Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Eric Voskuil via bitcoin-dev]

> On Nov 6, 2017, at 20:38, Devrandom  wrote:
> 
> A hard-fork is a situation where non-upgraded nodes reject a block mined and 
> relayed by upgraded nodes.

As Peter pointed out, that is the case here.

> This creates a fork that cannot heal regardless of what follows.

That is not a condition of the hard fork concept.

https://github.com/bitcoin/bips/blob/master/bip-0099.mediawiki
Softfork
A consensus fork wherein everything that was previously invalid remains invalid 
while blocks that would have previously considered valid become invalid. A 
hashrate majority of miners can impose the new rules. They have some deployment 
advantages like backward compatibility.
Hardfork
A consensus fork that makes previously invalid blocks valid. Hardforks require 
all users to upgrade.

The essential element of a hard fork is that the new rule may cause rejection 
of blocks that are not rejected by old rules (thereby requiring that all users 
adopt the new rule in order to avoid a split). The reason a hard fork is 
interesting is that it can create a chain split even if it is enforced by 
majority hash power.

That is not the case with a soft fork and it is not the case here. A split can 
occur. The fact that it is possible for the split to also eventually orphan the 
old nodes does not make it a soft fork. A soft fork requires that a hash power 
majority can impose the rule. However, under the proposed new rule the hash 
power majority (according to the new rule) cannot impose the rule on existing 
nodes.

> This proposal is not a hard-fork, because the non-upgraded node *will heal* 
> if the attack has less than 1/2 of the original-POW power in the long term.

Nothing about this proposal implies an attack. From the Motivation section:

Mitigate centralization pressures by introducing a POW that does not have 
economies of scale
Introduce an intermediary confirmation point, reducing the impact of mining 
power fluctuations

> The cost of such an attack is the cost of a normal "51%" attack, multiplied 
> by the fractional weight of the original POW (e.g. 0.75 or 0.5).
> 
> So rather than saying this is a hard-fork, I would say that this is a 
> soft-fork with reduced security for non-upgraded nodes.

Presumably this preference exists because it implies the new rule would not 
cause a chain split, making it more acceptable to a risk-averse economy. This 
is precisely why it should be described correctly.

> I would also say that the reduction in security is proportional to the 
> reduction in weight of the original POW at the time of attack.
> 
> As mentioned before, the original-POW weight starts at 1.0 and is reduced 
> over a long period of time.  I would set up the transition curve so that all 
> nodes upgrade by the time the weight is, say, 0.75.  In reality, nodes 
> protecting high economic value would upgrade early.

In reality you have no way to know if/when people would adopt this rule. What 
matters in the proposal is that people who do adopt it are well aware of its 
ability to split them from the existing economy.

e

>> On Mon, Nov 6, 2017 at 3:55 PM Eric Voskuil via bitcoin-dev 
>>  wrote:
>> If a block that would be discarded under previous rules becomes accepted 
>> after a rule addition, there is no reason to not simply call the new rule a 
>> hard fork. IOW it's perfectly rational to consider a weaker block as 
>> "invalid" relative to the strong chain. As such I don't see any reason to 
>> qualify the term, it's a hard fork. But Peter's observation (the specific 
>> behavior) is ultimately what matters.
>> 
>> e
>> 
>>> On Nov 6, 2017, at 12:30, Paul Sztorc via bitcoin-dev 
>>>  wrote:
>>> 
>>> +1 to all of Peter Todd's comments
>>> 
 On Nov 6, 2017 11:50 AM, "Peter Todd via bitcoin-dev" 
  wrote:
 On Wed, Nov 01, 2017 at 05:48:27AM +, Devrandom via bitcoin-dev wrote:
 
 Some quick thoughts...
 
 > Hi all,
 >
 > Feedback is welcome on the draft below.  In particular, I want to see if
 > there is interest in further development of the idea and also interested 
 > in
 > any attack vectors or undesirable dynamics.
 >
 > (Formatted version available here:
 > https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )
 >
 > # Soft-fork Introduction of a New POW
 
 First of all, I don't think you can really call this a soft-fork; I'd call 
 it a
 "pseudo-soft-fork"
 
 My reasoning being that after implementation, a chain with less total work 
 than
 the main chain - but more total SHA256^2 work than the main chain - might 
 be
 followed by non-supporting clients. It's got some properties of a 
 soft-fork,
 but it's security model is definitely different.
 
 > ### Aux POW intermediate block
 >
 > Auxiliary POW blocks are 

Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Devrandom via bitcoin-dev]

A hard-fork is a situation where non-upgraded nodes reject a block mined
and relayed by upgraded nodes.  This creates a fork that cannot heal
regardless of what follows.

This proposal is not a hard-fork, because the non-upgraded node *will heal*
if the attack has less than 1/2 of the original-POW power in the long term.

The cost of such an attack is the cost of a normal "51%" attack, multiplied
by the fractional weight of the original POW (e.g. 0.75 or 0.5).

So rather than saying this is a hard-fork, I would say that this is a
soft-fork with reduced security for non-upgraded nodes. I would also say
that the reduction in security is proportional to the reduction in weight
of the original POW at the time of attack.

As mentioned before, the original-POW weight starts at 1.0 and is reduced
over a long period of time.  I would set up the transition curve so that
all nodes upgrade by the time the weight is, say, 0.75.  In reality, nodes
protecting high economic value would upgrade early.

On Mon, Nov 6, 2017 at 3:55 PM Eric Voskuil via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> If a block that would be discarded under previous rules becomes accepted
> after a rule addition, there is no reason to not simply call the new rule a
> hard fork. IOW it's perfectly rational to consider a weaker block as
> "invalid" relative to the strong chain. As such I don't see any reason to
> qualify the term, it's a hard fork. But Peter's observation (the specific
> behavior) is ultimately what matters.
>
> e
>
> On Nov 6, 2017, at 12:30, Paul Sztorc via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> +1 to all of Peter Todd's comments
>
> On Nov 6, 2017 11:50 AM, "Peter Todd via bitcoin-dev" <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> On Wed, Nov 01, 2017 at 05:48:27AM +, Devrandom via bitcoin-dev wrote:
>>
>> Some quick thoughts...
>>
>> > Hi all,
>> >
>> > Feedback is welcome on the draft below.  In particular, I want to see if
>> > there is interest in further development of the idea and also
>> interested in
>> > any attack vectors or undesirable dynamics.
>> >
>> > (Formatted version available here:
>> > https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )
>> >
>> > # Soft-fork Introduction of a New POW
>>
>> First of all, I don't think you can really call this a soft-fork; I'd
>> call it a
>> "pseudo-soft-fork"
>>
>> My reasoning being that after implementation, a chain with less total
>> work than
>> the main chain - but more total SHA256^2 work than the main chain - might
>> be
>> followed by non-supporting clients. It's got some properties of a
>> soft-fork,
>> but it's security model is definitely different.
>>
>> > ### Aux POW intermediate block
>> >
>> > Auxiliary POW blocks are introduced between normal blocks - i.e. the
>> chain
>> > alternates between the two POWs.
>> > Each aux-POW block points to the previous normal block and contains
>> > transactions just like a normal block.
>> > Each normal block points to the previous aux-POW block and must contain
>> all
>> > transactions from the aux-POW block.
>>
>> Note how you're basically proposing for the block interval to be
>> decreased,
>> which has security implications due to increased orphan rates.
>>
>> > ### Heaviest chain rule change
>> >
>> > This is a semi-hard change, because non-upgraded nodes can get on the
>> wrong
>> > chain in case of attack.  However,
>>
>> Exactly! Not really a soft-fork.
>>
>> --
>> https://petertodd.org 'peter'[:-1]@petertodd.org
>>
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Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Eric Voskuil via bitcoin-dev]

If a block that would be discarded under previous rules becomes accepted after 
a rule addition, there is no reason to not simply call the new rule a hard 
fork. IOW it's perfectly rational to consider a weaker block as "invalid" 
relative to the strong chain. As such I don't see any reason to qualify the 
term, it's a hard fork. But Peter's observation (the specific behavior) is 
ultimately what matters.

e

> On Nov 6, 2017, at 12:30, Paul Sztorc via bitcoin-dev 
>  wrote:
> 
> +1 to all of Peter Todd's comments
> 
>> On Nov 6, 2017 11:50 AM, "Peter Todd via bitcoin-dev" 
>>  wrote:
>> On Wed, Nov 01, 2017 at 05:48:27AM +, Devrandom via bitcoin-dev wrote:
>> 
>> Some quick thoughts...
>> 
>> > Hi all,
>> >
>> > Feedback is welcome on the draft below.  In particular, I want to see if
>> > there is interest in further development of the idea and also interested in
>> > any attack vectors or undesirable dynamics.
>> >
>> > (Formatted version available here:
>> > https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )
>> >
>> > # Soft-fork Introduction of a New POW
>> 
>> First of all, I don't think you can really call this a soft-fork; I'd call 
>> it a
>> "pseudo-soft-fork"
>> 
>> My reasoning being that after implementation, a chain with less total work 
>> than
>> the main chain - but more total SHA256^2 work than the main chain - might be
>> followed by non-supporting clients. It's got some properties of a soft-fork,
>> but it's security model is definitely different.
>> 
>> > ### Aux POW intermediate block
>> >
>> > Auxiliary POW blocks are introduced between normal blocks - i.e. the chain
>> > alternates between the two POWs.
>> > Each aux-POW block points to the previous normal block and contains
>> > transactions just like a normal block.
>> > Each normal block points to the previous aux-POW block and must contain all
>> > transactions from the aux-POW block.
>> 
>> Note how you're basically proposing for the block interval to be decreased,
>> which has security implications due to increased orphan rates.
>> 
>> > ### Heaviest chain rule change
>> >
>> > This is a semi-hard change, because non-upgraded nodes can get on the wrong
>> > chain in case of attack.  However,
>> 
>> Exactly! Not really a soft-fork.
>> 
>> --
>> https://petertodd.org 'peter'[:-1]@petertodd.org
>> 
>> ___
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Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Devrandom via bitcoin-dev]

>
> Note how you're basically proposing for the block interval to be decreased,
>> which has security implications due to increased orphan rates.
>>
>
> Note that the total transaction rate and block size don't materially
> change, so I don't
> see why the orphan rate will change.  Normal blocks are constrained to have
> all of the txs of the aux blocks, so propagation time should stay the
> same.  Am I missing
> something?
>

Ah, yes, I'm missing that the expected time to find each type of block is
halved, so the orphan rate doubles.
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Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Devrandom via bitcoin-dev]

I am also concerned.  However, this proposal allows two POWs to coexist and
allows for gradual transitions. This is hopefully a less disruptive
approach since it allows cooperative miners to migrate over time.  And of
course, as a soft-fork it keeps backwards compatibility with existing
software.

On Thu, Nov 2, 2017 at 4:55 PM Tao Effect  wrote:

> Just going to throw in my support for a POW change, not any particular
> implementation, but the idea.
>
> Bitcoin is technically owned by China now. That's not acceptable.
>
> - Greg
>
> --
> Please do not email me anything that you are not comfortable also sharing with
> the NSA.
>
> On Oct 31, 2017, at 10:48 PM, Devrandom via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> Hi all,
>
> Feedback is welcome on the draft below.  In particular, I want to see if
> there is interest in further development of the idea and also interested in
> any attack vectors or undesirable dynamics.
>
> (Formatted version available here:
> https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )
>
> # Soft-fork Introduction of a New POW
>
> ## Motivation:
>
> - Mitigate mining centralization pressures by introducing a POW that does
> not have economies of scale
> - Introduce an intermediary confirmation point, reducing the impact of
> mining power fluctuations
>
> Note however that choice of a suitable POW will require deep analysis.
> Some pitfalls include: botnet mining, POWs that seem ASIC resistant but are
> not, unexpected/covert optimization.
>
> In particular, unexpected/covert optimizations, such as ASCIBOOST, present
> a potential centralizing and destabilizing force.
>
> ## Design
>
> ### Aux POW intermediate block
>
> Auxiliary POW blocks are introduced between normal blocks - i.e. the chain
> alternates between the two POWs.
> Each aux-POW block points to the previous normal block and contains
> transactions just like a normal block.
> Each normal block points to the previous aux-POW block and must contain
> all transactions from the aux-POW block.
> Block space is not increased.
>
> The new intermediate block and the pointers are introduced via a soft-fork
> restriction.
>
> ### Reward for aux POW miners
>
> The reward for the aux POW smoothly increases from zero to a target value
> (e.g. 1/2 of the total reward) over time.
> The reward is transferred via a soft-fork restriction requiring a coinbase
> output to an address published in the
> aux-POW block.
>
> ### Aux POW difficulty adjustment
>
> Difficulty adjustments remain independent for the two POWs.
>
> The difficulty of the aux POW is adjusted based on the average time
> between normal block found
> to aux block found.
>
> Further details are dependent on the specific POW.
>
> ### Heaviest chain rule change
>
> This is a semi-hard change, because non-upgraded nodes can get on the
> wrong chain in case of attack.  However,
> it might be possible to construct an alert system that notifies
> non-upgraded nodes of an upcoming rule change.
> All blocks are still valid, so this is not a hardforking change.
>
> The heaviest chain definition changes from sum of `difficulty` to sum of:
>
> mainDifficulty ^ x * auxDifficulty ^ y
>
> where we start at:
>
> x = 1; y = 0
>
> and end at values of x and y that are related to the target relative
> rewards.  For example, if the target rewards
> are equally distributed, we will want ot end up at:
>
> x = 1/2; y = 1/2
>
> so that both POWs have equal weight.  If the aux POW is to become
> dominant, x should end small relative to y.
>
>
> ## Questions and Answers
>
> - What should be the parameters if we want the aux POW to have equal
> weight? A: 1/2 of the reward should be transferred
> to aux miners and x = 1/2, y = 1/2.
>
> - What should be the parameters if we want to deprecate the main POW?  A:
> most of the reward should be transferred to
> aux miners and x = 0, y = 1.  The main difficulty will tend to zero, and
> aux miners will just trivially generate the
> main block immediately after finding an aux block, with identical content.
>
> - Wasted bandwidth to transfer transactions twice?  A: this can be
> optimized by skipping transactions already
> transferred.
>
> - Why would miners agree to soft-fork away some of their reward?  A: they
> would agree if they believe that
> the coins will increase in value due to improved security properties.
>
> ## Open Questions
>
> - After a block of one type is found, we can naively assume that POW will
> become idle while a block of the other type is being mined.  In practice,
> the spare capacity can be used to find alternative ("attacking") blocks or
> mine other coins.  Is that a problem?
> - Is selfish mining amplified by this scheme for miners that have both
> types of hardware?
>
> ## POW candidates
>
> - SHA256 (i.e. use same POW, but introduce an intermediate block for
> faster confirmation)
> - Proof of Space and Time (Bram Cohen)
> - Equihash
> - Ethash
>
> ## 

Re: [bitcoin-dev] Introducing a POW through a soft-fork

[Tao Effect via bitcoin-dev]

Just going to throw in my support for a POW change, not any particular 
implementation, but the idea.

Bitcoin is technically owned by China now. That's not acceptable.

- Greg

--
Please do not email me anything that you are not comfortable also sharing with 
the NSA.

> On Oct 31, 2017, at 10:48 PM, Devrandom via bitcoin-dev 
>  > wrote:
> 
> Hi all,
> 
> Feedback is welcome on the draft below.  In particular, I want to see if 
> there is interest in further development of the idea and also interested in 
> any attack vectors or undesirable dynamics.
> 
> (Formatted version available here: 
> https://github.com/devrandom/btc-papers/blob/master/aux-pow.md 
>  )
> 
> # Soft-fork Introduction of a New POW
> 
> ## Motivation:
> 
> - Mitigate mining centralization pressures by introducing a POW that does not 
> have economies of scale
> - Introduce an intermediary confirmation point, reducing the impact of mining 
> power fluctuations
> 
> Note however that choice of a suitable POW will require deep analysis.  Some 
> pitfalls include: botnet mining, POWs that seem ASIC resistant but are not, 
> unexpected/covert optimization.
> 
> In particular, unexpected/covert optimizations, such as ASCIBOOST, present a 
> potential centralizing and destabilizing force.
> 
> ## Design
> 
> ### Aux POW intermediate block
> 
> Auxiliary POW blocks are introduced between normal blocks - i.e. the chain 
> alternates between the two POWs.
> Each aux-POW block points to the previous normal block and contains 
> transactions just like a normal block.
> Each normal block points to the previous aux-POW block and must contain all 
> transactions from the aux-POW block.
> Block space is not increased.
> 
> The new intermediate block and the pointers are introduced via a soft-fork 
> restriction.
> 
> ### Reward for aux POW miners
> 
> The reward for the aux POW smoothly increases from zero to a target value 
> (e.g. 1/2 of the total reward) over time.
> The reward is transferred via a soft-fork restriction requiring a coinbase 
> output to an address published in the
> aux-POW block.
> 
> ### Aux POW difficulty adjustment
> 
> Difficulty adjustments remain independent for the two POWs.
> 
> The difficulty of the aux POW is adjusted based on the average time between 
> normal block found
> to aux block found.
> 
> Further details are dependent on the specific POW.
> 
> ### Heaviest chain rule change
> 
> This is a semi-hard change, because non-upgraded nodes can get on the wrong 
> chain in case of attack.  However,
> it might be possible to construct an alert system that notifies non-upgraded 
> nodes of an upcoming rule change.
> All blocks are still valid, so this is not a hardforking change.
> 
> The heaviest chain definition changes from sum of `difficulty` to sum of:
> 
> mainDifficulty ^ x * auxDifficulty ^ y
> 
> where we start at:
> 
> x = 1; y = 0
> 
> and end at values of x and y that are related to the target relative rewards. 
>  For example, if the target rewards
> are equally distributed, we will want ot end up at:
> 
> x = 1/2; y = 1/2
> 
> so that both POWs have equal weight.  If the aux POW is to become dominant, x 
> should end small relative to y.
> 
> 
> ## Questions and Answers
> 
> - What should be the parameters if we want the aux POW to have equal weight? 
> A: 1/2 of the reward should be transferred
> to aux miners and x = 1/2, y = 1/2.
> 
> - What should be the parameters if we want to deprecate the main POW?  A: 
> most of the reward should be transferred to
> aux miners and x = 0, y = 1.  The main difficulty will tend to zero, and aux 
> miners will just trivially generate the
> main block immediately after finding an aux block, with identical content.
> 
> - Wasted bandwidth to transfer transactions twice?  A: this can be optimized 
> by skipping transactions already
> transferred.
> 
> - Why would miners agree to soft-fork away some of their reward?  A: they 
> would agree if they believe that
> the coins will increase in value due to improved security properties.
> 
> ## Open Questions
> 
> - After a block of one type is found, we can naively assume that POW will 
> become idle while a block of the other type is being mined.  In practice, the 
> spare capacity can be used to find alternative ("attacking") blocks or mine 
> other coins.  Is that a problem?
> - Is selfish mining amplified by this scheme for miners that have both types 
> of hardware?
> 
> ## POW candidates
> 
> - SHA256 (i.e. use same POW, but introduce an intermediate block for faster 
> confirmation)
> - Proof of Space and Time (Bram Cohen)
> - Equihash
> - Ethash
> 
> ## Next Steps
> 
> - evaluate POW candidates
> - evaluate difficulty adjustment rules
> - simulate miner behavior to identify if there are incentives for detrimental 
> behavior patterns (e.g. block 

[bitcoin-dev] Introducing a POW through a soft-fork

[Devrandom via bitcoin-dev]

Hi all,

Feedback is welcome on the draft below.  In particular, I want to see if
there is interest in further development of the idea and also interested in
any attack vectors or undesirable dynamics.

(Formatted version available here:
https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )

# Soft-fork Introduction of a New POW

## Motivation:

- Mitigate mining centralization pressures by introducing a POW that does
not have economies of scale
- Introduce an intermediary confirmation point, reducing the impact of
mining power fluctuations

Note however that choice of a suitable POW will require deep analysis.
Some pitfalls include: botnet mining, POWs that seem ASIC resistant but are
not, unexpected/covert optimization.

In particular, unexpected/covert optimizations, such as ASCIBOOST, present
a potential centralizing and destabilizing force.

## Design

### Aux POW intermediate block

Auxiliary POW blocks are introduced between normal blocks - i.e. the chain
alternates between the two POWs.
Each aux-POW block points to the previous normal block and contains
transactions just like a normal block.
Each normal block points to the previous aux-POW block and must contain all
transactions from the aux-POW block.
Block space is not increased.

The new intermediate block and the pointers are introduced via a soft-fork
restriction.

### Reward for aux POW miners

The reward for the aux POW smoothly increases from zero to a target value
(e.g. 1/2 of the total reward) over time.
The reward is transferred via a soft-fork restriction requiring a coinbase
output to an address published in the
aux-POW block.

### Aux POW difficulty adjustment

Difficulty adjustments remain independent for the two POWs.

The difficulty of the aux POW is adjusted based on the average time between
normal block found
to aux block found.

Further details are dependent on the specific POW.

### Heaviest chain rule change

This is a semi-hard change, because non-upgraded nodes can get on the wrong
chain in case of attack.  However,
it might be possible to construct an alert system that notifies
non-upgraded nodes of an upcoming rule change.
All blocks are still valid, so this is not a hardforking change.

The heaviest chain definition changes from sum of `difficulty` to sum of:

mainDifficulty ^ x * auxDifficulty ^ y

where we start at:

x = 1; y = 0

and end at values of x and y that are related to the target relative
rewards.  For example, if the target rewards
are equally distributed, we will want ot end up at:

x = 1/2; y = 1/2

so that both POWs have equal weight.  If the aux POW is to become dominant,
x should end small relative to y.


## Questions and Answers

- What should be the parameters if we want the aux POW to have equal
weight? A: 1/2 of the reward should be transferred
to aux miners and x = 1/2, y = 1/2.

- What should be the parameters if we want to deprecate the main POW?  A:
most of the reward should be transferred to
aux miners and x = 0, y = 1.  The main difficulty will tend to zero, and
aux miners will just trivially generate the
main block immediately after finding an aux block, with identical content.

- Wasted bandwidth to transfer transactions twice?  A: this can be
optimized by skipping transactions already
transferred.

- Why would miners agree to soft-fork away some of their reward?  A: they
would agree if they believe that
the coins will increase in value due to improved security properties.

## Open Questions

- After a block of one type is found, we can naively assume that POW will
become idle while a block of the other type is being mined.  In practice,
the spare capacity can be used to find alternative ("attacking") blocks or
mine other coins.  Is that a problem?
- Is selfish mining amplified by this scheme for miners that have both
types of hardware?

## POW candidates

- SHA256 (i.e. use same POW, but introduce an intermediate block for faster
confirmation)
- Proof of Space and Time (Bram Cohen)
- Equihash
- Ethash

## Next Steps

- evaluate POW candidates
- evaluate difficulty adjustment rules
- simulate miner behavior to identify if there are incentives for
detrimental behavior patterns (e.g. block withholding / selfish mining)
- Protocol details

## Credits

Bram Cohen came up with a similar idea back in March:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-March/013744.html
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