A public blockchain, also known as an open or permissionless blockchain, is one where anybody can join the network freely and establish a node. Because of their open nature, these blockchains must be secured with cryptography and a consensus system like proof of work (PoW). A private or permissioned blockchain, on the other hand, requires each node to be approved before joining. Because nodes are considered to be trusted, the layers of security do not need to be as robust.