If you want universal uniqueness, 128 bit won't last you too long. The number of atoms in the universe is estimated to be around 10^80, so you'd need about 266

If you want universal uniqueness, 128 bit won't last you too long. The number of atoms in the universe is estimated to be around 10^80, so you'd need about 266 bits to address every single atom. Give it some headroom, go to 512 bits...