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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
31-Jul-2008 Limit maximum descriptor size to 20 kilobytes to prevent
abuse.
01-Aug-2008 Use first part of Diffie-Hellman handshake for replay
protection instead of rendezvous cookie.
01-Aug-2008 Remove improved hidden service protocol without client
authorization (2.1). It might get implemented in proposal
142.
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).
An adversary could try to abuse the fact that introduction points can be
encrypted by storing arbitrary, unrelated data in the hidden service
directory. This abuse can be limited by setting a hard descriptor size
limit, forcing the adversary to split data into multiple chunks. There
are some limitations that make splitting data across multiple descriptors
unattractive: 1) The adversary would not be able to choose descriptor IDs
freely and have to implement an own indexing structure. 2) Validity of
descriptors is limited to at most 24 hours after which descriptors need
to be republished.
The regular descriptor size in bytes is 745 + num_ipos * 837 + auth_data.
A large descriptor with 7 introduction points and 5 kilobytes of
authorization data would be 11724 bytes in size. The upper size limit of
descriptors should be set to 20 kilobytes, which limits the effect of
abuse while retaining enough flexibility in designing authorization
protocols.
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
first part of the Diffie-Hellman handshake has not been used in the past
60 minutes to prevent replay attacks by rogue introduction points. (The
reason for not using the rendezvous cookie to detect replays---even
though it is only sent once in the current design---is that it might be
desirable to re-use rendezvous cookies for multiple introduction requests
in the future.) 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 two specific authorization protocols that
make use of (parts of) the new authorization infrastructure:
1. The first 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.
2. The second 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 two protocol instances extend the existing hidden service protocol
version 2. Hidden services that perform client authorization may run in
parallel to other services running versions 0, 2, or both.
2.1. Service with large-scale client authorization
The first client authorization protocol aims at performing access control
while consuming as few 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 (here: "0") 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. Authorized client should be able
to efficiently find the session key that is encrypted for him/her, so
that 4 octet long client ID are generated consisting of descriptor cookie
and initilization vector. Descriptors always contain a number of
encrypted session keys that is a multiple of 16 by adding fake entries.
Encrypted session keys are ordered by client IDs in order to conceal
addition or removal of authorized clients by the service provider.
ATYPE Authorization type: set to 1. [1 octet]
ALEN Number of clients := 1 + ((clients - 1) 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 [(15 - ((clients - 1) 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 "1" 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.
2.2. 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 "1" encoded as auth-type in the remaining 4 of 132 bits
instead of "0" 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 2. [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 "2", 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 "2" in INTRODUCE2 cells that
it sends to the service.
2.3. Hidden service configuration
A hidden service that is meant to perform client authorization adds a
new option HiddenServiceAuthorizeClient to its hidden service
configuration. This option contains the authorization type which is
either "1" for the protocol described in 2.1 or "2" for the protocol in
2.2 and 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 2 or higher.
Tor stores all generated authorization data for the authorization
protocols described in Sections 2.1 and 2.2 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 2.2 is used, Tor also generates
and stores the following data:
"client-key" NL a public key in PEM format
2.4. 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.
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