Control light intensity via shader script

Hi, folks! I have various objects in scene that uses the directional light as their light source. The problem is the light intensity is set too high. Some objects' color in the scene is too intense that the texture gets saturated. Instead of having multiple directional light with their respective culling mask I wanted to create a shader that can adjust the object's light intesity. Here's my goals and restrictions:

1. Create a slider to control intensity.

2. The shadow must not be affected by the shader

3. The shader must reference the main directional light

4. Shader must be specular (BlinnPhong)

5. Surface shader preferred (vertex and fragment scripting is still a challenge for me)

This project is for mobile, so optimization is critical. Any help and direction would be appreciated. I am new to shader scripting so please excuse my ignorance.

Thanks in advance!