Hi Tor devs,
I'm part of the DeDiS team at EPFL and we are working on Cothority,
which is a generic framework for Collective Authority. We worked on a
proposal to apply Collective Signatures (CoSi) to D.A. consensus
document in the Tor ecosystem. While not adding too much complexity to
the Tor ecosystem, we believe it can bring more transparency,
accountability and security regarding the consensus documents. There is
also a section regarding the emergency procedure that D.A.s operators
are under when a relay is detected as malicious, and how CoSi could help
in this process.
I hope you will find it interesting and welcome all the feedbacks /
opinions you can have!
Thanks in advance,
Nicolas
<-------- tor_cosi.txt --------------->
Filename: tor_cosi.txt
Title: Tor CoSi
Author: Nicolas GAILLY
Created: 09.03.2016
Status: draft
0. Introduction
This document describes how to provide and use a decentralized witness
cosigning mechanism in order to gain proactive transparency and public
accountability for the Tor consensus documents. Directory
Authorities (DA)
send their document to this set of witnesses and embed the signature
within
the document. Tor relays and clients can choose to refuse a
consensus document
if it has not been accepted and signed by a threshold of witnesses.
1. Overview
A weakness of the current DA system is that if any 5 of the 9 DAsâ keys
are stolen or coerced they could be used to sign fake directories
that the
attacker might use secretly in another part of the world to
compromise Tor
clients in the attackerâs domain. We propose to address this class
of attacks
by incorporating decentralized witness cosigning (CoSi) into the
directory
signing process, which ensures that any consensus document must be
not only
signed by appropriate DAs, but also publicly witnessed, signed and
logged by a
larger group of servers acting as witnesses, before clients will
accept the
directory.
A Tor relay or client expects to receive an additional "CoSi" signature
alongside the consensus document. They verify if the signature is
correct and
whether a sufficient number of witnesses attested that the consensus
document
is valid or not. The Tor project would fix such a threshold in the
default
configuration but any user or relay is free to adjust this value to
its own
need. In order to verify the signature, they need to have the
individuals
public keys of all the witnesses beforehand. This "CoSi certificate"
can be
embedded in the software in the same way certificate pinning does.
2. Motivation
Tor's DAs are comprised of 9 servers (and one extra for the
bridges): four
of them are in the US and 5 of them are in the EU. Attacking these
central and
vital points of the Tor network is clearly within the reach of state
level
adversaries if they were to collaborate. Recent stories about
surveillance
show that such a collaboration is already happening.
For example, let's imagine a plausible situation where a state-level
attacker secretly coerces and/or steals the private keys of 5 of the
9 DAs,
takes them back to the Republic of Tyrannia where they control the
ISPs and
the countryâs Internet connectivity to the rest of the world. The
government
embeds those keys in their âGreat Firewallâ type devices, and uses
them to
secretly MITM attack targeted Tor users within Tyrannia by giving them
correctly-signed but completely false views of the Tor directory in
which all
of the available relays are run by the Tyrannian authorities. Since
this
attack does not attack the consensus documents that the legitimate
DAs are
regularly broadcasting to the rest of the world, neither the Tor
project nor
anyone else outside of Tyrannia will have the opportunity to see or
become
aware of the fake consensus documents, and not many people even inside
Tyrannia might have the opportunity to detect the attack if it is
carefully
targeted against the small number of suspected activists and
journalists the
government does not like.
The main goal of CoSi applied in Tor should be to ensure consensus
document
transparency: that is, ensure the property that any consensus
document that
any Tor client anywhere will accept has been observed and logged by a
significant number of parties throughout the world, so that any
misuse of a
quorum of 5 DA keys anywhere will be quickly detectable (soon, if not
necessarily during the signing process itself).
4. Design
The first part will talk more about the architecture of a "CoSi"
system and
the second part will go into more detail about how such a system can be
integrated in Tor.
4.1 Simplified Architecture
The designated leader arranges all the witnesses in a tree. The public
keys of each participants are aggregated to form an aggregate public
key. The
tree is only there for performance reasons and can be reconfigured
at any
point in time without affecting security (the latest experiments
showed that
we could run up to 8000 nodes with a 2 second delay for generating a
signature). Once the tree is constructed, its "certificate"
represents the
aggregate public key and the hash of a Merkle tree containing all the
individual public keys. This Merkle tree is needed in case some
nodes are
failing, so the verification of the signature can still be executed
without
any change by the client. These issues are covered later in the
âHandling
Exceptionsâ part. In a nutshell, a signature round broadcasts the
message down
the tree, and to each participant. At the end of a signature round,
the client
have a simple Schnorr signature to verify.
Besides contributing to the signing, each witness can and should perform
any readily feasible syntactic and semantic correctness checks on
the leaderâs
proposed statements before signing off on them. They can / should
probably
publish logs of the statements they witness or simply make available
a public
mirror of everything that its tree roster has been asked to sign.
An important point is that each signature is chained with the last
one, so
each witness also knows the full set of witnesses that accepted the last
signature and whether the leader is acting malicious or not.
4.2 CoSi in Tor
4.2.1 Setup
The CoSi system can be constructed using independent, diverse and
decentralized witnesses. In the context of Tor, one could imagine to
include
the DA themselves (one could be the leader), and some of the best
relays in
term of availability and reputation. Due to the decentralized nature of
CoSi, this system can also includes external organizations that are
favorable and trusted by Tor (e.g. EFF). Another criteria for
the tree construction includes the latency between witnesses. One can
collect information about the communication latencies between the
witnesses
and construct a shortest-path spanning tree using this data in order
to reduce
the global latency of the system. One important property is that the
system is under a closed membership policy. The certificate of the
CoSi system will have to be embedded in the Tor code in a similar
way as for
certificate pinning.
One obvious way to do it is to setup the set of witnesses as being
the DAs
and the set of (future) âbackup directory serversâ as their public
keys will
already be hard-coded into the Tor client software. That way the
deployment
cost is drastically reduced as software will already have the keys
to verify a
CoSi signature.
4.2.2 Operations
Once the tree is setup and each one knows about it (have the
certificate),
then the signature process happens for each consensus document. The CoSi
protocol produces a collective signature in response to the
initiation of the
protocol by a leader. This signature is then included in the consensus
document so clients don't have to request it from another party.
One issue for discussion is who should initiate CoSi protocol rounds and
at what times. For example, each of the 9 DAs (or whatever subset
is online)
could independently initiate CoSi rounds on each directory consensus
event,
producing up to nine separate, redundant collective signatures on each
directory consensus. Alternatively, the common case might be for
one of the 9
DAs to be the CoSi initiator at a given time, with a round-robin
leader-change
mechanism ensuring that another leader takes over if the prior one
becomes
unavailable.
A related issue for discussion is whether it could be problematic if
there
are two or more distinct collective signatures for a given directory
consensus, and whether it is a problem if distinct subsets of 5 DAs
might
(perhaps accidentally) produce multiple slightly different, though
valid and
legitimately-signed, consensus documents at about the same time. In
other
words, does Tor directory consensus âneedâ strong consistency with a
single
serialized timeline, as Byzantine consensus protocols are intended
to provide
- or is weak consistency with occasional cases of multiple concurrent
consensus documents and/or collective signatures acceptable?
4.2.3 Integration
First of all, integrating CoSi would *not* immediately affect the
fundamental structure or function of the current DAs: there could
still be 9
of them, of which any 5 can authorize the release of a new consensus
document,
as they do now. Secondly, CoSi would not necessarily change
anything about how
the 9 DAs decide on how to compute these directory consensus
documents; e.g.,
it would not prevent the DAs from working together to block the
inclusion of
(or assignment of bandwidth-weight to) relays that might be
perceived by the
DAs as doing bad things. Finally, full backward compatibility with
old Tor
clients and relay software may be maintained by treating the new
CoSi-generated
collective signature as just an additional signature that gets
attached to and
distributed with consensus documents. It may be treated as a special
â10th
virtual DAâ that does not help authorize decisions but just publicly
witnesses
the output of the regular 9 DAs. Old client and relay software can
simply
ignore that new collective signature, whereas new software might
look for it
and over time gradually increase the threshold number of witnesses
it expects
to see.
4.3 Optional: Break-the-glass Emergency Directory Adjustments
In case of emergency, the delay caused by having to coordinate among
5 DAs in
order to make anything happen (i.e. excluding a set of malicious
nodes) can be
problematic.
This section proposes a mechanism in which the CoSi witnesses can
accept and
witness not just âfull consensusâ documents (signed by 5 DAs), but
can also
accept âemergency adjustmentsâ, which are highly-constrained deltas
(diffs) to
an existing full consensus document signed by a smaller threshold of
DAs,
e.g., 2 or even just 1. For example, the CoSi witness cosigning
rules might
require that an emergency directory-adjustment must:
- be a diff against a âfreshâ, recent full consensus document
(perhaps *the*
most recent one),
- can make no modifications to the full consensus other than some
pre-defined
operations such as decreasing bandwidth weights assigned to relays,
- cannot affect the directory-wide total bandwidth weight by more
than X%
(e.g., 1% or .1%).
These suggestions are just a few imaginable rules to get the idea
across; the ârightâ rules would of course need much more
discussion. This
way, if one or two DAs discovers or even strongly suspects an attack
of some
kind, they can take emergency countermeasures against the attack and
be able
to roll them out to clients quickly without having to get a full 5
DAs out of
bed - but because the delta-consensus is still witness-cosigned
automatically
by (perhaps) all the DAs and a number of additional trusted relays,
we get the
strong accountability provision that the use of such a âbreak-the-glassâ
emergency provision will immediately become known to the other DAs
as soon as
they do get out of bed.
Such a break-the-glass emergency adjustment mechanism could be
designed, if
desired, so that ordinary clients and relays cannot immediately tell the
difference between a directory consensus produced via the normal
threshold of
5 DAs and one that was produced as a delta via the emergency adjustment
mechanism. Only the witness cosigners would necessarily need to
know which
collectively-signed directories were authorized via the full consensus
procedure or via a break-the-glass adjustment. So if itâs important
to keep
it secret from the general public the precise reason for a
particular directory
update, that can be accommodated. Only the more-trusted group of
witness
cosigners (and obviously all the DAs themselves) would necessarily know
the precise origin and administrative justification of a given directory
update. With even fancier crypto, even the witnesses would not
necessarily need
to know, but thatâs beyond the scope of this proposal and its
desirability may
be questionable at any rate.
4. Security implications
4.1 Cons
Since the structure is a tree, if any node fails, there must be some
failover mechanisms to restore connectivity between the children of the
failed node and the rest of the tree. Since the DA reach consensus every
hour [1], with the help of the gossiping network, the availability
problem
is not an issue.
4.2 Benefits
Technically, it is quite easy to implement witness cosigning if the
group
of witnesses is small. If we want the group of witnesses to be
large, however
â and we do, to ensure that compromising transparency would require
not just a
few but hundreds or even thousands of witnesses to be colluding
maliciously â
then gathering hundreds or thousands of individual signatures could
become
painful and inefficient. Worse, every client would need to check all
these
signatures individually. The key technical contribution of our
research is a
distributed protocol that makes large, decentralized witness
cosigning groups
practical. This decentralized approach enables the security of the whole
system to scale with the number of witnesses.
Not only does this system dramatically increase the cost of successfully
deploying an attack on the DA (the attacker would have to corrupt a
large
majority of the witnesses), it also decreases the incentive to
launch such an
attack because the threshold of witnesses that are required to sign the
document for the signature to be accepted can be locally set on each
client.
4.3 Differences between CoSi and Certificate Transparency
Prior transparency mechanisms have two weaknesses. First they do not
significantly
increase the number of secret keys an attacker must control to
compromise any
client device, and client devices cannot even retroactively detect such
compromise unless they can actively communicate with multiple well-known
Internet servers. For example, even with Certificate Transparency, an
attacker can forge an Extended Validation (EV) certificate for
Chrome after
compromising or coercing only three parties: one Certificate
Authority (CA)
and two log servers. Since many CAs and log servers are in US
jurisdiction,
such an attack is clearly within reach of the US government. If such
an attack
does occur, Certificate Transparency cannot detect it unless the
victim device
has a chance to communicate or gossip the fake certificate with
other parties
on the Internet â after it has already accepted and started using
the fake
digital certificate. In the case of Tor Transparency, the attack is
harder
because the attacker would have to compromise the three parties plus a
majority of Directory Authorities but facing a state-level adversary the
threat is still plausible. One way to increase the difficulty of the
attack
is to make sure the logs servers are scattered in different places of
the world.
Second, the attacker can still evade transparency by controlling the
clientâs
Internet access paths. For example, a compromised Internet service
provider
(ISP) or corporate Internet gateway can defeat retroactive transparency
mechanisms by persistently blocking a victim deviceâs access to
transparency
servers elsewhere on the Internet. Even if the userâs device is
mobile, a
state intelligence service such as Chinaâs âGreat Firewallâ could defeat
retroactive transparency mechanisms by persistently blocking
connections from
a targeted victimâs devices to external transparency servers, in the
same way
that China already blocks connections to many websites and Tor relays.
Using CoSi requires the clients to have the list of public keys of
all the
witnesses embedded in the software, like certificate pinning. In
order to
reduce the size of this CoSi certificate, we embed the aggregated
public key
of all the witnesses and a hash representing the root of a Merkle tree
containing the public key of all the witnesses. Using the
certificate this way
provides an universally-verifiable commitment to all the witnessesâ
public
keys, without the certificate actually containing them all.
5. Specifications
6. Compatibility
7. Implementation
Implementation in Go is open source at:
https://github.com/dedis/cothority
8. Performance
9. Acknowledgments
This proposals has received some valuable feedback from Bryan Ford,
Linus
Gasser, Ismail Khoffi, Philipp Jovanovic, and Ludovic Barman.
A. References
[1] https://collector.torproject.org
<-------------- END ----------------------->
Filename: tor_cosi.txt
Title: Tor Cosi
Author: Nicolas GAILLY, DeDiS lab, EPFL
Created: 09.03.2016
Status: draft
0. Introduction
This document describes how to provide and use a decentralized witness
cosigning mechanism in order to gain proactive transparency and public
accountability for the Tor consensus documents. Directory Authorities (DA)
send their document to this set of witnesses and embed the signature within
the document. Tor relays and clients can choose to refuse a consensus document
if it has not been accepted and signed by a threshold of witnesses.
1. Overview
A weakness of the current DA system is that if any 5 of the 9 DAsâ keys
are stolen or coerced they could be used to sign fake directories that the
attacker might use secretly in another part of the world to compromise Tor
clients in the attackerâs domain. We propose to address this class of attacks
by incorporating decentralized witness cosigning (CoSi) into the directory
signing process, which ensures that any consensus document must be not only
signed by appropriate DAs, but also publicly witnessed, signed and logged by a
larger group of servers acting as witnesses, before clients will accept the
directory.
A Tor relay or client expects to receive an additional "CoSi" signature
alongside the consensus document. They verify if the signature is correct and
whether a sufficient number of witnesses attested that the consensus document
is valid or not. The Tor project would fix such a threshold in the default
configuration but any user or relay is free to adjust this value to its own
need. In order to verify the signature, they need to have the individuals
public keys of all the witnesses beforehand. This "CoSi certificate" can be
embedded in the software in the same way certificate pinning does.
2. Motivation
Tor's DAs are comprised of 9 servers (and one extra for the bridges): four
of them are in the US and 5 of them are in the EU. Attacking these central and
vital points of the Tor network is clearly within the reach of state level
adversaries if they were to collaborate. Recent stories about surveillance
show that such a collaboration is already happening.
For example, let's imagine a plausible situation where a state-level
attacker secretly coerces and/or steals the private keys of 5 of the 9 DAs,
takes them back to the Republic of Tyrannia where they control the ISPs and
the countryâs Internet connectivity to the rest of the world. The government
embeds those keys in their âGreat Firewallâ type devices, and uses them to
secretly MITM attack targeted Tor users within Tyrannia by giving them
correctly-signed but completely false views of the Tor directory in which all
of the available relays are run by the Tyrannian authorities. Since this
attack does not attack the consensus documents that the legitimate DAs are
regularly broadcasting to the rest of the world, neither the Tor project nor
anyone else outside of Tyrannia will have the opportunity to see or become
aware of the fake consensus documents, and not many people even inside
Tyrannia might have the opportunity to detect the attack if it is carefully
targeted against the small number of suspected activists and journalists the
government does not like.
The main goal of CoSi applied in Tor should be to ensure consensus document
transparency: that is, ensure the property that any consensus document that
any Tor client anywhere will accept has been observed and logged by a
significant number of parties throughout the world, so that any misuse of a
quorum of 5 DA keys anywhere will be quickly detectable (soon, if not
necessarily during the signing process itself).
4. Design
The first part will talk more about the architecture of a "CoSi" system and
the second part will go into more detail about how such a system can be
integrated in Tor.
4.1 Simplified Architecture
The designated leader arranges all the witnesses in a tree. The public
keys of each participants are aggregated to form an aggregate public key. The
tree is only there for performance reasons and can be reconfigured at any
point in time without affecting security (the latest experiments showed that
we could run up to 8000 nodes with a 2 second delay for generating a
signature). Once the tree is constructed, its "certificate" represents the
aggregate public key and the hash of a Merkle tree containing all the
individual public keys. This Merkle tree is needed in case some nodes are
failing, so the verification of the signature can still be executed without
any change by the client. These issues are covered later in the âHandling
Exceptionsâ part. In a nutshell, a signature round broadcasts the message down
the tree, and to each participant. At the end of a signature round, the client
have a simple Schnorr signature to verify.
Besides contributing to the signing, each witness can and should perform
any readily feasible syntactic and semantic correctness checks on the leaderâs
proposed statements before signing off on them. They can / should probably
publish logs of the statements they witness or simply make available a public
mirror of everything that its tree roster has been asked to sign.
An important point is that each signature is chained with the last one, so
each witness also knows the full set of witnesses that accepted the last
signature and whether the leader is acting malicious or not.
4.2 CoSi in Tor
4.2.1 Setup
The CoSi system can be constructed using independent, diverse and
decentralized witnesses. In the context of Tor, one could imagine to include
the DA themselves (one could be the leader), and some of the best relays in
term of availability and reputation. Due to the decentralized nature of
CoSi, this system can also includes external organizations that are
favorable and trusted by Tor (e.g. EFF). Another criteria for
the tree construction includes the latency between witnesses. One can
collect information about the communication latencies between the witnesses
and construct a shortest-path spanning tree using this data in order to reduce
the global latency of the system. One important property is that the
system is under a closed membership policy. The certificate of the
CoSi system will have to be embedded in the Tor code in a similar way as for
certificate pinning.
One obvious way to do it is to setup the set of witnesses as being the DAs
and the set of (future) âbackup directory serversâ as their public keys will
already be hard-coded into the Tor client software. That way the deployment
cost is drastically reduced as software will already have the keys to verify a
CoSi signature.
4.2.2 Operations
Once the tree is setup and each one knows about it (have the certificate),
then the signature process happens for each consensus document. The CoSi
protocol produces a collective signature in response to the initiation of the
protocol by a leader. This signature is then included in the consensus
document so clients don't have to request it from another party.
One issue for discussion is who should initiate CoSi protocol rounds and
at what times. For example, each of the 9 DAs (or whatever subset is online)
could independently initiate CoSi rounds on each directory consensus event,
producing up to nine separate, redundant collective signatures on each
directory consensus. Alternatively, the common case might be for one of the 9
DAs to be the CoSi initiator at a given time, with a round-robin leader-change
mechanism ensuring that another leader takes over if the prior one becomes
unavailable.
A related issue for discussion is whether it could be problematic if there
are two or more distinct collective signatures for a given directory
consensus, and whether it is a problem if distinct subsets of 5 DAs might
(perhaps accidentally) produce multiple slightly different, though valid and
legitimately-signed, consensus documents at about the same time. In other
words, does Tor directory consensus âneedâ strong consistency with a single
serialized timeline, as Byzantine consensus protocols are intended to provide
- or is weak consistency with occasional cases of multiple concurrent
consensus documents and/or collective signatures acceptable?
4.2.3 Integration
First of all, integrating CoSi would *not* immediately affect the
fundamental structure or function of the current DAs: there could still be 9
of them, of which any 5 can authorize the release of a new consensus document,
as they do now. Secondly, CoSi would not necessarily change anything about how
the 9 DAs decide on how to compute these directory consensus documents; e.g.,
it would not prevent the DAs from working together to block the inclusion of
(or assignment of bandwidth-weight to) relays that might be perceived by the
DAs as doing bad things. Finally, full backward compatibility with old Tor
clients and relay software may be maintained by treating the new CoSi-generated
collective signature as just an additional signature that gets attached to and
distributed with consensus documents. It may be treated as a special â10th
virtual DAâ that does not help authorize decisions but just publicly witnesses
the output of the regular 9 DAs. Old client and relay software can simply
ignore that new collective signature, whereas new software might look for it
and over time gradually increase the threshold number of witnesses it expects
to see.
4.3 Optional: Break-the-glass Emergency Directory Adjustments
In case of emergency, the delay caused by having to coordinate among 5 DAs in
order to make anything happen (i.e. excluding a set of malicious nodes) can be
problematic.
This section proposes a mechanism in which the CoSi witnesses can accept and
witness not just âfull consensusâ documents (signed by 5 DAs), but can also
accept âemergency adjustmentsâ, which are highly-constrained deltas (diffs) to
an existing full consensus document signed by a smaller threshold of DAs,
e.g., 2 or even just 1. For example, the CoSi witness cosigning rules might
require that an emergency directory-adjustment must:
- be a diff against a âfreshâ, recent full consensus document (perhaps *the*
most recent one),
- can make no modifications to the full consensus other than some pre-defined
operations such as decreasing bandwidth weights assigned to relays,
- cannot affect the directory-wide total bandwidth weight by more than X%
(e.g., 1% or .1%).
These suggestions are just a few imaginable rules to get the idea
across; the ârightâ rules would of course need much more discussion. This
way, if one or two DAs discovers or even strongly suspects an attack of some
kind, they can take emergency countermeasures against the attack and be able
to roll them out to clients quickly without having to get a full 5 DAs out of
bed - but because the delta-consensus is still witness-cosigned automatically
by (perhaps) all the DAs and a number of additional trusted relays, we get the
strong accountability provision that the use of such a âbreak-the-glassâ
emergency provision will immediately become known to the other DAs as soon as
they do get out of bed.
Such a break-the-glass emergency adjustment mechanism could be designed, if
desired, so that ordinary clients and relays cannot immediately tell the
difference between a directory consensus produced via the normal threshold of
5 DAs and one that was produced as a delta via the emergency adjustment
mechanism. Only the witness cosigners would necessarily need to know which
collectively-signed directories were authorized via the full consensus
procedure or via a break-the-glass adjustment. So if itâs important to keep
it secret from the general public the precise reason for a particular directory
update, that can be accommodated. Only the more-trusted group of witness
cosigners (and obviously all the DAs themselves) would necessarily know
the precise origin and administrative justification of a given directory
update. With even fancier crypto, even the witnesses would not necessarily need
to know, but thatâs beyond the scope of this proposal and its desirability may
be questionable at any rate.
4. Security implications
4.1 Cons
Since the structure is a tree, if any node fails, there must be some
failover mechanisms to restore connectivity between the children of the
failed node and the rest of the tree. Since the DA reach consensus every
hour [1], with the help of the gossiping network, the availability problem
is not an issue.
4.2 Benefits
Technically, it is quite easy to implement witness cosigning if the group
of witnesses is small. If we want the group of witnesses to be large, however
â and we do, to ensure that compromising transparency would require not just a
few but hundreds or even thousands of witnesses to be colluding maliciously â
then gathering hundreds or thousands of individual signatures could become
painful and inefficient. Worse, every client would need to check all these
signatures individually. The key technical contribution of our research is a
distributed protocol that makes large, decentralized witness cosigning groups
practical. This decentralized approach enables the security of the whole
system to scale with the number of witnesses.
Not only does this system dramatically increase the cost of successfully
deploying an attack on the DA (the attacker would have to corrupt a large
majority of the witnesses), it also decreases the incentive to launch such an
attack because the threshold of witnesses that are required to sign the
document for the signature to be accepted can be locally set on each client.
4.3 Differences between CoSi and Certificate Transparency
Prior transparency mechanisms have two weaknesses. First they do not significantly
increase the number of secret keys an attacker must control to compromise any
client device, and client devices cannot even retroactively detect such
compromise unless they can actively communicate with multiple well-known
Internet servers. For example, even with Certificate Transparency, an
attacker can forge an Extended Validation (EV) certificate for Chrome after
compromising or coercing only three parties: one Certificate Authority (CA)
and two log servers. Since many CAs and log servers are in US jurisdiction,
such an attack is clearly within reach of the US government. If such an attack
does occur, Certificate Transparency cannot detect it unless the victim device
has a chance to communicate or gossip the fake certificate with other parties
on the Internet â after it has already accepted and started using the fake
digital certificate. In the case of Tor Transparency, the attack is harder
because the attacker would have to compromise the three parties plus a
majority of Directory Authorities but facing a state-level adversary the
threat is still plausible. One way to increase the difficulty of the attack
is to make sure the logs servers are scattered in different places of
the world.
Second, the attacker can still evade transparency by controlling the clientâs
Internet access paths. For example, a compromised Internet service provider
(ISP) or corporate Internet gateway can defeat retroactive transparency
mechanisms by persistently blocking a victim deviceâs access to transparency
servers elsewhere on the Internet. Even if the userâs device is mobile, a
state intelligence service such as Chinaâs âGreat Firewallâ could defeat
retroactive transparency mechanisms by persistently blocking connections from
a targeted victimâs devices to external transparency servers, in the same way
that China already blocks connections to many websites and Tor relays.
Using CoSi requires the clients to have the list of public keys of all the
witnesses embedded in the software, like certificate pinning. In order to
reduce the size of this CoSi certificate, we embed the aggregated public key
of all the witnesses and a hash representing the root of a Merkle tree
containing the public key of all the witnesses. Using the certificate this way
provides an universally-verifiable commitment to all the witnessesâ public
keys, without the certificate actually containing them all.
5. Specifications
6. Compatibility
7. Implementation
Implementation in Go is open source at:
https://github.com/dedis/cothority
8. Performance
9. Acknowledgements
This proposals has received some valuable feedback from Bryan Ford, Linus
Gasser, Ismail Khoffi, Philipp Jovanovic, and Ludovic Barman.
A. References
[1] https://collector.torproject.org
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