Don't use reserved name "_encrypt" for parameters;

Don't use reserved name "_encrypt" for parameters;
instead I've picked "enc", because that's what's in the prototypes.
("_encrypt" is reserved only as an external name, but still
using it in an application doesn't look like good style to me --
and it certainly isn't if the point is just avoiding shadowing,
which is apparently why the previous name "encrypt" was changed.)