Question

It was not me who wrote this code, it was the previous programmer. However, I noticed he didn’t provide a decryption algorithm, rendering the encryption useless.

How can I decrypt this?

function Encrypt(jstr: String): String;
var
  I: Integer;
  A: Real;
begin
  if Length(jstr) = 0 Then begin
    Result := '';
    Exit;
  end;
  A := 0;
    for I := 0 To Length(jstr) do
      A := A + (Ord(jstr[I]) * Pos(jstr[I],jstr)) / 33;
  Result := FormatFloat('0000000000.0000000000',A);
  if Pos(',',Result) > 0 then begin
    Insert('.',Result,Pos(',',Result));
    Delete(Result,Pos(',',Result),1);
  end;
end;

Thanks!

Answer 1

That function cannot be reversed. Since it takes input of arbitrary length and returns output of finite length, simple information theory tells you the futility of attempting to write a general inverse. Even for shorter input strings it seems to me that different input strings can result in the same encrypted string.

Even as a hash this function seems very brittle to me due to the bizarre use of floating point code. If I were you I would replace this function with something more fit for purpose.

Finally, I recommend that you undertake a review of all code produced by this developer. The low quality of this code and algorithm suggests to me that everything that this developer touched is liable to have defects.