ClearAll[MakeTranslationTable, PlayfairCipher, PlayfairDecipher]
MakeTranslationTable[tt_List] := Module[{poss, in, out},
  poss = Tuples[Tuples[Range[5], 2], 2];
  Table[
   If[p[[1, 1]] == p[[2, 1]],
    (* same row *)
    {in, out} = {p, {{p[[1, 1]], Mod[p[[1, 2]] + 1, 5, 1]}, {p[[2, 1]], Mod[p[[2, 2]] + 1, 5, 1]}}};
    ,
    If[p[[1, 2]] == p[[2, 2]],
     (* same column *)
     {in, out} = {p, {{Mod[p[[1, 1]] + 1, 5, 1], p[[1, 2]]}, {Mod[p[[2, 1]] + 1, 5, 1], p[[2, 2]]}}};
     ,
     (*rectangle*)
     {in, out} = {p, {{p[[1, 1]], p[[2, 2]]}, {p[[2, 1]], p[[1, 2]]}}};
     ]
    ];
   StringJoin[Extract[tt, in]] -> StringJoin[Extract[tt, out]]
   ,
   {p, poss}
   ]
  ]
PlayfairCipher[txt_String, key_String, iisj_ : True] :=
 Module[{text, tt},
  text = RemoveDiacritics[ToUpperCase[txt]];
  tt = RemoveDiacritics[ToUpperCase[key]] <> CharacterRange["A", "Z"];
  text //= StringReplace[Except[Alternatives @@ CharacterRange["A", "Z"]] -> ""];
  tt //= StringReplace[Except[Alternatives @@ CharacterRange["A", "Z"]] -> ""];
  If[iisj,
   tt //= StringReplace["J" -> "I"];
   text //= StringReplace["J" -> "I"];
   ,
   tt //= StringReplace["Q" -> ""];
   text //= StringReplace["Q" -> ""];
   ];
  tt //= Characters /* DeleteDuplicates;
  text = FixedPoint[StringReplace[#, x_ ~~ x_ :> x ~~ "X" ~~ x, 1] &, text];
  If[OddQ[StringLength[text]], text = text <> "X"];
  If[Length[tt] == 25,
   tt = Partition[tt, 5];
   tt = MakeTranslationTable[tt];
   text = StringPartition[text, 2];
   StringRiffle[text /. tt, " "]
   ,
   Print["Something went wrong!"]
   ]
  ]
PlayfairDecipher[txt_String, key_String, iisj_ : True] :=
 Module[{text, tt},
  text = RemoveDiacritics[ToUpperCase[txt]];
  tt = RemoveDiacritics[ToUpperCase[key]] <> CharacterRange["A", "Z"];
  text //= StringReplace[Except[Alternatives @@ CharacterRange["A", "Z"]] -> ""];
  tt //= StringReplace[Except[Alternatives @@ CharacterRange["A", "Z"]] -> ""];
  If[iisj,
   tt //= StringReplace["J" -> "I"];
   text //= StringReplace["J" -> "I"];
   ,
   tt //= StringReplace["Q" -> ""];
   text //= StringReplace["Q" -> ""];
   ];
  tt //= Characters /* DeleteDuplicates;
  If[OddQ[StringLength[text]], text = text <> "X"];
  If[Length[tt] == 25,
   tt = Partition[tt, 5];
   tt = MakeTranslationTable[tt];
   text = StringPartition[text, 2];
   StringRiffle[text /. (Reverse /@ tt), " "]
   ,
   Print["Something went wrong!"]
   ]
  ]
PlayfairCipher["Hide the gold in...the TREESTUMP!!!", "Playfair example"]
PlayfairDecipher[%, "Playfair example"]