Kenji Saito

Kenji Saito, Mitsuru Iwamura

In response to the COVID-19 pandemic, Bluetooth-based contact tracing has been deployed in many countries with the help of the developers of smartphone operating systems that provide APIs for privacy-preserving exposure notification. However, it has been assumed by the design that the OS developers, smartphone vendors, or governments will not violate people's privacy. We propose a privacy-preserving exposure notification under situations where none of the middle entities can be trusted. We believe that it can be achieved with small changes to the existing mechanism: random numbers are generated on the application side instead of the OS, and the positive test results are reported to a public ledger (e.g. blockchain) rather than to a government server, with endorsements from the medical institutes with blind signatures. We also discuss how to incentivize the peer-to-peer maintenance of the public ledger if it should be newly built. We show that the level of verifiability is much higher with our proposed design if a consumer group were to verify the privacy protections of the deployed systems. We believe that this will allow for safer contact tracing, and contribute to healthier lifestyles for citizens who may want to or have to go out under pandemic situations.

Kenji Saito, Satoki Watanabe

To achieve lightweight selective disclosure for protecting privacy of document holders, we propose an XML format for documents that can hide arbitrary elements using a cryptographic hash function and salts, which allows to be partially digitally signed and efficiently verified, as well as a JSON format that can be converted to such XML. The documents can be efficiently proven to exist by representing multiple such structures as a Merkle tree and storing its root in blockchain. We show that our proposal has advantages over known methods that represent the document itself as a Merkle tree and partially hide it.

Kenji Saito, Akimitsu Shiseki, Mitsuyasu Takada et al.

It is said that blockchain will contribute to the digital transformation of society in a wide range of ways, from the management of public and private documents to the traceability in various industries, as well as digital currencies. A number of so-called blockchain platforms have been developed, and experiments and applications have been carried out on them. But are these platforms really conducive to practical use of the blockchain concept? To answer the question, we need to better understand what the technology called blockchain really is. We need to sort out the confusion we see in understanding what blockchain was invented for and what it means. We also need to clarify the structure of its applications. This document provides a generic model of understanding blockchain and its applications. We introduce design patterns to classify the platforms. We categorize possible use cases by identifying the structure among applications, and organize the functional, performance, operational and legal requirements for each such case. Based on the categorization and criteria, we evaluated and compared the following platforms: Hyperledger Fabric, Hyperledger Iroha, Hyperledger Indy, Ethereum, Quorum/Hyperledger Besu, Ethereum 2.0, Polkadot, Corda and BBc-1. We have tried to be fair in our evaluations and comparisons, but we also expect to provoke discussion. The intended readers for this document is anyone involved in development of application systems who wants to understand blockchain and their platforms, including non-engineers and non-technologists. The assessments in this document will allow readers to understand the technological requirements for the blockchain platforms, to question existing technologies, and to choose the appropriate platforms for the applications they envision. The comparisons hopefully will also be useful as a guide for designing new technologies.

Hiroshi Watanabe, Kenji Saito, Satoshi Miyazaki et al.

In tracing the (robotically automated) logistics of large quantities of goods, inexpensive passive RFID tags are preferred for cost reasons. Accordingly, security between such tags and readers have primarily been studied among many issues of RFID. However, the authenticity of data cannot be guaranteed if logistics services can give false information. Although the use of blockchain is often discussed, it is simply a recording system, so there is a risk that false records may be written to it. As a solution, we propose a design in which a digitally signing, location-constrained and tamper-evident reader atomically writes an evidence to blockchain along with its reading and writing a tag. By semi-formal modeling, we confirmed that the confidentiality and integrity of the information can be maintained throughout the system, and digitally signed data can be verified later despite possible compromise of private keys or signature algorithms, or expiration of public key certificates. We also introduce a prototype design to show that our proposal is viable. This makes it possible to trace authentic logistics information using inexpensive passive RFID tags. Furthermore, by abstracting the reader/writer as a sensor/actuator, this model can be extended to IoT in general.

Kazuyuki Shudo, Reiki Kanda, Kenji Saito

We discuss the issue of what we call {\em incentive mismatch}, a fundamental problem with public blockchains supported by economic incentives. This is an open problem, but one potential solution is to make application portable. Portability is desirable for applications on private blockchains. Then, we present examples of middleware designs that enable application portability and, in particular, support migration between blockchains.