Résumé : We propose a probabilistic variant of the pi-calculus as a framework to specify randomized security protocols and their intended properties. In order to express and verify the correctness of the protocols, we develop a probabilistic version of the testing semantics. We then illustrate these concepts on an extended example: the Partial Secret Exchange, a protocol which uses a randomized primitive, the Oblivious Transfer, to achieve fairness of information exchange between two parties.