  The system proposed in this study uses zero-knowledge proof (ZKP) to verify
the traceability of wood recorded in a public blockchain. Wood is a byproduct
of several states, ranging from standing trees to logs, lumber, and wood
products (hereinafter ``wood objects''). The advantage of using the blockchain
for record keeping is that participants can freely record the information at
their discretion, without any restrictions. However, the openness of the
blockchain may allow a malicious third party to introduce disinformation. In
this study, we employ ZKP and near-field communication (NFC) chips to eliminate
the possibility of disinformation introduction. ZKP is used to prove/validate
changes in the state of wood objects, and the unique nonce associated with that
state is encrypted and recorded on an NFC chip. The nonce is concealed and id
of the wood object is defined as hash value of this nonce. We developed a
prototype system based on an Android application and an Ethereum smart
contract. We confirm that wood traceability and verification can be performed
using the prototype system.
