What you need is called OCR:
http://en.wikipedia.org/wiki/Optical_character_recognition[
^].
The fact that you are interested in base64 string, does not change the problem much. It becomes just a bit easier, just because the number of characters is less:
http://en.wikipedia.org/wiki/Base64[
^].
The OCR problem is very successfully solved, but still, this is a very difficult problem; and a good source code is hard to find. I'll tell you this:
the mere fact that you somehow came to the situation where you have some base64 string in a bitmap is highly pathological. Even if this is not your fault, this is a big ugly fault, nothing else. You are just trying to do bad business, no matter how much of success you are going to help. So, it makes serious discussion of the OCR not really interesting to me. Therefore, I'll just give you one link to my past answer:
i need an ocr for a camera image capture[
^].
On that page, you will see further links. I'll be very glad if it can help you.
It has a .NET wrapper you can use:
http://www.pixel-technology.com/freeware/tessnet2/[
^].
As the character set is very limited, one important product to know, Tesseract:
http://en.wikipedia.org/wiki/Tesseract_%28software%29[
^].
—SA
Submit an article or tip