Data commitments

Abstract

A computer-implemented method of using a ledger, wherein the ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the ledger. Th

Description

TECHNICAL FIELD The present disclosure relates to methods of committing to data using a ledger, such as a blockchain. For example, the methods may be used to commit to the issuance and / or transfer of data, such as digital assets. An example of digital assets that may be committed to using the embodiments herein is tokens of a token protocol, such as digital cash. BACKGROUND In [1] an architecture is presented for a digital payment system where assets can be issued obliviously. By utilising Chaum's blind signature protocol [2], a user can be assigned newly minted assets in such a way that the issuer does not know which asset were issued to whom. The primary focus of [1] is to assure the privacy of a recipient of newly minted tokens. The Millicent protocol [3] is a digital token framework where tokens are transferred peer-to-peer and then sent to local trusted brokers to validate. The protocol relies on secrets that are shared between users and brokers in a transaction format called Scrip. The token ruleset is enforced by local brokers rather than an authorised central network. In [4] a comprehensive review is provided in the state of the art in cross-chain technology. This divided into three broad categories: notary, hash lock, and side / chain relay, along with novel outliers. All protocols involve either trusted third parties, shared secrets, or new bridging platforms with distributed consensus. References: [1] Goodell, G., Toliver, D.R., Nakib, H.D.: A Scalable Architecture for Electronic Payments. In: Matsuo, S., et al. Financial Cryptography and Data Security. FC 2022 International Workshops. FC 2022. Lecture Notes in Computer Science, vol 13412. Springer, Cham. doi.org / 10.1007 / 978-3-031-32415-4_38. [2] Chaum, D.: Blind Signatures for Untraceable Payments. In: Advances in Cryptology: Proceedings of Crypto 82(3), pp. 199-203,1983. [3] Glassman, S., Manasse, M., Abadi, M., Gauthier, P., and Sobalvarro, P.: The Millicent Protocol for Inexpensive Electronic Commerce. In: WWW4: Proceedings of the Fourth International Conference on World Wide Web, December 1995, Pages 603-618. doi.org / 10.1145 / 3592626.3592675. [4] Li, L., Wu, J., Cui, W.: A review of blockchain cross-chain technology. In: IET Blockchain, Volume3, Issue3, September 2023, Pages 149-158. doi.org / 10.1049 / blc2.12032. SUMMARY According to one aspect disclosed herein, there is provided a computer-implemented method of committing to an exchange of data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a first user and comprises: obtaining a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of a first ledger controlled by a first user or a service provider; generating a second data packet, wherein the second data packet comprises second data, a reference to the first data packet, and a reference to a second entry of a second ledger controlled by a second user or the service provider; generating a commitment of the second data packet; and submitting a third entry of the first ledger to the first ledger, wherein the third entry of the ledger references the first entry of the ledger, and wherein the third entry comprises the commitment of the second data packet, or sending the commitment of the second data packet to the service provider for submitting the third entry of the first ledger to the first ledger. According to one aspect disclosed herein, there is provided a computer-implemented method of committing to the generation of first data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a data issuer and comprises: generating a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of the ledger controlled by the first user or a service provider; and sending the first data packet to the first user. Embodiments of the present disclosure enable parties to commit to the transfer, flow, exchange, issuance, redemption, etc. of data. In general the data may relate to any application or protocol. The data may be linked in the sense that each message is linked to a previous message, such that the messages form a chain or sequence of messages. The chain or sequence may have any number of messages. Here, message does not necessarily mean a communication, although that is an option. Commitments are made using a ledger (either public or private). The ledger may be distributed. For example, the ledger may be a blockchain, e.g. an output-based blockchain. The ledger is used to provide double-spend protection, or equivalent protection in the case where the ledger does not involve the spending or transfer of a native digital asset. That is, in general the ledger is used to ensure that entries of the ledger can only be validly referenced (e.g. spent, assigned, etc.) a single time. Commitments are made in the form of data (or commitment) packets. The data packet contains the relevant data, e.g. a financial transaction, a digital token, a communication, etc. The data packet also includes a reference to an existing entry of the ledger (e.g. a reference to a blockchain transaction) controlled by a party to which the data is being transferred, exchanged, issued, etc. Depending on the type of blockchain, the refence may be to an outpoint of a transaction. A commitment of the data packet, such as a hash digest, is then recorded in an entry (e.g. transaction) on the ledger that references the same existing entry of the ledger. This creates an immutable record of the data transfer, exchange, issuance, etc. to the party that controls the referenced entry, without revealing any information about the data itself. According to one aspect disclosed herein, there is provided a computer-implemented method of transferring a digital token using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a first token user and comprises: obtaining a first token packet, wherein the first token packet comprises first token data, an identifier of the first token user, and a reference to a first entry of a first ledger controlled by the first token user or a service provider; generating a second token packet, wherein the second token packet comprises second token data, a reference to the first token packet, an identifier of a second token user, and a reference to a second entry of a second ledger controlled by the second token user or the service provider; generating a commitment of the second token packet; and submitting a third entry of the ledger to the first ledger, wherein the third entry of the ledger references the first entry of the ledger, and wherein the third entry comprises the commitment of the second token packet, or sending the commitment of the second token packet to the service provider for submitting the third entry of the first ledger to the first ledger. According to one aspect disclosed herein, there is provided a computer-implemented method of issuing a token using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a token issuer and comprises: generating a first token packet, wherein the first token packet comprises first token data, an identifier of a first token user, and a reference to a first entry of a first ledger controlled by the first token user or a service provider; and sending the first token packet to the first token user. Embodiments of the present disclosure may be used to implement a protocol for transferring digital assets on a private network overlayed on a blockchain. Users maintain custody of their assets and transfers do not require a trusted third party. Double-spend protection is provided by existing blockchain infrastructure a novel way that maximises privacy. In fact, the scheme is more private than the underlying blockchain. In previous approaches, one embeds token data within a blockchain transaction. This is useful to leverage the existing infrastructure for transaction processing by blockchain nodes. However, since the format of a transaction is fixed, this approach can be limiting. For example, one must use the same cryptosystem (e.g. digital signature scheme or hash functions) as the underlying blockchain. Privacy is also sacrificed due to the public nature of blockchain. In contrast, embodiments described herein take a different approach by making use of a new fundamental unit of transfer called a data packet (or commitment packet) and embed a reference to a previous blockchain transaction (e.g. an unspent blockchain outpoint) inside this packet. When the token transfer is finalised on the private network, a corresponding blockchain record is created that ensures the token cannot be double-spent. The token ruleset is enforced by the users of the private network and not by blockchain nodes, making the scheme private and scalable. This allows the design of the commitment packets to be highly flexible. In particular, different cryptosystem than the underlying blockchain may be used. Blockchain transactions are used for double-spend protection, but these transactions just record data and do not necessarily need to transfer any native blockchain tokens. The management of blockchain transactions may be outsourced to a service provider without introducing any risk of unauthorised token transfer. Whilst the scheme is designed to work on UTXO-based blockchains, it is agnostic to the choice of underlying blockchain or distributed ledger. The blockchain is used for doublespend protection and to record a hash digest (or other type of commitment) in a transaction (256-bits of data is enough). Since minimal use of the blockchain is required, the protocol may be applied in almost any blockchain setting. In fact, the scheme may be implemented on multiple blockchains at once, and tokens can be transferred between chains without additional infrastructure or a trusted third party. No other known token protocols appear to have this property. Central Bank Digital Currency (CBDC) is considered as a motivating use case. For simplicity, assume there is a single trusted token issuer which is the central bank. The focus of the present disclosure is on proving tokens have not been double-spent by users, which is a critical element in proving the provenance of a token. Some advantages of the scheme disclosed herein are as follows. • Flexible. The structure of the commitment packets is highly customisable. The token issuer is free to choose cryptosystems that are optimised for client or usage requirements, such as hardware security modules, rather than for blockchain settings. • Interoperable. The protocol can may deployed on multiple blockchains at the same time. Users may transfer tokens between blockchains. For instance, users may have different preferred blockchains, and each user may use their preferred blockchain. • Secure. The blockchain is used minimally to provide double-spend protection. Users may outsource blockchain transaction management to a service provider without giving up custody of their assets. The protocol does not require shared secrets or shared encryption. • Private. No private information can be determined by inspecting the blockchain. Blockchain transactions are not related to one-another through on-chain logic. • Scalable. The token ruleset is not enforced by blockchain nodes. Unspent blockchain outpoints may be prepared in advance so that will not have a child transaction in the same block. This reduces overheads on blockchain nodes when processing transactions in our scheme. • Cost effective. In the simplest embodiments, blockchain transactions contain one input and one unspendable output. They have a small, predictable size and no smart contract logic. The protocol described herein is compatible with [1], One difference is that we use the blockchain to provide double spend protection rather than an authorised central network. The protocol described herein is similar to [3] but replaces trusted brokers with a blockchain solution, and does not rely on shared secrets. The protocol described herein does not require any of the features required by [4], 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, Figure 3 schematically illustrates an example system for committing to the flow of data according to some embodiments described herein, Figure 4 schematically illustrates the linking of transactions, and Figure 5 schematically illustrates the relationship between commitment packets and blockchain transactions according to some embodiments described herein. DETAILED DESCRIPTION OF EMBODIMENTS 3. DATA COMMITMENTS Figure 3 illustrates an example system for implementing the embodiments described herein. The system includes a first party (e.g. Alice 103a) and a second party (e.g. Bob 103b). In this example, the system also includes a token issuer 301. However note that not all embodiments relate to a token protocol. In general the token issuer 301 may be thought of as a generator of a first data item or data packet, e.g. a first message or transaction. This example system also includes one or more nodes 104 of a blockchain network 106. However note that some examples do not require a blockchain, and other ledgers may be used instead. Note that "first", "second", etc. are used herein merely as labels and do not imply an ordering unless the context requires otherwise. The token issuer 301 (or data issuer) generates a first data packet. Data packets are also referred to herein as commitment packets. The data packet includes first data (e.g. a digital token, a digital certificate, financial transaction data, an IPv6 packet, a communication, etc.). The first data packet may also include an identifier of a first party or user (e.g. Alice 103a) to whom ownership of the first data is to be transferred or assigned. The identifier may be, for example, a public key associated with Alice 103a. The first data packet also includes a reference to a first entry of a ledger that is controlled by Alice 103a. The ledger may take any suitable form. A requirement is that entries of the ledger, or at least certain fields of the entries of the ledger can only be validly referenced once. For example, the ledger may be a blockchain and the entries may be transactions. Depending on the type of blockchain, the transaction itself can only be validly referenced once, or certain fields (e.g. outputs) of the transactions can only be validly referenced once. The ledger (e.g. blockchain) may be a public ledger or a private ledger. Some examples may involve multiple ledgers and will be discussed below. The data packet may refer to a blockchain transaction controlled by Alice 103a or an output of a blockchain transaction controlled by Alice 103a. Here, "controlled by" means that only Alice 103a can validly reference (e.g. spend) the transaction / output. For instance, the transaction / output may be locked to Alice's public key and require knowledge of the corresponding private key. The first data packet may include a digital signature generated by the data issuer. Any suitable signature scheme may be used, e.g. ECDSA, Schnorr signatures, RSA, etc. In some examples, an ECDSA signature may be used which utilises a curve other than the curve used by some blockchains, such as Bitcoin. The data packet may take any suitable form. In some examples, the data packets include inputs and outputs, similar to those of the blockchain transactions shown in Figure 2. This allows data packets to be linked, if needed. The data issuer 301 then sends the first data packet to Alice 103a. The data packet may be sent in an encrypted form, if required, and / or via a secure communication channel. Alice 103a generates a second data packet. The second data packet is similar to the first data packet. It includes second data, e.g. a second data item. Some of the second data may be the same as the first data. The second data packet may include an identifier (e.g. a public key) of a second party or user (e.g. Bob 103b). The second data packet may be signed by Alice 103a, e.g. using a private key corresponding to Alice's public key included in the first data packet. The second data packet also includes a refence to a ledger entry (e.g. blockchain transaction or blockchain transaction output) controlled by Bob 103b (e.g. locked to Bob's public key). Note that the entry controlled by Bob 103b may be on the same ledger as the entry controlled by Alice 103a, or a different ledger. E.g. the second data packet may reference a transaction recorded on a different blockchain. The second data packet includes a reference to the first data packet. For instance, the first data packet may have a referenceable output, and the second data packet may have an input that references the output of the first data packet. Any suitable refencing technique may be used. Without such a reference, two commitment packets may point to the same ledger entry (e.g. blockchain outpoint) and one would not know which was the single correct previous data packet. Alice 103a generates a commitment of the second data packet. For example, Alice 103a may generate a hash of the second data packet. Any suitable hash function may be used, such as a SHA-based hash function, e.g. SHA256. In some examples, Alice 103a generates an entry of the ledger that includes the commitment and references the first entry of the ledger. She then submits the entry to the ledger. For example, if the ledger is a blockchain, Alice 103a generates a blockchain transaction that references the transaction / output controlled by Alice 103a. For example, the transaction may include an input that refences Alice's output. The transaction (e.g. the input) may include a signature corresponding to Alice's public key. In other examples, the first entry of the ledger is controlled by a service provider (i.e. a third party) rather than by Alice 103a. In these examples, Alice 103a sends the commitment of the second data packet to the service provider who submits a transaction to the blockchain containing the commitment. That is, the service provider manages the interaction with the blockchain. Alice 103a may send the second data packet to Bob 103b, e.g. in encrypted form. Alice 103a may also send the first data packet to Bob 103b, e.g. in encrypted form. Alice 103a may also send a proof of inclusion (e.g. an SPV proof) to Bob 103b which proves that the transaction containing the commitment of the second data packet has been recorded on the ledger. Bob 103b may perform similar actions to Alice 103a in order to transfer data (e.g. ownership of the data, such as a token) to another party, e.g. Charlie. Examples above involved Alice 103a receiving the first data packet from the data issuer 301. Alice 103a may instead receive the first data packet from a different user. That is, one or more data transfers (each involving respective users) may take place before the actions described above as being performed by Alice 103a are performed. In some examples, the first data packet may include more than one references to respective ledger entries. In these examples, generates a data packet for each reference and a corresponding commitment. Alice 103a may generate a single ledger entry (e.g. blockchain transaction) that includes a respective commitment of each commitment. Alternatively, Alice 103a may generate a different ledger entry for each respective commitment. Alice 103a submits the entry / entries to the ledger. Alice 103a may instead send the commitments to the service provider who creates the necessary transaction(s). 1.1 Commitment Cash This section describes an example token framework that uses the embodiments described above, and that is flexible and interoperable. It uses existing blockchain infrastructure in a novel way that maximises privacy. Rather than enforcing token transfer logic, the framework makes minimal use of the blockchain for double-spend protection only. The framework is described as being implemented using a UTXO-based blockchain, but it can be applied to any public or private ledger. In fact, tokens can hop between blockchains (ledgers) without the need of a trusted third party or cross-chain bridge. The token solution can be deployed immediately on almost any blockchain without modification. CBDCs are considered as a motivating use case. 1.1.1 Preliminaries This section discusses token issuance and proof of provenance, before explaining how a blockchain can be used to establish a unique sequence of messages on a private network. 1.1.1.1 Token issuance and proof of provenance Assume that there is a widely recognized token issuer with known public key PKlssuer. A central bank may be the issuer and the tokens may be CBDC. Tokens are issued in a message that is signed by PKIssuer. This message may contain the number of tokens issued, their denomination, the public key of the recipient, a unique issuance ID, and any other relevant information. Since the issuer is trusted , it is assumed that they will not issue the same tokens to two different recipients. After a token has been issued, when one user receives the token from another user a proof of provenance is required. This means a proof that the token was issued correctly, that all appropriate rules have been followed throughout the lifetime of the token, and that the token has not been double spent. Embodiments described herein are focused on proving a token has not been double spent. Users themselves are left to check that the token ruleset has been correctly applied throughout the lifetime of the token. Note that even if a token is newly minted and then spent only once, embodiments are still applicable as they afford double-spend protection on the single transfer between users. 1.1.1.2 Using blockchain for double-spend protection of private message sequences Blockchain transactions are typically used for payments involving two or more users. However, a blockchain transaction can alternatively be used by a single user to upload data to the blockchain 150. In this case, the transaction can have just one input and one unspendable output containing a data payload. The blockchain tokens in the input are consumed entirely in the form of transactions fees. Such transactions can be used as double-spend protection for sequences of messages. This is done as follows. Assume there exists an unspent blockchain transaction outpoint voutQ that is controlled by Alice 103a. This may have existed for some time and be buried many blocks deep. Suppose at time t = 0 an 'issuer' Isaac 301 creates a message m0 that contains a reference to vout0. Isaac 301 digitally signs this message and gives the message and signature to Alice 103a. At a later time t = 1, Alice 103a creates a new message m1 that contains a reference to m0 and Isaac's signature. This reference may be in the form a hash digest. Alice 103a commits to her message m1 by creating a blockchain transaction Tx± that has an input vouto and a single output that contains a data payload comprising a hash of m1. This output will not be used again and may be unspendable. See Figure 4 which shows transaction Tx± having one input and one unspendable output. The input points to a previous outpoint vout0. The output contains a data payload comprising the hash of a message m1. Alice 103a can now prove to a verifier Bob 103b that the message m, is the unique next message in a sequence of messages starting with m0 signed by Isaac 301. This is because the data integrity and double-spend protection of the blockchain 150 implies that there can be no other message mj embedded in another blockchain transaction Tx[ that also spends vout0. Suppose that in her message m1 Alice 103a also references a blockchain outpoint vou^ that is controlled by Bob 103b. Bob 103b can then create a new message m2 that references m}, and commits to this message in a blockchain transaction Tx2 that spends voutp This process can be iterated indefinitely. The blockchain transactions created by Alice 103a and Bob 103b are completely independent from one another, and no link to the messages m0,m1,m2,... can be inferred by observing the blockchain 150. The next section describes how digital signatures may be embedded into these messages to turn this into a token protocol. 1.1.2 Commitment Cash Protocol The example protocol described in this section enables digital tokens to be transferred using almost any blockchain. Each transfer is made up of two fundamental elements: 1. Commitment packet, 2. Blockchain transaction. A commitment packet is a message digitally signed by the sender (e.g. payer) and sent privately to the receiver (e.g. payee). The message contains the details of the token transfer such as the token amount, payer's signature, and payee's public key. The exact format of a commitment packet and chosen cryptosystem can be customised for each use case. Commitment packets operate on a private network and are not exposed publicly on the blockchain 150. A blockchain transaction is used to provide double-spend protection for a commitment packet. Each blockchain transaction contains at least one input and one (e.g. unspendable) output. In some examples, each transaction contains only one input and one output. Further inputs and outputs may be included, as described further below. It is created by the payer of the commitment packet, but does not require a destination address and contains no information about the payee. Throughout this section, the following actors are considered: • Isaac, the issuer of CBDC tokens, • Alice, a customer, • Bob, a merchant. 5 1.1.2.1 Commitment packets Commitment packets may have inputs, outputs, metadata, and an optional data payload. They are used to transfer tokens and may be characterised by having an output that references a blockchain outpoint. Note that the structure and format of the commitment 10 packet described below is just one example and others are possible. CPID1 Version: 0.1 Asset identifier: Blockland CBDC tokens Data payload: NULL Inputs Outputs Index Input Index Output 0 Previous outpoint: CPlDo\\O Signature: SIGPKa Signature scheme: ECDSA, secp256rl 0 Amount: x Public key: PKB Blockchain outpoint: VOUt-L Table 1. Commitment packet CP^ from Alice to Bob transferring x CBDC tokens. The first field in the metadata specifies the version number, the second field specifies which 15 assets are being transferred, and the third field contains an optional data payload. The input field specifies the output of a previous commitment packet that contains the tokens that will be transferred. This implicitly contains a reference to an unspent blockchain outpoint vout0 that is controlled by Alice 103a. The input field also contains a signature from Alice 103a and the signature scheme used. In this example, Alice uses secp256rl, which is different from secp256kl commonly used in blockchain 150. The final set of fields are the outputs. They contain the amount of tokens to be transferred, Bob's public key, and an unspent blockchain outpoint vou^ controlled by Bob 103b. This may be the same public key to which the output controlled by Bob 103b is locked. Each commitment packet may also have a unique identifier CPID that is the double-hash of the serialized fields. 1.1.2.2 Blockchain transactions When the commitment packet CP± is finalised, Alice 103a creates a blockchain transaction 7¾ with a single input voutQ and one unspendable output containing the hash of the commitment packet CP1. The transaction structure is the same as shown in Table 2 but setting m1 = CP3. Since each commitment packet is private to everyone except Alice 103a and Bob 103b, there is no way for an outside observer to link the blockchain transaction to the commitment packet. Recall that the commitment packet CP^ contains a reference to a previous blockchain outpoint vout0. This is because CP± references a previous commitment packet CP0 which itself references a previous blockchain outpoint. It also contains a reference to the next blockchain outpoint vout^ Therefore, the blockchain transaction Tx1 provides an immutable and unique link between voutQ and vou^ that is known only to Alice 103a and Bob 103b. The relationship between commitment packets and blockchain transactions is displayed in Figure 5. The arrows indicate a record of the object pointed to. The sequence can be iterated indefinitely. The security of the scheme relies only on commitment (e.g. hash) functions and the doublespent protection of blockchain outpoints. The scheme is private in the sense that to an outside observer there is no way to link blockchain transactions to commitment packets or to one-another. 1.1.3 Proof of provenance Bob 103b checks himself that the token ruleset has been applied correctly throughout the lifetime of the token (proof of provenance). He may do this as follows. Bob first checks that CP1 is self-consistent with the token ruleset. For example, he checks that the number of tokens in the output does not exceed the input, and that Alice's digital signature is valid. He then checks that the blockchain transaction Tx± has the correct structure and that it has been accepted onto the blockchain network 106. He is now sure that Alice 103a has correctly assigned him the tokens and they are not double-spent. Bob 103b then asks Alice 103a to send him CP0 so he can check that Alice 103a was the rightful owner of the tokens in the first place. He checks that CPQ issued the correct number of tokens to Alice 103a, that it referenced vout0, and that it contained a valid digital signature from the issuer Isaac 301. If all the above checks pass correctly, then Bob 103b is able to prove the provenance of the tokens and therefore that he is the rightful owner. In this simple example, Bob 103b only needed to check one previous token transfer as the tokens were issued directly to Alice 103a from the issuer 301. This is already enough for many use cases and is the desired architecture outlined in [1] as it allows for enhanced privacy models. However, if further transfers are required, e.g. from Bob to Charlie, then all previous commitment packets and blockchain transactions are to be passed on to Charlie and any subsequent users. If data transfers requirements become too burdensome, or greater privacy is needed, then there are existing solutions that can be applied, such as going back to the issuer 301 to melt and remint the tokens. 1.1.4 User flow Now consider an example interaction of Alice 103a transferring x CBDC tokens to Bob 103b. Setup 1. Isaac 301 issues Alice 103a a commitment packet CPQ assigning x CBDC tokens to her public key PKA. The commitment packet has one output that references a blockchain outpoint vouto that she controls. Exchange 2. Bob 103b creates a blockchain outpoint vout-L that he controls. This can be done a long time in advance and can be buried many blocks deep. 3. Bob 103b sends to Alice 103a the public key PKB that he would like to use to receive the tokens along with his blockchain outpoint vout1. 4. Alice 103a creates a commitment packet CPr assigning the tokens to Bob's public key. She digitally signs the commitment packet. 5. Alice 103a creates a blockchain transaction Tx± that has input voutQ and a single unspendable output containing hash^CP^. She does not yet broadcast the blockchain transaction to the blockchain network 106. 6. Alice 103a sends the commitment packets CP0, CP} and blockchain transaction Tx} to Bob 103b. Finalisation 7. Bob 103b broadcasts the blockchain transaction Txt to the network 106. Once the transaction is accepted, Alice 103a is unable to double-spend the tokens in her commitment packet as they are linked to vout0 which has now been spent. Future transfers 8. If Bob 103b would like to transfer his tokens to Charlie, he can use CP0, CP1, and Tx^ as proof of provenance. An SPV proof may be used by Bob to efficiently prove that Tx± has been published on the blockchain 150. 1.1.5 Extensions 1.1.5.1 Multiple inputs and outputs The commitment packet presented above has just one input and one output. This can be easily extended to include multiple inputs and outputs. Suppose the commitment packet contains two inputs that refer to two blockchain outpoints vout0 and vouto- There are two options. 1. Multiple blockchain transactions. In this case, Alice 103a creates one blockchain transaction for each blockchain outpoint in her inputs. The first transaction will have vout0 as input, and second transaction will have vout'o as input. Both will have one unspendable output containing the hash of the commitment packet. 2. Single blockchain transaction. In this case, one blockchain transaction is created with two inputs voutQ and voutg and a single unspendable output containing the hash of the commitment packet. 1.1.5.2 Outsourcing blockchain transaction management to a service provider As discussed above, there is no third party required in the exchange of tokens. The disadvantage is that the users Alice 103a and Bob 103b have to be able to create and spend blockchain transactions. To avoid this, a service provider may be introduced to manage the blockchain transactions on Alice 103a and Bob's 103b behalf. This service provider has limited scope. It is not involved in, nor has knowledge of, any token transfers. Its only role is to manage blockchain outpoints and create transactions with a hash digest given to them by their users. If a service provider is compromised, the worst consequence is that they can prevent a token transfer from taking place. The service provider does not have the ability to re-allocate tokens themselves. 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 "Tx”. 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 "Txo" in Figure 2. Tat? 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, UTXOoin the output 203 of / ^comprises a locking script [Checksig Pa] which requires a signature Sig Pa of Alice in order for UTXOoto 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. SIDE CHANNEL As shown in Figure 1, 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 107 with Bob 103b (at the instigation of either party or a third party). The side channel 107 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 152 between Alice and Bob without the transaction (yet) being registered onto the blockchain network 106 or making its way onto the chain 150, until one of the parties chooses to broadcast it to the network 106. 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 107 may be used to exchange any other transaction related data, such as keys, negotiated amounts or terms, data content, etc. The side channel 107 may be established via the same packet-switched network 101 as the blockchain network 106. Alternatively or additionally, the side channel 301 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 107 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 106. 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 107. 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 107, 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. 5. 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 of committing to an exchange of data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a first user and comprises: obtaining a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of a first ledger controlled by a first user or a service provider; generating a second data packet, wherein the second data packet comprises second data, a reference to the first data packet, and a reference to a second entry of a second ledger controlled by a second user or the service provider; generating a commitment of the second data packet; and submitting a third entry of the first ledger to the first ledger, wherein the third entry of the ledger references the first entry of the ledger, and wherein the third entry comprises the commitment of the second data packet, or sending the commitment of the second data packet to the service provider for submitting the third entry of the first ledger to the first ledger. The commitment may take any suitable form. For instance, the commitment may be a hash of data packet. Other commitments may be used, such as Pederson commitments. Statement 2. The method of statement 1, comprising sending the first data packet and / or the second data packet to the second user. Statement 3. The method of statement 1 or statement 2, comprising sending, to the second user, a proof of inclusion proving that the third entry is included on the ledger. Each ledger may be a public ledger or a private ledger. Some ledgers may be the same or different. In some examples there is a single ledger, i.e. the first and second ledger are the same ledger. Statement 4. The method of any preceding statement, wherein the ledger is a blockchain, and wherein each entry of the ledger is a blockchain transaction. Statement 5. The method of statement 4, wherein the blockchain is an output-based blockchain, and wherein each respective reference to a respective entry of the ledger comprises a respective outpoint of the respective entry. Statement 6. The method of any preceding statement, wherein the first data packet comprises a respective identifier of the first user and / or wherein the second data packet comprises a respective identifier of the second user. Statement 7. The method of statement 6, wherein each respective identifier of the respective user comprises a respective public key. Statement 8. The method of statement 6 and statement 7, wherein the respective entry controlled by the respective user is controlled by the respective public key of the respective user. The first and second ledgers may be different ledgers. For instance, The first and second ledgers may be different blockchains. Alternatively, the first and second ledgers are the same ledger, e.g. the same blockchain. Statement 9. The method of any preceding statement, wherein the first data packet comprises third data and a reference to a fourth entry of the first or second ledger controlled by the first token user, and wherein the method comprises: generating a third data packet, wherein the third data packet comprises fourth data, a reference to the first data packet, and a reference to the fourth entry of the first or second ledger controlled by a third user or the service provider; generating a commitment of the third data and wherein: a) the method comprises: submitting a fifth entry of the first or second ledger to the first or second ledger, wherein the fifth entry of the ledger references the fourth entry of the first or second ledger, and wherein the fifth entry comprises the commitment of the third data packet, or sending the commitment of the third data packet to the service provider for submitting the fifth entry of the first or second ledger to the first or second ledger; or b) wherein the third entry of the ledger references the fourth entry of the first or second ledger, and wherein the third entry comprises a commitment of the third data packet; and sending the third data packet to the third user. Statement 10. The method of any preceding statement, wherein each respective data comprises token data relating to a token protocol. Statement 11. The method of any preceding statement, wherein generating a respective data packet comprises signing the respective data packet. Statement 12. The method of statement 4 and statement 11, wherein signing the respective data packet comprises signing the respective data packet with a respective signature of a signature scheme other than a signature scheme used to validly sign transactions of the output-based blockchain. Statement 13. A computer-implemented method of committing to the generation of first data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a data issuer and comprises: generating a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of the ledger controlled by the first user or a service provider; and sending the first data packet to the first user. Statement 14. 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 13. 5 Statement 15. 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 13. According to another aspect disclosed herein, there may be provided a method comprising 10 the actions of the first user and the second user. According to another aspect disclosed herein, there may be provided a system comprising the computer equipment of the first user and the second user. According to another aspect disclosed herein, there may be provided a method comprising 15 the actions of the token issuer and the first and / or second token user. According to another aspect disclosed herein, there may be provided a system comprising the computer equipment of the token issuer and the first / and or second token user.

Claims

1. A computer-implemented method of committing to an exchange of data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a first user and comprises:obtaining a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of a first ledger controlled by a first user or a service provider;generating a second data packet, wherein the second data packet comprises second data, a reference to the first data packet, and a reference to a second entry of a second ledger controlled by a second user or the service provider;generating a commitment of the second data packet; andsubmitting a third entry of the first ledger to the first ledger, wherein the third entry of the ledger references the first entry of the ledger, and wherein the third entry comprises the commitment of the second data packet, or sending the commitment of the second data packet to the service provider for submitting the third entry of the first ledger to the first ledger.

2. The method of claim 1, comprising sending the first data packet and / or the second data packet to the second user.

3. The method of claim 1 or claim 2, comprising sending, to the second user, a proof of inclusion proving that the third entry is included on the ledger.

4. The method of any preceding claim, wherein the ledger is a blockchain, and wherein each entry of the ledger is a blockchain transaction.

5. The method of claim 4, wherein the blockchain is an output-based blockchain, and wherein each respective reference to a respective entry of the ledger comprises a respective outpoint of the respective entry.

6. The method of any preceding claim, wherein the first data packet comprises a respective identifier of the first user and / or wherein the second data packet comprises a respective identifier of the second user.

7. The method of claim 6, wherein each respective identifier of the respective user comprises a respective public key.

8. The method of claim 6 and claim 7, wherein the respective entry controlled by the respective user is controlled by the respective public key of the respective user.

9. The method of any preceding claim, wherein the first data packet comprises third data and a reference to a fourth entry of the first or second ledger controlled by the first token user, and wherein the method comprises:generating a third data packet, wherein the third data packet comprises fourth data, a reference to the first data packet, and a reference to the fourth entry of the first or second ledger controlled by a third user or the service provider;generating a commitment of the third data and wherein:c) the method comprises: submitting a fifth entry of the first or second ledger to the first or second ledger, wherein the fifth entry of the ledger references the fourth entry of the first or second ledger, and wherein the fifth entry comprises the commitment of the third data packet, or sending the commitment of the third data packet to the service provider for submitting the fifth entry of the first or second ledger to the first or second ledger; ord) wherein the third entry of the ledger references the fourth entry of the first or second ledger, and wherein the third entry comprises a commitment of the third data packet; andsending the third data packet to the third user.

10. The method of any preceding claim, wherein each respective data comprises token data relating to a token protocol.

11. The method of any preceding claim, wherein generating a respective data packet comprises signing the respective data packet.

12. The method of claim 4 and claim 11, wherein signing the respective data packet comprises signing the respective data packet with a respective signature of a signature scheme other than a signature scheme used to validly sign transactions of the output-based blockchain.

13. A computer-implemented method of committing to the generation of first data using one or more ledgers, wherein each ledger comprises a plurality of entries, wherein each entry can only be validly referenced a single time by a later entry of the respective ledger, wherein the method is performed by a data issuer and comprises:generating a first data packet, wherein the first data packet comprises first data, and a reference to a first entry of the ledger controlled by the first user or a service provider; andsending the first data packet to the first user.

14. 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 13.

15. 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 13.

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