tor-dev May 2016

tor-dev@lists.torproject.org

45 participants
38 discussions

Directory authority of type bridge?
by Nicholas R. Parker (RIT Student) 11 May '16

11 May '16

Hey again all, got another one for you. When we've started adding bridges to the network, they send a warning "Publishing server descriptor to directory authorities of type 'Bridge', but no authorities of that type listed!" Not sure how to have a directory authority as a bridge type given that if you set BridgeRelay 1 on an authority it drops the DirPort and just becomes a bridge relay. How does one list an authority as a bridge type? Nicholas R. Parker Rochester Institute of Technology 5thYear, BS/MS Computing Security

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Re: [tor-dev] [Proposal] Obfuscating the Tor Browser Bundle initial download
by Blake Hadley 09 May '16

09 May '16

> The environment you're were in was mounting a MITM attack to break TLS, > or has compromised your box, because the only component of the URL that > is visible otherwise is the host in the SNI field. > > In such an environment, gettor in general isn't unblockable because > there is no privacy/security for the request/response messages. > > Regards, > > -- > Yawning Angel Sorry, I'm not completely familiar with how TLS works. The environment requires an HTTPS proxy to reach the World Web Web. Do HTTP proxies inherently create a situation similar to MITM?

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[Proposal] Obfuscating the Tor Browser Bundle initial download
by Blake Hadley 09 May '16

09 May '16

Hey everyone, [How it's currently done] Distributed by gettor(a)torproject.com, the URL makes it pretty clear what you're downloading. Dropbox: https://www.dropbox.com/s/mz9ug2rzvj85791/torbrowser-install-5.5.5_en-US.ex… Google Drive: https://docs.google.com/uc?id=0B76pDbk5No54VHowTEprZnBfWlU&export=download GitHub: https://github.com/TheTorProject/gettorbrowser/releases/download/v5.5.5/tor… [Security problem] The download URL on Google Drive is somewhat obfuscated, but once the download is started, the filename that the browser requests is 'torbrowser[...]' An environment I was working in has started to block the files based on name, and it would be very easy for an adversary monitoring network traffic to detect users downloading it. [Solution proposed] When the user emails gettor, they could also request obfuscation. An application would randomize the filename and upload it to a mainstream host (Google, Dropbox, GitHub, AWS). Maybe even protect the file from scanning by making an AES encrypted ZIP file, and giving the user the password in the reply email. I'd be happy to make a proof-of-concept. What do you all think of this? Does anyone have any better ideas? Anything that uses less processing resource?

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Re: [tor-dev] [proposal] Post-Quantum Secure Hybrid Handshake Based on NewHope
by eikovi＠sigaint.org 09 May '16

09 May '16

Typos: > used as input to the SHAKE-256 extendable output function (XOF), as decribed deScribed > In the case for relays possessing Ed25519 identity keys (c.f. Tor proposal > ... > descriptor (c.f. Tor proposal #264) advertises support for the "Relay" > ... > (c.f. Tor proposal #249). > ... > We introduce a new sub-protocol number, "Relay=3", (c.f. Tor proposal #264 confer, cf., it's a single flying word > public keys already being in included within the "ntor-onion-key" entry. s/in included/included/ > poly_getnoise() first generates 4096 Bytes of uniformly random data. This can s/Bytes/bytes/ > mode). The output of the PRG is considered an array of 2048 16-bit integers > ... > Note further that the output of this function is secret; the PRG (and the PRnG was used previously > pseudocode description of a very naive inplace transformation of an input > ... > [0]; a pseudocode description of a very naive inplace transformation of an s/inplace/in-place/g

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Testing Network Node Availability
by Xiaofan Li 09 May '16

09 May '16

Hi all, We've finished building TOR on QUIC and everything works fine with chutney. However, we have an issue with the node availability when we test on real networks. We need this to work in order to evaluate the effect of HOL blocking with QuicTor vs. Tor. Specifically: 1. When using our QuicTor and normal TOR* without path restriction*, warning messages like: [warn] Failed to find node for hop 1 of our path. Discarding this circuit. Keeps showing up but the node would actually reach 100% bootstrap and all file transfer functionalities work fine. 2. However, our real issue is when I *restrict the path selection* to 3 pre-determined nodes for all exit circuits, the client will not reach 100% anymore and keeps hanging at 85% (or 80% sometimes). 3. We've thoroughly tested path restriction with chutney and it works. Our network has 11 nodes: 3 clients and 8 relays (2 of which are authorities). We already have assumeReachable and I've tuned up voting frequencies, just like chutney's default config. Any other configuration flag that could help propagating router availability info? Any idea at all? Li.

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Exit relay proportions for test network
by Nicholas R. Parker (RIT Student) 08 May '16

08 May '16

Quick question for all of you. At this stage of construction on a private tor network, I've reached the point where I can't quite figure out how many of our relays ought to be exit relays. I know that a 25 node network ought to have 4 authorities, 16 relays, and 5 clients, but is there a minimum required number of exit relays? Nicholas R. Parker Rochester Institute of Technology 5thYear, BS/MS Computing Security

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Tor Volunteer - Censorship analyser
by Lucia Di Marco 05 May '16

05 May '16

Hi everyone, my name is Lucia and I am a computer science student from UPC, Spain. I am also starting my final year project and I chose to develop the Censorship Analyser you had in the volunteer's page. A month ago I realize that this project idea was gone and I asked Philipp Winter about it. He told me that it is not listed as he cannot spend time on it, but I really wanted to do a project to help Tor community. My questions here are only two: do you think this project is not needed anymore now we have a lot of recent data about censorship in other countries (OONI Explorer) and other tools? If I develop this software (I will probably do it even if the answer to the first question is "yes"), can I ask the Tor community (tor-dev?) for help in testing and improving proposals? I will not ask it a lot as I have my project director and some friends of the college who will help me, but I think it would be great if more people test it as it will be more robust and adapted to Tor community needs. A quick review about the project: it will be a tool like OONI scripts but more basic and without using tor. It will also be more difficult to detect as I will study how to avoid being noticed by surveillance systems and with DPI methods for example. Also it will be user friendly, as one of its purpose is to reach all kind of users. Thanks for your time, Lucia.

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Proposal: Tor with collective signatures
by Nicolas Gailly 03 May '16

03 May '16

Hi all, Since the first post, I've received some comments off-list and consequently added more sections to the proposal. Particularly, it adds a detailed description of the messages that needs to be passed down between the witnesses, paragraphs to answer some questions, and a link to the official paper. I hope it gets clearer and answer most of your questions :) Please, feel free to ask questions on/off list, I'd be happy to discuss about it anytime ;) (irc: nikkolasg) Thanks, Nicolas version 0.2: - 3.2.1 Setup : explicitly using Fallback directory mirrors as witnesses - 3.2.2 Operation: what does a witness that refuse to sign do - 3.2.3 added incremental deployment section - 3.3 : added evolution of the CoSi set of witnesses section - 3.1 added signature description - 5 Specifications: 5.1: Protocol 5.2: Format ------------------------------------------------------------------------------------------ Filename: tor_cosi.txt Title: Tor Cosi Author: Nicolas GAILLY, DeDiS lab, EPFL Created: 09.03.2016 Status: draft Version: 0.2 0. Introduction This document describes how to provide and use a decentralized witness cosigning mechanism in order to gain proactive transparency and public accountability for the Tor consensus documents. Directory Authorities (DA) send their document to this set of witnesses and embed the signature within the document. Tor relays and clients can choose to refuse a consensus document if it has not been accepted and signed by a threshold of witnesses. 1. Overview A weakness of the current DA system is that if any 5 of the 9 DAs’ keys are stolen or coerced they could be used to sign fake directories that the attacker might use secretly in another part of the world to compromise Tor clients in the attacker’s domain. We propose to address this class of attacks by incorporating decentralized witness cosigning (CoSi) into the directory signing process, which ensures that any consensus document must be not only signed by appropriate DAs, but also publicly witnessed, signed and logged by a larger group of servers acting as witnesses, before clients will accept the directory. A Tor relay or client expects to receive an additional "CoSi" signature alongside the consensus document. They verify if the signature is correct and whether a sufficient number of witnesses attested that the consensus document is valid or not. The Tor project would fix such a threshold in the default configuration but any user or relay is free to adjust this value to its own need. In order to verify the signature, they need to have the individuals public keys of all the witnesses beforehand. This "CoSi certificate" can be embedded in the software in the same way certificate pinning does. 2. Motivation Tor's DAs are comprised of 9 servers (and one extra for the bridges): four of them are in the US and 5 of them are in the EU. Attacking these central and vital points of the Tor network is clearly within the reach of state level adversaries if they were to collaborate. Recent stories about surveillance show that such a collaboration is already happening. For example, let's imagine a plausible situation where a state-level attacker secretly coerces and/or steals the private keys of 5 of the 9 DAs, takes them back to the Republic of Tyrannia where they control the ISPs and the country’s Internet connectivity to the rest of the world. The government embeds those keys in their “Great Firewall” type devices, and uses them to secretly MITM attack targeted Tor users within Tyrannia by giving them correctly-signed but completely false views of the Tor directory in which all of the available relays are run by the Tyrannian authorities. Since this attack does not attack the consensus documents that the legitimate DAs are regularly broadcasting to the rest of the world, neither the Tor project nor anyone else outside of Tyrannia will have the opportunity to see or become aware of the fake consensus documents, and not many people even inside Tyrannia might have the opportunity to detect the attack if it is carefully targeted against the small number of suspected activists and journalists the government does not like. The main goal of CoSi applied in Tor should be to ensure consensus document transparency: that is, ensure the property that any consensus document that any Tor client anywhere will accept has been observed and logged by a significant number of parties throughout the world, so that any misuse of a quorum of 5 DA keys anywhere will be quickly detectable (soon, if not necessarily during the signing process itself). 3. Design The first part will talk more about the architecture of a "CoSi" system and the second part will go into more detail about how such a system can be integrated in Tor. For more details, please refer to the official paper [0]. 3.1 Simplified Architecture The designated leader arranges all the witnesses in a tree. The public keys of each participants are aggregated to form an aggregate public key. The tree is only there for performance reasons and can be reconfigured at any point in time without affecting security (the latest experiments showed that we could run up to 8000 nodes with a 2 second delay for generating a signature). Once the tree is constructed, its "certificate" represents the aggregate public key and all its individual public keys of each witnesses. At the end of a signature round, the client will be able to verify the CoSi signature (which uses Schnorr signatures) using the aggregate public key. A CoSi signature have two components: - Schnorr signature - Exceptions: bitmap of length equal to the size of the witness group used by absent witnesses or refusing-to-sign witnesses (see 3.2.2 Operations). Besides contributing to the signing, each witness can and should perform any readily feasible syntactic and semantic correctness checks on the leader’s proposed statements before signing off on them. They can / should probably publish logs of the statements they witness or simply make available a public mirror of everything that its tree roster has been asked to sign. 3.2 CoSi in Tor 3.2.1 Setup CoSi relies on a list of decentralized cosigning witnesses, an optional role to be supported by a future version of the standard Tor relay software. The set of witness servers will initially be defined as the D.A. and a subset of the Fallback Directory Mirrors [2], namely the set of relays that are (a) deemed stable enough by the same criteria as used for selecting Fallback Directory Mirrors, (b) are running a recent enough version of the Tor relay software to support the CoSi witness server role, and (c) have not opted out of playing this CoSi witness role. That way the deployment cost is drastically reduced as software will already have the keys to verify a CoSi signature. Another criteria for the tree construction includes the latency between witnesses. One can collect information about the communication latencies between the witnesses and construct a shortest-path spanning tree using this data in order to reduce the global latency of the system. 3.2.2 Operations Once the tree is setup and each one knows about it (have the certificate), then the signature process happens for each consensus document. The CoSi protocol produces a collective signature in response to the initiation of the protocol by a leader. This signature is then included in the consensus document so clients don't have to request it from another party. If a witness does not want to sign, it toggles the bit at its index in the bitmap. Its index is determined as the index in the list of witnesses from the CoSi certificate. The client will then see a "1" bit in the bitmap, and will subtract the corresponding public key of the witness from the aggregate public key. That way, the client is still able to verify the signature and it knows about which witnesses refused to sign off. The mechanism is similar for witnesses that went offline. The parent of an offline witness will set the bit in the bitmap of the failed witness. One issue for discussion is who should initiate CoSi protocol rounds and at what times. For example, each of the 9 DAs (or whatever subset is online) could independently initiate CoSi rounds on each directory consensus event, producing up to nine separate, redundant collective signatures on each directory consensus. Alternatively, the common case might be for one of the 9 DAs to be the CoSi initiator at a given time, with a round-robin leader-change mechanism ensuring that another leader takes over if the prior one becomes unavailable. A related issue for discussion is whether it could be problematic if there are two or more distinct collective signatures for a given directory consensus, and whether it is a problem if distinct subsets of 5 DAs might (perhaps accidentally) produce multiple slightly different, though valid and legitimately-signed, consensus documents at about the same time. In other words, does Tor directory consensus “need” strong consistency with a single serialized timeline, as Byzantine consensus protocols are intended to provide - or is weak consistency with occasional cases of multiple concurrent consensus documents and/or collective signatures acceptable? 3.2.3 Integration First of all, integrating CoSi would *not* immediately affect the fundamental structure or function of the current DAs: there could still be 9 of them, of which any 5 can authorize the release of a new consensus document, as they do now. Secondly, CoSi would not necessarily change anything about how the 9 DAs decide on how to compute these directory consensus documents; e.g., it would not prevent the DAs from working together to block the inclusion of (or assignment of bandwidth-weight to) relays that might be perceived by the DAs as doing bad things. Finally, full backward compatibility with old Tor clients and relay software may be maintained by treating the new CoSi-generated collective signature as just an additional signature that gets attached to and distributed with consensus documents. It may be treated as a special “10th virtual DA” that does not help authorize decisions but just publicly witnesses the output of the regular 9 DAs. Old client and relay software can simply ignore that new collective signature, whereas new software might look for it and over time gradually increase the threshold number of witnesses it expects to see. For incremental deployment purposes, it's reasonable to think that those Fallback Directory mirrors servers probably would not deply CoSi support all at the same time. Moreover, this set of witnesses might be a subset of the Fallback Directoy mirrors and some of them would not want to be cosigners. The hard-coded list of mirrors into the Tor client might need to be annotated with a “cosigner flag" for each such mirror, allowing some bootstrap relays to be cosigners and others not. The witness group is then simply the subset of fallback directory mirrors with the cosigner flag set. 3.3 Evolution of the CoSi set of witnesses One obvious solution for the evolution of the CoSi set of winesses lies into the version-ing mechanism of Tor. A particular Tor client version would be associated with a particular cosigning group which consists of the D.A. and the Fallback Directory Mirrors whose keys are embedded into the source code of this Tor version with this "cosigner flag" set. A client will have the latest CoSi set keys when and only when its Tor client would be upgraded - just like the list of directory authorities. 3.4 Optional: Break-the-glass Emergency Directory Adjustments In case of emergency, the delay caused by having to coordinate among 5 DAs in order to make anything happen (i.e. excluding a set of malicious nodes) can be problematic. This section proposes a mechanism in which the CoSi witnesses can accept and witness not just “full consensus” documents (signed by 5 DAs), but can also accept “emergency adjustments”, which are highly-constrained deltas (diffs) to an existing full consensus document signed by a smaller threshold of DAs, e.g., 2 or even just 1. For example, the CoSi witness cosigning rules might require that an emergency directory-adjustment must: - be a diff against a “fresh”, recent full consensus document (perhaps *the* most recent one), - can make no modifications to the full consensus other than some pre-defined operations such as decreasing bandwidth weights assigned to relays, - cannot affect the directory-wide total bandwidth weight by more than X% (e.g., 1% or .1%). These suggestions are just a few imaginable rules to get the idea across; the “right” rules would of course need much more discussion. This way, if one or two DAs discovers or even strongly suspects an attack of some kind, they can take emergency countermeasures against the attack and be able to roll them out to clients quickly without having to get a full 5 DAs out of bed - but because the delta-consensus is still witness-cosigned automatically by (perhaps) all the DAs and a number of additional trusted relays, we get the strong accountability provision that the use of such a “break-the-glass” emergency provision will immediately become known to the other DAs as soon as they do get out of bed. Such a break-the-glass emergency adjustment mechanism could be designed, if desired, so that ordinary clients and relays cannot immediately tell the difference between a directory consensus produced via the normal threshold of 5 DAs and one that was produced as a delta via the emergency adjustment mechanism. Only the witness cosigners would necessarily need to know which collectively-signed directories were authorized via the full consensus procedure or via a break-the-glass adjustment. So if it’s important to keep it secret from the general public the precise reason for a particular directory update, that can be accommodated. Only the more-trusted group of witness cosigners (and obviously all the DAs themselves) would necessarily know the precise origin and administrative justification of a given directory update. With even fancier crypto, even the witnesses would not necessarily need to know, but that’s beyond the scope of this proposal and its desirability may be questionable at any rate. 4. Security implications 4.1 Cons Since the structure is a tree, if any node fails, there must be some failover mechanisms to restore connectivity between the children of the failed node and the rest of the tree. Since the DA reach consensus every hour [1], with the help of the gossiping network, the availability problem is not an issue. 4.2 Benefits Technically, it is quite easy to implement witness cosigning if the group of witnesses is small. If we want the group of witnesses to be large, however – and we do, to ensure that compromising transparency would require not just a few but hundreds or even thousands of witnesses to be colluding maliciously – then gathering hundreds or thousands of individual signatures could become painful and inefficient. Worse, every client would need to check all these signatures individually. The key technical contribution of our research is a distributed protocol that makes large, decentralized witness cosigning groups practical. This decentralized approach enables the security of the whole system to scale with the number of witnesses. Not only does this system dramatically increase the cost of successfully deploying an attack on the DA (the attacker would have to corrupt a large majority of the witnesses), it also decreases the incentive to launch such an attack because the threshold of witnesses that are required to sign the document for the signature to be accepted can be locally set on each client. 4.3 Differences between CoSi and Certificate Transparency Prior transparency mechanisms have two weaknesses. First they do not significantly increase the number of secret keys an attacker must control to compromise any client device, and client devices cannot even retroactively detect such compromise unless they can actively communicate with multiple well-known Internet servers. For example, even with Certificate Transparency, an attacker can forge an Extended Validation (EV) certificate for Chrome after compromising or coercing only three parties: one Certificate Authority (CA) and two log servers. Since many CAs and log servers are in US jurisdiction, such an attack is clearly within reach of the US government. If such an attack does occur, Certificate Transparency cannot detect it unless the victim device has a chance to communicate or gossip the fake certificate with other parties on the Internet – after it has already accepted and started using the fake digital certificate. In the case of Tor Transparency, the attack is harder because the attacker would have to compromise the three parties plus a majority of Directory Authorities but facing a state-level adversary the threat is still plausible. One way to increase the difficulty of the attack is to make sure the logs servers are scattered in different places of the world. Second, the attacker can still evade transparency by controlling the client’s Internet access paths. For example, a compromised Internet service provider (ISP) or corporate Internet gateway can defeat retroactive transparency mechanisms by persistently blocking a victim device’s access to transparency servers elsewhere on the Internet. Even if the user’s device is mobile, a state intelligence service such as China’s “Great Firewall” could defeat retroactive transparency mechanisms by persistently blocking connections from a targeted victim’s devices to external transparency servers, in the same way that China already blocks connections to many websites and Tor relays. Using CoSi requires the clients to have the list of public keys of all the witnesses embedded in the software, like certificate pinning. In order to reduce the size of this CoSi certificate, we embed the aggregated public key of all the witnesses and a hash representing the root of a Merkle tree containing the public key of all the witnesses. Using the certificate this way provides an universally-verifiable commitment to all the witnesses’ public keys, without the certificate actually containing them all. 5. Specifications 5.1 Protocol We will describe quickly the protocol here; for a more detailed explanation, please refers to the academic paper [0]. The setup is as described in 3.2.1. The protocol in itself consists of four phases: - Announcement: The leader broadcast down the consensus document to its children, which in turn also broadcast to their children,etc. - Commitment: When the leaves of the CoSi tree get the consensus document, generate its random value v(i) and the corresponding commitment V(i) and sends V(i) up to its parent. If a leaf refuses to sign this consensus document, it does not create any commitment. Each intermediate node aggregate all the commitments of their children, add their own commitment (or nothing if it refuses to sign) and send the result up in the tree. The root gets the aggregated commitment V of all signing witnesses. - Challenge: The root then compute the challenge c = H( m || V ), with m being the consensus document and H being a collision resistant hash function that returns a scalar, and distribute the challenge down the tree like in the Announcement phase. - Response: Starting from the leaves, each witnesses compute its response r(i) = v(i) - c * x(i), where x(i) is the long term private key of the witness. If the witness refuses to sign, it simply set the n-th bit of the bitmap to "1", where n is the index of the witness in the "CoSi certificate" (the list of all individual public keys). Each intermediate nodes in the tree aggregate the responses and the bitmap of all its children, aggregate with its own response/bitmap and send that up in the tree. At the end of the protocol, the root gets the aggregated response r. The signature is the tuple (c,r) and must be included in the consensus document. If no exceptions occurred (i.e. the bitmap contains all "0"s), the signature can be verified using the aggregate public key of all witnesses using standard Schnorr verification algorithm [3]. If an exception occurs, the client needs to lookup the indexes where the bitmap contains "1"s. The client then lookup the corresponding public keys (from the list of public keys of witnesses) and subtract each of them from the aggregate public key. The client can then use this reduced public key to verify the signature as usual. 5.2 Format + The "CoSi certificate" is a list of all witnesse's ed25519 public keys and the aggregate public key of all individual public keys. Please note that while the current implementation only uses ed25519, it is completely possible to use any other elliptic curve implementations. + A CoSi signature contains: - the challenge c, an ed25519 scalar - the response r, an ed25519 scalar - the bitmap of exceptions, whose length is equal to the number of witnesses. + The messages sent during the four following phases are as follow: - Announcement: consensus document - Commitment: an ed25519 curve point - Challenge: an ed25519 scalar - Response: an ed25519 scalar and the exception bitmap 6. Compatibility 7. Implementation Implementation in Go is open source at: https://github.com/dedis/cothority 8. Performance 9. Acknowledgements This proposals has received some valuable feedback from Bryan Ford, Linus Gasser, Ismail Khoffi, Philipp Jovanovic, and Ludovic Barman. A. References [0] http://arxiv.org/pdf/1503.08768v3.pdf [1] https://collector.torproject.org [2] https://trac.torproject.org/projects/tor/wiki/doc/FallbackDirectoryMirrors [3] https://en.wikipedia.org/wiki/Schnorr_signature

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tor-dev May 2016