public SecureUtils.K2Output getTxDerivatives() {
        switch (phase) {
            case KEY_DISTRIBUTION:
                return oldDerivatives;
            case USING_NEW_KEYS:
            default:
                return derivatives;
        }
    }
Returns the NetworkKey based on the key refresh procedure phase.

@return key
mesh/src/main/java/no/nordicsemi/android/mesh/NetworkKey.java