In our links section there is a link (in "Libraries") to cryptographic support for Qt. AFAIR it uses Qt3 and I guess you use Qt4, but maybe you can use it anyway.

An alternative is to implement a QIODevice which you should place on top of the socket device which will handle encryption/decryption. In Qt Quarterly there was an article how to do it (and it is better to do it this way -- QCA uses some other mechanism to achieve the same goal).