Multicasting over public network

Just writing a technical proposal in work, and I'm thinking that
Multicasting could save a lot of bother. I'm just wondering though
if multicasting can be used over a public network (ie the internet)
and would security be an issue (ie only want a specific number
of hosts to receive the data, and dont want any non-authorised
users).

Any mention I've heard of multicasting before has always been
for internal networks where security wouldnt be that much of
an issue.