Using the register_constructor function enables custom keymat_t
implementations per IKE version. If no constructor is registered the
default behavior is preserved.
This nonce generator uses an RNG to generate nonces. The RNG quality is
currently set to RNG_WEAK which is the same value used in IKE init.
The plugin is enabled and thus built by default.
Nonce generators (nonce_gen_t) can be used to get or allocate nonces.
Users can request nonce generators from the crypto factory while nonce
plugins register/remove themselves to/from the crypto factory.