Patent Description:
A blockchain refers to a form of distributed data structure, wherein a duplicate copy of the blockchain is maintained at each of a plurality of nodes in a peer-to-peer (P2P) network. The blockchain comprises a chain of blocks of data, wherein each block comprises one or more transactions. Each transaction may point back to a preceding transaction in a sequence which may span one or more blocks. Transactions can be submitted to the network to be included in new blocks by a process known as "mining", which involves each of a plurality of mining nodes competing to perform "proof-of-work", i.e. solving a cryptographic puzzle based on a pool of the pending transactions waiting to be included in blocks.

Conventionally the transactions in the blockchain are used to convey a digital asset, i.e. a number of digital tokens. However, a blockchain can also be exploited in order to layer additional functionality on top of the blockchain. For instance, blockchain protocols may allow for storage of additional user data in an output of a transaction. Modern blockchains are increasing the maximum data capacity that can be stored within a single transaction, enabling more complex data to be incorporated. For instance this may be used to store an electronic document in the blockchain, or even audio or video data.

Each node in the network can have any one, two or all of three roles: forwarding, mining and storage. Forwarding nodes propagate transactions throughout the nodes of the network. Mining nodes validate transactions and insert them into candidate blocks for which they attempt to identify a valid proof-of-work solution. Storage nodes each store their own copy of the mined blocks of the blockchain. In order to have a transaction recorded in the blockchain, a party sends the transaction to one of the nodes of the network to be propagated. Mining nodes which receive the transaction may race to mine the transaction into a new block. Each node is configured to respect the same node protocol, which will include one or more conditions for a transaction to be valid. Invalid transactions will not be propagated nor mined into blocks. Assuming the transaction is validated and thereby accepted onto the blockchain, the additional user data will thus remain stored at each of the nodes in the P2P network as an immutable public record.

<NPL>) discloses a payment system in which a depositor who locks an amount of tokens on the blockchain, and supervises payers to make off-chain payments. Therefore, payers can pay to multiple payees off-chain without double-spending.

<NPL>) discloses research showing that layer-two protocols enable blockchains to scale without modification on the base layer.

<NPL>) discloses a decentralized system whereby transactions are sent over a network of micropayment channels whose transfer of value occurs off-blockchain.

One of the fundamental principles of the blockchain is that parties are unable to double spend an unspent transaction output (UTXO). Each blockchain transaction submitted to the blockchain network must contain an input that references an output of a previously submitted transaction, e.g. a transaction that has been accepted onto the blockchain network. To be validated and accepted onto the blockchain network, the referenced output must be an unspent transaction output. If the referenced output has already been spent, the transaction will be rejected. This prevents double spending of outputs that have been recorded on the blockchain.

Blockchain transactions may also be sent peer-to-peer between parties without being transmitted to the blockchain network. That is, a first party may transmit a blockchain transaction via an off-chain side channel to a second party. For example, the blockchain transaction may be a template transaction to which the second party may add an input to the template transaction, e.g. to fund the transaction or to attest to the contents of the transaction. As another example, the second party may add an output to the transaction, e.g. to define a spending condition of the transaction. The second party may complete the transaction and then submit it to the blockchain network to be recorded in the blockchain.

The second party may (intentionally or inadvertently) attempt to double spend the output of a previous transaction referenced by the input of the newly submitted transaction. For example, unbeknownst to the second party, the first party may have included an input in the transaction template that references a previously spent transaction output. When the second party submits the completed transaction to the blockchain network (e.g. after adding an output to the transaction template), the submitted transaction will be rejected. This may have negative consequences for the second party. For instance, the second party may have traded goods or services to the first party in return for digital assets transferred to the second party via the transaction (or at least believed to be transferred to the second party at the time of submitting the transaction to the blockchain). Other potential consequences include being liable to fraud charges or being banned (e.g. blacklisted) from using the blockchain.

There is therefore a need to mitigate the risk of double spend attempts.

According to one aspect disclosed herein, there is provided a computer-implemented method for enabling a second party to determine whether to accept a blockchain transaction from a first party, the method being performed by a first party and comprising: generating a first blockchain transaction, wherein the first blockchain transaction comprises an input for unlocking an output of a blockchain transaction previously transmitted to one or more nodes of a blockchain network for inclusion in the blockchain; generating a signature based on i) the first blockchain transaction and ii) one or more time indicators, each time indicator indicating when the first blockchain transaction was generated and/or transmitted to a second party; and transmitting to the second party, the first blockchain transaction, the signature and the one or more time indicators, wherein at least the first blockchain transaction is transmitted to the second party via an off-chain communication channel, and wherein the second party (103b) is configured to accept the first blockchain transaction based on one or more conditions, a first one of the one or more conditions being that the first party (103a) has transmitted the signature and the one or more first time indicators.

Building on the example provided above, the first party may be a customer and the second party may be a merchant. The first blockchain transaction may be used to transfer a digital asset to the second party in return for goods or services offered by the merchant. The merchant may be set up to accept <NUM>-conf transactions. A "<NUM>-conf" transaction is a transaction that has not received any confirmations from any nodes of the network that the transaction has been included in a block of the blockchain.

Returning to the example, the merchant accepts the first blockchain transaction as payment for goods or services without checking that the first blockchain transaction being a "double-spend" attempt. an input of the first blockchain transaction spends an output of a previous blockchain transaction which has already been spent. Transmitting the first blockchain transaction along with the signature and the indicator (e.g. a timestamp) can prove the order of transaction issuance to mitigate the risk of double-spend attempts. The signature is based on the transaction and the time at which the first transaction is generated and/or transmitted to the merchant. This, in effect, means that the second party attests to the time at which the first blockchain transaction was generated and/or transmitted. In the event of a double spend, the merchant can use the signature as proof that the first party intentionally double-spent the output of the previous transaction. Thus, the appropriate authorities can take action against the first party. The merchant can also use the signature to prove, e.g. to the merchant's bank or insurer, that the merchant has been a victim of fraud.

The problem of double spend attempts also arises in peer-to-peer networks such as Internet-of-Things networks. Peer-to-Peer (P2P) architectures offer a more secure and efficient solution compared to centralised architectures, whereby neighbours interact directly with one-another without using any centralized node or agent between them. Blockchain technology is the foundation for secure P2P communication and is promising to revolutionize the development of loT systems. Blockchain enables the integration of payment and control into one network; existing infrastructure can be used to piggyback messages regarding device-state changes; and decentralised control of data on the network enables faster user-device interaction. Combined with blockchain technology, traditional loT devices that perform roles in the physical world will be able to message and exchange value simultaneously. Public blockchains serve as a global payment network as well as a general-purpose commodity ledger with strong cryptographic security built into its protocol that automatically addresses several of the risks associated with loT. However, if the next generation of blockchain based systems for loT devices are to become a reality, blockchain-based control methods for loT need to overcome challenges that are inherent in open systems. These include device control mechanisms that may not be inherent to the blockchain itself. Such a control mechanism is described in GB1915842.

In order to guarantee that communication between nodes occurs with minimal latency, loT control mechanisms may rely on P2P transaction propagation, transaction malleability and <NUM>-conf transactions. However, a malicious or faulty node may wish to have actions performed by other nodes on the network without paying for it, or without wanting the record of the interaction being left on-chain. In this scenario, the malicious node may create a transaction Tx<NUM> that contains a command message and that spends some input(s) of a previous transaction. At the same time, the node broadcasts another transaction Tx<NUM>' which spends the same inputs, therefore guaranteeing that the network will reject Tx<NUM> once it has been finalised by the responding node. Whilst in a trusted network it can be assumed that nodes are not intentionally malicious, the receiver of a partially complete transaction cannot guarantee that once they perform the action (based on the command message) and finalise the transaction with an additional signature, that the transaction they broadcast will be accepted. Furthermore, the creator of the partially complete transaction cannot be prevented from re-spending an input if the action has not been performed and the transaction has not been broadcast.

According to another aspect disclosed herein, there is provided a computer-implemented method of determining whether to accept a blockchain transaction from a first party, the method being performed by a second party and comprising: receiving a first blockchain transaction from the first party via an off-chain communication channel, wherein the first blockchain transaction comprises an input for unlocking an output of a blockchain transaction previously transmitted to one or more nodes of a blockchain network for inclusion in the blockchain; determining whether the first party has transmitted, to the second party, (a) a signature generated based on the first blockchain transaction and one or more first time indicators, each first time indicator indicating when the first blockchain transaction was generated and/or transmitted, and (b) the one or more time indicators; and accepting the first blockchain transaction based on one or more conditions, a first one of the one or more conditions being that the first party has transmitted the signature and the one or more first time indicators.

The present invention mitigates against the risk of the double spend attempts caused by malicious actors. To build on the scenarios described above, the first and second parties may be nodes of an loT network, wherein the first blockchain transaction (a "control transaction") is used by the first party (first node) to instruct the second party (second node) to control an end device. For example, the first node may be a master node whereas the second node may be a slave node. The nodes may be set up to accept and act on <NUM>-conf transactions. In the event that a malicious party has gained control of the first node, the malicious party may attempt to control the second node by replaying a transaction or spending a spent output of a previous transaction. The second node may be set up to only accept control transactions that are accompanied by the signature and indicator. Alternatively, the second node may only accept transactions that are accompanied with a timestamp that indicates a time that is within a threshold of the time at which the first blockchain transaction was received.

It will be appreciated that there are many scenarios where the risk of double spend attempts may arise when blockchain transaction are sent off-chain between parties. The present invention is applicable to all such scenarios.

<FIG> shows an example system <NUM> for implementing a blockchain <NUM> generally. The system <NUM> comprises a packet-switched network <NUM>, typically a wide-area internetwork such as the Internet. The packet-switched network <NUM> comprises a plurality of nodes <NUM> arranged to form a peer-to-peer (P2P) overlay network <NUM> within the packet-switched network <NUM>. Each node <NUM> comprises computer equipment of a peers, with different ones of the nodes <NUM> belonging to different peers. Each node <NUM> comprises processing apparatus comprising one or more processors, e.g. one or more central processing units (CPUs), accelerator processors, application specific processors and/or field programmable gate arrays (FPGAs). Each node also comprises memory, i.e. computer-readable storage in the form of a non-transitory computer-readable medium or media. The memory may comprise one or more memory units employing one or more memory media, e.g. a magnetic medium such as a hard disk; an electronic medium such as a solid-state drive (SSD), flash memory or EEPROM; and/or an optical medium such as an optical disk drive.

The blockchain <NUM> comprises a chain of blocks of data <NUM>, wherein a respective copy of the blockchain <NUM> is maintained at each of a plurality of nodes in the P2P network <NUM>. Each block <NUM> in the chain comprises one or more transactions <NUM>, wherein a transaction in this context refers to a kind of data structure. The nature of the data structure will depend on the type of transaction protocol used as part of a transaction model or scheme. A given blockchain will typically use one particular transaction protocol throughout. In one common type of transaction protocol, the data structure of each transaction <NUM> comprises at least one input and at least one output. Each output specifies an amount representing a quantity of a digital asset belonging to a user <NUM> to whom the output is cryptographically locked (requiring a signature of that user in order to be unlocked and thereby redeemed or spent). Each input points back to the output of a preceding transaction <NUM>, thereby linking the transactions.

At least some of the nodes <NUM> take on the role of forwarding nodes 104F which forward and thereby propagate transactions <NUM>. At least some of the nodes <NUM> take on the role of miners <NUM> which mine blocks <NUM>. At least some of the nodes <NUM> take on the role of storage nodes <NUM> (sometimes also called "full-copy" nodes), each of which stores a respective copy of the same blockchain <NUM> in their respective memory. Each miner node <NUM> also maintains a pool <NUM> of transactions <NUM> waiting to be mined into blocks <NUM>. A given node <NUM> may be a forwarding node <NUM>, miner <NUM>, storage node <NUM> or any combination of two or all of these.

In a given present transaction 152j, the (or each) input comprises a pointer referencing the output of a preceding transaction 152i in the sequence of transactions, specifying that this output is to be redeemed or "spent" in the present transaction 152j. In general, the preceding transaction could be any transaction in the pool <NUM> or any block <NUM>. The preceding transaction 152i need not necessarily exist at the time the present transaction 152j is created or even sent to the network <NUM>, though the preceding transaction 152i will need to exist and be validated in order for the present transaction to be valid. Hence "preceding" herein refers to a predecessor in a logical sequence linked by pointers, not necessarily the time of creation or sending in a temporal sequence, and hence it does not necessarily exclude that the transactions 152i, 152j be created or sent out-of-order (see discussion below on orphan transactions). The preceding transaction 152i could equally be called the antecedent or predecessor transaction.

The input of the present transaction 152j also comprises the signature of the user 103a to whom the output of the preceding transaction 152i is locked. In turn, the output of the present transaction 152j can be cryptographically locked to a new user 103b. The present transaction 152j can thus transfer the amount defined in the input of the preceding transaction 152i to the new user 103b as defined in the output of the present transaction 152j. In some cases a transaction <NUM> may have multiple outputs to split the input amount between multiple users (one of whom could be the original user 103a in order to give change). In some cases a transaction can also have multiple inputs to gather together the amounts from multiple outputs of one or more preceding transactions, and redistribute to one or more outputs of the current transaction.

The above may be referred to as an "output-based" transaction protocol, sometimes also referred to as an unspent transaction output (UTXO) type protocol (where the outputs are referred to as UTXOs). A user's total balance is not defined in any one number stored in the blockchain, and instead the user needs a special "wallet" application <NUM> to collate the values of all the UTXOs of that user which are scattered throughout many different transactions <NUM> in the blockchain <NUM>.

An alternative type of transaction protocol may be referred to as an "account-based" protocol, as part of an account-based transaction model. In the account-based case, each transaction does not define the amount to be transferred by referring back to the UTXO of a preceding transaction in a sequence of past transactions, but rather by reference to an absolute account balance. The current state of all accounts is stored by the miners separate to the blockchain and is updated constantly. In such a system, transactions are ordered using a running transaction tally of the account (also called the "position"). This value is signed by the sender as part of their cryptographic signature and is hashed as part of the transaction reference calculation. In addition, an optional data field may also be signed the transaction. This data field may point back to a previous transaction, for example if the previous transaction ID is included in the data field.

With either type of transaction protocol, when a user <NUM> wishes to enact a new transaction 152j, then he/she sends the new transaction from his/her computer terminal <NUM> to one of the nodes <NUM> of the P2P network <NUM> (which nowadays are typically servers or data centres, but could in principle be other user terminals). This node <NUM> checks whether the transaction is valid according to a node protocol which is applied at each of the nodes <NUM>. The details of the node protocol will correspond to the type of transaction protocol being used in the blockchain <NUM> in question, together forming the overall transaction model. The node protocol typically requires the node <NUM> to check that the cryptographic signature in the new transaction 152j matches the expected signature, which depends on the previous transaction 152i in an ordered sequence of transactions <NUM>. In an output-based case, this may comprise checking that the cryptographic signature of the user included in the input of the new transaction 152j matches a condition defined in the output of the preceding transaction 152i which the new transaction spends, wherein this condition typically comprises at least checking that the cryptographic signature in the input of the new transaction 152j unlocks the output of the previous transaction 152i to which the input of the new transaction points. In some transaction protocols the condition may be at least partially defined by a custom script included in the input and/or output. Alternatively it could simply be a fixed by the node protocol alone, or it could be due to a combination of these. Either way, if the new transaction 152j is valid, the current node forwards it to one or more others of the nodes <NUM> in the P2P network <NUM>. At least some of these nodes <NUM> also act as forwarding nodes 104F, applying the same test according to the same node protocol, and so forward the new transaction 152j on to one or more further nodes <NUM>, and so forth. In this way the new transaction is propagated throughout the network of nodes <NUM>.

In an output-based model, the definition of whether a given output (e.g. UTXO) is spent is whether it has yet been validly redeemed by the input of another, onward transaction 152j according to the node protocol. Another condition for a transaction to be valid is that the output of the preceding transition 152i which it attempts to spend or redeem has not already been spent/redeemed by another valid transaction. Again if not valid, the transaction 152j will not be propagated or recorded in the blockchain. This guards against double-spending whereby the spender tries to spend the output of the same transaction more than once. An account-based model on the other hand guards against double-spending by maintaining an account balance. Because again there is a defined order of transactions, the account balance has a single defined state at any one time.

In addition to validation, at least some of the nodes <NUM> also race to be the first to create blocks of transactions in a process known as mining, which is underpinned by "proof of work". At a mining node <NUM>, new transactions are added to a pool of valid transactions that have not yet appeared in a block. The miners then race to assemble a new valid block <NUM> of transactions <NUM> from the pool of transactions <NUM> by attempting to solve a cryptographic puzzle. Typically this comprises searching for a "nonce" value such that when the nonce is concatenated with the pool of transactions <NUM> and hashed, then the output of the hash meets a predetermined condition. the predetermined condition may be that the output of the hash has a certain predefined number of leading zeros. A property of a hash function is that it has an unpredictable output with respect to its input. Therefore this search can only be performed by brute force, thus consuming a substantive amount of processing resource at each node <NUM> that is trying to solve the puzzle.

The first miner node <NUM> to solve the puzzle announces this to the network <NUM>, providing the solution as proof which can then be easily checked by the other nodes <NUM> in the network (once given the solution to a hash it is straightforward to check that it causes the output of the hash to meet the condition). The pool of transactions <NUM> for which the winner solved the puzzle then becomes recorded as a new block <NUM> in the blockchain <NUM> by at least some of the nodes <NUM> acting as storage nodes <NUM>, based on having checked the winner's announced solution at each such node. A block pointer <NUM> is also assigned to the new block 151n pointing back to the previously created block 151n-<NUM> in the chain. The proof-of-work helps reduce the risk of double spending since it takes a large amount of effort to create a new block <NUM>, and as any block containing a double spend is likely to be rejected by other nodes <NUM>, mining nodes <NUM> are incentivised not to allow double spends to be included in their blocks. Once created, the block <NUM> cannot be modified since it is recognized and maintained at each of the storing nodes <NUM> in the P2P network <NUM> according to the same protocol. The block pointer <NUM> also imposes a sequential order to the blocks <NUM>. Since the transactions <NUM> are recorded in the ordered blocks at each storage node <NUM> in a P2P network <NUM>, this therefore provides an immutable public ledger of the transactions.

Note that different miners <NUM> racing to solve the puzzle at any given time may be doing so based on different snapshots of the unmined transaction pool <NUM> at any given time, depending on when they started searching for a solution. Whoever solves their respective puzzle first defines which transactions <NUM> are included in the next new block 151n, and the current pool <NUM> of unmined transactions is updated. The miners <NUM> then continue to race to create a block from the newly defined outstanding pool <NUM>, and so forth. A protocol also exists for resolving any "fork" that may arise, which is where two miners <NUM> solve their puzzle within a very short time of one another such that a conflicting view of the blockchain gets propagated. In short, whichever prong of the fork grows the longest becomes the definitive blockchain <NUM>.

In most blockchains the winning miner <NUM> is automatically rewarded with a special kind of new transaction which creates a new quantity of the digital asset out of nowhere (as opposed to normal transactions which transfer an amount of the digital asset from one user to another). Hence the winning node is said to have "mined" a quantity of the digital asset. This special type of transaction is sometime referred to as a "generation" transaction. It automatically forms part of the new block 151n. This reward gives an incentive for the miners <NUM> to participate in the proof-of-work race. Often a regular (non-generation) transaction <NUM> will also specify an additional transaction fee in one of its outputs, to further reward the winning miner <NUM> that created the block 151n in which that transaction was included.

Due to the computational resource involved in mining, typically at least each of the miner nodes <NUM> takes the form of a server comprising one or more physical server units, or even whole a data centre. Each forwarding node <NUM> and/or storage node <NUM> may also take the form of a server or data centre. However in principle any given node <NUM> could take the form of a user terminal or a group of user terminals networked together.

The memory of each node <NUM> stores software configured to run on the processing apparatus of the node <NUM> in order to perform its respective role or roles and handle transactions <NUM> in accordance with the node protocol. It will be understood that any action attributed herein to a node <NUM> may be performed by the software run on the processing apparatus of the respective computer equipment. Also, the term "blockchain" as used herein is a generic term that refers to the kind of technology in general, and does not limit to any particular proprietary blockchain, protocol or service.

Also connected to the network <NUM> is the computer equipment <NUM> of each of a plurality of parties <NUM> in the role of consuming users. These act as payers and payees in transactions but do not necessarily participate in mining or propagating transactions on behalf of other parties. They do not necessarily run the mining protocol. Two parties <NUM> and their respective equipment <NUM> are shown for illustrative purposes: a first party 103a and his/her respective computer equipment 102a, and a second party 103b and his/her respective computer equipment 102b. It will be understood that many more such parties <NUM> and their respective computer equipment <NUM> may be present and participating in the system, but for convenience they are not illustrated. Each party <NUM> may be an individual or an organization. Purely by way of illustration the first party 103a is referred to herein as Alice and the second party 103b is referred to as Bob, but it will be appreciated that this is not limiting and any reference herein to Alice or Bob may be replaced with "first party" and "second party" respectively.

The computer equipment <NUM> of each party <NUM> comprises respective processing apparatus comprising one or more processors, e.g. one or more CPUs, GPUs, other accelerator processors, application specific processors, and/or FPGAs. The computer equipment <NUM> of each party <NUM> further comprises memory, i.e. computer-readable storage in the form of a non-transitory computer-readable medium or media. This memory may comprise one or more memory units employing one or more memory media, e.g. a magnetic medium such as hard disk; an electronic medium such as an SSD, flash memory or EEPROM; and/or an optical medium such as an optical disc drive. The memory on the computer equipment <NUM> of each party <NUM> stores software comprising a respective instance of at least one client application <NUM> arranged to run on the processing apparatus. It will be understood that any action attributed herein to a given party <NUM> may be performed using the software run on the processing apparatus of the respective computer equipment <NUM>. The computer equipment <NUM> of each party <NUM> comprises at least one user terminal, e.g. a desktop or laptop computer, a tablet, a smartphone, or a wearable device such as a smartwatch. The computer equipment <NUM> of a given party <NUM> may also comprise one or more other networked resources, such as cloud computing resources accessed via the user terminal.

The client application or software <NUM> may be initially provided to the computer equipment <NUM> of any given party <NUM> on suitable computer-readable storage medium or media, e.g. downloaded from a server, or provided on a removable storage device such as a removable SSD, flash memory key, removable EEPROM, removable magnetic disk drive, magnetic floppy disk or tape, optical disk such as a CD or DVD ROM, or a removable optical drive, etc..

The client application <NUM> comprises at least a "wallet" function. This has two main functionalities. One of these is to enable the respective user party <NUM> to create, sign and send transactions <NUM> to be propagated throughout the network of nodes <NUM> and thereby included in the blockchain <NUM>. The other is to report back to the respective party the amount of the digital asset that he or she currently owns. In an output-based system, this second functionality comprises collating the amounts defined in the outputs of the various <NUM> transactions scattered throughout the blockchain <NUM> that belong to the party in question.

The instance of the client application <NUM> on each computer equipment <NUM> is operatively coupled to at least one of the forwarding nodes 104F of the P2P network <NUM>. This enables the wallet function of the client <NUM> to send transactions <NUM> to the network <NUM>. The client <NUM> is also able to contact one, some or all of the storage nodes <NUM> in order to query the blockchain <NUM> for any transactions of which the respective party <NUM> is the recipient (or indeed inspect other parties' transactions in the blockchain <NUM>, since in embodiments the blockchain <NUM> is a public facility which provides trust in transactions in part through its public visibility). The wallet function on each computer equipment <NUM> is configured to formulate and send transactions <NUM> according to a transaction protocol. Each node <NUM> runs software configured to validate transactions <NUM> according to a node protocol, and in the case of the forwarding nodes 104F to forward transactions <NUM> in order to propagate them throughout the network <NUM>. The transaction protocol and node protocol correspond to one another, and a given transaction protocol goes with a given node protocol, together implementing a given transaction model. The same transaction protocol is used for all transactions <NUM> in the blockchain <NUM> (though the transaction protocol may allow different subtypes of transaction within it). The same node protocol is used by all the nodes <NUM> in the network <NUM> (though it many handle different subtypes of transaction differently in accordance with the rules defined for that subtype, and also different nodes may take on different roles and hence implement different corresponding aspects of the protocol).

As mentioned, the blockchain <NUM> comprises a chain of blocks <NUM>, wherein each block <NUM> comprises a set of one or more transactions <NUM> that have been created by a proof-of-work process as discussed previously. Each block <NUM> also comprises a block pointer <NUM> pointing back to the previously created block <NUM> in the chain so as to define a sequential order to the blocks <NUM>. The blockchain <NUM> also comprises a pool of valid transactions <NUM> waiting to be included in a new block by the proof-of-work process. Each transaction <NUM> (other than a generation transaction) comprises a pointer back to a previous transaction so as to define an order to sequences of transactions (N. sequences of transactions <NUM> are allowed to branch). The chain of blocks <NUM> goes all the way back to a genesis block (Gb) <NUM> which was the first block in the chain. One or more original transactions <NUM> early on in the chain <NUM> pointed to the genesis block <NUM> rather than a preceding transaction.

When a given party <NUM>, say Alice, wishes to send a new transaction 152j to be included in the blockchain <NUM>, then she formulates the new transaction in accordance with the relevant transaction protocol (using the wallet function in her client application <NUM>). She then sends the transaction <NUM> from the client application <NUM> to one of the one or more forwarding nodes 104F to which she is connected. this could be the forwarding node 104F that is nearest or best connected to Alice's computer <NUM>. When any given node <NUM> receives a new transaction 152j, it handles it in accordance with the node protocol and its respective role. This comprises first checking whether the newly received transaction 152j meets a certain condition for being "valid", examples of which will be discussed in more detail shortly. In some transaction protocols, the condition for validation may be configurable on a per-transaction basis by scripts included in the transactions <NUM>. Alternatively the condition could simply be a built-in feature of the node protocol, or be defined by a combination of the script and the node protocol.

On condition that the newly received transaction 152j passes the test for being deemed valid (i.e. on condition that it is "validated"), any storage node <NUM> that receives the transaction 152j will add the new validated transaction <NUM> to the pool <NUM> in the copy of the blockchain <NUM> maintained at that node <NUM>. Further, any forwarding node 104F that receives the transaction 152j will propagate the validated transaction <NUM> onward to one or more other nodes <NUM> in the P2P network <NUM>. Since each forwarding node 104F applies the same protocol, then assuming the transaction 152j is valid, this means it will soon be propagated throughout the whole P2P network <NUM>.

Once admitted to the pool <NUM> in the copy of the blockchain <NUM> maintained at one or more storage nodes <NUM>, then miner nodes <NUM> will start competing to solve the proof-of-work puzzle on the latest version of the pool <NUM> including the new transaction <NUM> (other miners <NUM> may still be trying to solve the puzzle based on the old view of the pool <NUM>, but whoever gets there first will define where the next new block <NUM> ends and the new pool <NUM> starts, and eventually someone will solve the puzzle for a part of the pool <NUM> which includes Alice's transaction 152j). Once the proof-of-work has been done for the pool <NUM> including the new transaction 152j, it immutably becomes part of one of the blocks <NUM> in the blockchain <NUM>. Each transaction <NUM> comprises a pointer back to an earlier transaction, so the order of the transactions is also immutably recorded.

<FIG> illustrates an example transaction protocol. This is an example of an UTXO-based protocol. A transaction <NUM> (abbreviated "Tx") is the fundamental data structure of the blockchain <NUM> (each block <NUM> comprising one or more transactions <NUM>). The following will be described by reference to an output-based or "UTXO" based protocol. However, this not limiting to all possible embodiments.

In a UTXO-based model, each transaction ("Tx") <NUM> comprises a data structure comprising one or more inputs <NUM>, and one or more outputs <NUM>. Each output <NUM> may comprise an unspent transaction output (UTXO), which can be used as the source for the input <NUM> of another new transaction (if the UTXO has not already been redeemed). The UTXO includes a value specifying an amount of a digital asset. This represents a set number of tokens on the (distributed) ledger. The UTXO may also contain the transaction ID of the transaction from which it came, amongst other information. The transaction data structure may also comprise a header <NUM>, which may comprise an indicator of the size of the input field(s) <NUM> and output field(s) <NUM>. The header <NUM> may also include an ID of the transaction. In embodiments the transaction ID is the hash of the transaction data (excluding the transaction ID itself) and stored in the header <NUM> of the raw transaction <NUM> submitted to the miners <NUM>.

Note that whilst each output in <FIG> is shown as a UTXO, a transaction may additionally or alternatively comprise one or more unspendable transaction outputs.

Say Alice 103a wishes to create a transaction 152j transferring an amount of the digital asset in question to Bob 103b. In <FIG> Alice's new transaction 152j is labelled " Tx<NUM>". It takes an amount of the digital asset that is locked to Alice in the output <NUM> of a preceding transaction 152i in the sequence, and transfers at least some of this to Bob. The preceding transaction 152i is labelled "Tx<NUM>" in <FIG>. Tn<NUM> and Tx<NUM> are just an arbitrary labels. They do not necessarily mean that Tx<NUM> is the first transaction in the blockchain <NUM>, nor that Tx<NUM> is the immediate next transaction in the pool <NUM>. Tx<NUM> could point back to any preceding (i.e. antecedent) transaction that still has an unspent output <NUM> locked to Alice.

The preceding transaction Tx<NUM> may already have been validated and included in the blockchain <NUM> at the time when Alice creates her new transaction Tx<NUM>, or at least by the time she sends it to the network <NUM>. It may already have been included in one of the blocks <NUM> at that time, or it may be still waiting in the pool <NUM> in which case it will soon be included in a new block <NUM>. Alternatively Txo and Tx<NUM> could be created and sent to the network <NUM> together, or Txo could even be sent after Tx<NUM> if the node protocol allows for buffering "orphan" transactions. The terms "preceding" and "subsequent" as used herein in the context of the sequence of transactions refer to the order of the transactions in the sequence as defined by the transaction pointers specified in the transactions (which transaction points back to which other transaction, and so forth). They could equally be replaced with "predecessor" and "successor", or "antecedent" and "descendant", "parent" and "child", or such like. It does not necessarily imply an order in which they are created, sent to the network <NUM>, or arrive at any given node <NUM>. Nevertheless, a subsequent transaction (the descendent transaction or "child") which points to a preceding transaction (the antecedent transaction or "parent") will not be validated until and unless the parent transaction is validated. A child that arrives at a node <NUM> before its parent is considered an orphan. It may be discarded or buffered for a certain time to wait for the parent, depending on the node protocol and/or miner behaviour.

One of the one or more outputs <NUM> of the preceding transaction Tx<NUM> comprises a particular UTXO, labelled here UTXO<NUM>. Each UTXO comprises a value specifying an amount of the digital asset represented by the UTXO, and a locking script which defines a condition which must be met by an unlocking script in the input <NUM> of a subsequent transaction in order for the subsequent transaction to be validated, and therefore for the UTXO to be successfully redeemed. Typically the locking script locks the amount to a particular party (the beneficiary of the transaction in which it is included). the locking script defines an unlocking condition, typically comprising a condition that the unlocking script in the input of the subsequent transaction comprises the cryptographic signature of the party to whom the preceding transaction is locked.

The locking script (aka scriptPubKey) is a piece of code written in the domain specific language recognized by the node protocol. A particular example of such a language is called "Script" (capital S). The locking script specifies what information is required to spend a transaction output <NUM>, for example the requirement of Alice's signature. Unlocking scripts appear in the outputs of transactions. The unlocking script (aka scriptSig) is a piece of code written the domain specific language that provides the information required to satisfy the locking script criteria. For example, it may contain Bob's signature. Unlocking scripts appear in the input <NUM> of transactions.

So in the example illustrated, UTXO<NUM> in the output <NUM> of Txocomprises a locking script [Checksig PA] which requires a signature Sig PA of Alice in order for UTXO<NUM> to be redeemed (strictly, in order for a subsequent transaction attempting to redeem UTXO<NUM> to be valid). [Checksig PA] contains the public key PA from a public-private key pair of Alice. The input <NUM> of Tx<NUM> comprises a pointer pointing back to Tx<NUM> (e.g. by means of its transaction ID, TxID<NUM>, which in embodiments is the hash of the whole transaction Tx<NUM>). The input <NUM> of Tx<NUM> comprises an index identifying UTXO<NUM> within Tx<NUM>, to identify it amongst any other possible outputs of Tx<NUM>. The input <NUM> of Tx<NUM> further comprises an unlocking script <Sig PA> which comprises a cryptographic signature of Alice, created by Alice applying her private key from the key pair to a predefined portion of data (sometimes called the "message" in cryptography). What data (or "message") needs to be signed by Alice to provide a valid signature may be defined by the locking script, or by the node protocol, or by a combination of these.

When the new transaction Tx<NUM> arrives at a node <NUM>, the node applies the node protocol. This comprises running the locking script and unlocking script together to check whether the unlocking script meets the condition defined in the locking script (where this condition may comprise one or more criteria). In embodiments this involves concatenating the two scripts: <MAT> where "| |" represents a concatenation and "<. >" means place the data on the stack, and "[. ]" is a function comprised by the unlocking script (in this example a stack-based language). Equivalently the scripts may be run one after another, with a common stack, rather than concatenating the scripts. Either way, when run together, the scripts use the public key PA of Alice, as included in the locking script in the output of Tx<NUM>, to authenticate that the locking script in the input of Tx<NUM> contains the signature of Alice signing the expected portion of data. The expected portion of data itself (the "message") also needs to be included in Txoorder to perform this authentication. In embodiments the signed data comprises the whole of Tx<NUM> (so a separate element does to need to be included specifying the signed portion of data in the clear, as it is already inherently present).

The details of authentication by public-private cryptography will be familiar to a person skilled in the art. Basically, if Alice has signed a message by encrypting it with her private key, then given Alice's public key and the message in the clear (the unencrypted message), another entity such as a node <NUM> is able to authenticate that the encrypted version of the message must have been signed by Alice. Signing typically comprises hashing the message, signing the hash, and tagging this onto the clear version of the message as a signature, thus enabling any holder of the public key to authenticate the signature.

If the unlocking script in Tx<NUM> meets the one or more conditions specified in the locking script of Tx<NUM> (so in the example shown, if Alice's signature is provided in Tx<NUM> and authenticated), then the node <NUM> deems Tx<NUM> valid. If it is a mining node <NUM>, this means it will add it to the pool of transactions <NUM> awaiting proof-of-work. If it is a forwarding node 104F, it will forward the transaction Tx<NUM> to one or more other nodes <NUM> in the network <NUM>, so that it will be propagated throughout the network. Once Tx<NUM> has been validated and included in the blockchain <NUM>, this defines UTXO<NUM> from Tx<NUM> as spent. Note that Tx<NUM> can only be valid if it spends an unspent transaction output <NUM>. If it attempts to spend an output that has already been spent by another transaction <NUM>, then Tx<NUM> will be invalid even if all the other conditions are met. Hence the node <NUM> also needs to check whether the referenced UTXO in the preceding transaction Tx<NUM> is already spent (has already formed a valid input to another valid transaction). This is one reason why it is important for the blockchain <NUM> to impose a defined order on the transactions <NUM>. In practice a given node <NUM> may maintain a separate database marking which UTXOs <NUM> in which transactions <NUM> have been spent, but ultimately what defines whether a UTXO has been spent is whether it has already formed a valid input to another valid transaction in the blockchain <NUM>.

Note that in UTXO-based transaction models, a given UTXO needs to be spent as a whole. It cannot "leave behind" a fraction of the amount defined in the UTXO as spent while another fraction is spent. However the amount from the UTXO can be split between multiple outputs of the next transaction. the amount defined in UTXO<NUM> in Tx<NUM> can be split between multiple UTXOs in Tx<NUM>. Hence if Alice does not want to give Bob all of the amount defined in UTXO<NUM>, she can use the remainder to give herself change in a second output of Tx<NUM>, or pay another party.

In practice Alice will also usually need to include a fee for the winning miner, because nowadays the reward of the generation transaction alone is not typically sufficient to motivate mining. If Alice does not include a fee for the miner, Tx<NUM> will likely be rejected by the miner nodes <NUM>, and hence although technically valid, it will still not be propagated and included in the blockchain <NUM> (the miner protocol does not force miners <NUM> to accept transactions <NUM> if they don't want). In some protocols, the mining fee does not require its own separate output <NUM> (i.e. does not need a separate UTXO). Instead any different between the total amount pointed to by the input(s) <NUM> and the total amount of specified in the output(s) <NUM> of a given transaction <NUM> is automatically given to the winning miner <NUM>. say a pointer to UTXO<NUM> is the only input to Tx<NUM>, and Tx<NUM> has only one output UTXO<NUM>. If the amount of the digital asset specified in UTXO<NUM> is greater than the amount specified in UTXO<NUM>, then the difference automatically goes to the winning miner <NUM>. Alternatively or additionally however, it is not necessarily excluded that a miner fee could be specified explicitly in its own one of the UTXOs <NUM> of the transaction <NUM>.

Note also that if the total amount specified in all the outputs <NUM> of a given transaction <NUM> is greater than the total amount pointed to by all its inputs <NUM>, this is another basis for invalidity in most transaction models. Therefore such transactions will not be propagated nor mined into blocks <NUM>.

Alice and Bob's digital assets consist of the unspent UTXOs locked to them in any transactions <NUM> anywhere in the blockchain <NUM>. Hence typically, the assets of a given party <NUM> are scattered throughout the UTXOs of various transactions <NUM> throughout the blockchain <NUM>. There is no one number stored anywhere in the blockchain <NUM> that defines the total balance of a given party <NUM>. It is the role of the wallet function in the client application <NUM> to collate together the values of all the various UTXOs which are locked to the respective party and have not yet been spent in another onward transaction. It can do this by querying the copy of the blockchain <NUM> as stored at any of the storage nodes <NUM>, e.g. the storage node <NUM> that is closest or best connected to the respective party's computer equipment <NUM>.

Note that the script code is often represented schematically (i.e. not the exact language). For example, one may write [Checksig PA] to mean [Checksig PA] = OP_DUP OP_HASH160 <H(Pa)> OP_EQUALVERIFY OP_CHECKSIG. " refers to a particular opcode of the Script language. OP_CHECKSIG (also called "Checksig") is a Script opcode that takes two inputs (signature and public key) and verifies the signature's validity using the Elliptic Curve Digital Signature Algorithm (ECDSA). At runtime, any occurrences of signature ('sig') are removed from the script but additional requirements, such as a hash puzzle, remain in the transaction verified by the 'sig' input. As another example, OP_RETURN is an opcode of the Script language for creating an unspendable output of a transaction that can store metadata within the transaction, and thereby record the metadata immutably in the blockchain <NUM>. the metadata could comprise a document which it is desired to store in the blockchain.

The signature PA is a digital signature. In embodiments this is based on the ECDSA using the elliptic curve secp256k1. A digital signature signs a particular piece of data. In embodiments, for a given transaction the signature will sign part of the transaction input, and all or part of the transaction output. The particular parts of the outputs it signs depends on the SIGHASH flag. The SIGHASH flag is a <NUM>-byte code included at the end of a signature to select which outputs are signed (and thus fixed at the time of signing).

The locking script is sometimes called "scriptPubKey" referring to the fact that it comprises the public key of the party to whom the respective transaction is locked. The unlocking script is sometimes called "scriptSig" referring to the fact that it supplies the corresponding signature. However, more generally it is not essential in all applications of a blockchain <NUM> that the condition for a UTXO to be redeemed comprises authenticating a signature. More generally the scripting language could be used to define any one or more conditions. Hence the more general terms "locking script" and "unlocking script" may be preferred.

<FIG> shows a further system <NUM> for implementing a blockchain <NUM>. The system <NUM> is substantially the same as that described in relation to <FIG> except that additional communication functionality is involved. The client application on each of Alice and Bob's computer equipment 102a, 120b, respectively, comprises additional communication functionality. That is, it enables Alice 103a to establish a separate side channel <NUM> with Bob 103b (at the instigation of either party or a third party). The side channel <NUM> enables exchange of data separately from the P2P network. Such communication is sometimes referred to as "off-chain". For instance this may be used to exchange a transaction <NUM> between Alice and Bob without the transaction (yet) being published onto the network P2P <NUM> or making its way onto the chain <NUM>, until one of the parties chooses to broadcast it to the network <NUM>. Alternatively or additionally, the side channel <NUM> may be used to exchange any other transaction related data, such as keys, negotiated amounts or terms, data content, etc..

The side channel <NUM> may be established via the same packet-switched network <NUM> as the P2P overlay network <NUM>. Alternatively or additionally, the side channel <NUM> may be established via a different network such as a mobile cellular network, or a local area network such as a local wireless network, or even a direct wired or wireless link between Alice and Bob's devices <NUM>, 102b. Generally, the side channel <NUM> as referred to anywhere herein may comprise any one or more links via one or more networking technologies or communication media for exchanging data "off-chain", i.e. separately from the P2P overlay network <NUM>. Where more than one link is used, then the bundle or collection of off-chain links as a whole may be referred to as the side channel <NUM>. Note therefore that if it is said that Alice and Bob exchange certain pieces of information or data, or such like, over the side channel <NUM>, then this does not necessarily imply all these pieces of data have to be send over exactly the same link or even the same type of network.

<FIG> illustrates an example implementation of the client application <NUM> for implementing embodiments of the presently disclosed scheme. The client application <NUM> comprises a transaction engine <NUM> and a user interface (Ul) layer <NUM>. The transaction engine <NUM> is configured to implement the underlying transaction-related functionality of the client <NUM>, such as to formulate transactions <NUM>, receive and/or send transactions and/or other data over the side channel <NUM>, and/or send transactions to be propagated through the P2P network <NUM>, in accordance with the schemes discussed above and as discussed in further detail shortly.

The UI layer <NUM> is configured to render a user interface via a user input/output (I/O) means of the respective user's computer equipment <NUM>, including outputting information to the respective user <NUM> via a user output means of the equipment <NUM>, and receiving inputs back from the respective user <NUM> via a user input means of the equipment <NUM>. For example the user output means could comprise one or more display screens (touch or non-touch screen) for providing a visual output, one or more speakers for providing an audio output, and/or one or more haptic output devices for providing a tactile output, etc. The user input means could comprise for example the input array of one or more touch screens (the same or different as that/those used for the output means); one or more cursor-based devices such as mouse, trackpad or trackball; one or more microphones and speech or voice recognition algorithms for receiving a speech or vocal input; one or more gesture-based input devices for receiving the input in the form of manual or bodily gestures; or one or more mechanical buttons, switches or joysticks, etc..

Note: whilst the various functionality herein may be described as being integrated into the same client application <NUM>, this is not necessarily limiting and instead they could be implemented in a suite of two or more distinct applications, e.g. one being a plug-in to the other or interfacing via an API (application programming interface). For instance, the functionality of the transaction engine <NUM> may be implemented in a separate application than the UI layer <NUM>, or the functionality of a given module such as the transaction engine <NUM> could be split between more than one application. Nor is it excluded that some or all of the described functionality could be implemented at, say, the operating system layer. Where reference is made anywhere herein to a single or given application <NUM>, or such like, it will be appreciated that this is just by way of example, and more generally the described functionality could be implemented in any form of software.

<FIG> gives a mock-up of an example of the user interface (Ul) <NUM> which may be rendered by the UI layer <NUM> of the client application 105a on Alice's equipment 102a. It will be appreciated that a similar UI may be rendered by the client 105b on Bob's equipment 102b, or that of any other party.

By way of illustration <FIG> shows the UI <NUM> from Alice's perspective. The UI <NUM> may comprise one or more UI elements <NUM>, <NUM>, <NUM> rendered as distinct UI elements via the user output means.

For example, the UI elements may comprise one or more user-selectable elements <NUM> which may be, such as different on-screen buttons, or different options in a menu, or such like. The user input means is arranged to enable the user <NUM> (in this case Alice 103a) to select or otherwise operate one of the options, such as by clicking or touching the UI element on-screen, or speaking a name of the desired option (N. the term "manual" as used herein is meant only to contrast against automatic, and does not necessarily limit to the use of the hand or hands). The options enable the user (Alice) to generate transactions and send them to another user (Bob), and to generate a signature of a transaction in accordance with the described embodiments.

Alternatively or additionally, the UI elements may comprise one or more data entry fields <NUM>, through which the user can input data to be included in the generated transaction and/or a message to be signed. These data entry fields are rendered via the user output means, e.g. on-screen, and the data can be entered into the fields through the user input means, e.g. a keyboard or touchscreen. Alternatively the data could be received orally for example based on speech recognition.

Alternatively or additionally, the UI elements may comprise one or more information elements <NUM> output to output information to the user. this/these could be rendered on screen or audibly.

It will be appreciated that the particular means of rendering the various UI elements, selecting the options and entering data is not material. The functionality of these UI elements will be discussed in more detail shortly. It will also be appreciated that the Ul <NUM> shown in <FIG> is only a schematized mock-up and in practice it may comprise one or more further UI elements, which for conciseness are not illustrated.

Embodiments of the present invention will be described with reference to the example system of <FIG>. A first party (Alice) 103a is configured to communicate with a second party (103b) via an off-chain communication channel, e.g. the side channel <NUM>. It will be understood that some or all of the actions attributed to Alice 103a and Bob 103b are performed by the respective computer equipment of Alice 103a and Bob 103b.

Alice 103a generates a first blockchain transaction Tx<NUM>. The first transaction Tx<NUM> contains an input that references an output of a previous transaction. As explained with reference to <FIG>, a previous transaction Tx<NUM> is a transaction that has already been transmitted to one or more nodes <NUM> of the blockchain network <NUM>. Note that the previous transaction does not necessarily have to have been included in a block of the blockchain <NUM>. It is sufficient for the previous transaction to have been transmitted to one or more nodes <NUM>. When a transaction is submitted to nodes <NUM> of the network <NUM>, the nodes will first add the transaction to their respective transaction memory pool (also known as a "mempool"). Mining nodes <NUM> attempt to form a block <NUM> out of transactions included in the mempool.

Alice 103a also generates a signature of a message. The message is at least partially based on the first transaction Tx<NUM>. For example, the message to be signed may comprise part or all of the first transaction Tx<NUM>. Additionally or alternatively, the message may comprise a transaction identifier TxID<NUM> of the first transaction Tx<NUM>. The message is also at least partially based on one or more time indicators. Each time indicator indicates (or represents, or corresponds to) a time at which the first transaction Tx<NUM> was generated and/or transmitted to the second party 103b. For instance, a time indicator may be a timestamp, e.g. the time (such as the UNIX time) at which the first transaction Tx<NUM> was generated by Alice 103a. As another examples, a time indicator may be a current block height of the blockchain <NUM>. The block height is the number of blocks preceding a particular block <NUM> on a blockchain <NUM>. For example, the genesis block <NUM> has a height of zero because zero blocks preceded it. Such a time indicator has, on average, a granularity of ten minutes since new blocks are added to the blockchain <NUM> approximately every ten minutes at present. A time indicator may in general be any indicator that at least approximately indicates when the transaction was generated or transmitted. Other examples of time indicators include: blockchain related time indicators, e.g. a current total number of transactions submitted to the blockchain, a current total number of unspent transaction outputs (UTXOs) on the blockchain <NUM>, a current blockchain size, a current hash rate, a total number of unique addresses, etc.; and/or blockchain related time indicators, e.g. current date and/or time, the current GBP-USD exchange rate, a company stock price, a snapshot of a stock market index, etc. One, some or all of the above time indicators may be used to generate the signature.

The message may comprise the first transaction Tx<NUM> concatenated with one or more time indicators. Additionally or alternatively, the message may comprise the identifier TxID<NUM> of the first transaction Tx<NUM> concatenated with one the one or more time indicators.

Alice 103a transmits the first transaction Tx<NUM>, the signature, and the one or more indicators on which the signature is based, to Bob 103b. Alice 103a sends the first transaction Tx<NUM> to Bob 103b via the side channel <NUM> (i.e. an off-chain channel). Alice 103a may also send the signature and indicator(s) via the same side channel <NUM>. Alice 103a may instead use a different side channel. In some examples, Alice 103a may include the signature and/or the indicator(s) in a different blockchain transaction and submit that transaction to the blockchain network <NUM>. The different blockchain transaction acts as an immutable record of the signature and indicator(s) in case of a dispute with Bob 103b.

Bob 103b, upon receiving the first transaction from Alice 103a, checks whether Alice 103a has also sent the signature and the time indicator(s). The particular time indicator(s) may be prescribed by Bob 103a or by a protocol to which Alice 103a and Bob 103b (are meant to) adhere.

In general, Alice 103a may use any signature scheme to generate the signature. Preferably Alice 103a signs the message using a private key corresponding to a public key which is associated with (or can be linked to) Alice 103a. Alice 103a may use the Elliptic Curve Digital Signature Algorithm (ECDSA) to sign the message with her private key. The public key may be a certified public key. The public key may be known to, or accessible by, Bob 103b prior to receiving the first transaction.

In some examples, the first transaction may be a partial transaction, i.e. a partially complete blockchain transaction. That is, the partial transaction may be missing one or more inputs and/or outputs that are necessary in order for the transaction to be accepted by the nodes <NUM> of the blockchain network <NUM>. For example, the partial transaction may not include any outputs. As another example, the partial transaction may include one or more outputs which together lock an amount of digital asset greater than the amount of digital asset unlocked by the input(s) of the partial transaction.

In other examples, the partial transaction may be a transaction that is technically a valid transaction and which could be accepted onto the blockchain, but which can also be added to by the Bob 103b. That is, the partial transaction may be configured to allow Bob <NUM> to add one or more inputs or one or more outputs to the transaction. Alice 103a can use different signature flags to sign different components of the transaction, thus allowing Bob <NUM> to add inputs and/or outputs depending on which flag(s) have been used. For instance, the signature flag "SIGHASH_ANYONECANPAY" is a signature hash type which signs only the current input, thus allowing Bob 103b to add other inputs and outputs.

Bob 103a may accept and act on the first transaction Tx<NUM>, i.e. perform one or more actions in response to receiving the first transaction. For example, Bob 103a may send something (goods, services, (digital) assets, etc.), or provide access to something (e.g. a transport network, an office or other type of building) to Alice 103a in return for the first transaction Tx<NUM>. Additionally or alternatively, Bob <NUM> may add one or more inputs and/or outputs to the first transaction Tx<NUM> in response to receiving the first transaction Tx<NUM>. As another example, Bob 103b may be configured to initiate a process in response to receiving the first transaction Tx<NUM>, such as initiating control of a device controllable by Bob 103b. That is, Bob 103b may be configured to control one or more devices which are connected to Bob 103b (e.g. via a wired or wireless connection). Control of devices in response to receiving a transaction will be described below. Any of the above actions may be performed automatically by Bob in response to receiving the first transaction Tx<NUM>.

Bob 103a may only act on the first transaction under one or more conditions. That is, Bob 103a may choose whether or not to accept the first transaction only if at least one condition is met. A first condition which has to be met in order for Bob 103a to accept the first transaction is that Alice 103a must have sent the signature (i.e. the signature based on the first transaction and the time indicator(s)) and the time indicator(s) to Bob 103b. The condition may be met if Alice 103a sends the signature and indicator(s) with (e.g. at the same time as) the first transaction. Alternatively, Alice 103a have previously sent the signature and time indicator(s) to Bob 103b if the time indicator(s) indicate when the first transaction was generated. If Alice 103a does not send the signature and time indicator(s), Bob 103b may reject the first transaction.

Another example of a condition that may have to be met in order for Bob 103b to accept the first transaction is that one, some or all of the time indicator(s) indicate that the first transaction was generated and/or transmitted to Bob 103b within a threshold period of time from when Bob 103b actually receives the first transaction. That is, Bob 103b may determine one or more time indicator(s) himself, each of which correspond to one of the time indicators sent by Alice 103a, and then check that there are not any large discrepancies between the received and determined time indicators. For example, Alice 103a may send the UNIX time at which the first transaction was generated. Bob 103b may determine the UNIX time at which the first transaction was received. If the two times differ by more than a predetermined (i.e. specified) threshold, Bob 103b may reject the first transaction.

Another example of a condition that may have to be met in order for Bob 103b to accept the first transaction is that the signature must have been generated using a private key corresponding to a public key associated with Alice 103a. Various techniques for verifying that a signature has been generated using a private key corresponding to a particular public key are known in the art. Bob 103b may use an expected public key to verify that the signature has indeed been signed using the corresponding private key. Bob 103b may already have Alice's public key, or he may access it from a public record. As an example, Alice's public key may be certified by a certificate authority and, for instance, recorded in an (OP_RETURN) output of a blockchain transaction <NUM>. If the signature has not been generated using a private key corresponding to an expected public key, Bob 103b may reject the first transaction.

As described above, the first transaction may be a partial transaction. If one, some or all of the conditions for accepting the first transaction have been met, Bob 103b may add one or more inputs and/or outputs to the first transaction in order to complete the transaction. Bob 103b may then submit the completed transaction to the blockchain network <NUM>. Additionally or alternatively, Bob may send the completed transaction back to Alice 103a. Alice 103a may then submit the completed transaction to the blockchain network <NUM>.

<FIG> illustrates another example system <NUM> for implementing embodiments of the present invention. The example system <NUM> comprises a first network <NUM> of one or more end devices (i.e. computing devices) <NUM> and one or more bridging nodes <NUM> (i.e. computing devices which run a blockchain client application <NUM> and therefore act as a bridge between the blockchain network <NUM> and the first network <NUM>). For clarity, the first network <NUM> will be referred to as an loT network, i.e. a network of computing devices interconnected by the internet. However, it will be appreciated that the first network need not be an loT network and, in general, may be any P2P network. Typically the end devices <NUM> and bridging nodes <NUM> are embedded in everyday devices. An end device <NUM> may take one of a variety of forms, e.g. user devices (e.g. smart TVs, smart speakers, toys, wearables, etc.), smart appliances (e.g. fridges, washing machines, ovens, etc.), meters or sensors (e.g. smart thermostats, smart lighting, security sensors, etc.). Similarly, a bridging node <NUM> may also take a variety of forms, which may include, but is not limited to, the same forms as which an end device may take. A node <NUM> may also take the form of dedicated server equipment, a base station, an access point, a router, and so on. In some examples, each device may have a fixed network (e.g. IP) address. For instance, one, some or all of the end devices may be a stationary device (e.g. a smart light, or smart central heating controller, etc.), as opposed to a mobile device. In this example system <NUM>, Alice 103a and Bob 103b each take the form of a bridging node <NUM>.

The loT network is a packet-switched network <NUM>, typically a wide-area internetwork such as the Internet. The nodes <NUM> and devices <NUM> of the packet-switched network <NUM> are arranged to form a peer-to-peer (P2P) overlay network <NUM> within the packet-switched network <NUM>. Each node <NUM> comprises respective computer equipment, each comprising respective processing apparatus comprising one or more processors, e.g. one or more central processing units (CPUs), accelerator processors, application specific processors and/or field programmable gate arrays (FPGAs). Each node <NUM> also comprises memory, i.e. computer-readable storage in the form of a non-transitory computer-readable medium or media. The memory may comprise one or more memory units employing one or more memory media, e.g. a magnetic medium such as a hard disk; an electronic medium such as a solid-state drive (SSD), flash memory or EEPROM; and/or an optical medium such as an optical disk drive.

Each node <NUM> of the loT network is also a blockchain node <NUM>. These nodes <NUM> are arranged as bridging nodes (gateway nodes) which act as a bridge (gateway) between the first network <NUM> and the blockchain network <NUM>. A blockchain node <NUM> may be a "listening node". A listening node runs a client application <NUM> that keeps a full copy of the blockchain, validates and propagate new transactions and blocks but does not actively mine or generate new blocks. Alternatively, a node may be a "simplified payment verification node" (SPV node). An SPV node runs a lightweight client that can generate and broadcast bitcoin transactions and monitor addresses indirectly but does not keep a full copy of the blockchain.

Each node <NUM> of the loT network is configured to control an end device <NUM> either directly or indirectly. A node <NUM> that is directly connected to an end device <NUM> can directly control that device. A node <NUM> that is not directly connected to an end device <NUM> can only indirectly control that device, e.g. by forwarding a control message to the end node via one or more intermediary nodes. Each node <NUM> is connected to one or more mining nodes <NUM>.

<FIG> also illustrates a network <NUM> of mining nodes <NUM> which is a subset of the blockchain network <NUM>. Mining nodes have been discussed above with reference to <FIG>. The mining nodes <NUM> are configured to mine valid transactions (e.g. transactions transmitted from the loT nodes) to the blockchain <NUM>.

As shown in <FIG>, the nodes <NUM> form part of both the P2P network <NUM> and the blockchain P2P network <NUM>, whereas the mining nodes <NUM> form part of only the blockchain P2P network <NUM>. Whilst the end devices <NUM> are shown in <FIG> as forming part of only the P2P loT network <NUM>, it is not excluded that the end devices <NUM> could also be blockchain nodes <NUM>.

<FIG> illustrates an example loT network <NUM> topology. The loT network <NUM> may control a master node 503a, one or more sets <NUM> of one or more intermediary nodes 503b, 503c, and a set of end devices <NUM>. The master node 502a is configured to control one or more intermediary nodes 503b, 503c. If the loT network <NUM> comprises multiple sets (e.g. layers) 601a, 601b of intermediary nodes, the master node 503a is configured to directly control the first set (layer) 601a of intermediary nodes ("server nodes" 503b) and to indirectly control one or more further sets (layers) 601b of intermediary nodes (e.g. a layer of "slave nodes" 503c). The master node 503a is a controlling node with the ability to override and control server and slave nodes. Each server node 503b is a node with the ability to control slave nodes 503c. Each slave node 503c is a node under the control of the server nodes 503b and the master node 503a. As an example, to instruct end device 502a, the master node 503a would issue a command to slave node 503c via servant node 503b.

Whilst the example loT network of <FIG> shows only two layers of intermediary nodes (server nodes and slave nodes), other examples may comprise one or more further sets of intermediary nodes, e.g. between the master node 503a and server nodes 503b, and/or between the server nodes 503b and slave nodes 503c. As shown, each node is connected to one or more other nodes via a respective connection <NUM>, and each end device <NUM> is connected to one or more slave nodes via a respective connection <NUM>. One or more nodes (e.g. the master node) are referred to below as controlling nodes. Each controlling node is a node <NUM> that can instruct other nodes to perform an action through issuing commands.

The loT network nodes <NUM> may correspond to hierarchies in scope of functionality, in superiority of instructions/prerogatives, and/or in span of access. In some implementations, a hierarchical set of SPV nodes implement an "loT controller" with three levels of hierarchy, corresponding to the master 503a, server 503b and slave nodes 503c of <FIG> and <FIG>. The master node 503a instructs one or more server nodes 503b, and each server node instructs one or more slave nodes 503c. Each slave node 503c receives instructions from one or more server nodes 503b. Every slave node 503c communicates with one or more loT end-devices <NUM>, and these are the direct channels of communication between the loT-controller <NUM> and the loT end-devices <NUM>. The states of execution of the loT controller <NUM> are recorded in blockchain transactions Tx. Each loT node - master, server, or slave - has the capacity to create and broadcast corresponding transactions Tx to the blockchain network <NUM>. Each slave node monitors for trigger and/or confirmation signals from end-devices <NUM>, and every loT node <NUM> has the capacity to interact with any other loT node with the purpose of executing the overall logic of the loT controller.

The master node, server node(s) and slave node(s) can each independently connect to nodes <NUM> on the blockchain network <NUM>, operate a blockchain wallet <NUM> (e.g. to watch blockchain addresses) and possibly run a full node (although this is not required). The master node 503a is configured to monitor the activity of other loT nodes both directly and indirectly under their control, issue commands to these nodes in the form of blockchain transactions Tx and respond to alerts. The server node 503b is configured to watch multiple addresses, including addresses not directly controlled by the server node 503b. Server nodes 503b can be commanded to perform actions by a master node 503a. The slave node 503c is configured to monitor the activities of end devices <NUM> directly under their control. Slave nodes 503c are under the direct command of server nodes 503b and can also be commanded to perform actions by the master node 503a. The slave nodes 503c act as gateway nodes for the end devices <NUM> (i.e. a gateway between the end device and the blockchain network <NUM>). The end device <NUM> is configured to connect to nearby slave devices. They report on end device state using off-chain messaging protocol.

Note that whilst a distinction is made between an loT node <NUM> and an end device <NUM> in that end devices <NUM> are controlled by loT nodes <NUM> but do not themselves control loT nodes <NUM>, an end device <NUM> may also be a node <NUM> of the blockchain network <NUM>. That is, in some examples an end device <NUM> may operate a blockchain protocol client or wallet application <NUM>.

The loT network <NUM> strikes a balance between centralisation and decentralisation by combining a command and control hierarchy with use of a blockchain network infrastructure. Users of the network <NUM> may create their own multilevel control hierarchy which includes client-server as well as peer-to-peer relationships between devices. The network architecture comprises three layers: an loT network <NUM>, a blockchain P2P network <NUM> (i.e. full and lightweight blockchain clients, e.g. the master, servant and slave nodes are lightweight clients operating SPV wallets <NUM>), and a blockchain mining network <NUM> (a subset of the blockchain P2P network that validates, propagates and stores the transactions propagated by the loT nodes). The blockchain network <NUM> acts as backend infrastructure and there is an overlap between the loT network <NUM> and the blockchain P2P network <NUM>.

The first network (e.g. an loT network) comprises one or more bridging nodes and one or more devices which can be controlled by one or more of the bridging nodes. The bridging nodes are also nodes of a blockchain network. That is, they are part of the loT network and the blockchain network in the sense that they can connect both to the loT network (e.g. to communicate with other network nodes and devices) and to the blockchain network (e.g. to transmit transactions to the blockchain and to identify and read from transactions recorded on the blockchain). These nodes act as a gateway or bridge between the first network and the blockchain network. They need not also have the roles of mining nodes, forwarding nodes or storage nodes of the blockchain network, though that is not excluded either. In some examples, one or more of the devices s of the first network may also be a node of the blockchain network.

A requesting node (the first bridging node) generates a blockchain transaction (the first transaction). The transaction is signed by the requesting node and contains an output that includes command data. The command data includes an identifier of the device to be controlled, and a command message specifying the command. For example, the device may be a smart washing machine, and the command message may be an instruction to start a washing cycle. The transaction may be transmitted to the blockchain and/or transmitted off-chain to a second node (e.g. a responding node or an approval node). A responding node is a bridging node that directly controls the end device having the specified identifier. An approval node is a bridging node which has authority to grant approval of the command to control the end device. To continue with the washing machine example, the second node may be a smart home hub which controls the washing machine. The first node may be, for instance, a user device such as a laptop, mobile phone, etc..

The first blockchain transaction may be a partially complete transaction. Partially complete in this context means that additional inputs (e.g. signatures) may need to be added to complete the transaction. In some instances, the partial transaction may be invalid without the additional inputs, meaning that if it was transmitted to the blockchain network it would be rejected by the network.

The responding node obtains the first transaction, e.g. from the blockchain itself (it the first transaction has been transmitted to the blockchain) or directly from the requesting node. The responding node then identifies the device to be controlled using the device identifier in the command data, and the command message specifying the command. The responding node then transmits the command to the identified end device. In other words, the responding node instructs the end device to perform an action based on the command data in the first transaction.

Preferably, the responding node updates the first blockchain transaction by signing the obtained first transaction, and then transmits the updated transaction to the blockchain network. The updated transaction acts as a record of the command request and an acknowledgement of the command being carried out by the end device.

The blockchain transactions are a template for digitally signed messages that can be propagated across a global network without discrimination. Nodes use the blockchain as a data carrier network by encoding commands, status updates and other related actions into an output of a transaction (e.g. an unspendable output). In order to minimize communication latency, transactions encoding command data are sent peer-to-peer (i.e. between nodes directly) before being broadcast to the blockchain network.

Together, the bridging nodes of the first network operate a decentralised loT communication protocol using blockchain transactions. Blockchain protocols allow for high capacity and low fee microtransaction throughput as well as a scalable network infrastructure, thus allowing devices to be connected reliably and at a global scale, while communicating at minimal costs. By combining a multilevel control hierarchy and a blockchain based communication protocol, the request and loT communication protocol provides for: large scale P2P communication using low-fee microtransactions, integration of value transfer and control into one platform, low barriers to entry for loT network devices, secure timestamped storage of loT communication data, and loT metadata accessible for auditing and performance monitoring.

Embodiments of the present disclosure provide for a protocol for nodes of a network (e.g. an loT network) <NUM> to use blockchain transactions Tx to issue command requests, instruct devices based on those command requests and issue command acknowledgements. Whilst embodiments will be described with respect to an loT network <NUM>, in general, the teaching of the present disclosure could be applied to any network comprising nodes which operate a blockchain protocol client application <NUM>, and end devices controllable by at least a subset of those nodes.

A first bridging node <NUM> of the network <NUM> (e.g. master node 503a, or server node 503b) generates a first transaction Tx<NUM> which comprises an input signed by the first node and an output comprising command data. The command data comprises an identifier of an end device <NUM> to be controlled and a command message for controlling the end device <NUM>. The first node may be the originator of the command. That is, the first node may generate the command data.

The first node may transmit the first transaction Tx<NUM> to a second bridging node <NUM> of the first network <NUM> (e.g. slave node 503c) that controls the end device <NUM>. The first transaction Tx<NUM> may be transmitted off-chain, i.e. without being transmitted to the blockchain. For instance, the first transaction Tx<NUM> may be sent directly from the first node to the second node, e.g. over the internet. For example, the first node may be a server node 503b and the second node may be a slave node 503c. Alternatively, the first transaction Tx<NUM> may be sent indirectly, e.g. via one or more intermediary nodes. As an example, the first transaction Tx<NUM> may be sent from a master node 503a to a slave node 503c via a server node 503b. The second node may be connected to the end device <NUM> via a wired or wireless connection, e.g. via an Ethernet or Wi-Fi connection.

Referring to <FIG>, in these examples the first node may be comprised by the computer equipment 102a of Alice 103a and the second node may be comprised by the computer equipment 102b of Bob 103b. As explained previously, Alice and Bob may use a side channel (e.g. side channel <NUM>) to exchange a transaction without the transaction (yet) being published onto the blockchain network <NUM> or making its way onto the chain <NUM>, until one of the parties chooses to broadcast it to the network <NUM>.

The second node may obtain the first transaction Tx<NUM> directly or indirectly from the first node, e.g. the first transaction Tx<NUM> may be forwarded to the second node via one or more intermediary nodes. The second node uses the command data to transmit a control instruction to the end device <NUM> identified by the device identifier ("Device ID") in the command data. The control message in the command data may define a desired action of the end device <NUM>. The control message may be configured to cause the second node to transmit a particular one of several possible instructions to the end device <NUM>.

Alternatively, the second node may be configured to send a single instruction to the end device <NUM>, i.e. the second node only ever sends the same instruction to the end device. This may be the case, for instance, if the end device <NUM> is a simple device like a sensor, and the instruction is a request for a sensor reading. The command (i.e. the instruction for the end device) may be transmitted to the device off-chain over a wired or wireless connection, e.g. using Wi-Fi. Alternatively, if the device is also a node of the network, the command may be transmitted via a blockchain transaction Tx.

In some embodiments, a request and response cycle for device and controller communication may be implemented by the first and second nodes. The request (command) is issued as a partially complete transaction containing an output which comprises the command data (e.g. an OP_RETURN payload). The response (acknowledgment of the command) is the broadcasting of a finalised transaction containing the signature of both the requester and responder nodes. Transaction malleability enables this method of communication as the message receiver can add inputs and outputs whilst unable to alter the command data (e.g. the OP_RETURN payload).

The first transaction Tx<NUM> transmitted from the first node to the second node may be transmitted without a second output. the transaction comprises a single output (the output comprising the command data). In order to complete the partial transaction, the second may update the transaction by adding an input and an output to the first transaction. The input comprises a signature of the second node, i.e. a signature generated using a private key of the second node. The output is an output locked to a public key of the second node, e.g. a P2PKH output. To spend a P2PKH output, an input of the spending transaction must comprise a public key such that the hash (e.g. OP_HASH160) of the public key matches the public key hash in the P2PKH output. A P2PKH output challenges the spender to provide two items: a public key such that the hash of the public key matches the address in the P2PKH output, and a signature that is valid for the public key and the transaction message, not necessarily in that order. The public key may correspond to the private key used to generate the signature. Alternatively, the signature may be linked to a first public key, and the output may be locked to a different public key. The second node may then transmit the completed transaction to the blockchain network <NUM>. The completed transaction (referred to as a command transaction in these embodiments) is available in the blockchain <NUM> for other nodes to view, e.g. the first node, and acts as a record of the command carried out by the device. That is, once a transaction is broadcast, an independent observer can see which public key issued the command/message and which public key responded to it.

<FIG> illustrate an example partial first transaction Tx<NUM> (partial) and an example updated first transaction Tx<NUM> (complete). The partial first transaction comprises a single input 701a and a single output 702a. The updated first transaction includes the input 701b and output 702b added by the second node. A SIGHASH_SINGLE signature type can be used to achieve the desired level of transaction malleability. For example, a node with public key PK<NUM> sends an instruction to a node with public key PK<NUM>. The instruction is encoded in an unspendable output (e.g. an OP_RETURN output) of a transaction signed using SIGHASH_SINGLE signature type (<FIG>). The partially complete transaction is valid. On completion of the instruction, the second node with PK<NUM> adds an output locked to their address. The second node with PK<NUM> then finalises the transaction by signing the entire transaction using SIGHASH_ALL signature type (see <FIG>).

In alternative embodiments, the first transaction Tx<NUM> transmitted from the first node to the second node may be transmitted with a second output. The second output is locked to a public key of the second node. For example, the second output may be a P2PKH to the second node's public key.

In order to complete the first transaction Tx<NUM>, the second node updates the first transaction by adding an input to the first transaction. The first transaction Tx<NUM> now includes two inputs and two outputs. The second input comprises a public key of the second node. The public key in the second input may or may not be the same as the public key to which the second output is locked. Once completed, the updated first transaction (referred to as a command transaction in these embodiments) is sent to the blockchain network <NUM> for inclusion in the blockchain <NUM>. Once a command transaction is broadcast any independent observer can see which public key issued the command/message and which public key responded to it.

The second output locked to the public key of the second node may transfer an amount of the digital asset which is greater than the amount of the digital asset referenced by the first input of the first transaction. In that case, the first transaction Tx<NUM> is a partially complete transaction that would not be deemed valid by other nodes of the blockchain network <NUM>. That is, the first transaction Tx<NUM> would not satisfy the consensus rules followed by the blockchain nodes and thus would not be mined into a block <NUM> of the blockchain <NUM>. When updating the first transaction Tx<NUM>, the second node would have to ensure that the combined amount of digital asset referenced by the first and second inputs is greater than the amount of the digital asset locked to the second output.

<FIG> illustrate an example partial first transaction Tx<NUM> (partial) and an example updated first transaction Tx<NUM> (complete). The first transaction includes the command data in a first output 802a and a second output 802b locked to the public key of the second node. The updated first transaction includes the additional input 801b added by the second node. If the first node with PK<NUM> sends an instruction to the second node with PK<NUM> that they want carried out by the second node with PK<NUM> only, they can send a partially complete transaction which locks both outputs 802a, 802b but does not pay a fee (and therefore will not be mined or propagated). In order to redeem the digital asset locked to PK<NUM>, the second node with PK<NUM> will need to provide an input 801b that pays the fee. To issue a command using a partially complete transaction, the SIGHASH flag for < SigPK<NUM>> is set to SIGHASH_ANYONECANPAY and contains an OP_RETURN output with the command data. This means that, whilst the command data included in the first output 802a is fixed, anyone can add an additional input. The public key that received the command can add an additional input 801b to redeem the funds in the input 801a. To secure the new input 801b and prevent further transaction malleability the receiver of the funds adds a minimal value (dust) input and signs the transaction outputs using SIGHASH_ALL.

Note that a SIGHASH flag is a flag added to signatures in transaction inputs to indicate which part of the transaction the signature signs. The default is SIGHASH_ALL (all parts of the transaction other than the ScriptSig are signed). The unsigned parts of the transaction can be modified.

In more detail, a SIGHASH flag is a single byte appended to the end of a signature i.e. if SIG is the signature found in the transaction then SIG = [DER encoded ECDSA signature] + [<NUM>-byte SIGHASH flag]. The SIGHASH <NUM>-byte is also appended to the transaction before creating the hash value, i.e. ECDSA Sig = [r s] where <MAT> sk and k are private and ephemeral keys, respectively.

An example request and response algorithm is provided below, with reference to <FIG>, <FIG> and <FIG>. A controlling device 503b is configured to communicate with other nodes on the network <NUM> and can calculate the shortest route of communication to any other node on the network. For example, PKserv identifies that PKslave is the nearest controller to a device with device_ID.

Step <NUM>: A controlling device 503b with public key PKserv sends a partial command Tx<NUM> (see <FIG>) to a second controlling device 503c with public key PKslave. The loT message contained in the transaction specifies the command and target device with device_ID.

Step <NUM>: The second controlling device (PKslave) checks that the signature for the transaction is valid and that the message contained within the loT message payload is valid in accordance with the rules of the network <NUM>.

Step <NUM>. The second controlling device (PKslave) sends a command message ("Msg") to the device (device_ID) via off chain communication (e.g. wired connection, Bluetooth, IP-to-IP).

Step <NUM>: Upon completion of the command requested action, the device (device_ID) sends a command completion or acknowledgment message ("ack") back to the second controlling device (PKslave).

Step <NUM>: The second controller (PKslave) adds a second input and signature and finalises the transaction (see <FIG>). This will signal that the second controller confirms the completion of the command.

Step <NUM>: The second controller (PKslave) broadcasts the finalised transaction to the blockchain (mining) network <NUM>.

When the second node (e.g. a slave node) obtains a command transaction, the second node transmits a command (Msg) to the device identified by the device identifier (Device_ID) in the command data. In some examples, the device <NUM> may transit an acknowledgement message (Ack) to the second node to indicate that it has received the command and/or that it has actioned the command. In these examples, the second node may only update the first transaction (and then broadcast the updated transaction) on condition that it has received an acknowledgement from the device. This provides further supporting evidence of the end device has executed a command.

Due to resource constraints that most everyday small electronic devices have, they may not be able to easily monitor the blockchain <NUM> and/or even communicate with loT network components outside of their immediate location, and therefore the control of end devices <NUM> is performed locally (second node to device) and off-chain. Messages to and from end devices may take the form of the raw command data (e.g. OP_RETURN payloads) without the additional transaction metadata. This ensures that the data packets containing the messages remain small and computationally intensive operations (such as elliptic curve mathematics) are not required. In some embodiments, the command data included in the transactions is encrypted.

In summary, nodes <NUM> on the loT network <NUM> communicate directly using transactions containing loT command data, as well as by connecting to the blockchain network <NUM> to broadcast transactions. The blockchain <NUM> is used as a permanent data store for recording commands and status updates from loT network components as well as issuing reports and alerts related to loT devices <NUM>. The protocol may make use of one or more of the following features.

As set out above, Alice 103a and Bob 103b may each perform actions associated with the bridging nodes <NUM> of <FIG>. That is, Alice 103a and Bob <NUM> may each comprise or operate a respective bridging node. Alice may be a master node 503a or an intermediate node 503b, whereas Bob may be an intermediate node 503b, 503c. In the scenario shown in <FIG>, Alice 103a operates a server node 503b and transmits a partial transaction Tx<NUM> to Bob 103b who operates a slave node 503c. The partial transaction Tx<NUM> comprises a command for controlling an end device <NUM>. In a fully trusted system, Bob 103b can assume that Alice 103a is not acting maliciously and therefore accept the partial transaction Tx<NUM>, control the end device <NUM> based on the command, and submit a completed transaction Tx<NUM> to the blockchain network <NUM> in order to store a record of the command on the blockchain <NUM>. However, the system may not be a fully trusted system, or it may be susceptible to faulty nodes.

<FIG> and <FIG> illustrate an example system which has been compromised by a malicious actor <NUM>. In the example of <FIG>, the server node 503b issuing the partial transaction Tx<NUM> to the slave node 503c is controlled by the malicious actor <NUM>. The malicious actor <NUM> commands the server node 503b to issue the command transaction Tx<NUM> to the slave node 503c and at the same time submits the same command transaction Txi to the blockchain network <NUM>. That is, the malicious actor <NUM> issues a double spend transaction. Not knowing that the command transaction Tx<NUM> is a double spend, the slave node <NUM> instructs the end device <NUM> to carry out the command. The transaction Tx<NUM>' will reach the blockchain network <NUM> before the slave node 503c has a chance to send the complete transaction Tx<NUM> to the blockchain network <NUM>, as shown in <FIG>. In the example of <FIG>, the end device <NUM> returns an acknowledgement message to the slave node 503c. In response, the slave node 503c completes the command transaction Tx<NUM> and submits it to the blockchain network <NUM>. However, the command transaction Tx<NUM>' issued by the server node 502b controlled by the malicious actor <NUM> has already been propagated to other nodes <NUM> of the blockchain network <NUM>. Therefore, when these nodes <NUM> receive the command transaction Tx<NUM> from the slave node 503c, they will reject it as it will be a double spend attempt. This means there will not be any record on the blockchain of the command being carried out.

In order to mitigate the risk of double spend transactions, Bob 103b (e.g. the slave node 503c) requires Alice 130a (e.g. the server node 503b) to send a signature of the command transaction and one or more time indicator(s). In some examples, a condition for Bob 103b instructing an end device <NUM> under his control to carry out a command is that the Alice 103a has sent the signature and the time indicator(s) with the command transaction. If Alice 103a does not send the signature and the time indicator(s), Bob 103b may reject the command transaction and not issue the command to the end device <NUM>. As another example, Bob 103b may only update the command transaction, e.g. complete the command transaction if Alice 103a has sent the signature and the time indicator(s) with the (partial) command transaction. If Alice 103a does not send the signature and the time indicator(s), Bob 103b may reject the command transaction and not update the command transaction. In this manner, a malicious actor <NUM> cannot cause Bob 103b to control end devices to carry out commands without any record of the commands being recorded on the blockchain <NUM>.

In some examples, Bob 103b may only update the partial transaction and submit it to the blockchain network <NUM> once the command instruction (or message) has been transmitted to the end device. In other examples, Bob 103b may only update the partial transaction and submit it to the blockchain network <NUM> once he receives an acknowledgement from the end device that the instruction has been performed.

In summary, while <NUM>-conf double spend attempts cannot be fully prevented, the present invention proves the order of transaction issuance in order to mitigate the risk of a double spend attempt. As a particular example, assuming that Node<NUM> wishes to issue a command by propagating a transaction Tx<NUM> to Node<NUM>, the partially complete transaction Tx<NUM> is sent from Node<NUM> to Node<NUM> along with two other pieces of data: a signature and a timestamp <MAT> Where TxID<NUM> = H<NUM>(Tx<NUM>) (transaction ID), H(·) is a hash function, and t is the UNIX time. More specifically, H<NUM>(Tx<NUM>) = SHA<NUM>(SHA<NUM>(Tx<NUM>)). This additional signature and timestamp can be stored and later used as proof that Node<NUM> sent Tx<NUM> to Node<NUM> at approximately time t. In the event of a double spend, this additional data can be used to penalise Node<NUM> or at least assess network performance issues should the double spend be executed by a non-malicious node. Note that in the context of the loT network <NUM>, P2P replay attacks are equivalent to a double spend attack in the blockchain network <NUM>. The inclusion of a timestamp in command transactions prevents playback of authentic data since a large discrepancy in the time of data transmission would be easily detected by the recipient node. In some examples, all devices on a network (e.g. bridging nodes and end devices of an loT network) are configured to keep a local timestamped list of TxIDs from when they are first seen on that network.

Transmitting via an off-chain communication channel means transmitting via a channel other than the blockchain network. For instance, transmitting the blockchain transaction via an off-chain communication channel means transmitting the transaction without sending it to nodes of the blockchain network for inclusion in the blockchain. As an example, the transaction may be sent via TLS communication, email, a messaging application, Wi-Fi, Bluetooth, NFC, etc..

The blockchain may comprise the previous blockchain transaction. Alternatively, the previous blockchain transaction may be a transaction in a respective memory pool (mempool) of the one or more nodes of the blockchain network.

The off-chain communication channel used to transmit the signature and/or the at least one time indicator may be the same communication channel or different communication channels.

In embodiments, the signature may be generated using a private key corresponding to a public key associated with the first party.

In embodiments, the first blockchain transaction may be a partial blockchain transaction that requires one or more inputs and/or one or more inputs to be added to the partial blockchain transaction in order for the partial blockchain transaction to be accepted by one or more nodes of the blockchain network.

Note that a partial blockchain transaction may also be referred to as a partially complete blockchain transaction.

In embodiments, the second party may be configured to control one or more devices, wherein the first blockchain transaction comprises an output comprising command data, and wherein the command data comprises a command message for causing the second party to control at least one of the one or more devices.

The command message may be configured to cause the second party to transmit a control instruction to the at least one of the one or more device. The command message may be encrypted.

In embodiments, the first party and second party may be respective nodes of a first network, the first network being a peer-to-peer network.

In embodiments, the first network and the blockchain network may be different networks.

In embodiments, the first and second parties may be respective nodes of the first network and the blockchain network.

In embodiments, the first network may comprise the one or more devices, each device having a respective device identifier, and wherein the command data comprises a respective identifier of the at least the one of the one or more devices.

In embodiments, the first network may comprise a master layer comprising a master node, one or more intermediary layers each comprising a plurality of intermediate nodes, and a device layer comprising the one or more devices; wherein the first party is the master node or a respective one of the plurality of intermediate nodes, and wherein the second party is a respective one of the plurality of intermediate nodes.

In embodiments, the one or more devices may be Internet-of-Things devices.

Note that the off-chain communication channel may be a transport layer security (TLS) communication channel. Other options include Bluetooth or other near-field communication methods.

The threshold time period may be any suitable time period (e.g. one second, ten seconds, one hour, etc.) and may depend on the particular use case. The second party may set the threshold, or the threshold may be set by a protocol adhered to by the first and second parties.

The control instruction may be transmitted to the at least one of the one or more devices via an off-chain communication channel, which may or may not be the same off-chain communication channel via which the first blockchain transaction is transmitted from the first party to the second party.

Claim 1:
A computer-implemented method for enabling a second party to determine whether to accept a blockchain transaction from a first party, the method being performed by a first party (103a) and comprising:
generating a first blockchain transaction, wherein the first blockchain transaction comprises an input for unlocking an output of a blockchain transaction previously transmitted to one or more nodes (<NUM>) of a blockchain network (<NUM>) for inclusion in the blockchain (<NUM>);
generating a signature based on i) the first blockchain transaction and ii) one or more time indicators, each time indicator indicating when the first blockchain transaction was generated and/or transmitted to a second party (103b); and
transmitting to the second party, the first blockchain transaction, the signature and the one or more time indicators, wherein at least the first blockchain transaction is transmitted to the second party via an off-chain communication channel, and wherein the second party (103b) is configured to accept the first blockchain transaction based on one or more conditions, a first one of the one or more conditions being that the first party (103a) has transmitted the signature and the one or more first time indicators.