Abstract: A block chain may be used to certify the existence, integrity, and/or ownership of a file or communication. The present disclosure describes receiving a plurality of data units; hashing the plurality of data units to provide a plurality of hashes, individual hashes being unique cryptographic identifiers of corresponding data units such that an individual hash verifiably relates to a corresponding data unit and the individual hashes cannot be used by themselves to obtain corresponding data units; temporarily storing the hashes; generating a first cryptographic structure based on the plurality of hashes; publishing the first cryptographic structure on the block chain; providing proofs associated with individual ones of the plurality of data units that allow independent verification that the data units are certified; and verifying certification of data units based on roots of reconstructed cryptographic structures.