Suppose two beings are omniscient about Earth but not each other. One wants to point out one specific human being on this planet to the other.

They must do this in the minimum number of ASCII characters such that they can be quite confident the other being will know exactly who they are referencing.

No previous common knowledge except common knowledge of the planet Earth and everything that can be logically deduced from that common knowledge.

Now suppose you pick a given person. How would you evaluate whether a proposed string has this property?

If they had a pre-agreed codebook, you’d need roughly log₂(8×10⁹) ≈ 33 bits ≈ 5 ASCII characters to index any human. If they are logically omniscient and omniscient about Earth, but do not have a pre-agreed codebook, how good can they get at minimax? How close to five chars?