Methods, based on hash trees, of proving a time of origination and a source of digital redactable documents is described. A document is represented via a number of redactable data units masked using a list or tree of blinding values and hashed using a list or tree of hashes, allowing for a selective subsequent revealing of the content of the document, any part of which can be redacted out. Depending on whether a data unit (or multiple data units corresponding to the same branch of the tree) is to be revealed or redacted, a respective blinding value or a respective hash value is provided. The described methods allow the verifier to confirm the time of origination and the source of the document by recovering the final hash notwithstanding any number of the data units being redacted. The described methods optimize the amount of data that has to be stored by the document originator and transmitted to the verifier for hash verification.
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Schejbal, Jan, "Redactable Hashing for Time Verification", Technical Disclosure Commons, (January 24, 2021)