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 distributed peer-to-peer (P2P) network (referred to below as a "blockchain network") and widely publicised. The blockchain comprises a chain of blocks of data, wherein each block comprises one or more transactions. Each transaction, other than so-called "coinbase transactions", points back to a preceding transaction in a sequence which may span one or more blocks going back to one or more coinbase transactions. Coinbase transactions are discussed further below. Transactions that are submitted to the blockchain network are included in new blocks. New blocks are created by a process often referred to as "mining", which involves each of a plurality of the nodes competing to perform "proof-of-work", i.e. solving a cryptographic puzzle based on a representation of a defined set of ordered and validated pending transactions waiting to be included in a new block of the blockchain. It should be noted that the blockchain may be pruned at some nodes, and the publication of blocks can be achieved through the publication of mere block headers.

The transactions in the blockchain may be used to for one or more of the following purposes: to convey a digital asset (i.e. a number of digital tokens), to order a set of entries in a virtualised ledger or registry, to receive and process timestamp entries, and/or to time-order index pointers. A blockchain can also be exploited in order to layer additional functionality on top of the blockchain. For example blockchain protocols may allow for storage of additional user data or indexes to data in a transaction. There is no pre-specified limit to the maximum data capacity that can be stored within a single transaction, and therefore increasingly more complex data can be incorporated. For instance this may be used to store an electronic document in the blockchain, or audio or video data.

Nodes of the blockchain network (which are often referred to as "miners") perform a distributed transaction registration and verification process, which will be described in more detail later. In summary, during this process a node validates transactions and inserts them into a block template for which they attempt to identify a valid proof-of-work solution. Once a valid solution is found, a new block is propagated to other nodes of the network, thus enabling each node to record the new block on the blockchain. In order to have a transaction recorded in the blockchain, a user (e.g. a blockchain client application) sends the transaction to one of the nodes of the network to be propagated. Nodes which receive the transaction may race to find a proof-of-work solution incorporating the validated transaction into a new block. Each node is configured to enforce the same node protocol, which will include one or more conditions for a transaction to be valid. Invalid transactions will not be propagated nor incorporated into blocks. Assuming the transaction is validated and thereby accepted onto the blockchain, then the transaction (including any user data) will thus remain registered and indexed at each of the nodes in the blockchain network as an immutable public record.

The node who successfully solved the proof-of-work puzzle to create the latest block is typically rewarded with a new transaction called the "coinbase transaction" which distributes an amount of the digital asset, i.e. a number of tokens. The detection and rejection of invalid transactions is enforced by the actions of competing nodes who act as agents of the network and are incentivised to report and block malfeasance. The widespread publication of information allows users to continuously audit the performance of nodes. The publication of the mere block headers allows participants to ensure the ongoing integrity of the blockchain.

In an "output-based" model (sometimes referred to as a UTXO-based model), the data structure of a given transaction comprises one or more inputs and one or more outputs. Any spendable output comprises an element specifying an amount of the digital asset that is derivable from the proceeding sequence of transactions. The spendable output is sometimes referred to as a UTXO ("unspent transaction output"). The output may further comprise a locking script specifying a condition for the future redemption of the output. A locking script is a predicate defining the conditions necessary to validate and transfer digital tokens or assets. Each input of a transaction (other than a coinbase transaction) comprises a pointer (i.e. a reference) to such an output in a preceding transaction, and may further comprise an unlocking script for unlocking the locking script of the pointed-to output. So consider a pair of transactions, call them a first and a second transaction (or "target" transaction). The first transaction comprises at least one output specifying an amount of the digital asset, and comprising a locking script defining one or more conditions of unlocking the output. The second, target transaction comprises at least one input, comprising a pointer to the output of the first transaction, and an unlocking script for unlocking the output of the first transaction.

In such a model, when the second, target transaction is sent to the blockchain network to be propagated and recorded in the blockchain, one of the criteria for validity applied at each node will be that the unlocking script meets all of the one or more conditions defined in the locking script of the first transaction. Another will be that the output of the first transaction has not already been redeemed by another, earlier valid transaction. Any node that finds the target transaction invalid according to any of these conditions will not propagate it (as a valid transaction, but possibly to register an invalid transaction) nor include it in a new block to be recorded in the blockchain.

An alternative type of transaction model is an account-based model. In this 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 nodes separate to the blockchain and is updated constantly.

<CIT> discloses techniques for accessing and storing blockchain data. A client computer system submits a blockchain-agnostic request to a computing resource service provider and the service provider processes the request.

<CIT> discloses a distributed ledger and transaction computing network fabric over which large numbers of transactions are processed concurrently.

<CIT> discloses a web session-based blockchain-linked service which provides a variety of public blockchain-based services to a user to be used on a web session through a service providing system having a web service providing server and a blockchain node server integrated linked thereto.

<CIT> discloses a system of enabling a user to transact using cryptocurrency of a value defined in relation to a different medium of exchange or financial instrument.

Users (and other types of parties, e.g. organisations, autonomous entities, etc.) typically connect directly with one or more blockchain nodes of the blockchain network. That is, a user operating a client application can connect with a blockchain node in order to e.g. submit a transaction to the network, obtain a transaction from the blockchain, query for the existence of specific unspent transaction outputs (UTXOs) etc. In order to carry out these operations a reliable connection between the user and the node(s) is required. It would therefore be desirable to boost connectivity between users and nodes in order to ensure a reliable connection to the blockchain network is maintained.

For instance, consider a scenario in which a user (say Alice) operating a lightweight client application attempts to submit a transaction to the blockchain network, or query the existence of one or more UTXOs that another user (say Bob) is attempting to assign to Alice, e.g. in return for goods or services. If Alice cannot connect to the blockchain network she is unable to send the transaction, or she may be unable to verify that Bob is not attempting to double-spend UTXOs that have already been referenced by an already validated transaction. Of course, this is case-dependent as there are other ways to verify UTXOs depending on whether additional information is available (e.g. a Merkle path of the UTXO). Alice may be unable to connect to a blockchain node due to an issue with the connection to the node itself, or because the node to which Alice ordinarily connects is no longer acting as a node on the network.

According to one aspect disclosed herein, there is provided a computer-implemented method of transmitting blockchain transactions to a blockchain network, wherein the method is performed by a first party and comprises: transmitting at least part of a blockchain transaction to an internet server via an internet service hosted by the internet server, wherein the internet server is configured to connect to one or more nodes of the blockchain network, and to transmit a blockchain transaction to the one or more blockchain nodes, wherein the transmitted blockchain transaction comprises the at least part of the blockchain transaction; and receiving, from the internet server via the internet service, a list of contacts, which are other parties associated with the first party, wherein for each contact in the list, the list comprises a respective one or more blockchain addresses associated with that contact. According to another aspect disclosed herein, there is provided a computer-implemented method of transmitting blockchain transactions to a blockchain network, wherein the method is performed by an internet server configured to connect to one or more blockchain nodes and comprises: receiving, from a first party via an internet service hosted by the internet server, at least part of a blockchain transaction; transmitting a blockchain transaction to the one or more blockchain nodes, wherein the blockchain transaction comprises the at least part of the blockchain transaction; maintaining a list of, which are other parties associated with the first party, wherein the list of contacts comprises, for each contact, one or more blockchain addresses associated with that contact; and transmitting the list of contacts to the first party.

The first party (e.g. Alice) forms a connection to an internet server (e.g. a web server or a mail server) that is connected to one or more blockchain nodes. Alice therefore does not need to be connected directly to the blockchain network, and instead can exploit the internet server which may have a permanent connection to one or more nodes. The internet server acts as a gateway between Alice and the blockchain network. Rather than Alice having to rely on her connection(s) to the network, Alice can utilise the internet server which may maintain many respective connections to different nodes. If the server is experiencing an issue with one or more particular connections, or one or more of the nodes have dropped off the blockchain network, the server can instead route Alice's transaction to the network via one or more different nodes.

In some instances, the internet server may maintain a permanent connection to the blockchain network via at least one node, e.g. by maintaining a connection to the same node over a given period, or by maintaining connections to different nodes at different points over that period.

In some embodiments, the internet service may be a web page such as a social media site or the like. The web page may have an embedded function configured to receive or generate one or more components of a transaction, e.g. a signature generated using Alice's private key. The transaction component(s) or a complete transaction are then sent to the server hosting the web page, which then forwards the transaction to the blockchain network. Depending on the particular set up, the server may generate the transaction itself, e.g. by filling in a transaction template with the components supplied to or generated by the web page.

In other embodiments, the internet service may be a mail service, i.e. an email application. The email application may also have an embedded function that allows users to interact with the blockchain network from within the application. As detailed below, the mail server hosting the email application may maintain a list of peers (contacts) linked with or otherwise known to Alice that also operate a blockchain client application. A respective public key or blockchain address (e.g. a public key hash) may be stored in association with Alice's contacts. This may be used to indicate to Alice which of her contacts she can assign her digital assets to, and to which blockchain address of her contacts to use in order to do so. Note that these examples apply similarly to the web page implementations mentioned above.

<FIG> shows an example system <NUM> for implementing a blockchain <NUM>. The system <NUM> may comprise of a packet-switched network <NUM>, typically a wide-area internetwork such as the Internet. The packet-switched network <NUM> comprises a plurality of blockchain nodes <NUM> that may be arranged to form a peer-to-peer (P2P) network <NUM> within the packet-switched network <NUM>. Whilst not illustrated, the blockchain nodes <NUM> may be arranged as a near-complete graph. Each blockchain node <NUM> is therefore highly connected to other blockchain nodes <NUM>.

Each blockchain node <NUM> comprises computer equipment of a peer, with different ones of the nodes <NUM> belonging to different peers. Each blockchain 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), and other equipment such as application specific integrated circuits (ASICs). 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 blockchain nodes <NUM> in the distributed or blockchain network <NUM>. As mentioned above, maintaining a copy of the blockchain <NUM> does not necessarily mean storing the blockchain <NUM> in full. Instead, the blockchain <NUM> may be pruned of data so long as each blockchain node <NUM> stores the block header (discussed below) of each block <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 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 as property, an example of which is a user <NUM> to whom the output is cryptographically locked (requiring a signature or other solution 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.

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>. Each transaction <NUM> (other than a coinbase 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.

Each of the blockchain nodes <NUM> is configured to forward transactions <NUM> to other blockchain nodes <NUM>, and thereby cause transactions <NUM> to be propagated throughout the network <NUM>. Each blockchain node <NUM> is configured to create blocks <NUM> and to store a respective copy of the same blockchain <NUM> in their respective memory. Each blockchain node <NUM> also maintains an ordered set (or "pool") <NUM> of transactions <NUM> waiting to be incorporated into blocks <NUM>. The ordered pool <NUM> is often referred to as a "mempool". This term herein is not intended to limit to any particular blockchain, protocol or model. It refers to the ordered set of transactions which a node <NUM> has accepted as valid and for which the node <NUM> is obliged not to accept any other transactions attempting to spend the same output.

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 ordered set <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 input authorisation, for example 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 or entity 103b. The present transaction 152j can thus transfer the amount defined in the input of the preceding transaction 152i to the new user or entity 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 or entities (one of whom could be the original user or entity 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.

According to an output-based transaction protocol such as bitcoin, when a party <NUM>, such as an individual user or an organization, wishes to enact a new transaction 152j (either manually or by an automated process employed by the party), then the enacting party sends the new transaction from its computer terminal <NUM> to a recipient. The enacting party or the recipient will eventually send this transaction to one or more of the blockchain nodes <NUM> of the network <NUM> (which nowadays are typically servers or data centres, but could in principle be other user terminals). It is also not excluded that the party <NUM> enacting the new transaction 152j could send the transaction directly to one or more of the blockchain nodes <NUM> and, in some examples, not to the recipient. A blockchain node <NUM> that receives a transaction checks whether the transaction is valid according to a blockchain node protocol which is applied at each of the blockchain nodes <NUM>. The blockchain node protocol typically requires the blockchain node <NUM> to check that a 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 such an output-based transaction protocol, this may comprise checking that the cryptographic signature or other authorisation of the party <NUM> 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 assigns, wherein this condition typically comprises at least checking that the cryptographic signature or other authorisation in the input of the new transaction 152j unlocks the output of the previous transaction 152i to which the input of the new transaction is linked to. The condition may be at least partially defined by a script included in the output of the preceding transaction 152i. Alternatively it could simply be fixed by the blockchain node protocol alone, or it could be due to a combination of these. Either way, if the new transaction 152j is valid, the blockchain node <NUM> forwards it to one or more other blockchain nodes <NUM> in the blockchain network <NUM>. These other blockchain nodes <NUM> apply the same test according to the same blockchain 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 blockchain nodes <NUM>.

In an output-based model, the definition of whether a given output (e.g. UTXO) is assigned (e.g. spent) is whether it has yet been validly redeemed by the input of another, onward transaction 152j according to the blockchain node protocol. Another condition for a transaction to be valid is that the output of the preceding transaction 152i which it attempts to redeem has not already been redeemed by another transaction. Again if not valid, the transaction 152j will not be propagated (unless flagged as invalid and propagated for alerting) or recorded in the blockchain <NUM>. This guards against double-spending whereby the transactor tries to assign 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 validating transactions, blockchain nodes <NUM> also race to be the first to create blocks of transactions in a process commonly referred to as mining, which is supported by "proof-of-work". At a blockchain node <NUM>, new transactions are added to an ordered pool <NUM> of valid transactions that have not yet appeared in a block <NUM> recorded on the blockchain <NUM>. The blockchain nodes then race to assemble a new valid block <NUM> of transactions <NUM> from the ordered set 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 a representation of the ordered pool of pending 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. Note that this is just one particular type of proof-of-work puzzle, and other types are not excluded. 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 blockchain node <NUM> that is trying to solve the puzzle.

The first blockchain 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 blockchain 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 first blockchain node <NUM> propagates a block to a threshold consensus of other nodes that accept the block and thus enforce the protocol rules. The ordered set of transactions <NUM> then becomes recorded as a new block <NUM> in the blockchain <NUM> by each of the blockchain nodes <NUM>. 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 significant amount of effort, for example in the form of hash, required to create a proof-of-work solution signals the intent of the first node <NUM> to follow the rules of the blockchain protocol. Such rules include not accepting a transaction as valid if it assigns the same output as a previously validated transaction, otherwise known as double-spending. Once created, the block <NUM> cannot be modified since it is recognized and maintained at each of the blockchain nodes <NUM> in the blockchain network <NUM>. 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 blockchain node <NUM> in a network <NUM>, this therefore provides an immutable public ledger of the transactions.

Note that different blockchain nodes <NUM> racing to solve the puzzle at any given time may be doing so based on different snapshots of the pool of yet-to-be published transactions <NUM> at any given time, depending on when they started searching for a solution or the order in which the transactions were received. Whoever solves their respective puzzle first defines which transactions <NUM> are included in the next new block 151n and in which order, and the current pool <NUM> of unpublished transactions is updated. The blockchain nodes <NUM> then continue to race to create a block from the newly-defined ordered pool of unpublished transactions <NUM>, and so forth. A protocol also exists for resolving any "fork" that may arise, which is where two blockchain nodes104 solve their puzzle within a very short time of one another such that a conflicting view of the blockchain gets propagated between nodes <NUM>. In short, whichever prong of the fork grows the longest becomes the definitive blockchain <NUM>. Note this should not affect the users or agents of the network as the same transactions will appear in both forks.

According to the bitcoin blockchain (and most other blockchains) a node that successfully constructs a new block <NUM> is granted the ability to newly assign an additional, accepted amount of the digital asset in a new special kind of transaction which distributes an additional defined quantity of the digital asset (as opposed to an inter-agent, or inter-user transaction which transfers an amount of the digital asset from one agent or user to another). This special type of transaction is usually referred to as a "coinbase transaction", but may also be termed an "initiation transaction" or "generation transaction". It typically forms the first transaction of the new block 151n. The proof-of-work signals the intent of the node that constructs the new block to follow the protocol rules allowing this special transaction to be redeemed later. The blockchain protocol rules may require a maturity period, for example <NUM> blocks, before this special transaction may be redeemed. Often a regular (non-generation) transaction <NUM> will also specify an additional transaction fee in one of its outputs, to further reward the blockchain node <NUM> that created the block 151n in which that transaction was published. This fee is normally referred to as the "mining fee", and is discussed blow.

Due to the resources involved in transaction validation and publication, typically at least each of the blockchain nodes <NUM> takes the form of a server comprising one or more physical server units, or even whole a data centre. However in principle any given blockchain node <NUM> could take the form of a user terminal or a group of user terminals networked together.

The memory of each blockchain node <NUM> stores software configured to run on the processing apparatus of the blockchain node <NUM> in order to perform its respective role or roles and handle transactions <NUM> in accordance with the blockchain node protocol. It will be understood that any action attributed herein to a blockchain node <NUM> may be performed by the software run on the processing apparatus of the respective computer equipment. The node software may be implemented in one or more applications at the application layer, or a lower layer such as the operating system layer or a protocol layer, or any combination of these.

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 users may interact with the blockchain network <NUM> but do not participate in validating transactions or constructing blocks. Some of these users or agents <NUM> may act as senders and recipients in transactions. Other users may interact with the blockchain <NUM> without necessarily acting as senders or recipients. For instance, some parties may act as storage entities that store a copy of the blockchain <NUM> (e.g. having obtained a copy of the blockchain from a blockchain node <NUM>).

Some or all of the parties <NUM> may be connected as part of a different network, e.g. a network overlaid on top of the blockchain network <NUM>. Users of the blockchain network (often referred to as "clients") may be said to be part of a system that includes the blockchain network <NUM>; however, these users are not blockchain nodes <NUM> as they do not perform the roles required of the blockchain nodes. Instead, each party <NUM> may interact with the blockchain network <NUM> and thereby utilize the blockchain <NUM> by connecting to (i.e. communicating with) a blockchain node <NUM>. 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 <NUM>, 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 <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 party <NUM> to create, authorise (for example sign) and send transactions <NUM> to one or more bitcoin nodes <NUM> to then be propagated throughout the network of blockchain 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.

Note: whilst the various client functionality may be described as being integrated into a given client application <NUM>, this is not necessarily limiting and instead any client functionality described herein may instead be implemented in a suite of two or more distinct applications, e.g. interfacing via an API, or one being a plug-in to the other. More generally the client functionality could be implemented at the application layer or a lower layer such as the operating system, or any combination of these. The following will be described in terms of a client application <NUM> but it will be appreciated that this is not limiting.

The instance of the client application or software <NUM> on each computer equipment <NUM> is operatively coupled to at least one of the blockchain nodes <NUM> of the 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 blockchain 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. As set out above, each blockchain node <NUM> runs software configured to validate transactions <NUM> according to the blockchain node protocol, and to forward transactions <NUM> in order to propagate them throughout the blockchain network <NUM>. The transaction protocol and the 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>. The same node protocol is used by all the nodes <NUM> in the network <NUM>.

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 or more blockchain nodes <NUM> to which she is connected. this could be the blockchain node <NUM> that is best connected to Alice's computer <NUM>. When any given blockchain node <NUM> receives a new transaction 152j, it handles it in accordance with the blockchain 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 blockchain node <NUM> that receives the transaction 152j will add the new validated transaction <NUM> to the ordered set of transactions <NUM> maintained at that blockchain node <NUM>. Further, any blockchain node <NUM> that receives the transaction 152j will propagate the validated transaction <NUM> onward to one or more other blockchain nodes <NUM> in the network <NUM>. Since each blockchain node <NUM> applies the same protocol, then assuming the transaction 152j is valid, this means it will soon be propagated throughout the whole network <NUM>.

Once admitted to the ordered pool of pending transactions <NUM> maintained at a given blockchain node <NUM>, that blockchain node <NUM> will start competing to solve the proof-of-work puzzle on the latest version of their respective pool of <NUM> including the new transaction <NUM> (recall that other blockchain nodes <NUM> may be trying to solve the puzzle based on a different pool of transactions154, but whoever gets there first will define the set of transactions that are included in the latest block <NUM>. Eventually a blockchain node <NUM> will solve the puzzle for a part of the ordered 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.

Different blockchain nodes <NUM> may receive different instances of a given transaction first and therefore have conflicting views of which instance is 'valid' before one instance is published in a new block <NUM>, at which point all blockchain nodes <NUM> agree that the published instance is the only valid instance. If a blockchain node <NUM> accepts one instance as valid, and then discovers that a second instance has been recorded in the blockchain <NUM> then that blockchain node <NUM> must accept this and will discard (i.e. treat as invalid) the instance which it had initially accepted (i.e. the one that has not been published in a block <NUM>).

An alternative type of transaction protocol operated by some blockchain networks 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 nodes of that network, 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.

<FIG> illustrates an example transaction protocol. This is an example of a 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 is not limiting to all possible embodiments. Note that while the example UTXO-based protocol is described with reference to bitcoin, it may equally be implemented on other example blockchain networks.

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 nodes <NUM>.

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>. Txoand Tx<NUM> are just 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 a block <NUM> of 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 ordered set <NUM> in which case it will soon be included in a new block <NUM>. Alternatively Tx<NUM> and Tx<NUM> could be created and sent to the network <NUM> together, or Tx<NUM> 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 blockchain 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 blockchain 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 node 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) which is used by the blockchain network. 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 Tx<NUM> comprises 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 a representation (i.e. a hash) of 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). The data (or "message") that 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 blockchain 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:
<Sig PA> <PA> || [Checksig PA] where "| |" represents a concatenation and "<. >" means place the data on the stack, and "[. ]" is a function comprised by the locking script (in this example a stack-based language). Equivalently the scripts may be run one after the other, 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 unlocking 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 order to perform this authentication. In embodiments the signed data comprises the whole of Tx<NUM> (so a separate element does not 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 using her private key, then given Alice's public key and the message in the clear, another entity such as a node <NUM> is able to authenticate that the message must have been signed by Alice. Signing typically comprises hashing the message, signing the hash, and tagging this onto the message as a signature, thus enabling any holder of the public key to authenticate the signature. Note therefore that any reference herein to signing a particular piece of data or part of a transaction, or such like, can in embodiments mean signing a hash of that piece of data or part of the transaction.

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 blockchain node <NUM> deems Tx<NUM> valid. This means that the blockchain node <NUM> will add Tx<NUM> to the ordered pool of pending transactions <NUM>. The blockchain node <NUM> will also forward the transaction Tx<NUM> to one or more other blockchain nodes <NUM> in the network <NUM>, so that it will be propagated throughout the network <NUM>. 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 blockchain node <NUM> also needs to check whether the referenced UTXO in the preceding transaction Tx<NUM> is already spent (i.e. whether it 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 blockchain 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>.

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 included in a block <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 bitcoin node <NUM> that successfully includes her transaction <NUM> in a block <NUM>. If Alice does not include such a fee, Tx<NUM> may be rejected by the blockchain nodes <NUM>, and hence although technically valid, may not be propagated and included in the blockchain <NUM> (the node protocol does not force blockchain nodes <NUM> to accept transactions <NUM> if they don't want). In some protocols, the transaction fee does not require its own separate output <NUM> (i.e. does not need a separate UTXO). Instead any difference 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 blockchain node <NUM> publishing the transaction. 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 may be assigned by the node <NUM> that wins the proof-of-work race to create the block containing UTXO<NUM>. Alternatively or additionally however, it is not necessarily excluded that a transaction fee could be specified explicitly in its own one of the UTXOs <NUM> of the transaction <NUM>.

Alice and Bob's digital assets consist of the 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 bitcoin nodes <NUM>.

Note that the script code is often represented schematically (i.e. not using the exact language). For example, one may use operation codes (opcodes) to represent a particular function. " refers to a particular opcode of the Script language. As an example, OP_RETURN is an opcode of the Script language that when preceded by OP_FALSE at the beginning of a locking script creates an unspendable output of a transaction that can store data within the transaction, and thereby record the data immutably in the blockchain <NUM>. the data could comprise a document which it is desired to store in the blockchain.

Typically an input of a transaction contains a digital signature corresponding to a public key PA. In embodiments this is based on the ECDSA using the elliptic curve secp256k1. A digital signature signs a particular piece of data. In some embodiments, for a given transaction the signature will sign part of the transaction input, and some or all of the transaction outputs. The particular parts of the outputs it signs depends on the SIGHASH flag. The SIGHASH flag is usually 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 typically 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 typically 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.

As shown in <FIG>, the client application on each of Alice and Bob's computer equipment 102a, 120b, respectively, may comprise additional communication functionality. This additional functionality 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 blockchain network. Such communication is sometimes referred to as "off-chain" communication. For instance this may be used to exchange a transaction <NUM> between Alice and Bob without the transaction (yet) being registered 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>. Sharing a transaction in this way is sometimes referred to as sharing a "transaction template". A transaction template may lack one or more inputs and/or outputs that are required in order to form a complete transaction. 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 blockchain 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 102a, 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 blockchain 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 (UI) 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 one or more nodes <NUM> to be propagated through the blockchain 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 (UI) <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 (e.g. Alice) to submit data to an internet server, e.g. some or all of a blockchain transaction, or to generate a signature.

Alternatively or additionally, the UI elements may comprise one or more data entry fields <NUM>, through which the user can enter data such as a signature or blockchain address. 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 UI <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.

<FIG> illustrates an example system <NUM> for implementing embodiments of the present invention. The system comprises an internet server <NUM>, a first party 402a and the blockchain network <NUM>, or at least one or more nodes <NUM> of the blockchain network <NUM>. The system may also include a second party 402b. Each of the first and second parties 402a, 402b may take the role of Alice 103a or Bob 103b as described with reference to <FIG>. That is, the first party 402a may comprise respective computing equipment 102a and operate a client application 105a configured to perform some or all of the actions associated with Alice 103a. Similarly, the second party 402b may comprise respective computing equipment 102b and operate a client application 105b configured to perform some or all of the actions associated with Bob 103b. Note that the first party 402a may perform some of the actions associated with Bob 103b, and vice versa.

The internet server <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), and other equipment such as application specific integrated circuits (ASICs). The internet server <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. The memory on the internet server <NUM> may store software arranged to run on the processing apparatus. The software may be initially provided to the internet server <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..

Internet servers per se are well known and so their generic functions will not be described here in detail. In general, the internet server <NUM> may take any of the following forms: a database server, a mail server, a web server, a game server, or an application server. The internet server <NUM> may instead take a different form. A database server is a server which uses a database application that provides database services to other computer programs or to computers. A web server is a server that is configured to satisfy client requests on the World Wide Web. A web server typically hosts one or more websites and processes incoming network requests over HTTP and several other related protocols. An application server typically provides both the facilities to create web applications and a server environment to run them. A game server (also sometimes referred to as a host) is a server which is the authoritative source of events in a multiplayer video game. The server transmits enough data about its internal state to allow its connected clients to maintain their own accurate version of the game world for display to players. They also receive and process each player's input.

The internet server <NUM> is configured to host (i.e. serve) an internet service to its connected clients. The type of internet service hosted by the internet server <NUM> will depend on the form of the internet server <NUM>. If the internet server <NUM> is a web server, the internet service will be a website, whereas if the internet server <NUM> is a mail server, the internet server will be a mailbox. Similarly, a game server will host an internet service in the form of an online video game, a database server will host an internet service in the form of an online database and an application server will host an internet service in the form of an application run using the internet.

To submit a blockchain transaction to the blockchain network, instead of connecting directly to one or more blockchain nodes <NUM>, the first party 402a (i.e. the client application 105a operated by the first party 402a) is configured to connect to an internet server <NUM> via an internet service hosted by the internet server <NUM>. The first party 402a provides the internet server, again via the internet service, with one or more components of a blockchain transaction. That is, the first party 402a transmits at least part of a blockchain transaction to the internet server <NUM> via the internet service. In some examples, the first party 402a provides a complete blockchain transaction. In other examples, the first party 402a transmits one or more inputs and/or one or more outputs of a blockchain transaction for completing a transaction template. Note that transmitting an input or output is taken to mean transmitting input data (e.g. an unlocking script or part of an unlocking script) or output data (e.g. a locking script or part of a locking script). For instance, the first party 402a may transmit a signature, generated using a private key owned by the first party 402a, to the internet server <NUM>. As another example, the first party 402a may supply a public key or public key hash, which may be linked with the first party 402a or the second party 402b.

The internet service hosted by the internet server <NUM> may comprise one or more data entry fields (i.e. input fields) for entering data. The first party 402a may enter transaction data, e.g. signatures, public key, transaction identifiers, transaction outpoints, etc., using the data entry fields. Similarly, the internet service may comprise one or more functions for generating transaction data, e.g. a signature or public key hash.

Note that preferably the first party 402a has a secure communication channel with the internet server <NUM>, and/or the first party 402a uses one or more cryptographic techniques to preserve security and/or privacy. These are discussed below.

The internet server <NUM> is configured to obtain the one or more transaction components and to transmit a transaction comprising those components to one or more blockchain nodes <NUM>. If the first party 402a has supplied a complete transaction, the internet server <NUM> forwards the transaction to the blockchain nodes <NUM>. Alternatively, the internet server <NUM> may generate a transaction based on the transaction component(s), e.g. by entering the component(s), i.e. data, into a transaction template.

<FIG> illustrates an example flow of a transaction from the first party 402a to the blockchain network via the internet server <NUM>. As shown, in reality there may be many different parties <NUM> (e.g. users) which are served by multiple internet servers <NUM>. Any given internet server <NUM> may serve a single party <NUM> or multiple parties <NUM>. Similarly, an internet server <NUM> may be connected to only one blockchain node <NUM> or to multiple blockchain nodes <NUM>.

The internet server <NUM> may maintain a permanent connection to the blockchain network <NUM> via one or more respective connections to respective blockchain nodes <NUM>. Alternatively, the internet server <NUM> may periodically connect to one or more blockchain nodes <NUM>, or the internet server <NUM> may connect on an ad hoc basis.

The internet server <NUM> may maintain a connection to a minimum number of blockchain nodes. For instance, if the number of connections drops below a predefined minimum, the internet server <NUM> may connect to one or more additional nodes to take the number of connections back to at least the predefined minimum.

In contrast, the first party 402a need not have a permanent connection to the internet server <NUM> and instead may connect to the internet server <NUM>, via the internet service, on an ad hoc basis, e.g. when using the internet service. However it is also not excluded that the first party 402a may be permanently connected to the internet server <NUM> via the internet service.

In embodiments in which the system <NUM> comprises a second party 402b, the second party 402b may transmit a request message to the first party 402a. The request message may be transmitted directly to the first party 402a, e.g. via a side channel <NUM>. Alternatively, the request message may be transmitted via the internet server <NUM>. That is, the second party 402a may transmit a request message, using the internet service hosted by the internet server <NUM>, to the internet server <NUM>, which then presents the request message to the first party 402a using the internet service. As an illustrative example, the request message may be entered and presented using a website (e.g. a social media sites) hosted by a web server <NUM>.

The request message may comprise a request to assign a number of digital assets, owned by the first party 402a, to the second party 402b, e.g. in return for goods or services. Additionally or alternatively, the request message may comprise one or more components of a blockchain transaction. For instance, the request message may comprise a locking script comprising a public key (or hash thereof) linked to the second party 402b.

The first party 402a may transmit the blockchain transaction (or at least the one or more components of the blockchain transaction) to the internet server <NUM> in response to receiving the request message. As an example, the second party 402b may send a blockchain address to the first party 402a (e.g. via the internet server <NUM>), and the first party 402a may send a signature for unlocking a UTXO to the internet server <NUM>. The internet server <NUM> may then formulate a transaction based on those components for sending to the blockchain network <NUM>.

Note that the first party 402a may perform any of the actions associated with the request message that have been described as being performed by the second party 402b, with any necessary adaptations. That is, the first party 402a may send a request message to the second party 402b, e.g. to assign an amount of the digital asset to a blockchain address linked to the first party 402a.

In some embodiments, after the first party 402a sends the blockchain transaction (or component(s) thereof) to the internet server <NUM>, and after the internet server <NUM> subsequently submits the blockchain transaction to the blockchain network <NUM>, the internet server <NUM> may transmit an update message to the first party 402a. The update message is presented to the first party 402a via the internet service.

In general, the update message comprises information concerning the transaction that has been submitted to the blockchain network <NUM>. The update message may comprise the transaction itself, i.e. a copy of the transaction that comprises the component(s) supplied by the first party 402a and that was submitted to the network <NUM>. The update message may, as an additional or alternative option, comprise an indication that the transaction has been successfully published on the blockchain <NUM>. The indication may comprise a block header of the block <NUM> in which the transaction has been included. In some examples, the update message may comprise a Merkle path for verifying that the transaction has been included in a block <NUM>. A Merkle path comprises a set of hashes that, together with a hash of the transaction, is used to calculate a Merkle root of a Merkle tree. If the Merkle root, generated using the Merkle path and the hash of the transaction, corresponds to a block header of a block <NUM> of the blockchain <NUM>, the first party 402a can be certain that the transaction was included in that block <NUM>.

The internet server <NUM> is configured to maintain a list of contacts (i.e. other parties <NUM>) associated with the first party 402a. For each contact, the internet server <NUM> records one or more blockchain addresses (e.g. public keys and/or public key hashes) linked with that contact, i.e. owned by that contact. The internet server <NUM> is configured to transmit the list of contacts and/or blockchain addresses to the first party 402a, via the internet service. The first party 402a may use this information to connect with the first party's contacts. In this sense, connecting with a contact may comprise assigning digital assets owned by the first party 402a to one of the contacts, e.g. the second party 402b.

As mentioned above, the internet server <NUM> may be a web server that hosts a web service, e.g. a website. That is, the web server <NUM> may serve one or more web pages to the first party 402a. Preferably, the first party 402a (i.e. the client application operated by the first party 402a) and the web server <NUM> communicate using Hypertext Transfer Protocol (HTTP). For instance, the transaction (or components thereof) may be transmitted to the web server <NUM> using HTTP. Similarly, the request message may be sent to/from the web server <NUM> using HTTP. The update message may also be sent to the first party 402a from the web server <NUM> using HTTP. Whilst HTTP is preferred, it is not excluded that data may be sent between the first party 402a and the web server using an alternative protocol, e.g. Simple Mail Transfer Protocol (SMTP) or File Transfer Protocol (FTP).

Alternatively, the internet server <NUM> may be a mail server that hosts a mail service, e.g. an email application. From the perspective of the first party 402a, the mail server may be an incoming mail server and/or an outgoing mail server. The first party 402a and the mail server may communicate using one Simple Mail Transfer Protocol (SMTP) for outgoing messages, and one of a Post Office Protocol (POP), e.g. POP3, or Internet Message Access Protocol (IMAP) for incoming messages.

The website or email application may include an embedded function configured to submit the blockchain transaction, or components thereof, to the web server or mail server respectively. For instance, an existing email application may be modified to include a data entry field or "button" that is configured to receive and/or generate transaction data. The website (e.g. a social media site or other type of web page) may also include a similarly configured function.

The following provides further details regarding web servers and mail servers configured to implement embodiments of the invention.

The internet server <NUM> may be a web <NUM> server. Web <NUM> refers to the second generation of the world wide web. Web <NUM> provides an interactive and dynamic web experience for users to share information online via social media, blogs and other web-based communities. Although the back-end infrastructure of the web remains the same, the front-end has been enriched with web browser technologies supported by, for example, AJAX and JavaScript frameworks to support new user information flows and dynamic content that is responsive to user input.

Web <NUM> servers (and web servers in general) may be used to propagate transactions to blockchain nodes <NUM> on the blockchain network <NUM>. A user 402a may operate a browser-based client application, which may operate to a simplified payment verification (SPV) method. The application may appear as a browser extension on a web <NUM> platform, for instance on a social media site. The user may create a transaction using the applications API. The application then propagates the transaction via one or more web <NUM> servers to one or more blockchain nodes <NUM>. Those blockchain nodes then validate the transaction and may send an update back to the client application, e.g. via the web <NUM> server. <FIG> schematically illustrates this process.

SPV client applications require a high level of connectivity and so may make use of the present invention. Such applications also sometimes require a high degree of security due to sensitive data being included in transactions that will be published on the blockchain <NUM>. Therefore the data submitted to the web server may be encrypted, e.g. by the user 402a or by the web service.

Connecting to the blockchain network <NUM> using web servers may also be used to bootstrap new users during network discovery. URL Flux-based Command and Control (UFCC) is a centralised architecture for communication using web <NUM> services. UFCC manages peer-lists within a network via web links to active peers. The links are typically uploaded to popular web <NUM> services that promise <NUM>-hour online availability. For an SPV client application, a UFCC communication system can be implemented to connect with other online peers (i.e. operating a respective instance of the SPV client application) during a bootstrap procedure. Blockchain nodes <NUM> may use a similar bootstrapping process to discover other blockchain nodes <NUM> when joining the blockchain network <NUM>. For instance, a blockchain node <NUM> may download a list of other blockchain nodes <NUM>, (like the list of contacts on a social media account) to see which blockchain nodes <NUM> are active (e.g. 'online') in the blockchain network <NUM>, in order to broadcast transactions to online nodes <NUM>. The list is maintained by the internet server <NUM>, which may transmit the list to a node <NUM>, e.g. upon request. The list may contain a list of identifiers (e.g. network address, public key, etc.). Each identifier is associated with a different node <NUM>.

The internet server may be a mail server. The mail server may be used to serve a client application in analogy to an email account. The mail server monitors the addresses (e.g. public keys or public key hashes) of a user <NUM> (account holder) to consolidate UTXOs and store the account information on the mail server. The information may be presented to the user when the user accesses their account.

The mail server may be non-custodial i.e. the server does not store digital keys on behalf of the first party 402a. The server <NUM> may file know-your-customer (KYC) information that proves the identity of the account holder (first party 402a). This information could be stored on- or off-chain. For instance, the server <NUM> may store a pointer to the location of a digital identity certificate embedded in a blockchain transaction. The server <NUM> monitors the relevant transaction ID in the UTXO set to ensure that the user's certified key(s) are up-to-date, i.e. the digital certificate has not expired. The server <NUM> may only submit the transaction to the blockchain <NUM> if the user's certified key has not expired. Note that this may also apply to other types of internet servers <NUM>.

As mentioned above, cryptographic techniques may be used to ensure a user's security and/or privacy is retained when propagating transactions over internet servers <NUM>. For instance, the first party 402a may implement digital key management to cycle through unique digital keypairs that are used to sign individual transactions, e.g. the same key is not used more than once to sign a transaction. Similarly, a second party 402b (e.g. a merchant) can ensure know-your-customer (KYC) and/or anti-money laundering (AML) regulatory compliance by requesting that the first party 402a signs the transaction with a certified key.

Since digital keypairs are preferably only used once, a user's digital keypairs can be cryptographically linked to their certified identity using hierarchically deterministic keys that are derived from a certified root key, see e.g. <CIT>. Note that this method can also be combined with a secret value distribution method to embed a shared secret into the key derivation path, thereby creating a private link between the sender and the recipient, see e.g. <CIT>. As another example, additional measures can be taken to protect a user's private keys that are linked to larger funds. A user may partly sign the transaction with a digital signature derived from the share of a private key. A threshold number of signatures must be reached to broadcast the complete transaction. Note that the full private key (combination of all key shares) never exists, which means that it can never be stolen from the user's web wallet by a malicious agent. The skilled person will be familiar with threshold signatures schemes. Finally, the first party 402a may use an encrypted browser, whereby traffic can be encrypted using a TLS client / server key encryption in accordance with the HTTPS communication protocol. This may be an SSL server certificate for the web wallet issuer and an SSL client certificate both based on the elliptic curve digital signature algorithm (ECDSA) keys of both the user and the server.

<FIG> and <FIG> illustrate example use cases of the present invention. <FIG> illustrates a merchant (second party 402b) conducting the sale of a product to a user (first party 402a) using the blockchain <NUM>. <FIG> illustrates a merchant receiving a refund request for a product by a user using the blockchain <NUM>. It will be appreciated that this example is for illustrative purposes only and the technique may be applied more generally to other use cases. That is, embodiments of the invention are not limited to customer-merchant type interactions.

The sequence shown in <FIG> is as follows. It will be appreciated that some of the steps are optional. At step <NUM>, a merchant 402b sends a payment request to the internet server <NUM>. The payment request may comprise a request for an amount of the digital asset. The payment request may comprise a locking script, or part of a locking script, for locking the amount of the digital asset to a blockchain address associated with the merchant 402b. Note that preferably the payment request only includes part of the locking script, as opposed to a complete locking script, if the internet server <NUM> is operated by a trusted third party and/or a secure communication channel is used. At step <NUM>, the internet server <NUM> saves the request for sending to the customer 402a when the customer comes online, e.g. accesses a web service or email application. At step <NUM>, the customer 402a comes online and the internet server <NUM> sends the request to the customer 402a. At step <NUM>, the customer 402a responds to the request. The customer 402a may reject the request and so send a reject message back to the internet server <NUM>, which would then forward to the merchant 402b. Alternatively, as shown in <FIG>, the customer 402a may accept the request. This may involve the customer sending a signature back to the internet server <NUM>, e.g. a signature for unlocking a UTXO of a previous transaction.

The internet server <NUM> may then take one of several options, two of which are shown. In one example, at step 5a, the internet server <NUM> sends a signed transaction directly to the blockchain network <NUM>. For instance, the internet server <NUM> may use the data contained in the payment request and the signature from the customer 402a to complete a transaction template. At steps 6a and 7a, the internet server <NUM> sends updates to the merchant 402b and customer 402a respectively. The updates may comprise a Merkle path for verifying that the transaction has been included in a block <NUM>, or other data relating to the transaction.

In another example, at step 5b, the internet server <NUM> sends the signed transaction to the merchant 402b. At step 6b, the merchant may check and finalise the transaction, e.g. signing the transaction, before sending to the internet server <NUM>. Alternatively, the merchant may submit the transaction to the blockchain network <NUM> if it has a direct connection to a node <NUM>. At step 7b, the internet server <NUM> sends the transaction to the blockchain network <NUM>. At steps 8b and 9b, the internet server <NUM> sends updates to the merchant 402b and customer 402a respectively.

The sequence shown in <FIG> is as follows. It will be appreciated that some of the steps are optional. The steps of <FIG> are similar to those of <FIG>. At step <NUM>, a customer 402a sends a refund request to the internet server <NUM>. The refund request may comprise a request for an amount of the digital asset. The refund request may comprise a locking script, or part of a locking script, for locking the amount of the digital asset to a blockchain address associated with the customer 402a. As for the payment request, preferably the refund request only includes part of the locking script, as opposed to a complete locking script, if the internet server <NUM> is operated by a trusted third party and/or a secure communication channel is used. At step <NUM>, the internet server <NUM> saves the request for sending to the merchant 402a when the merchant comes online, e.g. accesses a web service or email application. At step <NUM>, the merchant 402b comes online and the internet server <NUM> sends the request to the merchant 402b. The internet server <NUM> may send the customer's certified key as proof for the merchant 402b to accept the request. At step <NUM>, the merchant 402b responds to the request. The merchant 402b may reject the request and so send a reject message back to the internet server <NUM>, which would then forward to the customer 402a. Alternatively, as shown in <FIG>, the merchant 402a may accept the request. This may involve the merchant 402b sending a signature back to the internet server <NUM>, e.g. a signature for unlocking a UTXO of a previous transaction. The internet server <NUM> may save the full transaction and Merkle path if sent by the merchant 402b. Alternatively, the merchant 402b may download the input transaction and look for the Merkle path. At step <NUM>, the internet server <NUM> sends a signed transaction directly to the blockchain network <NUM>. For instance, the internet server <NUM> may use the data contained in the refund request and the signature from the merchant 402b to complete a transaction template. At steps <NUM> and <NUM>, the internet server <NUM> sends updates to the merchant 402b and customer 402a respectively. The updates may comprise a Merkle path for verifying that the transaction has been included in a block <NUM>, or other data relating to the transaction.

In summary, existing web and mail server infrastructures may be used to boost the connectivity of users (and other parties) <NUM> to the blockchain network <NUM>. Blockchain transactions may be broadcast over web servers (e.g. web <NUM> servers) to blockchain nodes <NUM>. Network discovery can be improved during the bootstrap procedure of new users to active peers that are also connected to web or email services. The invention is particularly advantageous for lightweight client applications using an SPV method where a high degree of network connectivity is required. Since lightweight clients do not require access to the entire blockchain, transactions can be propagated via web or mail services. Due to their typically large user bases and <NUM>-hour online availability, web <NUM> services (and other internet services) can be highly effective in boosting transaction propagation for blockchain client applications. In addition to HTTP-based applications, SMTP and IMAP/POP protocols can be utilised to propagate transactions using a mail server, in analogy to email accounts.

Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The scope of the disclosure is not limited by the described embodiments but only by the accompanying claims.

For instance, some embodiments above have been described in terms of a bitcoin network <NUM>, bitcoin blockchain <NUM> and bitcoin nodes <NUM>. However it will be appreciated that the bitcoin blockchain is one particular example of a blockchain <NUM> and the above description may apply generally to any blockchain. That is, the present invention is in by no way limited to the bitcoin blockchain. More generally, any reference above to bitcoin network <NUM>, bitcoin blockchain <NUM> and bitcoin nodes <NUM> may be replaced with reference to a blockchain network <NUM>, blockchain <NUM> and blockchain node <NUM> respectively. The blockchain, blockchain network and/or blockchain nodes may share some or all of the described properties of the bitcoin blockchain <NUM>, bitcoin network <NUM> and bitcoin nodes <NUM> as described above.

In preferred embodiments of the invention, the blockchain network <NUM> is the bitcoin network and bitcoin nodes <NUM> perform at least all of the described functions of creating, publishing, propagating and storing blocks <NUM> of the blockchain <NUM>. It is not excluded that there may be other network entities (or network elements) that only perform one or some but not all of these functions. That is, a network entity may perform the function of propagating and/or storing blocks without creating and publishing blocks (recall that these entities are not considered nodes of the preferred bitcoin network <NUM>).

In non-preferred embodiments of the invention, the blockchain network <NUM> may not be the bitcoin network. In these embodiments, it is not excluded that a node may perform at least one or some but not all of the functions of creating, publishing, propagating and storing blocks <NUM> of the blockchain <NUM>. For instance, on those other blockchain networks a "node" may be used to refer to a network entity that is configured to create and publish blocks <NUM> but not store and/or propagate those blocks <NUM> to other nodes.

Claim 1:
A computer-implemented method of transmitting blockchain transactions to a blockchain network (<NUM>), wherein the method is performed by a first party (402a) and comprises:
transmitting at least part of a blockchain transaction to an internet server (<NUM>) via an internet service hosted by the internet server (<NUM>), wherein the internet server (<NUM>) is configured to connect to one or more nodes (<NUM>) of the blockchain network (<NUM>), and to transmit a blockchain transaction to the one or more blockchain nodes (<NUM>), wherein the transmitted blockchain transaction comprises the at least part of the blockchain transaction;
characterised by
receiving, from the internet server (<NUM>) via the internet service, a list of contacts, which are other parties (402b) associated with the first party (402a), wherein for each contact in the list, the list comprises a respective one or more blockchain addresses associated with that contact.