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add proposal 114-distributed-storage.txt

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doc/spec/proposals/000-index.txt
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@ -32,4 +32,5 @@ Proposals by number:
111 Prioritizing local traffic over relayed traffic [OPEN]
112 Bring Back Pathlen Coin Weight [OPEN]
113 Simplifying directory authority administration [OPEN]
114 Distributed Storage for Tor Hidden Service Descriptors [OPEN]

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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
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 from new security threads 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 was added, hidden services would 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 to store even bigger numbers of descriptors.
Second, besides of 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 needed 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
router 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 shall only learn as few 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.
All nodes:
All nodes can combine the network lists received from all directory nodes
to one routing list containing only those nodes that store and serve
hidden service descriptors and which are contained in the majority of
network lists. A node only trusts its own routing list and never learns
about routing information from other nodes. This list should only be
created on demand by those nodes that are involved in the new hidden
service protocol, i.e. hidden service directory node, hidden service
provider, and hidden service client.
All nodes 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 to prevent 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 a freely chosen
identifier.
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.
Hidden service providers 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.) Amonst 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. [usage of a derived key as encryption key
instead of the shared key itself suggested 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 (introduction point IP, port, public service key)
} encrypted with h(time-period + cookie + 'introduction')
} 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 decision 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) desciptor-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 256 bits instead
of 80 bits. This requires a secure hash function with an output of 256
instead of 160 bits, e.g. SHA-256. [extending the descriptor-id length
from 80 to 256 bits suggested by LO]
2) permanent-id: This is the first half of the onion address that a client
passes to his OP. The onion address should be easy to memorize.
Therefore, the overall length of an onion address should not be
extended over the existing 80 bits, so that 40 bits is the maximum
length of the permanent-id. However, the question remains open, if an
onion address of 40+40=80 bits can generate a descriptor-id with enough
entropy to justify 256 instead of 80 bits. Otherwise, the onion address
would need to be extended to 128, 160, 224, or 256 bits, making it
harder to memorize for human-beings.
3) cookie: This is the second half of the onion address that is passed to
an OP. It should have the same size as permanent-id.