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By extending rot13 to Unicode characters, it supports encrypting emoji message automatically!

No. Emojis go from U+10000 to U+1FFFF, and this rotates chars U+0 through U+FFFF (hence the U+8000 middle point).

Right you'd need rot 0x88000 to cover all 17 planes. Downside would be that the space is not fully packed so you'd get a lot of invalid characters.

Pedantic point: some (early) emoji have codes below this