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 are intended to extend existing hidden service protocol versions 0 and 2 and shall therefore be considered hidden service protocol version 3. All changes in this version 3 are designed to be fully backward-compatible to version 2 and can be run in parallel to version 0. 2.1. Service with large-scale client authorization The first client authorization protocol aims at performing access control while consuming as little 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 "1" 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. ### What would be a simple solution to include n encrypted session keys ### in the descriptor? The format may be binary and has no strict upper ### size limit. An authorized client should be able to efficiently find ### the session key that is encrypted for him/her. It should be ### impossible to track certain authorized clients over time by finding ### that the session key was encrypted for them in different descriptors. ### It should be hard to determine the exact number of authorized ### clients. ### ### Here comes the voodoo I've conceived: ### ### ATYPE Authorization type: set to 1. [1 octet] ### ALEN Number of authorized clients 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 [(16 - (number-of-clients 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 "2" encoded as auth-type in the remaining 4 of 132 bits instead of "1" 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 implements this proposal and that is meant to use the new protocols adds version 3 to the list of supported hidden service protocols: HiddenServiceVersion version,version,... (Default: 0, 2, 3) If the service shall perform client authorization, another config option is set to either "1" for the protocol described in 2.1 or "2" for the protocol in 2.2. This config option also includes 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 3 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: "service-address" client-specific-onion-address NL "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.