We present Quicksilver/OCaml, an Objective Caml library for
communicating data in a possibly hostile network environment without
losing type safety or abstraction.  It is entirely implemented at user
level, i.e., without modifying the compiler or the runtime at all.
The key ideas are (1) representing types by (tuples of) functions, (2)
abstraction by encryption, and (3) distinction of type representations
for input and output.  We show by examples that the library is useful
for implementing applications where not only _safety_, but also
_security_ against malicious attackers is required.