asym_key_ids
struct asymmetric_key_ids *kids = prep->payload.data[asym_key_ids];
struct asymmetric_key_ids *kids = key->payload.data[asym_key_ids];
key->payload.data[asym_key_ids] = NULL;
prep->payload.data[asym_key_ids] = NULL;
prep->payload.data[asym_key_ids] = kids;
return key->payload.data[asym_key_ids];