yes, this is the old mirror codes.
What we did for them was write them in 2 columns A - M and Z - N and then make up phrases, obv AZ don't need doing but further through the alphabet you do; so, for us -
KP was Kevin Pieterson (cricket lovers)
EV - evil and vile
HS - head and shoulders
GT - gin and tonic (the shame!)
I later found out that my child just "knew" these from the off and was patronising me rather in my need for a list; but I found it fun (fun being a relative term here, fun cf doing ghastly codes, I mean). I think he takes after his father and felt sorry for his dimbo mother.