One online tool for BASE64 is Motobit Software's BASE64 encoder/decoder.
To use it to convert StarTools masks back into importable PNG files;
This should result in a download of the mask as a PNG file which can be imported into the StarTools mask editor, as welll as other applications.
StarTools does more than just help you make "pretty pictures".
As of 1.7, enhanced mouse controls are implemented; Zoom in.
The Unified De-Noise module offers detail-aware, astro-specific noise reduction.
StarTools' Detail-aware Wavelet Sharpening allows you to bring out faint structural detail in your images.
You can convert everything you see to a format you find convenient. Give it a try!