I've read the wikipedia article on socks and cannot find information about encryption between client and proxy-server. There is only information about authentication.Is socks5 connection a plain-text?
SOCKS5 is there encryption between client and proxy server?
proxy
SOCKS5 is just a transport protocol on top of TCP/UDP but below application layer.  Thus it's comparable with TCP and UDP, too.  There's no inherent encryption in SOCKS, but you application has to care about this (it's not a VPN technology, but a proxy at last).  If you want encryption, the protocol you speak inside of the SOCKS-channel has to provide it.