Filename: 114-distributed-storage.txt Title: Distributed Storage for Tor Hidden Service Descriptors Version: $Revision$ Last-Modified: $Date$ Author: Karsten Loesing Created: 13-May-2007 Status: Open Change history: 13-May-2007 Initial proposal 14-May-2007 Added changes suggested by Lasse Overlier 30-May-2007 Changed descriptor format, key length discussion, typos Overview: The basic idea of this proposal is to distribute the tasks of storing and serving hidden service descriptors from currently three authoritative directory nodes among a large subset of all onion routers. The two reasons to do this are better scalability and improved security properties. Further, this proposal suggests changes to the hidden service descriptor format to prevent new security threats coming from decentralization and to gain even better security properties. Motivation: The current design of hidden services exhibits the following performance and security problems: First, the three hidden service authoritative directories constitute a performance bottleneck in the system. The directory nodes are responsible for storing and serving all hidden service descriptors. At the moment there are about 1000 descriptors at a time, but this number is assumed to increase in the future. Further, there is no replication protocol for descriptors between the three directory nodes, so that hidden services must ensure the availability of their descriptors by manually publishing them on all directory nodes. Whenever a fourth or fifth hidden service authoritative directory is added, hidden services will need to maintain an equally increasing number of replicas. These scalability issues have an impact on the current usage of hidden services and put an even higher burden on the development of new kinds of applications for hidden services that might require storing even bigger numbers of descriptors. Second, besides posing a limitation to scalability, storing all hidden service descriptors on three directory nodes also constitutes a security risk. The directory node operators could easily analyze the publish and fetch requests to derive information on service activity and usage and read the descriptor contents to determine which onion routers work as introduction points for a given hidden service and need to be attacked or threatened to shut it down. Furthermore, the contents of a hidden service descriptor offer only minimal security properties to the hidden service. Whoever gets aware of the service ID can easily find out whether the service is active at the moment and which introduction points it has. This applies to (former) clients, (former) introduction points, and of course to the directory nodes. It requires only to request the descriptor for the given service ID --- which can be performed by anyone anonymously. This proposal suggests two major changes to approach the described performance and security problems: The first change affects the storage location for hidden service descriptors. Descriptors are distributed among a large subset of all onion routers instead of three fixed directory nodes. Each storing node is responsible for a subset of descriptors for a limited time only. It is not able to choose which descriptors it stores at a certain time, because this is determined by its onion ID which is hard to change frequently and in time (only routers which are stable for a given time are accepted as storing nodes). In order to resist single node failures and untrustworthy nodes, descriptors are replicated among a certain number of storing nodes. A simple replication protocol makes sure that descriptors don't get lost when the node population changes. Therefore, a storing node periodically requests the descriptors from its siblings. Connections to storing nodes are established by extending existing circuits by one hop to the storing node. This also ensures that contents are encrypted. The effect of this first change is that the probability that a single node operator learns about a certain hidden service is very small and that it is very hard to track a service over time, even when it collaborates with other node operators. The second change concerns the content of hidden service descriptors. Obviously, security problems cannot be solved only by decentralizing storage; in fact, they could also get worse if done without caution. At first, a descriptor ID needs to change periodically in order to be stored on changing nodes over time. Next, the descriptor ID needs to be computable only for the service's clients, but should be unpredictable for all other nodes. Further, the storing node needs to be able to verify that the hidden service is the true originator of the descriptor with the given ID even though it is not a client. Finally, a storing node should learn as little information as necessary by storing a descriptor, because it might not be as trustworthy as a directory node; for example it does not need to know the list of introduction points. Therefore, a second key is applied that is only known to the hidden service provider and its clients and that is not included in the descriptor. It is used to calculate descriptor IDs and to encrypt the introduction points. This second key can either be given to all clients together with the hidden service ID, or to a group or a single client as authentication token. In the future this second key could be the result of some key agreement protocol between the hidden service and one or more clients. A new text-based format is proposed for descriptors instead of an extension of the existing binary format for reasons of future extensibility. Design: The proposed design is described by the changes that are necessary to the current design. Changes are grouped by content, rather than by affected specification documents. Tor clients and servers: All participants can combine the network status lists received from all directory authorities to one routing list containing only those servers that store and serve hidden service descriptors and which are contained in the majority of network status lists. A participant only trusts its own routing list and never learns about routing information from other parties. This list should only be created on demand by Tor clients and servers that are involved in the new hidden service protocol, i.e. hidden service directory node, hidden service provider, and hidden service client. All parties that are involved in the new hidden service protocol calculate the clock skew between their local time and the times of directory authorities. If the clock skew exceeds 1 minute (as opposed to 30 minutes as in the current implementation), the user is warned upon performing the first operation that is related to hidden services. However, the local time is not adjusted automatically, because then they would be open to attacks based on false times from directory authorities. Hidden service directory nodes: Every onion router can decide whether it wants to store and serve hidden service descriptors by setting a new config option HiddenServiceDirectory 0|1 to 1. This option should be 1 by default for those onion routers that have their directory port open, because the smaller the group of storing nodes is, the poorer the security properties are. HS directory nodes include the fact that they store and serve hidden service descriptors in router descriptors that they send to directory authorities. HS directory nodes accept publish and fetch requests for hidden service descriptors and store/retrieve them to/from their local memory. (It is not necessary to make descriptors persistent, because after disconnecting, the onion router would not be accepted as storing node anyway, because it is not stable.) All requests and replies are formatted as HTTP messages. Requests are directed to the router's directory port and are contained within BEGIN_DIR cells. A HS directory node stores a descriptor only when it thinks that it is responsible for storing that descriptor based on its own routing table. Every HS directory node is responsible for the descriptor IDs in the interval of its n-th predecessor in the ID circle up to its own ID (n denotes the number of replicas). A HS directory node replicates descriptors for which it is responsible by downloading them from other HS directory nodes. Therefore, it checks its routing table periodically every 10 minutes for changes. Whenever it realizes that a predecessor has left the network, it establishes a connection to the new n-th predecessor and requests its stored descriptors in the interval of its (n+1)-th predecessor and the requested n-th predecessor. Whenever it realizes that a new onion router has joined with an ID higher than its former n-th predecessor, it adds it to its predecessors and discards all descriptors in the interval of its (n+1)-th and its n-th predecessor. Authoritative directory nodes: Directory nodes include a new flag for routers that decided to provide storage for hidden service descriptors and that are stable for a given time. The requirement to be stable prevents a node from frequently changing its onion key to become responsible for an identifier it wants to target. Hidden service provider: When setting up the hidden service at introduction points, a hidden service provider does not pass its own public key, but the public key of a freshly generated key pair. It also includes this public key in the hidden service descriptor together with the other introduction point information. The reason is that the introduction point does not need to know for which hidden service it works, and should not know it to prevent it from tracking the hidden service's activity. Each hidden service provider publishes a new descriptor whenever its content changes or a new publication period starts for this descriptor. If the current publication period would only last for less than 60 minutes, the hidden service provider publishes both a current descriptor and one for the next period. Publication is performed by sending the descriptor to all hidden service directories that are responsible for keeping replicas for the descriptor ID. Hidden service client: Instead of downloading descriptors from a hidden service authoritative directory, a hidden service client downloads it from a randomly chosen hidden service directory that is responsible for keeping replica for the descriptor ID. When contacting an introduction point, the client does not use the public key of the hidden service provider, but the freshly-generated public key that is included in the hidden service descriptor. Hidden service descriptor: The descriptor ID needs to change periodically in order for the descriptor to be stored on changing nodes over time. It further may only be computable by a hidden service provider and all of his clients to prevent unauthorized nodes from tracking the service activity by periodically checking whether there is a descriptor for this service. Finally, the hidden service directory needs to be able to verify that the hidden service provider is the true originator of the descriptor with the given ID. Therefore, the ID is derived from the public key of the hidden service provider, the current time period, and a shared secret between hidden service provider and clients. Only the hidden service provider and the clients are able to generate future IDs, but together with the descriptor content the hidden service directory is able to verify its origin. The formula for calculating a descriptor ID is as follows: descriptor-id = h(permanent-id + h(time-period + cookie)) "permanent-id" is the hashed value of the public key of the hidden service provider, "time-period" is a periodically changing value, e.g. the current date, and "cookie" is a shared secret between the hidden service provider and its clients. (The "time-period" should be constructed in a way that periods do not change at the same moment for all descriptors by including the "permanent-id" in the construction.) Amongst other things, the descriptor contains the public key of the hidden service provider, the value of h(time-period + cookie), and the signature of the descriptor content with the private key of the hidden service provider. The introduction points that are included in the descriptor are encrypted using a key that is derived from the same shared key that is used to generate the descriptor ID. [correction to use another key than h(time-period + cookie) as encryption key for introduction points made by LO] A new text-based format is proposed for descriptors instead of an extension of the existing binary format for reasons of future extensibility. The complete hidden service descriptor format looks like this: { descriptor-id = h(permanent-id + h(time-period + cookie)) permanent-public-key (with permanent-id = h(permanent-public-key)) h(time-period + cookie) timestamp { list of intro points (ID, IP, onion port, onion key, service key) } encrypted with cookie } signed with permanent-private-key A hidden service directory can verify that a descriptor was created by the hidden service provider by checking if the descriptor-id corresponds to the permanent-public-key and if the signature can be verified with the permanent-public-key. A client can download the descriptor by creating the same descriptor-id and verify its origin by performing the same operations as the hidden service directory. Security implications: The security implications of the proposed changes are grouped by the roles of nodes that could perform attacks or on which attacks could be performed. Attacks by authoritative directory nodes Authoritative directory nodes are not anymore the single places in the network that know about a hidden service's activity and introduction points. Thus, they cannot perform attacks using this information, e.g. track a hidden service's activity or usage pattern or attack its introduction points. Formerly, it would only require a single corrupted authoritative directory operator to perform such an attack. Attacks by hidden service directory nodes A hidden service directory node could misuse a stored descriptor to track a hidden service's activity and usage pattern by clients. Though there is no countermeasure against this kind of attack, it is very expensive to track a certain hidden service over time. An attacker would need to run a large number of stable onion routers that work as hidden service directory nodes to have a good probability to become responsible for its changing descriptor IDs. For each period, the probability is: 1-(N-c choose r)/(N choose r) for N-c>=r and 1 else with N as total number of hidden service directories, c as compromised nodes, and r as number of replicas The hidden service directory nodes could try to make a certain hidden service unavailable to its clients. Therefore, they could discard all stored descriptors for that hidden service and reply to clients that there is no descriptor for the given ID or return an old or false descriptor content. The client would detect a false descriptor, because it could not contain a correct signature. But an old content or an empty reply could confuse the client. Therefore, the countermeasure is to replicate descriptors among a small number of hidden service directories, e.g. 5. The probability of a group of collaborating nodes to make a hidden service completely unavailable is in each period: (c choose r)/(N choose r) for c>=r and N>=r, and 0 else with N as total number of hidden service directories, c as compromised nodes, and r as number of replicas A hidden service directory could try to find out which introduction points are working on behalf of a hidden service. In contrast to the previous design, this is not possible anymore, because this information is encrypted to the clients of a hidden service. Attacks on hidden service directory nodes An anonymous attacker could try to swamp a hidden service directory with false descriptors for a given descriptor ID. This is prevented by requiring that descriptors are signed. Anonymous attackers could swamp a hidden service directory with correct descriptors for non-existing hidden services. There is no countermeasure against this attack. However, the creation of valid descriptors is more expensive than verification and storage in local memory. This should make this kind of attack unattractive. Attacks by introduction points Current or former introduction points could try to gain information on the hidden service they serve. But due to the fresh key pair that is used by the hidden service, this attack is not possible anymore. Attacks by clients Current or former clients could track a hidden service's activity, attack its introduction points, or determine the responsible hidden service directory nodes and attack them. There is nothing that could prevent them from doing so, because honest clients need the full descriptor content to establish a connection to the hidden service. At the moment, the only countermeasure against dishonest clients is to change the secret cookie and pass it only to the honest clients. Specification: The proposed changes affect multiple sections in several specification documents that are only mentioned in the following. The detailed specification will follow as soon as the design decisions above are final. dir-spec-v2.txt 2.1 The router descriptor format needs to include an additional flag to denote that a router is a hidden service directory. 3 The network status format needs to be extended by a new status flag to denote that a router is a hidden service directory. 4 The sections on directory caches need to be extended by new sections for the operation of hidden service directories, including replication of descriptors. rend-spec.txt 1.2 The new descriptor format needs to be added. 1.3 Instead of Bob's public key, the hidden service provider uses a freshly generated public key for every introduction point. 1.4 Bob's OP does not upload his service descriptor to the authoritative directories, but to the hidden service directories. 1.6 Alice's OP downloads the service descriptors similarly as Bob published them in 1.4. 1.8 Alice uses the public key that is included in the descriptor instead of Bob's permanent service key. tor-spec.txt 6.2.1 Directory streams need to be used for connections to hidden service directories. Compatibility: The proposed design is meant to replace the current design for hidden service descriptors and their storage in the long run. There should be a first transition phase in which both, the current design and the proposed design are served in parallel. Onion routers should start serving as hidden service directories, and hidden service providers and clients should make use of the new design if both sides support it. But hidden service providers should continue publishing descriptors of the current format, and authoritative directories should store and serve these descriptors. After the first transition phase, hidden service providers should stop publishing descriptors on authoritative directories, and hidden service clients should not try to fetch descriptors from the authoritative directories. However, the authoritative directories should continue serving hidden service descriptors for a second transition phase. After the second transition phase, the authoritative directories should stop serving hidden service descriptors. Implementation: There are three key lengths that might need some discussion: 1) descriptor-id, formerly known as onion address: It is generated by OPs internally and used for storing and looking up descriptors. There is no need to remember a descriptor-id for a human. In order to reduce the success rate of collisions it could be extended to the full output of SHA-1 of 160 bits instead of 80 bits. [extending the descriptor-id length suggested by LO] 2) permanent-id: This is the first part of the onion address that a client passes to his OP. The overall onion address should be easy to memorize. Therefore, its overall length should only be extended from the existing 80 bits to as few bits as necessary. The length of the permanent-id has an influence on the probability that an adversary creates an own key pair that leads to the same descriptor-id in a given time-period as an honest service's key. 32 bits should provide sufficient protection to avoid collisions, given the fact that key generation is expensive and the attack needed to be performed for every time-period. 3) cookie: This is the second part of the onion address that is passed to an OP. In order to provide confidentiality of introduction points, this secret key should have 128 bits. In total, this leads to an onion address of 160 bits instead of the current 80 bits.