[Bitcoin-development] Coinbase TxOut Hashcash

[Peter Todd <pete@petertodd.org>]

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It has been previously(1) proposed that hashcash using the same PoW
function as the Bitcoin block hashing algorithm be used to create
hashcash whose value is denominated in Bitcoins. This poses two problems
however: widespread use of such hashcash would harm overall network
security and determining the value of the hashcash requires knowing the
revenue miners can gain from transaction fees at a given block height -
a non-computable function. However, with some modifications we can
extend the idea to directly denominate the hashcash in Bitcoins at the
cost of a small increase in proof size.

Recall that the fundemental problem is the need to do some work to make
digest D have value V, resulting in a proof that can be given to a third
party. We want V to be denominated in Bitcoins, and we want the actual
economic cost to create P to be as close as possible to the face-value
V. Finally should computing P result in a valid Bitcoin block header,
the creator of the proof should have a strong incentive to publish their
header to the P2P network and extend the current best chain.


# Proof structure

Lets look at the elements of the proof from the block header to the
digest.


## PoW Block Header

This must be a valid block header. It is particularly important to
ensure that the header can be linked to the actual blockchain, although
the header itself does not need to be a part of the chain, and hence the
block hash does not need to meet the difficulty requirements.


### Previous Block Headers

The proof may optionally include one or more previous block headers in
the event that the PoW block header's previous block is an orphan.
Unlike the PoW block header, these block headers MUST meet the
difficulty requirements although an implementation MAY skip actually
checking the difficulty if a difficulty retarget has not happened or the
PoW is timestamped. (see below)


## Partial Transaction and Merkle Path

The partial transaction consists of a SHA256 midstate followed by
exactly one transaction output. The merkle path to the PoW block header
MUST prove the transaction was the coinbase transaction and not any
other transaction.


## Transaction Output

The last transaction output must have a scriptPubKey consisting of
exactly one PUSHDATA op which pushes H(D | N) to the stack. Its value,
V', is the basis for determining the value of the proof of work. V' must
satisfy V' < k*Vi(h) where Vi is the inflation reward for the PoW block
height and k < 1 For a number of reasons, including making sure there
are strong incentives for broadcasting succesful PoW solutions, the
value of k should be chosen fairly conservatively; the author suggests k
= 1/10 as a ballpark figure. Finally N is some fixed value specific to
hashcash of this form to ensure the txout proof can-not be reused.

Vi can also be calculated as the median of the last n "anyone-can-spend"
outputs seen in coinbases when the value of the inflation reward falls
low enough that using the inflation reward is impractical.


## Timestamp

If the proof-of-work is used after a difficulty retarget the PoW needs
to be timestamped in the block chain with a merkle path leading to a
valid block header. The difficulty used for calculating the value of the
PoW then becomes the minimum of the difficulties of the PoW previous
block and the timestamp.


# Determining the actual value of the PoW

The proof proves that work was done to find a valid block header. That
block header, had it met the difficulty threshhold, could have created a
valid block worth at least the inflationary reward Vi(h) to the miner.

The coinbase transaction output and merkle path shows that were such a
block found, the miner would have then given away V' to whomever managed
to create a transaction spending it when the coinbase matured. The
coinbase takes 100 block to mature, so the chance of any one miner
collecting it is proportional to the hashing power they control.(*)

*) As with fidelity bonds we make the assumption that no party controls
more than 50% of the hashing power - the assumption underlying Bitcoin's
security anyway. If this assumption is proven incorrect or
insufficiently strong, possibly due to a cartel of miners banding
together to create low-cost PoW's, the output can use the provably
unspendable/prunable OP_RETURN <digest> scriptPubKey instead with a
non-zero value.

With P(block hash, target), the expected probability of a valid PoW
being found given the work required to create the block hash with the
given difficulty target, we can finally calculate the value of the PoW
in terms of expected cost: V = P(hash, target) * V'


# Pool implementation and 51% attack security

Because doing the work required to create coinbase txout hashcash is
sufficient to also create a valid block a pool can safely rent out
hashing power to create hashcash of this form on demand without making
it possible to rent large amounts of hashing power directly on short
notice. (though some extensions to GetBlockTemplate for hashers
verifying it may be required)

Because the anyone-can-spend txout is the basis for the value of the
hashcash the value remains computable even if transaction fees become a
larger proportion of the block reward in the future.

Unlike announce-commit sacrificies(2) proofs with very small values can
be easily created; the pool operator can make a trade-off between the
profit varience - remember that a block header with a valid PoW
represents a loss - and latency by adjusting the proof of work
difficulty and V'.

As an aside, note how the mechanism of a anyone-can-spend txout in a
coinbase can replace the announce portion of an announce-commit
sacrifice; a coinbase transaction is the only case where a single merkle
path proves that the transaction output was possible to spend in a
subsequent block, but was not yet spent; also an argument for allowing
coinbase transaction inputs.


# Application: Paying for additional flood-fill bandwidth

Additional messaging applications built on top of the Bitcoin P2P
network would be useful, yet there needs to be some general mechanism to
make DoS attacks expensive enough that they are impractical. For
instance a useful P2P network feature would be a mechanism to propose
trust-free coin mixes transaction outputs, propose specific txout sets,
and finally a mechanism to broadcast valid ANYONECANPAY signatures so
the inputs and outputs can become a valid transaction. By separating the
txout and signature broadcasts, who is paying for what output is made
very difficult to determine.

Of course such a mechanism will likely come under attack by those trying
to combat anonymity. However with the coinbase txout hashcash mechanism
those attackers are forced to either contribute to the security of the
Bitcoin network or incur much higher opporuntity costs for conducting
their attack than honest nodes pay. (remember how the choice of k = 10
makes for a large ratio of maximum V' value to Vi(h) inflation reward)

To reduce amortized proof size one proof can be used for multiple
payments with Rivest PayWords and similar techniques.


# PowPay - Off-chain, anonymous, probabalistic payments

By setting the special txout to a scriptPubKey spendable by the
recipient we can prove to a third party that work was done that with
probability P(hash,target) could have resulted in a txout spendable by
them of value V' Thus the expected value of the payment is V = P(h,t)*V'
The recipient needs to make the proof non-reusable, either by recording
all proofs submitted, or by requiring a nonce in the scriptPubKey: (*)

    <nonce> DROP {additional ops}

*) Note the implications for the IsStandardInput() test.

Because the recipient has no way of knowing how the sender paid to have
the hashing done on their behalf the source of the funds is unknown to
them. Additionally the payment can be of any amount less than a full
block reward, and the time varient between actual payments can be
reduced to, in theory, as little as the block interval itself with 100%
miner participation.


## Maximum Payment amount

Unlike coinbase txout hashcash the maximum value of a PowPay transaction
is strictly limited by the inflation reward; the trick of calculating
actual cost by prior sacrifices doesn't work because no honest sacrifice
is involved. In any case it is desirable for the mechanism to account
for a large percentage of total transaction value.

The issue is that should a valid block be found either the miner must
still have a strong incentive to broadcast that block that can be proven
to the recipient, or the miner must not be the one who controls that
decision.

The latter option is possible by inverting the relationship: now the
recipient constructs the block, and the sender simply arranges for a
valid PoW to be created - essentially the recipient acts as a mining
pool with an extremely high minimum work, and the sender provides
hashing power. With the 1MB blocksize the cost to operate the full
validating node required is low and attacks on block propagation are
difficult to successfully pull off.


### Supporting PowPay volume in excess of inflation reward + tx fees

To support overall PowPay volumes that are in excess of the inflation
reward and transaction fees the sender can provide the recipient with
signed transaction inputs subject to the constraint that only blocks
with PoW's generated by the sender can be used to spend them. For
instance a nonce in a well-known place can be provided by the sender and
included in a modified block header. By modifying the block hashing
algorithm so that PoW-withholding is not possible - a significantly more
serious problem in this application - the sender still is forced to send
all potential solutions to the recipient, including possible winning
ones. Provided that attacking block propagation is difficult the sender
can't prevent the reciver from spending their transaction inputs.


## Scalability

PowPay can provide much greater scalability than Bitcoin itself, in
terms of payments per second, however it is still limited in terms of
actual fund transfers to recipients per second. A naive implementation
would give a actual transfer every ten minutes maximum, and a highly
sophisticated solution 7/second. (albeit probably requiring a hardfork
to solve PoW withholding and/or use of third parties)

At the same time the proofs required become large with an increased
blocksize, and in the case of the inverted "recipient builds blocks"
mode the recipients either incur large costs running full nodes, or
greatly disrupt transaction flow for on-chain users by mining blocks
with no transactions in them at all. (remember that a recipient who
trusts someone else to construct the blocks for them is trusting that
third-party to do so correctly)

The latter is especially problematic because as the blocksize is
increased a higher percentage of the cost of mining goes to the overhead
required to run a validating node, rather than hashing, which has the
perverse effect of decreasing the cost of mining blocks with no
transactions in them at all. (or transactions that the miner knows have
not been revealed to other miners)

The analysis of this strange mixed bag of incentives is highly complex.


# Paying for mining

TxOut HashCash and PayPow both require the sender to somehow get someone
to mine on their behalf. The exact nature of these relationships will
vary and are beyond the scope of this paper.


# Eliminating PoW withholding

While the above examples have used economic incentives possible within
the existing Bitcoin system a structural incentive is possible as well.
A nonce N is chosen by the party paying for the PoW, such as a pool or
PowPay recipient, and H(n) is included in the block header.(*) The PoW
function is then modified to consider the PoW valid if the sum of the
expected hashes required to find H(B) and H(B | n) exceeds the current
difficulty target.

*) Note how the block header can be extended, while remaining fairly compatible
with existing ASIC mining hardware, by taking advantage of the fact that
ASIC's use the SHA256 midstate at a starting point for their PoW
calculations.(3)




1) "Re: [Bitcoin-development] Discovery/addr packets (was: Service bits
for pruned nodes)" - 2013-06-06 - Peter Todd <pete@petertodd.org> -
bitcoin-development email list

2) "Purchasing fidelity bonds by provably throwing away bitcoins" -
https://bitcointalk.org/index.php?topic=134827.0 - Peter Todd

3) "Re: 32 vs 64-bit timestamp fields" - 2013-06-09 - John Dillon
<john.dillon892@gmail.com> - bitcoin-development email list

-- 
'peter'[:-1]@petertodd.org
0000000000000039e49118426bbe6739360d35116e920d6502dcacd8e51bc74c

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