- tor-dev - lists.torproject.org

Even more notes on relay-crypto constructions
by Nick Mathewson 19 Oct '12

19 Oct '12

I should share with the list an update of where I am with a design for an improved relay crypto protocol. For background and motivation, please see the last thread on the topic [Prop202]. There are three main questions remaining for me in choosing among new relay crypto protocols. Basically, they are: "Am I comfortable with this system?", "Among systems I'm comfortable with, how good is this system?", and "Do I know how to implement this system?" Unfortunately, the stuff I am currently comfortable with and know that I could implement is not nearly as good as the stuff that I'm _nearly_ comfortable enough to use and I don't know how to implement. Let's talk about some designs in detail, using the same terminology as proposal 202. Note: As usual, this is probably largely wrong. If you find it on a mailing list archive years from now, please don't try to learn cryptography from it. What I'm looking for most right now is places where my reasoning is wrong, places where I'm mistaken about what tradeoffs I need need to make, and places where there's research that I should have read or remembered. I. MAC-AND-PAD DESIGNS These are the least involved to implement: all you need is an AEAD construction and a PRNG. Those are both pretty well-understood: you can build a good PRNG out of any stream cipher, and you can build the AEAD mode out of a stream cipher and a MAC, or out of various other constructions. As a wrinkle: it would be good to have a system that generates "two MACs for the price of one." That's because in our topology, we need the ability to address relay cells to any node in the circuit, and one easy way to do that here is to have a construction that generates two MACs, and uses one for "cells that should arrive here" and another for "cells that we should relay". We also want shorter MACs: 16 bytes per hop is excessive for our needs. Authenticators like GCM whose output is the raw result of a polynomial evaluation aren't safe to truncate: Ferguson [Ferguson] has a result showing that it's easier than it should be to perform forgery against truncated-MAC GCM constructions. On the other hand, polynomial authenticators seem safe to truncate (generally) if the encryption step comes _after_ the polynomial evaluation. See for example [Poly1305-Trunc]'s discussion of truncated Poly1305 -- I hope it's right. So to be concrete, let me suggest a few modes of operation. I believe I'm competent to implement these: AES-GCM+AES: AES-GCM, except that we add (modulo 2^128) the output of AES(nonce) to each GCM output, in hopes that it will make GCM safe to truncate. (This is probably crazy somehow.) AES-CTR + HMAC-SHA512/256. AES-CTR + Poly1305. Poly1305 requires nonces, but we can use a counter for those. Salsa20 + Poly1305. For a padding PRNG, we could use any stream cipher we like. In each case, we'd want to authenticate not only the content of the cell, but also the previous cell's authenticator and the position of the cell within the stream. AES-GCM+AES is going to be the fastest on CPUs with specific support for AES and GCM, but not on other architectures until/unless more chips grow instructions specialized for AES and GF(2^128). Salsa20+Poly1305 should be fastest on other architectures. This entire category of designs still has the problems that it had before: it leaks the length of the circuit to the last node in the circuit, and consumes a fairly large portion of each cell (for one truncated mac per hop). Further, it's going to be a bit fiddly (though not impossible) to get it to work with rendezvous circuits. II. WIDE-BLOCK DESIGNS Turn we now to designs where we encrypt each block of the cipher using a 509-byte SPRP, such that each block's SPRP is keyed dependently on the original key and on the encrypted value of the previous block. There be dragons here. Let's talk about some ways we could build that. I'll start by talking about wide-block encryption, then talk about getting the "unrecoverability" property where any missing or corrupted block makes all future blocks unrecoverable. The wide-block SPRP I've used before is LIONESS [Bear-Lion]. It requires two keyed hash operations and two stream cipher operations, so it's going to be at best half the speed of the mac-and-pad designs above: a little worse, maybe, since it forces us to rekey with every block. Might that be acceptable? Right now, AES is not a huge fraction of our runtime, and a relay does AES on each cell three times (TLS,relay crypto,TLS) and SHA1 on each cell twice (TLS,TLS). An exit does AES on each cell twice (TLS,relay) and SHA1 on each cell twice (TLS,relay). So for relays, with a naive SHA1-AES LIONESS, we'd be increasing the stream-cipher operations by 25% and the digests by 100%. On an exit, we'd be increasing the stream-cipher operations by 33% and the digests by 100%. If we were to go with LIONESS, we'd surely want to look into faster, better keyed-hash algorithms than SHA1. I don't know whether one of the polynomial MACs above would be good enough, or whether we need other cryptographic digest properties that those don't give us and we'd need to turn to SHA256 or something. I've heard some suggestions that we should look into BEAR or LION instead, but I'm leery of the idea. They are faster, requiring one fewer application of their primitives, but they are PRPs, not SPRPs -- meaning I think that they don't give you Ind-CCA2 [Bear-Lion, section 6]. I don't know a way to exploit this in the Tor network, but deviating from an ideal block cipher seems to me like one of those ideas that's practically begging for disaster. Can we get faster than LIONESS? Indeed we can! There are a pile of constructions. If I understand correctly, and I'm not missing anything, the ones we might want to use fall into two broad categories: those that (like LIONESS) split the message into a short bit and a long bit, then treat them differently; and those that apply a transformation on the message, encrypt the message, and transform it again. The first category (split, then frob each part based on the other once or twice) would appear to be more of a patent minefield, thanks to the XCB patent. The ones to look at here are [HCTR] and [HCH], which require two applications of a universal hash function (can be polynomial-based) and approximately one encryption pass. HCH is a little more unlike XCB. Neither is exactly trivial. We could build either one out of whatever field and stream cipher we wanted, as above, though bitsliced AES-CTR wouldn't be efficient in this case: we aren't generating enough stream at once for bitslicing to pay off. The second category (frob, encrypt, frob) is pretty elegant IMO. The best-explained of these I've seen so far are in a paper by Palash Sarkar [Efficient-Tweakable], though the earlier TET construction [TET] might also be cool. For these, you need an invertible block-wise (Almost) (Xor-)Universal hash function, typically implemented with GF(2^128). I'm not sure if you could use a different field. The multiplication operations here appear to be multiplication by a primitive element, and multiplication by a per-key element. The encryption step can be realized with a somewhat unorthodox counter-mode stream cipher, or a ciphertext-stealing ECB approach. I don't know what you'd need to do to substitute in an orthodox stream cipher for the one used in iHCH. Sarkar seems to see iHCH as a successor to HCH, which is a little worrisome given that HCH is a spiritual descendant of the patented XCB, but to me the two constructions (HCH, iHCH) look practically nothing alike except for their use of a counter mode step. I am a little intimidated by the idea of trying to implement one of these ciphers. Above, I haven't taken one of our requirements into account: that any change to a single cell must make all future cells unrecoverable. There are modes that are supposed to prevent this, and applying them to a decent wide-block cipher might solve the issue. IGE is one of them [IGE], but it turns out to be broken by an attacker who knows some plaintext. The Accumulated Block Chaining [ABC] construction is supposed to fix that; I'm not too sure whether it's correct or efficient. As a blunt-force approach to the problem, we could rekey between each block, using new keys based on a MAC of the last block. This would be pretty inefficient for primitives that need any serious amount of per-key computation. Finally, we could look into constructions that produce an extra secret output incidental to their regular operation, and which are easily rekeyed for each operation. In this whole field, we need to keep an eye out for the patents on CMC, EME, and XCB. "Joy." III. CHOICE OF CIPHERS AND OTHER PRIMITIVES I am hearing exactly two recommendations for encryption primitives nowadays: AES and Salsa20 (and its family). Nobody's recommending anything else as far as I can tell. AES is still the IBM of the block cipher world, which "Nobody Ever Got Fired For Using." It's a bit of a pain to use it in software, though. Its most obvious implementations have timing attacks due to table lookups [DJB-Timing]. OpenSSL has three fast x86 implementations: one of them uses vector permutations, one uses bit-slicing, and one uses the aesni instructions to invoke the chip's built-in AES capabilities. On high-end Intel chips, these take about 12-25, 7-9, and 1.5 cycles per byte respectively. The bitsliced one really needs to encrypt 4KiB at a time (yes, kibibytes), which makes it fine for counter mode but not so good for CBC. (Those numbers are from the OpenSSL source.) Key setup times are cheap for all of these (I think!), but the bitsliced implementation gets expensive if you change the key (or the stream position). Salsa20 (and its descendant, ChaCha) is the Obvious Second Choice for people who don't want to use AES. It is faster than AES on every platform except those with hardware support for AES; 4-7 cycles per byte seems typical for high-end Intel stuff. You would probably have to go out of your way to implement it in a way that was vulnerable to timing side-channel attacks. The only arguments for its insecurity that I'm aware of are that although it has fewer known flaws than AES, it has received less attention than AES, therefore is likelier to _unsuspected_ problems. The counterargument there is that there _are_ several dozen cryptographers who have tried to attack it (or attack reduced-round variants of it), several of whom have also done successful results against AES. [DJB-Comm] Per-key setup time is basically nonexistent. There are a lot of constructions out there that want to do multiplication in GF(2^128) as a basic operation. That turns out to be less than totally straightforward, though. On newer intel chips, you've got a "multiply without carry" instruction that can supposedly let you get to around 2 cycles per byte. On other platforms, you're reduced to worse trickery to try to get good performance without timing side-channels, for an amount of trickery dependent on what operation you're trying to do exactly. The easiest GF(2^128) operation to implement safely and quickly is multiplication by a known compile-time value. (It's easy enough that even I could do it.) Next easiest is multiply a large number of values by the same run-time-fixed value -- you do precomputation on the value in order to generate some tables. (The scary, fast variants use big tables; the still-a-little-scary variants use 256-byte tables, and get performance on high-end Intel boxes around 7-10 cycles per byte when doing GCM.) Full-on multiplication of two arbitrary GF(2^128) values is slowest still. As a further wrinkle with GF(2^128), OpenSSL doesn't seem to actually expose its "multiply in GF(2^128)" functions as far as I can tell: we would need to snarf such code from elsewhere. Oh! ARMv8 has an optional crypto instruction set that supports AES, SHA256, and GF(2^128) multiplication [ARMv8]. If it looks like most of the ARMs we care about are going to get it, that could factor into our planning. IV. HOW MUCH DOES PERFORMANCE MATTER HERE ANYWAY? A fine question. The right place too look would IMO be at profiles on a busy non-exit non-AESNI server, to see how much time is spent in AES there. Make sure you look at all the AES variants, since it's not uncommon to see more than one used at once. A ridiculously large amount of that time will be in bn_sqr4x_mont and its unindicted co-conspirators , but expect this to get cut a great deal when we move to ECC for TLS DHE and for circuit extension. So if we're willing to make a lot of assumptions, then you can figure out a worst-case performance impact like this. Take the fraction of the time spent in bn_and_friends and remove them from the profile. (Assume that circuit creation dominates over TLS renegotiation because hey why not.) Then assume that the AES calls used for relay crypto's counter mode turn into whatever new relay encryption thing we're proposing. See how much that change costs Tor's performance overall. V. ACKNOWLEDGMENTS Thanks to everybody who's schooled me about this stuff, especially Daniel J. Bernstein, Ian Goldberg, and Robert Ransom. Assume that the good ideas are here due to their patience and that the bad ideas are here due to my slowness on the uptake. Thanks to Susan Born for a much-needed copy-edit. Assume that every correctly structured sentence is one she fixed, and that the bad ones I messed up after her editing. VI. References [Prop202] https://lists.torproject.org/pipermail/tor-dev/2012-June/003649.html [Ferguson] Niels Ferguson, "Authentication weaknesses in GCM", 2005. http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/comments/CWC-GCM/Fergu… [DJB-Timing] Daniel J. Bernstein, "Cache-timing attacks on AES", 2004. http://cr.yp.to/papers.html#cachetiming [DJB-Comm] Daniel J. Bernstein, personal communication [Poly1305-Trunc] http://osdir.com/ml/encryption.poly1305/2005-09/msg00007.html [ARMv8] "ARMv8 Instruction Set Overview", 2011, http://board.flatassembler.net/download.php?id=5698 [Bear-Lion] Ross Anderson and Eli Biham. "Two Practical and Provably Secure Block Ciphers: BEAR and LION". http://www.cl.cam.ac.uk/~rja14/Papers/bear-lion.pdf [Efficient-Tweakable] Palash Sarkar, "Efficient Tweakable Enciphering Schemes from (Block-Wise) Universal Hash Functions". 2008. http://eprint.iacr.org/2008/004.pdf [IGE] Gligor and Donescu, "On Message Integrity in Symmetric Encryption" has the good IGE analysis: http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/ige/ige-… The original IGE proposal was by Carl Campell in an old NIST publication that I can't find online; the paper above has a reference for it if you want to chase it more. [ABC] Lars R. Knudsen, "Block Cipher Chaining Modes of Operation". http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/abc/abc-… [HCTR] Peng Wang, Dengguo Feng, and Wenling Wu. "HCTR: A variable-input-length enciphering mode." 2005. http://delta.cs.cinvestav.mx/~debrup/hctr.pdf [HCH] Debrup Chakraborty and Palash Sarkar. "HCH: A new tweakable enciphering scheme using the hash-encrypt-hash approach." http://biblioteca.cinvestav.mx/indicadores/texto_completo/cinvestav/2006/13… [TET] Shai Halevi. "Invertible universal hashing and the TET encryption mode." 2007. http://www.iacr.org/archive/crypto2007/46220405/46220405.pdf

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Proposal: Increase Acceptable Consensus Age
by Mike Perry 16 Oct '12

16 Oct '12

Also available at: https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/tolerate-old-… ---------------------------------- Title: Increase Acceptable Consensus Age Author: Mike Perry Created: 01-10-2012 Status: Open Target: 0.2.4.x+ Overview This proposal aims to extend the duration that clients will accept old consensus material under conditions where the directory authorities are either down or fail to produce a valid consensus for an extended period of time. Motivation Currently, if the directory authorities are down or fail to consense for 24 hours, the entire Tor network will cease to function. Worse, clients will enter into a state where they all need to re-bootstrap directly from the directory authorities, which will likely exacerbate any potential DoS condition that may have triggered the downtime in the first place. The Tor network has had such close calls before. In the past, we've been able to mobilize a majority of the directory authority operators within this 24 hour window, but that is only because we've been exceedingly lucky and the DoS conditions were accidental rather than deliberate. If a DoS attack was deliberately timed to coincide with a major US and European combined holiday such as Christmas Eve, New Years Eve, or Easter, it is very unlikely we would be able to muster the resources to diagnose and deploy a fix to the authorities in time to prevent network collapse. Description Based on the need to survive multi-day holidays and long weekends balanced with the need to ensure clients can't be captured on an old consensus forever, I propose that the consensus liveness constants be set at 3 days rather than 24 hours. This requires updating two consensus defines in the source, and one descriptor freshness variable. The descriptor freshness should be set to a function of the consensus freshness. See Implementation Notes for further details. Security Concerns: Using an Old Consensus Clients should not trust old consensus data without an attempt to download fresher data from a directory mirror. As far as I could tell, the code already does this. The minimum consensus age before we try to download new data is two hours. However, the ability to accept old consensus documents does introduce the ability of malicious directory mirrors to feed their favorite old consensus document to clients to alter their paths until they download a fresher consensus from elsewhere. Directory guards (Proposal 207) may exacerbate this ability. This proposal does not address such attacks, and seeks only a modest increase in the valid timespan as a compromise. Future consideration of these and other targeted-consensus attacks will be left to proposals related to ticket #7126[1]. Once those proposals are complete and implemented, raising the freshness limit beyond 3 days should be possible. Implementation Notes There appear to be at least three constants in the code involved with using potentially expired consensus data. Two of them (REASONABLY_LIVE_TIME and NS_EXPIRY_SLOP) involve the consensus itself, and two (OLD_ROUTER_DESC_MAX_AGE and TOLERATE_MICRODESC_AGE) deal with descriptor liveness. Two additional constants ROUTER_MAX_AGE and ROUTER_MAX_AGE_TO_PUBLISH are only used when submitting descriptors for consensus voting. FORCE_REGENERATE_DESCRIPTOR_INTERVAL is the maximum age a router descriptor will get before a relay will re-publish. It is set to 18 hours. OLD_ROUTER_DESC_MAX_AGE is set at 5 days. TOLERATE_MICRODESC_AGE is set at 7 days. The consensus timestamps are used in networkstatus_get_reasonably_live_consensus() and networkstatus_set_current_consensus(). OLD_ROUTER_DESC_MAX_AGE is checked in routerlist_remove_old_routers(), router_add_to_routerlist(), and client_would_use_router(). It is my opinion that we should combine REASONABLY_LIVE_TIME and NS_EXPIRY_SLOP into a single define, and make OLD_ROUTER_DESC_MAX_AGE a function of REASONABLY_LIVE_TIME and FORCE_REGENERATE_DESCRIPTOR_INTERVAL: #define REASONABLY_LIVE_TIME (3*24*60*60) #define NS_EXPIRY_SLOP REASONABLY_LIVE_TIME #define OLD_ROUTER_DESC_MAX_AGE \ (REASONABLY_LIVE_TIME+FORCE_REGENERATE_DESCRIPTOR_INTERVAL) Based on my review of the above code paths, these changes should be all we need to enable clients to use older consensuses for longer while still attempting to fetch new ones. 1. https://trac.torproject.org/projects/tor/ticket/7126 -- Mike Perry

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tor decrypt packet
by esolve esolve 16 Oct '12

16 Oct '12

HI, I capture packets on the tor client using tcpdump and I want to decrypt the captured packets for analysis. I think there are two steps 1 obtain the session keys 2 use some tools to decrypt the packets Are there any ways, tools, methodology to decrypt the packets? thanks!

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Re: [tor-dev] Proposal 211: Internal Mapaddress for Tor Configuration Testing
by Nick Mathewson 15 Oct '12

15 Oct '12

On Thu, Oct 11, 2012 at 5:38 AM, Mike Perry <mikeperry(a)torproject.org> wrote: > Also at: > https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/mapaddress-ch… > > --------------------------------------------------------------- > > Title: Internal Mapaddress for Tor Configuration Testing > Author: Mike Perry > Created: 08-10-2012 > Status: Open > Target: 0.2.4.x+ > > > Overview > > This proposal describes a method by which we can replace the > https://check.torproject.org/ testing service with an internal XML > document provided by the Tor client. > > Motivation > > The Tor Check service is a central point of failure in terms of Tor > usability. If it is ever out of sync with the set of exit nodes on the > Tor network or down, user experience is degraded considerably. Moreover, > the check itself is very time-consuming. Users must wait seconds or more > for the result to come back. Worse still, if the user's software *was* > in fact misconfigured, the check.torproject.org DNS resolution and > request leaks out on to the network. > > Design Overview > > The system will have three parts: an internal hard-coded IP address > mapping (127.84.111.114:80), a hard-coded mapaddress to a DNS name > (selftest.torproject.org:80), and a DirPortFrontPage-style simple > HTTP server that serves an XML document for both addresses. The use of XML and HTTP here are both reasons for some unhappiness. Both of them pull in a fair amount of complexity that I'd prefer not to need. (Yes, Tor already has a sort of an HTTP implementation, but at least clients aren't currently required to run what amounts to a local HTTP server.) I seriously wonder whether the benefits of HTTP (easier to access from within a locked-down web browser environment) aren't actually the _defects_ of HTTP here: it's easier to poke it from a web page. I understand that your design takes some steps to prevent browser-based attacks on this, but I'm not currently sure how to become sure that that it solves them all. Right now, I'm nervous. > Upon receipt of a request to the IP address mapping, the system will > create a new 128 bit randomly generated nonce and provide it > in the XML document. > > Requests to http://selftest.torproject.org/ must include a valid, > recent nonce as the GET url path. Upon receipt of a valid nonce, > it is removed from the list of valid nonces. Nonces are only valid > for 60 seconds or until SIGNAL NEWNYM, which ever comes first. So, I'm not totally sure what the nonce field is for. The idea as I understand it is that when you connect to the IPv4 address, you get a nonce, and later when you connect to the hostname, you provide that nonce, and Tor tells you "yes" if you gave it the same nonce. What does that protect against? My first thought is that you're trying to prevent the case where a malicious local DNS server maps "selftest.torproject.org" to some IP address in their control, and then just runs a server at that IP address to say "yes I'm Tor". But that doesn't make sense, since you could just make one of those that said "yes I'm Tor" no matter what you say for the nonce. Also, how useful is the followup DNS check? If it's checking that DNS leaks aren't happening... You're going to need torbrowser or something of equivalent complexity for this to work at all; isn't it easier then for torbrowser to make sure that it set up SOCKS ? > The list of pending nonces should not be allowed to grow beyond 10 > entries. This means that any webpage could flush out the list of pending nonces. Does that matter? > The timeout period and nonce limit should be configurable in torrc. > > Design: XML document format for http://127.84.111.114 > [...] > Security Considerations > > XML was chosen over JSON due to the risks of the identifier leaking > in a way that could enable websites to track the user[1]. Well, that's a nuclear-powered-flyswatter! If I read that page right, the problem with using JSON is that it can be parsed and executed as Javascript, and the advantage of XML is that it's unlikely to be syntactically correct javascript, then maybe instead we should If that's the issue, I'd strongly suggest that instead of going with a more complex data format, we could add a layer of encoding over the json, or use an even simpler format. > Because there are many exceptions and circumvention techniques > to the same-origin policy, we have also opted for strict controls > on dns-nonce lifetimes and usage, as well as validation of the Host > header and SOCKS4A request hostnames. Of course, this all comes down to the fact that we're using http. Can we spell out why we need HTTP for this? > 1. http://www.hpenterprisesecurity.com/vulncat/en/vulncat/dotnet/javascript_hi… > -- Nick

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Re: [tor-dev] Proposal 210: Faster Headless Consensus Bootstrapping
by Nick Mathewson 15 Oct '12

15 Oct '12

On Thu, Oct 11, 2012 at 5:32 AM, Mike Perry <mikeperry(a)torproject.org> wrote: > Also at: > https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/consensus-boo… > > ------------------------------------------------------------------------- > > Title: Faster Headless Consensus Bootstrapping > Author: Mike Perry > Created: 01-10-2012 > Status: Open > Target: 0.2.4.x+ > > > Overview and Motiviation > > This proposal describes a way for clients to fetch the initial > consensus more quickly in situations where some or all of the directory > authorities are unreachable. This proposal is meant to describe a > solution for bug #4483. > > Design: Bootstrap Process Changes > > The core idea is to attempt to establish bootstrap connections in > parallel during the bootstrap process, and download the consensus from > the first connection that completes. > > Connection attempts will be done in batches of three. Only one > connection will be performed to one of the canonical directory > authorities. Two connections will be performed to randomly chosen hard > coded directory mirrors. I misread this paragraph at first. I thought you were suggesting 3 parallel directory downloads when in fact you were discussing 3 parallel TLS connections, with only the first one that finishes actually getting a download. [...] > Design: Fallback Dir Mirror Selection Out of scope for this proposal; relevant for proposal 206. > Performance: Additional Load with Current Parameter Choices > > This design and the connection count parameters were chosen such that > no additional bandwidth load would be placed on the directory > authorities. In fact, the directory authorities should experience less > load, because they will not need to serve the consensus document for a > connection in the event that one of the directory mirrors complete their > connection before the directory authority does. To be clear, it's the part of this proposal that's shared with proposal 206 (directory sources) that would lower load on the authorities. > However, the scheme does place additional TLS connection load on the > fallback dir mirrors. Because bootstrapping is rare and all but one of > the TLS connections will be very short-lived and unused, this should not > be a substantial issue. How do we know that bootstrapping is rare? > The dangerous case is in the event of a prolonged consensus failure > that induces all clients to enter into the bootstrap process. In this > case, the number of initial TLS connections to the fallback dir mirrors > would be 2*C/100, or 10,000 for C=500,000 users. If no connections > complete before the five retries, this could reach as high as 50,000 > connection attempts, but this is extremely unlikely to happen in full > aggregate. > > However, in the no-consensus scenario today, the directory authorities > would already experience C/9 or 55,555 connection attempts. The > 5-retry scheme increases their total maximum load to about 275,000 > connection attempts, but again this is unlikely to be reached > in aggregate. Additionally, with this scheme, even if the dirauths > are taken down by this load, the dir mirrors should be able to survive > it. This looks like an argument of the form "The outcome would be horrible, but the current outcome is also horrible, so we wouldn't break stuff any worse." Right? I wonder if in this case the answer isn't to actually back off from fetching after N minutes or M servers, like a sane system. Or to treat "hey, that's not a good consensus!" as different from "couldn't connect to directory server" in terms of what it means for how we back off. -- Nick

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Proposal 207: Directory guards
by Nick Mathewson 15 Oct '12

15 Oct '12

Filename: 207-directory-guards.txt Title: Directory guards Author: Nick Mathewson Created: 10-Oct-2012 Status: Open Target: 0.2.4.x Motivation: When we added guard nodes to resist profiling attacks, we made it so that clients won't build general-purpose circuits through just any node. But clients don't use their guard nodes when downloading general-purpose directory information from the Tor network. This allows a directory cache, over time, to learn a large number of IPs for non-bridge-using users of the Tor network. Proposal: In the same way as they currently pick guard nodes as needed, adding more guards as those nodes are down, clients should also pick a small-ish set of directory guard nodes, to persist in Tor's state file. Clients should not pick their own guards as directory guards, or pick their directory guards as regular guards. When downloading a regular directory object (that is, not a hidden service descriptor), clients should prefer their directory guards first. Then they should try more directories from a recent consensus (if they have one) and pick one of those as a new guard if the existing guards are down and a new one is up. Failing that, they should fall back to a directory authority (or a directory source, if those get implemented-- see proposal 206). If a client has only one directory guard running, they should add new guards and try them, and then use their directory guards to fetch multiple descriptors in parallel. Discussion: The rule that the set of guards and the set of directory guards need to be disjoint, and the rule that multiple directory guards need to be providing descriptors, are both attempts to make it harder for a single node to capture a route. Open questions and notes: What properties does a node need to be a suitable directory guard? If we require that it have the Guard flag, we'll lose some nodes: only 74% of the directory caches have it (weighted by bandwidth). We may want to tune the algorithm used to update guards. For future-proofing, we may want to have the DirCache flag from 185 be the one that nodes must have in order to be directory guards. For now, we could have authorities set it to Guard && DirPort!=0, with a better algorithm to follow. Authorities should never get the DirCache flag.

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[patch] fix cparser/firm compiler warnings
by Christian Grothoff 14 Oct '12

14 Oct '12

Hi! We're trying to compile Tor with cparser and got some warnings and an error. src/or/dirserv.c:803:1: error: declaration 'was_router_added_t dirserv_add_extrainfo(extrainfo_t*, const char**)' is incompatible with 'int dirserv_add_extrainfo(extrainfo_t*, const char**)' (declared at line 96:12) src/or/dirserv.c:81:14: warning: unnecessary static forward declaration for 'char* format_versions_list(config_line_t*)' [-Wredundant-decls] src/or/dirserv.c:2186:8: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:2210:8: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:2752:51: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:3009:51: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:3280:112: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:3483:54: warning: statement is empty [-Wempty-statement] src/or/dirserv.c:3603:129: warning: statement is empty [-Wempty-statement] 1 error(s), 47 warning(s) The attached patch fixes those the warnings that are unambiguously due to inconsistent style ("if (cond) {body};" -- removing extra semicolon) and fixes the warning due to inconsistently declared a return type (once declared as 'enum', once as 'int'). Is there a better place for sending patches that do not really require broad discussion? Happy hacking! Christian

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txtorcon 0.6
by meejah＠meejah.ca 12 Oct '12

12 Oct '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 txtorcon 0.6 is now tagged and released. txtorcon is a Twisted-based Python asynchronous implementation of the Tor control protocol. It includes a state-tracking abstraction, configuration abstraction, Twisted endpoint support for hidden services, 96%+ unit-test coverage and many examples. New in this release: . debian packaging (mmaker); . psutil fully gone; . *changed API* for launch_tor() to use TorConfig instead of args; . TorConfig.save() works properly with no connected Tor; . fix incorrect handling of 650 immediately after connect; . pep8 compliance; . use assertEqual in tests; . messages with embdedded keywords work properly; . fix bug with setup.py + pip; . issue #15 reported along with patch by Isis Lovecruft; . consolidate requirements (from aagbsn); . increased test coverage and various minor fixes; . https URIs for ReadTheDocs Code and built documentation may be obtained: https://github.com/meejah/txtorcon https://txtorcon.readthedocs.org/en/latest/index.html git clone git://github.com/meejah/txtorcon.git You may also install it directly: pip install txtorcon easy_install txtorcon The sha256sum output is: 4f939e217ea3149175bb1285fa7296edc02ac245a71a60ec77b9cf48511e991e txtorcon-0.6.tar.gz thanks, meejah -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Processed by Mailcrypt 3.5.8 <http://mailcrypt.sourceforge.net/> iQEcBAEBAgAGBQJQd5uqAAoJEMJgKAMSgGmnTCkIAOGT6QaE+Q8jaBOedE79j/B3 QYMXh1SSfrvxn2x/utmjIgRbvCrCE+lzkxv9T9GXXjAhKSRN2xyyhRxq/Mbxvucd FWb2zHyZIjsTcc//veHYNpRNEWORl1ZIG2kU0muHQPpxY5ZAvtEkW+RA5zMJbhHp LF/JyltsAEu1Ja6HMFsfPmlLZiOtWBkBi3MgGGenUVJsScrtGRNCjWutGEkXxKXe 9pw2W0IGH5YmBAUiAuSW6q4cnyPQroqjSgO+RFfKMVGJNAaSqgUE8UCkcCifyjjM 2cDtGNA50kU0obT8QLgsiRyavePCQSfn9Jr1y8/Ck45Toa9a4FOQbBlOOyQSgrY= =bORU -----END PGP SIGNATURE-----

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Proposal: Internal Mapaddress for Tor Configuration Testing
by Mike Perry 11 Oct '12

11 Oct '12

Also at: https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/mapaddress-ch… --------------------------------------------------------------- Title: Internal Mapaddress for Tor Configuration Testing Author: Mike Perry Created: 08-10-2012 Status: Open Target: 0.2.4.x+ Overview This proposal describes a method by which we can replace the https://check.torproject.org/ testing service with an internal XML document provided by the Tor client. Motivation The Tor Check service is a central point of failure in terms of Tor usability. If it is ever out of sync with the set of exit nodes on the Tor network or down, user experience is degraded considerably. Moreover, the check itself is very time-consuming. Users must wait seconds or more for the result to come back. Worse still, if the user's software *was* in fact misconfigured, the check.torproject.org DNS resolution and request leaks out on to the network. Design Overview The system will have three parts: an internal hard-coded IP address mapping (127.84.111.114:80), a hard-coded mapaddress to a DNS name (selftest.torproject.org:80), and a DirPortFrontPage-style simple HTTP server that serves an XML document for both addresses. Upon receipt of a request to the IP address mapping, the system will create a new 128 bit randomly generated nonce and provide it in the XML document. Requests to http://selftest.torproject.org/ must include a valid, recent nonce as the GET url path. Upon receipt of a valid nonce, it is removed from the list of valid nonces. Nonces are only valid for 60 seconds or until SIGNAL NEWNYM, which ever comes first. The list of pending nonces should not be allowed to grow beyond 10 entries. The timeout period and nonce limit should be configurable in torrc. Design: XML document format for http://127.84.111.114 To avoid the need to localize the message in Tor, Tor will only provide a XML object with connectivity information. Here is an example form: <tor-test> <tor-bootstrap-percent>100</tor-bootstrap-percent> <tor-version-current>true</tor-version-current> <dns-nonce>4977eb4842c7c59fa5b830ac4da896d9</dns-nonce> <tor-test/> The tor-bootstrap-percent field represents the results of the Tor client bootstrap status as integer percentages from bootstrap_status_t. The tor-version-current field represents the results of the Tor client consensus version check. If the bootstrap process has not yet downloaded a consensus document, this field will have the value null. The dns-nonce field contains a 128-bit secret, encoded in base16. This field is only present for requests that list the Host: header as 127.84.111.114. Design: XML document format for http://selftest.torproject.org/nonce <tor-test> <tor-bootstrap-percent>100</tor-bootstrap-percent> <tor-version-current>true</tor-version-current> <dns-nonce-valid>true</dns-nonce-valid> <tor-test/> The first two fields are the same as for the IP address version. The dns-nonce-valid field is only true if the Host header matches selftest.torproject.org and the nonce is current and valid. Upon receipt of a valid nonce, that nonce is removed from the list of valid nonces. Design: Request Servicing Care must be taken with the dns-nonce generation and usage, to prevent users from being tracked through leakage of nonce value to application content. While the usage of XML appears to make this impossible due to stricter same-origin policy enforcement than JSON, same-origin enforcement is still fraught with exceptions and loopholes. In particular: Any requests that contain the Origin: header MUST be ignored, as the Origin: header is only included for third party web content (CORS). dns-nonce fields MUST be omitted if the HTTP Host: header does not match the IP address 127.84.111.114. Requests to selftest.torproject.org MUST return false for the dns-nonce-valid field if the HTTP Host: header does not match selftest.torproject.org, regardless of nonce value. Further, requests to selftest.torproject.org MUST validate that 'selftest.torproject.org' was the actual hostname provided to SOCKS4A, and not some alternate address mapping (due to DNS rebinding attacks, for example). Design: Application Usage Applications will use the system in two steps. First, they will make an HTTP request to http://127.84.111.114:80/ over Tor's SOCKS port and parse the resulting XML, if any. If the request at this stage fails, the application should inform the user that either their Tor client is too old, or that it is misconfigured, depending upon the nature of the failure. If the request succeeds and valid XML is returned, the application will record the value of the dns-nonce field, and then perform a second request to http://selftest.torproject.org/nonce_value. If the second request succeeds, and the dns-nonce-valid field is true, the application may inform the user that their Tor settings are valid. If the second request fails, or does not provide the correct dns-nonce, the application will inform the user that their Tor DNS proxy settings are incorrect. If either tor-bootstrap-percent is not 100, or tor-version-current is false, applications may choose to inform the user of these facts using properly localized strings and appropriate UI. Security Considerations XML was chosen over JSON due to the risks of the identifier leaking in a way that could enable websites to track the user[1]. Because there are many exceptions and circumvention techniques to the same-origin policy, we have also opted for strict controls on dns-nonce lifetimes and usage, as well as validation of the Host header and SOCKS4A request hostnames. 1. http://www.hpenterprisesecurity.com/vulncat/en/vulncat/dotnet/javascript_hi… -- Mike Perry

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Proposal: Faster Headless Consensus Bootstrapping
by Mike Perry 11 Oct '12

11 Oct '12

Also at: https://gitweb.torproject.org/user/mikeperry/torspec.git/blob/consensus-boo… ------------------------------------------------------------------------- Title: Faster Headless Consensus Bootstrapping Author: Mike Perry Created: 01-10-2012 Status: Open Target: 0.2.4.x+ Overview and Motiviation This proposal describes a way for clients to fetch the initial consensus more quickly in situations where some or all of the directory authorities are unreachable. This proposal is meant to describe a solution for bug #4483. Design: Bootstrap Process Changes The core idea is to attempt to establish bootstrap connections in parallel during the bootstrap process, and download the consensus from the first connection that completes. Connection attempts will be done in batches of three. Only one connection will be performed to one of the canonical directory authorities. Two connections will be performed to randomly chosen hard coded directory mirrors. If no connections complete within 5 seconds, another batch of three connections will be launched. Otherwise, the first connection to complete will be used to download the consensus document and the others will be closed, after which bootstrapping will proceed as normal. If at any time, the total outstanding bootstrap connection attempts exceeds 15, no new connection attempts are to be launched until existing connection attempts experience full timeout. Design: Fallback Dir Mirror Selection The set of hard coded directory mirrors from #572 shall be chosen using the 100 Guard nodes with the longest uptime. The fallback weights will be set using each mirror's fraction of consensus bandwidth out of the total of all 100 mirrors. This list of fallback dir mirrors should be updated with every major Tor release. In future releases, the number of dir mirrors should be set at 20% of the current Guard nodes, rather than fixed at 100. Performance: Additional Load with Current Parameter Choices This design and the connection count parameters were chosen such that no additional bandwidth load would be placed on the directory authorities. In fact, the directory authorities should experience less load, because they will not need to serve the consensus document for a connection in the event that one of the directory mirrors complete their connection before the directory authority does. However, the scheme does place additional TLS connection load on the fallback dir mirrors. Because bootstrapping is rare and all but one of the TLS connections will be very short-lived and unused, this should not be a substantial issue. The dangerous case is in the event of a prolonged consensus failure that induces all clients to enter into the bootstrap process. In this case, the number of initial TLS connections to the fallback dir mirrors would be 2*C/100, or 10,000 for C=500,000 users. If no connections complete before the five retries, this could reach as high as 50,000 connection attempts, but this is extremely unlikely to happen in full aggregate. However, in the no-consensus scenario today, the directory authorities would already experience C/9 or 55,555 connection attempts. The 5-retry scheme increases their total maximum load to about 275,000 connection attempts, but again this is unlikely to be reached in aggregate. Additionally, with this scheme, even if the dirauths are taken down by this load, the dir mirrors should be able to survive it. Implementation Notes: Code Modifications The implementation of the bootstrap process is unfortunately mixed in with many types of directory activity. The process starts in update_consensus_networkstatus_downloads(), which initiates a single directory connection through directory_get_from_dirserver(). Depending on bootstrap state, a single directory server is selected and a connection is eventually made through directory_initiate_command_rend(). There appear to be a few options for altering this code to perform multiple connections. Without refactoring, one approach would be to make multiple calls to directory_initiate_command_routerstatus() from directory_get_from_dirserver() if the purpose is DIR_PURPOSE_FETCH_CONSENSUS and the only directory servers available are the authorities and the fallback dir mirrors. The code in directory_initiate_command_rend() would then need to be altered to maintain a list of the dircons created for this purpose as well as avoid immediately queuing the directory_send_command() request for the DIR_PURPOSE_FETCH_CONSENSUS purpose. A flag would need to be set on the dircon to be checked in connection_dir_finished_connecting(). The function connection_dir_finished_connecting() would need to be altered to examine the list of pending dircons, determine if this one is the first to complete, and if so, then call directory_send_command() to download the consensus and close the other pending dircons. An additional timer would need to be installed to re-call update_consensus_networkstatus_downloads() or a related helper after 5 seconds. connection_dir_finished_connecting() would cancel this timer. The helper would check the list of pending connections and ensure it never exceeds 15. -- Mike Perry

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