A method of obtaining, in an electronic circuit, at least one first key intended to be used in a cryptographic mechanism, on the basis of at least one second key contained in the same circuit, the first key being stored in at least one first storage element of the circuit, the first storage element being reinitialized automatically after a duration independent of the fact that the circuit is or is not powered. Also described are applications of this method to encrypted transmissions, usage controls, as well as an electronic circuit implementing these methods.