World: r4wp

You have a better chance of me seeing such a question if you pose 
it in !R3 Extensions

When R3 was open sourced, I presume the SDK was not also open sourced? 
I am looking for the 'C' source code for the RSA, AES and Blowfish 
encryption functions.

The SDK is only for REBOL2, which was not open sourced.

To find source codes, you will need to search with google or so. 
"blowfish source code c" first result looks promising http://www.schneier.com/blowfish-download.html