[exim] Encrypt operator?

Howdy. Forgive the possible RTFM-request but I've been looking up and down
ch11.html repeatedly and haven't found what I'm looking for.

Is there an operator that allows you to encrypt data, but with something
reversible (i.e. not md5 hash, but rather des or aes or something similar)?

I could always resort to linking to perl.o and using ${perl but that seems
like overkill.

I'd like to drop info into some added X- headers that we could use to trace
the message's history if we get it back via bounce or something like AOL
scomp, but without making it too visible. We already add some of this stuff
in X- headers but I've never been too comfortable about it being so public
(though obviously nothing sensitive or otherwise that would cause privacy
concerns, but still).

Anybody know of something I might've missed? I see an item on the Wishlist
(239) for probably the same thing. It mentions that there's no crypto code
in Exim, but (speaking as someone who can just barely write a "Hello world"
program in C) would it be possible to use OpenSSL functions (or GnuTLS) if
Exim is already being linked against those?