Schannel on Windows XP supports DES, Trip-DES (168bit) and RC4 (among others). On Windows Vista AES128 is supported as well. I'm not sure what's the exact reason the server and client choose AES128 on Windows Vista. But I assume AES128 is considered safe enough. A bigger encryption key also requires more CPU resources and results generally in slower transfers. Right now you cannot force SmartFTP to use a bigger key because we see little benefit in such an option.