Secure data access in an immutable ledger system

A method and system that enables a data owner to write data in an encrypted manner to an immutable ledger, and yet still be able to grant read access to specific data elements, as they were written at particular moments in time, to a requesting party. Examples therefore provide a process for encryption of data onto an immutable ledger in a time indexed manner, together with a process by which a third party can request access to the data stored in the immutable ledger from the data owner, and the data owner can provide them with certain decryption keys that allow the third party to read the data directly from the immutable ledger, again based on time-indexed queries. The data the third party can read is restricted to specific elements only of the data written, and further restricted to within a time range or to a specific point in time.