Blockchain-enabled broadcast encryption

A computer-implemented method of accessing broadcasted messages using blockchain transactions, wherein each user is associated with a respective user encryption key, wherein a broadcasting party maintains a first key graph comprising a plurality of nodes, wherein the key graph comprises a root layer comprising a first root node, followed by one or more inner layers, each inner layer comprising a respective plurality of inner nodes, followed by a leaf layer comprising a plurality of leaf nodes, wherein the root node is a parent node of the respective plurality of inner nodes of a first one of the inner layers, wherein each node of each respective inner layer other than a final inner layer is a respective parent node of one or more respective inner nodes a next inner layer, wherein each inner node of a final one of the inner layers is a respective parent node of one or more leaf nodes of the leaf layer, wherein each node is mapped to a respective encryption key, each leaf node being mapped to a respective user encryption key of a first set of users, wherein the first root node is mapped to a first message encryption key, wherein a respective key mapped to a respective child node is used to encrypt a respective key mapped to a respective parent node,

• and wherein the method is performing by a first user and comprises:
• providing, to the broadcasting party, a first input of a first message transaction, wherein the first message transaction comprises a first output locked to a first public key of the first user and/or a public key of the broadcasting party, wherein the first input is signed by the first user;
• obtaining the first message transaction, wherein the first message transaction comprises an output comprising a first set of encrypted data items, wherein each respective encrypted data item in the first set of encrypted data items comprises one or more respective encryption keys mapped to one or more respective parent nodes of the first key graph, encrypted using a respective encryption key mapped to a respective child node of the respective parent node of the one or more respective parent nodes of a respective lowest layer of the first key graph,
• obtaining, from the broadcasting party, a first encrypted message generated by encrypting a first message with the first message encryption key;
• decrypting at least one of the respective encrypted data items, using the respective encryption key used to encrypt the respective encrypted data item, to obtain the one or more respective encryption keys;
• and
• decrypting the first encrypted message, using at least the obtained one or more respective encryption keys, to obtain the first message.

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 a method of enabling users to access broadcasted messages using blockchain transactions, wherein each user is associated with a respective user encryption key, and wherein the method is performing by a broadcasting party and comprises, for a first message;
• determining a first set of users eligible to access the first message;
• maintaining a first key graph comprising a plurality of nodes, wherein the key graph comprises a root layer comprising a first root node, followed by one or more inner layers, each inner layer comprising a respective plurality of inner nodes, followed by a leaf layer comprising a plurality of leaf nodes, wherein the root node is a parent node of the respective plurality of inner nodes of a first one of the inner layers, wherein each node of each respective inner layer other than a final inner layer is a respective parent node of one or more respective inner nodes a next inner layer, wherein each inner node of a final one of the inner layers is a respective parent node of one or more leaf nodes of the leaf layer, wherein each node is mapped to a respective encryption key, each leaf node being mapped to a respective user encryption key of the first set of users, wherein the first root node is mapped to a first message encryption key, wherein a respective key mapped to a respective child node is used to encrypt a respective key mapped to a respective parent node;
• generating a first encrypted message by encrypting the first message with the first message encryption key;
• determining a first set of encrypted data items, wherein each respective encrypted data item in the first set of encrypted data items comprises one or more respective encryption keys mapped to one or more respective parent nodes of the first key graph, encrypted using a respective encrypted key mapped to a respective child node of the respective parent node of the one or more respective parent nodes of a respective lowest layer of the first key graph, wherein each respective user of the first set of users is configured to decrypt at least one of the respective encrypted data items;
• obtaining a first message transaction, wherein the first message transaction comprises:
• a respective input associated with each respective user of the first set of users, wherein the respective input is signed by the respective user,
• a respective input associated with the broadcasting party, wherein the respective input is signed by the broadcasting party,
• a respective output associated with each user of the first set of users, wherein each respective output is locked to respective public key of the respective user and/or a public key of the broadcasting party, and
• a respective output comprising the first set of encrypted data items;
• causing the first message transaction to be submitted to a blockchain network;
• and
• broadcasting the first encrypted message to at least the first set of users.

A non-transitory computer-readable storage medium comprising computer program code configured so as, when run on one or more processors, the one or more processors perform a method of enabling users to access broadcasted messages using blockchain transactions, wherein each user is associated with a respective user encryption key, and wherein the method is performing by a broadcasting party and comprises, for a first message:

• determining a first set of users eligible to access the first message;
• maintaining a first key graph comprising a plurality of nodes, wherein the key graph comprises a root layer comprising a first root node, followed by one or more inner layers, each inner layer comprising a respective plurality of inner nodes, followed by a leaf layer comprising a plurality of leaf nodes, wherein the root node is a parent node of the respective plurality of inner nodes of a first one of the inner layers, wherein each node of each respective inner layer other than a final inner layer is a respective parent node of one or more respective inner nodes a next inner layer, wherein each inner node of a final one of the inner layers is a respective parent node of one or more leaf nodes of the leaf layer, wherein each node is mapped to a respective encryption key, each leaf node being mapped to a respective user encryption key of the first set of users, wherein the first root node is mapped to a first message encryption key, wherein a respective key mapped to a respective child node is used to encrypt a respective key mapped to a respective parent node;
• generating a first encrypted message by encrypting the first message with the first message encryption key;
• determining a first set of encrypted data items, wherein each respective encrypted data item in the first set of encrypted data items comprises one or more respective encryption keys mapped to one or more respective parent nodes of the first key graph, encrypted using a respective encrypted key mapped to a respective child node of the respective parent node of the one or more respective parent nodes of a respective lowest layer of the first key graph, wherein each respective user of the first set of users is configured to decrypt at least one of the respective encrypted data items;
• obtaining a first message transaction, wherein the first message transaction comprises:
• a respective input associated with each respective user of the first set of users, wherein the respective input is signed by the respective user,
• a respective input associated with the broadcasting party, wherein the respective input is signed by the broadcasting party,
• a respective output associated with each user of the first set of users, wherein each respective output is locked to respective public key of the respective user and/or a public key of the broadcasting party, and
• a respective output comprising the first set of encrypted data items;
• causing the first message transaction to be submitted to a blockchain network;
• and
• broadcasting the first encrypted message to at least the first set of users.