sub encrypt {
    my($proto, $target, $data) = @_;
    my $id = $target->email;
    my $key = $target->pgp_key;
    my $ring = Crypt::OpenPGP::KeyRing->new(Data => $key)
            or $proto->error("new user KeyRing failed");
    my $pgp = Crypt::OpenPGP->new(PubRing => $ring)
            or $proto->error("new Context failed");
    my $kb = $ring->find_keyblock_by_uid($id)
            or $proto->error("find user keyblock failed", $ring);
    my $alg = $kb->preferred_sk_alg;
    $pgp->encrypt(
        Data => $data,
        Recipients => [ $id ],
        Armour => 1,
        ($alg ? (Cipher => $alg) : ()), # use default if no preference located
    );
}