Astoria is a research-prototype Tor client designed to reduce the chance that an autonomous system (AS)—a network operator or routing domain—can observe both ends of a Tor connection and correlate a user’s traffic. It does not stop the NSA, guarantee anonymity, or replace Tor Browser. Its authors reported substantial reductions in modeled exposure in a 2015 study, but Astoria’s own README warns that it is not secure enough for browsing sensitive content.
What Astoria is—and what “avoid NSA attacks” means
Astoria is an AS-aware Tor client research prototype, not a mainstream browser or a supported consumer privacy product. Its goal was to make certain traffic-correlation attacks harder by choosing Tor relays with knowledge of the Internet paths traffic might take.
The “NSA attacks” framing is too broad. The relevant threat is an observer that can monitor network paths and compare traffic entering and leaving the Tor network. That observer might be a single AS, cooperating ASes, or a state-level adversary with broader visibility. The research examined those modeled capabilities; it did not establish that Astoria defeats the NSA or a global passive observer.
Tor routes a connection through relays, but those relays do not determine every network path taken between the user, relays, and destination. Internet routing may be asymmetric: traffic in the two directions can traverse different networks. Consequently, an AS might observe both the client-to-entry path and the exit-to-destination path even though the Tor circuit uses distinct relays. Seeing timing and volume at both ends can help an observer infer that the flows belong to the same connection.
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How Astoria tries to reduce correlation risk
Astoria combines a modified Tor client with a separate path-prediction toolkit. The toolkit uses network-topology information to estimate the AS paths that traffic may cross. Astoria then selects entry and exit relays to reduce the chance that the same AS, sibling ASes, or a state-level observer can see both sides of a connection.
Relay selection is a constrained problem, not a guarantee that a perfectly safe route exists. The paper describes an optimization approach that minimizes modeled adversary exposure while accounting for relay capacity. If every available choice carries some risk, the design seeks a less-exposed path rather than making the risk disappear.
The repository README describes inputs including CAIDA AS-topology data and precomputed country data, and a Mono-based toolkit. These are components of the documented research setup, not evidence that the system has current topology data or is compatible with present-day Tor.
What the published evaluation found
The authors—Rishab Nithyanand, Oleksii Starov, Adva Zair, Phillipa Gill, and Michael Schapira—reported the following vulnerability estimates in their 2015 study:
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| Modeled adversary | Ordinary Tor circuits reported vulnerable | Astoria circuits reported vulnerable |
|---|---|---|
| AS-level adversary | Up to 40% | 2% |
| Colluding AS-level adversaries | 42% | Under 5% |
| State-level adversary | 85% | 25% |
These are results from the authors’ historical study, not present-day measurements or guarantees for an individual user. They depend on the study’s locations, websites, topology data, guard sets, attack model, and measurement period. In the NDSS evaluation, the authors also found that for 8% of requests from China and Iran, more than 95% of possible circuits were vulnerable under the study’s model. That finding describes those tested requests and conditions; it should not be generalized to all users in those countries or to today’s network.
Astoria versus ordinary Tor: security trade-offs and speed
The published performance comparison came from the authors’ NDSS evaluation, published in 2016. In that test setup, median page-load time was 5.9 seconds for vanilla Tor and 8.3 seconds for Astoria. Astoria was slower partly because it could not preconstruct and reuse circuits to the same degree and had to compute paths and check for potential attackers. The paper also reports path-computation overhead.
The authors said relay selection remained close to the load-balancing target used in their study, so the design did not simply optimize individual users’ paths without regard to relay capacity. These results are specific to that evaluation; they do not predict current performance.
| Question | Astoria | Ordinary Tor |
|---|---|---|
| Relay-path selection | Uses predicted AS paths to reduce modeled shared-observer exposure. | Does not use Astoria’s AS-aware path-prediction and relay-selection method. |
| Correlation resistance in the published study | Lower modeled vulnerability in the authors’ 2015 results; not a guarantee. | Higher modeled vulnerability in that study; the reported percentages are not current measurements. |
| Page-load performance in the published test | 8.3-second median page-load time in the authors’ NDSS evaluation. | 5.9-second median page-load time in the same evaluation. |
| Practical status | Research prototype; README warns against sensitive browsing. | Not a like-for-like implementation comparison of current Tor Browser; this study compared Astoria with vanilla Tor. |
Is Astoria still safe to use?
There is no basis here for treating Astoria as production security software. Its README explicitly says it is constantly changing and should not be considered secure enough for browsing sensitive content. The repository page described in the project documentation provides no release or package information, and the evaluation dates to 2015–2016. The study therefore does not establish compatibility with modern Tor clients, current routing conditions, current guard algorithms, or present-day adversaries.
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Do not substitute Astoria for Tor Browser when you need a maintained browser for private browsing. Nor should you assume that an AS-aware route protects against an observer able to see both ends by other means. The defensible claim is narrower: Astoria was designed to reduce opportunities for AS-level traffic correlation under the researchers’ modeled conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What it takes to run the research prototype
The repository instructions describe building a modified Tor client and starting the path-prediction toolkit separately. They are setup notes for research software, not a recommendation to use it for sensitive activity.
- Build the modified Tor client. The README gives the command
sh autogen.sh && ./configure && make && make install. - Prepare the prediction toolkit and data. The documented setup requires Mono, CAIDA AS-topology input, and precomputed country files.
- Start the prediction services before the modified Tor binary. The toolkit supplies the path predictions used for relay selection.
- Allow for startup and browsing delays. The README estimates about 15 minutes for initialization and cautions that the first page loads may take a couple of minutes. These are repository estimates, not independently reproduced timings.
Because the README warns against sensitive use and the setup depends on a modified client and supporting data, these instructions are best understood as a description of the prototype’s architecture—not as a safe installation guide for ordinary browsing.
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