Committing to undetermined data

Abstract

Computer-implemented methods of committing to and verifying data comprising a determined component and one or more undetermined components. A commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output. A committing party: obtains, for at least one undetermined component, a commitment transaction that references the blockchain transaction and / or the blockchain transaction output and includes the undetermined component and / or an obfuscated version of the undetermined component; and causes the commitment transaction to be submitted to one or more blockchain nodes of a blockchain network. A verifying party: obtains, for at least one undetermined component, the commitment transaction; verifies that the commitment transaction references the blockchain transaction and / or the blockchain transaction output; obtains a proof of inclusion for the commitment transaction; and uses the proof of inclusion to verify that the commitment transaction is recorded on the blockchain.

Description

[0001] COMMITTING TO UNDETERMINED DATA

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method of committing to undetermined data and / or to a method of controlling who can provide the undetermined data, once determined, a method of providing the undetermined data, a method of determining the undetermined data, and a method of verifying the undetermined data.

[0004] BACKGROUND

[0005] It is often useful to create an unalterable record of a piece of data at a certain point in time. This provable data integrity can be achieved by cryptographically committing the data. Two useful types of commitment are hash functions and digital signatures. These commitments can either be stored in a secure location or published on mediums such as a blockchain or a national newspaper.

[0006] SUMMARY

[0007] A problem arises if a party wishes to make a commitment but they do not know all of the data. Suppose that some (or all) of the data is undetermined at the time t = 0 of the commitment but will be given later at time t = 1. As an example, consider the birth of a baby who hasn't been given a name yet. The hospital would like to create an official certificate of birth. Some of the data is already known, such as the date and time of birth, the hospital name, the country, and the parent's names. But there is one important piece of information that is not yet determined: the baby's name.

[0008] According to one aspect disclosed herein, there is provided a computer-implemented method of committing to data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a committing party and comprises: for at least one undetermined component: obtaining a respective commitment transaction that references the respective blockchain transaction and / or the respective blockchain transaction output and includes the undetermined component and / or an obfuscated version of the undetermined component; and causing the respective commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

[0009] According to one aspect disclosed herein, there is provided a computer-implemented method of verifying data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a verifying party and comprises: for at least one undetermined component: obtaining a respective commitment transaction comprising the undetermined component and / or an obfuscated version of the undetermined component; verifying that the respective commitment transaction references the respective blockchain transaction and / or the respective blockchain transaction output; obtaining a respective proof of inclusion for the respective commitment transaction; and using the respective proof of inclusion to verify that the respective commitment transaction is recorded on the blockchain.

[0010] According to another aspect disclosed herein, there is provided a computer-implemented method of determining data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a determining party and comprises: obtaining the commitment; obtaining the determined component; obtaining the one or more undetermined components from the respective blockchain transactions; and determining the data based on the one or more undetermined components.

[0011] Also described is a computer-implemented method of committing to access control of data, wherein the data comprises a determined component and one or more undetermined components, wherein the method is performed by an access control party and comprises: generating a commitment based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output; and publishing and / or providing the commitment to one or more parties.

[0012] Embodiments of the present disclosure allow for meaningful commitments of data to be created even if some of the data is undetermined. This is achieved using the blockchain by nominating a transaction or unspent transaction output (UTXO) at the time of the commitment, and adding the data at a later time when the transaction or UTXO is spent. Effectively, the technique allows a party to commit a data reference (where the data is or will be), instead of the data. The double spend protection and immutability of the blockchain ensures that this data is uniquely determined at the later time. The same technique may be used for updating the data. For example, an individual's address or name might change on their verifiable credentials such as their passport or driving license.

[0013] Embodiments of the present disclosure provide some or all of the following advantages. The double-spend protection of the blockchain ensures that undetermined data can have only one finalised state. The embodiments are compatible with existing signature and commitment algorithms, and are private in the sense that anyone observing the blockchain will learn nothing about the data unless it is intentionally exposed. Blockchain transaction sizes are minimal and of constant form. This keeps transaction fees low and predictable. No complex smart contract logic is needed. A service provider may be used so that users do not need to sign blockchain transactions nor hold tokens. The blockchain provides an immutable timestamp of when the data was finalised. To validate the finalised data, a verifier does not need to be a participant in a blockchain network.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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:

[0016] Figure 1 is a schematic block diagram of a system for implementing a blockchain,

[0017] Figure 2 schematically illustrates some examples of transactions which may be recorded in a blockchain, Figure 3 is a schematic block diagram of an example system for implementing embodiments of the present disclosure,

[0018] Figure 4 is a schematic diagram of an undetermined data commitment using a transaction outpoint vout_0, whereby the finalised data is provided in a spending transaction at a later time,

[0019] Figure 5 schematically illustrates the use of SPV proofs to show that transactions Tx_0 and Tx_l have been published on the blockchain,

[0020] Figure 6 schematically illustrates using a transaction chain to update data, where each transaction has one input and one output,

[0021] Figure 7 schematically illustrates a Pedersen commitment being used to link the finalised data to the blockchain transaction,

[0022] Figure 8 schematically illustrates a transaction that provides an electronic signature as the finalised data, and

[0023] Figure 9 schematically illustrates a transaction that provides a person's name as the finalised data.

[0024] DETAILED DESCRIPTION OF EMBODIMENTS

[0025] 1. COMMITMENTS OF UNDETERMINED DATA

[0026] Figure 3 shows an example system 300 for implementing the embodiments described herein. The system 300 includes an access control party 301, a committing party 302, a verifying party 303 and a determining party 304. Each party is shown separately but in some examples one or more parties may be combined as a single party. Each party is configured to communicate with one, some or all of the other parties via respective (direct or indirect) connections. Only some of the possible connections are shown in Figure 3 for simplicity. Each party operates respective computer equipment. For example, each party may operate the computer equipment described below as being operated by Alice 103a and Bob 103b. Moreover, each party may be configured to perform any or all of the actions described below as being performed by Alice 103a and / or Bob 103b. The system 300 also includes one or more blockchain nodes 104 or a blockchain network 106. At least some of the parties are configured to communicate with the blockchain network 106 via respective (direct or indirect) connections. Whilst all parties are shown as being connected to the blockchain network 106 in Figure 3, this is not essential in all examples. The access control party 301 is configured to generate a commitment of data that includes one or more undetermined (e.g. unknown, missing, etc.) components. That is, at the time of generating the commitment, the data is incomplete. For example, the data may be missing a signature, or there may be multiple options to choose from for a given field of the data (e.g. a date) and the option has not been chosen or decided upon. The missing data may not be knowable until a future date. For instance, the missing data may be an application number of a patent application that is to be assigned once filed.

[0027] The access control party 301 generates the commitment based on (i.e. as a function of) the determined component (i.e. known part) of the data. The commitment is also based on a blockchain transaction (e.g. a transaction identifier (TxID) of a blockchain transaction) and / or an unspent output (UTXO) of a blockchain transaction (e.g. a transaction identifier and output index of a blockchain transaction). In some examples, the commitment is also based on a salt value. For simplicity, UTXO will be used below to mean "identifier of a transaction output".

[0028] The commitment may be generated by hashing the determined component and the TxID and / or UTXO (and optionally the salt). In this case the commitment is (or at least comprises) a hash digest. For example, the inputs to the hash function may be combined (e.g. by concatenation) before hashing. The hash function may be a SHA-based hash function, such as SHA256. A HMAC function may be used as the hash function. In some examples, the determined component and the TxID and / or UTXO (and optionally the salt) may form leaves of a hash tree (e.g. a Merkle tree). The hash digest is thus a root of a hash tree.

[0029] The commitment may instead by generating a Pederson commitment based on the determined component and the TxID and / or UTXO (and optionally the salt). Other types of zero-knowledge proofs may be used to generate the commitment.

[0030] The commitment may also be generated based on metadata relating to the undetermined component of the data. For example, the metadata may specify one or more properties of the undetermined data, e.g. a type of data (such as a value, an integer, a string, etc.), a length of the data (e.g. in bytes), a description of the data, an identifier of an intended supplier of the data, etc.

[0031] So far only a single undetermined component has being considered. More generally, the data may have multiple undetermined components, e.g. a name and a signature. One or more of the undetermined components may be associated with a single transaction and / or transaction output. One or more of the undetermined components may be associated with a different transaction and / or transaction output. In the case of multiple undetermined components, the commitment is based on the determined component and each transaction and / or UTXO associated with an undetermined component. For example, if there are three undetermined components, each associated with a different UTXO, the commitment is based on three UTXOs. If instead two of the three UTXOs are associated with the same UTXO, then the commitment is based on two UTXOS - one for one of the undetermined components and one for the other two undetermined components.

[0032] The access control party 301 may publish the commitment. For example, the commitment may be published on the blockchain 150. The access control party 301 may send the commitment to the blockchain network 106 to be recorded on the blockchain 150, or the access control party 301 may send the commitment to an intermediate party who then sends the commitment to the blockchain network 106.

[0033] Additionally or alternatively, the access control party 301 may send the commitment to one or more parties, e.g. the committing party 302, the verifying party 303 and / or the determining party 304.

[0034] As mentioned, each undetermined component is associated with a transaction and / or UTXO. Each transaction and UTXO may be controlled by a party. For example, the transaction or UTXO may be locked to a public key associated with a party. Each transaction or UTXO may be controlled by the same party. Alternatively, each transaction or UTXO ,may be controlled by a different party. In some examples, a given party may control multiple ones of the transactions and / or UTXOs. In some examples, the access control party 301 may control one or more of the transactions and / or UTXOs. A given transaction and / or UTXO may be controlled by multiple different parties, e.g. by being locked to multiple public keys. For instance, a UTXO may be controlled by the access control party 301 and the committing party 302.

[0035] The access control party 301 may sign the commitment, e.g. using an ECDSA signature. The signature may be signed using a private key corresponding to a public key known to be associated with the access control party 301. For example, the public key may be a certified public key issued to the access control party 301. The access control party 301 may published the signature, e.g. on the blockchain. For example, the commitment and signature may be included in the same transaction. The access control party 301 may send the signature to one or more parties, e.g. the committing party 302, the verifying party 303 and / or the determining party 304. The signature proves that the commitment was made by the access control party 301.

[0036] The access control party 301 may obtain a proof of inclusion (e.g. an SPV proof) of each transaction that the commitment is based on, i.e. one proof per transaction. The access control party 301 may request the proofs from a blockchain node 104. It is also not excluded that the access control party 301 may generate the proofs. The access control party 301 may publish the prods and / or send them to one or more parties, e.g. the committing party 302, the verifying party 303 and / or the determining party 304.

[0037] The determined component of the data may be known to one or more parties, e.g. the committing party 302. If not, the access control party 301 may send the determined component of the data to one or more parties, e.g. the committing party 302, the verifying party 303 and / or the determining party 304. The determined component may be published by the access control party 301, e.g. on the blockchain 150. For example, the determined component may be included in the same transaction as the commitment (and optionally, the signature that signs the commitment).

[0038] The committing party 302 is configured to provide, or at least commit, to one or more of the undetermined components. Examples will be described in terms of the committing party providing / committing to one undetermined component, but more generally the committing party 302 may provide / commit to any or all of the undetermined components.

[0039] The committing party 3012 obtains a commitment transaction that references the transaction or UTXO associated with an undetermined component of the data. The commitment transaction includes (e.g. in an output of the transaction) the undetermined component and / or a commitment of the undetermined component. To avoid confusion, the "commitment of the undetermined component" will be referred to as an "obfuscated version of the undetermined component", or simply, an "obfuscated component". The obfuscated component may be generated by hashing and / or encrypting the undetermined component. The obfuscated component may be a Pederson commitment (or other type of zero-knowledge proof) of the undetermined component. The obfuscated component may be based on the commitment (i.e. the commitment generated by the access control party 301) and / or a salt value. The commitment transaction may comprise the commitment generated by the access control party 301.

[0040] The committing party 302 may generate the commitment transaction, or receive the commitment transaction from a different party. The commitment party 302 may generate the undetermined component or receive the undetermined component from a different party. The commitment party 302 may generate the obfuscated component or receive the obfuscated component from a different party.

[0041] The committing party 302 submits the commitment transaction to the blockchain network 106. Alternatively, the committing party 302 may send the commitment transaction to a different party (e.g. the access control party 301) who then sends the commitment transaction to the blockchain network 106. Once recorded on the blockchain 150, the commitment transaction immutably fixes the (previously) undetermined component.

[0042] The transaction or UTXO referenced by the commitment transaction may be locked to a public key of the committing party 302. The committing party 302 may sign the commitment transaction using a private key corresponding to that public key. The referenced transaction or UTXO may be locked to multiple public keys (e.g. a public key controlled by the access control party 301), in which case the committing party 302 may send the commitment transaction to those parties to be signed, or receive the commitment transaction having already been signed by those parties.

[0043] In some examples, prior to generating the commitment transaction, the committing party

[0044] 302 may verify that the referenced transaction is recorded on the blockchain 150. The committing party 302 may obtain and use a proof of inclusion to check whether the referenced transaction is on the blockchain 150. The committing party 302 may receive the proof (e.g. an SPV proof) from the access control party 301.

[0045] Similarly, the committing party 302 may obtain a proof of inclusion (e.g. an SPV proof) of the commitment transaction being recorded on the blockchain 150. The proof of inclusion may be received from a blockchain node 104 or another party, or the committing party 302 may generate the proof. The committing party 302 may send the proof to one or more parties, e.g. the access control party 301, the verifying party 303 and / or the determining party 304.

[0046] In some examples, the committing party 302 may update the previously undetermined component. To do so, the committing party 302 obtains an updated commitment transaction which includes an updated version of the previously undetermined component, and causes the updated commitment transaction to be submitted to the blockchain network 106. The commitment transaction may be locked to a public key controlled by the committing party 302, and the updated commitment transaction may contain a signature corresponding to the public key. For instance, a UTXO of the commitment transaction may be locked to the committing parties' public key.

[0047] The verifying party 303 is configured to verify the commitment of one or more undetermined components of the data. Examples will be described in terms of the verifying party 303 verifying one undetermined component, but more generally the verifying party

[0048] 303 may verify any or all of the undetermined components.

[0049] For a given undetermined component of the data, the verifying party 303 obtains the commitment transaction that commits to that undetermined party, i.e. the commitment transaction that contains the previously undetermined (and now determined) component or the obfuscated version thereof. The verifying party 303 may receive the commitment transaction from one of the parties, e.g. the committing party 302, or from a blockchain node 104. The verifying party 303 verifies that the commitment transaction references the transaction and / or UTXO associated with the undetermined component, i.e. the transaction and / or UTXO upon which the commitment generated by the access control party 301 is based.

[0050] The verifying party 303 verifies that the commitment transaction is recorded on the blockchain 150. The verifying party 303 may obtain and use a proof of inclusion to check whether the commitment transaction is on the blockchain 150. The verifying party 303 may receive the proof (e.g. an SPV proof) from one of the parties, e.g. the committing party 302.

[0051] The verifying party 303 may obtain a public key associated with a party that is responsible for providing the undetermined component, e.g. the committing party 302. The verifying party also obtains a signature and verifies that the signature signs a message that includes the undetermined component or the obfuscated version, and that the signature is valid for the obtained public key. The signature may be included in the commitment transaction.

[0052] The verifying party 303 may obtain the commitment and the determined component, e.g. from the access control party 301 or the committing party 302. The verifying party 303 may verify that the commitment is a commitment of the determined component and the undetermined component as extracted from the commitment transaction. For example, the verifying party 303 may check that a hash of the determined component and the undetermined component is the same as the commitment. In some examples, the verifying party 303 may verify that the undetermined component satisfies one or more conditions stipulated in metadata which forms the commitment.

[0053] The determining party 304 is configured to determine the complete data. The determining party 304 obtains the commitment and the determined component. The commitment and determined component may be sent to the determining party 304 by one of the parties, e.g. the access control party 301, the committing party 302, and / or the verifying party 303. The determining party 304 also obtains the one or more undetermined components of the data from the respective commitment transactions. The determining party 304 may obtain the commitment transactions from the blockchain 150 or from one of the parties, e.g. the committing party 302. The determining party 304 uses the determined component and the undetermined component(s) to determine the whole data. That is, the determining party 304 completes the data using the undetermined component(s).

[0054] The determining party 304 may perform an action in response to, or based on, the completed data. For example, the completed data may be a contract, that once completed allows the determining party 304 to act according to the terms of the contract. The completed data may be a network address, which is used by the determining party to send messages. The completed data may be an identifier of another party (e.g. the committing party 302) and the determining party 304 may grant access to a resource the party based on the identifier.

[0055] The access control party 301 may perform any action described as being perform by any of the committing party 302, the verifying party 303 and the determining party 304. The committing party 302 may perform any action described as being perform by any of the access control party 301, the verifying party 303 and the determining party 304. The verifying party 303 may perform any action described as being perform by any of the access control party 301, the committing party 302 and the determining party 304. The determining party 304 may perform any action described as being perform by any of the access control party 301, the committing party 302, and the verifying party 303.

[0056] 1.1 Commitments of Undetermined Data - Further Examples

[0057] This section provides further examples of the embodiments described above. Some features are optional.

[0058] Throughout this section we will consider a scenario with two time frames: an early time t = 0 and a later time t = 1. One can imagine the second time as being one day after the first time, for example. We will consider a message m that is split into a part mxthat is known for all time (t = 0 and t = 1), and a part m2that is not yet known at time t = 0 but is determined to be the value d at the later time t = 1.

[0059] We would like to make a commitment Comm of some data m at time t = 0 where some of the data m2<= m is not known. This commitment could be the hash of the data, for example.

[0060] At time t = 0, there exists a transaction Tx0on the blockchain containing an output voutQ. When we make the data commitment Comm, instead of waiting for the undetermined data we instead record the output voutQas a placeholder for the undetermined data. One can think of this as setting m2= voutQat time t = 0, and therefore Comm = Comm(m1,vout0). As a simple example, we can set Comm = hash^m^ | | voitt0).

[0061] At a later time t = 1, the data is settled to a finalised value d. A transaction Tx is created that spends voutQand has an (unspendable) output containing a hash of the finalised data. A schematic diagram of transactions Tx0and Tx is given in Figure 4.

[0062] Transaction Tx is broadcast to the blockchain network 106 and published in a block. The data is now completely fixed. We can think of this as setting m2= d at time t = 1. The double-spend protection of the blockchain 150 means there is a unique link from the commitment Comm to the data record in Tx at time t = 1.

[0063] If required, the data record in Tx may contain a pointer to the commitment Comm. For example, the output of Tx may be modified to be OP_RETURN hash(d | | Comm) or OP_RETURN HMAC(Comm, d). This means there is a unique link linking the data record in Tx at time t = 1 to the commitment Comm.

[0064] The spending of the output voutQmay be controlled by the data owner, service provider, or some other party. Whilst this party can control what this data record is, they can never provide more than one data record, and the record they provide will exist forever on the blockchain. To keep things simple, we may assume that the vout0is embedded many blocks deep in the blockchain 150. Such outpoints may be created at time t = — 1 long before the commitment Comm. The outpoints may be provided by a dedicated service provider.

[0065] The locking script of voutQmay be configured to completely fix the structure of Tx . For example, it may fix that Tx has precisely one input and one unspendable output.

[0066] A salt may be included in the hash preimage to increase privacy, if required.

[0067] If a signature of the data m is required at time t = 0, this signature may sign the commitment.

[0068] 1.1.1 SPV proofs for efficient data verification

[0069] Simplified Payment Verification (SPV) is a method to prove that a transaction has appeared on a blockchain without having access to all block data. Instead, an SPV proof requires a verifier to have access to the latest chain of block headers and a Merkle proof of inclusion of the transaction in a block. As of now, the chain of block headers in Bitcoin is approx. 64 MB in size, and a transaction Merkle proof is less than 1 kB. The only cryptographic primitive involved in an SPV proof is a hash function, which means that it does not rely on the trust or security of public key cryptography. These features make SPV a very useful and efficient protocol.

[0070] In the method presented above, recall that at time t = 0 we require a transaction output voutQthat has been published many blocks deep in the blockchain 150. An SPV proof of the transaction Tx0that contains voutQmay be given to whomever is making a commitment or signature of the undetermined data. They will then be convinced that voutQis indeed a valid blockchain output.

[0071] At time t = 1, the data is finalised and recorded in a transaction on the blockchain 150. If a third party verifier wants to validate the data, they may be given transaction Tx0and Tx and an SPV proof of Tx . The verifier may explicitly check the SPV proof of Tx is valid and that Tx spends the output of Tx0. (Note that no additional SPV proof of Tx0is required.) A schematic diagram of the elements of the SPV proofs of Tx0and Tx at different times is given in Figure 5.

[0072] Using SPV proofs, the method presented above may be validated by a third party even if they are not a participant of a blockchain network. They just have to be given an up-to-date list of block headers and an SPV proof of Tx .

[0073] The information available at the two different time points is summarised inTable 1 below.

[0074] Table 1: Information at different points in time

[0075] 1.1.2 Updating data

[0076] The method may be extended to updates of data. In this case we may use a transaction chain to specify how the data is updated. In a transaction chain each transaction has one input corresponding to the previous transaction, and on output specifying the next transaction. In the simplest example of a transaction chain, each transaction has just one input and one output as shown in Figure 6.

[0077] Alternatively, each data element may nominate the next UTXO to be used to update the data.

[0078] 1.1.3 Merkle trees for privacy and efficiency Recall that the data is split into a determined part and an undetermined part m = (m1,m2), and at time t = 0 we have m2= voutQ. It may be useful to structure these two parts as leaves of a Merkle tree, and to use the Merkle root as the commitment Comm = hashf hashf m ) 11 hashf voutQ) ) .

[0079] This is useful because the determined part and the undetermined parts can easily be verified against the commitment without knowledge of one another. This is more efficient for large data sizes and also increases privacy. It is also extensible to include multiple different determined parts and multiple individual undetermined parts.

[0080] If a digital signature of the data is required at time t = 0, then this signature may sign the Merkle root Comm.

[0081] 1.1.4 Pedersen commitments for maximum privacy

[0082] Rather than using a hash function to link the finalised data to transaction Txltwe may instead use a Pedersen commitment to increase privacy. The issue with using a hash function is that anyone with access to the hash preimage (and salt if used) can verify the timestamped record of the finalised data on the blockchain. If this is shared with an unauthorised party then this unauthorised party may verify the link. This presents a privacy challenge.

[0083] To overcome this, techniques such as Pederson commitments may be used to allow the link between the data and the blockchain record to only be verifiable by authorised parties.

[0084] These authorised parties are called designated verifiers and the link is verified using a zeroknowledge proof. Since this proof is linked to a designated verifier's public key it can only be verified by them.

[0085] Recall that the original data commitment Comm contains a reference to voutQ. Instead of using a hash function, this commitment could instead contain a Pedersen commitment. For example, we could set Comm = Pedersen^ m 11 voitt0). This gives the user control of who can verify that voutQis related to the commitment. 1.1.5 Example Step-by-Step Workflow

[0086] Suppose Alice 103a would like Bob 103b to cryptographically sign a message m consisting of and m2, where m2is a placeholder for some data that is either unknown at the time of signing or private to Alice or Bob.

[0087] The most intuitive template is to have m2= voutQ, a transaction outpoint as a reference for the intended data. The value of voutQmay be as small as the dust value, which is 1 satoshi at the time of writing. The locking script of voutQmay be constructed to provide secure access control over who can provide the intended data. For example, a P2PKH locking script for a public key of Bob's implies that only Bob can provide the intended data, while a multi-sig locking script will make sure that the given data in the spending transaction is approved by all required parties. Without loss of generality, assume that Daisy is expected to provide data. The locking script will be P2PKH with PKDaisy. There are a few options in terms of implementation:

[0088] 1. Daisy can include the intended data in the spending transaction of voutQ.

[0089] 2. Daisy can include a commitment or hash of the data in the spending transaction and present the data separately off-chain to a verifier.

[0090] 3. Daisy can delegate the right to another entity by spending voutQto the intended entity.

[0091] It is also possible to use PUSHTX or an equivalent to enforce only one or more but not all options for Daisy.

[0092] Another template is to have m2= metaData | \voutQ, where metaData may be used to specify what data one should expect from the spending voutQ. A few examples of metaData is given below.

[0093] Table 1: possible restrictions to be included in metadata

[0094] This approach is particularly useful when the data is unknown to the signer Bob. By adding enough restrictions in metaData, Bob can be sure that he did not sign anything unexpected or unintended.

[0095] As a verifier, Charlie is given m,SigBob, PKBob,Tx-iand the Merkle proof of Tx .

[0096] 1. Charlie verifies SigBobon m with respect to PKBob.

[0097] 2. Charlie parses m as m and m2and extracts metaData and voutQfrom m2.

[0098] 3. Charlie verifies the Merkle proof of Tx and checks that Tx spends vout0.

[0099] 4. Charlie extracts the data payload d from Tx and checks that all restrictions in metaData are not violated.

[0100] Assuming all steps pass successfully, then the full message is represented by m and d.

[0101] If only the hash or the commitment of the data is given in the spending transaction, then Charlie is expected to receive d as an input and verifies its integrity against the hash value or the commitment.

[0102] If the data can be updated or delegated as specified in the metaData, then Charlie should trace the spending chain of voutQto the latest transaction. An alternative for Charlie is to take the entire transaction chain in transaction IDs and the Merkle proof for the latest transaction as part of the inputs, then query the spending status of the latest transaction to ensure that its outputs are still unspent.

[0103] 1.1.6 Use Cases

[0104] In this section we provide four use cases for the protocol for committing to undetermined data.

[0105] 1. Document signing 2. Digital certificates

[0106] 3. Associating IPv6 addresses with identity

[0107] 4. Decentralised identifiers (DIDs)

[0108] The first three use cases all involve digital signatures over undetermined data. The fourth use case (DIDs) does not involve signatures. Instead, we consider a blockchain commitment with some data undetermined.

[0109] 1.1.6.1 Document signing

[0110] Any reference to "document" may be replaced with the term "message" or "data item". The document, message or data item may be human-readable or non-human readable. The document, message or data item may be user generated or computer generated.

[0111] Let eSig refer to an electronic signature (not cryptographic) and dSig refer to a cryptographic digital signature.

[0112] We will consider a scenario where there is a document m1that requires an eSig and a dSig. It will be useful to have in mind the example below.

[0113] It is important that the dSig signs both the and eSig so that they have integrity (i.e. they cannot be changed in the future). We begin our scenario at time t = 0 with a document but no eSig or dSig. We are then asked to provide a dSig without having the eSig. This is a problem because we need the dSig to sign the eSig so that the eSig has integrity.

[0114] Instead of waiting for the eSig, we can use the method described above. We can treat eSig as undetermined data and sign a placeholder for the eSig. According to the method, this placeholder will be a transaction outpoint voutQ.

[0115] Concretely, at t = 0 we gather together the document m and the placeholder voutQinto a Merkle tree and calculate the root root = hash ) 11 hash voutQ) ).

[0116] The dSig then signs this root dSig(root).

[0117] At this point the digital signature can be verified against the document mi. At a late time t = 1, the electronic signature can be provided by the user. A hash of this is embedded in a transaction that spends voutQ.

[0118] Despite the eSig being created afterwards, the dSig is still linked uniquely to the eSig through the spending of voutQand the double-spend protection of the blockchain. An example transaction is shown in Figure 7.

[0119] 1.1.6.2 Digital certificates

[0120] Digital certificates such as X.509 are a collection of attributes that are attested to using a digital signature. In the current state of the art, all attributes must be known at the point of signing. Using the methods presented herein, we may consider a case where one of the attributes is unknown. Let us revisit the birth certificate example given in the introduction. The certificate will be given as follows.

[0121] Table 2: Example of digital certificate with an undetermined attribute in position four.

[0122] The digital certificate is complete in the sense that it has a signature that attests to the correctness of the attributes. However, the baby's name is attribute position 4 is as yet undetermined. At a later time, the parents decide to name their baby Charlie. A blockchain transaction Tx is created containing a hash of this name. It is broadcast to the blockchain network and so is completely fixed.

[0123] Since the name Charlie is easy to guess, salt is added so that the baby's name is private to anyone looking at the blockchain. An example transaction is shown in Figure 8.

[0124] 1.1.6.3 Associating IPv6 addresses with Identity

[0125] Internet Protocol version 6 (IPv6) is the latest version of the communication protocol used to locate and communicate with the global system of interconnected devices we call the internet. Addresses in IPv6 have 128-bits and have the format: 2001:db8:abcd:0012:0000:0000:0000:0000

[0126] Each IPv6 address is split into eight 2-byte groupings separated by a colon. The groupings have the following functions:

[0127] • Network address - the first three groupings of numbers (first 48 bits) in the subnet mask

[0128] • Subnet address - the fourth grouping of numbers (the 49th through 64th bits) in the subnet mask

[0129] • Device address - the last four groupings of numbers (the last 64 bits) in the subnet mask The IPv6 address space is extremely large. This means that an IPv6 address can be used globally for a single purpose and this will not change over time, unlike IPv4 where addresses need to be reused. Therefore, IPv6 address can be more easily linked to a user's identity and managed by the user themselves.

[0130] In fact, a user can generate their own IPv6 address and self-register it on the network.

[0131] Users are expected to have many IPv6 addresses that correspond to multiple devices. The question then becomes 'how do we know which IPv6 addresses correspond to a given person'? This is a challenge because we need to associate IPv6 address with human names, and IPv6 are long numbers that are not easy read (unlike email addresses, for example). One solution is to introduce a globally accessible repository of IPv6 address with their associated owners. We can imagine this repository is controlled by a trusted authority with public key PKIPv6Auth.

[0132] Such authorities may perform an equivalent role for certain local subnetworks. These local networks are identified by the first four groupings of an IPv6 address.

[0133] Suppose the IPv6 address authority grants each user a limited number of IPv6 address that they are allowed to register. For example, each user is allowed five IPv6 address. These addresses may not be determined in advance. The techniques disclosed herein may be used to create a certificate to register five address that are determined in the future by the user. An example of such a certificate is given below.

[0134] Table 3: A digital certificate linking an identity to multiple IPv6 addresses.

[0135] The outpoints are controlled by the user Alice herself. Suppose she would like to specify her first IPv6 address. To do this, she creates a transaction of the form given below.

[0136] Once the transaction is sent to the blockchain network her IPv6 address is permanently associated with the certificate. Alice is able to create these IPv6 addresses at any time in the future. However, our method limits her to linking only five addresses to the certificate above. This limitation is guaranteed by the double-spend protection of the blockchain, and no further involvement from the IPv6 Authority is necessary after issuing the certificate.

[0137] 1.1.6.4 Decentralised identifiers (DIDs)

[0138] Decentralised Identifiers (DIDs) are used to enable verifiable and decentralised digital identity. They are generally self-issued and self-maintained. Verifiable credentials (VCs) are set of claims that can be cryptographically verified by intended verifiers. While DID is fully decentralised, VC can be issued by trusted entities. A user can leverage the trustworthiness of VCs to increase the trustworthiness of their DID by linking them cryptographically. For example, when a driving license is issued in the form of a public key certificate, a user may request their DID to be included. On the other hand, in the future, a virtual world may become more significant to a user, and privacy is needed to protect information leaking from the virtual world to the real world. Therefore, it is natural that a user may not want to reveal their DID in the virtual world to the certificate authority in the real world. Moreover, it is also possible that having a driving license may become an attribute that the user may want to link to future virtual identities that the user has not yet registered or established in a virtual world. Effectively, there are two scenarios:

[0139] 1. DID is private and should be kept private from the credential issuers; and / or

[0140] 2. DID is unknown at the time the credential is issued.

[0141] One option to address both scenarios is for the user to commit an unspent transaction outpoint voutuser, say, SHA256(voutuser), and include it in the driving license. As a verifier, given the driving license of the user and a spending transaction of voutuser, they

[0142] 1. check that the driving license is authentic;

[0143] 2. extract the commitment of voutuserand verify that it matches SHA256(voutuser);

[0144] 3. read DID from the spending transaction and verify that the user is the owner of the DID.

[0145] Note that if the user commits to a vout that they have no control of, then they cannot add any data to the spending transaction. If they add another person's DID, then they cannot prove to the verifier that they own the DID. Therefore, they cannot gain anything by cheating. Consequently, the certificate issuer does not need to be concerned about signing their commitment of voutuserwithout knowing what their DID is.

[0146] After a successful verification, the verifier will be convinced that the user with their DID indeed holds a driving licence. Note that, the verifier does not even need to read any information on the driving licence other than the commitment. As the user, they successfully linked their DID to a certificate that is issued by a trusted entity, making their DID more trustworthy. At the same time, the privacy is still preserved from the trusted entity as they have no means to independently identify the DID of the user efficiently. Conversely, when creating DID, a user may want to link to credentials that would be obtained in the future, or some existing credentials that would be updated in the future. They can follow a similar approach as described above, adding one or more placeholders in their DID document specifying unspent transaction outpoints that are under their control. They can even include unspent transaction outpoints that are under credential issuer's control to directly provide creditability for the corresponding credentials. To summarise, the described techniques may be used for four scenarios in the context of DID and VC.

[0147] 1. DID is unknow at the time of issuing a VC.

[0148] 2. DID should be kept private to the VC issuer.

[0149] 3. VC as an attribute in a DID document that is unknown at the time of creating the DID.

[0150] 4. VC as an attribute in a DID document that is expected to be updated from time to time.

[0151] 2. EXAMPLE SYSTEM OVERVIEW

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] 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 T1 used as part of a transaction model or scheme. A given blockchain will use one particular transaction protocol throughout.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] 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).

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] 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.

[0172] 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.

[0173] 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.

[0174] 3. UTXO-BASED MODEL

[0175] 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.

[0176] 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.

[0177] 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 " TxT. 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.

[0178] 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.

[0179] 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.

[0180] 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.

[0181] So in the example illustrated, UTXOo in the output 203 of TAT? comprises 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 UTXOo to 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.

[0182] 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.

[0183] 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).

[0184] 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.

[0185] 4. FURTHER REMARKS

[0186] 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.

[0187] For instance, some embodiments above have been described in terms of a bitcoin network

[0188] 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.

[0189] 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).

[0190] 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.

[0191] 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.

[0192] 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.

[0193] 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.

[0194] Statement 1. A computer-implemented method of committing to data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a committing party and comprises: for at least one undetermined component: obtaining a respective commitment transaction that references the respective blockchain transaction and / or the respective blockchain transaction output and includes the undetermined component and / or an obfuscated version of the undetermined component; and causing the respective commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

[0195] Statement 2. The method of statement 1, wherein the respective commitment transaction comprises the commitment and / or wherein the obfuscated version of the undetermined component is based on the commitment.

[0196] Statement 3. The method of statement 1 or statement 2, wherein the respective blockchain transaction and / or the respective blockchain transaction output referenced by the respective commitment transaction is locked to a public key of the second party, and wherein the respective commitment transaction comprises a signature corresponding to the public key of the second party.

[0197] Statement 4. The method of any preceding statement, wherein the obfuscated version of the undetermined component is based on a salt value.

[0198] Statement 5. The method of any preceding statement, comprising: obtaining a proof of inclusion for the respective blockchain transaction and / or a respective blockchain transaction comprising the respective blockchain transaction output; and using the proof of inclusion to verify that the respective blockchain transaction has been recorded on the blockchain.

[0199] Statement 6. The method of any preceding statement, comprising generating the underdetermined component and / or the obfuscated version of the undetermined component.

[0200] Statement 7. The method of any of statements 1 to 6, comprising receiving the underdetermined component and / or the obfuscated version of the undetermined component.

[0201] Statement 8. The method of any preceding statement, wherein the obfuscated version of the undetermined component comprises a hash of at least the undetermined component.

[0202] Statement 9. The method of any of statements 1 to 7, wherein the obfuscated version of the undetermined component comprises a Pederson commitment of at least the undetermined component.

[0203] Statement 10. The method of any of statements 1 to 7, wherein the obfuscated version of the undetermined component comprises an encrypted version of at least the undetermined component. Statement 11. The method of any preceding statement, wherein the respective commitment transaction and / or an output of the respective commitment transaction is controlled by the second party and / or one or more other parties.

[0204] Statement 12. The method of statement 11, comprising: obtaining a respective updated commitment transaction that references the respective commitment transaction and / or the output of the respective commitment transaction and includes an updated version of the undetermined component and / or an obfuscated version of the updated version of the undetermined component; and causing the respective updated commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

[0205] Statement 13. The method of any preceding statement, comprising: publishing and / or providing the undetermined commitment to one or more parties.

[0206] Statement 14. The method of any preceding statement, comprising: obtaining a respective proof of inclusion on the blockchain of the respective commitment transaction; and publishing and / or providing the respective proof of inclusion to one or more parties.

[0207] Statement 15. The method of any preceding statement, wherein: the determined component comprises part of a message to be signed and at least one undetermined component comprises a signature signing the message; or the determined component comprises one or more completed parts of a digital certificate and at least one undetermined component comprises an uncompleted part of the digital certificate; or the determined component comprises one or more parts associated with a network address issuer and at least one undetermined component comprises a network address; or the determined component comprises a verifiable credential and at least one undetermined component comprises a decentralised identifier; or the determined component comprises at least part of a decentralised identifier and at least one undetermined component comprises a verifiable credential.

[0208] Statement 16. The method of any preceding statement, comprising: generating the commitment; and publishing and / or providing the commitment to one or more parties.

[0209] Statement 17. 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 16.

[0210] Statement 18. 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 16.

[0211] Statement 19. A computer-implemented method of verifying data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a verifying party and comprises: for at least one undetermined component: obtaining a respective commitment transaction comprising the undetermined component and / or an obfuscated version of the undetermined component; verifying that the respective commitment transaction references the respective blockchain transaction and / or the respective blockchain transaction output; obtaining a respective proof of inclusion for the respective commitment transaction; and using the respective proof of inclusion to verify that the respective commitment transaction is recorded on the blockchain. Statement 20. The method of statement 19, comprising: obtaining a public key associated with a predetermined party; obtaining a signature that signs at least the undetermined component and / or the obfuscated version of the undetermined component; and verifying that the signature is valid for the public key.

[0212] Statement 21. The method of statement 19 or statement 20, comprising receiving a reference of the respective blockchain transaction and / or the respective blockchain transaction output.

[0213] Statement 22. The method of any of statements 19 to 21, comprising: obtaining the determined component; obtaining the commitment; and verifying that the commitment is based on the determined component and the respective blockchain transaction and / or the respective blockchain transaction output.

[0214] Statement 23. The method of any of statements 19 to 22, wherein the commitment is based on metadata indicating one or more expected properties of the undetermined component, and wherein the method comprises verifying that the undetermined component satisfies the one or more expected properties.

[0215] Statement 24. A computer-implemented method of determining data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a determining party and comprises: obtaining the commitment; obtaining the determined component; obtaining the one or more undetermined components from the respective blockchain transactions; and determining the data based on the one or more undetermined components. Statement 25. The method of statement 24, comprising: performing one or more actions based on the determined data.

[0216] Statement 26. The method of statement 24 or statement 25, wherein said obtaining of the commitment and / or determined component comprises receiving the commitment and / or determined component.

[0217] Statement 27. The method of any of statements 24 to 26, wherein said obtaining of the commitment and / or determined component comprises extracting the commitment and / or determined component from a public resource.

[0218] Statement 28. The method of any of statements 24 to 27, comprising verifying that the commitment is based on the determined component and the respective blockchain transaction and / or the respective blockchain transaction output.

[0219] Statement 29. The method of any of statement 24 to 28, comprising, for each undetermined component: obtaining a respective commitment transaction comprising the undetermined component and / or an obfuscated version of the undetermined component; and verifying that the respective commitment transaction references the respective blockchain transaction and / or the respective blockchain transaction output.

[0220] Statement 30. The method of statement 29, comprising, for each undetermined component: obtaining a respective proof of inclusion for the respective commitment transaction; and using the respective proof of inclusion to verify that the respective commitment transaction is recorded on the blockchain.

[0221] Statement 31. The method of any of statements 24 to 30, wherein the commitment is based on metadata indicating one or more expected properties of the one or more undetermined components, and wherein the method comprises verifying that the one or more undetermined components satisfy the one or more expected properties.

[0222] Statement 32. The method of any of statements 24 to 31, wherein: the determined component comprises part of a message to be signed and at least one undetermined component comprises a signature signing the message; or the determined component comprises one or more completed parts of a digital certificate and at least one undetermined component comprises an uncompleted part of the digital certificate; or the determined component comprises one or more parts associated with a network address issuer and at least one undetermined component comprises a network address; or the determined component comprises a verifiable credential and at least one undetermined component comprises a decentralised identifier; or the determined component comprises at least part of a decentralised identifier and at least one undetermined component comprises a verifiable credential.

[0223] Statement 33. 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 24 to 32.

[0224] Statement 34. 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 24 to 32.

[0225] Statement 35. A computer-implemented method of committing to access control of data, wherein the data comprises a determined component and one or more undetermined components, wherein the method is performed by an access control party and comprises: generating a commitment based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output; and publishing and / or providing the commitment to one or more parties.

[0226] Statement 36. The method of statement 35, wherein the commitment comprises a hash of i) the determined component and ii) for each undetermined component, a respective identifier of the respective blockchain transaction and / or the respective blockchain transaction output.

[0227] Statement 37. The method of statement 35, wherein the commitment comprises a root of a hash tree generated based on i) the determined component and ii) for each undetermined component, a respective identifier of the respective blockchain transaction and / or the respective blockchain transaction output.

[0228] Statement 38. The method of any of statements 35 to 38, wherein one or more of the respective blockchain transactions and / or one or more of the respective blockchain transaction outputs are controlled by a respective party.

[0229] Statement 39. The method of statement 38, wherein one or more of the respective blockchain transactions and / or the respective blockchain transaction outputs are controlled by a different respective party.

[0230] Statement 40. The method of statement 38, wherein one or more of the respective blockchain transactions and / or the respective blockchain transaction outputs are controlled by a same respective party.

[0231] Statement 41. The method of any of statements 38 to 40, wherein one or more of the respective blockchain transactions and / or one or more of the respective blockchain transaction outputs are controlled by a respective public key.

[0232] Statement 42. The method of any of statements 38 to 41, wherein the respective party controls one or more of the respective blockchain transactions and / or the respective blockchain transaction outputs. Statement 43. The method of any of statements 38 to 42, wherein one or more of the respective blockchain transactions and / or one or more of the respective blockchain transaction outputs are controlled by multiple respective parties.

[0233] Statement 44. The method of any of statements 35 to 43, wherein each respective blockchain transaction is a different blockchain transaction and / or wherein each respective blockchain transaction output is a different blockchain transaction output.

[0234] Statement 45. The method of any of statements 35 to 43, wherein one or more of the respective blockchain transaction are a same blockchain transaction and / or wherein one or more of the respective blockchain transaction outputs are a same blockchain transaction output.

[0235] Statement 46. The method of any of statements 35 to 45, wherein the commitment is based on a salt value.

[0236] Statement 47. The method of any of statements 35 to 46, comprising: generating a signature based on the commitment; and publishing and / or providing the signature to one or more parties.

[0237] Statement 48. The method of any of statements 35 to 47, comprising, for each undetermined component: obtaining a respective proof of inclusion on the blockchain of the respective blockchain transaction and / or the respective blockchain transaction output; and publishing and / or providing the respective proof of inclusion to one or more parties.

[0238] Statement 49. The method of any of statements 35 to 48, wherein the commitment comprises a Pederson commitment.

[0239] Statement 50. The method of any of statements 35 to 49, wherein the commitment is generated based on iii) metadata indicating one or more expected properties of the undetermined component. Statement 51. The method of any of statements 35 to 50, comprising: publishing and / or providing the determined component to one or more parties.

[0240] Statement 52. The method of any of statements 35 to 51, wherein: the determined component comprises part of a message to be signed and at least one undetermined component comprises a signature signing the message; or the determined component comprises one or more completed parts of a digital certificate and at least one undetermined component comprises an uncompleted part of the digital certificate; or the determined component comprises one or more parts associated with a network address issuer and at least one undetermined component comprises a network address; or the determined component comprises a verifiable credential and at least one undetermined component comprises a decentralised identifier; or the determined component comprises at least part of a decentralised identifier and at least one undetermined component comprises a verifiable credential.

[0241] Statement 53. The method of any of statements 35 to 52, comprising: for at least one undetermined component: obtaining a respective commitment transaction that references the respective blockchain transaction and / or the respective blockchain transaction output and includes the undetermined component; and causing the respective commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

[0242] Statement 54. 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 35 to 53. Statement 55. 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 35 to 53. According to another aspect disclosed herein, there may be provided a method comprising the actions of any combination of the access control party, the committing party and the verifying party. According to another aspect disclosed herein, there may be provided a system comprising any combination of the computer equipment of the access control party, the committing party and the verifying party.

Claims

CLAIMS1. A computer-implemented method of committing to data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a committing party and comprises: for at least one undetermined component: obtaining a respective commitment transaction that references the respective blockchain transaction and / or the respective blockchain transaction output and includes the undetermined component and / or an obfuscated version of the undetermined component; and causing the respective commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

2. The method of claim 1, wherein the respective commitment transaction comprises the commitment and / or wherein the obfuscated version of the undetermined component is based on the commitment.

3. The method of claim 1 or claim 2, wherein the respective blockchain transaction and / or the respective blockchain transaction output referenced by the respective commitment transaction is locked to a public key of the second party, and wherein the respective commitment transaction comprises a signature corresponding to the public key of the second party.

4. The method of any preceding claim, wherein the obfuscated version of the undetermined component is based on a salt value.

5. The method of any preceding claim, comprising: obtaining a proof of inclusion for the respective blockchain transaction and / or a respective blockchain transaction comprising the respective blockchain transaction output; andusing the proof of inclusion to verify that the respective blockchain transaction has been recorded on the blockchain.

6. The method of any preceding claim, comprising generating the underdetermined component and / or the obfuscated version of the undetermined component.

7. The method of any of claims 1 to 6, comprising receiving the underdetermined component and / or the obfuscated version of the undetermined component.

8. The method of any preceding claim, wherein the obfuscated version of the undetermined component comprises a hash of at least the undetermined component.

9. The method of any of claims 1 to 7, wherein the obfuscated version of the undetermined component comprises a Pederson commitment of at least the undetermined component.

10. The method of any of claims 1 to 7, wherein the obfuscated version of the undetermined component comprises an encrypted version of at least the undetermined component.

11. The method of any preceding claim, wherein the respective commitment transaction and / or an output of the respective commitment transaction is controlled by the second party and / or one or more other parties.

12. The method of claim 11, comprising: obtaining a respective updated commitment transaction that references the respective commitment transaction and / or the output of the respective commitment transaction and includes an updated version of the undetermined component and / or an obfuscated version of the updated version of the undetermined component; and causing the respective updated commitment transaction to be submitted to one or more blockchain nodes of a blockchain network.

13. The method of any preceding claim, comprising: publishing and / or providing the undetermined commitment to one or more parties.

14. The method of any preceding claim, comprising: obtaining a respective proof of inclusion on the blockchain of the respective commitment transaction; and publishing and / or providing the respective proof of inclusion to one or more parties.

15. The method of any preceding claim, wherein: the determined component comprises part of a message to be signed and at least one undetermined component comprises a signature signing the message; or the determined component comprises one or more completed parts of a digital certificate and at least one undetermined component comprises an uncompleted part of the digital certificate; or the determined component comprises one or more parts associated with a network address issuer and at least one undetermined component comprises a network address; or the determined component comprises a verifiable credential and at least one undetermined component comprises a decentralised identifier; or the determined component comprises at least part of a decentralised identifier and at least one undetermined component comprises a verifiable credential.

16. The method of any preceding claim, comprising: generating the commitment; and publishing and / or providing the commitment to one or more parties.

17. A computer-implemented method of verifying data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undetermined component, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a verifying party and comprises: for at least one undetermined component:obtaining a respective commitment transaction comprising the undetermined component and / or an obfuscated version of the undetermined component; verifying that the respective commitment transaction references the respective blockchain transaction and / or the respective blockchain transaction output; obtaining a respective proof of inclusion for the respective commitment transaction; and using the respective proof of inclusion to verify that the respective commitment transaction is recorded on the blockchain.

18. The method of claim 17, comprising: obtaining a public key associated with a predetermined party; obtaining a signature that signs at least the undetermined component and / or the obfuscated version of the undetermined component; and verifying that the signature is valid for the public key.

19. The method of claim 17 or claim 18, comprising receiving a reference of the respective blockchain transaction and / or the respective blockchain transaction output.

20. The method of any of claims 17 to 19, comprising: obtaining the determined component; obtaining the commitment; and verifying that the commitment is based on the determined component and the respective blockchain transaction and / or the respective blockchain transaction output.

21. The method of any of claims 17 to 20, wherein the commitment is based on metadata indicating one or more expected properties of the undetermined component, and wherein the method comprises verifying that the undetermined component satisfies the one or more expected properties.

22. A computer-implemented method of determining data, wherein the data comprises a determined component and one or more undetermined components, wherein a commitment is based on i) the determined component and ii) for each undeterminedcomponent, a respective blockchain transaction and / or a respective blockchain transaction output, and wherein the method is performed by a determining party and comprises: obtaining the commitment; obtaining the determined component; obtaining the one or more undetermined components from the respective blockchain transactions; and determining the data based on the one or more undetermined components.

23. The method of claim 22 comprising: performing one or more actions based on the determined data.

24. The method of claim 22 or claim 23, wherein said obtaining of the commitment and / or determined component comprises receiving the commitment and / or determined component.

25. The method of any of claims 22 to 24, wherein said obtaining of the commitment and / or determined component comprises extracting the commitment and / or determined component from a public resource.

26. The method of any of claims 22 to 25, comprising verifying that the commitment is based on the determined component and the respective blockchain transaction and / or the respective blockchain transaction output.

27. The method of any of claim 22 to 26, comprising, for each undetermined component: obtaining a respective commitment transaction comprising the undetermined component and / or an obfuscated version of the undetermined component; and verifying that the respective commitment transaction references the respective blockchain transaction and / or the respective blockchain transaction output.

28. The method of claim 27, comprising, for each undetermined component:obtaining a respective proof of inclusion for the respective commitment transaction; and using the respective proof of inclusion to verify that the respective commitment transaction is recorded on the blockchain.

29. The method of any of claims 22 to 28, wherein the commitment is based on metadata indicating one or more expected properties of the one or more undetermined components, and wherein the method comprises verifying that the one or more undetermined components satisfy the one or more expected properties.

30. The method of any of claims 22 to 29 wherein: the determined component comprises part of a message to be signed and at least one undetermined component comprises a signature signing the message; or the determined component comprises one or more completed parts of a digital certificate and at least one undetermined component comprises an uncompleted part of the digital certificate; or the determined component comprises one or more parts associated with a network address issuer and at least one undetermined component comprises a network address; or the determined component comprises a verifiable credential and at least one undetermined component comprises a decentralised identifier; or the determined component comprises at least part of a decentralised identifier and at least one undetermined component comprises a verifiable credential.

31. 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 claims 1 to 30.

32. 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 30.

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