams need to be used for connections to hidden service
    directories.

Compatibility:

  The proposed design is meant to replace the current design for hidden service
  descriptors and their storage in the long run.

  There should be a first transition phase in which both, the current design
  and the proposed design are served in parallel. Onion routers should start
  serving as hidden service directories, and hidden service providers and
  clients should make use of the new design if both sides support it. But
  hidden service providers should continue publishing descriptors of the
  current format, and authoritative directories should store and serve these
  descriptors.

  After the first transition phase, hidden service providers should stop
  publishing descriptors on authoritative directories, and hidden service
  clients should not try to fetch descriptors from the authoritative
  directories. However, the authoritative directories should continue serving
  hidden service descriptors for a second transition phase.

  After the second transition phase, the authoritative directories should stop
  serving hidden service descriptors.

Implementation:

  There are three key lengths that might need some discussion:

    1) descriptor-id, formerly known as onion address: It is generated by OPs
       internally and used for storing and looking up descriptors. There is no
       need to remember a descriptor-id for a human. In order to reduce
       the success rate of collisions it could be extended to the full output
       of SHA-1 of 160 bits instead of 80 bits. [extending the descriptor-id
       length suggested by LO]

    2) permanent-id: This is the first part of the onion address that a client
       passes to his OP. The overall onion address should be easy to memorize.
       Therefore, its overall length should only be extended from the existing
       80 bits to as few bits as necessary. The length of the permanent-id has
       an influence on the probability that an adversary creates an own key
       pair that leads to the same descriptor-id in a given time-period as an
       honest service's key. 32 bits should provide sufficient protection to
       avoid collisions, given the fact that key generation is expensive and
       the attack needed to be performed for every time-period.

    3) cookie: This is the second part of the onion address that is passed to
       an OP. In order to provide confidentiality of introduction points, this
       secret key should have 128 bits. In total, this leads to an onion
       address of 160 bits instead of the current 80 bits.