Cryptography as a field to foster interactions between mathematics and informatics, and algorithms. Analysis of a didactical situation

We present the conception and analysis of a situation dealing with the principles of public-key cryptography and aiming at exploring informatics and mathematical concepts and methods. We rely on the Theory of Didactical Situations to design a situation (based on an unplugged activity) about public-key cryptography using graphs. After the preliminary analysis of the content, we conceived a didactical situation and developed its a priori analysis. The description of the associated solving strategies illustrates the interplay between mathematics and informatics, and the role of algorithms and algorithmic thinking.