A method for preventing ransomware attacks on a computing system. By controlling the access to a calling interface through which cryptographic functions, such as the random number generator, can be accessed to generate strong encryption keys the method allows to efficiently terminate cryptographic ransomware attacks on the system before they can start doing any damage. If the access to the cryptographic functions, such as the random number generator, is not granted, the ransomware is unable to build a strong encryption key, and it is unable to deploy its intended effect.