The robustness of programs using cryptography and the related fields must reside in the keys and the algorithms, not in unpublished implementation tricks. More, implementation glitches can ruin demonstrated algorithms. Therefore, here are the details of this implementation so that one can (in)validate its robustness.
The remaining values are the Di, one value (two characters) for each character of the secret
At the present time, the program implements strictly the Shamir's method. Some authors have proposed improvements, in particular by using variations that allow to detect individual invalid shares. One of them is Martin Tompa in How to Share a Secret with Cheaters.
In order to maintain the compatibility of the shares generated with the current version of ShareSecret and future ones that may provide such improvements, this byte is used as a "flag".
Currently, its Least Significant Bit is set to 0, meaning "no cheaters detection". It will be set to 1 when/if such a detection is implemented and the corresponding option is selected. The others bits are random. This means that this value is currently a random even number.
Here is the Scilab prototype used to get the maths right.
It is my intention to keep allowing free usage of ShareSecret on this website.
However, should this website not be available for any reason, you will still be able to recover your secret using the stand-alone version (which you should definitely download while the site is still there...)
Here is a cross-operating system version using Adobe AIR, available for Windows, Mac OS X and Linux. You need to install the Adobe AIR runtime.