This activity about cryptographic techniques illustrates how to accomplish a simple, but nevertheless seemingly impossible task—making a fair, random choice by flipping a coin between two people who don’t necessarily trust each other, and who are connected only by a telephone. Use this activity to introduce learners to cryptographic protocols and internet security/privacy issues. Variations, extensions, background information, and further reading suggestions are included in the PDF.


  • Computer Science > General
  • Mathematics > General
  • Education > General

Education Levels:

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