Horodocs: A scalable, sustainable, robust and privacy compliant system to securely timestamp digital evidence and documents

Human activities produce more and more digital traces. Criminal activities are no exception: criminals often operate on computers, carry mobile phones, use GPS devices, or are recorded by surveillance cameras. Moreover, analyses of analog traces can produce results in a digital form. As digital information (evidence or results) becomes highly relevant in today's investigations, there is a pressing need for a trustworthy way to strengthen the chain of custody for digital content, especially its integrity component. The Horodocs timestamping system responds to the need for a scalable, robust, trustworthy, independently verifiable, chronological ledger preventing backdating and enabling integrity verification of a digital file. In order to make the system scalable and limit costs, submitted file hash values are grouped together into a local, temporary Merkle tree, called the Horodocs tree; this tree is discarded after its root value has been used to record both a derived identifier and an encrypted random control value on the Ethereum blockchain. 1 The main innovation resides in the way information about the Horodocs tree is provided to each participant having requested a timestamp during the lifespan of this tree. Each submitter gets a receipt with enough information to verify the timestamp for the hash values that were submitted to the Horodocs system: the receipt is only valid for the hash values of the original file and allows one to recalculate the root value of the corresponding discarded Horodocs tree independently. The root value is required to find the record in the Ethereum blockchain and to recover and decrypt the stored random control value to validate the date and time of the timestamp. Throughout its conception, the Horodocs system has been developed with a concern for strong robustness against backdating, privacy-by-design, transparency, usability, scalability, sustainability, automation, as well as cost and energy savings. • Strengthening digital evidence integrity in the chain of custody is needed. • Horodocs provides a trustworthy way to timestamp digital files. • Privacy-by-design has been used in the development of the Horodocs system. • Receipts sent to submitters allow anonymous and independent timestamp verification. • Salted hash values combined in Horodocs trees allow scalability and cost control.