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uses IdCoder, IdCoderMIME, IdGlobal;

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const CK_ENC_VAR_RANGE=10;

function CKEncrypt(const Source:string; Salt:string): string;
var
  i, lenSalt, key, keyWeight: Word;
  InString: string;
begin
  Result := '';
  if (Salt = '') then begin
    Result := Source;
  end else begin
    InString := TIdEncoderMIME.EncodeString(Source, IndyTextEncoding_UTF8);

    lenSalt := Length(Salt);

    keyWeight := 0;
    for i := 1 to lenSalt do keyWeight := keyWeight + ord(Salt[i]);
    keyWeight := keyWeight mod CK_ENC_VAR_RANGE;

    for i := 1 to Length(InString) do
    begin
      key := ( ord(Salt[(i mod lenSalt) + 1]) mod CK_ENC_VAR_RANGE);
      Result := Result + CHAR(Byte(InString[i]) + key - keyWeight);
    end;
  end;
end;

function CKDecrypt(const InString:string; Salt:string): string;
var
  i, lenSalt, key, keyWeight: Word;
  Target: string;
begin
  if (Salt = '') then begin
    Target := InString;
  end else begin
    lenSalt := Length(Salt);

    keyWeight := 0;
    for i := 1 to lenSalt do keyWeight := keyWeight + ord(Salt[i]);
    keyWeight := keyWeight mod CK_ENC_VAR_RANGE;

    Target := '';
    for i := 1 to Length(InString) do
    begin
      key := ( ord(Salt[(i mod lenSalt) + 1]) mod CK_ENC_VAR_RANGE);
      Target := Target + CHAR(Byte(InString[i]) - key + keyWeight);
    end;
    Result := TIdDecoderMIME.DecodeString(Target, IndyTextEncoding_UTF8);
  end;
end;

{ Testng }
procedure TForm1.Button1Click(Sender: TObject);
begin
  Memo2.Text := CKEncrypt(Memo1.Text, 'foo');
  Memo3.Text := CKDecrypt(Memo2.Text, 'foo');
end;