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proposal 121: updated most parts of the concrete authorization protocol

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svn:r14497
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Karsten Loesing 2008-04-28 21:50:30 +00:00
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doc/spec/proposals/121-hidden-service-authentication.txt
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@ -15,6 +15,7 @@ Change history:
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
Overview:
@ -22,7 +23,7 @@ Overview:
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 onion proxy before contacting the rendezvous point. A
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 a
first instance of an authorization protocol for the presented
@ -71,9 +72,9 @@ Motivation:
(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 and rejecting unauthorized requests already at the introduction
them, rejecting unauthorized requests already at the introduction
point (ideally without leaking presence information at any of these
points).
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
@ -98,7 +99,7 @@ Motivation:
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.
resources of the service.
Authorization often implies authentication, i.e. proving one's identity.
However, when performing authorization within the Tor network, untrusted
@ -107,8 +108,8 @@ Motivation:
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 identity
from the hidden service is not aimed by this proposal.
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
@ -128,10 +129,11 @@ Details:
(1) when downloading and decrypting parts of the hidden service
descriptor,
(2) at the introduction point, and
(3) at Bob's onion proxy before contacting the rendezvous point.
(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 all of these points to authorized clients only.
restrict access to some or all of these points to authorized clients
only.
1.1. Client authorization at directory
@ -147,7 +149,13 @@ Details:
service descriptors):
descriptor-id =
H(permanent-id | H(time-period | descriptor-cookie | replica))
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
@ -166,10 +174,7 @@ Details:
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. However, we advise to use a small
number of descriptor cookies for efficiency reasons and for improving the
ability to hide presence of a service (see security implications at the
end of this document).
is applied by a service provider.
Although this part of the proposal is meant to describe a general
infrastructure for authorization, changing the way of using the
@ -186,7 +191,7 @@ Details:
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 is important
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
@ -222,7 +227,7 @@ Details:
limit.
The current ESTABLISH_INTRO cells as described in section 1.3 of
rend-spec don't contain either authorization data or version
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:
@ -313,18 +318,18 @@ Details:
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. The only change
is to switch the lengths of AUTHT and AUTHL, which we assume to be a typo
in current rend-spec. The proposed format of v3 INTRODUCE2 cells is as
follows:
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]
ATYPE An address type (typically 4) [1 octet]
ADDR Rendezvous point's IP address [4 or 16 octets]
PORT Rendezvous point's OR port [2 octets]
AUTHT The auth type that is supported [1 octet]
AUTHL Length of auth data [2 octets]
AUTHD Auth data [variable]
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]
@ -332,11 +337,12 @@ Details:
g^x Diffie-Hellman data, part 1 [128 octets]
The maximum possible length of authorization data is related to the
enclosing INTRODUCE1 cell. A v3 INTRODUCE2 cell with IPv6 address and
enclosing INTRODUCE1 cell. A v3 INTRODUCE2 cell with
1024 bit = 128 octets long public keys without any authorization data
occupies 321 octets, plus 58 octets for hybrid public key encryption (see
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 v1 INTRODUCE1 cell requires 24 octets. This leaves only 95
surrounding v1 INTRODUCE1 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.
@ -378,12 +384,12 @@ Details:
- 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 95 octets in the INTRODUCE1 cell.
- one part of 110 octets in the INTRODUCE1 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 95 octets in the INTRODUCE2 cell.
- 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.
@ -397,10 +403,8 @@ Details:
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. In the latter case a hidden server
would use one file per provided service, and a client would use one file
per server she wants to access. The exact format of these special files
depends on the authorization protocol used.
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
@ -410,142 +414,91 @@ Details:
2. An authorization protocol based on group and user passwords
In the following we discuss an authorization protocol for the proposed
authorization architecture that performs authorization at all three
proposed authorization points. The protocol relies on two symmetrically
shared keys: a group key and a user key. The reason for this separation
as compared to using a single key for each user is the fact that the
number of descriptor cookies should be limited, so that the group key
will be used for authenticating at the directory, whereas two keys
derived from the user key will be used for performing authorization at
the introduction and the hidden service.
authorization architecture that performs authorization at the directory
and the hidden service, but not at the introduction point.
The protocol relies on a distinct asymmetric (client-key) and a
symmetric key (descriptor-cookie) for
each client. The asymmetric key replaces the service's permanent key and
the symmetric key is used as descriptor cookie as described above.
2.1. Client authorization at directory
The server creates groups of users that shall be able to access his
service. He provides all users of a certain group with the same group key
which is a password of arbitrary length.
The symmetric key of 128 bits length is used as descriptor cookie for
publishing/fetching
hidden service descriptors and for encrypting/decrypting the contained
introduction points. Further, the asymmetric key replaces the service's
permanent key that is used to encode and sign a v2 hidden service descriptor.
The result is a v2 hidden service descriptor with the following format:
descriptor-id =
H(H(client-key)[:10] | H(time-period | descriptor-cookie | replica))
descriptor-content = {
descriptor-id,
version,
client-key,
H(time-period | descriptor-cookie | replica),
timestamp,
protocol-versions,
{ introduction-points } encrypted with descriptor-cookie
} signed with private-key
The group key is used as input to derive a 128 bit descriptor cookie from
it. We propose to apply a secure hash function and use the first 128 bits
of output:
descriptor-cookie = H(group-key)
Hence, there will be a distinct hidden service descriptor for every group
of users. All descriptors contain the same introduction points and the
authorization data required by the users of the given group. Whenever a
server decides to remove authorization for a group, he can simply stop
Whenever a
server decides to remove authorization for a client, he can simply stop
publishing hidden service descriptors using the descriptor cookie.
The fact that there needs to be a separate
hidden service descriptor for each user leads to a large number of
such descriptors. However, this is the only way for a service
provider to remove a client's authorization without remains. We assume
that distributing the directory of hidden service descriptors as
implemented by proposal 114 provides the necessary scalability to do so.
2.2. Client authorization at introduction point
The idea for authenticating at the introduction point is borrowed from
authorization at the rendezvous point using a rendezvous cookie. A
rendezvous cookie is created by the client and encrypted for the server
in order to authenticate the server at the rendezvous point. Likewise,
the so-called introduction cookie is created by the server and encrypted
for the client in order to authenticate the client at the introduction
point.
More precise, the server creates a new introduction cookie when
establishing an introduction point and includes it in the ESTABLISH_INTRO
cell that it sends to the introduction point. This introduction cookie
will be used by all clients during the complete time of using this
introduction point. The server then encrypts the introduction cookie for
all authorized clients (as described in the next paragraph) and includes
it in the introduction-point-specific part of the hidden service
descriptor. A client reads and decrypts the introduction cookie from the
hidden service descriptor and includes it in the INTRODUCE1 cell that it
sends to the introduction point. The introduction point can then compare
the introduction cookie included in the INTRODUCE1 cell with the value
that it previously received in the ESTABLISH_INTRO cell. If both values
match, the introduction point passes the INTRODUCE2 cell to the hidden
service.
For the sake of simplicity, the size of an introduction cookie should be
only 16 bytes so that they can be encrypted using AES-128 without using
a block mode. Although rendezvous cookies are 20 bytes long, the 16 bytes
of an introduction cookie should still provide similar, or at least
sufficient security.
Encryption of the introduction cookie is done on a per user base. Every
client shares a password of arbitrary length with the server, which is
the so-called user key. The server derives a symmetric key from the
client's user key by applying a secure hash function and using the first
128 bits of output as follows:
encryption-key = H(user-key | "INTRO")
It is important that the encryption key does not allow any inference on
the user key, because the latter will also be used for authorization at
the hidden service. This is ensured by applying the secure one-way
function H.
The 16 bytes long, symmetrically encrypted introduction cookies are
encoded in binary form in the authorization data object of a hidden
service descriptor. Additionally, for every client there is a 20 byte
long client identifier that is also derived from the user key, so that
the client can identify which value to decrypt. The client identifier is
determined as follows:
client-id = H(user-key | "CLIENT")
The authorization data encoded to the hidden service descriptor consists
of the concatenation of pairs consisting of 20 byte client identifiers
and 16 byte encrypted introduction cookies. The authorization type
number for the encrypted introduction cookies as well as for
ESTABLISH_INTRO and INTRODUCE1 cells is "1".
There is no need to perform authorization at the introduction point in
this protocol. Only authorized clients can decrypt the introduction
point part of a hidden service descriptor. This contains the
introduction key that was introduced by proposal 114 and that is required
to get an INTRODUCE1 cell passed at the introduction point.
2.3. Client authorization at hidden service
Authorization at the hidden service also makes use of the user key,
because whoever is authorized to pass the introduction point shall be
authorized to access the hidden service. Therefore, the server and client
derive a common value from the user key, which is called service cookie
and which is 20 bytes long:
service-cookie = H(user-key | "SERVICE")
The client is supposed to include this service cookie, preceded by the 20
bytes long client ID, in INTRODUCE2 cells that it sends to the server.
Authorization at the hidden service also makes use of the
descriptor cookie. The client include this descriptor cookie,
in INTRODUCE2 cells that it sends to the server.
The server compares authorization data of incoming INTRODUCE2 cells with
the locally stored value that it would expect. The authorization type
number of this protocol for INTRODUCE2 cells is "1".
Passing a derived value of a client's user key will make clients
identifiable to the hidden service. Although there might be ways to limit
identifiability, an authorized client can never be sure that he stays
anonymous to the hidden service. For example, if we created a service
cookie that is the same for all users and encrypted it for all users, and
if we further included a checksum of this service cookie in the
descriptor to prove that all users have the same value, a client would
never know if he is the only valid user contained in this descriptor,
with the other users only be fakes created by the hidden service.
Therefore, we did not make attempts to hide a client's identity from a
hidden service. Another reason was that we would not be able to apply a
connection limit of 10 requests per hour and user that helps prevent some
threats.
2.4. Providing authorization data
The authorization data that needs to be provided by servers consists of
a number of group keys, each having a number of user keys assigned. These
data items could be provided by two new configuration options
"HiddenServiceAuthGroup group-name group-key" and "HiddenServiceAuthUser
user-name user-key" with the semantics that a group contains all users
directly following the group key definition and before reaching the next
group key definition for a hidden service.
The Tor client of a hidden service needs to know the client keys
and descriptor cookies of all authorized clients. We decided to
create a new configuration option that specifies a comma-separated list
of human-readable client names:
On client side, authorization data also consists of a group and a user
key. Therefore, a new configuration option "HiddenServiceAuthClient
onion-address group-key user-key" could be introduced that could be
written to any place in the configuration file. Whenever the user would
try to access the given onion address, the given group and user key
would be used for authorization.
HiddenServiceAuthorizeClient client-name,client-name,...
When a hidden service is configured, the client keys and descriptor
cookies for all configured client names are either read from a file
or generated and appended to that file. The file format is:
"client-name" human-readable client identifier NL
"descriptor-cookie" 128-bit key ^= 22 base64 chars NL
"client-key" NL a public key in PEM format
On client side, we propose to add a new configuration option that
contains a service name, the service identifier (H(client-key)[:10]),
and the descriptor cookie that are required to access a hidden service.
The configuration option has the following syntax:
HidServAuth service-name service-address descriptor-cookie
Whenever the user tries to access the given onion address, the given
descriptor cookie is used for authorization.
Security implications:
In the following we want to discuss attacks and non-attacks by dishonest
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
@ -560,11 +513,12 @@ Security implications:
(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, because descriptors need to contain a
publicly visible signature of the server (see proposal 114 for a more
extensive discussion of the v2 descriptor format). A possible protection
would be to reduce the number of concurrently used descriptor cookies and
increase the number of hidden service directories in the network.
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
@ -581,7 +535,7 @@ Security implications:
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 server.
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
@ -590,8 +544,9 @@ Security implications:
(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 need to contain an
introduction cookie that is different for every introduction point.
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
@ -610,32 +565,16 @@ Security implications:
are working correctly.
(8) The rendezvous point could attempt to identify either server or
client: No, this remains impossible as it was before, because the
client: This remains impossible as it was before, because the
rendezvous cookie does not contain any identifiable information.
(9) An authenticated client could try to break the encryption keys of the
other authenticated clients that have their introduction cookies
encrypted in the hidden service descriptor: This known-plaintext attack
can be performed offline. The only useful countermeasure against it could
be safe passwords that are generated by Tor. However, the attack would
not be very useful as long as encryption keys do not reveal information
on the contained user key.
(10) An authenticated client could swamp the server with valid INTRODUCE1
(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.
(11) An authenticated client could attempt to break the service cookie of
another authenticated client to obtain access at the hidden service: This
requires a brute-force online attack. There are no countermeasures
provided, but the question arises whether the outcome of this attack is
worth the cost. The service cookie from one authenticated client is as
good as from another, with the only exception of possible better QoS
properties of certain clients.
Compatibility:
An implementation of this proposal would require changes to hidden
@ -643,8 +582,60 @@ Compatibility:
understand the new formats. However, both servers and clients would
remain compatible to regular hidden services without authorization.
Further, the implementation of introduction points would have to be
changed, so that they understand the new cell versions and perform
authorization. But again, the new introduction points would remain
compatible to the existing hidden service protocol.
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.