CryptoGuru PoC SIG, 2017-12-27 Public Domain v1.00 The Burst Dymaxion An Arbitrary Scalable, Energy Efficient and Anonymous Transaction Network Based on Colored Tangles Seán Gauld1 *, Franz von Ancoina, Robert Stadler2 Abstract We describe the concept and implementation of a proposed layer to the Burstcoin cryptocurrency called “The Burst Dymaxion”. This layer implements an arbitrary scalable, energy efficient and anonymous transaction network based on colored tangles. Tangles are DAGs that can be seen as generalization of the blockchain. Coloring is a simple tagging technique used in cryptocurrencies to allow for coexistence of various instances of a class in a common data context. The Burstcoin network has several unique properties that make it our premier choice for implementation: its Proof-of-Capacity (PoC) consensus algorithm is energy efficient, fair and anti-centralistic, the blockchain is very compact and implements data scrubbing to reduce blockchain bloat and it has smart contracts. The coin has over 3 years of heritage and a community to bootstrap with, yet it is small enough to allow for significant improvements without a long and paralyzing scaling debate. The original Burstcoin blockchain is used as underlying layer to open and close an arbitrary number of general purpose transaction channels, similar to the Lightning Network proposal of Bitcoin, but using IOTA-like tangles for propagation and verification. Each of these channels is not limited in terms of capacity and number of transactions. This concept takes the best traits of the original Burstcoin, IOTA, Monero, ZCash and the newest Bitcoin proposals to create a currency suitable for truly global use. It allows e.g. banks, clearing houses, remittance processors and virtually all other market participants to use quasi-private, yet decentral and trustless transaction channels with desired properties. Keywords Burstcoin – DAG – Dymaxion – Lightning-Network – PoC – Ring-Signature – Scaling – Tangle – zk-SNARK 1 PoC Consortium - a CryptoGuru SIG 2 Stronzo Bestiale Institute of Technology *Corresponding author: [email protected] Contents 3.4 Blockchain Enforcing . . . . . . . . . . . . . . . . . . . . 12 4 Security Considerations 12 Introduction 1 4.1 Collusive Nodes Attack . . . . . . . . . . . . . . . . . . . 12 1 Building Blocks 3 4.2 Spamming and DoS Scenarios . . . . . . . . . . . . . 13 1.1 Burstcoin Blockchain . . . . . . . . . . . . . . . . . . . . 3 1.2 Smart Contracts . . . . . . . . . . . . . . . . . . . . . . . 3 5 Results and Discussion 13 1.3 DAGs and The Tangle . . . . . . . . . . . . . . . . . . . .4 5.1 Prototype Performance . . . . . . . . . . . . . . . . . . . 13 1.4 zk-SNARK vs. Ring Signature . . . . . . . . . . . . . .4 5.2 Adoption Process . . . . . . . . . . . . . . . . . . . . . . . 15 1.5 Blockchain Binding with ACCTs . . . . . . . . . . . . 5 Acknowledgments 15 1.6 Lightning Network . . . . . . . . . . . . . . . . . . . . . . 6 A SpaceMint Paper Errata . . . . . . . . . . . . . . . . . . 16 1.7 Coloring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 B The Burst/Qora ACCT Process . . . . . . . . . . . . . 17 2 Putting it All Together 7 C Burstcoin CIPs . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.1 Dymaxion Tangle vs. IOTA Tangle . . . . . . . . . . . .7 References 23 2.2 Blockchain-Dymaxion Interaction . . . . . . . . . . . .7 2.3 Using Nodes P2P for ad-hoc DLs . . . . . . . . . . . 8 2.4 Dymaxion Anonymity . . . . . . . . . . . . . . . . . . . . 9 Introduction 3 Implementation Details 10 Cryptographic currencies, or cryptocurrencies for short, have been around for about 8 years now. After an existence on the 3.1 Opening a DL . . . . . . . . . . . . . . . . . . . . . . . . . 10 fringes, they are being perceived increasingly as disruptive 3.2 A Transaction within a DL . . . . . . . . . . . . . . . . . 11 technology that may change the way how we will see and use 3.3 Closing a DL . . . . . . . . . . . . . . . . . . . . . . . . . . 12 money in the future.
The Burst Dymaxion — 2/23 Figure 1. Cap Gemini/BNP Paribas: Number of Worldwide Non-Cash Transactions 2010-2020 (est.) Despite their technical elegance and possibilities, cryptocur- 1.5% of the global non-cash transaction volume - see also figure rencies lack mass adoption because of two critical factors: us- 1). You would need a 400MiB blocksize to meet that demand. ability and scalability. While usability can be addressed with This would imply a blockchain growth of over 56GiB per day. sufficient engineering and development effort, scalability is Aside from the feasibility considerations, it is questionable often a theoretical problem of its own. if a transaction for the acquisition of a sack of rice made in The inherent features of a good cryptocurrency - decentral- China 2017 should be kept for all eternity on the blockchain and ization and trustless design - often go contrary to traditional thus in memory or on mass storage of all participating nodes methods of upscaling centralistic processes. While Bitcoin’s world-wide. pioneer achievement, the blockchain, solved the problem of a A concept to overcome many of the restrictions a typical decentralized trust, its inventor certainly left much headroom blockchain imposes on scalability was presented in the IOTA for scaling that concept for a truly global use. white paper. In this novel system, transactions are not broad- In fact, speaking of “truly global use”, let’s fathom the scale cast over a network of participating nodes, some of which of this concept. Assume you would like to support one hundred (miners) try to put them into a block. Instead, network nodes million people world-wide to be able to use a cryptocurrency can validate transactions and by this validation work actually like Bitcoin with an average of just 1 transactions per day (about earn the right to perform transactions themselves. The nodes
The Burst Dymaxion — 3/23 have unified roles and form a kind of systolic array for data Proof-of-Work (PoW) systems and is responsible for making processing. The data being a transaction propagated along a Burstcoin energy efficient. Efficiency - while similar to Proof- directed acyclic graph. of-Stake (PoS) systems, is still achieved by a low barrier to Due to it’s success and thus increasing usage, Bitcoin started entry, where you do not need to acquire a significant stake of to experience scalability issues, now commonly hitting its the- the currency in order to be able to participate in the mining oretical tx/s limit. A long scaling debate about pushing this consensus. limit and removing other protocol constraints continues to the present day. One of the suggested extensions to the Bitcoin protocol is called the Lightning Network (LN).1 It is basically a network of two-party ledger entries for participants who have subscribed to a bidirectional payment channel. While a LN solves many of the problems associated with high transaction volume, micropayments and their cost, it is not a silver bullet. E.g. it does quite little to increase the limits on the number of concurrent users. Dymaxion is a term that Richard Buckminster Fuller as- sociated with much of his work. It is a portmanteau of the words dynamic, maximum, and tension and sums up the goal of his study, "maximum gain of advantage from minimal energy input." We use this term for the Burst Dymaxion to honor the achievements and influence of this true generalist and also be- cause the geodesic dome - a structure Fuller helped to popular- ize - has very similar properties to what is proposed in this paper for transaction propagation: Elements of the dome - while each limited in resilience - distribute mechanical stress throughout the whole structure, making geodesic domes able to withstand very heavy loads for their size. In our proposal, the Dymaxion is a set of tangle-based lightning networks, each single one used for propagating transactions. Each single of these networks distributes “transactional stress” among its whole network of participating nodes, allowing for arbitrary scalability. The more nodes participate, the higher the transactional capacity of each of the currently active networks becomes. As any number of these tangle-based LNs can co-exist at the same time, we speak of Dymaxion layers (DL). We therefore present nothing short of an energy-efficient cryptocurrency that is able to sustain the total global load of non-cash transactions. Below all of this is a small and very moderately growing Figure 2. The main chain (black) consists of the longest series energy-efficient blockchain with Proof-of-Capacity consensus of blocks from the genesis block (green) to the current block used for bookkeeping the opening and closing of the Dymaxion (orange). Orphan blocks (purple) exist outside of the main layers above it. See also central figure 7. chain on so called forks† . 1. Building Blocks 1.2 Smart Contracts 1.1 Burstcoin Blockchain A blockchain can be seen as a distributed database that is en- Burstcoin is a digital cryptocurrency and payment system de- suring consistency and validity by verification work done by rived from the Nxt cryptocurrency. As such it is based on the a majority of participating nodes in this blockchains network. blockchain technology (Figure 2 shows the general concept of While cryptocurrencies such as Bitcoin speak of the blockchain a blockchain with the most essential terms explained), but is as a public ledger, others such as Ethereum put an emphasis on using a special class of consensus algorithm called proof-of- smart contracts - and not only monetary transactions - to store capacity (PoC). This algorithm allows for the usage of storage in their blockchain. space instead of raw computational power as is being used in A smart contract is a procedural way, usually done by a com- 1 Ethereum has a very similar concept under the name Raiden Network puter program, to facilitate, verify, or enforce the negotiation
The Burst Dymaxion — 4/23 or performance of a contract. Compared to their “inanimate” main proposition of this concept, is its feasibility to provide a paper-based ancestors, smart contracts also fulfill the role of good infrastructure for microtransactions, mainly targeted at otherwise needed lawyers (verification), notaries (validation) the IoT industry. and executors (enforcing). For this very reason, smart contracts Any microtransaction-capable infrastructure seeks to over- are seen as a disruptive technology to future digital economies. come the problem of paying a fee that is larger than the amount Burstcoin has been given a smart contract system in 2014. of value being transferred. The smaller the value of a micro- The formalism used is called AT (Automated Transaction) and transaction becomes, the more applications open up to it (tip- has been proposed and implemented by CIYAM. ping, machine-to-machine transactions, microservices, spam As turing-complete formalism, ATs are both powerful (ex- protection etc.) but the harder it gets to lower the transaction pressiveness) and dangerous (verifiability) and have been used cost below the value being transferred. Ideally the transaction only sparsely as templates to facilitate simpler smart contracts cost would be zero, but then again - what would the incentive (SCs), such as lotteries, crowdfunding and an asset exchange. for a transaction processor be? Because of the expressiveness of the formalism, other possible IOTA answers this dilemma by unifying the roles of net- applications are basically limitless, yet have to be designed work participants. There is no more discrimination between a with great care to avoid situations such as the DAO debacle transaction (tx) issuer and a tx verifier - a node has both these that led to the hard fork and community split of Ethereum and roles. The cost for issuing a tx is to provide the verification to Ethereum Classic.2 at least two other txs. Doing this, the network security is kept up by issuing transactions. It is assumed that the nodes check 1.3 DAGs and The Tangle if the approved transactions are not conflicting. If a node finds A directed acyclic graph (DAG) is a restricted form of a finite that a transaction is in conflict with the tangle history, the node directed graph that has no directed cycles. In short, it is a will not approve the conflicting transaction. If a node issues data structure with interesting computational properties and a new transaction that approves conflicting transactions, then widespread use in computer science. We assume the reader has it risks that other nodes will not approve its new transaction, basic understanding of graph theory and encourage to acquire which will fall into oblivion. it otherwise. Figure 3 shows an example of a simple DAG. While not discussing the implementation, the IOTA white paper suggests there being a hidden PoW in the validation process: For a node to issue a valid transaction, the node must solve a cryptographic puzzle similar to those in the Bitcoin blockchain. This is achieved by finding a nonce such that the hash of that nonce concatenated with some data from the approved transaction has a particular form. The “incentive to participate” for a node (assume a node has no need to perform transactions, so why should it validate other transactions) is achieved by nodes keeping statistics on their peers, e.g. how many new transactions are received from a neighbor. If one particular node is “too lazy”, it will be dropped by its neighbors. Therefore, even if a node does not issue transactions, and hence has no direct incentive to share new transactions that approve its own transaction, it still has incentive to participate. 1.4 zk-SNARK vs. Ring Signature The acronym zk-SNARK stands for “Zero-Knowledge Suc- Figure 3. A simple directed acyclic graph consisting of 5 cinct Non-Interactive Argument of Knowledge,” and refers to a vertices (a-e) and 8 edges connecting them. The arrows stand proof construction where one can prove possession of certain in- for a direction and there are no loops - therefore acyclic. formation, e.g. a secret key, without revealing that information, and without any interaction between the prover and verifier. The IOTA white paper  describes a blockchain-less ap- A ring signature is a type of digital signature that can be proach of ensuring transactions in a cryptocurrency with the performed by any member of a group of users that each have use of a so called “Tangle”, which is in essence a DAG. The keys (to sign messages). Therefore, a message signed with a 2 https://www.coindesk.com/ ring signature is endorsed by someone in a particular group of ethereum-classic-explained-blockchain/ people. One of the security properties of a ring signature is that
The Burst Dymaxion — 5/23 it should be computationally infeasible to determine which of Both ring signatures as well as zk-SNARKs do have their the group members’ keys was used to produce the signature. advantages and drawbacks in anonymizing transactions, which Both concepts are central elements of cryptocurrencies with leaves either of these functionalies as optional feature in the a high focus on anonymity, namely ZCash (zk-SNARKs) setup parameter space for created tangles. and Monero (ring signatures). 1.5 Blockchain Binding with ACCTs Atomic Cross-Chain Trading (ACCT) is a mechanism, where two (or more) parties own coins in separate cryptocurrencies, and want to exchange them without having to trust a third party. The term “atomic” here means indivisible, and refers to the fact that sending coins on one chain and sending other coins on the other chain cannot be performed independent of each other. If these transactions could be performed independent of each other, then while one party could fulfil their side of the bargain and send some coins on one chain, the other party would have the option of going back on his end of the bargain and simply not following through with the protocol, ending up with both coins. A decentral exchange where two parties, A and B, want to exchange tokens can be based on the following process:5 1. A generates some random data (called the secret) x. 2. A generates T x1 (the payment) containing an output with the chain-trade smart contract in it. It allows coin release either by signing with the two keys (Ak and Bk ) or with Figure 4. Rivest, Shamir, Tauman ring signature scheme (secret x, Bk ). This transaction is not broadcast. The chain release script contains hashes, not the actual secrets Ring signatures as referenced by figure 4 are a pretty plain themselves. and straightforward way of a multi-sig application: Given n entities each with pub/priv keys, (P1 , S1 ), (P2 , S2 ), ..., (Pn , Sn ). 3. A generates T x2 (the contract), which spends T x1 and Party i can compute a ring signature σ on a message m, on has an output going back to Ak . It has a lock time in the input (m, Si , P1 ...Pn ). Anyone can check the validity of a ring future and the input has a sequence number of zero, so it signature given σ, m, and the public keys involved, P1 ...Pn . If a can be replaced. A signs T x2 and sends it to B, who also ring signature is properly computed, it should pass the check. signs it and sends it back. Compared to this, zk-SNARKs seem outright “magical” at 4. A broadcasts T x1 and T x2 . B can now see the coins but first glance. You can verify the correctness of computations cannot spend them because it does not have an output without having to execute them and you will not even learn going to him, and the tx is not finalized anyway. what was executed – just that it was done correctly. In real- ity, zk-SNARKs can be dissected into four relatively simple 5. B performs the same scheme in reverse on the alternative ingredients: chain. The lock time for B should be much larger than the lock time for A. Both sides of the trade are now pending 1. represent as a polynomial problem but incomplete. 2. random sample for succinct check 3. homomorphic encoding / encryption 6. Since A knows the secret, A can claim his coins imme- 4. zero knowledge by checking structure diately. However, A, in the process of claiming his coin, reveals the secret x to B, who then uses it to finish the We will not explain these in full here, but encourage the other side of the trade with (x, Bk ). reader to visit good introductions and explaining blogs for zk- SNARKs, . ACCTs have the disadvantage that both chains need to We will not discuss the concrete implementation of both implement the ACCT (of course in addition to being capable to concepts here, as there are already off-the-shelf implementa- provide smart contracts) in order for this to work. tions on github3 4 If Burst was to provide some ACCT with - say - Bitcoin, 3 https://github.com/apuljain/ the ACCT handling would have to be implemented not only on Linkable-Ring-Signature 4 https://github.com/scipr-lab/libsnark 5 see Appendix B for a visualization
The Burst Dymaxion — 6/23 Figure 5. Burst/Qora ACCT prototype the Burstcoin side, but also in Bitcoin client(s). While not en- In a way, the formation of a lightning network relies on transi- tirely impossible, it is unlikely to get arbitrary cryptocurrencies tive properties of separate P2P communications. development teams to provide that kind of interoperability. If, The problem with this approach is similar to the problem on the other hand, the implementation of ACCTs happened in of premonetary trade. Achieving transitive pathways for m out a situation where the same development team exercises control of n network participants becomes increasingly difficult if n over both “chains” in question, then this approach certainly is not much larger than m. The old explanation why money seems viable. was invented in the first place holds true for a LN itself: If A Burst already does have a prototype ACCT implementation6 had twenty hens and wanted to exchange them for one pig, he with the cryptocurrency Qora - see figure 5. either had to find someone willing to do exactly that trade, or two other people - say B and C - where C might have had a 1.6 Lightning Network pig wanting two goats and finally B wanting twenty hens and The term “Lightning Network” became widely known in 2016 having two goats. The categories in that example translate to from a publication by Poon and Dryja in the context of the incompatible monetary volume and time in a LN. Bitcoin scaling debate. In essence, it is an off-chain protocol It is possible that operation of a LN is well feasible with running on a P2P network of nodes for making micropayments the help of powerful intermediaries/facilitators who command of digital currencies. Said nodes form a scale-free network large financial resources. In this case, however, we can expect of bidirectional payment channels without delegating custody the LN to exhibit a decentral and not a distributed topology - of funds or trust to third parties. Yet, these payment channels compare figures 6 and 8. are bound by smart contracts to the underlying blockchain to ensure enforcability in case of uncooperative participants. 1.7 Coloring At the basis of this mechanism are so called Hash Time- In an abstract sense, coloring is a simple tagging technique Locked Contracts (HTLC) implemented as smart contracts on used to allow distinction, thus coexistence, of various instances the blockchain. When opening a payment channel, participants of a class in a common data context. must commit an amount in a transaction which is registered on In cryptocurrencies, colored coins is a concept that allows the blockchain. This can be seen a collateral and guarantees attaching metadata to transactions and by this leveraging the enforcability of transactions (so-called commitments) done off- coins infrastructure for issuing and trading immutable digital chain in the payment channel. Given their similarity, ACCTs assets that can represent real world value. are probably the origin of the technique now called HTLCs. Colored coins are a method to track the origin of Burst As introduced for Bitcoin, this system is conceptually not coins, so that a certain set of coins can be set aside and con- an independent overlay network; it is more a deferral of state on served, allowing a party to acknowledge them in various ways. the standard blockchain, as the enforcement is still occurring Such coins can be used to represent arbitrary digital tokens, on the blockchain itself (albeit deferred to future dates and such as stocks, bonds, smart property and can even represent transactions). real-world objects. Because the payment channels are bi-directional between When a coin is "colored", it can be traded on the Burst two parties only, forming a network where n parties can par- network just like any other coin in the system. This allows ticipate in transactions between each other depends on finding BURST to be exchanged for whatever object the colored coin “paths” between two parties that can go through intermediaries. represents. This concept forms the basis for the Burst Asset 6 based on http://ciyam.org/at/at_atomic.html Exchange.
The Burst Dymaxion — 7/23 2.1 Dymaxion Tangle vs. IOTA Tangle While DAGs have been used as part of graph theory for several decades now and are mathematically well understood today, their use in cryptocurrencies as generalization of the blockchain is a novelty. We are convinced of the advantages tangles bring to ques- tions of scalability and decentral design, but not every aspect of the current iri (IOTA reference implementation) seems to be the best possible design choice. Our biggest concern is the cryptographic hash function used in IOTA: Curl. The IOTA vulnerability report has shown already practical signature forgery attacks and while IOTA no longer uses the Curl hash function to hash transactions as part of the IOTA signing process, Curl is still used for other purposes in IOTA. Similarly disturbing seems the claim of one of the IOTA founders to have placed that vulnerability into Curl on purpose, to be able to shut down copycats. Even leaving the ethical aspect aside - publishing IOTA source under GPL3 license and allegedly placing a cryptographic vulnerability in its core hash function - experience shows, that control over backdoors and vulnerabilities seldom remains in the “right hands”. By Figure 6. Decentral (left) vs. distributed topology. extension, this puts IOTA itself at risk. It seems more apropriate, to use the iri only as design guid- ance in some aspects, using standard libraries and standard Since 2014 it has been suggested, that various coloured hashing algorithms complemented by own peer-reviewed im- coin protocols could be of interest to banks and major finan- plementation. cial institutions,7 because of its inherent applicability to their Moreover, the biggest difference being the underlying Burst requirements to use self-issued, quasi-private, yet decentral and blockchain allowing Dymaxion tangles to be opened and closed trustless transaction channels with desired properties. against a point of reference, as well as the existence of an Since then various implementations like CoinSpark8 and arbitray number of logically separated (colored) Dymaxion MultiChain9 have evolved from the original concept by Rosen- tangles compared to just one IOTA tangle. A future IOTA could feld. very well live within the Dymaxion, maybe even in its native While our proposed combination of Colored Coins and form. tangle-based Lightning Network (which we call colored tan- gles) is a brand new concept, prototype implementations for 2.2 Blockchain-Dymaxion Interaction the Bitcoin LN do already exist.10 Figure 7 is the top-level view on the presented concept. The Burst blockchain acts as a fundamental bookkeeping chain 2. Putting it All Together where opening and closing of the Dymaxion layer is recorded. Payment processors, banks, exchanges etc. may use ACCTs As we have seen, all building blocks for our proposal are already (ACTTs really) to open and close payment channels with de- in place, not only as theoretical concepts, but well developed sired properties (network size, validation reqirements, etc.) and and even tested in real-world application use. These are solved keep them open for a certain period (TTL) - e.g. one day for problems. daytrading on Wallstreet. The realization of the Burst Dymaxion therefore seems The tangles work asynchronously with no inherent clock more like an engineering rather than a research task, but the signal contrary to the 4-minute target time for block generation major contribution of this paper is the seamless integration in the Burst blockchain. Transaction latency in a Dymaxion of said components into a new framework with significant layer is effectively network latency plus validation latency synergistic gains. ttx = tnl + tvl (1) 7 https://is.gd/vz3oiN with network latency effectively being the packet time between 8 http://coinspark.org/developers/ two peers as sum of network-hop latencies, often measured 9 https:/www.multichain.com/ in milliseconds, and validation time being PoW, PoC or PoS 10 https://is.gd/7RcPqf runtime on the node - measured in seconds.
The Burst Dymaxion — 8/23 Figure 7. Dymaxion Top-Level view: set of colored tangles over the Burst blockchain. Each tangle representing an independent Lightning Network for P2P transaction channels. Block 92 is magnified to show inner structure containing transactions some of which are opening and closing tangle LNs. With the average block time on the Burst blockchain being tive are called peers. This infrastructure is already in place said 240 seconds, this can lead to a situation of potentially for Burst and there is a communication protocol between these short-lived tangles that can be opened and closed intra-block nodes to ensure transactions are propagated, blocks are down- i.e. within one and the same block on the Burst blockchain (see loaded for new or outdated nodes (blockchain syncing) etc. - see green tangle). The application for this could be a voting where https://burstwiki.org/wiki/The_Burst_API some hundred or a thousand globally distributed voters would Similar to the Lightning Network Daemon (lnd11 ), we im- have to give their vote within a short period. plement a Burst Dymaxion Daemon (bdd) that can - but doesn’t Some tangles might live for a day or longer and bear the need to be - an integral part of the Burst wallet12 . transactional load of a financial institute or an exchange and In any case, the bdd would be active on nodes that do only at the end of the day or the respective period close and participate in specific colored tangle networks. The decision record that state in the blockchain. whether to participate or not, would be defined on dynamic node As mentioned in section 1.5, ACCT stands for Atomic Cross capabilities as defined in the respective CIP - see appendix C. Chain Transaction. We will use a slightly modified ACTT - One particular detail of the DL network infrastructure ac- for Atomic Chain-Tangle Transaction from now on. Of course, quisition - node discovery - is presented in the next section (see figure 7 is a very simplified view if you consider that basi- 3.1). A tangle-based network is made for a high degree of de- cally every block pictured potentially could - without any Burst centralization and transaction performance, but has a problem blockchain scaling CIP - have up to 255 incoming or outgoing upon startup if there is not a network of sufficient size in place. ACTTs. We see only three ACTTs for block 92, one incoming The Burst Dymaxion solves this problem by effectively ACTT for blocks 90 and 95 and one outgoing for block 91 providing the equivalent of a cloud service for nodes. Figure 8 and even this simplification already suggests a multiple of the shows the Burstcoin core network - running the PoC blockchain traditional blockchain-only transaction capacity. protocol. This base network has been running for over 3 years We will discuss the implementation details (opening, clos- now and will always form a source of nodes potentially avail- ing, enforcing) in section 3. able upon DL initiation. 2.3 Using Nodes P2P for ad-hoc DLs 11 https://github.com/lightningnetwork/lnd Each cryptocurrency network consists of nodes and each of 12 BRS- Burst Reference Software: https://github.com/ these nodes has contact to other nodes which from its perspec- PoC-Consortium/burstcoin
The Burst Dymaxion — 9/23 Figure 8. Burst cryptocurrency core network of nodes as of December 22nd. Node discovery by recursive descent. 2.4 Dymaxion Anonymity tell. If it were a severe bug, potentially somebody Both ring signatures as well as zk-SNARKs are part of the could inflate the money supply by hundreds of mil- DL implementation and upon opening a DL can be chosen lions of dollars, making a profit while lowering the - optionally - to conceal transactions that do happen within price of Zcash for speculators. There are several that specific DL. This has consequences for the respective DL examples of major cryptocurrency bugs that have record (see section 3.3 as well as for monetary supply on the led to a massive misallocation of the quantity of DL. cryptocurrency that should have been in circula- For various reasons, there are no plans to bring RS or zk- tion.13 SNARK anonymity on-chain. Due to the cutting edge nature of ZK-Snarks, in our opinion there is not enough peer-review of Contrary to this, even in the worst case of a bug in the the underlying cryptography to make it mandatory or part of the anonymity functions, due to the TTL of a DL any possible base chain. Participants of a DL who desire anonymity can limit damage is contained and limited to the collaterals used when the value-at-stake by providing zero collateral. An application forming the DL. Even if a “DL goes wrong”, this has no effect scenario would be dissidents that had to carry only clearing on other past, present or future DLs and certainly not on the cost for on-chain operations and use the DL for encrypted base chain. communication, voting or similar. While zk-SNARKs and the Pinocchio protocol do allow We believe a global payment system should be agnostic of for concealment of even the value being transferred, said prob- any moral, legal, monetary or social requirements as these are lem with an uncontrolled supply of tokens may not be worth most often not global in nature. That’s why groups wanting to the risk. use the DL for anonymous value-transfer activities can do so. Ring Signatures provide a very nice alternative for com- Enabling the anonymity features of a DL is supposed to be the plementary scenarios with anonymity requirements. There is result of a cost-benefit consideration. no risk of uncontrolled supply and in case the DL has a larger One such potential cost to anonymous DL mode of opera- number of members RS can guarantee a sufficient level of tion comes from the fact that: anonymity with good transaction performance levels. While anonymity on-tangle can be covered for a large pa- In Zcash, where there is no “certain scarcity” and rameter space of requirements, on-chain anonymity is some- if god forbid, Zcash had a bug that allowed for peo- what weak by itself. Many addresses are re-used and even ple to generate more Zcash coins than the intended money supply, then it is possible that nobody could 13 http://zcoin.io/zcoin-and-zcash/
The Burst Dymaxion — 10/23 named. Their connection to on-tangle transactions (via collater- 3.1 Opening a DL als) can prevent any anonymization effort on-tangle. Opening a Dymaxion Layer (DL), is the main operation for Fortunately there are two mechanisms to improve on-chain a tangle to spring into existence. While more complex, the anonymity. The somewhat crude approach is a mixing ser- operation is conceptually very similar to an ACCT, creating an vice, that theoretically can provide sufficient concealment of asset or the BTC Lightning Network initiation process. source and target payment streams, but from a cryptographic point of view it is more an obfuscation functionality than real DL Initiation anonymization. A Tangle Initiator creates an on-chain ACTT (which is effec- The other mechanism for on-chain anonymity is a little- tively a HTLC) with several tangle parameters including, but known feature of CIYAM AT: anonymous ACCTs. not limited to: One might think, that anonymous ACCTs are not possible, Consensus Type PoC, PoW and PoS will be supported. PoC because the AT itself has no secrets, but with slight changes to is the preferred way for full nodes with attached storage the process it can be altered to allow just that. (“Plots”), PoW and PoS available for various applica- Normally an ACCT would involve putting the same "hash" tion scenarios, where small tangle nodes (IoT) possibly into two ATs (each residing on a different blockchain, or in our cannot perform PoC or some even PoW. case chain and tangle) and then sending the secret to unlock both in order to make the transaction take place - see appendix Time-To-Live (TTL) Global Time-Lock for the tangle. A B. point in the future where the tangle will fold. Maxi- It turns out, there is no need for the secret to be “identical” mum TTL could be decades, but we propose in a first - so the on-chain AT would store the hash release a max. TTL of 217 blocks on the Burst blockchain e47823401ae24e6885de22e1c427c9df\ (ca. 1 year). Must be bigger than the Subscribe-Time. e477ce5e1095f3c2bf4dcbc35f2c7ee0 Subscribe-Time Deadline for subscribers to sign their collat- while the AT on-tangle could store erals on-tangle. Maximum 360 blocks (ca. 24h). 0d5a2b016e90b3db4d42d5a4c420f75b\ Collateral Collateral in BURST by the tangle initiator. This b131bb4465bdfebb8037daadf0174a54 collateral is used as backing for whatever Units (see be- Both hashes seem completely unrelated, and to some ob- low) the initiator issues. The collateral can be as low as server they are, yet the ATs do have the knowledge that one 0, but then any currency, asset, bonds, futures, shares is a SHA256 of “Burst123”, while the other is the SHA256 or whatever the issued Units represent must be backed of “Tangle123”. (this is of course a trivial example of how to by other means (promises - similar to current Burst As- do this - the in-production version does use a more elaborated set exchange) and might not have as much trust as a callenge-response process than just using fixed strings with collateral-backed issuance. The collateral is blocked and some concatenated id). unspendable for the issuer until the tangle folds. Techni- Assuming we will have numerous ATs operating across nu- cally, the collateral is defined by the BURST address and merous blockchain-tangle DLs all doing transfers at around the it’s UTXo which the initiator names and signs. same time it is pretty clear that any observer aiming for “total awareness” would have to record the total global sum of trans- Features A DL can be created with more parameters defin- actions the Dymaxion is performing. It seems highly unlikely ing its behavior. The most significant probably being for this to succeed when observing a “globally decentralized anonymous transactions and their type, where the partici- transaction machine”. pants (initiator and subscribers) can perform transactions using ring signatures or zk-SNARKs (see 1.4). In this 3. Implementation Details case, records of the tangle transactions (see 3.3) can be kept, but do not contain any information to make the The life cycle of a Dymaxion Layer consists of the following participants of the tangle identifiable. steps Units Number and types of units issued, can be applied e.g. 1. Initiation for different types of stocks like common, preferred for (a) Initiator set up an IPO issuance. Other than that each type of Unit can (b) Creating the DL (Node Discovery, AT) be a 64bit number of Quants. (c) Subscribe Phase Subscribers Optional list of addresses who are subscribers to 2. Operation the tangle - the participants. This is a pull operation, that can happen without interaction of the subscriber himself. 3. Closing It has no effect on the availability of the funds on these (a) Node DL shutdown broadcast particular addresses. All are spendable or can receive (b) Clearing more BURST. Until the subscriber himself does not sign
The Burst Dymaxion — 11/23 his participation on-tangle, the particular address is still detached from the tangle. If this parameter is given, we speak of a private DL else it is a public DL - open to everybody, in which case tx fees for clearing of balances upon folding are paid by the subscribers themself. Cost Creating a tangle implies a minimum transaction fee, which is independent of the collateral. It consists of a fixed and a variable part. The fixed part being a constant fee going to the miners in the block where the tangle opening was recorded .The variable part depends on the number of subscribers - see equation 2 - and is locked to a time until the tangle folds. This fee is intended to cover the tx cost for clearing operations for the participants. Fvar = Nsubscribers ∗ Fordinary−payment (2) Although the on-chain accounts are protected from any form of hijacking - simply being referenced by the tangle initia- tor has no effect - an initiator has also no motivation to reference an arbitrary number of on-chain accounts, as this will raise the Figure 9. Parallel node discovery for 3 colored tangles (DL tangle opening fee. Unspent tx fee (reserved for a subscriber network infrastructure acquisition): Peer lookup, check against who never signed on-tangle) is forfeited for the tangle initiator node capabilities. and goes to the miners of the Burst blockchain block where the folding of the tangle is recorded. The tangle initiator is technically just the first subscriber to While there are defined maximum values for TTL and sub- the tangle, therefore his collateral is defined the same way as scribe time, there is no minimum time given, except the con- for all subsequent subscribers: by providing a signature for the straint respective Burst address with X funds at the time of signing. tttl > tst (3) By successfully recording the tangle to the Burst blockchain, it will get a unique ID - its “color” - and by definition all units must be met. This allows for short lived dymaxion layers that on this colored tangle are limited to this context. As of now, can be opened and closed within one block and its approx. 240 there is no inter-tangle transaction possible, all value transfer seconds life time. Naturally these kind of short-lived DLs are desired to happen between two different colored tangles has to probably more suited for machine2machine IoT transactions go over the Burst blockchain as intermediary. instead. Node discovery Figure 9 shows the step immediately following the DL initia- 3.2 A Transaction within a DL tion: node discovery. In this step, a DL initiation requirement is For consistency reasons, to describe a transaction process on the broadcast to the peers of the node that got the tangle initiation DL, we would like to use parts of the IOTA terminology: sites requirement (not necessarily the node of the tangle initiator). are transactions represented on the tangle graph. The network The peers record this requirement, broadcast it to their is composed of nodes which issue and validate transactions. peers and check the DL parameters against their own capability As in IOTA, the main idea of the tangle is the following: to configuration (see C) and in case the node’s configuration does issue a transaction, nodes must work to approve other transac- allow for handling the requested DL, the node becomes part of tions. Therefore, nodes that issue a transaction are contributing the DL network. to the network’s security. In section 5.1 we discuss the latency and throughput bench- So the price for a transaction on-tangle is not named in fees, marks for tangle initiation under various connectivity scenarios but in validation work done for other transactions. IOTA uses and geographic situation. some kind of “low-effort PoW” to validate a transaction. This design choice makes sense, if you target IoT devices and lots DL Subscription of them. The problem is, it works only with sufficient security Within the Subscribe Time as defined by the tangle initiator, if such a large scale tangle network is already in place. each subscriber must sign his address on-tangle to become The Burst Dymaxion implementation will support all three participant of the newly formed DL. If a list of subscribers has types of consensus algorithms, but we will start off with just been given, the subscriber must be part of that list (private DL) PoC until the network has reached sufficient size and more and the respective tangle can be seen as an invite-only. If no experience from network operation is gained. such list has been given, the DL is open to everyone with an The on-tangle PoC validation will be quite similar to BURST address on the Burst blockchain. mining and also using regular PoC2 plot files, although the re-
The Burst Dymaxion — 12/23 quirements for the plot size of a node will be much lower - in recorded with its hash value so forging the records, although the order of 1-5 GB plot size. The node will validate transac- they may have been archived by a single entity only, is consid- tions also by finding deadlines to the tx hash of the transaction ered impossible. This attribute of immutability is an important to validate. legal requirement in archiving financial documentation. In case it can’t find a deadline below a certain threshold, the Even if all tangle participants choose or are required to keep validation has failed and the node can’t validate that particular the tangle records (similar to bank statements), the storage is transaction. in their responsibility and thus only of their local interest. The If a node manages to validate two transactions this way, it blockchain is free to just record openings and closings of the can use its solution to send a transaction itself and the cycle tangles working above it and sustaining the high-volume trans- repeats (with another set of on-tangle nodes). actional load. Tangle storage is left completely to the private Because this type of validation is verifiable (nodes can domain only to those who have interest keeping the records, yet verify the PoC validation solution found by other nodes), there these records’ immutability is ensured by the decentral Burst is no need for a central entity to issue any specific transactions blockchain. as this is currently the case with the IOTA coordinator. Moreover, there is no gain for nodes to amass more PoC 3.4 Blockchain Enforcing space for validation, as it will not give them more “validation Security of off-chain transactions is enforced by blockchain power”. The PoC validation is a minimum-threshold satisfi- smart-contracts without creating an on-blockchain transaction ability problem, which is either met or not. Should a miner for individual payments. with several hundred TB of PoC2 plots decide to use these for The blockchain serves the function of an arbiter, so it is on-tangle tx validation (which we assume will be more the rule possible to conduct transactions off-blockchain without limita- than the exception), it will not give him any more power than a tions. Transactions can be made off-chain with confidence of regular 1-5 GB node has, as the tx validation PoC2 search is a on-blockchain enforceability and deterministic results. 1st fit. Enforcing the transfer of collaterals after folding of the tan- gle does not require cooperation from any counterparty. Clear- 3.3 Closing a DL ing operations are solely on the ACTT running both on-chain When a Dymaxion layer is closed, we speak of the tangle as well as on-tangle. folding. This is because upon initiation of closing the layer, While subscribed parties can choose to not participate in clearing of all participants’ (initiator and subscribers) balances any on-tangle transactions, their on-chain collaterals remain happens and after the final balances are in the Burst blockchain frozen until the tangle is folded. - as ordinary payment transactions with the respective fee from the tangle setup cost - the whole structure of the tangle is scrapped. As of now, there are two possible conditions when a 4. Security Considerations tangle will fold: 4.1 Collusive Nodes Attack In a tangle network a possible attack scenario is a set of collu- • the tangle TTL has expired sive nodes causing either double spends ot modifying transac- • there are no subscribers (except initiator) to the tangle tions along the graph while validating them for each other and after the subscribe-time causing a so called parasite chain - see , pg. 20. The IOTA paper introduces a family of Markov Chain The first condition is defined by the tangle intiator when Monte Carlo (MCMC) algorithms to counter the problems of creating the dymaxion layer, whereas the second condition is de- bad tip selection from collusive attackers. fined by the subscribers who either do not show up (subscribe) The Dymaxion implementation uses instead a Byzantine on the tangle at all, or who unsubscribe. Consensus Algorithm. From a mathematical model point of Unsubscribing from a tangle does not free up the sub- view, attackers in the network can be considered “faults” in scribers funds on-chain or on-tangle. These still remain locked a distributed computing protocol. Byzantine fault tolerance until the tangle folds. (BFT) is achieved if the non-malicious nodes have a majority agreement on their strategy like handling or default values for Tangle records missing or corrupted messages. If a tangle participant chooses to keep the records, he is free to We use the BFT-SMaRt14 - a Byzantine fault-tolerant state do so, but these are of no relevance to the funds that are now machine replication library to build dependable protocols. Given on the Burst blockchain. f as the number of faulty nodes, the total number of nodes Some participants, most often the initiator if it’s a financial needs to be 3 f − 1 for a correct operation of the transaction institution, are required by law to keep these records and they propagation on the tangle (2 f − 1 honest nodes). can do so, but these records have no impact on blockchain size. This puts an upper boundary for the maximum tolerable On the other hand, the Burst blockchain can serve as war- portion of malevolent nodes at almost 33%. If there are more rant of validity of these privately kept records, as the tangle entry and exit points are stored. The final tangle state is being 14 https://bft-smart.github.io/library/
The Burst Dymaxion — 13/23 than 1/3rd malevolent nodes, trustable transaction validation battle-tested in the wake of the spamming attacks that occured on-tangle is not possible without further security precautions. in July 2017. While we believe that an attack with such a high number of colluding nodes is infeasible for large global networks, this theoretical limit also exists for the IOTA network as a whole, 5. Results and Discussion no matter the tip selection algorithm used. 5.1 Prototype Performance Contrary to this, a Dymaxion layer has additional security We performed various benchmarks with prototypes for DL measures in place. As described in section 3.1, the node discov- initiation consisting of node discovery latency timings, inter- ery run picks potentially from the total set of nodes in the Burst node communication throughput and various cost metrics. network a subset suitable and available for the initiated colored The benchmarks are done with a scripted prototype and are tangle. Given N as the total number of nodes in the Burstcoin preliminary. As such they can be expected to improve, but as network, and T as the number of available and suitable nodes you can clearly see from the numbers, perfomance is good even for the Dymaxion layer tangle, where T ⊂ N (T is proper sub- in the worst case scenario with many adverse effects coming set of N), collusive nodes would have to take up to 33% of the into play. whole Burst network N and not only a subset T . We also would like to point out, that all members of set Latency N are constantly vetted as part of the Burst network and once misbehaving are blacklisted by other nodes in the network, which effectively bars them from being chosen into set T . Even if a tangle initiator would work in collusion with such a network, the tangle initiation parameters cannot be set in a way that would favor the preferred choice of malicious nodes and by this lure the tangle subscribers into a trap to steal their collaterals. 4.2 Spamming and DoS Scenarios Transactions on-tangle are free from fees. If they were truly zero-cost transactions, there would be nothing at stake for some attacker who would like to issue a huge amount of transactions for some own benefit (spamming) or to simply congest the network and hinder other transactions to be validated (DoS). In the worst case, the attacker could amass a processing power that would allow him to execute a whole class of majority attacks with dire consequences to the network as such. Figure 10. Node discovery at reference point IOTA currently counters this problem by the use of a so- (wallet.burst.cryptoguru.org) called Coordinator - a specific node run by the IOTA foundation to make specific transactions called Milestones - to stabilize the Figure 10 shows node discovery latency from our point network until it has reached a size that would make majority of reference, the PoCC online wallet, hosted as some german attacks infeasible. ISP and representing currently the best case scenario for the The Burst Dymaxion design starts at a much better initial prototype. As you can see, the maximum response time from situation. Instead of a centralized entity to rely on, the under- a peer answering (a potential tangle node) is below 0.8s with lying Burst network and the blockchain form already points the majority of peers having answered within 0.1s. This can be of reference upon entry and exit of a tangle. Moreover, nodes considered the best case scenario, where a broadcasting node propagating on-tangle transactions are required to perform a with a good connectivity is also in a region of dense Burst nodes PoC for tx validation. presence. PoS and PoW consensus algorithms for on-tangle transac- For comparison, figure 10 shows discovery latencies from tions will be added as DLs will run on devices not capable to a node with regular internet connectivity (cable) for a home perform a PoC consensus (or are defined for a low resource network, situated near a region with good Burst nodes coverage, requirement operation see C), but the initial mode of operation but no nodes in the own country yet. This can be considered will allow for the network to grow in a decentralized manner the standard situation in areas where the Burst network starts to without any entity to exercise central control over any aspect of proliferate. The PoCC network observer gives a good overview it. of areas with dense Burst nodes presence and their neighboring As mentioned in section 4.1, nodes in the Burst network regions. are vetted and blacklisted when misbehaving (e.g. spamming) Even in this case, almost half of the answering peers remain already. This is actually a functionality that has been added and under 0.1s with the slowest peer answering with a latency of
The Burst Dymaxion — 14/23 Figure 12. Node discovery from a Burst node situated in Figure 11. Node discovery from a Burst node situated in Tokyo - ISP network Prague - home network around 800ms. Even for the very specific case of a flash tan- gle to be formed and folded within one Burst block (ca. 240 seconds) this would leave over nv = (240 − ti − t f )/tavg (4) validation steps within the tangle for transactions. ti being the tangle initation time, t f the tangle folding time, tavg being the median response time per node, 0.085228s in the “Prague” case. Depending on the width of the tangle w = nn /2, this allows for at least nv ∗ w (5) Figure 13. Node discovery from a Burst node situated in on-tangle transactions until folding the tangle potentially in the same block. (238/0.085228) ∗ 10 = 27925 transactions in this Sydney - ISP network case. So even some pathologically small tangle consisting of only with a tangle formed in Europe are shown in figure 13. The 20 nodes, could perform intra-block almost 28k transactions in maximum latency is 1147ms, the median is 631ms which re- a 240s time window (over 10 million tx per day). This is the sults to (237/0.631) ∗ 10 = 3756 transactions during a single green tangle depicted in figure 7. Burst block time (1.35 million tx per day). While this number isn’t even the best possible case, let’s look at some worst case scenarios. Tangle initiation, folding Throughput and operation from a region far away from dense Burst nodes Besides latency, we measured throughput for the peer initiated presence. Once Burst nodes proliferate to form a truly global to form a DL. Table 1 shows the results for the prototype im- and omnipresent network, such a situation may not even exist, plementation. The results are representative, but measuring but it is always good to have a limn→∞ xn estimate. throughput for long-distance node-node communication is nat- Figure 12 shows the situation for a node far away from urally highly volatile, as time of day, global network load or other Burst nodes. We can see a maximum of 1094ms latency regional ISP activity can have an influence. with the median being at 582ms. In this case our formulas 4 “One-off” measurements handle one tx validation (connect, and 5 resolve to roughly (237/0.582) ∗ 10 = 4072 transactions validate) then exit, so we can rule out any network caching during a regular Burst block time (1.46 million tx per day).15 effects, although we can assume for a long-running tangle The most exiled node the PoCC could benchmark is situ- better network throughput performance as peer connections ated in Sydney, Australia. Measured latencies from this node will be in router caches. 15 theoretically 237.812 seconds, but we are rounding to account for further The high performance between “Sydney” and “Tokyo” nodes processing delays is probably due to the fact that these - while geographically
The Burst Dymaxion — 15/23 min \ max Germany Prague Tokyo Sydney is always considered the premier option (e.g. PoC2 CIP in sec- tion C) Germany – 22.30 98.70 68.60 If more rigorous changes are required with significant conse- Prague 11.70 – 3.82 3.24 quences (hard-forking, re-plotting), the cost-benefit evaluation Tokyo 81.80 3.26 – 152.00 must weigh significantly in favor to the benefit aspect. Sydney 52.40 2.63 150.00 – It is evident, the Dymaxion as whole represents the most Table 1. one-off node-node connection throughput in Mbit/s significant update to the Burst cryptocurrency. The claim is to provide a cryptocurrency capable of sustaining the total global load of non-cash transactions. To the best of our knowledge distributed - are operated by the same ISP and evidently have a any Burst stakeholder section should root for this to happen. dedicated connectivity. In general, each change should undergo the CIP workflow The poor connectivity between the “Prague” node and the (see also C). Each feature would have two thresholds that need two “Sydney” and “Tokyo” nodes suggests that it is probably to be met in order for it to become active: 1) an activation block not a good idea to initiate a tangle from your home network. In height, which should be set far enough in the future, so every fact, we assume tangle initiation will be done by large corpo- node operator has enough time to prepare for it, and 2) a defined rations or institutions with dedicated hardware and excellent percentage of signalling nodes to support this feature, which connectivity. would by definition be Burst nodes capable of the feature in Still, let’s examine what throughput-limited performance question. we could expect if a tangle was to be formed between geograph- ically distributed nodes such as in this case. The payload for an ordinary non-anonymous transaction is roughly 200byte. An Acknowledgments anonymous ring signature takes up to 3 kB, while zk-SNARK The authors would like to thank many members of the crypto- tx is roughly 2kB. graphic community for their support and valuable input. Anton Even in our worst-case scenario (Prague-Sydney), we can Yip provided great insight into Burst CIYAM ATs and their see that throughput is not a limiting factor. Theoretically, even applicability to ACCTs. Burstcoin community member Quibus a 2.63 Mbit/s throughput (336.64 kB/s) could allow for took the time for a forensic dilligence of the Burst plotting and • 1683 tx/s ordinary payments mining process. Also, his proposal of a backward-compatible and un-gameable PoC2 can be considered a milestone for Burst. • 168 tx/s RS-anonymous transactions Tom Créance (@Gadrah) helped with the visual representation of some figures. Sergey Blagodarenko for providing insight • 112 tx/s zk-SNARK-anonymous transactions and code on the PoC1 gameability problem. We also would However, compared to the latency-limited 3756 tx per Burst like to express our gratitude to the numerous authors of Bitcoin, block time - giving us “only” 15 tx/s for a pathologically small IOTA, Monero and ZCash. Without the giants who kindly let and excessively relocated tangle, throughput limits are not the us stand on their shoulders, we would not have had the building issue. blocks necessary for the Burst Dymaxion. 5.2 Adoption Process Burst has been around since 2014, yet it is still small enough to allow for significant improvements without a long and paralyz- ing scaling debate. It is natural if cryptocurrencies with large market capitalization have to be very conservative with their changes to the code base given the amounts at stake. While Burst is more flexible in this aspect, as a cryptocur- rency there are certain principles development must adhere to. Changes to the code base must have the approval of the major- ity of users, which in this case are wallet/node operators (not miners). Blindly forking the coin can result in community split and is undesireable in general. Establishing The Burst Dymaxion will be an incremental process consisting of the implementation of several Burst CIPs (see section C - partially C). Each of these steps will carefully adhere to a well defined and transparent process with evident benefits resulting from its adoption. Because the Burst Dymaxion is a layer on top of the Burst blockchain, virtually all essential components of Burst will re- main unchanged and if there is a change, backward-compatibility
The Burst Dymaxion — 16/23 Appendices SHABAL256 operations have to be performed, as each SHA- BAL256 delivers a 32byte (256bit) hash value only. A SpaceMint Paper Errata Because the amount of input data that is being given to the SpaceMint: A Cryptocurrency Based on Proofs of Space SHABAL256 is capped at 4096 bytes, it’s also not " 8 million is a paper about a proposed cryptocurrency named SpaceMint 256bit blocks" that are being hashed in total to get all scoops in which is based on a ”proof of space” concept - some of the a nonce, but exactly 33292288 bytes, therefore a total 1040384 authors presented in another paper. of 256bit blocks. So roughly 18 th of the claimed value in the In section 2 "Related Work", the authors make some claims SpaceMint paper. about Burstcoin as it is the only coin implementing a Proof-of- Capacity consensus - somewhat related to the Proof-of-Space Ad “time-memory tradeoffs” As mentioned in the previous described in the paper. In particular they make 3 claims about paragraph, many estimates of the authors are based on the Burst weaknesses: insufficient information about how Burstcoin works at the time available to them. This results in certain follow-up errors that • By requiring the examination of a constant fraction (0.24%) skew the computations. E.g. there are 8192 hashes computed of reserved disk space, Burst is - according to the authors and not 4097. So for the attack as described by the authors, - inefficient compared to SpaceMint which requires only there is actually only 1/8192th of the total plot space needed a logarithmically proportional amount of examination. (the final 32 bytes to perform the XOR). Now with the current PoC used with Burstcoin, there indeed • Verification is problematic in Burstcoin, because “a miner are time-memory tradeoffs possible, roughly as the authors has to verify 8 million blocks to verify another miners describe. The Burstcoin developers have been aware of this claim”. possibility to use less Capacity and invest more Work for a • Burstcoin being susceptible to time-memory tradeoffs, specific range of Scoops (at this moment the highest 64 Scoops thus allowing miners to mine using PoW and using just a 4096 - 4032 are prone to this attack in a somewhat viable way). small fraction of space to be at the same rate as “honest This attack on PoC mining fairness was possible in theory, miners”. but not feasible economically, because the PoW required for this mode of operation consumed far more energy than a PoC We would like to clarify and refute some of these claims, mining style. However recent advancements in hardware and because they seem to have originated in the SpaceMint authors’ GPU performance do show, that economical feasibility is just a incomplete understanding of the Burstcoin PoC and even some matter of time. simple arithmetic mistakes. Therefore this situation must be addressed. A PoC2 CIP (see C) is underway to ensure the Burstcoin blockchain is not Ad “inefficient examination” It is correct, that Burstcoin gameable in any way - not even with the advent of any theoreti- requires per round - each block every 240 seconds - 1/4096th of cal SHABAL256 ASIC devices postulated by the SpaceMint the space reserved on disk to examine16 . The authors also point authors. out correctly this being 0.024%. Unfortunately in their model comparison with SpaceMint, using a 1TB mining space, they Summary Burst is neither inefficient in plot examination dur- transform this 0.024% into 24 gigabyte of data to be examined ing the mining process, nor is the load put on a verifier “too for Burst, while SpaceMint allegedly requires only 24 megabyte high”. We agree that there is a small theoretical possibility to be examined. for “time-memory tradeoffs”, but as of this moment its security Due to lack of availability we were not able to verify the impact is low. Burst will undergo a series of upgrades and this claimed requirements for SpaceMint, but 0.024% of 1TB cor- issue will be addressed by a PoC2 CIP (see section C). rectly translates into 240 megabyte, thus a factor of 100 lower We would also like to add that reducing mining and verifica- than the authors’ wrong comparison value. We therefore be- tion effort is not necessarily the ultimate goal. The SpaceMint lieve, that the claim of Burst inefficiency merely exists due to authors are aware of so called Nothing-at-stake problems: this error in the SpaceMint paper. When replacing PoW with a different type of proof Ad “problematic verification” The authors avow that their that is computationally easy to generate (such as assessment of the Burst plotting and mining process is only PoSpace), a series of problems arise which are their best guess, based on the - admittedly - sparse and informal collectively known as nothing-at-stake problems. specification at the time of writing the paper. They base their Intuitively, because mining is cheap, miners can claims mainly on the old, and even at the time of its publication (1) mine on multiple chains, and (2) try multiple not exact Mining/Plotting diagram. 17 blocks per chain, at very little additional cost. (3) The biggest deviation from the actual situation results from These two problems potentially allow for double- the fact, that in order to compute one scoop (64 bytes), two spending attacks and slow down consensus. 16 a so called Scoop 17 See also https://is.gd/bwPjCb We believe the Burst mining process is a premier example
The Burst Dymaxion — 17/23 Figure 14. Proof-of-Capacity 2.0: backward-compatible plots with scoops consisting of interleaved SHABAL256 hashes. Preventing time-memory tradeoff for high-range scoops. of an equilibrium between energy-efficiency and nothing-at- stake problem prevention. Figure 17. Burst/Qora ACCT step 3: Alice sends after examining the AT from Bob, the key to the Qora AT. • Alice (the Initiator) wants to trade Burst for Qora. Figure 15. Burst/Qora ACCT step 1: Alice deploys an ACCT AT on Burst that will contain the responder’s address, quantity, • Bob (the Responder) wants to trade Qora for Burst. password and expiration time. Two hashes are made of the password: key and lock Now there are only 4 steps necessary to be provided by the AT mechanism running on both the Burst and the Qora chains - see figure 15 to 18. The communication between Initiator and Responder, like sending the details of the generated ATs and INitiator to Resipient key in case everything went well is of course done off-chain and the mode of operation depends on what communication channels are established (website, phone, face to face,...). C Burstcoin CIPs Burstcoin Capability Improvement Proposals18 establish a pro- Figure 16. Burst/Qora ACCT step 2: Upon examining Alices cess, defined by the Burst community, similar to BIPs19 (Bit- AT, Bob deploys an ACCT AT on Qora with the initiator’s coin Improvement Proposals) and EIPs20 (Ethereum). address, quantity, lock sent from Alice and expiration time CIPs (short for "Capability Improvement Proposal" or even (less than than what Alice set). "Coin Improvement Proposal") are meant to advance further development of Burstcoin and describe proposed standards for the Burstcoin platform, including core protocol specifications, B The Burst/Qora ACCT Process client APIs, nomenclature and contract standards. The general process of an ACCT is described in section 1.5. 18 https://burstwiki.org/wiki/CIP For the concrete Burst/Qora realization, which to the best of 19 https://en.bitcoin.it/wiki/Bitcoin_Improvement_ our knowledge was the 1st ACCT ever realized, please see the Proposals depicted reference below. 20 https://github.com/ethereum/EIPs
The Burst Dymaxion — 18/23 Figure 18. Burst/Qora ACCT step 4: The Qora AT sends the Amount to Alice’s address and reveals the key to Bob for the Burst AT. Bob sends the key to the Burst AT and receives the payment. What’s not covered by CIPs are changes or improvements to the coin that can be done without any change to the protocol or API, such as UI or usability improvements. Improved wallet- Figure 19. PoC1 time-memory tradeoff PoW miner speed. UI or initializing a Burst address with a public key without an Y-axis shows nonces/minute on 1 CPU core, X-axis the nonce outgoing transaction fall into that category. number. source: Sergey Blagodarenko PoC2 The “time-memory tradeoffs” mentioned in the SpaceMint pa- As can be seen in figure 20, PoC1 optimized plots provide per are real and show a weakness of the PoC (henceforth PoC1) significant advantages compared to unoptimized plots. The consensus used in Burst. A proof-of-concept implementation number of seeks of a unoptimized PoC1 plot is basically n- exists (see figure 19) - improving its speed is only an engineer- times the number of nonces in that plot, which can be in the ing task. While they currently do not represent a fatal threat to millions. Burstcoin (one can assume around 2% of all mining capacity Compared to this, the difference between seek times of going to dishonest miners), it is good practice for a cryptocur- a PoC1 plot interpreted as a PoC2 (unoptimized PoC2) and rency, as well as a sign of responsible development, to address “native” PoC2 is merely a factor of 2, as for each PoC2 scoop, such issues in a timely manner. Moreover a Burst blockchain two PoC1 scoops have to be read (consisting of the two 32-byte being the backbone of many high-volume payment channels SHABAL256 hashes). must address security issues even if they haven’t yet crossed This factor 2 applies to both unoptimized as well as opti- the threshold to practical applicability. mized PoC1 plots. Therefore several Burst core developers discussed ways to address this problem in a way that would not only fix the issue, Burst Units Nomenclature but also do so with minimum impact to the current stakeholders, Burst is divisible, similar to many other cryptocurrencies, into in this specific case miners who are vested with a large capacity 100 million parts. Up to now, BURST had an equivalent value of plots. in the low cent range, so nomenclature of fractions of BURST The PoC2 proposal is a minimally invasive way to achieve seemed not important. time-memory tradeoff resistance, while keeping the currently Similar to the nomenclature of Bitcoin units21 we propose used plots functional. Figure 14 shows the concept of hash a simple naming scheme for the fractions of BURST - see table interleaving to re-shuffle SHABAL256 hashes in scoops in a 2. way so that each scoop represents an equal amount of hashing While the canonical SI-names milliBURST (mBURST) and effort. microBURST (µBURST, or uBURST) are probably best used Software used for the mining process can operate on both in technical documentation and protocol specifications, the PoC1 as well as PoC2 format, where PoC1 requires twice the more intuitive names like Burst-cent (BC → “Bessie”) instead 1 reads compared to PoC2 and works on both optimized as well of centi-BURST or 1000 th of this (MilliBessie → “Maybel”) as unoptimized PoC1 plots. can be used for human2human communication. For better PoC2 performance a PoC1 → PoC2 converter We denote the smallest unit of BURST as “Planck”. will be offered. 21 https://en.bitcoin.it/wiki/Units
The Burst Dymaxion — 19/23 Figure 20. PoC1 unoptimized and optimized plots. Optimized plots reduce HDD seek time significantly, by an order of magnitude equal to the number of nonces present. Figure 21. Comparison constant and linear-progressive fee Decimal (BURST) Canonical Name Alternate Name structure inclusion guideline. A linear-progressive fee structure 1.00000000 BURST Burst does allow for microtransactions, while preventing spamming 0.01000000 cBURST Bessie and ensuring miner rewards in same height as in the 0.00100000 mBURST – constant-fee system under high-load conditions. 0.00001000 – Maybel 0.00000100 uBURST – 0.00000001 – Planck way tx fees could be above this upper limit and there is no way - even if users would like to spend more than the minimum of 1 0.12345678 digit reference – BURST per transaction - to get more than 255 transactions into Table 2. Units of BURST and its fractions a block. We propose to make transaction cost and block size dy- namic values to be better able to cope with varying transactional Dynamic Block Size and Tx Cost load on the Burst blockchain. Currently, the minimum tx fee is 1 Burst and the maximum Given the Burst blockchain is stored in a relational database, number of transactions that can go in a block is 255. Both these varying block size is not much of an issue. Yet, space on the hard-coded values do work well only for a narrow parameter blockchain is to be considered a scarce, thus precious resource. space of Burst value and transactional load on the network. Cost of a transaction should therefore depend on how much If we imagine a scenario where Burst rises to a value of 1, space it will occupy within a block and thus on the blockchain. 10, 100 USD - or even BTC levels, an immutable transaction Furthermore, the cost of a transaction should depend on fee of 1 Burst would certainly be inhibitory to network usage. network load, which by extension means fill state of forged While that might not be a big problem in a future scenario where blocks. Currently values for the number of transactions and Burst is used as (valuable) collateral in opening and closing the maximum block payload are hard coded in the burst wallet Dymaxion layers, there are other arguments against keeping source: these values as-is. Hard-capping the lower bound of tx cost to 1 Burst effec- MAX_NUMBER_OF_TRANSACTIONS = 255; tively dismisses possible Burst microtransactions, because a MAX_PAYLOAD_LENGTH = transaction of 0.0012 Burst with transaction cost of 1 Burst is MAX_NUMBER_OF_TRANSACTIONS * 176; illogical. On the other hand, simply lowering transactional cost could If we assume a regular block filled with the current max. lead to spam attacks on the network. The low value of Burst capacity of 255 ordinary payments, the total tx fee would be together with fixed cost per transaction, independent of transac- 255 Burst and the payload would be 255 ∗ 176 = 44800 bytes. tion size in bytes, already did enable a spamming attack in July This means, the current protocol expects - roughly, without 2017 disturbing network operation significantly. headers - to be paid 1 BURST per 176 byte of tx payload. At this moment, the transaction fee possible for one block Compared to this, the bitcoin network22 offers a fee structure of are in the range from 0 Burst (for empty blocks) to 255 000 Burst (for a block filled with 255 generated assets). There is no 22 https://bitcoinfees.earn.com/
The Burst Dymaxion — 20/23 1 to 430 Satoshi per byte, with best-case 0-block delay starting Together, these two modifications (linear-progressive fee at 270 Satoshi/byte. and max 1020 tx/block), would ensure that nominally, miner As of December 2017, the transaction cost structure com- earnings will remain the same. “Nominally” meaning same parison between Burstcoin and Bitcoin (1 BURST = ca. 150 network load, same price. Sat), shows 0.85 Satoshi/Byte in the Burstcoin network and Moreover micro-transactions should be possible and under therefore a roughly 317x higher cost for Bitcoin block space high-load conditions miner profit would be a multiple of what and ca. 900x higher cost for network transactions, as a Bitcoin it is now. By enabling limited micro-transaction capability on- transaction is roughly 500 bytes in size23 . chain now, Burst would open to new applications and markets. In Fiat, this means that at a price of around 3 US-cent per Still, with the progressive fee structure spamming would be out BURST, one byte of payload costs 0.017 cent in the Burstcoin of the question and financially strong market participants could network and around 5.4 cent in the Bitcoin network. basically “throw money at the problem”, when e.g. a financial If the hardcoded tx cost for the Burst network was to remain institute would need to open a Dymaxion layer in the current at 1 BURST minimum, Burst would reach the same level of block, it most probably could do so at a premium. tx cost that Bitcoin has today at a price of 176 ∗ 5.4 = 950.4 US-cent, therefore around $9.50. Dynamic Node Capabilities The network consisting of P2P nodes will never be homogenous as hardware capabilities of the nodes will always differ. In our Tx no. Tx fee Total fees opinion it is not desireable to make artificial (1-dimensional) 1 0.00735 0.00735 distinction between so-called “super-nodes” and “regular-nodes” 100 0.73500 37.11750 as some cryptocurrencies do. Neither does it seem to be the 255 1.87425 239.90400 right design decision if some minimum capability requirements 510 3.74850 957.74175 on nodes are imposed to all nodes (such as available memory) 765 5.62275 2153.51325 thus leaving potential capacities unused. 1020 7.49700 3827.21850 We believe a fine-grained configurability of nodes, extend- Table 3. Progressive Tx fee reference: per-tx fee and total ing on current principles, has to be implemented. While today block tx-fee for various number of transactions in a maximum the BRS configuration options allow to allocate number of 1020 tx/block model. CPUs - or enabling GPU support to the processing capacity and also define how much network traffic and peers a node is willing to cope with, more fundamental settings are not possible. If we look at todays price levels, Burstcoin miners were In the wke of the spam attacks, limits on mempool have ensuring the network for a maximum payment of 91800 Burst been hard-coded in the wallet. While this proved to be a very daily. On average, the network has been doing around 5000 effective measure against memory DoS attacks - what the spam tx/day. So in addition to the block reward, the transaction fee attack effectively was - it prevents nodes with higher resources reward per day, for the whole Burstcoin network is around $150 to make use of their full potential to support the network. We in December 2017. propose the mempool size being configurable to better adjust In order to cope with future development of BURST price to the node capabilities. and transaction volume, we propose a progressive tx fee struc- Other parameters, similar to packet introspection of routing ture guideline, where wallets can decide and priorize what protocols but in this case applying to parameters of transaction transaction to include in the current block depending on the datagrams, could define policies for nodes, e.g. which trans- fill-state of the current block and memory pool backlog - see actions to relay and which not. This is common practice e.g. figure 21. The area of the blue rectangle and triangle is roughly in the Bitcoin network, where a node can decide to support the same (255 vs. 240). In order to be included in the next low-fee transactions (by forwarding them) or not. block a transaction currently in mempool must provide a higher We believe a structure in the network will evolve from these tx fee than the currently free slot in the block requires. If the parameters, better adapted to the underlying node capabilities - transaction can fulfill this requirement, it is included in the or what node operators are willing to provide - forming natu- block, if it can’t it will wait in mempool for the next block, rally a hierarchy of nodes with backbones and super-backbones where this process repeats. emerging from that. The progressive approach opens the door for more on-chain On the other end of the scale it will also allow nodes to scalability. Instead of limiting the max. number of transac- participate which are not capable of doing so today. Very small tions to 255, we could now theoretically have an open limit embedded devices from the ubiquitous IoT, that can support and solve much of the scalability issues blockchains have per the network serving even merely as repeaters with very little se. Instead, we propose a conservative extension of the max. resource requirements. number of tx.transactions to 1020 (4-fold). With the same linear In section C we talked about transactions that have to re- progressive rule, the reference values for a 1020-tx “triangle” main in mempool in case they could not be included in the are shown in table 3. current block. Because nodes have limits for mempool size, 23 https://en.bitcoin.it/wiki/Transaction_fees transactions with the lowest fees are being discarded from mem-
The Burst Dymaxion — 21/23 pool first if maximum mempool size is reached. Acting as a spam prevention, this also ensures a priorization of high-fee priority transactions. Nodes with more resources and higher mempool storage parameters will still have these low-fee transactions in mempool while others may have already discarded them. So while it will not be impossible for a low-fee transaction to “survive” until it gets included in a block (dip in traffic volume), tx-fee is certainly proportional to chance of being included in a timely manner or at all. Dymaxion The Burst Dymaxion is not only the most significant update to the Burst cryptocurrency, it is the biggest technological update any cryptocurrency has ever received. Each of the compo- nents features a full outstanding protocol feature by itself, so establishing the Dymaxion as a whole needs to be done as an incremental process, split up into several separate CIPs: • adding ring signatures library • adding zk-SNARKs library • adding parts of the IOTA iri • adding ACCT (ACTT) templates As mentioned earlier, all of these components are already Figure 22. Symmetric Feistel cipher transformation for in place, and some may be at the time of the publication part of individualization of a PoC3 plot file.‡ the BRS repository. The integration and enabling of these fea- tures will be subject to a community-approved roadmap defined by block-height and adoption rate (percentage of supporting The protocol would allow these files to be announced to the wallets in use) for the features as described in section 5.2. network (SHA256, size) and voted on by nodes for validation Also the features themself will be establishen in an incre- inclusion. If a very high percentage (say 95% threshold) of the mental process. While there is support for all major consensus nodes would vote in favor of adding these files to the pool of algorithm types, only PoC - as already present - will be enabled “dual-use plots” which also implies storing these files themself, in the first instance of the Dymaxion. each node would find deadlines in these files based on a suc- cinct test of values of a virtual hard-to-pebble trees laid Post-Dymaxion: PoC3 over individualised versions of these files. Once in the pool The advantages of PoC in comparison to PoW or PoS have of accepted plots, PoC3 plots would need to remain there, be- been mentioned and examined under multiple perspectives cause while for mining their presence is optional, for ab-initio already. While PoC and its designated successor PoC2 are validation of the blockchain when resyncing their presence is energy-efficient, which in our opinion is the only sustainable mandatory.24 way for a cryptocurrency with global impact, the space used First, each PoC3-accepted file would undergo a Feistel ci- for the PoC consensus is of no other use than to perfom mining pher transformation (fig. 22) with the numeric Id of an account and tx validation for the Burst blockchain. to individualize it as plot and counter grinding attacks. The Critics have pointed out that the disk space used for Burst mining process would assume the file being mapped on a RB- is “lost” or “wasted” otherwise as plots are not really usable Tree(figure 23) layout as seen in figure 24. for anything else. This is formally true and because of this we Let the basic operation of the Feistel encryption with the propose establishing a Proof-of-Capacity 3.0 consensus some round function F and sub-keys K0 , K1 , . . . , Kn for the rounds time after the Dymaxion comes to full effect. 0, 1, . . . , n be as follows: This PoC3 will exist in parallel to PoC2. It will be based Split the plaintext block into two equal pieces, (L0 , R0 ), in on dual-use data instead of the Burst mining-only plots of PoC our case two 32-byte chunks. and PoC2. Dual-use means real-world data, like movies, audio, Wikipedia archive files, OpenStreetMap GIS data and more. In 24 For the future, we can picture a situation with a distinction of “full PoC general, large immutable files of permanent interest to all - not nodes” containing all PoC3 and being able to make such a resync and “restricted the private word document, holiday picture or browser cache. PoC nodes” being able to sync blockchain from the full nodes
The Burst Dymaxion — 22/23 in a logarithmic way, also the position of a submitted deadline on this path modifies this deadline logarithmically (in the case of this binary tree structure this resolves to simply halving the 1 value for each level n : v = 2n−1 , so 12 for level 2, 14 for level 3 etc. Using Feistel ciphers allows for individualization, yet loss- less retrieval of arbitrary data. The symmetry guarantees the cipher being of same length as the original data, paving the way for a dual-use Proof-of-Capacity consensus. The Burst blockchain will thus not only form the funda- mental layer for a truly global transaction network, it can also take over a custodian role in globally distributed redundant Figure 23. Red-Black binary tree structure for traversing PoC3 storage. This means it can be used for the safe preservation of plots. Each node being a 64-byte data chunk, each depth all information that has been acquired by our civilization and represents next level as depicted in the data layout of the next that is of permanent interest. figure. Figure 24. Data layout of a PoC3 “plot”. Each level twice the size, maximum 64 levels allow for a plot size of up to 2 YiB (yobibyte). For each round i = 0, 1, . . . , n, compute Li+1 = Ri Ri+1 = Li ⊕ F(Ri , Ki ). Then the ciphertext is Rn+1 , Ln+1 ). Decryption of a ci- phertext (Rn+1 , Ln+1 ) is accomplished by computing for i = n, n − 1, . . . , 0 Ri = Li+1 Li = Ri+1 ⊕ F(Li+1 , Ki ). Then (L0 , R0 ) is the plaintext again. Of course by “plaintext” in cryptography nomenclature we mean the more generic term original data. Similar to the current Burst mining process, a scoop would define the starting point in the data layout (index in the lowest level). The algorithm would traverse the RB tree of 64-byte scoops, where the path decision (start, then red or black for each level) would simply be defined by the bit sequence of the previous block-id 1=B, 0=R. A maximum of 64 steps allows for a maximum PoC3 plot file size of 2 YiB (yobibyte). The values along the traversed path would be concatenated and hashed. All values along the path as well as their hash value are then broadcast to the network, where any nodes can take over the validation of the submitted data. As the length of the path defines the size of the underlying plot
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