How to detect if a string is Base64Encoded or not?

Question

Which is the best method to detect if a string is Base64Encoded or not (using Delphi)?

Answer 1

Best you can do is try to decode it. If the decode fails then the input was not base64 encoded. It the string successfully decodes then the input might have been base64 encoded.

Answer 2

You can check if the string only contains Base64 valids chars

function StringIsBase64(const InputString : String ) : Boolean;
const
  Base64Chars: Set of AnsiChar = ['A'..'Z','a'..'z','0'..'9','+','/','='];
var
  i : integer;
begin
  Result:=True;
   for i:=1 to Length(InputString) do
   {$IFDEF UNICODE}
   if not CharInSet(InputString[i],Base64Chars) then
   {$ELSE}
   if not (InputString[i] in Base64Chars) then
   {$ENDIF}
   begin
     Result:=False;
     break;
   end;
end;

The = char is used for padding so you can add an aditional valiation to the function for padded base64 strings checking if the length of the string is mod 4