gistfile1.txt

decode[text_String, key_String] := Module[{textCode, keyCode},
  textCode = 
   Cases[ToCharacterCode[
      ToUpperCase@
       text], _?(IntervalMemberQ[Interval@{65, 90}, #] &)] - 65;
  keyCode = 
   Cases[ToCharacterCode[
      ToUpperCase@
       key], _?(IntervalMemberQ[Interval@{65, 90}, #] &)] - 65;
  keyCode = If[Length[textCode] < Length[keyCode],
    keyCode[[;; Length@textCode]],
    PadRight[keyCode, Length@textCode, keyCode]];
  FromCharacterCode[Mod[textCode - keyCode, 26] + 65]]
table1 := Table[
   a[[2]] -> a[[1]], {a, 
    Transpose[{CharacterRange["A", "Z"], Range[0, 25]}]}]; 
table2 := Table[
   a[[1]] -> a[[2]], {a, 
    Transpose[{CharacterRange["A", "Z"], Range[0, 25]}]}]; 
konvert[val_] := PadLeft[IntegerDigits[val, 26], 3, "A"] /. table1;
konvertBack[str_] := FromDigits[Characters[str] /. table2, 26]
step[st_] := Block[{str, a, b, c},
   str := StringJoin[konvert[st]];
   a := decode["UHG", str];
   b := decode["NZCBXBXUSXDBXIWZWMMLLVLHM", str];
   c := decode["WEQ", str];
   {str, a, b, c}
   ];
Export["3st.txt", ParallelTable[step[i], {i, 0, 26^3}], "Text"]