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Tor Shared Random Subsystem Specification
This document specifies how the commit-and-reveal shared random subsystem of
Tor works. This text used to be proposal 250-commit-reveal-consensus.txt.
Table Of Contents:
1. Introduction
1.1. Motivation
1.2. Previous work
2. Overview
2.1. Introduction to our commit-and-reveal protocol
2.2. Ten thousand feet view of the protocol
2.3. How we use the consensus [CONS]
2.3.1. Inserting Shared Random Values in the consensus
2.4. Persistent State of the Protocol [STATE]
2.5. Protocol Illustration
3. Protocol
3.1 Commitment Phase [COMMITMENTPHASE]
3.1.1. Voting During Commitment Phase
3.1.2. Persistent State During Commitment Phase [STATECOMMIT]
3.2 Reveal Phase
3.2.1. Voting During Reveal Phase
3.2.2. Persistent State During Reveal Phase [STATEREVEAL]
3.3. Shared Random Value Calculation At 00:00UTC
3.3.1. Shared Randomness Calculation [SRCALC]
3.4. Bootstrapping Procedure
3.5. Rebooting Directory Authorities [REBOOT]
4. Specification [SPEC]
4.1. Voting
4.1.1. Computing commitments and reveals [COMMITREVEAL]
4.1.2. Validating commitments and reveals [VALIDATEVALUES]
4.1.4. Encoding commit/reveal values in votes [COMMITVOTE]
4.1.5. Shared Random Value [SRVOTE]
4.2. Encoding Shared Random Values in the consensus [SRCONSENSUS]
4.3. Persistent state format [STATEFORMAT]
5. Security Analysis
5.1. Security of commit-and-reveal and future directions
5.2. Predicting the shared random value during reveal phase
5.3. Partition attacks
5.3.1. Partition attacks during commit phase
5.3.2. Partition attacks during reveal phase
6. Discussion
6.1. Why the added complexity from proposal 225?
6.2. Why do you do a commit-and-reveal protocol in 24 rounds?
6.3. Why can't we recover if the 00:00UTC consensus fails?
7. Acknowledgements
1. Introduction
1.1. Motivation
For the next generation hidden services project, we need the Tor network to
produce a fresh random value every day in such a way that it cannot be
predicted in advance or influenced by an attacker.
Currently we need this random value to make the HSDir hash ring
unpredictable (#8244), which should resolve a wide class of hidden service
DoS attacks and should make it harder for people to gauge the popularity
and activity of target hidden services. Furthermore this random value can
be used by other systems in need of fresh global randomness like
Tor-related protocols (e.g. OnioNS) or even non-Tor-related (e.g. warrant
canaries).
1.2. Previous work
Proposal 225 specifies a commit-and-reveal protocol that can be run as an
external script and have the results be fed to the directory authorities.
However, directory authority operators feel unsafe running a third-party
script that opens TCP ports and accepts connections from the Internet.
Hence, this proposal aims to embed the commit-and-reveal idea in the Tor
voting process which should make it smoother to deploy and maintain.
2. Overview
This proposal alters the Tor consensus protocol such that a random number is
generated every midnight by the directory authorities during the regular voting
process. The distributed random generator scheme is based on the
commit-and-reveal technique.
The proposal also specifies how the final shared random value is embedded
in consensus documents so that clients who need it can get it.
2.1. Introduction to our commit-and-reveal protocol
Every day, before voting for the consensus at 00:00UTC each authority
generates a new random value and keeps it for the whole day. The authority
cryptographically hashes the random value and calls the output its
"commitment" value. The original random value is called the "reveal" value.
The idea is that given a reveal value you can cryptographically confirm that
it corresponds to a given commitment value (by hashing it). However given a
commitment value you should not be able to derive the underlying reveal
value. The construction of these values is specified in section [COMMITREVEAL].
2.1. Ten thousand feet view of the protocol
Our commit-and-reveal protocol aims to produce a fresh shared random value
every day at 00:00UTC. The final fresh random value is embedded in the
consensus document at that time.
Our protocol has two phases and uses the hourly voting procedure of Tor.
Each phase lasts 12 hours, which means that 12 voting rounds happen in
between. In short, the protocol works as follows:
Commit phase:
Starting at 00:00UTC and for a period of 12 hours, authorities every
hour include their commitment in their votes. They also include any
received commitments from other authorities, if available.
Reveal phase:
At 12:00UTC, the reveal phase starts and lasts till the end of the
protocol at 00:00UTC. In this stage, authorities must reveal the value
they committed to in the previous phase. The commitment and revealed
values from other authorities, when available, are also added to the
vote.
Shared Randomness Calculation:
At 00:00UTC, the shared random value is computed from the agreed
revealed values and added to the consensus.
This concludes the commit-and-reveal protocol every day at 00:00UTC.
2.3. How we use the consensus [CONS]
The produced shared random values need to be readily available to
clients. For this reason we include them in the consensus documents.
Every hour the consensus documents need to include the shared random value
of the day, as well as the shared random value of the previous day. That's
because either of these values might be needed at a given time for a Tor
client to access a hidden service according to section [TIME-OVERLAP] of
proposal 224. This means that both of these two values need to be included
in votes as well.
Hence, consensuses need to include:
(a) The shared random value of the current time period.
(b) The shared random value of the previous time period.
For this, a new SR consensus method will be needed to indicate which
authorities support this new protocol.
2.3.1. Inserting Shared Random Values in the consensus
After voting happens, we need to be careful on how we pick which shared
random values (SRV) to put in the consensus, to avoid breaking the consensus
because of authorities having different views of the commit-and-reveal
protocol (because maybe they missed some rounds of the protocol).
For this reason, authorities look at the received votes before creating a
consensus and employ the following logic:
- First of all, they make sure that the agreed upon consensus method is
above the SR consensus method.
- Authorities include an SRV in the consensus if and only if the SRV has
been voted by at least the majority of authorities.
- For the consensus at 00:00UTC, authorities include an SRV in the consensus
if and only if the SRV has been voted by at least AuthDirNumAgreements
authorities (where AuthDirNumAgreements is a newly introduced consensus
parameter).
Authorities include in the consensus the most popular SRV that also
satisfies the above constraints. Otherwise, no SRV should be included.
The above logic is used to make it harder to break the consensus by natural
partioning causes.
We use the AuthDirNumAgreements consensus parameter to enforce that a
_supermajority_ of dirauths supports the SR protocol during SRV creation, so
that even if a few of those dirauths drop offline in the middle of the run
the SR protocol does not get disturbed. We go to extra lengths to ensure
this because changing SRVs in the middle of the day has terrible
reachability consequences for hidden service clients.
2.4. Persistent State of the Protocol [STATE]
A directory authority needs to keep a persistent state on disk of the on
going protocol run. This allows an authority to join the protocol seamlessly
in the case of a reboot.
During the commitment phase, it is populated with the commitments of all
authorities. Then during the reveal phase, the reveal values are also
stored in the state.
As discussed previously, the shared random values from the current and
previous time period must also be present in the state at all times if they
are available.
2.5. Protocol Illustration
An illustration for better understanding the protocol can be found here:
https://people.torproject.org/~asn/hs_notes/shared_rand.jpg
It reads left-to-right.
The illustration displays what the authorities (A_1, A_2, A_3) put in their
votes. A chain 'A_1 -> c_1 -> r_1' denotes that authority A_1 committed to
the value c_1 which corresponds to the reveal value r_1.
The illustration depicts only a few rounds of the whole protocol. It starts
with the first three rounds of the commit phase, then it jumps to the last
round of the commit phase. It continues with the first two rounds of the
reveal phase and then it jumps to the final round of the protocol run. It
finally shows the first round of the commit phase of the next protocol run
(00:00UTC) where the final Shared Random Value is computed. In our fictional
example, the SRV was computed with 3 authority contributions and its value
is "a56fg39h".
We advice you to revisit this after you have read the whole document.
3. Protocol
In this section we give a detailed specification of the protocol. We
describe the protocol participants' logic and the messages they send. The
encoding of the messages is specified in the next section ([SPEC]).
Now we go through the phases of the protocol:
3.1. Commitment Phase [COMMITMENTPHASE]
The commit phase lasts from 00:00UTC to 12:00UTC.
During this phase, an authority commits a value in its vote and
saves it to the permanent state as well.
Authorities also save any received authoritative commits by other authorities
in their permanent state. We call a commit by Alice "authoritative" if it was
included in Alice's vote.
3.1.1. Voting During Commitment Phase
During the commit phase, each authority includes in its votes:
- The commitment value for this protocol run.
- Any authoritative commitments received from other authorities.
- The two previous shared random values produced by the protocol (if any).
The commit phase lasts for 12 hours, so authorities have multiple chances to
commit their values. An authority MUST NOT commit a second value during a
subsequent round of the commit phase.
If an authority publishes a second commitment value in the same commit
phase, only the first commitment should be taken in account by other
authorities. Any subsequent commitments MUST be ignored.
3.1.2. Persistent State During Commitment Phase [STATECOMMIT]
During the commitment phase, authorities save in their persistent state the
authoritative commits they have received from each authority. Only one commit
per authority must be considered trusted and active at a given time.
3.2. Reveal Phase
The reveal phase lasts from 12:00UTC to 00:00UTC.
Now that the commitments have been agreed on, it's time for authorities to
reveal their random values.
3.2.1. Voting During Reveal Phase
During the reveal phase, each authority includes in its votes:
- Its reveal value that was previously committed in the commit phase.
- All the commitments and reveals received from other authorities.
- The two previous shared random values produced by the protocol (if any).
The set of commitments have been decided during the commitment
phase and must remain the same. If an authority tries to change its
commitment during the reveal phase or introduce a new commitment,
the new commitment MUST be ignored.
3.2.2. Persistent State During Reveal Phase [STATEREVEAL]
During the reveal phase, authorities keep the authoritative commits from the
commit phase in their persistent state. They also save any received reveals
that correspond to authoritative commits and are valid (as specified in
[VALIDATEVALUES]).
An authority that just received a reveal value from another authority's vote,
MUST wait till the next voting round before including that reveal value in
its votes.
3.3. Shared Random Value Calculation At 00:00UTC
Finally, at 00:00UTC every day, authorities compute a fresh shared random
value and this value must be added to the consensus so clients can use it.
Authorities calculate the shared random value using the reveal values in
their state as specified in subsection [SRCALC].
Authorities at 00:00UTC start including this new shared random value in
their votes, replacing the one from two protocol runs ago. Authorities also
start including this new shared random value in the consensus as well.
Apart from that, authorities at 00:00UTC proceed voting normally as they
would in the first round of the commitment phase (section [COMMITMENTPHASE]).
3.3.1. Shared Randomness Calculation [SRCALC]
An authority that wants to derive the shared random value SRV, should use
the appropriate reveal values for that time period and calculate SRV as
follows.
HASHED_REVEALS = H(ID_a | R_a | ID_b | R_b | ..)
SRV = SHA3-256("shared-random" | INT_8(REVEAL_NUM) | INT_4(VERSION) |
HASHED_REVEALS | PREVIOUS_SRV)
where the ID_a value is the identity key fingerprint of authority 'a' and R_a
is the corresponding reveal value of that authority for the current period.
Also, REVEAL_NUM is the number of revealed values in this construction,
VERSION is the protocol version number and PREVIOUS_SRV is the previous
shared random value. If no previous shared random value is known, then
PREVIOUS_SRV is set to 32 NUL (\x00) bytes.
To maintain consistent ordering in HASHED_REVEALS, all the ID_a | R_a pairs
are ordered based on the R_a value in ascending order.
3.4. Bootstrapping Procedure
As described in [CONS], two shared random values are required for the HSDir
overlay periods to work properly as specified in proposal 224. Hence
clients MUST NOT use the randomness of this system till it has bootstrapped
completely; that is, until two shared random values are included in a
consensus. This should happen after three 00:00UTC consensuses have been
produced, which takes 48 hours.
3.5. Rebooting Directory Authorities [REBOOT]
The shared randomness protocol must be able to support directory
authorities who leave or join in the middle of the protocol execution.
An authority that commits in the Commitment Phase and then leaves MUST have
stored its reveal value on disk so that it continues participating in the
protocol if it returns before or during the Reveal Phase. The reveal value
MUST be stored timestamped to avoid sending it on wrong protocol runs.
An authority that misses the Commitment Phase cannot commit anymore, so it's
unable to participate in the protocol for that run. Same goes for an
authority that misses the Reveal phase. Authorities who do not participate in
the protocol SHOULD still carry commits and reveals of others in their vote.
Finally, authorities MUST implement their persistent state in such a way that they
will never commit two different values in the same protocol run, even if they
have to reboot in the middle (assuming that their persistent state file is
kept). A suggested way to structure the persistent state is found at [STATEFORMAT].
4. Specification [SPEC]
4.1. Voting
This section describes how commitments, reveals and SR values are encoded in
votes. We describe how to encode both the authority's own
commitments/reveals and also the commitments/reveals received from the other
authorities. Commitments and reveals share the same line, but reveals are
optional.
Participating authorities need to include the line:
"shared-rand-participate"
in their votes to announce that they take part in the protocol.
4.1.1. Computing commitments and reveals [COMMITREVEAL]
A directory authority that wants to participate in this protocol needs to
create a new pair of commitment/reveal values for every protocol
run. Authorities SHOULD generate a fresh pair of such values right before the
first commitment phase of the day (at 00:00UTC).
The value REVEAL is computed as follows:
REVEAL = base64-encode( TIMESTAMP || H(RN) )
where RN is the SHA3 hashed value of a 256-bit random value. We hash the
random value to avoid exposing raw bytes from our PRNG to the network (see
[RANDOM-REFS]).
TIMESTAMP is an 8-bytes network-endian time_t value. Authorities SHOULD
set TIMESTAMP to the valid-after time of the vote document they first plan
to publish their commit into (so usually at 00:00UTC, except if they start
up in a later commit round).
The value COMMIT is computed as follows:
COMMIT = base64-encode( TIMESTAMP || H(REVEAL) )
4.1.2. Validating commitments and reveals [VALIDATEVALUES]
Given a COMMIT message and a REVEAL message it should be possible to verify
that they indeed correspond. To do so, the client extracts the random value
H(RN) from the REVEAL message, hashes it, and compares it with the H(H(RN))
from the COMMIT message. We say that the COMMIT and REVEAL messages
correspond, if the comparison was successful.
Participants MUST also check that corresponding COMMIT and REVEAL values
have the same timestamp value.
Authorities should ignore reveal values during the Reveal Phase that don't
correspond to commit values published during the Commitment Phase.
4.1.4. Encoding commit/reveal values in votes [COMMITVOTE]
An authority puts in its vote the commitments and reveals it has produced and
seen from the other authorities. To do so, it includes the following in its
votes:
"shared-rand-commit" SP VERSION SP ALGNAME SP IDENTITY SP COMMIT [SP REVEAL] NL
where VERSION is the version of the protocol the commit was created with.
IDENTITY is the authority's SHA1 identity fingerprint and COMMIT is the
encoded commit [COMMITREVEAL]. Authorities during the reveal phase can
also optionally include an encoded reveal value REVEAL. There MUST be only
one line per authority else the vote is considered invalid. Finally, the
ALGNAME is the hash algorithm that should be used to compute COMMIT and
REVEAL which is "sha3-256" for version 1.
4.1.5. Shared Random Value [SRVOTE]
Authorities include a shared random value (SRV) in their votes using the
following encoding for the previous and current value respectively:
"shared-rand-previous-value" SP NUM_REVEALS SP VALUE NL
"shared-rand-current-value" SP NUM_REVEALS SP VALUE NL
where VALUE is the actual shared random value encoded in hex (computed as
specified in section [SRCALC]. NUM_REVEALS is the number of reveal values
used to generate this SRV.
To maintain consistent ordering, the shared random values of the previous
period should be listed before the values of the current period.
4.2. Encoding Shared Random Values in the consensus [SRCONSENSUS]
Authorities insert the two active shared random values in the consensus
following the same encoding format as in [SRVOTE].
4.3. Persistent state format [STATEFORMAT]
As a way to keep ground truth state in this protocol, an authority MUST
keep a persistent state of the protocol. The next sub-section suggest a
format for this state which is the same as the current state file format.
It contains a preamble, a commitment and reveal section and a list of
shared random values.
The preamble (or header) contains the following items. They MUST occur in
the order given here:
"Version" SP version NL
[At start, exactly once.]
A document format version. For this specification, version is "1".
"ValidUntil" SP YYYY-MM-DD SP HH:MM:SS NL
[Exactly once]
After this time, this state is expired and shouldn't be used nor
trusted. The validity time period is till the end of the current
protocol run (the upcoming noon).
The following details the commitment and reveal section. They are encoded
the same as in the vote. This makes it easier for implementation purposes.
"Commit" SP version SP algname SP identity SP commit [SP reveal] NL
[Exactly once per authority]
The values are the same as detailed in section [COMMITVOTE].
This line is also used by an authority to store its own value.
Finally is the shared random value section.
"SharedRandPreviousValue" SP num_reveals SP value NL
[At most once]
This is the previous shared random value agreed on at the previous
period. The fields are the same as in section [SRVOTE].
"SharedRandCurrentValue" SP num_reveals SP value NL
[At most once]
This is the latest shared random value. The fields are the same as in
section [SRVOTE].
5. Security Analysis
5.1. Security of commit-and-reveal and future directions
The security of commit-and-reveal protocols is well understood, and has
certain flaws. Basically, the protocol is insecure to the extent that an
adversary who controls b of the authorities gets to choose among 2^b
outcomes for the result of the protocol. However, an attacker who is not a
dirauth should not be able to influence the outcome at all.
We believe that this system offers sufficient security especially compared
to the current situation. More secure solutions require much more advanced
crypto and more complex protocols so this seems like an acceptable solution
for now.
Here are some examples of possible future directions:
- Schemes based on threshold signatures (e.g. see [HOPPER])
- Unicorn scheme by Lenstra et al. [UNICORN]
- Schemes based on Verifiable Delay Functions [VDFS]
For more alternative approaches on collaborative random number generation
also see the discussion at [RNGMESSAGING].
5.2. Predicting the shared random value during reveal phase
The reveal phase lasts 12 hours, and most authorities will send their
reveal value on the first round of the reveal phase. This means that an
attacker can predict the final shared random value about 12 hours before
it's generated.
This does not pose a problem for the HSDir hash ring, since we impose an
higher uptime restriction on HSDir nodes, so 12 hours predictability is not
an issue.
Any other protocols using the shared random value from this system should
be aware of this property.
5.3. Partition attacks
This design is not immune to certain partition attacks. We believe they
don't offer much gain to an attacker as they are very easy to detect and
difficult to pull off since an attacker would need to compromise a directory
authority at the very least. Also, because of the byzantine general problem,
it's very hard (even impossible in some cases) to protect against all such
attacks. Nevertheless, this section describes all possible partition attack
and how to detect them.
5.3.1. Partition attacks during commit phase
A malicious directory authority could send only its commit to one single
authority which results in that authority having an extra commit value for
the shared random calculation that the others don't have. Since the
consensus needs majority, this won't affect the final SRV value. However,
the attacker, using this attack, could remove a single directory authority
from the consensus decision at 24:00 when the SRV is computed.
An attacker could also partition the authorities by sending two different
commitment values to different authorities during the commit phase.
All of the above is fairly easy to detect. Commitment values in the vote
coming from an authority should NEVER be different between authorities. If
so, this means an attack is ongoing or very bad bug (highly unlikely).
5.3.2. Partition attacks during reveal phase
Let's consider Alice, a malicious directory authority. Alice could wait
until the last reveal round, and reveal its value to half of the
authorities. That would partition the authorities into two sets: the ones
who think that the shared random value should contain this new reveal, and
the rest who don't know about it. This would result in a tie and two
different shared random value.
A similar attack is possible. For example, two rounds before the end of the
reveal phase, Alice could advertise her reveal value to only half of the
dirauths. This way, in the last reveal phase round, half of the dirauths
will include that reveal value in their votes and the others will not. In
the end of the reveal phase, half of the dirauths will calculate a
different shared randomness value than the others.
We claim that this attack is not particularly fruitful: Alice ends up
having two shared random values to choose from which is a fundamental
problem of commit-and-reveal protocols as well (since the last person can
always abort or reveal). The attacker can also sabotage the consensus, but
there are other ways this can be done with the current voting system.
Furthermore, we claim that such an attack is very noisy and detectable.
First of all, it requires the authority to sabotage two consensuses which
will cause quite some noise. Furthermore, the authority needs to send
different votes to different auths which is detectable. Like the commit
phase attack, the detection here is to make sure that the commitment values
in a vote coming from an authority are always the same for each authority.
6. Discussion
6.1. Why the added complexity from proposal 225?
The complexity difference between this proposal and prop225 is in part
because prop225 doesn't specify how the shared random value gets to the
clients. This proposal spends lots of effort specifying how the two shared
random values can always be readily accessible to clients.
6.2. Why do you do a commit-and-reveal protocol in 24 rounds?
The reader might be wondering why we span the protocol over the course of a
whole day (24 hours), when only 3 rounds would be sufficient to generate a
shared random value.
We decided to do it this way, because we piggyback on the Tor voting
protocol which also happens every hour.
We could instead only do the shared randomness protocol from 21:00 to 00:00
every day. Or to do it multiple times a day.
However, we decided that since the shared random value needs to be in every
consensus anyway, carrying the commitments/reveals as well will not be a
big problem. Also, this way we give more chances for a failing dirauth to
recover and rejoin the protocol.
6.3. Why can't we recover if the 00:00UTC consensus fails?
If the 00:00UTC consensus fails, there will be no shared random value for
the whole day. In theory, we could recover by calculating the shared
randomness of the day at 01:00UTC instead. However, the engineering issues
with adding such recovery logic are too great. For example, it's not easy
for an authority who just booted to learn whether a specific consensus
failed to be created.
7. Acknowledgements
Thanks to everyone who has contributed to this design with feedback and
discussion.
Thanks go to arma, ioerror, kernelcorn, nickm, s7r, Sebastian, teor, weasel
and everyone else!
References:
[RANDOM-REFS]:
http://projectbullrun.org/dual-ec/ext-rand.html
https://lists.torproject.org/pipermail/tor-dev/2015-November/009954.html
[RNGMESSAGING]:
https://moderncrypto.org/mail-archive/messaging/2015/002032.html
[HOPPER]:
https://lists.torproject.org/pipermail/tor-dev/2014-January/006053.html
[UNICORN]:
https://eprint.iacr.org/2015/366.pdf
[VDFS]:
https://eprint.iacr.org/2018/601.pdf
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