[bitcoin-dev] BIP: OP_PRANDOM

[bitcoin-dev] BIP: OP_PRANDOM

[Matthew Roberts]

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== Background

OP_PRANDOM is a new op code for Bitcoin that pushes a pseudo-random number
to the top of the stack based on the next N block hashes. The source of the
pseudo-random number is defined as the XOR of the next N block hashes after
confirmation of a transaction containing the OP_PRANDOM encumbered output.
When a transaction containing the op code is redeemed, the transaction
receives a pseudo-random number based on the next N block hashes after
confirmation of the redeeming input. This means that transactions are also
effectively locked until at least N new blocks have been found.


== Rational

Making deterministic, verifiable, and trustless pseudo-random numbers
available for use in the Script language makes it possible to support a
number of new smart contracts. OP_PRANDOM would allow for the simplistic
creation of purely decentralized lotteries without the need for complicated
multi-party computation protocols. Gambling is also another possibility as
contracts can be written based on hashed commitments, with the winner
chosen if a given commitment is closest to the pseudo-random number.
OP_PRANDOM could also be used for cryptographically secure virtual asset
management such as rewards in video games and in other applications.


== Security

Pay-to-script-hash can be used to protect the details of contracts that use
OP_PRANDOM from the prying eyes of miners. However, since there is also a
non-zero risk that a participant in a contract may attempt to bribe a miner
the inclusion of multiple block hashes as a source of randomness is a must.
Every miner would effectively need to be bribed to ensure control over the
results of the random numbers, which is already very unlikely. The risk
approaches zero as N goes up.

There is however another issue: since the random numbers are based on a
changing blockchain, its problematic to use the next immediate block hashes
before the state is “final.” A safe default for accepting the blockchain
state as final would need to be agreed upon beforehand, otherwise you could
have multiple random outputs becoming valid simultaneously on different
forks.

A simple solution is not to reveal any commitments before the chain height
surpasses a certain point but this might not be an issue since only one
version will eventually make it into the final chain anyway -- though it is
something to think about.


== Outro

I'm not sure how secure this is or whether its a good idea so posting it
here for feedback

Thoughts?

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Johnson Lau]

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Using the hash of multiple blocks does not make it any safer. The miner of the last block always determines the results, by knowing the hashes of all previous blocks.

> 
> == Security
> Pay-to-script-hash can be used to protect the details of contracts that use OP_PRANDOM from the prying eyes of miners. However, since there is also a non-zero risk that a participant in a contract may attempt to bribe a miner the inclusion of multiple block hashes as a source of randomness is a must. Every miner would effectively need to be bribed to ensure control over the results of the random numbers, which is already very unlikely. The risk approaches zero as N goes up.


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Re: [bitcoin-dev] BIP: OP_PRANDOM

[James MacWhyte]

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Matthew,

Other than gambling, do you have any specific examples of how this could be
useful?

On Fri, May 20, 2016, 20:34 Johnson Lau via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Using the hash of multiple blocks does not make it any safer. The miner of
> the last block always determines the results, by knowing the hashes of all
> previous blocks.
>
>
> == Security
>
> Pay-to-script-hash can be used to protect the details of contracts that
> use OP_PRANDOM from the prying eyes of miners. However, since there is also
> a non-zero risk that a participant in a contract may attempt to bribe a
> miner the inclusion of multiple block hashes as a source of randomness is a
> must. Every miner would effectively need to be bribed to ensure control
> over the results of the random numbers, which is already very unlikely. The
> risk approaches zero as N goes up.
>
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Matthew Roberts]

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Good point, to be honest. Maybe there's a better way to combine the block
hashes like taking the first N bits from each block hash to produce a
single number but the direction that this is going in doesn't seem ideal.

I just asked a friend about this problem and he mentioned using the hash of
the proof of work hash as part of the number so you have to throw away a
valid POW if it doesn't give you the hash you want. I suppose its possible
to make it infinitely expensive to manipulate the number but I can't think
of anything better than that for now.

I need to sleep on this for now but let me know if anyone has any better
ideas.



On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:

> Using the hash of multiple blocks does not make it any safer. The miner of
> the last block always determines the results, by knowing the hashes of all
> previous blocks.
>
>
> == Security
>
> Pay-to-script-hash can be used to protect the details of contracts that
> use OP_PRANDOM from the prying eyes of miners. However, since there is also
> a non-zero risk that a participant in a contract may attempt to bribe a
> miner the inclusion of multiple block hashes as a source of randomness is a
> must. Every miner would effectively need to be bribed to ensure control
> over the results of the random numbers, which is already very unlikely. The
> risk approaches zero as N goes up.
>
>
>

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Eric Martindale]

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Matthew,

You should take a look at OP_DETERMINISTICRANDOM [1] from the Elements
Project.  It aims to achieve a similar goal.

Code is in the `alpha` branch [2].

[1]: https://www.elementsproject.org/elements/opcodes/
[2]:
https://github.com/ElementsProject/elements/blob/alpha/src/script/interpreter.cpp#L1252-L1305

On Fri, May 20, 2016 at 8:29 AM Matthew Roberts via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Good point, to be honest. Maybe there's a better way to combine the block
> hashes like taking the first N bits from each block hash to produce a
> single number but the direction that this is going in doesn't seem ideal.
>
> I just asked a friend about this problem and he mentioned using the hash
> of the proof of work hash as part of the number so you have to throw away a
> valid POW if it doesn't give you the hash you want. I suppose its possible
> to make it infinitely expensive to manipulate the number but I can't think
> of anything better than that for now.
>
> I need to sleep on this for now but let me know if anyone has any better
> ideas.
>
>
>
> On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:
>
>> Using the hash of multiple blocks does not make it any safer. The miner
>> of the last block always determines the results, by knowing the hashes of
>> all previous blocks.
>>
>>
>> == Security
>>
>> Pay-to-script-hash can be used to protect the details of contracts that
>> use OP_PRANDOM from the prying eyes of miners. However, since there is also
>> a non-zero risk that a participant in a contract may attempt to bribe a
>> miner the inclusion of multiple block hashes as a source of randomness is a
>> must. Every miner would effectively need to be bribed to ensure control
>> over the results of the random numbers, which is already very unlikely. The
>> risk approaches zero as N goes up.
>>
>>
>>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Jeremy]

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nack -- not secure.

OP_PRANDOM also adds extra validation overhead on a block potentially
composed of transactions all spending an OP_PRANDOM output from all
different blocks.

I do agree that random numbers are highly desirable though.

I think it would be much better for these use cases to add OP_XOR back and
then use something like Blum's fair coin-flipping over the phone. OP_XOR
may have other uses too.

I have a write-up from a while back which does Blum's without OP_XOR using
OP_SIZE for off-chain probabilistic payments if anyone is interested. No
fork needed, but of course it is more limited and broken in a number of
ways.

(sorry to those of you seeing this twice, my first email bounced the list)

--
@JeremyRubin <https://twitter.com/JeremyRubin>
<https://twitter.com/JeremyRubin>

On Fri, May 20, 2016 at 2:32 PM, Eric Martindale via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Matthew,
>
> You should take a look at OP_DETERMINISTICRANDOM [1] from the Elements
> Project.  It aims to achieve a similar goal.
>
> Code is in the `alpha` branch [2].
>
> [1]: https://www.elementsproject.org/elements/opcodes/
> [2]:
> https://github.com/ElementsProject/elements/blob/alpha/src/script/interpreter.cpp#L1252-L1305
>
> On Fri, May 20, 2016 at 8:29 AM Matthew Roberts via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Good point, to be honest. Maybe there's a better way to combine the block
>> hashes like taking the first N bits from each block hash to produce a
>> single number but the direction that this is going in doesn't seem ideal.
>>
>> I just asked a friend about this problem and he mentioned using the hash
>> of the proof of work hash as part of the number so you have to throw away a
>> valid POW if it doesn't give you the hash you want. I suppose its possible
>> to make it infinitely expensive to manipulate the number but I can't think
>> of anything better than that for now.
>>
>> I need to sleep on this for now but let me know if anyone has any better
>> ideas.
>>
>>
>>
>> On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:
>>
>>> Using the hash of multiple blocks does not make it any safer. The miner
>>> of the last block always determines the results, by knowing the hashes of
>>> all previous blocks.
>>>
>>>
>>> == Security
>>>
>>> Pay-to-script-hash can be used to protect the details of contracts that
>>> use OP_PRANDOM from the prying eyes of miners. However, since there is also
>>> a non-zero risk that a participant in a contract may attempt to bribe a
>>> miner the inclusion of multiple block hashes as a source of randomness is a
>>> must. Every miner would effectively need to be bribed to ensure control
>>> over the results of the random numbers, which is already very unlikely. The
>>> risk approaches zero as N goes up.
>>>
>>>
>>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Sergio Demian Lerner]

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Bitcoin Beacon paper relevant here

Basically is suggest using deciding a random bit on the majority 1s or 0s
of lsb bits taken from last block hashes.

Iddo Bentov∗ Technion, Ariel Gabizon,  David Zuckerman

We examine a protocol πbeacon that outputs unpredictable and publicly
verifiable randomness, meaning that the output is unknown at the time that
πbeacon starts, yet everyone can verify that the output is close to uniform
after πbeacon terminates. We show that πbeacon can be instantiated via
Bitcoin under sensible assumptions; in particular we consider an adversary
with an arbitrarily large initial budget who may not operate at a loss
indefinitely.
In case the adversary has an infinite budget, we provide an impossibility
result that stems from the similarity between the Bitcoin model and
Santha-Vazirani sources. We also give a hybrid protocol that combines
trusted parties and a Bitcoin-based beacon.

On Sun, May 22, 2016 at 10:30 AM, Jeremy via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> nack -- not secure.
>
> OP_PRANDOM also adds extra validation overhead on a block potentially
> composed of transactions all spending an OP_PRANDOM output from all
> different blocks.
>
> I do agree that random numbers are highly desirable though.
>
> I think it would be much better for these use cases to add OP_XOR back and
> then use something like Blum's fair coin-flipping over the phone. OP_XOR
> may have other uses too.
>
> I have a write-up from a while back which does Blum's without OP_XOR using
> OP_SIZE for off-chain probabilistic payments if anyone is interested. No
> fork needed, but of course it is more limited and broken in a number of
> ways.
>
> (sorry to those of you seeing this twice, my first email bounced the list)
>
> --
> @JeremyRubin <https://twitter.com/JeremyRubin>
> <https://twitter.com/JeremyRubin>
>
> On Fri, May 20, 2016 at 2:32 PM, Eric Martindale via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Matthew,
>>
>> You should take a look at OP_DETERMINISTICRANDOM [1] from the Elements
>> Project.  It aims to achieve a similar goal.
>>
>> Code is in the `alpha` branch [2].
>>
>> [1]: https://www.elementsproject.org/elements/opcodes/
>> [2]:
>> https://github.com/ElementsProject/elements/blob/alpha/src/script/interpreter.cpp#L1252-L1305
>>
>> On Fri, May 20, 2016 at 8:29 AM Matthew Roberts via bitcoin-dev <
>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> Good point, to be honest. Maybe there's a better way to combine the
>>> block hashes like taking the first N bits from each block hash to produce a
>>> single number but the direction that this is going in doesn't seem ideal.
>>>
>>> I just asked a friend about this problem and he mentioned using the hash
>>> of the proof of work hash as part of the number so you have to throw away a
>>> valid POW if it doesn't give you the hash you want. I suppose its possible
>>> to make it infinitely expensive to manipulate the number but I can't think
>>> of anything better than that for now.
>>>
>>> I need to sleep on this for now but let me know if anyone has any better
>>> ideas.
>>>
>>>
>>>
>>> On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:
>>>
>>>> Using the hash of multiple blocks does not make it any safer. The miner
>>>> of the last block always determines the results, by knowing the hashes of
>>>> all previous blocks.
>>>>
>>>>
>>>> == Security
>>>>
>>>> Pay-to-script-hash can be used to protect the details of contracts that
>>>> use OP_PRANDOM from the prying eyes of miners. However, since there is also
>>>> a non-zero risk that a participant in a contract may attempt to bribe a
>>>> miner the inclusion of multiple block hashes as a source of randomness is a
>>>> must. Every miner would effectively need to be bribed to ensure control
>>>> over the results of the random numbers, which is already very unlikely. The
>>>> risk approaches zero as N goes up.
>>>>
>>>>
>>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>>
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>

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Re: [bitcoin-dev] BIP: OP_PRANDOM

[Sergio Demian Lerner]

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Missing link to paper: https://arxiv.org/abs/1605.04559

Another relevant paper:

On Bitcoin as a public randomness source
Joseph Bonneau, Jeremy Clark, and Steven Goldfeder
https://eprint.iacr.org/2015/1015.pdf

On Tue, May 24, 2016 at 11:30 AM, Sergio Demian Lerner <
sergio.d.lerner@gmail.com> wrote:

> Bitcoin Beacon paper relevant here
>
> Basically is suggest using deciding a random bit on the majority 1s or 0s
> of lsb bits taken from last block hashes.
>
> Iddo Bentov∗ Technion, Ariel Gabizon,  David Zuckerman
>
> We examine a protocol πbeacon that outputs unpredictable and publicly
> verifiable randomness, meaning that the output is unknown at the time that
> πbeacon starts, yet everyone can verify that the output is close to uniform
> after πbeacon terminates. We show that πbeacon can be instantiated via
> Bitcoin under sensible assumptions; in particular we consider an adversary
> with an arbitrarily large initial budget who may not operate at a loss
> indefinitely.
> In case the adversary has an infinite budget, we provide an impossibility
> result that stems from the similarity between the Bitcoin model and
> Santha-Vazirani sources. We also give a hybrid protocol that combines
> trusted parties and a Bitcoin-based beacon.
>
> On Sun, May 22, 2016 at 10:30 AM, Jeremy via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> nack -- not secure.
>>
>> OP_PRANDOM also adds extra validation overhead on a block potentially
>> composed of transactions all spending an OP_PRANDOM output from all
>> different blocks.
>>
>> I do agree that random numbers are highly desirable though.
>>
>> I think it would be much better for these use cases to add OP_XOR back
>> and then use something like Blum's fair coin-flipping over the phone.
>> OP_XOR may have other uses too.
>>
>> I have a write-up from a while back which does Blum's without OP_XOR
>> using OP_SIZE for off-chain probabilistic payments if anyone is interested.
>> No fork needed, but of course it is more limited and broken in a number of
>> ways.
>>
>> (sorry to those of you seeing this twice, my first email bounced the list)
>>
>> --
>> @JeremyRubin <https://twitter.com/JeremyRubin>
>> <https://twitter.com/JeremyRubin>
>>
>> On Fri, May 20, 2016 at 2:32 PM, Eric Martindale via bitcoin-dev <
>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> Matthew,
>>>
>>> You should take a look at OP_DETERMINISTICRANDOM [1] from the Elements
>>> Project.  It aims to achieve a similar goal.
>>>
>>> Code is in the `alpha` branch [2].
>>>
>>> [1]: https://www.elementsproject.org/elements/opcodes/
>>> [2]:
>>> https://github.com/ElementsProject/elements/blob/alpha/src/script/interpreter.cpp#L1252-L1305
>>>
>>> On Fri, May 20, 2016 at 8:29 AM Matthew Roberts via bitcoin-dev <
>>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>>
>>>> Good point, to be honest. Maybe there's a better way to combine the
>>>> block hashes like taking the first N bits from each block hash to produce a
>>>> single number but the direction that this is going in doesn't seem ideal.
>>>>
>>>> I just asked a friend about this problem and he mentioned using the
>>>> hash of the proof of work hash as part of the number so you have to throw
>>>> away a valid POW if it doesn't give you the hash you want. I suppose its
>>>> possible to make it infinitely expensive to manipulate the number but I
>>>> can't think of anything better than that for now.
>>>>
>>>> I need to sleep on this for now but let me know if anyone has any
>>>> better ideas.
>>>>
>>>>
>>>>
>>>> On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:
>>>>
>>>>> Using the hash of multiple blocks does not make it any safer. The
>>>>> miner of the last block always determines the results, by knowing the
>>>>> hashes of all previous blocks.
>>>>>
>>>>>
>>>>> == Security
>>>>>
>>>>> Pay-to-script-hash can be used to protect the details of contracts
>>>>> that use OP_PRANDOM from the prying eyes of miners. However, since there is
>>>>> also a non-zero risk that a participant in a contract may attempt to bribe
>>>>> a miner the inclusion of multiple block hashes as a source of randomness is
>>>>> a must. Every miner would effectively need to be bribed to ensure control
>>>>> over the results of the random numbers, which is already very unlikely. The
>>>>> risk approaches zero as N goes up.
>>>>>
>>>>>
>>>>>
>>>> _______________________________________________
>>>> bitcoin-dev mailing list
>>>> bitcoin-dev@lists.linuxfoundation.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>
>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>>
>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>>
>

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