Explicit value transactions

Abstract

A computer-implemented method comprising: creating a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transac

Description

TECHNICAL FIELD The present disclosure relates to a method, computer equipment and a computer program for determining the value of an input of blockchain transaction. BACKGROUND In a blockchain transaction (e.g., a Bitcoin transaction), the transaction information does not explicitly contain how much value (which in a Bitcoin transaction would be how many satoshis or how many bitcoins) the transaction has. For example, the inputs of a blockchain transaction do not explicitly state how much value the inputs have. Here, the value of an input is the value of the output a previous transaction referenced by that output. SUMMARY To check whether a total value of the outputs of a current transaction is less than or equal to a total value of the inputs of that transaction requires fetching the previous transaction or transactions based on the transaction ID and index references in the inputs of the current transaction. While this is part of the validation process for blockchain nodes, it is not an easy task for users of the blockchain. It would therefore be desirable to provide a technique that enables the value of the transaction (or the value of an input of a transaction) to be easily determined. According to one aspect disclosed herein, there is provided a computer-implemented method comprising: creating a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; sending the second blockchain transaction to one or more parties. According to another aspect disclosed herein, there is provided a computer-implemented method comprising: obtaining a first blockchain transaction, wherein the first blockchain transaction comprises an output locking a first value of a digital asset; receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references the output of the first blockchain transaction; determining whether the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; and when the input of the second blockchain transaction comprises a second value based on the first value, sending the second blockchain transaction to one or more parties. According to another aspect disclosed herein, there is provided a computer implemented method comprising: receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; and validating the second blockchain transaction according to a defined ruleset without obtaining the first blockchain transaction. BRIEF DESCRIPTION OF THE DRAWINGS To assist understanding of embodiments of the present disclosure and to show how such embodiments may be put into effect, reference is made, by way of example only, to the accompanying drawings in which: Figure 1 is a schematic block diagram of a system for implementing a blockchain, Figure 2 schematically illustrates some examples of transactions which may be recorded in a blockchain, and Figure 3 schematically illustrates an example system for implementing embodiments of the present disclosure. DETAILED DESCRIPTION OF EMBODIMENTS 1. EXPLICIT VALUE TRANSACTIONS Examples described herein provide an overlay node that can regulate a policy that the value of an input of a blockchain transaction should be explicitly included as part of the unlocking script of that input and / or that the value of the input should be included as part of a locking script of an output of the blockchain transaction. In the latter case, the chosen output may be the output that has an index corresponding to the input of the blockchain transaction. Additionally or alternatively, the output containing the value may include an index of the input of the blockchain transaction. Using the policy, a transaction can be valid to native blockchain nodes 104 and invalid to the overlay node. This allows receivers of the transaction or any interested entities to read the input values with ease and without the need to fetch previous transactions referenced by the transaction. This reduces processing requirements required to determine values of the blockchain transaction. An overlay node ("Olivia" herein) can regulate a policy that: • requires parties in a blockchain network 106 to add input value(s) of a blockchain transaction to an input of the blockchain transaction; and / or • requires parties in the blockchain network 106 to add input value(s) of a blockchain transaction to an output of a blockchain transaction (e.g. having an index corresponding to an index of the input of blockchain transaction). The policy may require the output to include the index of the input. As shown in Figure 3, an example system may comprise an overlay node (Olivia) 301, a first party (Alice) 103a, a second party (Bob) 103b and one or more nodes 104 of a blockchain network 106. In a particular example, Alice 103a submits a transaction (the second transaction discussed below) to Olivia 301 for processing. Olivia 301 may then submit the transaction to the blockchain network 106 if it passes one or more checks (discussed below). Additionally or alternatively, Alice 103a may submit the transaction to Bob 103b. Bob 103b may be a tax authority, auditor, or the like. A first blockchain transaction (TxID0) may comprise an output that locks a first value (v0) of a digital asset (e.g., an amount of satoshis or bitcoins). A second blockchain transaction (TxID^ created by a first party (Alice) 103a may reference the output of TxID0 that locks v0 of the digital asset. The policy requires Alice 103a to include a second value v0' in at least one of: an input of TxlD^, an output of TxlD1 (when v0' is included in the output of TxID1, an index of the input of TxID^ having the value of v0' is also included in the output of TxID^. The second value vQ' is based on the first value v0. In some examples v0'= v0 + the cost of processing transaction TxlD^ In some examples, v0'= v0. Table 1 shows an example of a transaction complying with the policy, where TxID± includes a second value v0' an input of TxID1. TxID1 Version 1 Locktime 0 In-count 1 Out-count 1 Input list Output list Outpoint Unlocking script Value Locking script TxIDo\\O < Vq X SigAUce ><PK-Alice > V1 [P2PKH Bob] Table 1: Transaction showing the format of a compliant transaction. [P2PKH Bob] is shorthand for a locking script that enforces a pay-to-public-key-hash script based on a public key associated with Bob 103b. The transaction spends the input TxIDo\\O which contains a value of v0. In accordance with the policy, Alice adds Vq to the unlocking script. The transaction is then sent to Olivia. Upon receiving the transaction, Olivia can conduct the following check: • Fetch TxID0 and read the value of the output with index 0; • Check that the value v0 is equal to the sum of the cost of processing transaction TxID^nd v0' in the unlocking script of TXID1, or simply v0 if no transaction fees are being paid. If there are multiple inputs, the check can be performed for each input. If the transaction passes all checks, it is then passed on to other blockchain nodes 104 and / or to a second party "Bob" 103b. By performing this check at the overly node (Olivia), processing requirements of other parties (e.g., Alice 103a and Bob 103b) can be reduced. This is useful in situations where processing capacity of Alice and Bob are limited (e.g., if Alice and / or Bob comprise mobile devices, for example). Table 2 shows another example of a transaction complying with the policy, where TxIDt includes a second value v0' with an index of the input of TxID1 having the value of v0' is also included in the output of TxID^ TxID1 Version 1 Locktime 0 In-count 1 Out-count 1 Input list Output list Outpoint Unlocking script Value Locking script TxIDo\\O < SigAnce > P^Alice > Vi [P2PKH Bob]< v0' >lndex=0 Table 2: Transaction showing the format of a compliant transaction. The transaction spends the input TxIDQ\\0 which contains a value of v0. In accordance with the policy, Alice adds Vq to the locking script of the output along with an index of the input of TxIDy having the value of v0' (including the index is optional). The transaction is then sent to Olivia. Upon receiving the transaction, Olivia can conduct the following check: • Fetch TxID0 and read the value of the output with index 0; • Check that the value v0 is equal to the sum of the cost of processing transaction TxID^nd v0' in the locking script of TXID1. If there are multiple inputs, the check can be performed for each input. If the transaction passes all checks, it is then passed on to other blockchain nodes 104 and / or a second party "Bob" 103b. By performing this check at the overly node (Olivia), processing requirements of other parties (e.g., Alice 103a and Bob 103b) can be reduced. Again, this is useful in situations where processing capacity of blockchain nodes of Alice and Bob are limited (e.g., if Alice and / or Bob comprise mobile devices, for example). If a policy-invalid transaction is sent to the blockchain network 106 directly by a malicious user, it will be published as a valid blockchain transaction. However, the malicious user risks burning their layer-2 tokens, if the transaction is a token transaction, and the receiver may claim that they did not receive the tokens. Olivia, or a service provider, can validate TXID^ without having to obtain TXID0 according to the defined ruleset of the policy, as Olivia can obtain the value v0' without having to obtain TXID0. To validate TXIDV Olivia can determine an amount of digital asset paid to a blockchain node for accepting TXID1. Olivia can use this along with the value v0' to determine v0. As the value v0' is not signed, the integrity cannot be offered by e.g., the native blockchain signature scheme. However, the transaction ID can be used to ensure that v0' is not modified. Olivia can record the transaction ID and communicate it to the sender (e.g. Alice 103a) and the receiver (e.g. Bob 103b) before submitting it to the blockchain network 106. After the transaction is included in a block, Olivia can further communicate the Merkle proof (or an alternative inclusion proof) to the sender and the receiver. Any changes made to v0’ would result in a different transaction ID and render the Merkle proof invalid. In some examples where the value v0' is included in an output of TxID1, the input ofTxID1 may comprise a signature that signs a message based on the output of TxID-^. As a result, the value v0' will have integrity. In some examples where the value v0' is included in an input of TxIDlt the output of TxID} may comprise a signature that signs a message based on an input of TxID1. As a result, the value v0' will have integrity. In the latter case, a nonnative blockchain signature scheme may be used, e.g. RSA signatures. In some examples, the output of TxID0 referenced by the input of TxlD0 may comprise a locking script that is configured to verify that the input of TxID1 comprises the v0'. The skilled person will be familiar with the concept of a locking script that can verify that a value provided in an unlocking script of a spending transaction matches a value provided in the locking script. For instance, an opcode such as OP_EQUALVERIFY may be used to check for equality. In some examples where the value v0' is included in an output of TxID± with an index of the corresponding input, an 0P_RETURN data payload or OP_PUSHDATA payload can be used to show that the output comprises the value v0' of the corresponding input. In some examples, Olivia can connect directly to one or more trusted blockchain nodes 104 to ensure integrity of the value vQ'. The policy described above is useful for any payment scenario as it makes the input value explicit to relevant entities without the need to fetch previous transaction or transactions. For example, Alice 103a wants to prove to the tax authority that she made a payment to Bob 103b of 1 BSV and paid 10 satoshis transaction fee, where the fee is tax deductible. Previously, Alice 103a must submit two transactions: the transaction she is spending, and the spending transaction. The tax authority must then derive the fee from both transactions. With the technique above, Alice 103a can just submit one transaction - the spending transaction. This cuts down on the amount of data being sent between parties, and the amount of processing by both parties. 2. EXAMPLE SYSTEM OVERVIEW 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 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 timeorder 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. 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. Figure 1 shows an example system 100 for implementing a blockchain 150. The system 100 may comprise a packet-switched network 101, typically a wide-area internetwork such as the Internet. The packet-switched network 101 comprises a plurality of blockchain nodes 104 (often referred to as "miners") that may be arranged to form a peer-to-peer (P2P) network 106 within the packet-switched network 101. Whilst not illustrated, the blockchain nodes 104 may be arranged as a near-complete graph. Each blockchain node 104 is therefore highly connected to other blockchain nodes 104. Each blockchain node 104 comprises computer equipment of a peer, with different ones of the nodes 104 belonging to different peers. Each blockchain node 104 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 150 comprises a chain of blocks of data 151, wherein a respective copy of the blockchain 150 is maintained at each of a plurality of blockchain nodes 104 in the distributed or blockchain network 106. As mentioned above, maintaining a copy of the blockchain 150 does not necessarily mean storing the blockchain 150 in full. Instead, the blockchain 150 may be pruned of data so long as each blockchain node 150 stores the block header (discussed below) of each block 151. Each block 151 in the chain comprises one or more transactions 152, 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. A blockchain node 104 may be configured to forward transactions 152 to other blockchain nodes 104, and thereby cause transactions 152 to be propagated throughout the network 106. A blockchain node 104 may be configured to create blocks 151 and to store a respective copy of the same blockchain 150 in their respective memory. A blockchain node 104 may also maintain an ordered set (or "pool") 154 of transactions 152 waiting to be incorporated into blocks 151. The ordered pool 154 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 104 has accepted as valid and for which the node 104 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. Spending or redeeming does not necessarily imply transfer of a financial asset, though that is certainly one common application. More generally spending could be described as consuming the output, or assigning it to one or more outputs in another, onward transaction. In general, the preceding transaction could be any transaction in the ordered set 154 or any block 151. The preceding transaction 152i need not necessarily exist at the time the present transaction 152j is created or even sent to the network 106, 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. Due to the resources involved in transaction validation and publication, typically at least each of the blockchain nodes 104 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 104 could take the form of a user terminal or a group of user terminals networked together. The memory of each blockchain node 104 stores software configured to run on the processing apparatus of the blockchain node 104 in order to perform its respective role or roles and handle transactions 152 in accordance with the blockchain node protocol. It will be understood that any action attributed herein to a blockchain node 104 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. Any given blockchain node may be configured to perform one or more of the following operations: validating transactions, storing transactions, propagating transactions to other peers, performing consensus (e.g. proof-of-work) / mining operations. In some examples, each type of operation is performed by a different node 104. That is, nodes may specialise in particular operation. For example, a nodes 104 may focus on transaction validation and propagation, or on block mining. In some examples, a blockchain node 104 may perform more than one of these operations in parallel. Any reference to a blockchain node 104 may refer to an entity that is configured to perform at least one of these operations. Also connected to the network 101 is the computer equipment 102 of each of a plurality of parties 103 in the role of consuming users. These users may interact with the blockchain network 106 but do not participate in validating transactions or constructing blocks. Some of these users or agents 103 may act as senders and recipients in transactions. Other users may interact with the blockchain 150 without necessarily acting as senders or recipients. For instance, some parties may act as storage entities that store a copy of the blockchain 150 (e.g. having obtained a copy of the blockchain from a blockchain node 104). Some or all of the parties 103 may be connected as part of a different network, e.g. a network overlaid on top of the blockchain network 106. Users of the blockchain network (often referred to as "clients") may be said to be part of a system that includes the blockchain network 106; however, these users are not blockchain nodes 104 as they do not perform the roles required of the blockchain nodes. Instead, each party 103 may interact with the blockchain network 106 and thereby utilize the blockchain 150 by connecting to (i.e. communicating with) a blockchain node 106. Two parties 103 and their respective equipment 102 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 103 and their respective computer equipment 102 may be present and participating in the system 100, but for convenience they are not illustrated. Each party 103 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 102 of each party 103 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 102 of each party 103 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 102 of each party 103 stores software comprising a respective instance of at least one client application 105 arranged to run on the processing apparatus. It will be understood that any action attributed herein to a given party 103 may be performed using the software run on the processing apparatus of the respective computer equipment 102. The computer equipment 102 of each party 103 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 102 of a given party 103 may also comprise one or more other networked resources, such as cloud computing resources accessed via the user terminal. The client application 105 may be initially provided to the computer equipment 102 of any given party 103 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 105 comprises at least a "wallet" function. This has two main functionalities. One of these is to enable the respective party 103 to create, authorise (for example sign) and send transactions 152 to one or more bitcoin nodes 104 to then be propagated throughout the network of blockchain nodes 104 and thereby included in the blockchain 150. 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 152 transactions scattered throughout the blockchain 150 that belong to the party in question. Note: whilst the various client functionality may be described as being integrated into a given client application 105, 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 105 but it will be appreciated that this is not limiting. The instance of the client application or software 105 on each computer equipment 102 is operatively coupled to at least one of the blockchain nodes 104 of the network 106. This enables the wallet function of the client 105 to send transactions 152 to the network 106. The client 105 is also able to contact blockchain nodes 104 in order to query the blockchain 150 for any transactions of which the respective party 103 is the recipient (or indeed inspect other parties' transactions in the blockchain 150, since in embodiments the blockchain 150 is a public facility which provides trust in transactions in part through its public visibility). The wallet function on each computer equipment 102 is configured to formulate and send transactions 152 according to a transaction protocol. As set out above, each blockchain node 104 runs software configured to validate transactions 152 according to the blockchain node protocol, and to forward transactions 152 in order to propagate them throughout the blockchain network 106. 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 152 in the blockchain 150. The same node protocol is used by all the nodes 104 in the network 106. 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" or "nonce"). 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. Some account-based transaction models share several similarities with the output-based transaction model described herein. For example, as mentioned above, the data field of an account-based transaction may point back to a previous transaction, which is equivalent to the input of an output-based transaction which references an outpoint a previous transaction. Thus both models enable linking between transactions. As another example, an account-based transaction contains a "recipient" field (in which a receiving address of an account is specified) and a "value" field (in which an amount of digital asset may be specified). Together the recipient and value fields are equivalent to the output of an outputbased transaction which may be used to assign an amount of digital asset to a blockchain address. Similarly, an account-based transaction has a "signature" field which includes a signature for the transaction. The signature is generated using the sender's private key and confirms the sender has authorized this transaction. This is equivalent to an input / unlocking script of an output-based transaction which, typically, includes a signature for the transaction. When both types of transaction are submitted to their respective blockchain networks, the signatures are checked to determine whether the transaction is valid and can be recorded on the blockchain. On an account-based blockchain, a "smart contact" refers to a transaction that contains a script configured to perform one or more actions (e.g. send or "release" a digital asset to a recipient address) in response to one or more inputs (provided by a transaction) meeting one or more conditions defined by the smart contact's script. The smart contract exists as a transaction on the blockchain, and can be called (or triggered) by subsequent transactions. Thus, in some examples, a smart contract may be considered equivalent to a locking script of an output-based transaction, which can be triggered by a subsequent transaction, and checks whether one or more conditions defined by the locking script are met by the input of the subsequent transaction. 3. UTXO-BASED MODEL Figure 2 illustrates an example transaction protocol. This is an example of a UTXO-based protocol. A transaction 152 (abbreviated "Tx") is the fundamental data structure of the blockchain 150 (each block 151 comprising one or more transactions 152). 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") 152 comprises a data structure comprising one or more inputs 202, and one or more outputs 203. Each output 203 may comprise an unspent transaction output (UTXO), which can be used as the source for the input 202 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 201, which may comprise an indicator of the size of the input field(s) 202 and output field(s) 203. The header 201 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 201 of the raw transaction 152 submitted to the nodes 104. Say Alice 103a wishes to create a transaction 152j transferring an amount of the digital asset in question to Bob 103b. In Figure 2 Alice's new transaction 152j is labelled "Txi". It takes an amount of the digital asset that is locked to Alice in the output 203 of a preceding transaction 152i in the sequence, and transfers at least some of this to Bob. The preceding transaction 152i is labelled "Tx" in Figure 2. 7¼ and Txi are just arbitrary labels. They do not necessarily mean that Txo is the first transaction in the blockchain 151, nor that Txi is the immediate next transaction in the pool 154. Txi could point back to any preceding (i.e. antecedent) transaction that still has an unspent output 203 locked to Alice. 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 106, or arrive at any given blockchain node 104. 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 104 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 203 of the preceding transaction Txo comprises a particular UTXO, labelled here UTXOo. 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 202 of a subsequent transaction in order for the subsequent transaction to be validated, and therefore for the UTXO to be successfully redeemed. 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 203, for example the requirement of Alice's signature. Locking 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 202 of transactions. So in the example illustrated, UTXOo in the output 203 of Tat? com prises a locking script [Checksig Pa] which requires a signature Sig Pa of Alice in order for UTXOo to be redeemed (strictly, in order for a subsequent transaction attempting to redeem UTXOoto be valid). [Checksig Pa] contains a representation (i.e. a hash) of the public key Pa from a publicprivate key pair of Alice. The input 202 of Txi comprises a pointer pointing back to Txi (e.g. by means of its transaction ID, TxIDo, which in embodiments is the hash of the whole transaction Txo}. The input 202 of Txi comprises an index identifying UTXOo within Txo, to identify it amongst any other possible outputs of Txo. The input 202 of Txi 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 Txi arrives at a blockchain node 104, 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). 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. "OP_..." 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 150. E.g. 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 secp256kl. 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 4-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 150 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. 4. FURTHER REMARKS 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 106, bitcoin blockchain 150 and bitcoin nodes 104. However it will be appreciated that the bitcoin blockchain is one particular example of a blockchain 150 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 106, bitcoin blockchain 150 and bitcoin nodes 104 may be replaced with reference to a blockchain network 106, blockchain 150 and blockchain node 104 respectively. The blockchain, blockchain network and / or blockchain nodes may share some or all of the described properties of the bitcoin blockchain 150, bitcoin network 106 and bitcoin nodes 104 as described above. In preferred embodiments of the invention, the blockchain network 106 is the bitcoin network and bitcoin nodes 104 perform at least all of the described functions of creating, publishing, propagating and storing blocks 151 of the blockchain 150. 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 106). In other embodiments of the invention, the blockchain network 106 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 151 of the blockchain 150. 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 151 but not store and / or propagate those blocks 151 to other nodes. Even more generally, any reference to the term "bitcoin node" 104 above may be replaced with the term "network entity" or "network element", wherein such an entity / element is configured to perform some or all of the roles of creating, publishing, propagating and storing blocks. The functions of such a network entity / element may be implemented in hardware in the same way described above with reference to a blockchain node 104. Some embodiments have been described in terms of the blockchain network implementing a proof-of-work consensus mechanism to secure the underlying blockchain. However proof-of-work is just one type of consensus mechanism and in general embodiments may use any type of suitable consensus mechanism such as, for example, proof-of-stake, delegated proof-of-stake, proof-of-capacity, or proof-of-elapsed time. As a particular example, proof-of-stake uses a randomized process to determine which blockchain node 104 is given the opportunity to produce the next block 151. The chosen node is often referred to as a validator. Blockchain nodes can lock up their tokens for a certain time in order to have the chance of becoming a validator. Generally, the node who locks the biggest stake for the longest period of time has the best chance of becoming the next validator. It will be appreciated that the above embodiments have been described by way of example only. More generally there may be provided a method, apparatus or program in accordance with any one or more of the following Statements. Statement 1: A computer-implemented method comprising: creating a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; sending the second blockchain transaction to one or more parties. Statement 2: The computer-implemented method of Statement 1, wherein the one or more parties comprises a blockchain node. Statement 3: The computer-implemented method of Statement 1 or Statement 2, wherein the one or more parties comprise an overlay node, wherein the overlay node is configured to verify whether the input of the second blockchain transaction comprises the second value. Statement 4: The computer-implemented method of Statement 3, the method comprising: receiving a transaction identifier of the second blockchain transaction and a Merkle proof of the second blockchain transaction from the overlay node. Statement 5: The computer-implemented method of any preceding Statement, wherein the input of the second blockchain transaction comprises a signature that signs a message based on the output of the second blockchain transaction. Statement 6: The computer-implemented method of Statement 5, wherein the output of the second blockchain transaction comprises the second value. Statement 7: The computer-implemented method of any of Statements 1 to 4, wherein the output of the second blockchain transaction comprises a signature that signs a message based on the input of the second blockchain transaction. Statement 8: The computer-implemented method of any preceding Statement, wherein the output of the first blockchain transaction comprises a locking script, and wherein the locking script is configured to verify that the input of the second blockchain transaction comprises the second value. Statement 9: The computer-implemented method of any preceding Statement, wherein the first value is equal to the second value. Statement 10: A computer-implemented method comprising: obtaining a first blockchain transaction, wherein the first blockchain transaction comprises an output locking a first value of a digital asset; receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references the output of the first blockchain transaction; determining whether the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; and when the input of the second blockchain transaction comprises a second value based on the first value, sending the second blockchain transaction to one or more parties. Statement 11: The computer-implemented method of Statement 10, wherein receiving the second blockchain transaction comprises receiving the second blockchain transaction from a blockchain node. Statement 12: The computer-implemented method of Statement 11, wherein receiving the second blockchain transaction comprises only accepting the second blockchain transaction from a trusted blockchain node. Statement 13: The computer-implemented method of any of Statements 10 to 12, wherein the one or more parties comprise a sending party of the second blockchain transaction and a receiving party of the second blockchain transaction, the method comprising: sending, to the sending party and / or the receiving party, a transaction identifier for the second blockchain transaction before submitting the second blockchain transaction to one or more blockchain nodes. Statement 14: The computer-implemented method of Statement 13, the method comprising: sending, to the sending party and the receiving party, a Merkle proof for the second blockchain transaction after submitting the second blockchain transaction to the one or more blockchain nodes. Statement 15: The computer-implemented method of any of Statements 10 to 14, wherein the input of the second transaction comprises a signature that signs a message based on the output of the second transaction. Statement 16: A computer implemented method comprising: receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; validating the second blockchain transaction according to a defined ruleset without obtaining the first blockchain transaction. Statement 17: The computer-implemented method of Statement 16, wherein validating the second blockchain transaction comprises: determining an amount of digital asset paid to a blockchain node for accepting the second blockchain transaction; and / or determining the first value of the digital asset locked by the output of the first blockchain transaction. Statement 18: Computer equipment comprising: memory comprising one or more memory units; and processing apparatus comprising one or more processing units, wherein the memory stores code arranged to run on the processing apparatus, the code being configured so as when on the processing apparatus to perform the method of any of Statements 1 to 17. Statement 19: A computer program embodied on computer-readable storage and configured so as, when run on one or more processors, to perform the method of any of Statements 1 to 17. According to another aspect disclosed herein, there may be provided a method comprising the actions of Olivia. According to another aspect disclosed herein, there may be provided a system comprising the computer equipment of Olivia. According to another aspect disclosed herein, there may be provided a method comprising the actions of Alice. According to another aspect disclosed herein, there may be provided a system comprising the computer equipment of Alice. According to another aspect disclosed herein, there may be provided a method comprising the actions of Bob. According to another aspect disclosed herein, there may be provided a system comprising the computer equipment of Bob.

Claims

1. A computer-implemented method comprisingcreating a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction;sending the second blockchain transaction to one or more parties.

2. The computer-implemented method of claim 1, wherein the one or more parties comprises a blockchain node.

3. The computer-implemented method of claim 1 or claim 2, wherein the one or more parties comprise an overlay node, wherein the overlay node is configured to verify whether the input of the second blockchain transaction comprises the second value.

4. The computer-implemented method of claim 3, the method comprising:receiving a transaction identifier of the second blockchain transaction and a Merkle proof of the second blockchain transaction from the overlay node.

5. The computer-implemented method of any preceding claim, wherein the input of the second blockchain transaction comprises a signature that signs a message based on the output of the second blockchain transaction.

6. The computer-implemented method of claim 5, wherein the output of the secondblockchain transaction comprises the second value.

7. The computer-implemented method of any of claims 1 to 4, wherein the output of the second blockchain transaction comprises a signature that signs a message based on the input of the second blockchain transaction.

8. The computer-implemented method of any preceding claim, wherein the output of the first blockchain transaction comprises a locking script, and wherein the locking script is configured to verify that the input of the second blockchain transaction comprises the second value.

9. The computer-implemented method of any preceding claim, wherein the first value is equal to the second value.

10. A computer-implemented method comprising:obtaining a first blockchain transaction, wherein the first blockchain transaction comprises an output locking a first value of a digital asset;receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references the output of the first blockchain transaction;determining whether the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; andwhen the input of the second blockchain transaction comprises a second value based on the first value, sending the second blockchain transaction to one or more parties.

11. The computer-implemented method of claim 10, wherein receiving the second blockchain transaction comprises receiving the second blockchain transaction from a blockchain node.

12. The computer-implemented method of claim 11, wherein receiving the second blockchain transaction comprises only accepting the second blockchain transaction from a trusted blockchain node.

13. The computer-implemented method of any of claims 10 to 12, wherein the one or more parties comprise a sending party of the second blockchain transaction and a receiving party of the second blockchain transaction, the method comprising:sending, to the sending party and / or the receiving party, a transaction identifier for the second blockchain transaction before submitting the second blockchain transaction to one or more blockchain nodes.

14. The computer-implemented method of claim 13, the method comprising: sending, to the sending party and / or the receiving party, a Merkle proof for the second blockchain transaction after submitting the second blockchain transaction to the one or more blockchain nodes.

15. The computer-implemented method of any of claims 10 to 14, wherein the input of the second transaction comprises a signature that signs a message based on the output of the second transaction, and wherein the method comprises validating the signature; and / or wherein the output of the second transaction comprises a signature that signs a message based on the input of the second transaction, and wherein the method comprises validating the signature.

16. A computer implemented method comprising:receiving a second blockchain transaction, wherein the second blockchain transaction comprises an input that references an output of a first blockchain transaction and the output of the first blockchain transaction locks a first value of a digital asset, wherein the input of the second blockchain transaction comprises a second value based on the first value and / or an output of the second blockchain transaction comprises the second value and an index of the input of the second blockchain transaction; andvalidating the second blockchain transaction according to a defined ruleset without obtaining the first blockchain transaction.

17. The computer-implemented method of claim 16, wherein validating the second blockchain transaction comprises:determining an amount of digital asset paid to a blockchain node for accepting the second blockchain transaction; and / ordetermining the first value of the digital asset locked by the output of the first blockchain transaction.

18. Computer equipment comprising:memory comprising one or more memory units; andprocessing apparatus comprising one or more processing units, wherein the memory stores code arranged to run on the processing apparatus, the code being configured so as when on the processing apparatus to perform the method of any of claims 1 to 17.

19. A computer program embodied on computer-readable storage and configured so as, when run on one or more processors, to perform the method of any of claims 1 to 17.

Images