Since IoT devices have small computing power and limited battery size, it is desirable that the number of communications be as small as possible. To solve such a request, we propose a calculation method based on triangulation. The feature of our method is that the number of communications is one round trip, and sufficient computational security is realized with small entropy secret information using W-OTS+ and HMAC. In this paper, "small entropy" means a 8-digit integer (about 24 [bit]) that can be memorized by humans. We show an example calculation and analyze the relationship between accuracy of position measurement and security. From the results, we conclude that our method has sufficient security and enough practicality using small entropy secret information.
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