[ecfisa]

Hola camiz.

Un modo es:

Código Delphi [-]
...
type
  TForm1 = class(TForm)
  ...
  private
    procedure FieldGetText(Sender: TField; var Text: String; DisplayText: Boolean);
  ...
  end;
...

implementation

....

procedure TForm1.FormCreate(Sender: TObject);
var
  i: Integer;
begin
  // abrir DataSet, etc, etc...
  with DBGrid1.DataSource.DataSet do
    for i:= 0 to FieldCount-1 do
      Fields[i].OnGetText := FieldGetText;
end;

procedure TForm1.FieldGetText(Sender: TField; var Text: String;
  DisplayText: Boolean);
begin
  Text := Descifrar(Sender.AsString);  // Aquí va tu función de descifrar
end;

...

procedure TForm1.FormDestroy(Sender: TObject);
var
  i: Integer;
begin
   with DBGrid1.DataSource.DataSet do
     for i:= 0 to FieldCount-1 do
       Fields[i].OnGetText := nil
end;

Saludos