[tor-dev] is the consensus document unpredictable / unique?

Razvan Dragomirescu razvan.dragomirescu at veri.fi
Sun Jun 26 12:32:55 UTC 2016

Thank you s7r, Tom,

I'll try to explain what I'm doing - I'm working on something called
SIM4Things - it's an Internet of Things project, giving Internet-connected
objects a persistent, cryptographically secure identity and a way to reach
similar objects. The closest analogy is the SIM card in the GSM / mobile
world (hence the name :) ). The identity is actually an RSA keypair, stored
in a tamper-resistant microSD form factor secure element (like this one
https://www.swissbit.com/ps-100u/ ).

The project does multiple things - first, it gives the node an identity -
the RSA private key inside is used to sign a hidden service descriptor (a
la OnionBalance) that is then published. As long as the device has access
to the smartcard, it can sign descriptors. Once the card is removed, it can
no longer do that.

Second, using hidden services means that the devices become accessible at a
single .onion address regardless of how they connect to the Internet and
how many firewalls and/or NAT gateways they are behind.

I'm very close to having a fully functional proof of concept on this tiny
. It runs OpenWRT. A Python script using STEM connects to Tor and to the
internal smartcard, fetches the hidden service descriptor as published by
Tor and modifies / re-signs it to point to the address associated with its
public key (keeps the introduction points, rewrites everything else). I
know this will no longer work with Prop 224 but afaik Prop224 is still 1-2
years away. Once the new descriptor is published, the node can talk to any
other similar node over the Tor network.

I want to offer the same guarantees that a regular SIM card inside your
phone would offer - as long as you have the SIM, you can join the network
and talk to other nodes. Once the SIM is gone, you should no longer be able
to do so. It should also be impossible (or very hard) to clone such a SIM
card and it should be impossible (or hard) to generate hidden service
descriptors in advance (that would allow you to join the network even after
the SIM has been removed).

So, to summarize - I'm doing a SIM card for the Internet of Things. The SIM
is a microSD tamper-resistant secure element with an RSA key inside. It
gives the node an identity (strongly tied to the physical SIM) and a way to
talk to similar nodes, with no central server or censorship opportunity.

If you have any questions, feel free to ask.


Razvan Dragomirescu
Chief Technology Officer
Cayenne Graphics SRL

On Sun, Jun 26, 2016 at 1:29 AM, s7r <s7r at sky-ip.org> wrote:

> Hello,
> If you hash the consensus entirely, yes that should produce unique
> hashes every time that are unpredictable until the consensus is available.
> However, you cannot guarantee it will be the same value for everyone at
> a given time, because consensus documents overlap and two clients/relays
> might work properly but not use exactly the same consensus at a given
> time (at 13:45 "Y" uses consensus document valid after 12:00 and "Z"
> uses consensus document valid after 13:00. Both are within their
> valid-until limits).
> I don't recommend that your rely on what you've suggested because of
> pour security properties and too complicated design with too many
> possible failure cases to be worth it.
> Very soon Tor will include an unique random value we call "Consensus
> Shared Randomness [SRVALUE]" in every consensus, you can just use that.
> This is proposal 250. This seams like a better standardized upstream
> solution with infinite better security properties, so I'd use this as a
> cookie. This has the advantage of having the same unique value on all
> nodes all the time: just filter and pair consensus SRVALUE + consensus
> valid-after timestamp and everyone will be requesting the SRVALUE of the
> same consensus, therefor producing the same result or fail if none.
> Last but not least, how will your system work in practice? The hidden
> service private key will be stored on a smartcard and it cannot be
> copied, it will only sign descriptors at the request of the host. So far
> so good, but the smartcard has to stay plugged in the host all the time,
> or at least all the time the hidden service is running, so what's the
> security property here?
> If you think you can manually plug in the smartcard rarely just to sign
> descriptors and keep it somewhere else physically most of the time, this
> will not work. In the wild things happen that demand new descriptors to
> be signed in an unpredictable way: introduction points go offline;
> HSDirs go offline, too many INTRODUCE2 cells received on a single
> introduction point circuit.
> And if the private key is on a smartcard, and the smartcard is plugged
> in the host all the time, what's the gain? I am not saying there isn't
> any, I just don't see it at this moment. One I can think of is that
> malware and/or someone hacking can't copy the private key and hijack the
> hidden service, but the risk remains in case someone physically sizes
> the server ("host").
> Do note that next generation of hidden services will support natively
> offline keys and actually allow the hidden service to run properly: the
> offline keys will generate blinded signing keys, which are used to sign
> descriptor signing keys.
> On 6/26/2016 12:52 AM, Razvan Dragomirescu wrote:
> > Hello everyone,
> >
> > I couldn't find a detailed description of the Tor consensus, so I'm
> > checking that my understanding of it is correct. Basically, would it be
> > correct to assume that the consensus document (or a hash thereof) for a
> > date in the future is an unpredictable value that will also be unique to
> > all nodes inquiring about it at that time?
> >
> > I'm thinking of using a hash of the consensus document -
> > like - as a
> > descriptor cookie in a hidden service. This way, an attacker cannot
> > generate or publish a hidden service descriptor for the future (one with
> > a correct cookie). A client can fetch the consensus at the time it wants
> > to connect, hash it, then use that as the descriptor cookie to determine
> > the correct descriptor id and decrypt the introduction point list.
> >
> > Does anyone see any issues with this? In my project, the hidden service
> > private key is on a smartcard, so it can't be copied, you can only ask
> > the smartcard to sign something with it for you - and I'm trying to
> > prevent an attacker from generating hidden service descriptors in
> > advance,to be used without the smartcard. If future descriptors depend
> > on an unpredictable future value (the hash of the consensus at that
> > time), an attacker can only generate descriptors for past and current
> > time periods.
> >
> > Thank you,
> > Razvan
> >
> > --
> > Razvan Dragomirescu
> > Chief Technology Officer
> > Cayenne Graphics SRL
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> tor-dev at lists.torproject.org
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