Résumé : We propose a general transformation that maps a protocol secure in an extremely weak sense (essentially in a model where no adversary is present) into a protocol that is secure against a fully active adversary which interacts with an unbounded number of protocol sessions, and has absolute control over the network. The transformation works for arbitrary protocols with any number of participants, written with usual cryptographic primitives. Our transformation provably preserves a large class of security properties that contains secrecy and authenticity. An important byproduct contribution of this paper is a modular protocol development paradigm where designers focus their effort on an extremely simple execution setting -- security in more complex settings being ensured by our generic transformation. Conceptually, the transformation is very simple, and has a clean, well motivated design. Each message is tied to the session for which it is intended via digital signatures and on-the-fly generated session identifiers, and prevents replay attacks by encrypting the messages under the recipient's public key.