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Improved circuit-setup protocol [was: Re: Designing and implementing improved circuit-setup protocol [was: GSoC 2011]]
by Nick Mathewson 08 Apr '11

08 Apr '11

On Thu, Mar 31, 2011 at 5:52 AM, Robert Ransom <rransom.8774(a)gmail.com> wrote: Hi! I'm going to wait on a full review of your create/extend proposal till it's done, but I though I could potentially answer some questions and offer some comments: 1) I think CREATE cells need to get a field asking for a specific way of handling RELAY cells. If we ever update the relay crypto (that is, the stuff described in 5.5 and 6.1 today) to do something other than AES128_CTR for the encryption, SHA1 for the digest, we'll need a way to ask for it. I suppose that we could do that via adding a new handshake type for a new desired link protocol, but it seems like the two could be more or less orthogonal. 2) I don't get the rationale behind the variable-length type names. Elsewhere in our protocol we'd just use a 1- or 2-byte type field. Do we win a lot by letting these be longer? 3) By convention, everything in our protocol is in "network" (big-endian) byte-order. 4) There needs to be an authenticated way for the client to know which handshake types and link specifier types a given node supports. This could be as simple as a series of statements like "Tor versions xyz and later will support handshake type Y", or as complex as advertising them in router descriptors. 5) I like the idea of separating the link specifiers from the handshake type in EXTENDED cells. 6) I also think it'd be smart to figure out the actual link specifier and handshake types that we want to move to before we decide that we are confident that this format is right. Those can be a different proposal, though. 7) Here's a first cut of what I think might go in a link specifier format: * V4Address -- an ipv4 address, set to 0 if there is no IPv4 address for the node [4 bytes] * V4ORPort [2 bytes] * V6Address -- an ipv6 address, set to 0 if there is no IPv6 address for the node [16 bytes] * V6ORPort [2 bytes] * SHA256 of RSA1024 identity key [32 bytes] When we decide what longer identity keys should look like, we can add a new link specifier format that supports those. 8) This is totally back-of-the-envelope stuff, but it might be a good starting point for crypto discussion. Here's a first cut of what I think might go in an improved RSA handshake: * First 8 bytes of the SHA256 hash of the onion key [8 bytes] (This is here so that onion key rotation can work without having to sometimes try the wrong onion key incorrectly.) * PK-encrypted: * Padding [PK_PAD_LEN bytes] * SHA256 hash of all remaining fields. [32 bytes] * Symmetric key seed [16 bytes] * The first part of g^x. [as much will fit in the PK-encrypted portion] * Symmetrically encrypted: * The rest of g^x. * 0 bytes for padding. Here's a first cut of what I think might go in a hypothetical diffie-hellman based handshake, for use with something like Curve25519. (I'm using g^x and g^y notation here as if this were diffie-hellman in Z_p, since I don't yet trust myself to write ECC stuff correctly. I'm assuming that the node's public onion key is g^x.) * SHA256 of all remaining fields. [32 bytes] * First 8 bytes of the SHA256 hash of the onion key (8 bytes) * g^y1 [DH_LEN bytes] * Encrypted using a symmetric key based on g^(x*y1): * g^y2 [DH_LEN bytes] * 0 bytes for padding In both cases, we'll want a new key derivation function. yrs, -- Nick

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[Patch] src/test.c
by Gisle Vanem 07 Apr '11

07 Apr '11

I don't think it's a good idea to use hard-coded paths. Even in a test source-file. Easy patch: --- Git-latest\src\test\test.c Wed Mar 30 11:58:28 2011 +++ src\test\test.c Thu Mar 31 14:06:14 2011 @@ -86,7 +86,7 @@ #ifdef MS_WINDOWS // XXXX tor_snprintf(temp_dir, sizeof(temp_dir), - "c:\\windows\\temp\\tor_test_%d", (int)getpid()); + "%s\\tor_test_%d", getenv("TEMP"), (int)getpid()); r = mkdir(temp_dir); #else tor_snprintf(temp_dir, sizeof(temp_dir), "/tmp/tor_test_%d", (int) getpid()); ---------- --gv

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Embedding Parrot in Tor as GSoC Project
by Jonathan "Duke" Leto 07 Apr '11

07 Apr '11

Howdy, Parrot recently added a GSoC proposal idea to embed Parrot into Tor. It would be great to get the feedback from Tor developers. Also, this could be under the Parrot or the Tor GSoC project, whichever makes the most sense. The proposal idea could use more detail, but the high-level view that I imagine is: 1) Allow Parrot to talk to libtor (this will use NCI - Native Call Interface) via PIR 2) Ability to create Parrot interpreter objects from within Tor via C 3) Write glue code for a High Level Language (HLL) to talk to libtor There are various HLL's to choose from: Lua, TCL, Perl 6, Winxed, NQP and others. Duke Thanks for your help, Duke [0] http://trac.parrot.org/parrot/wiki/GSoc2011 -- Jonathan "Duke" Leto jonathan(a)leto.net http://leto.net

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GSoC - Torbutton for Thunderbird
by Sukhbir Singh 05 Apr '11

05 Apr '11

Hi! I am interested in participating in Google's Summer of Code this year and implementing the Torbutton equivalent for Thunderbird [0]. I have been discussing the project with Mike Perry and we feel that it is possible to implement this. The first step was removing the hostname Thunderbird puts in the outgoing message header, which we easily fixed. That gave us the initial motivation to pursue this further. There are a number of issue that still remain, however we feel that those can be easily worked out. We have put all probable solutions to Torbutton requirements from [1] in the attachment with this mail and are looking for your suggestions/ comments about this. We have not submitted an application for this to Google yet; I intend to do that after gauging the response towards this proposal from the mailing list. Let's hope that we can make this a success, -- Thanks, Sukhbir Singh [0] - https://www.torproject.org/getinvolved/volunteer.html.en#torbuttonForThunde… [1] - https://www.torproject.org/torbutton/en/design/#requirements Attachment - Plain text

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Torbutton for Thunderbird
by Sukhbir Singh 04 Apr '11

04 Apr '11

Hi! I am interested in participating in Google's Summer of Code this year and implementing the Torbutton equivalent for Thunderbird [0]. I have been discussing the project with Mike Perry and we feel that it is possible to implement this. The first step was removing the hostname Thunderbird puts in the outgoing message header, which we easily fixed. That gave us the initial motivation to pursue this further. There are a number of issue that still remain, however we feel that those can be easily worked out. We have put all probable solutions to Torbutton requirements from [1] in the attachment with this mail and are looking for your suggestions/ comments about this. We have not submitted an application for this to Google yet; I intend to do that after gauging the response towards this proposal from the mailing list. Let's hope that we can make this a success, -- Thanks, Sukhbir Singh [0] - https://www.torproject.org/getinvolved/volunteer.html.en#torbuttonForThunde… [1] - https://www.torproject.org/torbutton/en/design/#requirements Attachment - Plain text

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First draft: Thandy Package Format spec
by Erinn Clark 31 Mar '11

31 Mar '11

Also available here: https://gitweb.torproject.org/erinn/thandy.git/blob/refs/heads/thp-draft-sp… Title: Specification for a TBB/Thandy package format. Status: Draft Authors: nickm, erinn Started-On: 9 Feb 2011 Finished-On: Introduction On some platforms, in some environments, Thandy can use existing platform-provided mechanisms for packages. But for the Tor Browser Bundle, and for Windows, we can't use built-in packaging systems (because they put data in a database or registry, because they require root). Requirements, Goals: Thandy has these requirements for a packaging system: - It needs to be able to install/upgrade packages. - It needs to be able to check which version of a package is installed For use in TBB, we we need a few more features: - It needs to be able to remove packages - It needs to be able to leave cofiguration files alone when upgrading To use this right, Thandy needs these features: - download packages as-needed in the background - report when packages are ready to install - Have a way to upgrade the TBB as it restarts To avoid big problems, package installation needs these features: - Idempotence - Validatability - Spending as little time as possible in non-functional states. (We can't get true atomicity on some OSs, but we should get as much as reasonable as we can.) Nongoals: This is a packagesystem for TBB and similar things for use with Thandy. It is not a more general system, a replacement for rpm/deb, an all-purpose software distribution mechanism, or a dessert topping. This is not the spec for Thandy. This is not a spec for interfaces to Thandy. It lists some interfaces that are necessary but it doesn't explain how they work. This is not the spec for any modes of operation for thandy, interfaces to thandy, or a thandy net installer. We need specs for those, of course. Though this document has suggestions on how to make good packages and install them well, there may be additional requirements for a high-quality installer not listed here. We're trying to design this system to _support_ being the most solid tool it can be, but we're not trying to figure out every possible implementation detail here. Mode of operation: While TBB is running, Vidalia should periodically launch Thandy, telling it, "Fetch packages as needed" or "Tell me if you could fetch packages." While TBB is running, Vidalia should periodically launch Thandy, asking it, "Are there packages downloaded and ready to install?" If so, it should tell the user. When TBB first starts, it should start a launcher program that asks Thandy, "Are there packages downloaded and ready to install?" If so, it should tell Thandy to install them. To make Thandy and the launcher self-upgrade, here's a two-step process: the install process installs new stuff into a new directory to the side of the old directory, and then launches a new "replacer" program to move the new stuff over the old stuff. The replacer, when it's done, re-launches the launcher. "It's not pretty, and you can't dance to it." - Frank Zappa The "thandy installable" file format: The file metaformat is a zip file, with the file extension .thp. It has these directories: content/ meta/ The "content" tree has the actual package contents, in a directory layout mirroring the layout of the installed package. The "meta" tree has information about the package. The meta tree has one required file, "package.json". Its contents are a single json object, with the following required fields: 'format-version' The number 1. An installer SHOULD NOT try to install a package with a format-version that it doesn't recognize. 'manifest' A list of files relative to the thp's content directory. Each file is an object with these fields: 'name': the name of the file relative to thp's content directory 'digest': optionally, a 2-tuple of a digest algorithm and a hexadecimal digest value. The following algorithms are supported: SHA256. 'length': optionally, the length of the file in bytes 'isconfig': optionally, a boolean. If it is present and true, this file is considered "configuration". 'package-name' The name of this package. There shouldn't be two packages with the same name installed at once in the same hierarchy. Ex: "Tor". 'package-version' The version of the package as a human-readable string. This should include both the version of the thing being packaged, and the version of of the package itself. Ex: "0.2.2.35-rc-7" is the seventh release of a thp file for Tor 0.2.2.35-rc. 'package-version-tuple' The version of the package as a list of numbers and strings such that a lexical comparison of two package version tuples is a correct version comparison. Ex: [ 0, 2, 2, 35, "rc", 7 ] 'timestamp' The time when this thp was generated, as a YYYY-MM-DD HH:MM:SS string, relative to UTC. Ex: "2011-03-02 17:33:07" 'additional-files' Optional: A list of files or file sets relative to the install root, to decribe files that the package is responsible for, even if they're not distributed with the package. This is used to kill off temporary and cache files on uninstall. Each file is an object with these fields: 'name': The name of the file relative to the install root. This name may contain "*" and "**" file-globbing patterns. 'isconfig': as for manifest. 'install-order' Optional: number between 0 and 100 inclusive to indicate that this package must be installed before or after others. Defaults to "50". 'options' Optional: a map from option strings to values. Known options are: 'cycle-install': This package should be installed to a separate install root from the currently installed package, then moved over. 'platform' Optional. The OS and CPU type that this package is for. 'require-features' Optional. A list of strings naming installer features that the installer needs to support to install this package correctly. An installer SHOULD NOT try to install the package if it does not recognize and support all members of this list. Ex: [ "pythonscripts" ] 'require-packages' Optional. A list of objects for all packages that must be installed before this package can be installed. Each object has these fields: 'package-name' 'min-version-tuple' 'scripts' Optional: a map from scripting language to set of scripts. Supported scripting languages are python2, sh, none. (We can add more later.) If the 'scripts' field is present, the installer SHOULD NOT install the package unless it supports one or more of the scripting languages. Each set of scripts contains one or more of the following fields: 'checkinst' 'preinst' 'postinst' 'prerm' 'postrm' Each names a file relative to meta/scripts in the thp zip file. Implementations SHOULD NOT generate other files or subdirectories of the main zip root directory; implementations MUST ignore files and subdirectories that they do not recognize. The package database: The installer keeps a directory that contains files describing the status of the packages we have installed. It has a subdirectory: "pkg-status". "pkg-status" has, for each package, two files: packagename.json, and packagename.status. Optionally, it has a packagename.json.new file. The packagename.json file contains a copy of the package.json file from the most recent successfully installed version of the the package. The packagename.status file contains a json object containing at least the field 'status' set to one of the following: "INSTALLED", "IN-PROGRESS". If a package install or upgrade is in-progress, packagename.json.new has the package.json file from the new version of the package. Scripts: Each script should be callable by a language- and platform-specific calling convention for invoking a script with named arguments. The arguments to the script are: THP_PACKAGE_NAME THP_OLD_VERSION (absent if we are doing a fresh installation) THP_NEW_VERSION THP_OLD_INSTALL_ROOT THP_INSTALL_ROOT THP_JSON_FILE THP_VERBOSE (flag: "1" if the script should log verbosely.) THP_PURGE (flag: "1" if we are doing a remove and we want to get rid of absolutely everything.) THP_TEMP_DIR For sh and python2 scripts, these arguments are passed as environment variables. Scripts need a way to signal success or failure. For sh and python2 scripts, this is done via return value. The 'none' script type must never have any scripts. Installers should never choose it if they support any other script type: it only exists to tell the installer that the scripts are optional. Directories: All installation happens relative to a single "install root". Thandy maintains a cache directory of its own, containing (among other things) downloaded thp files. The thp installer has a database directory explaining package status. Steps of operation: Checking for and downloading packages is done by thandy, and out-of-scope here, except inasmuch as Thandy needs the thp installer to say which version (if any) of package X is installed. The installer can do this by looking at the X.status file and the X.json file. To validate a package, the thp installer verifies that all files listed in the manifest are in fact installed with the sizes and digests listed, unless they are config files. {THIS NEXT PART IS A DRAFT AND NEEDS MORE THOUGHT! IT COULD BE WAY MORE ATOMIC} Installing and updating a downloaded package is done by Thandy teling the thp installer, "install/update these packages" with a list of thp files. To do this, the installer: * Grabs a lockfile. * Finds the current version, if any, of all of the packages. * Runs the checkinst script if present for every package that might need installing or updating. If any fails, the update can't happen. * For each package, sorted in topological order by 'requires' relationships, with ties broken by 'install-order' fields: * Overwrite the status file for the package in the database, changing the status to "IN-PROGRESS". Copy the package's package.json file to packagename.json.new in the database. * Run the package's preinst script. If that fails, abort. * Unpack all files in the package's content fork to their target locations. If files tagged 'config' are present, ignore them. * If any files are listed in the manifest for the old version of the package but not for the new version, and they are not config files, remove them. (Have an option to override this?) * Run the postinst script * Replace the packagename.json file with the packagename.json.new file. * Change the package status to "INSTALLED". CHANGES TO MAKE TO THE INSTALL PROCESS ABOVE: - Instead of overwrite-as-we-go, perhaps have overwrite as the very last step? - Perhaps checkpoint all files first for easy rollback? - Specify how to do the cycle-install feature. "Is it... atomic?" "Yes! VERY atomic!" -- The 5000 Fingers of Dr T Future directions and open questions: F.1 Configuration file updates We need a smart way to handle configuration file updates and changes in the future. There are several three-way merge tools available that we can model our behavior on, or re-use the code of, such as Debian's ucf tool. F.1 Binary patching There are auto-update tools (courgette for Chrome, as an example) which do smart binary patching, thus often drastically reducing the download time of updates. When the tool is more mature, we should look into ways to do this.

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Designing and implementing improved circuit-setup protocol [was: GSoC 2011]
by Nick Mathewson 31 Mar '11

31 Mar '11

Quoting the whole message in my reply, since we're moving this to tor-dev where it belongs. On Wed, Mar 23, 2011 at 1:36 PM, Robert Ransom <rransom.8774(a)gmail.com> wrote: > On Wed, 23 Mar 2011 12:54:17 -0400 > Nick Mathewson <nickm(a)freehaven.net> wrote: > >> On Wed, Mar 23, 2011 at 8:10 AM, George Kadianakis <desnacked(a)gmail.com> wrote: >> > Hey Robert and Nick, >> > >> > I hate bureaucratic procedures and yet I have to send my GSoC >> > application 'till the 8th of April; so I want to get done with it. >> > >> > My application should have "a project proposal, why you'd like to >> > execute on this particular project, and the reason you're the best >> > individual to do so." (that was taken from GSoC FAQ). >> > >> > So regarding the project proposal, I talked with Robert yesterday and he >> > told me: >> > 02:18:24 <rransom> asn: Implementing the new circuit protocol will be >> > a good GSoC project for you. I think identity key >> > migration (and thus link protocol migration) will >> > require too much beyond the new circuit protocol. >> > I suppose that by "new circuit protocol" you mean the section 3.2 of >> > xxx-crypto-migration.txt, Robert. >> > >> > How do you feel about this, Nick? >> >> It could be helpful. I don't have a good sense of how much work this >> would be, since I don't have a good sense of what the new protocol >> would actually be. Robert -- how would you think this would break >> down into a summer's worth of work? > > The first step in the Great Tor Crypto Migration is to add new CREATE2 > and EXTEND2 RELAY cell types. They can be used with the existing > circuit-extension handshake and link protocol initially, but will be > extensible to support new ones. > > Further steps, all independent of each other: > > * Add 128-byte and 2048-byte RELAY cells and a circuit-configuration > cell, initially to allow the client to change the cell size to be > used on a circuit. > > * Refactor Tor's cryptographic primitive abstractions to accommodate > public-key encryption primitives, public-key signature primitives, > symmetric authenticated encryption, symmetric block encryption, and > hashes. > > * Implement one or more new link protocols that do not constrain a > relay's choice of identity key cryptosystem. > > Further mutually independent steps building on those above: > > * Modify the directory protocol and implementation to support relays > with multiple identity keys. > > * Implement a new circuit-extension handshake (the part that involves > ‘onionskins’). > > * Implement a new circuit ciphersuite (the part that mangles cell data > so that relay A can't see what data relay C sees). Hm. These steps all stretch pretty far beyond what's just described in 3.2 of xxx-crypto-migration. I think they're probably more than we can promise to design before summer, and possibly more than a typical gsoc scope all put together. >> I'm also a little concerned about the interaction of 3.2 and 3.3 >> ("Relay crypto") : I'll be surprised if it turns out that we can >> design a good circuit extend protocol without thinking about the >> countours of a new relay protocol. (Not that you'd have to build both >> at once, but we should think about them all as we design.) > > It's actually the other way around -- we need a new EXTEND cell before > we can use a new link protocol. (Otherwise, we would have to build in > a covert channel (i.e. a backdoor for people who want to block Tor by > handshake) in the new link protocol to indicate client and server link > protocol versions, and that really *really* sucks.) I'm talking about the stuff in 3.3: the relay protocol, where we process cells. Link protocol stuff is 3.1. Also, I'm talking about *design order*, not *implementation order*: The different parts of the Tor protocol are not sufficiently orthogonal that we can do them independently. Thus, we need to get most of the design changes sketched out before we can be reasonably say that any part of the redesign does what the other parts need. >> Maybe we should get a protocol sketch together this week if the app is >> due April 8. > > Yes. I have the EXTEND2 cell draft written; I bogged down on writing > explanatory text (I thought I didn't have enough in the draft, but > didn't know what to add). > > >> Either way, I think this might be drifting into design work, so if >> there are no objections, I'd suggest that we follow up on the tor-dev >> mailing list. :) > > Good idea. > > > Robert Ransom >

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(revised) Proposal 180: Pluggable Transports for Circumvention
by Nick Mathewson 21 Mar '11

21 Mar '11

We sent out an earlier version of this, well, earlier. Here it is more complete and more revised. I've tried to incorporate people's comments. There's some worthwhile stuff it doesn't do yet, like provide automatic and easy ways to compose plugins. I think it should work well enough, but more comment is always worthwhile. yrs, -- Nick ===== Filename: 180-pluggable-transport.txt Title: Pluggable transports for circumvention Author: Jacob Appelbaum, Nick Mathewson Created: 15-Oct-2010 Status: Open Overview This proposal describes a way to decouple protocol-level obfuscation from the core Tor protocol in order to better resist client-bridge censorship. Our approach is to specify a means to add pluggable transport implementations to Tor clients and bridges so that they can negotiate a superencipherment for the Tor protocol. Scope This is a document about transport plugins; it does not cover discovery improvements, or bridgedb improvements. While these requirements might be solved by a program that also functions as a transport plugin, this proposal only covers the requirements and operation of transport plugins. Motivation Frequently, people want to try a novel circumvention method to help users connect to Tor bridges. Some of these methods are already pretty easy to deploy: if the user knows an unblocked VPN or open SOCKS proxy, they can just use that with the Tor client today. Less easy to deploy are methods that require participation by both the client and the bridge. In order of increasing sophistication, we might want to support: 1. A protocol obfuscation tool that transforms the output of a TLS connection into something that looks like HTTP as it leaves the client, and back to TLS as it arrives at the bridge. 2. An additional authentication step that a client would need to perform for a given bridge before being allowed to connect. 3. An information passing system that uses a side-channel in some existing protocol to convey traffic between a client and a bridge without the two of them ever communicating directly. 4. A set of clients to tunnel client->bridge traffic over an existing large p2p network, such that the bridge is known by an identifier in that network rather than by an IP address. We could in theory support these almost fine with Tor as it stands today: every Tor client can take a SOCKS proxy to use for its outgoing traffic, so a suitable client proxy could handle the client's traffic and connections on its behalf, while a corresponding program on the bridge side could handle the bridge's side of the protocol transformation. Nevertheless, there are some reasons to add support for transportation plugins to Tor itself: 1. It would be good for bridges to have a standard way to advertise which transports they support, so that clients can have multiple local transport proxies, and automatically use the right one for the right bridge. 2. There are some changes to our architecture that we'll need for a system like this to work. For testing purposes, if a bridge blocks off its regular ORPort and instead has an obfuscated ORPort, the bridge authority has no way to test it. Also, unless the bridge has some way to tell that the bridge-side proxy at 127.0.0.1 is not the origin of all the connections it is relaying, it might decide that there are too many connections from 127.0.0.1, and start paring them down to avoid a DoS. 3. Censorship and anticensorship techniques often evolve faster than the typical Tor release cycle. As such, it's a good idea to provide ways to test out new anticensorship mechanisms on a more rapid basis. 4. Transport obfuscation is a relatively distinct problem from the other privacy problems that Tor tries to solve, and it requires a fairly distinct skill-set from hacking the rest of Tor. By decoupling transport obfuscation from the Tor core, we hope to encourage people working on transport obfuscation who would otherwise not be interested in hacking Tor. 5. Finally, we hope that defining a generic transport obfuscation plugin mechanism will be useful to other anticensorship projects. Non-Goals We're not going to talk about automatic verification of plugin correctness and safety via sandboxing, proof-carrying code, or whatever. We need to do more with discovery and distribution, but that's not what this proposal is about. We're pretty convinced that the problems are sufficiently orthogonal that we should be fine so long as we don't preclude a single program from implementing both transport and discovery extensions. This proposal is not about what transport plugins are the best ones for people to write. We do, however, make some general recommendations for plugin authors in an appendix. We've considered issues involved with completely replacing Tor's TLS with another encryption layer, rather than layering it inside the obfuscation layer. We describe how to do this in an appendix to the current proposal, though we are not currently sure whether it's a good idea to implement. We deliberately reject any design that would involve linking more code into Tor's process space. Design overview To write a new transport protocol, an implementer must provide two pieces: a "Client Proxy" to run at the initiator side, and a "Server Proxy" to run at the server side. These two pieces may or may not be implemented by the same program. Each client may run any number of Client Proxies. Each one acts like a SOCKS proxy that accepts connections on localhost. Each one runs on a different port, and implements one or more transport methods. If the protocol has any parameters, they are passed from Tor inside the regular username/password parts of the SOCKS protocol. Bridges (and maybe relays) may run any number of Server Proxies: these programs provide an interface like stunnel: they get connections from the network (typically by listening for connections on the network) and relay them to the Bridge's real ORPort. To configure one of these programs, it should be sufficient simply to list it in your torrc. The program tells Tor which transports it provides. The Tor consensus should carry a new approved version number that is specific for pluggable transport; this will allow Tor to know when a particular transport is known to be unsafe safe or non-functional. Bridges (and maybe relays) report in their descriptors which transport protocols they support. This information can be copied into bridge lines. Bridges using a transport protocol may have multiple bridge lines. Any methods that are wildly successful, we can bake into Tor. Specifications: Client behavior We extend the bridge line format to allow you to say which method to use to connect to a bridge. The new format is: "bridge method address:port [[keyid=]id-fingerprint] [k=v] [k=v] [k=v]" To connect to such a bridge, the Tor program needs to know which local SOCKS proxy will support the transport called "method". It then connects to this proxy, and asks it to connect to address:port. If [id-fingerprint] is provided, Tor should expect the public identity key on the TLS connection to match the digest provided in [id-fingerprint]. If any [k=v] items are provided, they are configuration parameters for the proxy: Tor should separate them with semicolons and put them in the user and password fields of the request, splitting them across the fields as necessary. If a key or value value must contain a semicolon or a backslash, it is escaped with a backslash. The "id-fingerprint" field is always provided in a field named "keyid", if it was given. Method names must be C identifiers. Example: if the bridge line is "bridge trebuchet www.example.com:3333 rocks=20 height=5.6m" AND if the Tor client knows that the 'trebuchet' method is provided by a SOCKS5 proxy on 127.0.0.1:19999, the client should connect to that proxy, ask it to connect to www.example.com, and provide the string "rocks=20;height=5.6m" as the username, the password, or split across the username and password. There are two ways to tell Tor clients about protocol proxies: external proxies and managed proxies. An external proxy is configured with ClientTransportPlugin method socks4 address:port [auth=X] or ClientTransportPlugin method socks5 address:port [username=X] [password=Y] as in "ClientTransportPlugin trebuchet socks5 127.0.0.1:9999". This example tells Tor that another program is already running to handle 'trubuchet' connections, and Tor doesn't need to worry about it. A managed proxy is configured with ClientTransportPlugin <method> exec <path> [options] as in "ClientTransportPlugin trebuchet exec /usr/libexec/trebuchet --managed" This example tells Tor to launch an external program to provide a socks proxy for 'trebuchet' connections. The Tor client only launches one instance of each external program with a given set of options, even if the same executable and options are listed for more than one method. If instead of a transport name, the torrc lists "*" for a managed proxy, tor uses that proxy for all transports that it supports. So "ClientTransportPlugin * exec /usr/libexec/tor/foobar" tells Tor that it should use the foobar plugin for everything that it supports. If two proxies support the same method, Tor should use whichever one is listed first. The same program can implement a managed or an external proxy: it just needs to take an argument saying which one to be. See "Managed proxy behavior" for more information on the managed proxy interface. Server behavior Server proxies are configured similarly to client proxies. When launching a proxy, the server must tell it what ORPort it has configured, and what address (if any) it can listen on. The server must tell the proxy which (if any) methods it should provide if it can; the proxy needs to tell the server which methods it is actually providing, and on what ports. When a client connects to the proxy, the proxy may need a way to tell the server some identifier for the client address. It does this in-band. As before, the server lists proxies in its torrc. These can be external proxies that run on their own, or managedproxies that Tor launches. An external server proxy is configured as ServerTransportPlugin method proxy address:port param=val.. as in ServerTransportPlugin trebuchet proxy 127.0.0.1:999 rocks=heavy The param=val pairs and the address are used to make the bridge configuration information that we'll tell users. A managed proxy is configured as ServerTransportPlugin method exec /path/to/binary [options] or ServerTransportPlugin * exec /path/to/binary [options] When possible, Tor should launch only one binary of each binary/option pair configured. So if the torrc contains ClientTransportPlugin foo exec /usr/bin/megaproxy --foo ClientTransportPlugin bar exec /usr/bin/megaproxy --bar ServerTransportPlugin * exec /usr/bin/megaproxy --foo then Tor will launch the megaproxy binary twice: once with the option --foo and once with the option --bar. Managed proxy interface When the Tor client launches a client proxy from the command line, it communicates via environment variables. At a minimum, it sets: {Client and server} HOME, PATH -- as you'd expect. "STATE_LOCATION" -- a directory where the proxy should store state if it wants to. This directory is not required to exist, but the proxy SHOULD be able to create it if it doesn't. The proxy SHOULD NOT store state elsewhere. "MANAGED_TRANSPORT_VER=1" -- To tell the proxy which versions of this configuration protocol Tor supports. Future versions will give a comma-separated list. Clients MUST accept comma-separated lists containing any version that they recognize, and MUST work correctly even if some of the versions they don't recognize are non-numeric. {Client only} "CLIENT_TRANSPORTS" -- a comma-separated list of which methods this client should enable, or * if all methods should be enabled. The proxy SHOULD ignore methods that it doesn't recognize. {Server only} "EXT_SERVER_PORT=addr:portnum" -- A port (probably on localhost) that speaks the extended server protocol. "ORPORT=addr:portnum" -- Our regular ORPort in a form suitable for local connections. "BINDADDR=addr" -- An address on which to listen for local connections. This might be the advertised address, or might be a local address that Tor will forward ports to. It MUST be an address that will work with bind(). "SERVER_TRANSPORTS=..." -- A comma-separated list of server methods that the proxy should support, or * The transport proxy replies by writing NL-terminated lines to stdout. The metaformat is Keyword OptArgs NL OptArgs = Args | Args = SP ArgChar | Args ArgChar ArgChar = Any character but NUL or NL Keyword = KeywordChar | Keyword KeywordChar KeyWordChar = All alphanumeric characters, dash, and underscore. Tor MUST ignore lines with keywords that it doesn't recognize. First, the proxy writes "VERSION 1" to say that it supports this protocol. It must either pick a version that Tor told it about, or pick no version at all, and say "ERROR no-version\n" and exit. The proxy should then open its ports. If running as a client proxy, it should not use fixed ports; instead it should autoselect ports to avoid conflicts. A client proxy should by default only listen on localhost for connections. A server proxy SHOULD try listen at a consistent port, though it SHOULD pick a different one if the port it last used is now allocated. A client or server proxy then should tell which methods it has made available and how. It does this by printing zero or more CMETHOD and SMETHOD lines to its stdout. These lines look like: CMETHOD methodname SOCKS4/SOCKS5 address:port [ARGS=arglist] \ [OPT-ARGS=arglist] as in CMETHOD trebuchet SOCKS5 127.0.0.1:19999 ARGS=rocks,height \ OPT-ARGS=tensile-strength The ARGS field lists mandatory parameters that must appear in every bridge line for this method. The OPT-ARGS field lists optional parameters. If no ARGS or OPT-ARGS field is provided, Tor should not check the parameters in bridge lines for this method. The proxy should print a single "CMETHODS DONE" line after it is finished telling Tor about the client methods it provides. If it tries to supply a client method but can't for some reason, it should say: CMETHOD-ERROR methodname "Message" A proxy should tell Tor about the server methods it is providing by printing zero or more SMETHOD lines. These lines look like: SMETHOD methodname address:port [Options] If there's an error setting up a configured server method, the proxy should say: SMETHOD-ERROR methodname "message" The 'address:port' part of an SMETHOD line is the address to put in the bridge line. The ARGS: part is a list of key-value pairs that the client needs to know. The Options part is a list of space-separated K:V flags that Tor should know about. Recognized options are: - FORWARD:1 If this option is set, and address:port is not a publicly accessible address, then the bridge needs to forward some other address:port to address:port via upnp-helper. - ARGS:k=v,k=v,k=v If this option is set, the K=V arguments are added to the extrainfo document. - DECLARE:K=V,... If this option is set, all the K=V options should be added as extension entries to the router descriptor. (See below) - USE-EXTPORT:1 If this option is set, the server plugin is using the extended server port. SMETHOD and CMETHOD lines may be interspersed. After the list SMETHOD line, the proxy says "SMETHODS DONE" The proxy SHOULD NOT tell Tor about a server or client method unless it is actually open and ready to use. Tor clients SHOULD NOT use any method from a client proxy or advertise any method from a server proxy UNLESS it is listed as a possible method for that proxy in torrc, and it is listed by the proxy as a method it supports. Proxies should respond to a single INT signal by closing their listener ports and not accepting any new connections, but keeping all connections open, then terminating when connections are all closed. Proxies should respond to a second INT signal by shutting down cleanly. The extended ORPort protocol. Server transports may need to connect to the bridge and pass additional information about client connections that the bridge would ordinarily receive from the kernel's TCP stack. To to this, they connect to the "extended server port" as given in SERVER_PORT, sent a short amount of information, wait for a response, and then send the user traffic on that port. The extended server port protocol is as follows: COMMAND [2 bytes, big-endian] BODYLEN [2 bytes, big-endian] BODY [Bodylen bytes] Commands sent from the transport to the server are: [0x0000] DONE: There is no more information to give. (body ignored) [0x0001] USERADDR: an address:port string that represents the user's address. If the transport doesn't actually do addresses, this shouldn't be sent. Replies sent from tor to the proxy are: [0x1001] OKAY: Send the user's traffic. (body ignored) [0x1002] DENY: Tor would prefer not to get more traffic from this address for a while. (body ignored) [We could also use an out-of-band signalling method to tell Tor about client addresses, but that's a historically error-prone way to go about annotating connections.] Advertising bridge methods: Bridges put the 'method' lines in their extra-info documents. method SP methodname SP address:port SP arglist NL The address:port parse are as returned from an SMETHOD line. The arglist is a K=V,... list as retuned in the ARGS part of the SMETHOD line. If the SMETHOD line includes a DECLARE: part, the routerinfo gets a new line: method-info SP methodname SP arglist NL Bridge authority behavior We need to specify a way to test different transport methods that bridges claim to support. We should test as many as possible. We should NOT require that we have a way to tra Bridgedb behavior: Bridgedb can, given a set of router descriptors and their corresponding extrainfo documents, generate a set of bridge lines for each descriptor. Bridgedb may want to avoid handing out methods that seem to get bridges blocked quickly. Implementation plan First, we should implement per-bridge socks settings (as described above in "manually configuring a client proxy for a bridge") and the extended-server-port mechanism. This will let bridges run transport proxies such that they can hand-generate bridge lines to give to clients for testing. Once that's done, we can improve usability a little bit by implementing external proxies. Once that's done, we can see if we need any managed proxies, or if the whole idea there is silly. If we do, the next most important part seems to be getting the client-side automatic part written. And once that's done, we can evaluate how much of the server side is easy for people to do and how much is hard. The "obfsproxy" obfuscating proxy is a likely candidate for an initial transport, as is Steven Murdoch's http thing or something similar. Notes on plugins to write: We should ship a couple of null plugin implementations in one or two popular, portable languages so that people get an idea of how to write the stuff. 1. We should have one that's just a proof of concept that does nothing but transfer bytes back and forth. 1. We should not do a rot13 one. 2. We should implement a basic proxy that does not transform the bytes at all 1. We should implement DNS or HTTP using other software (as goodesll did years ago with DNS) as an example of wrapping existing code into our plugin model. 2. The obfuscated-ssh superencipherment is pretty trivial and pretty useful. It makes the protocol stringwise unfingerprintable. 1. Nick needs to be told firmly not to bikeshed the obfuscated-ssh superencipherment too badly 1. Go ahead, bikeshed my day 1. If we do a raw-traffic proxy, openssh tunnels would be the logical choice. Appendix: recommendations for transports Be free/open-source software. Also, if you think your code might someday do so well at circumvention that it should be implemented inside Tor, it should use the same license as Tor. Use libraries that Tor already requires. (You can rely on openssl and libevent being present if current Tor is present.) Be portable: most Tor users are on Windows, and most Tor developers are not, so designing your code for just one of these platforms will make it either get a small userbase, or poor auditing. Think secure: if your code is in a C-like language, and it's hard to read it and become convinced it's safe, then it's probably not safe. Think small: we want to minimize the bytes that a Windows user needs to download for a transport client. Avoid security-through-obscurity if possible. Specify. Resist trivial fingerprinting: There should be no good string or regex to search for to distinguish your protocol from protocols permitted by censors. Imitate a real profile: There are many ways to implement most protocols -- and in many cases, most possible variants of a given protocol won't actually exist in the wild.

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Logging [Enhancement]
by grarpamp 18 Mar '11

18 Mar '11

- An FYI, found while grokking, thanks. - These various log entries could really have their format normalized sometime. - If it's going to tell us why it's giving up in one case, it should do so in all cases. - The fingerprints should be printed instead of the ambigous router names. - Eliminate personal pronouns 'We'. - Remove 'Have'. - Shorten seconds to sec. - Change 'get a connection' to 'build a circuit' - Remove quoting, parenthesis, etc. Or create, and consistently use, parseable entries. We tried for 15 seconds to connect to '[scrubbed]' using exit 'Unnamed'. Retrying on a new circuit. Have tried resolving or connecting to address '[scrubbed]' at 3 different places. Giving up. Tried for 123 seconds to get a connection to [scrubbed]:80. Giving up. Tried for 120 seconds to get a connection to [scrubbed]:80. Giving up. (waiting for circuit) Tried for 120 seconds to get a connection to [scrubbed]:0. Giving up. (waiting for socks info)

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DefenestraTor: Throwing out Windows in Tor
by Roger Dingledine 14 Mar '11

14 Mar '11

http://www.cacr.math.uwaterloo.ca/techreports/2011/cacr2011-06.pdf "Tor is the most widely used privacy enhancing technology for achieving online anonymity and resisting censorship. While conventional wisdom dictates that the level of anonymity offered by Tor increases as its user base grows, the most significant obstacle to Tor adoption contin- ues to be its slow performance. We seek to enhance Tor's performance by offering techniques to control congestion and improve flow control, thereby reducing unnecessary delays. To reduce congestion, we first evaluate small fixed-size circuit win- dows and a dynamic circuit window that adaptively resizes in response to perceived congestion. While these solutions improve web page response times and require modification only to exit routers, they generally offer poor flow control and slower downloads relative to Tor's current design. To improve flow control while reducing congestion, we implement N23, an ATM-style per-link algorithm that allows Tor routers to explicitly cap their queue lengths and signal congestion via back-pressure. Our results show that N23 offers better congestion and flow control, resulting in improved web page response times and faster page loads compared to Tor's current design and the other window-based approaches. We also argue that our proposals do not enable any new attacks on Tor users' privacy. " I haven't read it yet, but figured I'd share it with others here in the meantime. --Roger

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