Bridging digital and physical worlds for securityConventional authentication schemes used in blockchains are based on cryptographic credentials generated in the ‘digital world’, such as PINs/passwords, long-term secret keys, or digital signatures. Yet, such digital credentials are easy to copy and steal. We have designed security protocols which depend on the properties of the ‘physical world’ such as location, distance, and time — such physical properties are much harder to spoof or manipulate as they are governed by physics, i.e., physical laws. For example, to access the blockchain, a participant should have a correct ‘physical fingerprint’ which can be its location or a distance to an asset. These properties can be measured and observed and as such we can continuously verify the authenticity and integrity of the blockchains.