VeriBlock Whitepaper

Tuesday, June 11, 2019
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Proof-of-Proof: A Decentralized, Trustless, Transparent, and Scalable Means of Inheriting Proof-of-Work Security VeriBlock, Inc. Maxwell Sanchez Justin Fisher Version 1.0p Abstract The Proof-of-Proof consensus protocol enables blockchains to inherit proof-of-work security from other blockchains, creating an ecosystem wherein security originates on established blockchains like Bitcoin and extends to other blockchains. Such an ecosystem creates indirect scalability of Bitcoin by utilizing it as a security mechanism for purpose- built chains. Current progress in other areas of scalability, including off-chain transactional networks and sidechains, benefit from a hierarchical security model which enables all blockchains to operate under the security context of Bitcoin. We propose a means for this inheritance without the involvement or approval of Bitcoin miners, without any centralized entities or federated nodes, and without imposing any technological limitations on blockchains which adopt this protocol. 1 Introduction One of the largest issues facing blockchains today—their ability to reach and maintain consensus over blockchain data—has sparked a variety of debates over the complexity and security of a broad selection of upcoming technologies. The proof-of-work protocol used by Bitcoin has met two primary criticisms: (1) wasteful electricity consumption, and (2) weak on chains with less hash power. The criticism regarding inefficiency in consuming electricity stands on unsolid footing—so too could the filament of a lightbulb be similarly called an inefficient conductor, despite its ability to produce light. It is true, however, that smaller cryptocurrencies implementing proof-of-work are vulnerable to relatively low-cost attacks, especially when a larger chain utilizing the same hashing algorithm exists. In answer to these criticisms, a variety of alternative consensus mechanisms have been proposed and developed, including proof-of-stake where users hold balances in native tokens to mine, Practical Byzantine Fault Tolerance and the Ripple Protocol Consensus Algorithm 1

which adapt the ideas behind classical consensus algorithms like RAFT and Paxos to function on large-scale and trustless systems, and federated nodes or trusted nodes which act as network authorities and resolve consensus conflicts. Each of these consensus algorithms trade off some of the advantages of a proof-of-work consensus mechanism: thermodynamically-sound security expectations, trustless and permissionless involvement of miners, mathematically-verifiable replaying of network history for new nodes, significant opportunity costs to attack, etc. Our Proof-of-Proof consensus protocol addresses both of these concerns by recycling the hashing power of a powerful proof-of-work blockchain to secure an unlimited quantity of additional blockchains. 2 Previous Technologies There have been previous attempts to reuse the security of existing high-security blockchains. In 2011 several blockchains, including Namecoin, adopted merge mining and the AuxPoW protocol, which allowed Bitcoin miners to simultaneously mine both Bitcoin and one or more auxiliary blockchains. In 2013, Mastercoin (now Omni/Omnilayer) launched a protocol which runs on top of Bitcoin by embedding data in the Bitcoin blockchain. 2.1 Merged Mining (AuxPoW) Merged mining enables the miner of one parent blockchain to simultaneously mine on one or more auxiliary blockchains. The parent blockchain itself requires no modification to allow other blockchains to merge-mine using AuxPoW. To merge mine, a miner must first build valid block(s) for the auxiliary blockchain(s), and then include some proof of these blocks in the parent blockchain which they attempt to mine (often by embedding the auxiliary blockchain hash in the parent block coinbase transaction). If a miner successfully solves the proof-of-work below a target that satisfies one or more of the merge-mined or parent blockchains, the corresponding block(s) and the proof-of-work solution are combined and relayed to their respective blockchain(s). Merge-mining requires active participation of parent blockchain miners, and the percentage of the hash-rate which the auxiliary blockchain inherits is the percentage of the parent lo k hai s hashi g po e hi h is pe fo i g e ge-mining for the particular auxiliary blockchain. Merge- i i g does t s ale effe ti el to se u e a la ge u e of au ilia lo k hai s, because it would require that parent blockchain miners track and assemble blocks for a large quantity of auxiliary blockchains. It also forces the auxiliary blockchains to use the same hashing algorithm as the parent blockchain. Finally, in most implementations, the opportunity cost for parent blockchain miners to attack the auxiliary blockchain is only the cost of not merge-mining the auxiliary blockchain legitimately, as the miner can continue to mine the 2

parent blockchain (and merge-mine other blockchains) honestly while attempting an attack on another auxiliary network. 2.2 Layered Technologies Blockchains or pseudo-blockchains inherit the security of highly-secure blockchains by writing the entirety or near-entirety of their blockchain within another blockchain. E ha ed o a a e lie ts fo these te h ologies a t as odes on the parent blockchain network and look for embedded data which has special meaning to their blockchain. These data are then interpreted under their own rules to perform manipulations of the secondary or embedded blockchain. Some implementations of layered technologies: Omni/Omnilayer (formerly Mastercoin), Colored Coins, and Counterparty. Reorganizations in the parent blockchain result in reorganizations on the secondary/embedded blockchain. Generally, a transaction on the parent blockchain is created whenever a transaction on the secondary/embedded blockchain is created. This transactional data or a representation (hash) thereof is embedded into the parent blockchain transaction using a a iet of ea s i ludi g OP_RETURN a d i possi le add esses hi h e ed data a d do t ha e a k o o espo di g pu li /p i ate ke pai . Embedding a secondary blockchain within a parent blockchain imposes significant limitations on the secondary blockchain including block-time limitation and minimal storage capacity. For the sake of efficiency, this often requires the secondary blockchain to utilize the address format (and corresponding signature algorithm) of the parent blockchain. Users of the secondary blockchain must also own and spend tokens on the parent blockchain to transact on the secondary blockchain. Finally, these technologies have significant difficulty scaling beyond the transactional volume (measured in number of transactions, not necessarily size of transactions) supported by the parent blockchain. While technologies like Omni(layer) store a d t a s it t a sa tio atta h e ts o a to e t et o k, ea h u i ue t a sa tio o the Omni blockchain requires a transaction to be broadcast on Bitcoin as well. 2.3 ChainDB The ChainDB proposal for securing a chain to Bitcoin requires that ChainDB block-building nodes collaborate to build a Bitcoin transaction which denotes the next ChainDB block, limits the secured chain s i i u lo k time to the block time of Bitcoin, requires that fully- validating ChainDB nodes also be fully-validating Bitcoin nodes (although a model using SPV- like knowledge of Bitcoin embedded in the ChainDB chain would appear to function as well), and poses an attack vector wherein Bitcoin miners take the fee paid by legitimate ChainDB bidders, but still controls the ChainDB blockchain by including an alternate ChainDB block- defining transaction with an incredibly-large fee. Additionally, an attacker who wishes to modify a ChainDB blockchain would only need to pay fees that are each a few times greater than the block reward of the ChainDB block to have a significant chance of a multi-block rewrite on the ChainDB blockchain; a ChainDB blockchain would have potential security issues when significantly more value than its per-block coinbase is transferred per block. 3

2.4 Summary of Existing Technologies Existing technologies for reusing the proof-of-work security of one parent blockchain on auxiliary/secondary blockchains comes with significant drawbacks regarding level of security, limitations imposed on the auxiliary/secondary blockchains, and scalability concerns. 3 The Goal Proof-of-Proof aims to enable a security inheriting (SI) blockchain (analogous to a merge mining auxiliary blockchain or a layered technology secondary blockchain) to inherit the complete proof-of-work security of a security providing (SP) blockchain (analogous to a parent blockchain). This inheritance should not impose any non-trivial limitations on the SI blockchain, should not require the permission of the SP blockchain or the knowledge/involvement of SP blockchain miners, should not require any centralized network authority (including federated nodes), and should not leave the SI blockchain non-functional in the event that the SP blockchain fails. Additionally, non-mining users of the SI blockchain network should not have to interface with the SP blockchain network, nor should they be required to hold any of its native token. 4 PoP Protocol The PoP protocol introduces a new type of miner who performs periodic publications of one lo k hai s u e t state to another blockchain. These publications are referenced in the event of a potential blockchain reorganization. PoP requires a blockchain has some means of creating blocks, such as low-hashrate local PoW, PoS, etc. 4.1 Definitions Consensus Inheriting (CI) Blockchain: A blockchain secured by PoP, which inherits PoW from another blockchain. Consensus Providing (CP) Blockchain: An established, high-security blockchain which a SI blockchain inherits PoW from. Blockchain State Data: Data regarding the current state of a blockchain, such as a the most recent block header, block hash, merkle root of transactions, etc. PoP Miner: A new type of miner who performs publications of blockchain state data from a SI blockchain to a SP blockchain. 4.2 PoP Mining Process Overview PoP miners serve as the communication/transactional bridges between a SI blockchain and a SP blockchain. As often as they wish, PoP miners will take the most recent blockchain state data from the SI blockchain and publish it to the SP blockchain, along with some identifier, which allows them to later receive compensation by creating a SP blockchain transaction with 4

the SI blockchain state data and identifier embedded in it, and submits it to the SP blockchain network. Several different methods can be used for embedding the blockchain state data in a SP blockchain transaction: OP_RETURN, fake addresses, fake addresses in multisig, etc. The PoP miner then waits for the transaction to be included in a SP blockchain block, constructs some form of proof of publication, adds any identifying information necessary for them to take credit for the publication, and submits this proof back to the SI blockchain in the form of a special PoP mining transaction. 4.3 PoP Publication Data In order to take advantage of OP_RETURN, the SI blockchain state data along with some means of identifying the miner for payment needs to fit within 80 bytes. It is recommended that the entire block header of the SI blockchain be published to close a security vulnerability discussed later. By using 192-bit hashes for the previous block hash and merkle tree, the standard block header format consisting of a version, previous block hash, merkle tree hash, timestamp, nbits-style target, and nonce only occupies 64 bytes of space, leaving 16 bytes of the OP_RETURN data for PoP miner identifi atio su h as the fi st 1 tes of the i e s address). When the PoP miner submits their PoP mining transaction to the SI blockchain, they will include the full SI blockchain address whose first 16 bytes match these 16 bytes of miner identification. 4.4 PoP Mining Transactions The specialized PoP mining transaction demonstrates that SI blockchain state data was included in a SP blockchain transaction, which was included in a SP blockchain block. As such, it needs to contain the SI blockchain state data which was originally published (along with the miner identification), the SP blockchain transaction containing the SI blockchain state data, the merkle path (or another form of proof such as a witness for a cryptographic accumulator, if the SP blockchain uses a structure other than a merkle tree for transactions) which demonstrates inclusion of the transaction in a SP blockchain block, and the SP blockchain block header corresponding to the block in which the SI blockchain state data was published. Additio all , the i i g t a sa tio eeds to p o ide the full i e ide tifi atio if it as t included in its entirety in the published data (example: the full address whose first 16 bytes match the 16 bytes of miner identification published in an OP_RETURN). Finally, additional contextual information may be required, such as sufficient previous block hashes from the SP blockchain to enable the SI blockchain to construct and validate the entire SP blockchain blockchain up to the block holding the PoP i e s pu li atio . The si plest algo ith fo this 5

requires that the PoP miner submit adequate SP blockchain block headers to build from the SI blockchain et o k s p e iousl -known and highest-height SP blockchain header to the header of the block in which the PoP publication occurs. 4.5 PoP Mining Transaction Validation Peers on the SI blockchain validate a PoP mining transaction by checking the validity of the published SI blockchain state data, checking for its inclusion in the provided SP blockchain transaction, ensuring the SP blockchain transaction is included in the provided SP blockchain lo k heade s e kle t ee (or evaluating some other form of proof, such as a cryptographic accumulator witness), and ensuring that the provided block header(s) of the SP blockchain uild the lo gest PoW hai o the SP blockchain. Checking the validity of the SI blockchain state data only requires looking back in the SI blockchain for the block corresponding to the published state data. Checking for its inclusion in the provided SP blockchain transaction involves parsing the transaction and checking the data after OP_RETURN, or for the blockchain state data to appear in an encoded form, such as inside multisig addresses. Then, the SP blockchain transaction is hashed and the merkle path is followed, which should result in the merkle root embedded in the provided SP blockchain header. Since only the headers of a pure PoW blockchain are sufficient for determining consensus on blocks, SI blockchain peers have sufficient information to ensure that the PoP publication occurred in a valid SP blockchain block. 4.6 PoP Block Format In order for PoP mining transactions to later be used for consensus, they must be stored in the SI blockchain. Additionally, the block headers of the SP blockchain need to be stored in such a way that consensus of the SP blockchain can be tracked without requiring peers to interface with the SP blockchain network. As such, the blocks on a blockchain implementing PoP contain a special segment to hold the new SP blockchain block headers since the last SI blockchain lo k s i luded SP headers. In the diagram above, the green blockchain is a SI blockchain implementing PoP. The blue blocks are headers from the SP blockchain. By linking together the SP blockchain headers stored in the SI blockchain, the entire SP lo k hai s PoW o se sus a e o fi ed. I the event that a fork on the SP blockchain occurs, a SI block can include multiple competing blocks and allow SP blockchain headers embedded in future SI blockchain blocks to resolve the 6

conflict: Proof-of-Proof mining transactions can reference, as the SP blockchain block in which they published SI blockchain state data, any SP blockchain block headers stored in their enclosing block or in previous blocks (PoP mining transactions in purple): To facilitate this, block miners (PoW/PoS/etc.) take the block header data provided by the PoP mining transactions, and embed the zero or more SP blockchain headers necessary to provide context for the PoP mining transactions they wish to include in their block. 5 Fork Resolution with PoP The est fo k amongst all proposed forks is selected based on a cumulative score. In PoP, however, the score of a fork is calculated relative to another fork; the timeliness of PoP publications in the SP blockchain determine their weight, and the timeliness of a PoP publication of a SI blockchain block at a particular height is relative to the first publication of any SI blockchain block from any of the considered forks. 5.1 SP Blockchain Tracking In order for a peer on the SI blockchain network to perform fork resolution, the peer must construct and evaluate a version of the SP blockchain using all of the SP blockchain headers provided by all of the SI blockchain blocks (including those on every potential fork for which the client has knowledge). In order to do so, the peer collects every single SP blockchain header from the SI blockchain blocks, and determines consensus according to the rules of the SP lo k hai fi di g the hea iest o hai e ui i g the ost o putatio al po e to build). 7

By utilizing information available from all potential forks when reconstructing the SP blockchain, the peer can ensure that whatever final picture they get of the SP blockchain represents the state of the SP blockchain at the latest time any of the blocks in any of the forks were created, allowing for evaluation nearly as if the peer had direct access to the entirety of the SP blockchain. Through this mechanism, the relative weight of any two particular chains can be calculated on-demand by peers who join the network at a later point in time. 5.2 Fork Weight Calculation The weight of two competing forks is calculated by summing up all of the scores of all of the blocks for which two chains diverge. The scores of competing blocks between multiple forks are calculated relative to each other, following the algorithm: • For all competing blocks at height n, find all PoP mining transactions in each chain that at h said hai s lo k at height o For all PoP mining transactions endorsing any block at height n from all competing chains, find the one with the earliest publication (by block height) in the SP blockchain. Store this height as m o For each competing block n: ▪ For each PoP mining transaction endorsing block n: • If the PoP mining transaction publishes to a block in the longest known SP blockchain fork: o Determine the difference in SP blockchain publication height from m, add the value floor(1/((difference + 1) * (difference + 1))) to the score for the current block n 8

In the diagram above, the SI blockchain encounters a fork, with two competing chains (red and orange). The blue numbers inside PoP mining transactions represent the index of the SP blockchain which they published data to. Not shown for the sake of complexity are the SP blockchain blocks embedded in the two forks, which allow the reconstruction of the SP blockchain and subsequent ordering of PoP transactions. To determine which blockchain to accept, the score for each block at each index is calculated relative to the other block at the same index, and the scores for all blocks on each chain are added together: The first PoP publication for either competing block at fork index 0 was at index 1 of the illustrated SP blockchain. The red block at fork index 0, R0, is endorsed by three PoP publications which occur at index 1 of the SP blockchain, so its score is 3*(1/((1-1+1)*(1- 1+1))) = 300; R0 = 300. The orange block at fork index 0, O0, is endorsed by two PoP publications which occur at index 1 of the SP blockchain, and one PoP publication at index 2 in the SP blockchain, so its score is 2*(1/((1-1+1)*(1-1+1))) + 1*(1/((2-1+1)*(2-1+1))) = 225; O0 = 225. Similarly, R1 = 1*(1/((2-2+1)*(2-2+1))) = 100 and O1 = 2*(1/((3-2+1)*(3-2+1))) = 50. The last block in any chain never has any proof weight, because no block has come after it to contain PoP endorsements for it; R2 = 0 and O2 = 0. Summing these up, the weight of the red chain is 300 + 100 = 400, and the weight of the orange chain is 225 + 50 = 275. Since 400 > 275, the red chain is the more-endorsed blockchain. Despite the o a ge hai s i lusio of PoP i i g t a sa tio s o pa ed to the ed hai s inclusion of 4 PoP i i g t a sa tio s, the elati e ti eli ess of the ed hai s t a sa tio s caused it to have a higher proof weight, making it the better chain. 5.3 Fork Resolution Design Rationale The use of relative weighting of PoP mining transactions based on their timeliness in the SP blockchain makes it incredibly difficult for an attacker to produce a fork of any significant period of time into the future without forking the SP blockchain itself. As such, an attacker needs to generate their potential fork of the blockchain alongside the legitimate network, and must do so in full public view by publishing the block hashes of their CI-forking chain in the SP blockchain. Anyone can monitor the SP blockchain for forks being built and delay accepting transactions until the fork is resolved, and SI blockchain networks can implement additional features to invalidate these chains using mechanisms such as balance- ased oti g to i alidate the atta ke s hain before it is released onto the network. 5.4 Weighting Function The weighting function suggested for a PoP mining transaction is floor(1/((distance+1)^2)), where distance is the afore-described distance between the PoP i i g t a sa tio s corresponding SP blockchain block and the first SP blockchain block in which any block of any considered competing chain at the same index was first published to the SP blockchain. This formula can be tweaked to better fit the desired security profile of a particular SI blockchain network. Using a function which trends towards 0 faster will result in an existing blockchain 9

being easier to attack, but also increases the possibility of short-term in-step attacks, where an attacker attempts to get a single block of their CI-attacking chain into a SP blockchain block before the corresponding legitimate SI chain block occurs on the network. This suggested function will continue to be tweaked as we run more simulations of different attack scenarios. The following graph illustrates the suggested distance-vs-PoP-weight function. Distance vs. PoP Weight 100 90 80 70 PoP Weight 60 50 40 30 20 10 0 0 2 4 6 8 10 Distance Instead of calculating the function each time, a simple lookup table can be used: Distance Weight 0 100 1 25 2 11 3 6 4 4 5 2 6 2 7 1 8 1 9 1 >=10 0 Any PoP mining transaction corresponding to a SP blockchain block 10 or more SP blockchain blocks after the first publication of the relevant SI blockchain block index has no weight. As mentioned below as a solution to a potential vulnerability, a similar weighting scheme can also be applied to prioritize the relative scores of blocks closer to the forking point, to ensure that SI blockchain forks need to be announced to the SP blockchain early. 10

6 Potential Attack Vectors and Mitigations As with all consensus mechanisms, an adversarial party can attempt to force the network to reestablish consensus. On a properly-implemented PoP network, these attack vectors include forking the SP blockchain and building and proving an alternate SI blockchain. Some of the design decisions of PoP eliminate other potential attack vectors of simpler theoretical PoP-like implementations. 6.1 CP Blockchain Forking In the event that an adversarial party successfully forks the SP blockchain, they can re-write the forked SP blockchain blocks with new PoP data, enabling them to produce a SI blockchain with a higher PoP weight. The amount/length (measured in real-world time, not blocks) of the SI blockchain they are able to rewrite is approximately equal to the distance they successfully fork the SP blockchain for. Note that a fork of the SP blockchain without specific intention to fork the SI lo k hai o t result in a SI blockchain fork. However, such a reorganization of the SP blockchain will cause the SI lo k hai s PoP mining transactions which occurred in the forked SP blockchain blocks to no longer exist in the SP blockchain, and thus hold no weight. An area of further research is whether, if the attacker still includes the PoP publications in their new blocks to earn their transaction fees, some sort of process could be used by PoP miners to re-demonstrate their old proofs’ prese ce o the ew SP blockchain. In the event that the SP lo k hai fo ks ut does t do so to atta k the SI blockchain, and the SI blockchain s PoP mining transactions are impacted and no longer hold weight, the current security of the SI blockchain will drop down to its own intermediate (PoW/PoS/etc.) consensus mechanism until PoP miners publish new blockchain state data to the SP blockchain and provide PoP mining transactions back to the SI blockchain. 6.2 Building an Alternative High-Proof-Weight SI Blockchain Performing this attack requires that the adversarial party build an alternate SI chain which has a higher proof weight than the current best SI chain. In order to execute this attack successfully (due to proofs being evaluated on their timeliness), an attacker would need to build their attacking blockchain simultaneously with (or faster than) the current SI chain. This requires that the atta ke pu lish thei atta ki g hai s lo k hai state data to the SP blockchain promptly, allowing users of the network to see the pending attack and its properties. As such, anyone watching the SP blockchain would see what block(s) are at risk for the fork, how much st o ge o eake the u e t hai is o pa ed to the ad e sa ial pa t s hai , a d ould potentially use some means (like balance-based voting) to invalidate the adversarial chain before it is released to the network. 11

In another possible (although more difficult) implementation, the attacker would build an alternate SI chain whose earlier blocks have little-to-no proof weight when compared to the current chain, but whose later blocks are published extensively to the SP blockchain. This sort of attack would still be publicly visible due to the publications on the SP blockchain, but it would not necessarily reveal how far back the fork could occur, and would also not appear to users of the network at the time when some of the earlier blocks of the attacking chain were being built without proof weight. To mitigate this attack, blockchain networks simply weight blocks closer to the forking point with significantly more weight (so the sum might look something like 100 * weight0 + 70 * weight1 + 55 * weight2 … , aki g this atta k diffi ult o impossible to successfully perform. 6.3 Publishing Bogus SI Blockchain State Data In a version of a PoP implementation where the SI blockchain state data published to the SP lo k hai is t e ough to verify the potential validity of the data, an adversarial party could cause parties on the network to delay in accepting transactions by faking a potential fork hi h does t a tuall e ist. This atta k does ot e ui e o e po e i g the i te ediate o se sus, ut o l allo s the atta ke to e a uisa e e ause the et o k o t fo k if the atta ke a t p o ide the o plete lo ks fo hi h data tagged to the SP blockchain exists. This attack involves the attacker publishing apparently valid blockchain state data for hi h the do t a tuall ha e lo ks fo . In a SI blockchain which relies solely on PoW for immediate consensus, this can be mitigated by requiring, as PoP currently does, the publication of the entire block header. This way, the attacker cannot publish bogus SI blockchain data because the data would not be a valid PoW solution. In a SI blockchain employing PoS, it is also possible to publish additional information proving the ownership of coin age or similar network-asset-based mining resources, or which could be verified by informed network participants such as full nodes (txid containing the coinage claiming to be spent, etc.). 7 Combination with PoS PoP requires an intermediate method of creating blocks (or other discrete units of consensus) and maintaining short-term consensus pending PoP publications. Implementing PoP on a network using PoW as the intermediate consensus mechanism is straight-forward, given PoP s natural extension of PoW-like consensus. Implementing PoP on a PoS network requires additional considerations, and offers solutions for the long-standing issues with pure PoS. 7.1 Existing PoS Issues Note: Several implementations of PoS exist. PoS is not a single consensus algorithm, but rather a collection of several closely-related consensus algorithms which share the common trait of 12

balance-based (or u spe t-output-based) mining, in which miners use their native token balance to produce blocks, a d the resource expe diture of i i g is the ti e-value of the tokens. Some of the issues present in the original Peercoin implementation of PoS (such as long-range attacks due to fixed stake modifiers) have been solved by newer PoS implementations, and those solved issues will not be resurrected for discussion here. Two primary issues face the latest iteration of PoS: 1. There is no way to mathematically demonstrate the validity of a blockchain to a new node during bootstrapping the hai has eak su je ti it . 2. There is only a short-te solutio last lo ks to the othi g at stake p o le . Both of these issues fall back to the subjectivity of PoS—a number of private keys representing a iti al ass of et o k toke o e ship at a a it a poi t i a lo k hai s histo a be used to produce a more valid fork of the network weeks, months, or years into the past, a d does t e ui e p ese t o e ship of a toke s. Additio all , it s i possi le to p o e that no party has access to a critical mass of network tokens. The Slasher protocol presents a pu iti e s ste to sol e othi g at stake p o le s i the sho t-term. 7.2 Mathematical Demonstration of Validity During Bootstrapping Traditional PoW systems have an o je ti e defi itio of the est lo k hai , the lo k hai which requires the most cumulative work to build) and assuming that a bootstrapping node has unrestricted access to the blockchain network, the node will always be able to independently determine the best blockchain. In pure-PoS systems, the solution to the aforementioned critical mass ownership problem is simply to, as part of the protocol, prevent any node from forking back more than a certain number of blocks. Such a system results in a rolling checkpointing system wherein each node simply refuses to remove more than a certain number of blocks n f o thei u e t est lo k hai ie . As such, if a client uses old coin ownership to create a fork which begins more than n blocks ago, nodes on the network will simply reject the fork. However, a boostrapping client might be fed the illegitimate blockchain first, and subsequently refuse to fork back more than n blocks back on the illegitimate chain, permanently (without human intervention) preventing them from tracking the correct blockchain. PoP provides a simple solution to this problem, as a blockchain using PoP will have a mathematically- e ifia le est lo k hai defi ed the lo k hai s i lusio i a PoW SP blockchain. 7.3 More than n Blocks Nothing at Stake Solution The u e t solutio to the othi g at stake solution in the long-run is for clients to ignore forks that remove more than x blocks from the current blockchain, as explained above. Refusing to perform a blockchain reorganization more than n blocks deep presents one interesting attack vector: deploying a fork on the blockchain which forks exactly y blocks of 13

history during the propagation of a new block, leaving portions of the network (who ha e t yet seen the latest block and as such as willing to fork back x blocks) permanently desynchronized (without human intervention) from the portions of the network who had already received the new block, and refused to fork back x+1 blocks. This atta k s plausi ilit and potential damage increases with increasing block propagation times, which can result from larger and more-complex-to-validate blocks. Since we were unable to find any mention of this type of attack elsewhere, we describe it in Appendix A. Implementing PoP would ensure that the acquisition of a critical mass of ownership of coins at a gi e ti e i a lo k hai s histo ould t e used to atta k the et o k, e ause the accompanying PoP publications would be either non-existent or irrelevant due to untimeliness, allowing clients to remove the rule regarding maximum fork distance, since forking any significant portion of the blockchain would require successfully and simultaneously attacking the SP blockchain. In fact, successful non-contested PoP e do se e ts of a lo k a t as a effe ti e soft a i u fo ki g dista e, g o i g st o ge as the diffi ult of eati g a fo k f o efo e a certain point in history becomes exceedingly implausible due to the PoP weighting algorithms. 14

8 Appendix A: Limited Reorg Distance Fracturing Attack 8.1 Review of Short-Term Consensus Protection in PoS In the short term, pure-PoS lo k hai s a a oid othi g at stake issues e ui i g deposits (or freezing of assets) for a given period of time in order to enable PoS mining on those coins (a method considered for Ethereum, and termed Slasher by Ethereum developers). In normal PoS systems, there is a negligible cost in attempting to produce PoS blocks on top of all possible forks. When a PoS miner receives two or more competing blocks, it is in their best interest to attempt to build upon both chains, or potentially withhold any of the competing blocks which they are unable to mine atop. However, by freezing assets, the network can punish miners who p a ti e this eha io allo i g ou t hu te s to at h for this type of behavior, provide cryptographic proof (such as two signatures from the same miner which vote for competing blocks at the same height), and receive a portion of the frozen coins. Naïve short-term forking attacks require ownership of a significant portion of the total et o k s staki g oi s i ediatel efo e the atta k, aki g the largely implausible and uneconomical. The stake g i di g atta k is also i effe ti e i the sho t-term due to minimum coin age before staking and dynamic stake modifiers implemented by projects like Blackcoin and Neucoin. These issues do not exist in PoW networks, as computational power spent attempting to build o o e hai a t e spe t atte pti g to build on another chain. 8.2 Long-Term Attack In the long term, most of the means of protecting consensus are ineffective. Frozen deposits are eventually returned, long periods of time allow attackers to sell their entire position in the coin or acquire private keys which held a large portion of the staking coins long ago, and grinding attacks become possible since an attacker able to reliably rewrite a critical mass of the blockchain far in the past can explore a number of possible blockchains (each with different transactions/transaction order, which alter things like the stake modifier) bound only by their available computational power. This allows a critical mass of token ownership/control fa i the lo k hai s past to, gi e suffi ie t o putational power, create a more-valid blockchain than the current blockchain. A atta ke atte pti g to o du t this fo of atta k does t e essa il o a of the toke currently, and is more likely interested in causing a massive disruption in services than in performing double-spends. 8.3 Current Long-Term Attack Solution In order to eliminate the possibility of long-term attacks from rewriting potentially years of blockchain history, clients on PoS networks are simply programmed to not accept 15

reorganizatio s hi h fo k the et o k o e tha lo ks ago. This akes it possi le fo new bootstrapping peers to be permanently stranded on an incorrect fork, but under normal ope atio does t ause a pote tial dis uptio s to odes hi h a e al a s o e ted, or connect more often than it takes the network to add n blocks. The value n chosen as the maximum reorganization depth relies on several factors, including the period for which coins are locked up (if on a deposit-based PoS system, as Slasher proposes), the time expected for an attacker to acquire old private keys/sell their position in the currency, the speed of blocks on the network, etc. Too small of an n makes it possible for the network to easily desynchronize (since even extremely difficult attacks are plausible in very short timespans due to probability), and too large of an n makes long-term attacks more efficacious. 8.4 Issues with Maximum Reorganization Depth In the event that an attacker was capable of creating a fork which diverges from the correct blockchain n blocks ago, the attacker could release this fork while the current block is still propagating across the network (meaning some peers on the network are at a different block than others). On a blockchain like Neucoin where the maximum reorganization depth is 43,830 blocks, if the current block height on the network was 143,830, then releasing a fork which forks the network back to block 100,000 would be accepted by all peers. However, releasing the fork back to block 100,000 o e the et o k is at lo k 1 ,8 1 ould t p odu e a esults. 16


Due to network/processing latency of new blocks, there is a period between the entire network being on block 143,830 and the entire network being on block 143,831. During this period, carefully-distributed and well-connected nodes controlled by the attacker could release the fork to their peers as soon as block 143,831 is first observed on the network. As such, peers who are only aware of block 143,830 would accept the fork and overwrite 43,830 blocks of history (and would end on the last block of the fork, which could be 143,831). Ho e e , pee s hi h e ei e the legiti ate lo k 1 ,8 1 fi st do t a ept the fo k propagating across the network. At this point, the network is fractured into two segments, which both have an apparently-valid block 143,831. The two sides of the split a e t a le to reconcile and determine which blockchain is correct, since either side accepting the other side s lo k hai ould e ui e a eo ga izatio ,8 1 lo ks deep, hi h o lie ts ill perform without human intervention, as per protocol. 18

In a non-punitive PoS system where the optimal self-serving mining strategy involves mining on all available blockchain forks, it is possible that several competing forks stemming from a long-ago common point continue to be built in parallel. When the length since a common ancestor of these competing forks reaches n, clients are likely to permanently desynchronize from each other as they individually choose one of these forks and refuse to fork to any other fork since the other forks require organizations that are deeper than they are willing to perform. 8.5 Mitigation This attack requires that n be suffi ie tl la ge that a atta ke a su essfull uild a self- sustai i g lo k hai ased o p i ate ke s o ed i the past, or that n is sufficiently small that shorter-term attacks of a distance n are practical to produce. Setting n to a value as far as possi le f o oth e t e es sig ifi a tl edu es a atta ke s a ilit to pe fo this attack. Alternately, implementing PoP removes the need for a maximum reorganization depth (and makes it nearly impossible for a long reorganization to be produced), eliminating the potential for this permanent desynchronization attack. 19

9 Appendix B: The VeriBlock Blockchain Most blockchains will want to inherit consensus security from Bitcoin. To mitigate problems arising from the limited block size and rising transaction fees, 10-minute block time, 80-byte limit to data published with OP_RETURN, and the large amount of unorganized un-related data to sort through on the Bitcoin blockchain, we propose an intermediary aggregation blockchain: VeriBlock. VeriBlock is designed to secure directly with Bitcoin using PoP, and allow other blockchains to publish arbitrary blockchain state data directly to VeriBlock, which gets published by proxy in aggregate to Bitcoin. 9.1 Integrating with VeriBlock A blockchain securing with VeriBlock would use a provided library to track VeriBlock and Bitcoin consensus automatically, and then would change their block format and reward structure slightly to accommodate and reward PoP miners, and update the rules of their network consensus to query the VeriBlock library when resolving forks. 9.2 VeriBlock Design VeriBlock is a PoW-based network designed to handle simple transactions (no scripting) on a mini-blockchain secured directly to Bitcoin with PoP. A fast blocktime (such as 1 minute) reduces publication variability, the transactions support publication of larger arbitrary pieces of data (sufficient for PoS and dPoS networks to use), and VeriBlock PoW miners automatically follow several simple aggregation rules to provide summaries of published data (ordering PoP transactions based on a prefix of the published data, which groups all potentially-relevant information for each network together). Additionally, VeriBlock is designed to provide easy subscriptions to early attack notifications on any SI blockchain of interest. This provides an environment where a merchant, exchange, payment processor, etc. has one place (VeriBlock) to acquire security information about all of the blockchains they are interested in, making secure integration with third parties incredibly simple. 9.3 VeriBlock Benefits To go directly to Bitcoin, a blockchain needs to change their block header to be around 64 bytes (instead of the normal 80) for PoW networks, or choose to use far more expensive and diffi ult ea s of pu li atio like i possi le add esses, ultiple OP_RETURN i ultiple transactions, etc.). Additionally, the blockchain would need to implement maintaining full SPV-level consensus of Bitcoin themselves, and interested users would need to listen to the entire Bitcoin blockchain for early attack detection. Using VeriBlock, they can keep their current block format, publish larger amounts of blockchain state data (particularly useful for PoS networks which need to publish data pertaining to proving the existence of the stake weight a PoS miner is claiming to consume to mint a block), take advantage of a faster blocktime for publications and weight appropriately to force attacks to become visible sooner to be viable, and have a higher level 20

of decentralization of their PoP mining, because each PoP publication costs less to perform on VeriBlock than on Bitcoin. 9.4 VeriBlock Dependence A lo k hai hi h hoses to use Ve iBlo k fo se u it o t ease to fu tio in the event that the VeriBlock network fails. Existing PoP consensus is stored on the SI blockchains themselves and can still be used, and future consensus will simply fall back to the lo k hai s o al o se sus algo ith PoW/PoS/et . i the a se e of new PoP information. Additionally, blockchains willing to forgo the early attack detection benefits of VeriBlock (or willing to put the development effort in to monitor VeriBlock and Bitcoin simultaneously for potential attacks) can allow PoP miners to use VeriBlock or perform publications directly to Bitcoin, providing a PoP failover in the event of VeriBlock problems. 21

References [1] Peercoin Whitepaper [] [2] BlackCoin PoS Whitepaper v2 [] [3] Neucoin Whitepaper [] [4] Ethereum Blog Post on Weak Subjectivity [ learned-love-weak-subjectivity/] [5] OmniLayer Specifications [] [6] Counterparty Specifications [] [7] Colored Coins Specifications [ Specification] [8] Merged Mining Specifications [] [9] ChainDB Whitepaper [] 22