  The random number generators (RNGs) are an indispensable tool in
cryptography. Of various types of RNG method, those using radiations from
nuclear decays (radioactive RNG) has a relatively long history but their
security has never been discussed rigorously in the literature. In this paper
we propose a new method of the radioactive RNG that admits a simple and
rigorous proof of security. The security proof is made possible here by
exploiting the parity (space inversion) symmetry arising in the device, which
has previously been unfocused but is generically available for a nuclide which
decays by parity-conserving interactions.
