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Title: Initial thoughts on migrating Tor to new cryptography
Author: Nick Mathewson
Created: 12 December 2010
1. Introduction
Tor currently uses AES-128, RSA-1024, and SHA1. Even though these
ciphers were a decent choice back in 2003, and even though attacking
these algorithms is by no means the best way for a well-funded
adversary to attack users (correlation attacks are still cheaper, even
with pessimistic assumptions about the security of each cipher), we
will want to move to better algorithms in the future. Indeed, if
migrating to a new ciphersuite were simple, we would probably have
already moved to RSA-1024/AES-128/SHA256 or something like that.
So it's a good idea to start figuring out how we can move to better
ciphers. Unfortunately, this is a bit nontrivial, so before we start
doing the design work here, we should start by examining the issues
involved. Robert Ransom and I both decided to spend this weekend
writing up documents of this type so that we can see how much two
people working independently agree on. I know more Tor than Robert;
Robert knows far more cryptography than I do. With luck we'll
complement each other's work nicely.
A note on scope: This document WILL NOT attempt to pick a new cipher
or set of ciphers. Instead, it's about how to migrate to new ciphers
in general. Any algorithms mentioned other than those we use today
are just for illustration.
Also, I don't much consider the importance of updating each particular
usage; only the methods that you'd use to do it.
Also, this isn't a complete proposal.
2. General principles and tricks
Before I get started, let's talk about some general design issues.
2.1. Many algorithms or few?
Protocols like TLS and OpenPGP allow a wide choice of cryptographic
algorithms; so long as the sender and receiver (or the responder and
initiator) have at least one mutually acceptable algorithm, they can
converge upon it and send each other messages.
This isn't the best choice for anonymity designs. If two clients
support a different set of algorithms, then an attacker can tell them
apart. A protocol with N ciphersuites would in principle split
clients into 2**N-1 sets. (In practice, nearly all users will use the
default, and most users who choose _not_ to use the default will do so
without considering the loss of anonymity. See "Anonymity Loves
Company: Usability and the Network Effect".)
On the other hand, building only one ciphersuite into Tor has a flaw
of its own: it has proven difficult to migrate to another one. So
perhaps instead of specifying only a single new ciphersuite, we should
specify more than one, with plans to switch over (based on a flag in
the consensus or some other secure signal) once the first choice of
algorithms start looking iffy. This switch-based approach would seem
especially easy for parameterizable stuff like key sizes.
2.2. Waiting for old clients and servers to upgrade
The easiest way to implement a shift in algorithms would be to declare
a "flag day": once we have the new versions of the protocols
implemented, pick a day by which everybody must upgrade to the new
software. Before this day, the software would have the old behavior;
after this way, it would use the improved behavior.
Tor tries to avoid flag days whenever possible; they have well-known
issues. First, since a number of our users don't automatically
update, it can take a while for people to upgrade to new versions of
our software. Second and more worryingly, it's hard to get adequate
testing for new behavior that is off-by-default. Flag days in other
systems have been known to leave whole networks more or less
inoperable for months; we should not trust in our skill to avoid
similar problems.
So if we're avoiding flag days, what can we do?
* We can add _support_ for new behavior early, and have clients use it
where it's available. (Clients know the advertised versions of the
Tor servers they use-- but see 2.3 below for a danger here, and 2.4
for a bigger danger.)
* We can remove misfeatures that _prevent_ deployment of new
behavior. For instance, if a certain key length has an arbitrary
1024-bit limit, we can remove that arbitrary limitation.
* Once an optional new behavior is ubiquitous enough, the authorities
can stop accepting descriptors from servers that do not have it
until they upgrade.
It is far easier to remove arbitrary limitations than to make other
changes; such changes are generally safe to back-port to older stable
release series. But in general, it's much better to avoid any plans
that require waiting for any version of Tor to no longer be in common
use: a stable release can take on the order of 2.5 years to start
dropping off the radar. Thandy might fix that, but even if a perfect
Thandy release comes out tomorrow, we'll still have lots of older
clients and relays not using it.
We'll have to approach the migration problem on a case-by-case basis
as we consider the algorithms used by Tor and how to change them.
2.3. Early adopters and other partitioning dangers
It's pretty much unavoidable that clients running software that speak
the new version of any protocol will be distinguishable from those
that cannot speak the new version. This is inevitable, though we
could try to minimize the number of such partitioning sets by having
features turned on in the same release rather than one-at-a-time.
Another option here is to have new protocols controlled by a
configuration tri-state with values "on", "off", and "auto". The
"auto" value means to look at the consensus to decide wither to use
the feature; the other two values are self-explanatory. We'd ship
clients with the feature set to "auto" by default, with people only
using "on" for testing.
If we're worried about early client-side implementations of a protocol
turning out to be broken, we can have the consensus value say _which_
versions should turn on the protocol.
2.4. Avoid whole-circuit switches
One risky kind of protocol migration is a feature that gets used only
when all the routers in a circuit support it. If such a feature is
implemented by few relays, then each relay learns a lot about the rest
of the path by seeing it used. On the other hand, if the feature is
implemented by most relays, then a relay learns a lot about the rest of
the path when the feature is *not* used.
It's okay to have a feature that can be only used if two consecutive
routers in the patch support it: each router knows the ones adjacent
to it, after all, so knowing what version of Tor they're running is no
big deal.
2.5. The Second System Effect rears its ugly head
Any attempt at improving Tor's crypto is likely to involve changes
throughout the Tor protocol. We should be aware of the risks of
falling into what Fred Brooks called the "Second System Effect": when
redesigning a fielded system, it's always tempting to try to shovel in
every possible change that one ever wanted to make to it.
This is a fine time to make parts of our protocol that weren't
previously versionable into ones that are easier to upgrade in the
future. This probably _isn't_ time to redesign every aspect of the
Tor protocol that anybody finds problematic.
2.6. Low-hanging fruit and well-lit areas
Not all parts of Tor are tightly covered. If it's possible to upgrade
different parts of the system at different rates from one another, we
should consider doing the stuff we can do easier, earlier.
But remember the story of the policeman who finds a drunk under a
streetlamp, staring at the ground? The cop asks, "What are you
doing?" The drunk says, "I'm looking for my keys!" "Oh, did you drop
them around here?" says the policeman. "No," says the drunk, "But the
light is so much better here!"
Or less proverbially: Simply because a change is easiest, does not
mean it is the best use of our time. We should avoid getting bogged
down solving the _easy_ aspects of our system unless they happen also
to be _important_.
2.7. Nice safe boring codes
Let's avoid, to the extent that we can:
- being the primary user of any cryptographic construction or
protocol.
- anything that hasn't gotten much attention in the literature.
- anything we would have to implement from scratch
- anything without a nice BSD-licensed C implementation
Sometimes we'll have the choice of a more efficient algorithm or a
more boring & well-analyzed one. We should not even consider trading
conservative design for efficiency unless we are firmly in the
critical path.
2.8. Key restrictions
Our spec says that RSA exponents should be 65537, but our code never
checks for that. If we want to bolster resistance against collision
attacks, we could check this requirement. To the best of my
knowledge, nothing violates it except for tools like "shallot" that
generate cute memorable .onion names. If we want to be nice to
shallot users, we could check the requirement for everything *except*
hidden service identity keys.
3. Aspects of Tor's cryptography, and thoughts on how to upgrade them all
3.1. Link cryptography
Tor uses TLS for its link cryptography; it is easy to add more
ciphersuites to the acceptable list, or increase the length of
link-crypto public keys, or increase the length of the DH parameter,
or sign the X509 certificates with any digest algorithm that OpenSSL
clients will support. Current Tor versions do not check any of these
against expected values.
The identity key used to sign the second certificate in the current
handshake protocol, however, is harder to change, since it needs to
match up with what we see in the router descriptor for the router
we're connecting to. See notes on router identity below. So long as
the certificate chain is ultimately authenticated by a RSA-1024 key,
it's not clear whether making the link RSA key longer on its own
really improves matters or not.
Recall also that for anti-fingerprinting reasons, we're thinking of
revising the protocol handshake sometime in the 0.2.3.x timeframe.
If we do that, that might be a good time to make sure that we aren't
limited by the old identity key size.
3.2. Circuit-extend crypto
Currently, our code requires RSA onion keys to be 1024 bits long.
Additionally, current nodes will not deliver an EXTEND cell unless it
is the right length.
For this, we might add a second, longer onion-key to router
descriptors, and a second CREATE2 cell to open new circuits
using this key type. It should contain not only the onionskin, but
also information on onionskin version and ciphersuite. Onionskins
generated for CREATE2 cells should use a larger DH group as well, and
keys should be derived from DH results using a better digest algorithm.
We should remove the length limit on EXTEND cells, backported to all
supported stable versions; call these "EXTEND2" cells. Call these
"lightly patched". Clients could use the new EXTEND2/CREATE2 format
whenever using a lightly patched or new server to extend to a new
server, and the old EXTEND/CREATE format otherwise.
The new onion skin format should try to avoid the design oddities of
our old one. Instead of its current iffy hybrid encryption scheme, it
should probably do something more like a BEAR/LIONESS operation with a
fixed key on the g^x value, followed by a public key encryption on the
start of the encrypted data. (Robert reminded me about this
construction.)
The current EXTEND cell format ends with a router identity
fingerprint, which is used by the extended-from router to authenticate
the extended-to router when it connects. Changes to this will
interact with changes to how long an identity key can be and to the
link protocol; see notes on the link protocol above and about router
identity below.
3.2.1. Circuit-extend crypto: fast case
When we do unauthenticated circuit extends with CREATE/CREATED_FAST,
the two input values are combined with SHA1. I believe that's okay;
using any entropy here at all is overkill.
3.3. Relay crypto
Upon receiving relay cells, a router transforms the payload portion of
the cell with the appropriate key appropriate key, sees if it
recognizes the cell (the recognized field is zero, the digest field is
correct, the cell is outbound), and passes them on if not. It is
possible for each hop in the circuit to handle the relay crypto
differently; nobody but the client and the hop in question need to
coordinate their operations.
It's not clear, though, whether updating the relay crypto algorithms
would help anything, unless we changed the whole relay cell processing
format too. The stream cipher is good enough, and the use of 4 bytes
of digest does not have enough bits to provide cryptographic strength,
no matter what cipher we use.
This is the likeliest area for the second-system effect to strike;
there are lots of opportunities to try to be more clever than we are
now.
3.4. Router identity
This is one of the hardest things to change. Right now, routers are
identified by a "fingerprint" equal to the SHA1 hash of their 1024-bit
identity key as given in their router descriptor. No existing Tor
will accept any other size of identity key, or any other hash
algorithm. The identity key itself is used:
- To sign the router descriptors
- To sign link-key certificates
- To determine the least significant bits of circuit IDs used on a
Tor instance's links (see tor-spec §5.1)
The fingerprint is used:
- To identify a router identity key in EXTEND cells
- To identify a router identity key in bridge lines
- Throughout the controller interface
- To fetch bridge descriptors for a bridge
- To identify a particular router throughout the codebase
- In the .exit notation.
- By the controller to identify nodes
- To identify servers in the logs
- Probably other places too
To begin to allow other key types, key lengths, and hash functions, we
would either need to wait till all current Tors are obsolete, or allow
routers to have more than one identity for a while.
To allow routers to have more than one identity, we need to
cross-certify identity keys. We can do this trivially, in theory, by
listing both keys in the router descriptor and having both identities
sign the descriptor. In practice, we will need to analyze this pretty
carefully to avoid attacks where one key is completely fake aimed to
trick old clients somehow.
Upgrading the hash algorithm once would be easy: just say that all
new-type keys get hashed using the new hash algorithm. Remaining
future-proof could be tricky.
This is one of the hardest areas to update; "SHA1 of identity key" is
assumed in so many places throughout Tor that we'll probably need a
lot of design work to work with something else.
3.5. Directory objects
Fortunately, the problem is not so bad for consensuses themselves,
because:
- Authority identity keys are allowed to be RSA keys of any length;
in practice I think they are all 3072 bits.
- Authority signing keys are also allowed to be of any length.
AFAIK the code works with longer signing keys just fine.
- Currently, votes are hashed with both sha1 and sha256; adding
more hash algorithms isn't so hard.
- Microdescriptor consensuses are all signed using sha256. While
regular consensuses are signed using sha1, exploitable collisions
are hard to come up with, since once you had a collision, you
would need to get a majority of other authorities to agree to
generate it.
Router descriptors are currently identified by SHA1 digests of their
identity keys and descriptor digests in regular consensuses, and by
SHA1 digests of identity keys and SHA256 digests of microdescriptors
in microdesc consensuses. The consensus-flavors design allows us to
generate new flavors of consensus that identity routers by new hashes
of their identity keys. Alternatively, existing consensuses could be
expanded to contain more hashes, though that would have some space
concerns.
Router descriptors themselves are signed using RSA-1024 identity keys
and SHA1. For information on updating identity keys, see above.
Router descriptors and extra-info documents cross-certify one another
using SHA1.
Microdescriptors are currently specified to contain exactly one
onion key, of length 1024 bits.
3.6. The directory protocol
Most objects are indexed by SHA1 hash of an identity key or a
descriptor object. Adding more hash types wouldn't be a huge problem
at the directory cache level.
3.7. The hidden service protocol
Hidden services self-identify by a 1024-bit RSA key. Other key
lengths are not supported. This key is turned into an 80 bit half
SHA-1 hash for hidden service names.
The most simple change here would be to set an interface for putting
the whole ugly SHA1 hash in the hidden service name. Remember that
this needs to coexist with the authentication system which also uses
.onion hostnames; that hostnames top out around 255 characters and and
their components top out at 63.
Currently, ESTABLISH_INTRO cells take a key length parameter, so in
theory they allow longer keys. The rest of the protocol assumes that
this will be hashed into a 20-byte SHA1 identifier. Changing that
would require changes at the introduction point as well as the hidden
service.
The parsing code for hidden service descriptors currently enforce a
1024-bit identity key, though this does not seem to be described in
the specification. Changing that would be at least as hard as doing
it for regular identity keys.
Fortunately, hidden services are nearly completely orthogonal to
everything else.

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Title: Requirements for Tor's circuit cryptography
Author: Robert Ransom
Created: 12 December 2010
Overview
This draft is intended to specify the meaning of 'secure' for a Tor
circuit protocol, hopefully in enough detail that
mathematically-inclined cryptographers can use this definition to
prove that a Tor circuit protocol (or component thereof) is secure
under reasonably well-accepted assumptions.
Tor's current circuit protocol consists of the CREATE, CREATED, RELAY,
DESTROY, CREATE_FAST, CREATED_FAST, and RELAY_EARLY cells (including
all subtypes of RELAY and RELAY_EARLY cells). Tor currently has two
circuit-extension handshake protocols: one consists of the CREATE and
CREATED cells; the other, used only over the TLS connection to the
first node in a circuit, consists of the CREATE_FAST and CREATED_FAST
cells.
Requirements
1. Every circuit-extension handshake protocol must provide forward
secrecy -- the protocol must allow both the client and the relay to
destroy, immediately after a circuit is closed, enough key material
that no attacker who can eavesdrop on all handshake and circuit cells
and who can seize and inspect the client and relay after the circuit
is closed will be able to decrypt any non-handshake data sent along
the circuit.
In particular, the protocol must not require that a key which can be
used to decrypt non-handshake data be stored for a predetermined
period of time, as such a key must be written to persistent storage.
2. Every circuit-extension handshake protocol must specify what key
material must be used only once in order to allow unlinkability of
circuit-extension handshakes.
3. Every circuit-extension handshake protocol must authenticate the relay
to the client -- an attacker who can eavesdrop on all handshake and
circuit cells and who can participate in handshakes with the client
must not be able to determine a symmetric session key that a circuit
will use without either knowing a secret key corresponding to a
handshake-authentication public key published by the relay or breaking
a cryptosystem for which the relay published a
handshake-authentication public key.
4. Every circuit-extension handshake protocol must ensure that neither
the client nor the relay can cause the handshake to result in a
predetermined symmetric session key.
5. Every circuit-extension handshake protocol should ensure that an
attacker who can predict the relay's ephemeral secret input to the
handshake and can eavesdrop on all handshake and circuit cells, but
does not know a secret key corresponding to the
handshake-authentication public key used in the handshake, cannot
break the handshake-authentication public key's cryptosystem, and
cannot predict the client's ephemeral secret input to the handshake,
cannot predict the symmetric session keys used for the resulting
circuit.
6. The circuit protocol must specify an end-to-end flow-control
mechanism, and must allow for the addition of new mechanisms.
7. The circuit protocol should specify the statistics to be exchanged
between circuit endpoints in order to support end-to-end flow control,
and should specify how such statistics can be verified.
8. The circuit protocol should allow an endpoint to verify that the other
endpoint is participating in an end-to-end flow-control protocol
honestly.

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Filename: xxx-pluggable-transport.txt
Title: Pluggable transports for circumvention
Author: Jacob Appelbaum, Nick Mathewson
Created: 15-Oct-2010
Status: Draft
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 a 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 accept connections on localhost. Each one
runs on a different port, and implements one or more transport
methods. If the protocol has any parameters, they 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-server (or whatever the
option is): 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.
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
Bridge lines can now follow the extended format "bridge method
address:port [[keyid=]id-fingerprint] [k=v] [k=v] [k=v]". To connect
to such a bridge, a client must open a local connection to the SOCKS
proxy for "method", and ask it to connect to address:port. If
[id-fingerprint] is provided, it 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
NUL bytes and put them user and password fields of the request,
splitting them across the fields as necessary. The "id-fingerprint"
field is always provided in a field named "keyid", if it was given.
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\0height=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 trebuchet socks5 127.0.0.1:9999". This
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 trebuchet
/usr/libexec/tor-proxies/trebuchet [options]", and tells Tor to launch
an external program on-demand to provide a socks proxy for 'trebuchet'
connections. The Tor client only launches one instance of each
external program, even if the same executable is listed for more than
one method.
The same program can implement a managed or an external proxy: it just
needs to take an argument saying which one to be.
Client proxy behavior
When launched from the command-line by a Tor client, a transport
proxy needs to tell Tor which methods and ports it supports. It does
this by printing one or more CMETHOD: lines to its stdout. These look
like
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 "METHODS:DONE" line after it is
finished telling Tor about the methods it provides.
The transport proxy MUST exit cleanly when it receives a SIGTERM from
Tor.
The Tor client MUST ignore lines beginning with a keyword and a colon
if it does not recognize the keyword.
In the future, if we need a control mechanism, we can use the
stdin/stdout from Tor to the transport proxy.
A transport proxy MUST handle SOCKS connect requests using the SOCKS
version it advertises.
Tor clients SHOULD NOT use any method from a client proxy unless it
is both listed as a possible method for that proxy in torrc, and it
is listed by the proxy as a method it supports.
[XXXX say something about versioning.]
Server behavior
Server proxies are configured similarly to client proxies.
Server proxy behavior
[so, we can have this work like client proxies, where the bridge
launches some programs, and they tell the bridge, "I am giving you
method X with parameters Y"? Do you have to take all the methods? If
not, which do you specify?]
[Do we allow programs that get started independently?]
[We'll need to figure out how this works with port forwarding. Is
port forwarding the bridge's problem, the proxy's problem, or some
combination of the two?]
[If we're using the bridge authority/bridgedb system for distributing
bridge info, the right place to advertise bridge lines is probably
the extrainfo document. We also need a way to tell the bridge
authority "don't give out a default bridge line for me"]
Server behavior
Bridge authority behavior
Implementation plan
Turn this into a draft proposal
Circulate and discuss on or-dev.
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 goodell
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.
Specify: if you can't come up with a good explanation
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.
Appendix: Raw-traffic transports
This section describes an optional extension to the proposal above.
We are not sure whether it is a good idea.