asym_subtype
struct asymmetric_key_subtype *subtype = prep->payload.data[asym_subtype];
key->payload.data[asym_subtype] = NULL;
prep->payload.data[asym_subtype] = &public_key_subtype;
prep->payload.data[asym_subtype] = &public_key_subtype;
return key->payload.data[asym_subtype];