ERC-4527: QR Code transmission protocol for wallets

ERC-4527 proposes a standard process and data transmission protocol for transferring data via QR codes between offline signers, which hold private keys without network access, and watch-only wallets, which have network access. The proposal builds on the Uniform Resources (UR) protocol from BlockchainCommons, extending it with animated QR codes and CBOR encoding. It defines new UR types, eth-sign-request for sending unsigned data and eth-signature for returning signatures, along with reuse of the existing crypto-hdkey and crypto-keypath types for public key setup. The article argues QR codes offer better transparency, compatibility, user experience and a smaller attack surface than USB, Bluetooth or file transfer. The Test Cases and Reference Implementation are in the ur-registry-eth package released by the Keystone team.