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Add two authoritzation protocols to rend-spec.txt.

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Karsten Loesing 2010-07-29 16:07:00 +02:00
parent 9ecb64c44d
commit 1cf6da821c
1 changed files with 177 additions and 3 deletions
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doc/spec/rend-spec.txt
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@ -627,7 +627,7 @@
g^x Diffie-Hellman data, part 1 [128 octets]
OR (in the v3 intro protocol)
VER Version byte: set to 3. [1 octet]
AUTHT Auth type [1 octet]
AUTHT The auth type that is used [1 octet]
AUTHL Length of auth data [2 octets]
AUTHD Auth data [variable]
TS A timestamp [4 octets]
@ -736,7 +736,181 @@
2. Authentication and authorization.
Foo.
The rendezvous protocol as described in Section 1 provides a few options
for implementing client-side authorization. There are two steps in the
rendezvous protocol that can be used for performing client authorization:
when downloading and decrypting parts of the hidden service descriptor and
at Bob's Tor client before contacting the rendezvous point. A service
provider can restrict access to his service at these two points to
authorized clients only.
There are currently two authorization protocols specified that are
described in more detail below:
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.
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 initialization 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
3. Hidden service directory operation