glBlendFunc and glBlendFuncSeparate (white = alpha 0, black = alpha 1)

I need some help with OpenGL textures masking. I have it working but need to find some other blending function parameters to work in other way. Now I have

Code :

---

//Background 
...code...
    glBlendFunc(GL_ONE, GL_ZERO);
...code
 
//Mask
...code...
    glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ZERO);
...code...
 
//Foreground
...code
    glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
...code

---

Now it sets foreground's opacity to 0 (fills with background texture) where mask is transparent. I need it to react to mask's colors. I mean something like setting foregrounds opacity depending on mask's color. For example if mask is black (0.0,0.0,0.0) then the opacity of that place in foreground is 0 (is filled with background), and if mask is white (1.0,1.0,1.0) then the opacity of foreground is 1 (not filled with background). It can be in reverse consequence (white = opacity 0, black = opacity 1). I just need it to work depending on color.

Visualization:

	1st column is my current result. Circle in mask is transparent. 2nd column is example of result I am trying to get. Circle in mask is white. 3rd column is example of why I want to get it working just like as I said (white mask color = foreground alpha 0, black mask color = foreground alpha 1 or reverse)
1st row is background 2nd row is mask 3rd row is foreground 4th row is result	http://s9.postimage.org/5ydobxmsf/Untitled.png

I got some info that I should use shaders. And it was said to me that this code should help me out:

Code :

---

uniform sampler2D backTex,maskTex,foreText;
 
void main()
{
   float mask;
   vec4 result;
 
   mask = texture2D(maskTex,gl_TexCoord[0].st).r;
   result = texture2D(backTex,gl_TexCoord[0].st) * (1 - mask);
   result = result + texture2D(foreTex,gl_TexCoord[0].st) * mask;
 
   result.a = 1;
 
   gl_FragColor = result;
}

---

But I have no idea how to implement it to my code (objective-c)