Filename: 114-distributed-storage.txt
Title: Distributed Storage for Tor Hidden Service Descriptors
Author: Karsten Loesing
Created: 13-May-2007
Status: Closed
Implemented-In: 0.2.0.x
Change history:
13-May-2007 Initial proposal
14-May-2007 Added changes suggested by Lasse Øverlier
30-May-2007 Changed descriptor format, key length discussion, typos
09-Jul-2007 Incorporated suggestions by Roger, added status of specification
and implementation for upcoming GSoC mid-term evaluation
11-Aug-2007 Updated implementation statuses, included non-consecutive
replication to descriptor format
20-Aug-2007 Renamed config option HSDir as HidServDirectoryV2
02-Dec-2007 Closed proposal
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 three
reasons to do this are better robustness (availability), 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.
Status:
As of December 2007, the new hidden service descriptor format is implemented
and usable. However, servers and clients do not yet make use of descriptor
cookies, because there are open usability issues of this feature that might
be resolved in proposal 121. Further, hidden service directories do not
perform replication by themselves, because (unauthorized) replica fetch
requests would allow any attacker to fetch all hidden service descriptors in
the system. As neither issue is critical to the functioning of v2
descriptors and their distribution, this proposal is considered as Closed.
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. As of May 2007 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 more 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 first 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. A second replication protocol distributes descriptors among
non-consecutive nodes of the ID ring to prevent a group of adversaries from
generating new onion keys until they have consecutive IDs to create a 'black
hole' in the ring and make random services unavailable. 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
an 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 required changes to the current
design. These requirements are grouped by content, rather than by affected
specification documents or code files, and numbered for reference below.
Hidden service clients, servers, and directories:
/1/ Create routing list
All participants can filter the consensus status document received from the
directory authorities to one routing list containing only those servers
that store and serve hidden service descriptors and which are running for
at least 24 hours. A participant only trusts its own routing list and never
learns about routing information from other parties.
/2/ Determine responsible hidden service directory
All participants can determine the hidden service directory that is
responsible for storing and serving a given ID, as well as the hidden
service directories that replicate its content. Every hidden service
directory is responsible for the descriptor IDs in the interval from
its predecessor, exclusive, to its own ID, inclusive. Further, a hidden
service directory holds replicas for its n predecessors, where n denotes
the number of consecutive replicas. (requires /1/)
[/3/ and /4/ were requirements to use BEGIN_DIR cells for directory
requests which have not been fulfilled in the course of the implementation
of this proposal, but elsewhere.]
Hidden service directory nodes:
/5/ Advertise hidden service directory functionality
Every onion router that has its directory port open can decide whether it
wants to store and serve hidden service descriptors by setting a new config
option "HidServDirectoryV2" 0|1 to 1. An onion router with this config
option being set includes the flag "hidden-service-dir" in its router
descriptors that it sends to directory authorities.
/6/ Accept v2 publish requests, parse and store v2 descriptors
Hidden service directory nodes accept publish requests for hidden service
descriptors and store them to 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 has not been
running for at least 24 hours.) 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 hidden service directory node stores a
descriptor only when it thinks that it is responsible for storing that
descriptor based on its own routing table. Every hidden service 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
consecutive replicas). (requires /1/)
/7/ Accept v2 fetch requests
Same as /6/, but with fetch requests for hidden service descriptors.
(requires /2/)
/8/ Replicate descriptors with neighbors
A hidden service directory node replicates descriptors from its two
predecessors by downloading them once an hour. Further, it checks its
routing table periodically 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.
(requires /1/)
[Dec 02: This function has not been implemented, because arbitrary nodes
what have been able to download the entire set of v2 descriptors. An
authorized replication request would be necessary. For the moment, the
system runs without any directory-side replication. -KL]
Authoritative directory nodes:
/9/ Confirm a router's hidden service directory functionality
Directory nodes include a new flag "HSDir" for routers that decided to
provide storage for hidden service descriptors and that are running for at
least 24 hours. The last requirement prevents a node from frequently
changing its onion key to become responsible for an identifier it wants to
target.
Hidden service provider:
/10/ Configure v2 hidden service
Each hidden service provider that has set the config option
"PublishV2HidServDescriptors" 0|1 to 1 is configured to publish v2
descriptors and conform to the v2 connection establishment protocol. When
configuring a hidden service, a hidden service provider checks if it has
already created a random secret_cookie and a hostname2 file; if not, it
creates both of them. (requires /2/)
/11/ Establish introduction points with fresh key
If configured to publish only v2 descriptors and no v0/v1 descriptors any
more, a hidden service provider that is setting up the hidden service at
introduction points does not pass its own public key, but the public key
of a freshly generated key pair. It also includes these fresh public keys
in the hidden service descriptor together with the other introduction point
information. The reason is that the introduction point does not need to and
therefore should not know for which hidden service it works, so as to
prevent it from tracking the hidden service's activity. (If a hidden
service provider supports both, v0/v1 and v2 descriptors, v0/v1 clients
rely on the fact that all introduction points accept the same public key,
so that this new feature cannot be used.)
/12/ Encode v2 descriptors and send v2 publish requests
If configured to publish v2 descriptors, a 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 (= 2 x 30 minutes to allow
the server to be 30 minutes behind and the client 30 minutes ahead), 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. This includes two non-consecutive replicas that are
stored at 3 consecutive nodes each. (requires /1/ and /2/)
Hidden service client:
/13/ Send v2 fetch requests
A hidden service client that has set the config option
"FetchV2HidServDescriptors" 0|1 to 1 handles SOCKS requests for v2 onion
addresses by requesting a v2 descriptor from a randomly chosen hidden
service directory that is responsible for keeping replica for the
descriptor ID. In total there are six replicas of which the first and the
last three are stored on consecutive nodes. The probability of picking one
of the three consecutive replicas is 1/6, 2/6, and 3/6 to incorporate the
fact that the availability will be the highest on the node with next higher
ID. A hidden service client relies on the hidden service provider to store
two sets of descriptors to compensate clock skew between service and
client. (requires /1/ and /2/)
/14/ Process v2 fetch reply and parse v2 descriptors
A hidden service client that has sent a request for a v2 descriptor can
parse it and store it to the local cache of rendezvous service descriptors.
/15/ Establish connection to v2 hidden service
A hidden service client can establish a connection to a hidden service
using a v2 descriptor. This includes using the secret cookie for decrypting
the introduction points contained in the descriptor. 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. Whether or not a fresh key is used instead
of the key of the hidden service depends on the available protocol versions
that are included in the descriptor; by this, connection establishment is
to a certain extend decoupled from fetching the descriptor.
Hidden service descriptor:
(Requirements concerning the descriptor format are contained in /6/ and /7/.)
The new v2 hidden service descriptor format looks like this:
onion-address = h(public-key) + cookie
descriptor-id = h(h(public-key) + h(time-period + cookie + relica))
descriptor-content = {
descriptor-id,
version,
public-key,
h(time-period + cookie + replica),
timestamp,
protocol-versions,
{ introduction-points } encrypted with cookie
} signed with private-key
The "descriptor-id" needs to change periodically in order for the
descriptor to be stored on changing nodes over time. It 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, "descriptor-id" is derived from the "public-key" of the hidden
service provider, the current "time-period" which changes every 24 hours,
a secret "cookie" shared between hidden service provider and clients, and
a "replica" denoting the number of this non-consecutive replica. (The
"time-period" is constructed in a way that time periods do not change at
the same moment for all descriptors by deriving a value between 0:00 and
23:59 hours from h(public-key) and making the descriptors of this hidden
service provider expire at that time of the day.) The "descriptor-id" is
defined to be 160 bits long. [extending the "descriptor-id" length
suggested by LØ]
Only the hidden service provider and the clients are able to generate
future "descriptor-ID"s. Hence, the "onion-address" is extended from now
the hash value of "public-key" by the secret "cookie". The "public-key" is
determined to be 80 bits long, whereas the "cookie" is dimensioned to be
120 bits long. This makes a total of 200 bits or 40 base32 chars, which is
quite a lot to handle for a human, but necessary to provide sufficient
protection against an adversary from generating a key pair with same
"public-key" hash or guessing the "cookie".
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 "public-key" and if the signature can be verified with the
"public-key".
The "introduction-points" that are included in the descriptor are encrypted
using the same "cookie" that is shared between hidden service provider and
clients. [correction to use another key than h(time-period + cookie) as
encryption key for introduction points made by LØ]
A new text-based format is proposed for descriptors instead of an extension
of the existing binary format for reasons of future extensibility.
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 no longer 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 otherwise, 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 otherwise,
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.
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. Hidden
service providers should be allowed to publish descriptors of the current
format in parallel, and authoritative directories should continue storing and
serving 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. As of this point,
all v2 config options should be set to a default value of 1.
After the second transition phase, the authoritative directories should stop
serving hidden service descriptors.