I’m not sure there is really a secure way to transmit login information to the server, but then again there really isn’t a need for it to be ultra secure. I’ve seen websites that house personal information that have zero security (not good btw!). I think just making it difficult to sniff the traffic should be enough protection. There are two libraries that I know of for the Arduino that are for the tea encryption which is a fast, simple encryption protocol.
Try https://github.com/franksmicro/Arduino/tree/master/libraries/Xtea or https://github.com/Denhart/TEA-encryption-on-arduino