A method is provided for defining a filtering module between a first module processing information with a first sensitivity level, and a second module processing information with a second sensitivity level connected, in parallel with the filtering module, by a cryptographic module. The method includes defining a set of filtering rules in a language that can be compiled, defining the properties of messages whereof transmission is allowed between the first and second modules; validation processing the predefined set of rules, validating that a transmission authorization or refusal has in fact been provided by applying the set of rules to any information that may be provided at the input of the filtering module; compiling the predefined set of rules; and integrating the compiled set of rules into a rules database of the filtering module.