A secure transaction system for a plurality of users, comprising a network comprising a plurality of nodes; a plurality of user devices, each user device associated with a respective user and each located at a respective one of the nodes; a plurality of private ledgers, a public ledger that stores public data. At least some of the public data and at least some of the private data of the users are mutually dependent. Each user device is configured to perform a cryptographic process in dependence on data from its associated private ledger and data from the public ledger to generate cryptographically secure data item(s) that are processable by the other user devices to determine a property of the user and/or a transaction that the user desires or is offering to perform or has performed. The cryptographic process is such that the cryptographically secure data item(s) is verifiable by the other user devices without determining values of at least some of the data of the private ledger of the user and/or without determining the identity of the user.
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