Key derivation & communication protocols

Abstract

A computer-implemented method of secure communication between nodes of a network comprising a master node, a set of intermediate nodes, and a set of end devices. The network comprises a network path o

Description

The present disclosure relates to a method of deriving keys for use in secure communication between nodes and devices of a network (e.g. an loT network) and to methods of secure communication between nodes and devices of the network. BACKGROUND A Multilevel loT (MIOT) System, as described in international (PCT) patent applications PCT / IB2020 / 059319 and PCT / IB2020 / 059323, facilitates the creation of a customizable, multilevel control hierarchy for controlling a network of loT devices. This local network integrates with the blockchain to create a hybrid system that enables scalable loT device management and device-to-device communications. Permissioning new nodes onto the local MIOT network involves the issuance of certificates by the "master node". These certificates contain a variety of information relating to the node, including a public key, certified by a network administrator / controller, used to identify a given device or intermediate node on the network. Certified nodes can monitor the master node's address so they can see every certificate that has been issued and signed, or revoked, by the master node, and can therefore see every MIOT node's valid certified public key. A certificate transaction is created when a new node is permissioned onto the network, and certified nodes can see the new certificate, so they can identify, and validate any transactions sent to and from the public key within the certificate. The visibility of a node's public key within a certificate transaction is necessary to ensure that all nodes possess the most up-to-date peer list and are only able to communicate with other certified nodes. Unfinalized transaction templates and command messages can be sent off chain, using the IP address within a certificate transaction. Alternatively, a transaction can be sent to the blockchain, and itsTxID sent off-chain to the relevant devices, to be viewed on chain. SUMMARY An observable certified key within a transaction on a public blockchain presents a privacy risk if certified keys are used for frequent communication between nodes in an MIOT network. Although knowledge of this key would not enable a malicious third party to hijack the corresponding private key and make any modifications to the loT network, or decrypt any data within a transaction payload, transactions sent between these keys can be monitored in the hopes of finding any information that might reveal a security vulnerability in the MIOT Network. Therefore a more sophisticated private key derivation solution is required to obfuscate public keys used for loT communications from third parties, while still allowing MIOT nodes to update their peer lists and communicate with other certified nodes. Additionally, messages sent between MIOT nodes should ideally be encrypted, to ensure all data is confidential. Both man-in-the-middle attacks and blockchain monitoring would be ineffective in exposing loT data, if the data has been encrypted, and can only be decrypted by specified nodes. The master node preferably be able to customize which devices can encrypt and decrypt data on a case-by-case basis. Note that these problems apply to networks in general and not just the MIOT network discussed above or to loT networks in general. More generally, these problems apply to networks of devices in general. According to one aspect disclosed herein, there is provided a computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the master node and comprises: generating an encrypted command message by encrypting a command message with a respective encryption key shared between the master node and the target end device; generating a blockchain transaction comprising the encrypted command message; generating a first encrypted transit message by combining the blockchain transaction with a respective public key of one or more intermediate nodes of the respective network path and a respective public key of the target end device, and encrypting the combination with a respective encryption key shared between the master node and the initial intermediate node; and sending the first encrypted transit message to the initial intermediate node. According to one aspect disclosed herein, there is provided a computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the initial intermediate node of the network path and comprises: receiving a first encrypted transit message, wherein the first encrypted transit message is encrypted with a respective encryption key shared between the master node and the initial intermediate node; obtaining a first message by decrypting the first encrypted transit message with the respective encryption key, and i) wherein the first message comprises a blockchain transaction combined with a respective public key of one or more of the intermediate nodes of the respective network path and a respective public key of the target end device, and wherein the method comprises: generating a second encrypted transit message by removing, from the second message, a respective public key of a next intermediate node of the respective network path, and encrypting the result with a respective encryption key shared between the initial intermediate node and the next intermediate node; and sending the second encrypted transit message to the next intermediate node; or ii) wherein the first message comprises the blockchain transaction combined with the respective public key of the target end device, and wherein the method comprises sending the blockchain transaction to the target end device. According to one aspect disclosed herein, there is provided a computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the final intermediate node of the network path and comprises: receiving a second encrypted transit message, wherein the second encrypted transit message is encrypted with a respective encryption key shared between the final intermediate node and a previous intermediate node in the respective network path; obtaining the second message by decrypting the second encrypted transit message with the respective encryption key, wherein the second message comprises a blockchain transaction combined with a respective public key associated with the target end device, wherein the blockchain transaction comprises an encrypted command message; generating an acknowledgement message; generating an encrypted acknowledgement message by encrypting the acknowledgment message with the respective encryption key shared between the master node and the final intermediate node; and adding a signature corresponding to the respective public key of the final intermediate node, and the encrypted acknowledgement message, to the blockchain transaction; and sending the encrypted command message to the target end device. According to one aspect disclosed herein, there is provided a computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the target end device and comprises: receiving, from the final intermediate node, an encrypted command message, wherein the encrypted command message is encrypted with a respective encryption key shared between the master node and the target end device; obtaining the command message by decrypting the encrypted command message with the respective encryption key; and performing a command associated with the command message. According to one aspect disclosed herein, there is provided 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 any of the methods provided herein. According to one aspect disclosed herein, there is provided a computer program embodied on computer-readable storage and configured so as, when run on one or more processors, to perform and of the methods provided herein. The use of key masking allows nodes on the local network (e.g. an loT network such as the abovementioned MIOT network) to remain private to third parties, while being discoverable to other nodes of the network. The shared network secret used to derive these keys may be updated by the master node to ensure the keys remain private. The use of a key generation technique such as Elliptic-curve Diffie-Hellman (ECDH) between derived keys allows precise encryption customization by the master node. Which nodes can encrypt and decrypt different sections of data can be determined by the master node for each command. Messages can be encrypted by, and sent between, intermediate (i.e. gateway) nodes, adding another layer of security. The (loT) data is encrypted by the shared secret with the master node, ensuring the data is visible to certain permissioned actors on the system, and obfuscated to third parties, as well as other intermediate nodes. In short, the combination of masked key derivation using an encrypted shared secret, along with derived shared secret creation, allows for increased privacy of devices, as well as enhanced message and transaction template encryption. Embodiments disclosed herein may be used generally to manage a closed network of devices. Whilst example are primarily described in terms of a network of loT devices, the embodiments may be applied to any type of network of devices, connected via the internet or otherwise. Example applications include warehouse and inventory management, supply chain data collection (e.g. farming), and automated systems (e.g. public transport, or smart homes). BRIEF DESCRIPTION OF THE DRAWINGS To assist understanding of embodiments of the present disclosure and to show how such embodiments may be put into effect, reference is made, by way of example only, to the accompanying drawings in which: Figure 1 is a schematic block diagram of a system for implementing a blockchain, Figure 2 schematically illustrates some examples of transactions which may be recorded in a blockchain, Figure 3 schematically illustrates an example network of nodes, Figure 4 shows an example flow diagram for deriving keys, and Figures 5 to 7 schematically illustrate example messaging protocols. DETAILED DESCRIPTION OF EMBODIMENTS 1. KEY DERIVATION &MESSAGING PROTOCOLS Figure 3 illustrates an example network 300 topology. The network 300 may be an loT network,, i.e. a network of computing devices interconnected by the internet. For convenience, the network 300 will be referred to as an loT network so as to distinguish the network 300 from the blockchain network 106 discussed elsewhere herein. However the embodiments described herein apply more generally to any type of network in which there is a hierarchy of nodes. The nodes of the network 300 need not be connected or communicate via the internet, though that of course is one option. For example, the nodes may communicate via radio or cellular connections, or via a wired (e.g. Ethernet) connection. The loT network 300 includes at least one master node 301, one or more sets (or layers) of one or more intermediate nodes 302 (each layer having at least one intermediate node 302), and a set of end devices 303. The master node 301 is configured to control one or more intermediate nodes 302. If the loT network 501 comprises multiple sets / layers of intermediate nodes, the master node 301 may be configured to directly control the first set / layer of intermediate nodes (e.g. "servant nodes" 302a) and to indirectly control one or more further sets / layers of intermediate nodes (e.g. a layer of "slave nodes" 302b). Here, controlling a node may be taken to mean communicate directly with that node. The master node 301 may be a controlling node with the ability to override and control servant and slave nodes. Each servant node 302a may be a node with the ability to control slave nodes 302b. Each slave node 302b may be a node under the control of the servant nodes 302a and the master node 301. As an example, to instruct end device 303, the master node 301 may issue a command to the end device via a servant node 302a and a slave node 302b. The loT network is a packet-switched network 101, typically a wide-area internetwork such as the Internet. The nodes 301, 302 and devices 303 of the packet-switched network 101 are arranged to form a peer-to-peer (P2P) overlay network 300 within the packet-switched network 101. Each node 301, 302 and device 303 comprises respective computer equipment, each comprising respective processing apparatus comprising one or more processors, e.g. one or more central processing units (CPUs), accelerator processors, application specific processors and / or field programmable gate arrays (FPGAs). Each node 301, 302 and device 303 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. Typically the end devices 303, master node 301 intermediate nodes 302 will be embedded in everyday devices. An end device 303 may take one of a variety of forms, e.g. user devices (e.g. smart TVs, smart speakers, toys, wearables, etc.), smart appliances (e.g. fridges, washing machines, ovens, etc.), meters or sensors (e.g. smart thermostats, smart lighting, security sensors, etc.). Similarly, the master node 301 and intermediate nodes 302 may also take a variety of forms, which may include, but is not limited to, the same forms as which an end device may take. A node 301, 302 may also take the form of dedicated servant equipment, a base station, an access point, a router, and so on. In some examples, each device may have a fixed network (e.g. IP) address. For instance, one, some or all of the end devices may be a stationary device (e.g. a smart light, or smart central heating controller, etc.), as opposed to a mobile device. Whilst the example loT network 300 of Figure 3 shows only two layers of intermediate nodes (servant nodes 302a and slave nodes 302b), other examples may comprise one or more further sets of intermediate nodes, e.g. between the master node 301 and servant nodes 302a, and / or between the servant nodes 302a and slave nodes 302b. As shown, each node is connected to one or more other nodes via a respective connection, and each end device 303 is connected to one or more slave nodes via a respective connection. The connections may be wired or wireless. Each connection may be the same, or some connections may be different. The loT network nodes may correspond to hierarchies in scope of functionality, in superiority of instructions / prerogatives, and / or in span of access. In some implementations, a hierarchical set of SPV nodes implement an "loT controller" with three levels of hierarchy, corresponding to the master 301, servant 302a and slave nodes 302b of Figures 3. The master node 301 instructs one or more servant nodes 302a, and each servant node instructs one or more slave nodes 302b. Each slave node 302b receives instructions from one or more servant nodes 302a. Every slave node 302b communicates with one or more loT end-devices 303, and these are the direct channels of communication between the loT-controller 503 and the loT end-devices 303. The states of execution of the loT controller 503 may be recorded in blockchain transactions Tx. Each loT node - master, servant, or slave - may have the capability to create and broadcast corresponding transactions Tx to the blockchain network 106. The master node, servant node(s) and slave node(s) may each independently connect to nodes 104 on the blockchain network 106, and may each operate a blockchain wallet (e.g. to monitor blockchain addresses). The master node 301 may be configured to monitor the activity of other loT nodes both directly and indirectly under their control, issue commands to these nodes in the form of blockchain transactions Tx and respond to alerts. The servant nodes 302a may be configured to monitor multiple addresses, including addresses not directly controlled by the servant node 302a. Servant nodes 302a may be commanded to perform actions by a master node 301. The slave node 302b may be configured to monitor the activities of end devices 303 directly under their control. Slave nodes 302b are under the direct command of servant nodes 302a and may also be commanded to perform actions by the master node 301. The slave nodes 302b may act as gateway nodes for the end devices 303 (i.e. a gateway between the end device 303 and the blockchain network 106). The end device 303 is configured to connect to nearby slave devices. They report on end device state using an off-chain messaging protocol. Note that whilst a distinction is made between an loT node 301, 302 and an end device 303 in that end devices 303 are controlled by loT nodes but do not themselves control loT nodes, an end device 303 may also take the same physical form as an loT node and may also connect to the blockchain network 106. That is, in some examples an end device 303 may operate a blockchain protocol client or wallet application. The loT network 300 strikes a balance between centralisation and decentralisation by combining a command and control hierarchy with use of a blockchain network infrastructure. Users of the network 501 may create their own multilevel control hierarchy which includes client-servant as well as peer-to-peer relationships between devices. Embodiments disclosed herein may be used to generate shared secrets between nodes 301, 302 and devices 303 of the loT network 300. The master node 301 has (i.e. stores in memory) a private key, referred to as the master private key, and public key corresponding to that private key, referred to as master public key. The master node 301 is associated with the master public key in the sense that other nodes 302 and devices 303 of the network 300 are aware (or can at least discover) the link between the master node 301 and the master public key. Similarly, each intermediate node 302 has its own private key, referred to as an intermediate private key, and a corresponding intermediate public key. Each end device 303 also has its own private key, referred to as a device private key, and a corresponding device public key. The key-pairs may be elliptic curve key-pairs. The master node 301 is configured to generate a secret, referred to as a "network secret". The network secret takes the form of a private key, e.g. a 256 bit integer. The network secret may be randomly generated. The network secret may be generated by hashing some data. The master node 301 generates a shared secret for one, some or each intermediate node 302 (a different shared secret per intermediate node). For a given node 302, the master node 301 generates the shared secret based on the master private key and the intermediate public key associated with that node 302. The shared secret may be generated using ECDH. The shared secret may be a symmetric encryption key. The master node 301 then uses the shared secret to encrypt the network secret and send the encrypted network secret to the node 302. The master node 301 may perform the same process for one, some or each device 303. The network secret may be used to generate private and public keys. For example, the master node 301 may generate a private key (referred to as a derived private key) based on the master private key and the network secret. For one, some or each intermediate node 302, the master node 301 may generate a respective public key (referred to as a derived public key) based on the intermediate public key associated with that node and a public key corresponding to the network secret (referred to as the network public key). The master node 301 may perform the same process for one, some or each device 303. The derived private and public keys may be used to generate shared encryption keys, i.e. an encryption key known to a pair of nodes (e.g. the master node 301 and an intermediate node 302). For one, some or each intermediate node 302, the master node 301 may generate a respective shared encryption key based on its derived private key and the derived public key associated with the intermediate node 302. The shared encryption key may be a symmetric encryption key. The master node 301 may perform the same process for one, some or each device 303. Each intermediate node 302 is configured to generate one or more shared encryption keys, e.g. an encryption key shared with the master node 301. Examples will be discussed from the perspective of a "first intermediate node", but this is merely for convenience and it should be appreciated that "first" is being used merely as a label, and does not imply any ordering or priority. Each intermediate node 302 is configured to perform the same actions as the first intermediate node 302. The first intermediate node 302 is configured to receive a first encrypted network secret from the master node 301. Again, "first" is used merely as a label for the encrypted network secret generated for the first intermediate node, and does not necessarily mean it was the encrypted network secret that was first generated by the master node 301. The first intermediate node 302 generates a first shared secret based on its intermediate private key and the master public key, and uses the first shared secret to decrypt the first encrypted network secret, revealing the network secret. Similarly the master node 301, the first intermediate node 302 generates a derived private key based on its intermediate private key and the network secret. For the master node 301, the first intermediate node 302 generates a derived public key based on the master public key and the network public key. A shared encryption key may then be generated based on the derived private key of the first intermediate node 302 and the derived public key of the master node 301. For one, some or each other intermediate node 302, the first intermediate node 302 generates a derived public key based on the intermediate public key of that intermediate node and the network public key. A shared encryption key may then be generated based on the derived private key of the first intermediate node 302 and the derived public key of the intermediate node 302. The first intermediate node 302 may perform the same process for one, some or each device 303. In some examples, the master node 301 may update the network secret, e.g. at random, periodically, or in response to a trigger event, such as the leaking of the network secret. In these examples, the master node 301 may encrypt the updated network secret and share the encrypted updated network secret with each intermediate node and / or device using the techniques discussed above for the original network secret. Derived private and public keys may then be generated using the updated network secret. Then, shared encryption keys may be generated using the derived private and public keys generated based on the updated network secret. Embodiments disclosed herein may be used for secure node-to-node and node-to-device communication. The shared encryption keys described above may be used for said communication. Alternatively, encryption keys derived in an alternative way may be used. Examples involve sending a command message (i.e. an instruction) from the master node 301 to an end device 303, via one or more intermediate nodes, e.g. a servant node 302a and a slave node 302b. The device 303 to be commanded will be referred to as a target end device. The path (i.e. route) of nodes from the master node 301 to the target end device will be referred to as a network path. The network path may contain any number of intermediate nodes. In general the network path has an initial intermediate node and a final intermediate node. The initial and final intermediate nodes may be the same or different nodes. Three communication protocols are disclosed herein. The first will now be described. The master node 301 is configured to encrypt a command message with an encryption key shared between the master node 301 and the target end device 303. The encryption key may be a shared encryption key as discussed above, but in general any shared encryption key derivable by both the master node 301 and the target end device 303 may be used. The command message comprises a command / instruction interpretable by the target end device 303 and which causes the target end device to perform an operation, e.g. record a measurement and / or perform an action. The master node 301 combines (e.g. concatenates) the encrypted command message with the public key of each intermediate node 302 in the network path and the public key of the target end device. In some examples the public key of the initial intermediate node 302 is not included. The public keys may be the derived public keys described above, or simply the intermediate public keys of the intermediates nodes and the device public key of the target end device. The master node 301 encrypts the combination of the encrypted command message and public keys with an encryption key shared between the master node 301 and the initial intermediate node 302 in the network path, generating a first encrypted message. Again, the shared encryption key may be generated using the technique described above. The master node 301 sends the first encrypted message to the initial intermediate node 302. The encrypted message may also be referred to as an encrypted transit message, reflecting the fact that it is used to "transit" the encrypted command message. The initial intermediate node 302 receives the first encrypted message and decrypts it using the encryption key shared between the master node 301 and the initial intermediate node 302. If the first message includes the public key(s) of one or more intermediate nodes (i.e. there is at least one more intermediate node 302 in the network path), the public key of the next intermediate node 302 in the network path is removed (as is the public key of the initial intermediate node 302, if present). The encrypted command message and the remaining public keys (i.e. the public keys of the remaining intermediate node(s) 302 and the target end device 303) is encrypted with an encryption key shared between the initial intermediate node and the next intermediate node 302 in the network path, generating a second encrypted message. The initial intermediate node 302 sends the second encrypted message to the next intermediate node 302 in the network path. This process is repeated by each node 302 in the network path until the final intermediate node 302 in the network path receives and decrypt a message containing only the encrypted command message and the public key of the target end device 303. When this happens, the final intermediate node 302 sends the encrypted command message to the target end device 303. The target end device 303 is configured to receive the encrypted command message, and use the encryption key shared with the master node 301 to decrypt it to obtain the command message. The target end device 303 performs an action based on (i.e. associated with) the command message. The second communication protocol will now be described. The second protocol shares many similarities with the first protocol. The master node 301 includes the encrypted command message in a blockchain transaction. The encrypted command message may be included as part of an output of the transaction. The master node 301 may sign the blockchain transaction, e.g. with a signature generated using the master private key or the derived private key. The blockchain transaction is then encrypted with the encryption key shared between the master node 301 and the final intermediate node 302 in the network path. The shared encryption key may be generated as described above. The encrypted blockchain transaction is combined with the public keys of the nodes 302 and the target end device 303 (e.g. the derived public keys) in the network path. The combination of the encrypted blockchain transaction and the public keys is then encrypted with the encryption key shared between the master node 301 and the initial intermediate node 302 in the network path, generating a first encrypted message. The first encrypted message is then sent to the initial intermediate node 302. The process then continues in a similar way to the first communication protocol until the final intermediate node 302 receives an encrypted message. The final intermediate node 302 decrypts the encrypted message using an encryption key shared between the final inte4r5mediate node and the previous intermediate node in the network path, revealing the blockchain transaction. The final intermediate node 302 extracts the encrypted command message from the blockchain transaction and sends it to the target end device 303, and sends it to the target end device 303. The target end device 303 may generate data in response to performing the command and / or an acknowledgement acknowledging that the command has been performed. The target end device 303 may generate a data message that includes the data and / or the acknowledgement. The target end device 303 encrypts the data message using the encryption key shared with the master node 301, and sends the encrypted data message to the final intermediate node 302 in the network path. The final intermediate node 302 may submit the blockchain transaction to the blockchain network 106 at the point of sending the encrypted command message to the target end device 303, or in response to receiving the encrypted data message. The final intermediate node 302 may send the encrypted data message to the master node 301. Or, the final intermediate node 302 may include the encrypted data message in the blockchain transaction (e.g. as part of a different output) and then submit the blockchain transaction to the blockchain network 106. The final intermediate node 302 may sign the transaction with a signature generated with its intermediate private key or its derived private key. The master node 301 is configured to extract the encrypted data message from the blockchain transaction, use the shared encryption key to decrypt it and obtain the data and / or acknowledgement. The third communication protocol will now be described. The third communication protocol also involves a blockchain transaction. As in the case of the second communication protocol, the encrypted command message is included in a blockchain transaction. The third communication protocol differs in that the blockchain transaction is combined with the public keys of the intermediate node(s) and target end device in the network path, and then encrypted with an encryption key shared between the master node 301 and the initial intermediate node 302, generating an encrypted first message. The encrypted first message is then sent to the initial intermediate node 302. The initial intermediate node 302 decrypts the first encrypted message, revealing the blockchain transaction and public keys. The initial intermediate node 302 adds an encrypted acknowledgement message to the blockchain transaction. The acknowledgment message is encrypted with the encryption key shared between the master node 301 and the initial intermediate node 302. The initial intermediate node 302 may also sign the transaction with a signature generated using its intermediate private key or its derived private key. The initial intermediate node 302 removes the public key of the next intermediate node and encrypts the blockchain transaction and remaining public keys(s) with an encryption key shared between the initial intermediate node and the next intermediate node in the network path, and sends the encrypted message to the next intermediate node. The process is repeated for each intermediate node 302 in the network path, with each intermediate node adding an acknowledgement message and / or signature to the blockchain transaction, until the final intermediate node decrypts an encrypted message comprising the blockchain transaction and the public key of the target end device 303. The final intermediate node sends the encrypted message to the target end device. As described above, the target end device 303 may return an encrypted data message. The final intermediate node 302 adds the encrypted data message and a signature to the transaction before submitting the transaction to the blockchain network 106. 1.1 Example Implementations The following sections describe example implementations of the embodiments described above. These examples involve generating communication key-pairs (Pder = sder • G,sder), derived from certified keys (Pcert = scert ■ G, scert), for each node in a MIOT network (or blockchain-based networks more generally). The derived keys are generated using a network-wide shared secret Snet, created by the master node and distributed privately to all other nodes in the MIOT network. This ensures that all nodes in the network, including end devices, are able to determine each other node's derived public key, while third party observants of the network and blockchain are not. The derived key pair can therefore be used to sign blockchain transactions privately, retaining the advantage that all nodes within the network can determine which nodes sign transactions but without exposing this information to outside third parties. From the derived key-pairs, each pair of nodes in the network may further derive a shared secret between them which can be used for encrypting and securely sharing messages between any two nodes in the network privately. These pair-wise shared secrets may be generated using a Elliptic Curve Diffie Hellman (ECDH) key exchange (or a variant thereof), where the shared secret between nodes A and B is defined as SAB ■= Pder • sBer = sAer • PBer. A shared secret SAB may be used to derive an encryption key eAB used to privately encrypt and send messages between nodes A and B. The encryption key may be derived by hashing the shared secret as eAB := SHA256(Sab). Optionally, a more efficient method is to send the shared secret SAB to the new node with the TxID of the certificate transaction during the bootstrapping process. This section describes two protocols: 1. Setup protocol - used to establish an MIOT network, issue certified keys, distribute network secret to all nodes, and generate derived key pairs. 2. Messaging protocol - used by nodes on the network to securely exchange messages and commands, either directly or along a network path. The messaging protocol has three variants which offer different privacy properties for message exchange. Each messaging protocol makes use of shared secrets, i.e. secrets shared between pairs or nodes. In general the messaging protocols may use any shared secret and not necessarily only a shared secret derived according to the setup protocol. That is, the messaging protocols are not dependent on and may be used separate from the setup protocol. 1.1.1 Setup Protocol Initially, the network includes a set of nodes which have been onboarded to the network and assigned (i.e. provided with) a certified key-pair by the master node, or the nodes may self-generate their own private key and have the master node certify the corresponding public key. The master node then generates a network-wide secret Snet, which can be in the form of, for example, a random 256-bit integer. The master node, using its master key-pair (pm = sm . q sm^ derjves a shared secret Sm t with each node i as Sm t = sm • pfert where Pfert is the certified public key of node i. The master node then uses Smi to encrypt (e.g. by hashing to derive an encryption key, and using symmetric encryption such as AES) the network secret Snet and sends to node i, repeating this process for each node until all nodes possess the encrypted secret. Each device then generates their respective secret shared with the master as Smj = Pm • sfert and uses this to decrypt the secret Snet. Finally, using elliptic curve point addition, each node combines Snet with their certified key-pair to generate a derived key-pair (^pfer := p^n _|_ 5^. G>sfer := sfert + Snet). The full detail of this example Setup process is as follows (and shown in Figure 4): 1. The master node has private key s, and public key P = s • G 2. Using bootstrapping process certified key-pairs are issued to every certified node: pcert _ scert. £ where i = (Serv,Slave, End') 3. The master node uses a key exchange technicue such as ECDH between itself and every certified node to create a shared secret: SM_X = sM ■ Piert 4. The master node then generates a "network secret" Snet which may be a random 256-bit integer. 5. Snet is encrypted with SM_{ for each node "i", and is sent to i. 6. Using the network secret i and their certified public key Piert, nodes generate a derived key: Pfer = P™rt + (Snet ■ G) 7. These derived keys may then be used to generate new "derived shared secret keys" between every other node's derived key: = sfer ■ Pfer C J c J c J Each node may calculate the derived public key of every other node on the network by combining its certified public key (found within a certificate transaction) with the network secret Snet. Third parties with no knowledge of this secret would not be able to calculate these derived keys, and therefore cannot monitor MIOT node addresses or transactions. Leaking of the network secret would allow a malicious third party to monitor transactions sent to and from derived public keys but are still not able to recover the derived private keys, therefore signatures cannot be forged. Snet may be updated by the master node and sent to all certified nodes, who can easily re-calculate derived keys and derived shared secrets, re-establishing privacy. The transactions masked by the leaked network secret will still be visible (with the encrypted payload remaining private). Network secrets may be updated on a regular basis to mitigate this risk. In the final step of the setup, each pair of nodes may optionally derive pair-wise shared secrets with each other node of the network for future communications. Alternatively, this may be done on-the-fly when a message is sent, to reduce storage costs. 1.1.2 Messaging Protocol (Variant 1) Consider a message (e.g. an instruction for a sensor to take a reading) is to be sent from the master node to an end device. The message is to be sent along the relevant MIOT network path from the master node to the end device. In this variant, the intermediate / gateway nodes can see which end device is the intended recipient of the message, but not the message payload (i.e. the instruction) itself. This example protocol is as follows, and is shown schematically in Figure 5: 1. The master node encrypts command message M with the shared secret Smend shared between the master node'm' and the desired end device ‘end’, resulting in encrypted payload denoted by eend := e(M,Smiend). 2. The master node encrypts eend, concatenated with the derived public keys Psiave’ Pend of the nodes in the path to the end device, with the secret Sm serv shared between master and the servant, resulting in e.pr„ ■= {eend I\Psiave 11 Pend>Sm>serv} and sends eserv to the servant node. 3. The servant node decrypts eserv using shared key Smserv to recover eend and the path public keys P^, P^. 4. The servant node removes the derived public key Psfave of the next device in the path and encrypts eend, concatenated with the remaining key in the path Pgnd> using the secret Sservs(ave shared between servant and the slave resulting in esiave := >Sserv,slaved and sends to the slave. 5. The slave node decrypts eslave using shared key Sserv>slave to recover eend and P^. 6. The slave node removes the derived public key of the next device in the path Pp^d (i.e. the target device). Since the final path key has been removed, the slave node does not need to re-encrypt and simply sends eend to the device corresponding to pder ‘end ■ 7. The end device decrypts eend using secret key Sm end shared between the master and the end device to recover the original command message M. An advantage of this variant is that encrypted messages can only be decrypted by the specified recipient node or device using its ECDH key with the master node. The small size of encrypted messages, along with the absence of signing by gateway nodes, results in faster network transmission times. 1.1.3 Messaging Protocol (Variant 2 - Unfinalized Tx Template) In this variant, encrypted messages are appended to blockchain transaction outputs, rather than sent as ciphertext with encrypted derived public keys. The unfinalized transaction template is encrypted and sent off-chain between gateway / intermediate nodes. Gateway nodes can see the intended recipient, but are unable decrypt the transaction template, or the message within it. After being received by the closest slave node to the intended enddevice, the transaction template can be decrypted, and the encrypted message extracted from the transaction to be sent to the end-device. Once received, the end device can decrypt the command message (using its shared secret key with the master node), execute the command, and encrypt a new message (using its shared secret key with the master node) containing execution acknowledgement and any loT data. This new encrypted message is sent back to the slave node, which adds the message as a new output in the ready-made transaction, signs the transaction (E.G. with SIGHASH_ALL) and submits it to the blockchain to be published. This method removes the need for a slave node to generate its own transaction template, while also ensuring proof of the issuance of a command is saved as an output in the transaction, and can be viewed on chain by the master node. This example protocol is as follows, and is shown schematically in Figure 6: 1. The master node encrypts command message M with the shared secret Sm end shared between the master node ‘m’ and the desired end device 'end', resulting in encrypted payload denoted by eenA := e(M,Smend). 2. The payload eend is then appended to the output of a new transaction template (e.g. using the OP_PUSHDATA opcode) creating the unfinalized transaction Txslave which is encrypted with the shared secret between the master and the slave node closest to the desired end device Smsiave and signed by the master node, e.g. using SIGHASH_SINGLE | ANYONECANPAY. 3. The encrypted transaction Txsiave is concatenated with the derived public keys Psiave’^end of the nodes in the path to the end device, and encrypted with the secret Smserv shared between master and the servant, resulting in eserv := (Txslave\\P^ve\\ Pend’Sm,serv) and sends eserv to the servant node. 4. The servant node decrypts eserv using shared key Smserv to recover Txsiave and the path public keys P^ave'^end- 5. The servant node removes the derived public key Psfave of the next device in the path and concatenates Txslave, with the remaining key in the path P^nd’ then encrypts the using the secret Sservsiave shared between servant and the slave resulting in esJat,e := (Txstave\\P^,SserV:Siave), and sends to the slave. 6. The slave node decrypts eslave using shared key Sserv_stave to recover Txsiave and pder rend • 7. The slave node then decrypts Txsiave using shared key Smsiave and is then able to extract the encrypted message eend from the transaction template, and sends to the end device with public key Pend- 8. The end device decrypts eend using secret key SmiBnd shared between the master and the end device to recover the original command message M. The command message is executed resulting in loT Data payload D. 9. The end device encrypts D and a command execution acknowledgement to create a new message eData using the shared key between the end device and the master node Smend resulting in the encrypted payload eData := (£>|pC / <,5end,7n)- This is sent back to the slave node. 10. On being received, the slave node creates a new output in the transaction Txsiave and appends the new message eData. The slave signs (e.g. with SIGHASH_ALL) and sends to the blockchain 150 to be published. An advantage of this variant is that the encrypted transaction template allows the message to be signed by the master node, and its provenance stored on-chain. Without gateway node signatures, network speeds are fast. 1.1.3 Messaging Protocol (Variant 3 - Unfinalized Tx Template Signed Every Network Hop) This variant is similar to variant 2 in that it uses unfinalized transaction templates. However, instead of encrypting the template so it can only be decrypted by the slave node, every node that receives the transaction template can decrypt it. This allows every node to add, and sign, a new input to the transaction, before encrypting it and sending to the next node in the chain. It is important to note that the message within the transaction remains encrypted until it is received by the desired end device. This method will result in the final transaction that is submitted to the blockchain 150 containing the full message history, from creation by the master node, through a chain of gateway nodes, command confirmation and loT data from the end device, and finally a (e.g. SIGHASH_ALL) signature from the enddevice's controlling slave node. These new outputs from gateway nodes may also contain data such as the UNIX time for transaction submission. This example protocol is as follows, and is shown schematically in Figure 7: 1. The master node encrypts command message M with the shared secret Sm>end shared between the master node'm' and the desired end device 'end', resulting in encrypted payload denoted by eend := e(M,SmiBnd). 2. The payload eend is then appended to the output of a new transaction template (e.g. using the OP_PUSHDATA opcode) creating the unfinalized transaction Tx which is left un-encrypted and signed by the master node, e.g. using SIGHASH_SINGLE | ANYONECANPAY. 3. Tx is concatenated with the derived public keys PsieaVe> ^md of the nodes in the path to the end device, with the secret Smserv shared between the master node and the servant, resulting in Txm_serv := (Tx11PdiaVe 11 Pend>SmiServ) a^d sends Txm_serv to the servant node. 4. The servant node decrypts Txmaster_serv using shared key Smserv to recover Tx and the path public keys P^ve,P^. 5. The servant node removes the derived public key Pgi^g of the next device in the path, and creates a new input, which it signs with its derived public key PsdePv, e.g. using SIGHASH_SINGLE | ANYONECANPAY. The servant concatenates Tx with the remaining key in the path Pdnd, and encrypts using the secret Sservsiave shared between the servant and the slave resulting in Txserv_siave := (Wi’SServ,slaved and sends to the slave. 6. The slave node decrypts Txserv_slave using shared key Sservsiave to recover Tx and pder ‘encl ■ 7. The slave node creates a new input confirming its receipt of Tx and relaying of the message, and signs, e.g. with SIGHASH_SINGLE | ANYONECANPAY. 8. The end device decrypts eend using secret key SmiBnd shared between the master and the end device to recover the original command message M. The command message is executed resulting in loT Data payload D 9. The end device encrypts D and a command execution acknowledgement to create a new message eData using the shared key between the end device and the master node Smend resulting in the encrypted payload eData := (D|\ACK,Smiend). This is sent back to the slave node. 10. On being received, the slave node can add a new output to the transaction Tx and appends the new message eData. The slave signs (e.g. with SIGHASH_ALL) and sends to the blockchain 150. An advantage of this variant is that the transaction templates can be decrypted by every node in the chain to the end device. These nodes can sign this transaction, confirming it has been received, encrypted, and relayed onto the next node, creating a clear and immutable history of the transaction, the instruction and the resulting action. 2. EXAMPLE SYSTEM OVERVIEW A blockchain refers to a form of distributed data structure, wherein a duplicate copy of the blockchain is maintained at each of a plurality of nodes in a distributed peer-to-peer (P2P) network (referred to below as a "blockchain network") and widely publicised. The blockchain comprises a chain of blocks of data, wherein each block comprises one or more transactions. Each transaction, other than so-called "coinbase transactions", points back to a preceding transaction in a sequence which may span one or more blocks going back to one or more coinbase transactions. Coinbase transactions are discussed further below. Transactions that are submitted to the blockchain network are included in new blocks. New blocks are created by a process often referred to as "mining", which involves each of a plurality of the nodes competing to perform "proof-of-work", i.e. solving a cryptographic puzzle based on a representation of a defined set of ordered and validated pending transactions waiting to be included in a new block of the blockchain. It should be noted that the blockchain may be pruned at some nodes, and the publication of blocks can be achieved through the publication of mere block headers. The transactions in the blockchain may be used for one or more of the following purposes: to convey a digital asset (i.e. a number of digital tokens), to order a set of entries in a virtualised ledger or registry, to receive and process timestamp entries, and / or to timeorder index pointers. A blockchain can also be exploited in order to layer additional functionality on top of the blockchain. For example, blockchain protocols may allow for storage of additional user data or indexes to data in a transaction. There is no pre-specified limit to the maximum data capacity that can be stored within a single transaction, and therefore increasingly more complex data can be incorporated. For instance this may be used to store an electronic document in the blockchain, or audio or video data. In an "output-based" model (sometimes referred to as a UTXO-based model), the data structure of a given transaction comprises one or more inputs and one or more outputs. Any spendable output comprises an element specifying an amount of the digital asset that is derivable from the proceeding sequence of transactions. The spendable output is sometimes referred to as a UTXO ("unspent transaction output"). The output may further comprise a locking script specifying a condition for the future redemption of the output. A locking script is a predicate defining the conditions necessary to validate and transfer digital tokens or assets. Each input of a transaction (other than a coinbase transaction) comprises a pointer (i.e. a reference) to such an output in a preceding transaction, and may further comprise an unlocking script for unlocking the locking script of the pointed-to output. So consider a pair of transactions, call them a first and a second transaction (or "target" transaction). The first transaction comprises at least one output specifying an amount of the digital asset, and comprising a locking script defining one or more conditions of unlocking the output. The second, target transaction comprises at least one input, comprising a pointer to the output of the first transaction, and an unlocking script for unlocking the output of the first transaction. In such a model, when the second, target transaction is sent to the blockchain network to be propagated and recorded in the blockchain, one of the criteria for validity applied at each node will be that the unlocking script meets all of the one or more conditions defined in the locking script of the first transaction. Another will be that the output of the first transaction has not already been redeemed by another, earlier valid transaction. Any node that finds the target transaction invalid according to any of these conditions will not propagate it (as a valid transaction, but possibly to register an invalid transaction) nor include it in a new block to be recorded in the blockchain. An alternative type of transaction model is an account-based model. In this case each transaction does not define the amount to be transferred by referring back to the UTXO of a preceding transaction in a sequence of past transactions, but rather by reference to an absolute account balance. The current state of all accounts is stored by the nodes separate to the blockchain and is updated constantly. Figure 1 shows an example system 100 for implementing a blockchain 150. The system 100 may comprise a packet-switched network 101, typically a wide-area internetwork such as the Internet. The packet-switched network 101 comprises a plurality of blockchain nodes 104 (often referred to as "miners") that may be arranged to form a peer-to-peer (P2P) network 106 within the packet-switched network 101. Whilst not illustrated, the blockchain nodes 104 may be arranged as a near-complete graph. Each blockchain node 104 is therefore highly connected to other blockchain nodes 104. Each blockchain node 104 comprises computer equipment of a peer, with different ones of the nodes 104 belonging to different peers. Each blockchain node 104 comprises processing apparatus comprising one or more processors, e.g. one or more central processing units (CPUs), accelerator processors, application specific processors and / or field programmable gate arrays (FPGAs), and other equipment such as application specific integrated circuits (ASICs). Each node also comprises memory, i.e. computer-readable storage in the form of a non-transitory computer-readable medium or media. The memory may comprise one or more memory units employing one or more memory media, e.g. a magnetic medium such as a hard disk; an electronic medium such as a solid-state drive (SSD), flash memory or EEPROM; and / or an optical medium such as an optical disk drive. The blockchain 150 comprises a chain of blocks of data 151, wherein a respective copy of the blockchain 150 is maintained at each of a plurality of blockchain nodes 104 in the distributed or blockchain network 106. As mentioned above, maintaining a copy of the blockchain 150 does not necessarily mean storing the blockchain 150 in full. Instead, the blockchain 150 may be pruned of data so long as each blockchain node 150 stores the block header (discussed below) of each block 151. Each block 151 in the chain comprises one or more transactions 152, wherein a transaction in this context refers to a kind of data structure. The nature of the data structure will depend on the type of transaction protocol used as part of a transaction model or scheme. A given blockchain will use one particular transaction protocol throughout. A blockchain node 104 may be configured to forward transactions 152 to other blockchain nodes 104, and thereby cause transactions 152 to be propagated throughout the network 106. A blockchain node 104 may be configured to create blocks 151 and to store a respective copy of the same blockchain 150 in their respective memory. A blockchain node 104 may also maintain an ordered set (or "pool") 154 of transactions 152 waiting to be incorporated into blocks 151. The ordered pool 154 is often referred to as a "mempool". This term herein is not intended to limit to any particular blockchain, protocol or model. It refers to the ordered set of transactions which a node 104 has accepted as valid and for which the node 104 is obliged not to accept any other transactions attempting to spend the same output. In a given present transaction 152j, the (or each) input comprises a pointer referencing the output of a preceding transaction 152i in the sequence of transactions, specifying that this output is to be redeemed or "spent" in the present transaction 152j. Spending or redeeming does not necessarily imply transfer of a financial asset, though that is certainly one common application. More generally spending could be described as consuming the output, or assigning it to one or more outputs in another, onward transaction. In general, the preceding transaction could be any transaction in the ordered set 154 or any block 151. The preceding transaction 152i need not necessarily exist at the time the present transaction 152j is created or even sent to the network 106, though the preceding transaction 152i will need to exist and be validated in order for the present transaction to be valid. Hence "preceding" herein refers to a predecessor in a logical sequence linked by pointers, not necessarily the time of creation or sending in a temporal sequence, and hence it does not necessarily exclude that the transactions 152i, 152j be created or sent out-of-order (see discussion below on orphan transactions). The preceding transaction 152i could equally be called the antecedent or predecessor transaction. Due to the resources involved in transaction validation and publication, typically at least each of the blockchain nodes 104 takes the form of a servant comprising one or more physical servant units, or even whole a data centre. However in principle any given blockchain node 104 could take the form of a user terminal or a group of user terminals networked together. The memory of each blockchain node 104 stores software configured to run on the processing apparatus of the blockchain node 104 in order to perform its respective role or roles and handle transactions 152 in accordance with the blockchain node protocol. It will be understood that any action attributed herein to a blockchain node 104 may be performed by the software run on the processing apparatus of the respective computer equipment. The node software may be implemented in one or more applications at the application layer, or a lower layer such as the operating system layer or a protocol layer, or any combination of these. Any given blockchain node may be configured to perform one or more of the following operations: validating transactions, storing transactions, propagating transactions to other peers, performing consensus (e.g. proof-of-work) / mining operations. In some examples, each type of operation is performed by a different node 104. That is, nodes may specialise in particular operation. For example, a nodes 104 may focus on transaction validation and propagation, or on block mining. In some examples, a blockchain node 104 may perform more than one of these operations in parallel. Any reference to a blockchain node 104 may refer to an entity that is configured to perform at least one of these operations. Also connected to the network 101 is the computer equipment 102 of each of a plurality of parties 103 in the role of consuming users. These users may interact with the blockchain network 106 but do not participate in validating transactions or constructing blocks. Some of these users or agents 103 may act as senders and recipients in transactions. Other users may interact with the blockchain 150 without necessarily acting as senders or recipients. For instance, some parties may act as storage entities that store a copy of the blockchain 150 (e.g. having obtained a copy of the blockchain from a blockchain node 104). Some or all of the parties 103 may be connected as part of a different network, e.g. a network overlaid on top of the blockchain network 106. Users of the blockchain network (often referred to as "clients") may be said to be part of a system that includes the blockchain network 106; however, these users are not blockchain nodes 104 as they do not perform the roles required of the blockchain nodes. Instead, each party 103 may interact with the blockchain network 106 and thereby utilize the blockchain 150 by connecting to (i.e. communicating with) a blockchain node 106. Two parties 103 and their respective equipment 102 are shown for illustrative purposes: a first party 103a and his / her respective computer equipment 102a, and a second party 103b and his / her respective computer equipment 102b. It will be understood that many more such parties 103 and their respective computer equipment 102 may be present and participating in the system 100, but for convenience they are not illustrated. Each party 103 may be an individual or an organization. Purely by way of illustration the first party 103a is referred to herein as Alice and the second party 103b is referred to as Bob, but it will be appreciated that this is not limiting and any reference herein to Alice or Bob may be replaced with "first party" and "second "party" respectively. The computer equipment 102 of each party 103 comprises respective processing apparatus comprising one or more processors, e.g. one or more CPUs, GPUs, other accelerator processors, application specific processors, and / or FPGAs. The computer equipment 102 of each party 103 further comprises memory, i.e. computer-readable storage in the form of a non-transitory computer-readable medium or media. This memory may comprise one or more memory units employing one or more memory media, e.g. a magnetic medium such as hard disk; an electronic medium such as an SSD, flash memory or EEPROM; and / or an optical medium such as an optical disc drive. The memory on the computer equipment 102 of each party 103 stores software comprising a respective instance of at least one client application 105 arranged to run on the processing apparatus. It will be understood that any action attributed herein to a given party 103 may be performed using the software run on the processing apparatus of the respective computer equipment 102. The computer equipment 102 of each party 103 comprises at least one user terminal, e.g. a desktop or laptop computer, a tablet, a smartphone, or a wearable device such as a smartwatch. The computer equipment 102 of a given party 103 may also comprise one or more other networked resources, such as cloud computing resources accessed via the user terminal. The client application 105 may be initially provided to the computer equipment 102 of any given party 103 on suitable computer-readable storage medium or media, e.g. downloaded from a servant, or provided on a removable storage device such as a removable SSD, flash memory key, removable EEPROM, removable magnetic disk drive, magnetic floppy disk or tape, optical disk such as a CD or DVD ROM, or a removable optical drive, etc. The client application 105 comprises at least a "wallet" function. This has two main functionalities. One of these is to enable the respective party 103 to create, authorise (for example sign) and send transactions 152 to one or more bitcoin nodes 104 to then be propagated throughout the network of blockchain nodes 104 and thereby included in the blockchain 150. The other is to report back to the respective party the amount of the digital asset that he or she currently owns. In an output-based system, this second functionality comprises collating the amounts defined in the outputs of the various 152 transactions scattered throughout the blockchain 150 that belong to the party in question. Note: whilst the various client functionality may be described as being integrated into a given client application 105, this is not necessarily limiting and instead any client functionality described herein may instead be implemented in a suite of two or more distinct applications, e.g. interfacing via an API, or one being a plug-in to the other. More generally the client functionality could be implemented at the application layer or a lower layer such as the operating system, or any combination of these. The following will be described in terms of a client application 105 but it will be appreciated that this is not limiting. The instance of the client application or software 105 on each computer equipment 102 is operatively coupled to at least one of the blockchain nodes 104 of the network 106. This enables the wallet function of the client 105 to send transactions 152 to the network 106. The client 105 is also able to contact blockchain nodes 104 in order to query the blockchain 150 for any transactions of which the respective party 103 is the recipient (or indeed inspect other parties' transactions in the blockchain 150, since in embodiments the blockchain 150 is a public facility which provides trust in transactions in part through its public visibility). The wallet function on each computer equipment 102 is configured to formulate and send transactions 152 according to a transaction protocol. As set out above, each blockchain node 104 runs software configured to validate transactions 152 according to the blockchain node protocol, and to forward transactions 152 in order to propagate them throughout the blockchain network 106. The transaction protocol and the node protocol correspond to one another, and a given transaction protocol goes with a given node protocol, together implementing a given transaction model. The same transaction protocol is used for all transactions 152 in the blockchain 150. The same node protocol is used by all the nodes 104 in the network 106. An alternative type of transaction protocol operated by some blockchain networks may be referred to as an "account-based" protocol, as part of an account-based transaction model. In the account-based case, each transaction does not define the amount to be transferred by referring back to the UTXO of a preceding transaction in a sequence of past transactions, but rather by reference to an absolute account balance. The current state of all accounts is stored, by the nodes of that network, separate to the blockchain and is updated constantly. In such a system, transactions are ordered using a running transaction tally of the account (also called the "position" or "nonce"). This value is signed by the sender as part of their cryptographic signature and is hashed as part of the transaction reference calculation. In addition, an optional data field may also be signed the transaction. This data field may point back to a previous transaction, for example if the previous transaction ID is included in the data field. Some account-based transaction models share several similarities with the output-based transaction model described herein. For example, as mentioned above, the data field of an account-based transaction may point back to a previous transaction, which is equivalent to the input of an output-based transaction which references an outpoint a previous transaction. Thus both models enable linking between transactions. As another example, an account-based transaction contains a "recipient" field (in which a receiving address of an account is specified) and a "value" field (in which an amount of digital asset may be specified). Together the recipient and value fields are equivalent to the output of an outputbased transaction which may be used to assign an amount of digital asset to a blockchain address. Similarly, an account-based transaction has a "signature" field which includes a signature for the transaction. The signature is generated using the sender's private key and confirms the sender has authorized this transaction. This is equivalent to an input / unlocking script of an output-based transaction which, typically, includes a signature for the transaction. When both types of transaction are submitted to their respective blockchain networks, the signatures are checked to determine whether the transaction is valid and can be recorded on the blockchain. On an account-based blockchain, a "smart contact" refers to a transaction that contains a script configured to perform one or more actions (e.g. send or "release" a digital asset to a recipient address) in response to one or more inputs (provided by a transaction) meeting one or more conditions defined by the smart contact's script. The smart contract exists as a transaction on the blockchain, and can be called (or triggered) by subsequent transactions. Thus, in some examples, a smart contract may be considered equivalent to a locking script of an output-based transaction, which can be triggered by a subsequent transaction, and checks whether one or more conditions defined by the locking script are met by the input of the subsequent transaction. 3. UTXO-BASED MODEL Figure 2 illustrates an example transaction protocol. This is an example of a UTXO-based protocol. A transaction 152 (abbreviated "Tx") is the fundamental data structure of the blockchain 150 (each block 151 comprising one or more transactions 152). The following will be described by reference to an output-based or "UTXO" based protocol. However, this is not limiting to all possible embodiments. Note that while the example UTXO-based protocol is described with reference to bitcoin, it may equally be implemented on other example blockchain networks. In a UTXO-based model, each transaction ("Tx") 152 comprises a data structure comprising one or more inputs 202, and one or more outputs 203. Each output 203 may comprise an unspent transaction output (UTXO), which can be used as the source for the input 202 of another new transaction (if the UTXO has not already been redeemed). The UTXO includes a value specifying an amount of a digital asset. This represents a set number of tokens on the distributed ledger. The UTXO may also contain the transaction ID of the transaction from which it came, amongst other information. The transaction data structure may also comprise a header 201, which may comprise an indicator of the size of the input field(s) 202 and output field(s) 203. The header 201 may also include an ID of the transaction. In embodiments the transaction ID is the hash of the transaction data (excluding the transaction ID itself) and stored in the header 201 of the raw transaction 152 submitted to the nodes 104. Say Alice 103a wishes to create a transaction 152j transferring an amount of the digital asset in question to Bob 103b. In Figure 2 Alice's new transaction 152j is labelled "Txi". It takes an amount of the digital asset that is locked to Alice in the output 203 of a preceding transaction 152i in the sequence, and transfers at least some of this to Bob. The preceding transaction 152i is labelled “Txo” in Figure 2. Ztoand Txi are just arbitrary labels. They do not necessarily mean that Txo is the first transaction in the blockchain 151, nor that Txi is the immediate next transaction in the pool 154. Txi could point back to any preceding (i.e. antecedent) transaction that still has an unspent output 203 locked to Alice. The terms "preceding" and "subsequent" as used herein in the context of the sequence of transactions refer to the order of the transactions in the sequence as defined by the transaction pointers specified in the transactions (which transaction points back to which other transaction, and so forth). They could equally be replaced with "predecessor" and "successor", or "antecedent" and "descendant", "parent" and "child", or such like. It does not necessarily imply an order in which they are created, sent to the network 106, or arrive at any given blockchain node 104. Nevertheless, a subsequent transaction (the descendent transaction or "child") which points to a preceding transaction (the antecedent transaction or "parent") will not be validated until and unless the parent transaction is validated. A child that arrives at a blockchain node 104 before its parent is considered an orphan. It may be discarded or buffered for a certain time to wait for the parent, depending on the node protocol and / or node behaviour. One of the one or more outputs 203 of the preceding transaction Txo comprises a particular UTXO, labelled here UTXOo. Each UTXO comprises a value specifying an amount of the digital asset represented by the UTXO, and a locking script which defines a condition which must be met by an unlocking script in the input 202 of a subsequent transaction in order for the subsequent transaction to be validated, and therefore for the UTXO to be successfully redeemed. The locking script (aka scriptPubKey) is a piece of code written in the domain specific language recognized by the node protocol. A particular example of such a language is called "Script" (capital S) which is used by the blockchain network. The locking script specifies what information is required to spend a transaction output 203, for example the requirement of Alice's signature. Locking scripts appear in the outputs of transactions. The unlocking script (aka scriptSig) is a piece of code written the domain specific language that provides the information required to satisfy the locking script criteria. For example, it may contain Bob's signature. Unlocking scripts appear in the input 202 of transactions. So in the example illustrated, UTXOo in the output 203 of Txo com prises a locking script [Checksig Pa] which requires a signature Sig Pa of Alice in order for UTXOo to be redeemed (strictly, in order for a subsequent transaction attempting to redeem UTXOolo be valid). [Checksig Pa] contains a representation (i.e. a hash) of the public key P / ifrom a publicprivate key pair of Alice. The input 202 of Txi comprises a pointer pointing back to Txi (e.g. by means of its transaction ID, TxIDo, which in embodiments is the hash of the whole transaction Txo}. The input 202 of Txi comprises an index identifying UTXOo within Txo, to identify it amongst any other possible outputs of Txo. The input 202 of Txi further comprises an unlocking script <Sig Pa> which comprises a cryptographic signature of Alice, created by Alice applying her private key from the key pair to a predefined portion of data (sometimes called the "message" in cryptography). The data (or "message") that needs to be signed by Alice to provide a valid signature may be defined by the locking script, or by the node protocol, or by a combination of these. When the new transaction Txi arrives at a blockchain node 104, the node applies the node protocol. This comprises running the locking script and unlocking script together to check whether the unlocking script meets the condition defined in the locking script (where this condition may comprise one or more criteria). Note that the script code is often represented schematically (i.e. not using the exact language). For example, one may use operation codes (opcodes) to represent a particular function. "OP_..." refers to a particular opcode of the Script language. As an example, OP_RETURN is an opcode of the Script language that when preceded by OP_FALSE at the beginning of a locking script creates an unspendable output of a transaction that can store data within the transaction, and thereby record the data immutably in the blockchain 150. E.g. the data could comprise a document which it is desired to store in the blockchain. Typically an input of a transaction contains a digital signature corresponding to a public key Pa. In embodiments this is based on the ECDSA using the elliptic curve secp256kl. A digital signature signs a particular piece of data. In some embodiments, for a given transaction the signature will sign part of the transaction input, and some or all of the transaction outputs. The particular parts of the outputs it signs depends on the SIGHASH flag. The SIGHASH flag is usually a 4-byte code included at the end of a signature to select which outputs are signed (and thus fixed at the time of signing). The locking script is sometimes called "scriptPubKey" referring to the fact that it typically comprises the public key of the party to whom the respective transaction is locked. The unlocking script is sometimes called "scriptSig" referring to the fact that it typically supplies the corresponding signature. However, more generally it is not essential in all applications of a blockchain 150 that the condition for a UTXO to be redeemed comprises authenticating a signature. More generally the scripting language could be used to define any one or more conditions. Hence the more general terms "locking script" and "unlocking script" may be preferred. 4. FURTHER REMARKS Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The scope of the disclosure is not limited by the described embodiments but only by the accompanying claims. For instance, some embodiments above have been described in terms of a bitcoin network 106, bitcoin blockchain 150 and bitcoin nodes 104. However it will be appreciated that the bitcoin blockchain is one particular example of a blockchain 150 and the above description may apply generally to any blockchain. That is, the present invention is in by no way limited to the bitcoin blockchain. More generally, any reference above to bitcoin network 106, bitcoin blockchain 150 and bitcoin nodes 104 may be replaced with reference to a blockchain network 106, blockchain 150 and blockchain node 104 respectively. The blockchain, blockchain network and / or blockchain nodes may share some or all of the described properties of the bitcoin blockchain 150, bitcoin network 106 and bitcoin nodes 104 as described above. In preferred embodiments of the invention, the blockchain network 106 is the bitcoin network and bitcoin nodes 104 perform at least all of the described functions of creating, publishing, propagating and storing blocks 151 of the blockchain 150. It is not excluded that there may be other network entities (or network elements) that only perform one or some but not all of these functions. That is, a network entity may perform the function of propagating and / or storing blocks without creating and publishing blocks (recall that these entities are not considered nodes of the preferred bitcoin network 106). In other embodiments of the invention, the blockchain network 106 may not be the bitcoin network. In these embodiments, it is not excluded that a node may perform at least one or some but not all of the functions of creating, publishing, propagating and storing blocks 151 of the blockchain 150. For instance, on those other blockchain networks a "node" may be used to refer to a network entity that is configured to create and publish blocks 151 but not store and / or propagate those blocks 151 to other nodes. Even more generally, any reference to the term "bitcoin node" 104 above may be replaced with the term "network entity" or "network element", wherein such an entity / element is configured to perform some or all of the roles of creating, publishing, propagating and storing blocks. The functions of such a network entity / element may be implemented in hardware in the same way described above with reference to a blockchain node 104. Some embodiments have been described in terms of the blockchain network implementing a proof-of-work consensus mechanism to secure the underlying blockchain. However proof-of-work is just one type of consensus mechanism and in general embodiments may use any type of suitable consensus mechanism such as, for example, proof-of-stake, delegated proof-of-stake, proof-of-capacity, or proof-of-elapsed time. As a particular example, proof-of-stake uses a randomized process to determine which blockchain node 104 is given the opportunity to produce the next block 151. The chosen node is often referred to as a validator. Blockchain nodes can lock up their tokens for a certain time in order to have the chance of becoming a validator. Generally, the node who locks the biggest stake for the longest period of time has the best chance of becoming the next validator. It will be appreciated that the above embodiments have been described by way of example only. More generally there may be provided a method, apparatus or program in accordance with any one or more of the following Statements. Statement 1. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the master node and comprises: generating an encrypted command message by encrypting a command message with a respective encryption key shared between the master node and the target end device; generating a blockchain transaction comprising the encrypted command message; generating a first encrypted transit message by combining the blockchain transaction with a respective public key of one or more intermediate nodes of the respective network path and a respective public key of the target end device, and encrypting the combination with a respective encryption key shared between the master node and the initial intermediate node; and sending the first encrypted transit message to the initial intermediate node. Statement 2. The method of statement 1, wherein the respective encryption key shared between the master node and the target end device is based on a respective private key of the master node and a respective public key associated with the target end device, and wherein the respective encryption key shared between the master node and the initial intermediate node is based on the respective private key of the master node and a respective public key associated with the initial intermediate node. Statement 3. The method of statement 2, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the master node is based on the master private key and a network secret, wherein the respective public key associated with the target end device is based on the respective device public key and a public key corresponding to the network secret, and wherein the respective public key associated with the initial intermediate node is based on the respective intermediate public key and the public key corresponding to the network secret. Statement 4. The method of statement 3, comprising: generating the network secret; and for one or more respective intermediate nodes, including the initial intermediate node: generating a respective shared secret based on the master private key and the respective intermediate public key associated with the respective intermediate node; generating a respective encrypted network secret by encrypting the network secret with the respective shared secret; and sending the respective encrypted network secret to the respective intermediate node; and for one or more respective end devices, including the target end device: generating a respective shared secret based on the master private key and the respective device public key associated with the respective end device; generating a respective encrypted network secret by encrypting the network secret with the respective shared secret; and sending the respective encrypted network secret to the respective end device or a respective intermediate node configured to control the respective end device, for sending to the respective end device by said respective intermediate node. Statement 5. The method of any preceding statement, wherein the blockchain transaction is signed with a key associated with the master private key. Statement 6. The method of statement 5, wherein the signature is associated with a signature flag that allows additional signatures to be added to the blockchain transaction. Statement 7. The method of any preceding statement, wherein the blockchain transaction is published on the blockchain and comprises a data message generated by the target end device, and wherein the method comprises obtaining the blockchain transaction from the blockchain, and extracting the data message from the blockchain transaction. Statement 8. The method of statement 7, wherein the data message is an encrypted data message generated by the target end device encrypting data with the respective encryption key shared between the master node and the target end device, and wherein the method comprises obtaining the data by decrypting the encrypted data message with the respective encryption key. Statement 9. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the initial intermediate node of the network path and comprises: receiving a first encrypted transit message, wherein the first encrypted transit message is encrypted with a respective encryption key shared between the master node and the initial intermediate node; obtaining a first message by decrypting the first encrypted transit message with the respective encryption key, and i) wherein the first message comprises a blockchain transaction combined with a respective public key of one or more of the intermediate nodes of the respective network path and a respective public key of the target end device, and wherein the method comprises: generating a second encrypted transit message by removing, from the second message, a respective public key of a next intermediate node of the respective network path, and encrypting the result with a respective encryption key shared between the initial intermediate node and the next intermediate node; and sending the second encrypted transit message to the next intermediate node; or ii) wherein the first message comprises the blockchain transaction combined with the respective public key of the target end device, and wherein the method comprises sending the blockchain transaction to the target end device. Statement 10. The method of statement 9, wherein the respective encryption key shared between the master node and the initial intermediate node is based on a respective public key associated with the master node and a respective private key of the initial intermediate node, and wherein the respective encryption key shared between the initial intermediate node and the next intermediate node is based on the respective private key of the initial intermediate node and a respective public key associated with the next intermediate node. Statement 11. The method of statement 10, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the initial intermediate node is based on the respective intermediate private key and a network secret, wherein the public key associated with the master node is based on the master public key and a public key corresponding to the network secret, and wherein the respective public key associated with the next intermediate node is based on the respective intermediate public key and the public key corresponding to the network secret. Statement 12. The method of statement 11, comprising: receiving a first encrypted network secret, wherein the first encrypted network secret is encrypted with a first shared secret generated based on the master public key and the respective intermediate private key of the initial intermediate node; obtaining the network secret by decrypting the first encrypted network secret with the first shared secret; generating the respective private key of the initial intermediate node based on the respective intermediate private key and the network secret; generating the public key associated with the master node based on the master public key and a network public key corresponding to the network secret; generating the respective encryption key based on the respective private key of the initial intermediate node and the public key associated with the master node; and for one or more respective intermediate nodes, including the next intermediate node: generating a respective public key associated with the respective intermediate node based on the respective intermediate public key and the network public key; and generating a respective encryption key shared between the initial intermediate node and the respective intermediate node based on the respective private key of the initial intermediate node and the respective public key of the respective intermediate node. Statement 13. The method of any of statements 9 to 12, comprising signing the blockchain transaction with a key associated with the respective public key associated with the initial intermediate node. Statement 14. The method of any of statements 9 to 13, comprising: generating an acknowledgement message; generating an encrypted acknowledgement message by encrypting the acknowledgment message with the respective encryption key shared between the master node and the initial intermediate node; and adding the encrypted acknowledgement message to the blockchain transaction. Statement 15. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the final intermediate node of the network path and comprises: receiving a second encrypted transit message, wherein the second encrypted transit message is encrypted with a respective encryption key shared between the final intermediate node and a previous intermediate node in the respective network path; obtaining the second message by decrypting the second encrypted transit message with the respective encryption key, wherein the second message comprises a blockchain transaction combined with a respective public key associated with the target end device, wherein the blockchain transaction comprises an encrypted command message; generating an acknowledgement message; generating an encrypted acknowledgement message by encrypting the acknowledgment message with the respective encryption key shared between the master node and the final intermediate node; and adding a signature corresponding to the respective public key of the final intermediate node, and the encrypted acknowledgement message, to the blockchain transaction; and sending the encrypted command message to the target end device. Statement 16. The method of statement 15, wherein the respective encryption key shared between the final intermediate node and the previous intermediate node is based on a respective private key of the final intermediate node and a respective public key associated with the previous intermediate node, and wherein the respective encryption key shared between the final intermediate node and the master node is based on the respective private key of the final intermediate node and a respective public key associated with the master node. Statement 17. The method of statement 16, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the final intermediate node is based on the respective intermediate private key and a network secret, wherein the respective public key associated with the previous intermediate node is based on the respective intermediate public key and a public key corresponding to the network secret, and wherein the respective public key associated with the master node is based on the master public key and the public key corresponding to the network secret. Statement 18. The method of statement 17, comprising: receiving a second encrypted network secret, wherein the second encrypted network secret is encrypted with a second shared secret generated based on the master public key and the respective intermediate private key associated with the final intermediate node; obtaining the network secret by decrypting the second encrypted network secret with the second shared secret; generating the respective private key of the final intermediate node based on respective intermediate private key and the network secret; generating the public key associated with the previous intermediate node based on the respective intermediate public key and a network public key corresponding to the network secret; generating the respective encryption key shared between the final intermediate node and the previous intermediate node based on the respective private key of the final intermediate node and the respective public key associated with the previous intermediate node; generating the respective public key associated with the master node based on the master public key and the network public key; and generating the respective encryption key shared between the final intermediate node and the master node based on the respective private key of the final intermediate node and the respective public key associated with the master node. Statement 19. The method of any of statements 15 to 18, comprising: receiving, from the target end device, an encrypted data message; adding a signature corresponding to the respective public key of the final intermediate node, and the encrypted data message, to the blockchain transaction; and submitting the blockchain transaction to the blockchain. Statement 20. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the target end device and comprises: receiving, from the final intermediate node, an encrypted command message, wherein the encrypted command message is encrypted with a respective encryption key shared between the master node and the target end device; obtaining the command message by decrypting the encrypted command message with the respective encryption key; and performing a command associated with the command message. Statement 21. The method of statement 20, wherein the respective encryption key shared between the master node and the target end device is based on a respective private key of the target end device and a respective public key associated with the master node. Statement 22. The method of statement 21, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the target end device is based on the respective device private key and a network secret, and wherein the public key associated with the master node is based on the respective master public key and a public key corresponding to the network secret. Statement 23. The method of statement 22, comprising: receiving a third encrypted network secret, wherein the third encrypted network secret is encrypted with a third shared secret generated based on the master public key and the respective device private key; obtaining the network secret by decrypting the third encrypted network secret with the third shared secret; generating the respective private key of the target end device based on the respective device private key and the network secret; generating the public key associated with the master node based on the master public key and a network public key corresponding to the network secret; and generating the respective encryption key based on the respective private key of the target end device and the respective public key of the master node. Statement 24. The method of any of statements 20 to 23, comprising: generating a data message in response to performing the command; and sending the encrypted data message to the final intermediate node. Statement 25. The method of statement 24, wherein the data message is an encrypted data message generated by encrypting data with the respective encryption key shared between the master node and the target end device. Statement 26. The method of any preceding statement, wherein the network is an internet of things (loT) network and each respective end device is a respective sensor and / or actuator device. Statement 27. 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 26. Statement 28. 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 26.

Claims

1. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the master node and comprises:generating an encrypted command message by encrypting a command message with a respective encryption key shared between the master node and the target end device;generating a blockchain transaction comprising the encrypted command message;generating a first encrypted transit message by combining the blockchain transaction with a respective public key of one or more intermediate nodes of the respective network path and a respective public key of the target end device, and encrypting the combination with a respective encryption key shared between the master node and the initial intermediate node; andsending the first encrypted transit message to the initial intermediate node.

2. The method of claim 1, wherein the respective encryption key shared between the master node and the target end device is based on a respective private key of the master node and a respective public key associated with the target end device, and wherein the respective encryption key shared between the master node and the initial intermediate node is based on the respective private key of the master node and a respective public key associated with the initial intermediate node.

3. The method of claim 2, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the master node is based on the master private key and a network secret, wherein therespective public key associated with the target end device is based on the respective device public key and a public key corresponding to the network secret, and wherein the respective public key associated with the initial intermediate node is based on the respective intermediate public key and the public key corresponding to the network secret.

4. The method of claim 3, comprising:generating the network secret; andfor one or more respective intermediate nodes, including the initial intermediate node:generating a respective shared secret based on the master private key and the respective intermediate public key associated with the respective intermediate node;generating a respective encrypted network secret by encrypting the network secret with the respective shared secret; andsending the respective encrypted network secret to the respective intermediate node; andfor one or more respective end devices, including the target end device:generating a respective shared secret based on the master private key and the respective device public key associated with the respective end device;generating a respective encrypted network secret by encrypting the network secret with the respective shared secret; andsending the respective encrypted network secret to the respective end device or a respective intermediate node configured to control the respective end device, for sending to the respective end device by said respective intermediate node.

5. The method of any preceding claim, wherein the blockchain transaction is signed with a key associated with the master private key.

6. The method of claim 5, wherein the signature is associated with a signature flag that allows additional signatures to be added to the blockchain transaction.

7. The method of any preceding claim, wherein the blockchain transaction is published on the blockchain and comprises a data message generated by the target end device, andwherein the method comprises obtaining the blockchain transaction from the blockchain, and extracting the data message from the blockchain transaction.

8. The method of claim 7, wherein the data message is an encrypted data message generated by the target end device encrypting data with the respective encryption key shared between the master node and the target end device, and wherein the method comprises obtaining the data by decrypting the encrypted data message with the respective encryption key.

9. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising an initial intermediate node, and wherein the method is performed by the initial intermediate node of the network path and comprises:receiving a first encrypted transit message, wherein the first encrypted transit message is encrypted with a respective encryption key shared between the master node and the initial intermediate node;obtaining a first message by decrypting the first encrypted transit message with the respective encryption key, andi) wherein the first message comprises a blockchain transaction combined with a respective public key of one or more of the intermediate nodes of the respective network path and a respective public key of the target end device, and wherein the method comprises:generating a second encrypted transit message by removing, from the second message, a respective public key of a next intermediate node of the respective network path, and encrypting the result with a respective encryption key shared between the initial intermediate node and the next intermediate node; andsending the second encrypted transit message to the next intermediate node; orii) wherein the first message comprises the blockchain transaction combined with the respective public key of the target end device, and wherein the method comprises sending the blockchain transaction to the target end device.

10. The method of claim 9, wherein the respective encryption key shared between the master node and the initial intermediate node is based on a respective public key associated with the master node and a respective private key of the initial intermediate node, and wherein the respective encryption key shared between the initial intermediate node and the next intermediate node is based on the respective private key of the initial intermediate node and a respective public key associated with the next intermediate node.

11. The method of claim 10, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the initial intermediate node is based on the respective intermediate private key and a network secret, wherein the public key associated with the master node is based on the master public key and a public key corresponding to the network secret, and wherein the respective public key associated with the next intermediate node is based on the respective intermediate public key and the public key corresponding to the network secret.

12. The method of claim 11, comprising:receiving a first encrypted network secret, wherein the first encrypted network secret is encrypted with a first shared secret generated based on the master public key and the respective intermediate private key of the initial intermediate node;obtaining the network secret by decrypting the first encrypted network secret with the first shared secret;generating the respective private key of the initial intermediate node based on the respective intermediate private key and the network secret;generating the public key associated with the master node based on the master public key and a network public key corresponding to the network secret;generating the respective encryption key based on the respective private key of the initial intermediate node and the public key associated with the master node; andfor one or more respective intermediate nodes, including the next intermediate node:generating a respective public key associated with the respective intermediate node based on the respective intermediate public key and the network public key; andgenerating a respective encryption key shared between the initial intermediate node and the respective intermediate node based on the respective private key of the initial intermediate node and the respective public key of the respective intermediate node.

13. The method of any of claims 9 to 12, comprising signing the blockchain transaction with a key associated with the respective public key associated with the initial intermediate node.

14. The method of any of claims 9 to 13, comprising:generating an acknowledgement message;generating an encrypted acknowledgement message by encrypting the acknowledgment message with the respective encryption key shared between the master node and the initial intermediate node; andadding the encrypted acknowledgement message to the blockchain transaction.

15. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the final intermediate node ofthe network path and comprises:receiving a second encrypted transit message, wherein the second encrypted transit message is encrypted with a respective encryption key shared between the final intermediate node and a previous intermediate node in the respective network path;obtaining the second message by decrypting the second encrypted transit message with the respective encryption key, wherein the second message comprises a blockchain transaction combined with a respective public key associated with the target end device, wherein the blockchain transaction comprises an encrypted command message;generating an acknowledgement message;generating an encrypted acknowledgement message by encrypting the acknowledgment message with the respective encryption key shared between the master node and the final intermediate node; andadding a signature corresponding to the respective public key of the final intermediate node, and the encrypted acknowledgement message, to the blockchain transaction; andsending the encrypted command message to the target end device.

16. The method of claim 15, wherein the respective encryption key shared between the final intermediate node and the previous intermediate node is based on a respective private key of the final intermediate node and a respective public key associated with the previous intermediate node, and wherein the respective encryption key shared between the final intermediate node and the master node is based on the respective private key of the final intermediate node and a respective public key associated with the master node.

17. The method of claim 16, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the final intermediate node is based on the respective intermediate private key and a network secret, wherein the respective public key associated with the previous intermediate node is based on the respective intermediate public key and a public key corresponding to thenetwork secret, and wherein the respective public key associated with the master node is based on the master public key and the public key corresponding to the network secret.

18. The method of claim 17, comprising:receiving a second encrypted network secret, wherein the second encrypted network secret is encrypted with a second shared secret generated based on the master public key and the respective intermediate private key associated with the final intermediate node;obtaining the network secret by decrypting the second encrypted network secret with the second shared secret;generating the respective private key of the final intermediate node based on respective intermediate private key and the network secret;generating the public key associated with the previous intermediate node based on the respective intermediate public key and a network public key corresponding to the network secret;generating the respective encryption key shared between the final intermediate node and the previous intermediate node based on the respective private key of the final intermediate node and the respective public key associated with the previous intermediate node;generating the respective public key associated with the master node based on the master public key and the network public key; andgenerating the respective encryption key shared between the final intermediate node and the master node based on the respective private key of the final intermediate node and the respective public key associated with the master node.

19. The method of any of claims 15 to 18, comprising:receiving, from the target end device, an encrypted data message;adding a signature corresponding to the respective public key of the final intermediate node, and the encrypted data message, to the blockchain transaction; andsubmitting the blockchain transaction to the blockchain.

20. A computer-implemented method of secure communication between nodes of a network, wherein the network comprises a master node, a set of intermediate nodes, and a set of end devices, wherein each end device is controllable by at least one intermediate node, wherein the network comprises a network path of one or more intermediate nodes between the master node and a target end device, the network path comprising a final intermediate node, and wherein the method is performed by the target end device and comprises:receiving, from the final intermediate node, an encrypted command message, wherein the encrypted command message is encrypted with a respective encryption key shared between the master node and the target end device;obtaining the command message by decrypting the encrypted command message with the respective encryption key; andperforming a command associated with the command message.

21. The method of claim 20, wherein the respective encryption key shared between the master node and the target end device is based on a respective private key of the target end device and a respective public key associated with the master node.

22. The method of claim 21, wherein the master node has a master private key and is associated with a corresponding master public key, wherein each intermediate node has a respective intermediate private key and is associated with a corresponding intermediate public key, wherein each end device has a respective device private key and is associated with a corresponding respective device public key, wherein the respective private key of the target end device is based on the respective device private key and a network secret, and wherein the public key associated with the master node is based on the respective master public key and a public key corresponding to the network secret.

23. The method of claim 22, comprising:receiving a third encrypted network secret, wherein the third encrypted network secret is encrypted with a third shared secret generated based on the master public key and the respective device private key;obtaining the network secret by decrypting the third encrypted network secret with the third shared secret;generating the respective private key of the target end device based on the respective device private key and the network secret;generating the public key associated with the master node based on the master public key and a network public key corresponding to the network secret; andgenerating the respective encryption key based on the respective private key of the target end device and the respective public key of the master node.

24. The method of any of claims 20 to 23, comprising:generating a data message in response to performing the command;andsending the encrypted data message to the final intermediate node.

25. The method of claim 24, wherein the data message is an encrypted data message generated by encrypting data with the respective encryption key shared between the master node and the target end device.

26. The method of any preceding claim, wherein the network is an internet of things (loT) network and each respective end device is a respective sensor and / or actuator device.

27. Computer equipment comprising:memory comprising one or more memory units; andprocessing apparatus comprising one or more processing units, wherein the memory stores code arranged to run on the processing apparatus, the code being configured so as when on the processing apparatus to perform the method of any of claims 1 to 26.

28. 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 26.54

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