Is the plaintext refering to Public Key Crypography?

still confused ...so what does e correcpond to ? and a ?

Can we do this shorter way for example something like in this problem

t:="Ajfal; B fla;fj l;ajCf!";
aa1:=[seq(a1[i],i=1..length(a1))];
a2:=[seq(a1[i],i=1..length(a1))];
for i from 1 to nops(aa1) do
if (aa1[i]="A") then a2[i]:="a";
else if (aa1[i]="B") then a2[i]:="b";
else if (aa1[i]="C") then a2[i]:="c";
else if (aa1[i]="D") then a2[i]:="d";
else a2[i]:="-";
fi;fi;fi;fi;
od;
newt:=CharacterMap(a1,cat(op(a2)),t);

But then how do I know that letter corresponds to what?