A computer system to authenticate documents periodically appending a hash representing a document to a data structure, the data structure configured to store one or more hashes and creating a commitment for the data structure at pre-established intervals by creating a digest of the one or more hashes of the of the data structure, wherein the size of the commitment is constant regardless of the number of hashes in the data structure. A method, system, and computer product for verifying the existence of a document comprising postulating a state of existence of the document, obtaining, from a server, a proof of existence of the document at a time T, obtaining, from a trusted medium, a commitment for time T, and testing the postulate by determining whether there is a pre-established correspondence between the proof obtained from the server and the commitment obtained from the trusted medium, wherein a successful determination establishes the postulate is true and wherein a negative comparison establishes the postulate is false.