Helium Whitepaper

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Helium A Decentralized Wireless Network Amir Haleem Andrew Allen Andrew Thompson Marc Nijdam Rahul Garg Helium Systems, Inc. Release 0.4.2 (2018-11-14) Abstract to affect. These networks are more complicated to decentral- ize as they often require specialized hardware to function. The Internet of Things is an $800 billion industry, with over The Helium network is a wide-area wireless networking sys- 8.4 billion connected devices online, and spending predicted tem, a blockchain, and a protocol token. The blockchain runs to reach nearly $1.4 trillion by 2021 [1]. Most of these on a new consensus protocol, called the Helium Consensus devices need to connect to the Internet to function. However, Protocol, and a new kind of proof, called Proof-of-Coverage. current solutions such as cellular, WiFi, and Bluetooth are The Miners who are providing wireless network coverage in a suboptimal: they are too expensive, too power hungry, or too cryptographically verified physical location and time submit limited in range. proofs to the Helium network, and the Miners submitting the The Helium network is a decentralized wireless network best proofs are elected to an asynchronous byzantine fault tol- that enables devices anywhere in the world to wirelessly erant consensus group at a fixed epoch. The members of the connect to the Internet and geolocate themselves without consensus group receive encrypted transactions submitted by the need for power-hungry satellite location hardware or other Miners and forms them into blocks at an extremely high expensive cellular plans. Powering the Helium network is transaction rate. In addition to the blockchain protocol, the a blockchain with a native protocol token incentivizing a two- Helium Wireless protocol, WHIP, provides a bi-directional sided marketplace between coverage providers and coverage data transfer system between wireless Devices and the In- consumers. With the introduction of a blockchain, we inject ternet via a network of independent providers that does not decentralization into an industry currently controlled by rely on a single coordinator, where: (1) Devices pay to send monopolies. The result is that wireless network coverage & receive data to the Internet and geolocate themselves, (2) becomes a commodity, fueled by competition, available Miners earn tokens for providing network coverage, and (3) anywhere in the world, at a fraction of current costs. Miners earn fees from transactions, and for validating the integrity of the Helium network. Our secure and open-source primitives enable developers to build low-power, Internet-connected devices quickly and Note: This whitepaper represents a continuous work in cost-effectively. The Helium network has a wide variety of progress. We will endeavor to keep this document current applications across industries and is the first decentralized with the latest development progress. As a result of the on- wireless network of its kind. going and iterative nature of our development process, the resulting code and implementation is likely to differ from what is represented in this paper. 1. Introduction We invite the interested reader to peruse our GitHub repo The world is becoming decentralized. A multitude of plat- at https://github.com/helium as we continue to open- forms, technologies, and services are moving from central- source various components of the system over time. ized proprietary systems to decentralized, open ones. Peer- to-peer networks such as Napster (created by one of our 1.1 Key Components founders Shawn Fanning) [2] and BitTorrent paved the way for blockchain networks and crypto-currencies to be built. The Helium network is built around the following key com- Now Bitcoin, Ethereum, and other blockchain networks have ponents: shown the value of decentralized transaction ledgers. Exist- ing Internet services such as file storage, identity verification, Proof-of-Coverage We present a computationally inexpen- and the domain name system are being replaced by modern sive Proof-of-Coverage that allows Miners to prove they blockchain-based versions. While software-level decentraliza- are providing wireless network coverage. We anchor these tion has moved quickly, physical networks are taking longer proofs using a Proof-of-Serialization that allows miners 1

to prove they are accurately representing time relative to • Miners join the network by asserting their satellite-derived others on the network in a cryptographically secure way. location, a special type of transaction in the blockchain, and staking a token deposit. Helium Network We demonstrate an entirely new purpose- built blockchain network built to service WHIP and pro- • Miners specify the price they are willing to accept for vide a system for authenticating and identifying devices, data transport and Proof-of-Location services, and Routers providing cryptographic guarantees of data transmission specify the price they are willing to pay for their Device’s and authenticity, offer transaction primitives designed data. Miners are paid once they prove they have delivered around WHIP, and more. data to the Device’s specified Router. Helium Consensus Protocol We present a novel consensus • Miners participate in the creation of new blocks in the protocol construction that creates a permissionless, high blockchain by being elected to an asynchronous byzantine throughput, censor-resistant system by combining an asyn- fault tolerant consensus group. chronous byzantine fault tolerant protocol with identities • Miners are rewarded with newly minted protocol tokens presented via Proof-of-Coverage. for blocks that are created while they are part of the WHIP We introduce a new open-source and standards- consensus group. compliant wireless network protocol, called WHIP, de- • A Miner’s probability of being elected to the consensus signed for low power Devices over vast areas. This pro- group at a given epoch is based on the quality of the tocol is designed to run on existing commodity radio wireless network coverage they provide. chips available from dozens of manufacturers with no • The blockchain employs Proof-of-Coverage to guarantee proprietary technologies or modulation schemes required. that Miners are honestly representing the wireless network Proof-of-Location We outline a system for interpreting the coverage they are creating. physical geolocation of a Device using WHIP without the [Figure 1] shows a visual representation of the Helium need for expensive and power-hungry satellite location network. hardware. Devices can make immutable, secure, and verifiable claims about their location at a given moment in time which is recorded in the blockchain. 2. The Helium DWN DWN We present a decentralized wireless network (DWN) We introduce the core conmponents of the DWN. that provides wireless access to the Internet for Devices by way of multiple independent Miners and outlines the 2.1 Participants Helium network and WHIP specification by which par- ticipants in the Helium network should conform. Routers There are three types of participants in the Helium network: pay this network of Miners for sending data to and from Device, Miner, or a Router. the Internet, and Miners are rewarded with newly-minted tokens for providing network coverage and delivering De- Devices send and receive encrypted data from the Internet vice data to the Internet. using hardware compatible with WHIP [Section 2.4]. Data sent from Devices is fingerprinted, and that fingerprint stored in the blockchain. 1.2 System Overview Miners provide wireless network coverage to the Helium • The Helium network is a decentralized wireless network network via purpose-built hardware, called Hotspots [Sec- built around WHIP on a purpose-built blockchain with a tion 2.5], which provide a long-range bridge between native token. WHIP devices and the Internet. Users join the Helium network as Miners by purchasing or building a Hotspot • Devices take the form of hardware containing a radio chip that conforms to WHIP, and staking a token deposit pro- and firmware compatible with WHIP, and spend tokens portional to the density of other Miners operating in their by paying Miners to send data to and from the Internet. area [Section 5.3.3]. Miners participate in the Proof-of- • Miners earn tokens by providing wireless network cover- Coverage [Section 3] process to prove that they are contin- age via purpose-built hardware which provides a bridge uously providing wireless network coverage that Device between WHIP and Routers, which are Internet applica- can use. Miners join the Helium network with a score tions. [Section 3.3.4] that diminishes as blocks pass without valid proofs being submitted. At a given epoch, a new • Devices store their private keys in commodity key-storage group of Miners are elected to a consensus group which hardware and their public keys in the blockchain. mine new blocks in the blockchain and receive the block 2

Token Exchange Peer-to-Peer GPS GPS/GNSS Authentication Coverage Gateway Gateway Machine Router Token Token Gateway Blockchain Router Figure 1. System Overview reward and transaction fees for any transactions included The blockchain consists of blocks which contain a header and in the block once mined. As a Miner’s score drops their a list of transactions. There are several kinds of transactions, probability of being elected to the consensus group and outlined in [Section 5]. mining blocks diminishes. At a given epoch a given block consists of: Routers are Internet applications that purchase encrypted Device data from Miners. In locations with a sufficient Block Version number of Miners, Routers can pay several Miners to Block Height obtain enough copies of a packet to geolocate a Device Previous Block Hash without needing satellite location hardware, which we call Transactions 1..n Merkle Hash Proof-of-Location. Routers are the termination point for Threshold signature by the current consensus group Device data encryption. Devices record to the blockchain to which Routers a given Miner should send their data, As the Proof-of-Coverage [Section 3] is valuable to the net- such that any Hotspot on the Helium network can send work, Miners are required to submit their proofs at regu- any Device data to the appropriate Router. Routers are lar intervals. All Miners have a score, which decays over responsible for confirming to Hotspots that Device data time, and is boosted by submitting Proofs-of-Coverage to was delivered to the correct destination and that the Miner the blockchain. At a fixed epoch, a HoneyBadgerBFT [4] should be paid for their service. consensus group of the highest scoring Miners is elected. For that epoch, all transactions are encrypted and submitted to the consensus group for inclusion in the blockchain. The con- 2.2 Blockchain sensus group is responsible for decrypting transactions using threshold decryption, agreeing on the validity and ordering of The Helium network is a distributed ledger designed to pro- transactions, forming them into blocks, and appending them vide a cost-effective way to run application logic core to the to the blockchain for which the members of the consensus operation of a DWN, store immutable Device data fingerprints, group receive a reward. and furnish a transaction system. The Helium network is an immutable append-only list of transactions which achieves As the consensus group is validating transactions with- consensus using the Helium Consensus Protocol [Section 6]. out having to provide an associated block-proof (beyond Users internal and external to the DWN have access to the a threshold signature), there is practically no settlement time, blockchain, which is a new protocol built from scratch specif- and the transaction throughput is extremely high compared ically for the DWN. to a Nakamoto Consensus blockchain such as Bitcoin or 3

Ethereum. The Helium Consensus Protocol is outlined in 2.4 Wireless Protocol (WHIP) detail in [Section 6]. 2.4.1 Motivation 2.3 Physical Implementation Several Low Power Wide Area Network (LPWAN) technolo- gies are available today. These wireless technologies focus The Helium network is also a physical wireless network on creating long-range, low-power Internet communication instantiation. The participants in the Helium network can for sensors and other smart Devices. Typically these tech- be thought of as follows: nologies trade throughput for range, with data rates as low as 18 bits per second (bps) and range measured in miles. WHIP The Helium network uses a new open wireless pro- In comparison, a typical WiFi network has significantly tocol, called WHIP. WHIP is a long-range, low-power, higher data rates but ranges limited to only a few dozen wireless network protocol suitable for use with commod- feet. Several of these new technologies, such as LoRa [6] ity open-standards hardware. WHIP compatible hardware and RPMA [7], have gained good traction and there are can communicate over many square miles in dense urban many commercial products available compatible with these environments or hundreds of square miles in rural settings. systems. However, we believe a decentralized wireless net- WHIP compatible hardware can also last for several years work should use non-proprietary protocols and modulation using standard batteries. WHIP uses strong public key schemes and that participants in the Helium network should cryptography and authentication occurs using the Helium have the freedom to choose between competing hardware blockchain, and data is encrypted end-to-end between the vendors. We do not consider an open alliance built on top of device and corresponding Internet-hosted router. proprietary hardware to be an acceptable compromise. While there are many open-standard wireless networking stacks, Hotspots are physical network devices that provide wide- such as IEEE 802.15.4 [8] used in the first generation of our area wireless coverage and participate in the Helium net- wireless products, none meet our extremely long range and work. Hotspots transmit data back and forth between low power criteria. It is this lack of open solutions that drove Routers on the Internet and Devices while generat- the creation of a new protocol. ing Proofs-of-Coverage for the Helium network [Sec- tion 3]. Hotspots are manufactured using commodity open-standards components with no proprietary hardware. 2.4.2 Outline Hotspots can co-operate and geolocate Devices using the Helium network without any additional required hard- We introduce WHIP. WHIP is a highly secure, long range, ware. Each Hotspot can support thousands of connected low power, bi-directional wireless network protocol that is Devices, and provide coverage over many square miles. compatible with a wide range of existing radio transceivers Miners operating Hotspots specify the price they are will- operating in the sub-GHz unlicensed frequency spectrum. ing to accept for transport and Proof-of-Location services Authentication with the wireless network uses modern public- for Devices. key encryption and NIST P-256 ECC key pairs, with the public keys for all participants stored in the blockchain. Devices exist in the form of hardware products that contain The modulation format is simple and widely supported, easy a WHIP-compatible radio transceiver and communicate to implement and has excellent resistance to RF noise. There with Hotspots on the Helium network. WHIP is designed are dozens of vendors implementing radio transceivers com- to facilitate low power data transmission and reception, so patible with WHIP, such as Texas Instruments, Microchip, typically Devices exist in the form of battery-powered sen- and Silicon Labs. sors that can operate for several years using standard bat- teries (although mains-powered Devices also work quite WHIP is a narrowband wireless protocol which creates well). Devices can exist in a variety of forms, depending several channels within the unlicensed spectrum and employs on the product or use case, and a variety of transmission frequency hopping to switch between channels. Typically and reception strategies can be employed to optimize for frequency hopping requires a complex time-synchronized transmission/reception frequency or battery life. Device system that is limited in capacity. However, devices using manufacturers are encouraged to use hardware-based key WHIP do not need to coordinate with Hotspots on channel storage which can securely generate, store, and authenti- selection as Hotspots are capable of hearing all channels cate public/private key pairs without leaking the private within the available spectrum at any time. We choose narrow- key. band to accomplish the following goals: In this section, we expand on the components of the wireless Spectral Efficiency It is necessary to operate within unli- network. censed RF spectrum very efficiently. RF is a shared, lim- 4

ited resource, and therefore a focus on efficiency to in- Satellite location information is also correlated with packet crease capacity and improve robustness is necessary. arrival events to provide Proof-of-Location for Devices if multiple Hotspots observe the same packet. This allows Co-Existence Performance As the number of Devices and devices to locate themselves without requiring a GPS/GNSS networks increase, the ability to operate in noisy RF envi- transceiver physically, and therefore provide accurate location ronments without interference is a critical consideration. data at a fraction of the battery life and cost of competing Range Narrowband allows for extremely long-range com- methods. This method is described in detail in [Section 4]. munications, with data rates that scale both up and down We will make both a complete open-source reference design depending on the density of Hotspots. and a finished product available at launch of the Heilum network. 2.4.3 Implementation 2.6 Devices WHIP supports several data rates, channel bandwidths, and error-correction techniques. Hotspots and Devices dynami- A Device is any wireless hardware capable of communicating cally negotiate the combination of these options using a sig- with Hotspots via WHIP. WHIP is designed to facilitate low nalling packet delivered at the lowest bandwidth and symbol power data transmission and reception, so typically devices rate to ensure maximum range for the initial communication. would exist in the form of battery-powered sensors that can function for several years using standard batteries. The full WHIP specification will be made available by the WHIP is designed such that Devices can be manufactured Decentralized Device Network Alliance. using commodity hardware available from a wide variety of vendors with a very low-cost bill of materials (BOM). 2.5 Hotspots The technology in modern radio transceivers, such as the Texas Instruments CC1125 or STMicroelectronics S2-LP, Hotspots are physical network devices operated by Miners enables exceptionally long-range network systems that can that create wireless RF coverage over wide areas. They trans- be built without the need for proprietary modulation schemes mit data back and forth between Routers on the Internet and or physical layers. Some of these radios are available for Devices on the network, process blockchain transactions, around $1 at reasonable volumes. and create Proofs-of-Coverage for the Helium network [Sec- It is recommended that each Device use the Microchip tion 3]. Hotspots can connect to the Internet using any TCP/IP ECC508A or equivalent hardware-based key storage device, capable backhaul, such as Ethernet, WiFi or Cellular. Each which can securely generate, store, and authenticate pub- Hotspot contains a radio frontend chip capable of listening lic/private NIST P-256 ECC [3] key pairs without leaking to several MHz of radio spectrum at a time and can hear all the private key. Also, a wide array of defense mechanisms wireless traffic transmitted within that spectrum. In this con- prevent logical attacks on the encrypted data between the key figuration modulation and demodulation is done in software, storage device and its host MacDevicehine, along with physi- which is typically referred to as a Software Defined Radio cal protections on the security device itself. Users program (SDR). The benefit of this structure is that Hotspots can hear their key storage device as part of the onboarding process any Device traffic transmitted within the frequency range, and defined in the WHIP wireless specification using a defined no synchronization between the Hotspot and Device needs API. to occur. This allows Devices to remain inexpensive and rel- atively simple and reduces wireless protocol overhead. If a Miner wishes to minimize their Hotspot hardware costs, 2.7 Routers synchronized frequency hopping schemes are also permitted Routers are Internet-deployed applications that receive pack- within the specification as a cheaper alternative to a more ets from Devices via Hotspots and route them to appropriate expensive radio frontend. destinations such as an HTTP or MQTT endpoint. Hotspots require a GPS or GNSS receiver to obtain accurate Routers serve several functions on the Helium network, position and date/time information. This satellite-derived including: location is used in conjunction with other techniques to verify that a Hotspot is, in fact, providing wireless network coverage • Authenticating Devices with the Helium network; in the location it claims. Because satellite location messages • Receiving packets from Hotspots and routing them to the are easy to fabricate and do not necessarily prove that wireless Internet; RF coverage is being created, multiple mechanisms are required to validate this work as described in more detail • Delivering downlink messages, including OTA updates, in [Section 3]. to Devices via Hotspots; 5

• Providing delivery confirmations to ensure transport trans- we can obtain cryptographic proof of the approximate loca- actions are honest; tion and time of events occurring within the Helium network. • Providing authentication and routing mechanisms to third- party cloud services such as Google Cloud Platform or 3.1 Motivation Microsoft Azure; and Most existing blockchain networks such as Bitcoin [9] and • Storing and making available a full copy of the blockchain Ethereum [5] use a Proof-of-Work system that relies on an ledger by acting as a full node [Section 5.5] algorithmic puzzle that is asymmetric in nature. These proofs are extremely difficult to generate, but simple for a third When a Hotspot receives a data packet from a Device on the party to verify. Security on these networks is achieved by Helium network, it queries the blockchain to determine which the network-wide consensus that the amount of computing Router to use given the Device’s Helium network address. power required to generate a valid proof is difficult to forge, Anyone is free to host their own Router and define their and as subsequent blocks are added to the blockchain, the Devices’ traffic to be delivered there by any Hotspot on the cumulative difficulty of the chain becoming prohibitively Helium network. This ability allows users of the Helium difficult to fabricate. network to create VPN-like functionality whereby encrypted data is delivered only to a Router (or set of Routers) that they These computation-heavy proofs are, however, not otherwise specify and can optionally host themselves. useful to blockhain networks. We define useful as work that is valuable to a blockchain network beyond securing the Routers can implement a system called a channel which han- ledger. While there have been attempts in other networks to dles the authentication and routing of data to a specific third turn mining power into something useful, such as Ethereum party Internet application, such as Google Cloud Platform executing small programs called smart contracts, the majority IoT Core. These channel implementations can take advantage of the work is not useful or reusable. The mining process is of a Device’s onboard hardware security to create a secure, also extremely wasteful, as the determining factor in the work hardware-authenticated connection to a third party which is typically computational power, which consumes massive would otherwise be difficult to implement directly on an amounts of electricity and requires significant hardware to embedded microcontroller. We will make available an open execute. source reference implementation of a Channel that can be used to build additional interfaces to Internet services. The proofs used in the Helium network must be resistant to Sybil attacks in which dishonest Miners create pseudonymous We will also host a high-availability cloud router for anyone identities and use them to subvert the Helium network and to use and also provides and maintains an open-source router gain access to block rewards to which they should not be that is available either as source code or a binary package for entitled. This is a particularly difficult attack vector to manage a variety of operating systems and distributions. in a physical network like the Helium network. We must also The protocol specification required for implementing a router be resistant to a new attack vector: alternate reality attacks, is defined in the WHIP Wireless Specification document that which exist where a dishonest group of Miners are able to will be made available by the Decentralized Device Network simulate that wireless network coverage exists in the physical Alliance. world when it in fact does not. An example of this would be running the mining software on a single computer and simulating GPS coordinates and RF networking. 3. Proof-of-Coverage and We later propose the Helium Consensus Protocol [Section 6] Proof-of-Serialization that uses Proof-of-Coverage to both secure the blockchain and provide an extremely useful service to the Helium network, In the Helium network, Miners must prove that they are pro- which provides wireless network coverage that Devices can viding wireless network coverage that Devices are able to use use to send data to and from the Internet. to communicate with the Internet. Miners do this by comply- ing with the Proof-of-Coverage protocol which the Helium network and other Miners audit and verify. We use a Proof-of- 3.2 Inspiration Serialization to ensure that Miners are correctly representing their time in relation to others on the network, and obtain Proof-of-Coverage is an innovative proof that allows Miners cryptographic proof of dishonest behavior. Several compo- to prove that they are providing wireless network coverage nents of the Helium network, such as Proof-of-Coverage, use W in a specific region to a challenger, C. Proof-of-Coverage Proof-of-Serialization as a cryptographic “anchor” that root is an interactive protocol where a set of targets Tn assert that those occurrences with a cryptographic time proof. With a W exists in a specific GPS location L and then convinces C combination of Proof-of-Coverage and Proof-of-Serialization that Tn are in fact creating W and that said coverage must 6

have been created using the wireless RF network. Proof-of- 3. RF travels at the speed of light with (effectively) no Coverage is the first such protocol that attempts to prove latency the veracity of miners in a physical space, and then use it to Our goal is to verify whether Miners in a physical region achieve consensus on a blockchain network. are acting honestly and creating wireless network coverage With Proof-of-Coverage we aim to solve for the following: compatible with WHIP. To do this, a challenger C determin- istically constructs a multi-layer data packet O which begins • Prove that Miners are operating RF hardware and firmware at an initial target, T1 , and is broadcast wirelessly to a set of compatible with WHIP; sequential targets, Tn , each of which are only able to decrypt • Prove that Miners are located in the geography they claim the outer-most layer of O if they were the intended recipient. by having them communicate via RF; and Each target signs a receipt, Ks , delivers it to C, removes their layer of O, and broadcasts it for the next target. Essentially • Correctly identify which version of reality is correct when an “envelope of envelopes” only decipherable by the intended there is a conflict recipient. Proof-of-Coverage is inspired by the Guided Tour Protocol (GTP) [13] which devises a system for denial of service prevention by requiring a client c to make a request to a Challenger (C ) variety of “tour guide” computers Gn in order to gain access to a server s. The tour guides must be visited in a specific KT 1 KT KT L order and a hash of data exchanged which reveals the location of the next Gn in order. Only after every Gn has been visited can c gain access to s. O O1 OT OL Once c gets to the last stop of the tour, it submits evidence of the first and last stop to s who is able to verify that the Target 1 Target Target L (T1) (T ) (TL) first and last stops of the tour are correct without needing to contact Gn , and that c could only know the first and last stops Figure 2. Multi-Layer Data Packet Deconstruction if it had completed the tour correctly. While an extremely clever and innovative system, GTP 3.3.1 Selecting the Initial Target is not directly suitable as a proof in the Helium network as RF networking has limited range and therefore cannot We aim to deterministically locate a geographic reference tar- communicate with peers anywhere on the Helium network. get, T , for the challenger, C. Both C and T are Miners in the We aim to construct a proof loosely based on the ideas Helium network. T does not need to be geographically proxi- presented in GTP, but applicable to our protocol. mate to C. To locate T , C initially seeds verifiable entropy, η, into the selection process by signing the current block hash We combine Proof-of-Coverage with Proof-of-Serialization— with its private key. Since the probabilities associated to each a proof that allows Miners on the Helium network to achieve miner form a discrete probability distribution [Equation 1], cryptographic time consensus among decentralized clients. C uses the probability associated to each eligible Miner to We aim to achieve rough time synchronization in a secure locate T and applies the inverse cumulative distribution func- way that does not depend on any particular time server, and in tion using a uniform random number generated via η. This such a way that, if a time server does misbehave, then clients allows us to ensure that we always target potentially dishonest end up with cryptographic proof of that behavior. Miners as they have a lower score, thus increasing their prob- ability of being targeted by C. Given that a Miners score is 3.3 Constructing Proof-of-Coverage diminishing linearly over time [Section 3.3.4], it is necessary to create this inverse relationship to give low-scoring Miners With the Proof-of-Coverage protocol, we aim to construct a an opportunity to participate in the process and increase their proof that takes advantage of the following characteristics score. This diminishing score also incentivizes all the partic- of radio frequency (RF) communication that are unique and ipants to send receipts to C and broadcast the remainder of different to Internet communication: O. 1. RF has limited physical propagation and, therefore, dis- tance; 3.3.2 Constructing the multi-layer challenge 2. The strength of a received RF signal is inversely propor- Once T has been selected, C must construct a multi-layer tional to the square of the distance from the transmitter; challenge, O. O is a data packet broadcast over the Helium and network and received by geographically proximate targets Tn . 7

Geographically proximate is defined as within a radius of T , a network value T radius . Each layer of O, Ol , consists of a three- tuple of E (S, ψ, R), where E is a secure encryption function using the Elliptic-Curve Diffie-Hellman (ECDH) derived symmetric key, S is a nonce, ψ is the time to broadcast the next layer of the challenge and R is the remainder of O T1 consisting of recursive three-tuples. The maximum number of Ol is bounded by a network value, Omax . T2 T The construction logic of O by C is as follows: T3 1. A set of candidate nodes, Tn , are selected such that all members of Tn are within a contiguous radio network that TL also contains T ; 2. Two targets, T1 and TL , are selected by finding the highest scoring targets in Tn furthest from T ; 3. A weighted graph, Tg , is constructed from Tn such O1=E(S1,ψ,R) that members of Tg in radio range of each other are connected  by an edge weighted  by the value of 1 − score(Ta ) − score(Tb ) ; 4. The shortest path between T1 to T to TL is computed using Dijkstra’s algorithm [10] using the edge weights from the previous step; 5. An ephemeral public/private keypair Ek and E k-1 are generated; 6. A layer Ol is created and added to O, and S is encrypted with the combination of the public key of TL , retrieved from the blockchain as TLk and E k-1 as an ECDH ex- change to compute a shared secret, known only to both O parties C and TL ; and Figure 3. Construction of O 7. The previous step repeats with additional layers added to O until all TL → T1 have a layer Ol included in O 3. T records both the time of arrival β and the signal strength υ of O; The resulting O can be visually represented as depicted in [Figure 3]. 4. If successful, T then creates signed receipt Ks , where Ks = (S||β||υ) signed by the private key of T ; 3.3.3 Creating the Proof 5. T submits Ks to C via the Helium network, removes the outer most layer, and wirelessly broadcasts the remainder Once O has been constructed, it is delivered to T1 via the O; and Helium network and immediately broadcast by T1 via the Helium network. WHIP is not a point-to-point system, so 6. These steps repeat for T1 ..T ..TL , with TL being the last several Miners within proximity of T1 will hear O. In this target in the graph example, only the specific target T will be able to decrypt E and send a valid receipt back to the challenger, C. C expects to hear responses from Tg within a time threshold λ, otherwise it considers the Proof-of-Coverage to have con- We describe the approximate flow of Proof-of-Coverage cluded. Because C is the only party with complete knowledge creation as follows: of O, upper bounds of the values for β and υ are assigned by C which are used to verify that each layer of O was trans- 1. T1 receives O from C via the Helium network, decrypts mitted approximately where and when it was expected. The the outermost layer and immediately broadcasts it R via upper bound for β is limited by the speed of light τ between the Helium network; Tn and Tn − 1. Thus we know that, subject to some slight 2. T hears O and attempts to decrypt the value of E by using delays from reflection or multipath, the packet should not its private key where pk : Epk (S, ψ, R); arrive at Tg later than τ multiplied by the geographical dis- 8

  tance D plus some small episilon value, υ = τ × D +  . Using the above we can now construct a staleness-factor, δ, which would be used in determining the score of the Miner For υ, because of the inverse-square law, we can calculate M. the maximum RSSI (Received Signal Strength Indication) possible for a packet transmitted, µ, from Tg − 1 to Tg as µ = D12 . Hotspots that are closer than expected, or which are  0 2 −(8.h )  v0 = 0 02 transmitting at a higher power to mask their location disparity, δm = v 0 .(1 − min(0.25,v h v0 > 0 0) ) are unlikely to get µ correct, given that they do not know who   0 the next layer of O is addressed to. v .(1 − 10.v 0 .h02 ) v0 < 0 Once TL has delivered receipt to C, or λ has elapsed, the The above conditions strictly adhere to the following princi- Proof-of-Coverage is completed. The collection of signed ples: receipts, Ks , constitute the Proof-of-Coverage that C will submit to the Helium network. 1. A negative v indicates that the Miner is consistently failing verification. K L = (S L || β || υ) 2. If v = 0, then we do not have any trust information, therefore, we use a steep parabolic curve for the decay K T = (S T || β || υ) dependent on h0 . 3. If v > 0, then it implies that the Miner has been suc- K 1 = (S 1 || β || υ) cessfully verified consistently, hence, we use an inverse parabolic curve that crosses the Y axis at 1, where the width of the parabola increases as a factor of v up to Challenger Target 1 Target Target L (C ) (T1) (T ) (TL) 0.25. This implies that the more positive verifications [Section 3] the Miner has accrued, the slower its score decays as a factor of h0 . Figure 4. Proof-of-Coverage flow 4. Finally, if v < 0, then this is the inverse of the above case, wherein, a Miner has consistently been failing verification. 3.3.4 Scoring Therefore, we use a similar parabola as above; however, the width of the parabola decreases as a factor of v, leading The score allocated to a Miner, and therefore the resulting to a higher score decay for the Miner as a factor of h0 . score of the Proof-of-Coverage, is an integral part of the [Figure 5] shows the trends for each of the above functions. Helium Consensus Protocol described in [Section 6]. When Miners join the Helium network, they are assigned a score, 5 v0 < 0 4 φm . We consider any Miner with a score greater than φm to 3 2 be an honest miner. This score depreciates according to the 1 number of verifications the Miner has as well as the height 0 −1 since its last successful verification. As φm decreases the −2 −3 probability of the Miner M being the target for C increases, −4 v0 > 0 −5 such that the Helium network continually attempts to prove −6 δm −7 that the lowest scoring Miners are acting honestly, and giving −8 Miners a reasonable chance to improve their scores. −9 −10 −11 v0 = 0 In order to achieve this behavior we define the following −12 −13 invariants: −14 −15 −16 M, Miner −17 v, number of successful verfications for M - −18 −19 number of failed verifications for M −20 0 0.13 0.25 0.38 0.5 0.63 0.75 0.88 1 1.13 1.25 1.38 1.5 h, height since the last successful verification for M 0 h If we assume that the ideal verification interval for any Miner Figure 5. Trendlines for the scoring functions is close to 240 blocks (4 hours if we assume a 60 second block time), we scale these invariants to fit the scoring functions: Adhering to the above set of rules, we define the following 0 scoring function, which is essentially a variation of a sigmoid v, v/10.0 h0 , h/480 curve fluctuating between values (0, 1): 9

arctan(2.δm) + 1.58 φm = 3.16 This scoring function yields [Figure 6], which shows the variation of the score with the staleness-factor: 1 0.8 0.6 Figure 8. Snapshot of a random subset of the network after 10000 iterations φm 0.4 3.3.5 Target Selection 0.2 Due to the way scoring decays, there is a possibility that a given Miners’ score may become stale as that Miner may not be verified within a reasonable interval. We therefore 0 structure the target selection mechanism to give Miners a −10 −5 0 5 10 statistically greater chance to increase their score by being δm selected as a target as their score decays. This is accom- plished by biasing the probability of Miners being selected Figure 6. Scoring algorithm and the resulting staleness as potential targets based on their individual scores. factor Let the set of miners be defined as: [Figure 7] shows a snapshot of a random subset of the Helium N = {m1 , m2 , m3 . . . mn | n > 1} network at any blockchain height h. The Miners represent random locations with an illustrated score, while the edges Let the set of miner scores be defined as: are calculated using Dijkstra’s algorithm [10]. S = {φm , m ∈ N } We assign the target selection probability to each miner in the following way: 1 − φm P (m) = n (1) X n− φm i i=1 The above equation ensures that the Miner with the lowest score is assigned the highest probability of being selected as a potential target while the opposite holds for the Miner with the highest score. Figure 7. Snapshot of a random subset of the initial network Furthermore, it also asserts that the probabilities are inversely proportional to the score of an individual Miner. This allows After 10,000 iterations the Helium network appears as repre- us to successfully target potentially low scoring Miners and sented in [Figure 8]. improve the overall balance of the scoring system. The goal of this system is to ensure that the scoring algorithm Another valuable aspect of assigning the probability as shown considers that some Miners may attempt to act dishonestly. above is that all the probabilities together form a discrete However, because the calculated edge-weights (via Dijkstra’s probability distribution. A discrete probability distribution algorithm) and the target selection mechanism ensure that we satisfies the following equation: only boost the score of a Miner when it is being verified by other high scoring Miners, we believe that the system will X P (M = i) = 1 favor legitimate Miners and deter dishonest ones. i 10

3.3.6 Verifying the Proof 3. M generates a nonce, R, which is a SHA512 hash of the Proof-of-Coverage, which M has partially constructed; Once TL has delivered Ks , or λ has elapsed, the Proof- of-Coverage is considered complete. When C submits this 4. M then generates a salted hash commitment, K, called proof, via a special type of transaction, all receipts Ks from the proof-kernel, where K = H (R||M1 ||M2 ); T1 ...TL are included in the transaction published to the 5. M sends K to M1 . M1 replies with T , a signed message Helium network. As all the steps originally completed by including the current time T1 and K; and C are deterministic in nature with verifiable and recreatable randomness, it is simple for a verifying Miner, V , to recreate 6. M knows that the reply from M1 was not pre-generated the original steps and verify that the proof is legitimate. because it includes the nonce R that M generated Because M can not trust M1 , it will ask for another time Verifying Miners in the consensus group [Section 6] who see from M2 : the proof transaction are able to verify the Proof-of-Coverage by recreating the following steps: 1. For the second request, a new nonce R is generated using T truncated to 512-bits, blinded by XOR’ing a randomly 1. The verifying Miner, V , reconstructs the set of Miners N ; generated 512-bit number; 2. The random seed η can be verified by V to have been 2. M then generates a sub-proof-kernel, L = H (R||T ||K), created at approximately the correct time by the private and sends it to M2 ; key of C; 3. M2 replies with U , a signed message including the current 3. V then selects T from N , as seeding with η will result in time T2 and L; and the same target selection; 4. U is now a proof artifact that shows that M desired and 4. The set of candidate Tn are reconstructed from which T1 then proved a serialization between M1 and M2 and TL are determined; With only two servers, M can end up with proof that some- 5. Dijkstra’s algorithm is used to reconstruct the graph Tg ; thing is wrong, but no idea of the correct time But with half and a dozen or more independent servers, M will end up with 6. The Ks receipts contained in BC are verified to have been chain of proof of any server’s misbehaviour, signed by several signed by the private keys of T1 ..T ..TL others, and enough accurate replies to establish the correct time, Tt . Assuming these steps are completed successfully, the Proof- of-Coverage is verified the score of C is adjusted appropri- K=H(R||M 1 ||M 2 ) L=H(R||T||K) ately. K M L 3.4 Constructing Proof-of-Serialization To achieve cryptographic time consensus among decentral- ized clients, we implement a simplified form of Google’s M1 T U M2 Roughtime [12]. Roughtime is a protocol that aims to achieve T=(T 1 ||K) U=(T 1 ||L) rough time synchronization in a secure way that does not depend on any particular time server, and in such a way that, Figure 9. Creating Proof-of-Serialization if a time server does misbehave, clients end up with crypto- graphic proof of that behavior. 3.4.2 Verifying the Proof This section describes the construction of the Proof-of- Serialization protocol. If we assume that the times from M1 and M2 are significantly different, and the time from M2 is before M1 , then M has proof of misbehaviour. The reply from M2 implicitly shows 3.4.1 Creating the Proof that it was created later because of the way that M constructed the nonce. If the time from M2 is after M1 , then M can We outline the approximate process to achieve cryptographi- reverse the roles of M1 and M2 and repeat the process to cally secure time as follows: obtain, assuming steady clocks, a misordered proof as in the 1. To begin, a Miner M pseudo-randomly picks two Miners other case. M1 and M2 , with whom to prove contact serialization; To verify the correct time, it is necessary for M to repeat 2. It is assumed M has a public key for M1 and M2 , the time synchronization process with enough Miners to gain otherwise M should obtain it from the blockchain; consensus on the correct time: 11

1. A Miner M again pseudo-randomly selects n Miners 4.1 Motivation M1 ...Mn ; Location tracking is one of the most valuable use cases for 2. M generates a salted hash commitment, K, and delivers low power Devices. It is expected that there will be at least it to M1 , where K = H (R||M1 ||M2 ); 70 million asset tracking devices shipping by 2022 [14]. 3. M1 again responds with T , a signed message containing Today, Global Navigation Satellite Systems (GNSS) are the current time T1 and K; used by the majority of Devices which require geolocation services, with GPS being the most popular implementation. 4. M generates a sub-proof-kernel, L = H (R||T ||K), and GPS systems use a technique called Time of Arrival (TOA) sends it to the next Miner Mn ; to determine the location of a receiver in relation to 20 or so satellites orbiting the earth. GPS satellites synchronize 5. The next Miner replies with U , a signed message includ- their time using a high precision on-board clock and regular ing the current time and L; synchronization with control servers on the ground. GPS 6. These steps repeat through Mn until at least three time receivers receive precisely timestamped data from a number responses, Tn , are monotonic; and of satellites overhead and use a technique called trilateration to provide a precise location on earth. 7. Tn can then be confirmed to be Tt , the correct time GPS has matured into an extraordinarily reliable service used in a wide range of applications for providing both location K and time services. However, there are significant drawbacks T M to GPS particularly in the realm of low power Devices that the Helium network is designed to facilitate. It can take around 2 L minutes for a GPS receiver to achiehve lock with sufficient U satellites, which translates to drastically reduced battery life. As an example, a Device transmitting its location around 25 times a day may only last a month on a AA battery M1 M2 Mn compared with several years of life on the same battery without GPS. Using GPS indoors is generally impossible, as the GPS receiver typically needs line of sight with the Figure 10. Verifying Proof-of-Serialization sky in order to see the 3-4 satellites required to calculate an accurate location. GPS data is also delivered unencrypted, which leaves the system extremely vulnerable to spoofing, 3.4.3 Utilizing the Proven Time jamming, and other attack vectors. Once the correct time, Tt , has been determined via Proof- We are interested in low power implementations of location of-Serialization, it is used by M and included during proof services that, in conjunction with an immutable distributed construction as described in [Section 2.2]. The randomness, ledger, can be used to verify location and time. Given the η, used to compute O and, thus, obtain the Proof-of-Coverage above factors, we conclude that GPS is an unacceptable is tied to the previous block, which contains Tt . This allows mechanism for these requirements. us to prove, with relative certainty, that some piece of data D was created between the time of the previous block bt and 4.2 Constructing Proof-of-Location Tt . D in this case is the Proof-of-Coverage. Thus, we know that D must have been constructed between bt and Tt . This Our goal is to verify the physical geolocation of a given ensures that the Proof-of-Coverage cannot be pre-computed. Device, D, without using GNSS hardware. To do this, we rely on the fact that we have already determined and proven the physical geolocation and cryptographic time consensus 4. Proof-of-Location of a given Miner, M , using the Proof-of-Coverage and Proof- of-Serialization protocols described in [Section 3]. Using Proof-of-Coverage and Proof-of-Serialization, we achieve cryptographic proof of a Miners location and crypto- 4.2.1 Precise timestamping of RF data graphic time consensus among Miners. We can take advan- tage of these proofs to determine the physical geolocation There are a handful of techniques used by positioning systems of WHIP-compatible Devices and generate a new type of without the use of GNSS, which include Received Signal proof based on the Devices geolocations. We call this Proof- Strength Indication (RSSI), Time of Arrival (ToA), and of-Location. Time Differential of Arrival (TDoA). These techniques use 12

radio frequency transmissions, usually received by one or processor to sample this data, identify an appropriate packet, more receivers, combined with various algorithms based on and record the timestamp. Typically, a Field Programmable characterstics of those transmissions. Gate Array (FPGA) is used as the processor for this data as these types of processors are able to process data in a deter- Our conclusion is that TDoA is the most accurate but chal- ministic way. However, FPGAs are fairly expensive, power lenging technique to implement [15], [16], [17], [18]. TDoA, hungry, and emit significant heat. Instead, our Hotspot mining in simple terms, relies on the variance between precisely hardware uses a novel technique using commodity low-cost synchronized and recorded timing information between a components to process I/Q data and achieve timestamping transmitter and several receivers. As such, it is critical to ac- at this level of precision. As a comparative example, an ex- curately timestamp RF packets Devices emit, and synchronize isting low-cost LoRaWAN [23] access point is only capable the clocks of Miners on the Helium network. of providing timestamp data accurate within several millisec- An example timestamping flow is as follows: onds of precision - as radio waves travel at the speed of light, each millisecond equates to approximately 300,000 meters 1. A Device, D, broadcasts a packet P containing arbitrary of physical distance, which we deem practically useless for data via the Helium network; any accurate geolocation. Further information on the tech- 2. Several Miners, Mn , hear P , and record a timestamp Tn niques, components and schematics used in our Hotspot will of their reception time of P ; be released as open source software at launch of the Helium network. 3. Tn is created based on the nanosecond time received via GNSS and stamped using raw radio sample data received 4.2.2 Using timestamps to derive location by the Hotspot radio frontend; 4. A signed transaction including P and Tn are delivered to Now that the Devices Router, R, is in possession of a variety the router R belonging to D by Mn ; and of signed messages, which include the precise timestamps, Tn , it is possible to solve for the location of the Device D. 5. R has now received several copies of P , each of which A variety of TDoA algorithms exist such as [20], [21], [19] has a slightly varying value of Tn and [22]. If a sufficient density of Mn and, therefore, Tn are GPS/GNSS recorded for a given packet, the location of D can be derived down to a few meters depending on a variety of factors. We encourage the interested reader to read the cited papers for Gateway 1 Gateway 2 Gateway 3 further details on TDoA algorithms, as they are beyond the scope of this whitepaper. 4.2.3 Verifying Proof-of-Location Once R has computed a location of D, it may become nec- t2 essary to verify that the reported location of D was accurate at that given moment in time. As the Proof-of-Location is t1 deterministic and derived from information publicly avail- t3 able in the blockchain it is possible to reconstruct every step involved: • From the signatures contained within the timestamped packets, Tn , every Miner involved in providing times- tamps can be verified; Device • By inspecting the assert_location [Section 5.3.3] transaction, the claimed GPS location of those Miners Figure 11. Geolocation via TDoA can be determined; and • The Proofs-of-Coverage and scores [Section 3] for each Typically, it is challenging to accurately record these times- Miner can be retrieved from the blockchain and inspected tamps as any nanosecond-level variance in the timestamp can lead to significant variance in the resulting location so- By auditing the above steps the router operator can crypto- lution. To achieve this level of precision it is necessary to graphically prove (or disprove) the location of each of the use extremely high-bandwidth raw in-phase and quadrature Miners involved in providing the components for Proof-of- (I/Q) data from the Miner’s radio hardware and a fast enough Location for a given Device D. 13

The accuracy of the proof will depend heavily on the number potentially paying more for the transaction fees than the of Mn involved and, therefore, Tn received. Additional RF value being exchanged. This is a similar problem to buying factors, such as reflections and multipath, can significantly small-value items using credit cards today. The vendor affect the accuracy of the location calculation. pays a minimum fee on each credit card transaction, and under a certain charge they lose money on the transaction. These heavyweight transactions are clearly not suitable 5. Transactions for use as a micro transaction system within the Helium Transactions in the Helium network provide functionality that network. enables address-to-address transfers of protocol tokens, simi- lar to many existing blockchain networks, but also provide a Zero-fee Transactions While highly desirable from a device set of primitives that enable core functionality that is critical perspective, a true zero-fee blockchain would be fraught to the operation of a DWN. We will first address Helium’s need with spam transactions. It would be trivial to write a script for microtransactions and propose a new solution. to pollute the blockchain with transactions meant only to waste space on the blockchain and increase conges- 5.1 The Helium Nework’s Need for Microtransactions tion on the network. Some ostensibly zero-fee blockchain implementations solve this issue in clever ways, such as of- Devices Pay Per Packet The goal of the Helium network floading the work of processing and verifying transactions is to offer Internet data transport fees (the fees paid by to the transactors themselves. However, these implementa- Devices to Miners) that are an order of magnitude less than tions have their own issues, for example IOTA [24] has not anything currently available for this type of service. This yet proved it is capable of operating this type of system transport fee would need to be metered per-packet in order without the need for a centralized coordinator. to allow for maximum flexibility — this way, a Device could transact with any Miner, even just to send or receive State Channels State channels [31] allow two parties to a single packet without having previously established a exchange value, usually in small increments at a time, relationship with that Miner. with very limited risk. If one party thinks the other is All Transactions Occur On-Chain The Helium network is acting dishonestly, it can publish the final transaction in built on the philosophy that all transactions should occur the state channel to the blockchain and close the channel. on-chain; that is, blocks should be sized and mined with At most one payment is usually at risk. However, there a frequency such that every transaction which occurs on are several downsides: the payer has to lock up significant the Helium network should be stored in the blockchain. funds for the lifetime of the state channel, meaning they To accomplish this goal, the cost of mining must be low, may be unable to open state channels with other parties blocks must be large enough to encapsulate a large number or pay other dues; transactions in the state channel do not of transactions, and blocks must be created frequently appear on the main chain at all; and these implementations enough that transactions are processed quickly. are relatively complex to execute well (note that neither Lightning [29] nor Raiden [30] have become widely used Allow Devices to Persist Data to the Blockchain Because yet). the Helium network services a specific use, the DWN, blocks must additionally be able store fingerprints of data Payment in Arrear Payment in arrear, after the services sent from Devices along with the transaction, which pays have been rendered, is an extremely risky method in a Miner for its transport service. We believe that this a decentralized pseudo-anonymous system. There is no holistic tamper-proof data trail will enable entirely new mechanism to gain certainty around the intent or honesty use cases where the authenticity and veracity of sensor of the entities transacting, nor do you know if the entities data is critical. control the requisite funds when the debt comes due. This model only works when the parties involved trust each 5.2 Limitations of Existing Solutions other or have some other recourse to recover funds. Now that we have discussed the requirements of transactions within the Helium network, we outline the existing solutions for micropayments on a blockchain and address their short- 5.3 Types of Fees in Helium comings as they apply to the Helium network. Heavyweight Transactions This first option is suitable only In this section we outline the types of fees needed on the for larger transactions as the service fee is smaller than the Helium network, and propose solutions that take advantage of payment. This method does not work well for very small the unique characteristics of the Helium Consensus Protocol transactions as whoever pays the transaction fee ends up [Section 6]. 14

5.3.1 Transport Fees network, we root them in reality. The Helium network’s primary purpose is to facilitate a network of wireless Internet Devices using the Helium network to send and receive data coverage. In order to accomplish this in the long term, to and from the Internet must pay Miners what is known as a all of the economics of the system must align to make it transport fee. This fee compensates the Miner for delivering practical for the primary users to transact on the Helium data packets between the Device and the intended router network. If one set of fees were to outstrip the others, then on the Internet, and is unrelated to the transaction fee that the Helium network would quickly lose its utility for the key Miners earn for mining transactions as part of blocks that are user segment. recorded to the blockchain. The fee is negotiated between the Router to which the Device belongs, and the Miner, as To enable Miners and other light clients to determine an Devices are not directly connected to the blockchain. appropriate fee, full nodes [Section 5.5] will expose a fee suggestion API. This way resource constrained entities that Miners set the price they are willing to accept to transport do not maintain a complete copy of the blockchain will not data to and from the Internet on a per-byte basis. need to compute the fee from the most recent transactions. A Devices router pays Miners the transport fee on transmis- During the block submission process, Miners in the consensus sion or reception of the data. This means that the Miner will group [Section 6] will verify the correctness of the block receive the transport fee prior to the transaction being mined and ensure that no fee has deviated beyond the acceptable in a block and recorded into the blockchain. This entails threshold of δ. some risk for the Miner, as they must believe that the trans- Due to the censorship-resilience built into the Helium Consen- port payment is not malicious or fraudulent prior to it being sus Protocol [Section 6], there is no incentive to include larger confirmed in the blockchain. However, given how low the transaction fees. Unlike Bitcoin, where miners cherry-pick per-byte transport amount is likely to be, this risk seems toler- the transactions with the largest fees from their mempool to able. A Miner can blacklist a Device or organization address include in their blocks, Helium miners cannot see the contents if they continually abuse the system. of the transactions without collaborating with other members An example transport fee process is as follows: of the consensus group to decrypt them. Transactions with incorrect fees (either too high or low) will be rejected prior 1. A Miner, M , hears a packet, P , broadcast by Device D; to the block being appended to the blockchain. 2. M uses the address of D, attached to P , to identify a router, R, as the owner of D; 5.3.3 Staking Fees 3. M sends the signature, K(P ), of P and an offer of n The assert location transaction, mentioned below [Sec- tokens for transport to R; tion 5.4], has a special type of fee calculation, a dynamic fee. Because the Helium network reaches maximum usefulness 4. R receives K(P ) and the payment offer and determines at a specific density of Hotspots, we want the fees to incen- if it accepts the packet for the offered price, tivize the Helium network density to be as close to that ideal 5. Assuming R accepts the packet at the offered price, it as possible. To that end, the transaction fee for asserting a constructs a transaction T of value n payable to M and location can be thought of as the y coordinate on a curve with sends it to the Miner; and the formula: 4 y = (x − D) + F 6. Once M sees the transaction in the reply it delivers P to R and submits T to the consensus group for inclusion in where D is the ideal Hotspot density and F is the unit fee for the Helium network a location transaction. A sample graph of this function where D = 3 and F = 1 follows: 5.3.2 Transaction Fees As can be seen, Hotspots near the ideal network density are cheap to add, but establishing a new network or overpopu- Transaction fees are an essential part of most blockchain lating a network gets expensive very quickly. This serves to implementations. They incentivize Miners to include a trans- dis-incentivize Hotspot deployments that are not beneficial action in their draft block and ensure that spam transactions to the network. In particular, Alternate Reality Attacks and do not pollute the Helium network. warehouses full of Miners become prohibitively expensive. To determine the appropriate fee for a new transaction, the Miners who have not asserted their location, and therefore transactor will take the median of the past δ packet transport not paid the staking fee, will not be considered for inclusion fees, within some margin of error. Until δ packet transports in the consensus group [Section 6]. have occured on the Helium network, the fee will be fixed at a constant value α. By anchoring the transaction fee to Miners who move physical location will need to assert a new the current fees being charged for transport on the Helium location, and pay the new staking fee. 15

Property Description 80 payer address the address of the sender payee address the address of the recipient 60 nonce a monotoically increasing integer value an integer-based representation of the tokens to send Fee 40 signature the signature of the sender 20 5.5 Light Clients and Full Nodes Until now, we have discussed how to deal with microtrans- 0 actions in a cost-effective way, however we have not yet 0 1 2 3 4 5 6 addressed how to deal with the inevitable continuously in- Density creasing size of the blockchain. One requirement for the Figure 12. Staking fee vs Miner density Helium network is that all transactions occur on-chain. This means that the size of the full blockchain will eventually grow quite large. This is compounded by the fact that all Miners on 5.4 Primitives in The Helium Network the Helium network are Hotspot devices, relatively limited in computation power and storage space. Having discussed the philosophy of our transaction system and presented our approach to facilitating microtransactions We solve this constraint by allowing mining nodes to operate on the Helium network, we now delineate the transaction as light clients on the blockchain, pruning old blocks and primitives and their properties. transactions as needed and keeping only the latest ledger values. They will communicate over the peer-to-peer network add hotspot Registers a new Hotspot on the Helium net- with full nodes which maintain a complete history of the work, adding it to an existing account that will be respon- blockchain to verify transactions. sible for supplying its stake (required for mining) and will This raises a question: who is responsible for operating full receive mining rewards [Section 6] and fees earned by the nodes, and what is their incentive to do so? Routers are Hotspot software-only applications with access to scalable, cloud- based storage and will be required to operate full nodes in Property Description order to fulfill their purpose. We will operate a set of hosted hotspot address the public key address of the Hotspot routers that will make it easy for developers to launch prod- being added to the network ucts without needing to deploy their own router. However, owner address the address of the owner account many enterprise developers, who are required to maintain a signatures mutual signatures of the owner and higher standard of privacy, will want to host their own router. Hotspot Together, these routers will form a network of full nodes capa- ble of supporting resource constrained Hotspots and wallets operating light clients. assert location Asserts a Hotspot’s location in the form of geographic coordinates, requiring a dynamic stake 6. Helium Consensus Protocol Instead of an extremely computationally expensive and power Property Description hungry Proof-of-Work, Miners generate Proofs-of-Coverage hotspot address the address asserting its location [Section 3]. In this section we present how these useful proofs nonce a monotonically increasing integer can be used to create permissionless network consensus. latitude the latitude of the Hotspot longitude the longitude of the Hotspot 6.1 Motivation altitude the altitude of the Hotspot signature the signature of the Hotspot Many current generation blockchains rely on a computation- ally difficult Proof-of-Work to protect the Helium network against Sybil attacks, also known as Nakamoto Consensus. payment Moves tokens from one account, the payer, to The fact that the Proof-of-Work is computationally expensive another account, the payee, including the requisite fee. to create, but cheap to verify means that in order to propose 16

a new valid block to the Helium network there is evidence particular block being mined and is not otherwise useful that a significant amount of computation has been expended. or reusable on the network. An ideal consensus system Due to the fact that computation is limited by hardware cost, would contain work that is both useful and reusable to the power cost, physical space and computational efficiency of network beyond simply securing the blockchain. modern technology, Sybil attacks become impossible. How- High confirmed transaction rate Our ideal consensus pro- ever, this approach, while fundamental to the mainstream tocol would be able to process a very high number of adoption of blockchain technology, has several downsides. transactions per second, and once a transaction is seen Chief among the downsides is the power consumption; it is in a block it would be considered confirmed. Many exist- estimated that the Bitcoin network is consuming more power ing blockchains require a lengthy settlement time while than many small countries. Bitcoin’s Proof-of-Work is so the network achieves consensus which is not ideal in a wasteful it is now on the list of the top uses of electricity in system like the Helium network, which may experience a the world and whenever the value of Bitcoin goes up, so do very high number of transactions and where waiting for a the resources devoted to mining it. transaction to settle is not tenable. Related to the power problem is the mining pool problem. Transactions are censor-resistant Ideally, Miners would Many blockchains have mining pools where users band not be able to censor or otherwise pick and choose trans- together to, in parallel, mine a single block and listing the actions prior to mining them. This would not only nullify pool’s address as the party to get paid. The pool then shares any attempts to nefariously censor transactions, but would the block reward with the members of the pool. This ends allow for otherwise unattractive transactions (such as up defeating many of the advantages of decentralization as fixed-fee transactions) to be included in the blockchain. both Bitcoin and Ethereum have come to be dominated by less than 10 mining pools each. These large pools effectively The remainder of this section lays out our construction of a prevent independent parties from mining blocks on their own. consensus protocol with these design goals in mind that we This means that the consensus protocol for these blockchains refer to as the Helium Consensus Protocol. is effectively controlled by a very small number of mining pools and risks becoming further centralized. 6.2 Helium Consensus Protocol More recently there has been increased momentum around making blockchain consensus protocols less wasteful and We propose a unique consensus protocol around Proof-of- more useful to the network. Filecoin [25] has a Proof-of- Coverage to capture the useful work of verifying the Helium Spacetime and Ethereum [5] is moving towards a Proof-of- network as a replacement for Proof-of-Work, combined with Stake [26] approach. a variant of the HoneyBadgerBFT (HBFT) [4] asynchronous byzantine fault tolerant protocol. For the Helium network, we desire a consensus protocol with the following attributes: 6.2.1 HBFT Permissionless Nodes should be able to freely participate HBFT is an asynchronous atomic broadcast protocol designed in the Helium network without permission or approval to achieve optimal asymptotic efficiency, initially presented from any other entity, as long as those nodes operate in in 2016. In HBFT, the setting assumes a network of N accordance with the consensus rules. designated nodes with distinct well-known identities (P0 Extremely decentralized in nature Network consensus should through P N-1 ). In our HCP instantiation, this network of be designed such that there is no incentive available for nodes is known as the consensus group C. The consensus taking advantage of macro-economic factors, such as group receives transactions as input, and its goal is to reach cheaper access to electricity in certain geographies, and common agreement on an ordering of these transactions and that simply buying more hardware in the same location form them into blocks to be added to the blockchain. is either ineffective or cost prohibitive. Additionally, it The protocol proceeds in rounds, where after each round, a should be impossible for mining pools to form and for new batch of transactions is appended to the blockchain. At groups to collaborate in mining blocks. the beginning of each round, the group chooses a subset of Byzantine Fault Tolerant The protocol should be tolerant the transactions in its buffer and provides them as input to an of Byzantine failures [27] such that consensus can still instance of a randomized agreement protocol. At the end of be reached as long as a threshold of actors are acting the agreement protocol, the final set of transactions for this honestly. round is chosen. Based on useful work Achieving network consensus should HBFT relies on a threshold encryption scheme [28] that be useful and reusable to the network. Work performed in requires transactions be encrypted using a sharded public Nakamoto Consensus-based systems is only useful for the key, such that the consensus group must work together to 17

decrypt it. This means that no individual node is able to run the TPKE.DecShare(SKi , e) → σi function to produce decrypt or censor a particular transaction without colluding their decryption share. Members broadcast their σi to the with the majority of the group. other members of C, and once f + 1 members have seen σi shares they can proceed to the TPKE.Dec function using PK, 6.2.2 Applying Proof-of-Coverage to HBFT e and the σi shares and attempt to decrypt the transaction. Each member of C appends decrypted transactions to its own In the Helium network, miners are required to submit Proofs- instantiation of the next block kept in a local buffer. Double- of-Coverage to the Helium network at an epoch, ∆p . These spend and other malformed transactions are removed from proofs are submitted as a special type of transaction, and these blocks at this stage. subsequently recorded to the blockchain. As detailed in As members of the group cannot decrypt e on their own, [Section 3], Miners increase their scores as they submit valid a given member cannot censor a transaction prior to its proofs to the Helium network. At an epoch, ∆c , the highest inclusion in the candidate block without f + 1 members of C scoring Miners, N , are elected as the new HBFT consensus colluding as transactions are received. Any honest member group, C. of C that has t in the first B of its transaction queue will By using Proof-of-Coverage to elect the members of C we eventually be able to include t in a block as the other members are essentially substituting for well-known identities in the of C cannot decrypt the transaction until it has been agreed HBFT protocol. As we desire a permissionless network, we to, at which point it is too late to censor it. As the members can use Proofs-of-Coverage to determine whether Miners are of C for a ∆c epoch are selected based on their submitted acting honestly and reward the most honest Miners at a given Proofs-of-Coverage, making the members unpredictable, this epoch by electing them to the HBFT consensus group. type of collusion would be extremely challenging to execute. Once f + 1 nodes have agreed on the transactions for the 6.2.3 The consensus group block, a TPKE threshold signature is obtained over the block. This certifies that enough nodes to exceed the byzantine fault During ∆c , the currently elected consensus group is responsi- threshold have agreed on a block. Members of C that are ble for creating blocks and appending them to the blockchain. censoring or disagreeing on the contents of the block will All new transactions on the Helium network are submitted produce an incompatible signature share that cannot be used to the current members of the consensus group. New blocks to count towards the signature threshold. This block is then are created by C at a fixed interval ∆b and recorded to the gossiped via the Helium network to all Miners and added to blockchain. A token block reward is split among the members the blockchain. of C for every block submitted, along with the sum of all fees contained within valid transactions. In the unusual case C that there are no transactions during ∆b , an empty block is appended to the blockchain. σi σi B N N N 6.2.4 The mining process B Once the consensus group C has been elected for a given ∆c epoch, a distributed key generation phase occurs to bootstrap a threshold encryption key TPKE. TPKE is a cryptographic B e primitive that allows any party to encrypt a transaction to a master public key PK, such that C must work together to decrypt it. Once f + 11 correct members of C compute and B reveal decryption shares, σi , the transaction can be recovered. Once PK is generated via the TPKE.Setup function, a block containing PK is immediately submitted to the blockchain. M B Each member Nm in C receives a secret key share, SKi , of PK. Figure 13. The Consensus Group and Mining Miners on the Helium network submit new transactions t to C. Each member of C takes a random subset of the first 6.2.5 Conclusion B transactions in its queue and applies the TPKE.Enc(PK, t) → e function and submits them to the other member of We have presented the Helium Consensus Protocol which C. Once the members of C receive at least N − f e they combines a modern, asynchronous and highly efficient byzan- tine fault tolerant consensus protocol with a novel mecha- 1f is a protocol parameter equal to the number of tolerable byzantine faults nism for substituting permissioned identity with a useful and 18

reusable Proof-of-Coverage. The resultant protocol satisfies and direction was critical to some of the design decisions and the design requirements of being permissionless, decentral- evolution of this project. We also thank the Blockchain at ized, byzantine fault tolerant, based on useful work, and with Berkeley team for their help and detailed review of this work. a very high-rate censor proof transaction mechanism. We would also like to acknowledge many of the prior works We refer the interested reader to [4] for a detailed breakdown and inventions that have allowed us to create this project, and analysis of the HoneyBadgerBFT protocol. most notably Bitcoin [9] and Ethereum [5]. 7. Future Work This paper presents a well thought-out design for building the Helium network. However, we consider this to be just the beginning of the engineering, research and design of decentralized wireless networks. We believe that this tight integration of real-world hardware with a blockchain and a native token is a novel and valuable innovation that can be applied to other kinds of networks and wireless physical layers. We believe that the future of blockchains is not about who has the most hashing power or access to the cheapest electricity, but about blockchains where the mining proof is tied to providing a valuable, verifiable service. There are several initiatives that we either have or intend to undertake, including: • Investigate the applicability of applying these ideas to other physical layers such as WiFi, Bluetooth and Cellular • Explore the potential for the delivery of 5G 60GHz+ mmWave connectivity through a similar design • Research and implement more Proofs-of-Coverage to keep the Helium network secure as it grows • Game theoretical analysis of the incentive system • Formally prove the scoring algorithm used in the Proof- of-Coverage • Create and release the WHIP wireless specification • Manufacture Hotspots and Device modules for availability at launch of the Helium network • Investigate the deployment of a smart contract environ- ment beyond the basic DWN primitives • Continued work and evolution of Forward Error Correc- tion techniques Acknowledgments This document is the result of collaborative work by members of our team, and would not have been possible without the help, feedback, and review of our board of directors, advisors, investors and collaborators. We extend our most heartfelt thanks to all involved. We would also like to extend our thanks to Jeremy Rubin of the MIT Digital Currency Initiative. Your earliest feedback 19

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