Tor Passive- Active -Directory Attacks on onion network

Tor Passive- Active -Directory Attacks on onion network

Passive attacks

Observing user traffic patterns. Observing a user’s connection will not reveal her destination or data, but it will reveal traffic patterns (both sent and received). Profiling via user connection patterns requires further processing, because multiple application streams may be operating simultaneously or in series over a single circuit.

Observing user content. While content at the user end is encrypted, connections to responders may not be (indeed, the responding website itself may be hostile). While filtering content is not a primary goal of Onion Routing, Tor can directly use Privoxy and related filtering services to anonymize application data streams.

Option distinguishability. We allow clients to choose configuration options. For example, clients concerned about request linkability should rotate circuits more often than those concerned about traceability. Allowing choice may attract users with different needs; but clients who are in the minority may lose more anonymity by appearing distinct than they gain by optimizing their behavior [1].

End-to-end timing correlation. Tor only minimally hides such correlations. An attacker watching patterns of traffic at the initiator and the responder will be able to confirm the correspondence with high probability. The greatest protection currently available against such confirmation is to hide the connection between the onion proxy and the first Tor node, by running the OP on the Tor node or behind a firewall. This approach requires an observer to separate traffic originating at the onion router from traffic passing through it: a global observer can do this, but it might be beyond a limited observer’s capabilities.

End-to-end size correlation. Simple packet counting will also be effective in confirming endpoints of a stream. However, even without padding, we may have some limited protection: the leaky pipe topology means different numbers of packets may enter one end of a circuit than exit at the other.

Website fingerprinting. All the effective passive attacks above are traffic confirmation attacks, which puts them outside our design goals. There is also a passive traffic analysis attack that is potentially effective. Rather than searching exit connections for timing and volume correlations, the adversary may build up a database of “fingerprints” containing file sizes and access patterns for targeted websites. He can later confirm a user’s connection to a given site simply by consulting the database. This attack has been shown to be effective against SafeWeb [29]. It may be less effective against Tor, since streams are multiplexed within the same circuit, and fingerprinting will be limited to the granularity of cells (currently 512 bytes). Additional defenses could include larger cell sizes, padding schemes to group websites into large sets, and link padding or long-range dummies.4

Active attacks

Compromise keys. An attacker who learns the TLS session key can see control cells and encrypted relay cells on every circuit on that connection; learning a circuit session key lets him unwrap one layer of the encryption. An attacker who learns an OR’s TLS private key can impersonate that OR for the TLS key’s lifetime, but he must also learn the onion key to decrypt create cells (and because of perfect forward secrecy, he cannot hijack already established circuits without also compromising their session keys). Periodic key rotation limits the window of opportunity for these attacks. On the other hand, an attacker who learns a node’s identity key can replace that node indefinitely by sending new forged descriptors to the directory servers.

Iterated compromise. A roving adversary who can compromise ORs (by system intrusion, legal coercion, or extralegal coercion) could march down the circuit compromising the nodes until he reaches the end. Unless the adversary can complete this attack within the lifetime of the circuit, however, the ORs will have discarded the necessary information before the attack can be completed. (Thanks to the perfect forward secrecy of session keys, the attacker cannot force nodes to decrypt recorded traffic once the circuits have been closed.) Additionally, building circuits that cross jurisdictions can make legal coercion harder—this phenomenon is commonly called “jurisdictional arbitrage.” The Java Anon Proxy project recently experienced the need for this approach, when a German court forced them to add a backdoor to their nodes [51].

Run a recipient. An adversary running a webserver trivially learns the timing patterns of users connecting to it, and can introduce arbitrary patterns in its responses. End-to-end attacks become easier: if the adversary can induce users to connect to his webserver (perhaps by advertising content targeted to those users), he now holds one end of their connection. There is also a danger that application protocols and associated programs can be induced to reveal information about the initiator. Tor depends on Privoxy and similar protocol cleaners to solve this latter problem.

Run an onion proxy. It is expected that end users will nearly always run their own local onion proxy. However, in some settings, it may be necessary for the proxy to run remotely— typically, in institutions that want to monitor the activity of those connecting to the proxy. Compromising an onion proxy compromises all future connections through it.

DoS non-observed nodes. An observer who can only watch some of the Tor network can increase the value of this traffic by attacking non-observed nodes to shut them down, reduce their reliability, or persuade users that they are not trustworthy. The best defense here is robustness.

Run a hostile OR. In addition to being a local observer, an isolated hostile node can create circuits through itself, or alter traffic patterns to affect traffic at other nodes. Nonetheless, a hostile node must be immediately adjacent to both endpoints to compromise the anonymity of a circuit. If an adversary can run multiple ORs, and can persuade the directory servers that those ORs are trustworthy and independent, then occasionally some user will choose one of those ORs for the start and another as the end of a circuit. If an adversary controls m > 1 of N nodes, he can correlate at most ????m N 2 of the traffic— although an adversary could still attract a disproportionately large amount of traffic by running an OR with a permissive exit policy, or by degrading the reliability of other routers.

Introduce timing into messages. This is simply a stronger version of passive timing attacks already discussed earlier.

Tagging attacks. A hostile node could “tag” a cell by altering it. If the stream were, for example, an unencrypted request to a Web site, the garbled content coming out at the appropriate time would confirm the association. However, integrity checks on cells prevent this attack. Replace contents of unauthenticated protocols. When relaying an unauthenticated protocol like HTTP, a hostile exit node can impersonate the target server. Clients should prefer protocols with end-to-end authentication.

Replay attacks. Some anonymity protocols are vulnerable to replay attacks. Tor is not; replaying one side of a handshake will result in a different negotiated session key, and so the rest of the recorded session can’t be used. Smear attacks. An attacker could use the Tor network for socially disapproved acts, to bring the network into disrepute and get its operators to shut it down. Exit policies reduce the possibilities for abuse, but ultimately the network requires volunteers who can tolerate some political heat.

Distribute hostile code. An attacker could trick users into running subverted Tor software that did not, in fact, anonymize their connections—or worse, could trick ORs into running weakened software that provided users with less anonymity. We address this problem (but do not solve it completely) by signing all Tor releases with an official public key, and including an entry in the directory that lists which versions are currently believed to be secure. To prevent an attacker from subverting the official release itself (through threats, bribery, or insider attacks), we provide all releases in source code form, encourage source audits, and frequently warn our users never to trust any software (even from us) that comes without source.

Directory attacks

Destroy directory servers. If a few directory servers disappear, the others still decide on a valid directory. So long as any directory servers remain in operation, they will still broadcast their views of the network and generate a consensus directory. (If more than half are destroyed, this directory will not, however, have enough signatures for clients to use it automatically; human intervention will be necessary for clients to decide whether to trust the resulting directory.)

Subvert a directory server. By taking over a directory server, an attacker can partially influence the final directory. Since ORs are included or excluded by majority vote, the corrupt directory can at worst cast a tie-breaking vote to decide whether to include marginal ORs. It remains to be seen how often such marginal cases occur in practice. Subvert a majority of directory servers. An adversary who controls more than half the directory servers can include as many compromised ORs in the final directory as he wishes. We must ensure that directory server operators are independent and attack-resistant.

Encourage directory server dissent. The directory agreement protocol assumes that directory server operators agree on the set of directory servers. An adversary who can persuade some of the directory server operators to distrust one another could split the quorum into mutually hostile camps, thus partitioning users based on which directory they use. Tor does not address this attack.

Trick the directory servers into listing a hostile OR. Our threat model explicitly assumes directory server operators will be able to filter out most hostile ORs.

Convince the directories that a malfunctioning OR is working. In the current Tor implementation, directory servers assume that an OR is running correctly if they can start a TLS connection to it. A hostile OR could easily subvert this test by accepting TLS connections from ORs but ignoring all cells. Directory servers must actively test ORs by building circuits and streams as appropriate. The tradeoffs of a similar approach are discussed in deny Bob service by flooding his introduction points with requests. Because the introduction points can block requests that lack authorization tokens, however, Bob can restrict the volume of requests he receives, or require a certain amount of computation for every request he receives.

Attack an introduction point. An attacker could disrupt a location-hidden service by disabling its introduction points. But because a service’s identity is attached to its public key, the service can simply re-advertise itself at a different introduction point. Advertisements can also be done secretly so that only high-priority clients know the address of Bob’s introduction points or so that different clients know of different introduction points. This forces the attacker to disable all possible introduction points.

Compromise an introduction point. An attacker who controls Bob’s introduction point can flood Bob with introduction requests, or prevent valid introduction requests from reaching him. Bob can notice a flood, and close the circuit. To notice blocking of valid requests, however, he should periodically test the introduction point by sending rendezvous requests and making sure he receives them.

Compromise a rendezvous point. A rendezvous point is no more sensitive than any other OR on a circuit, since all data passing through the rendezvous is encrypted with a session key shared by Alice and Bob.


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