Paper 2024/790

Physical Ring Signature

Abstract

Ring signatures allow members of a group (called "ring") to sign a message anonymously within the group, which is chosen ad hoc at the time of signing (the members do not need to have interacted before). In this paper, we propose a physical version of ring signatures. Our signature is based on one-out-of-many signatures, a method used in many real cryptographic ring signatures. It consists of boxes containing coins locked with padlocks that can only be opened by a particular group member. To sign a message, a group member shakes the boxes of the other members of the group so that the coins are in a random state ("heads" or "tails", corresponding to bits $0$ and $1$), and opens their box to arrange the coins so that the exclusive "or" of the coins corresponds to the bits of the message they wish to sign. We present a prototype that can be used with coins, or with dice for messages encoded in larger (non-binary) alphabets. We suggest that this system can be used to explain ring signatures to the general public in a fun way. Finally, we propose a semi-formal analysis of the security of our signature based on real cryptographic security proofs.