Custom solution for MD5?

Hey everyone, wasn't sure where to post this but i figured i'd give it a shot. I'm trying to solve for some MD5 Hashes but not in the typical
sense.

I have an equation where i need to solve for a variable, it goes sort of like this:

StringaStringbStringc = Stringd(md5 hash of concatenation of Stringa+Stringb+Stringc)


We know the values of Stringa, Stringc, Stringd. We try to solve for Stringb

So with example values substituted we have,

1234(UNKNOWN_STRING_MD5_UPPERCASE)5678 = 4bd16ac7d6af4a922109bfffb0f6564d

Stringa=1234
Stringb=(UNKNOWN_STRING_MD5_UPPERCASE), which is the uppercase MD5 digest of a lowercase alphanumeric string, 8char max.
Stringc=5678
Stringd=4bd16ac7d6af4a922109bfffb0f6564d


We need to solve for Stringb, we know that Stringb is the (UPPERCASE)md5 result of a lowercase alphanumeric string, that has an 8 character limit.

We know that Stringd(4bd16ac7d6af4a922109bfffb0f6564d) is the md5 result of the concatenation of Stringa+Stringb+Stringc.

So in this case, solving for Stringb eventually provides us with the uppercase MD5 Value of "AE2B1FCA515949E5D54FB22B8ED95575", which is "testing" in plaintext, and it fits in the equation

So, we have:

Stringa=1234
Stringb=AE2B1FCA515949E5D54FB22B8ED95575
Stringc=5678
Stringd=4bd16ac7d6af4a922109bfffb0f6564d



MD5(1234AE2B1FCA515949E5D54FB22B8ED955755678) = 4bd16ac7d6af4a922109bfffb0f6564d


What i'm looking to do is have a script created that solves for stringb in this exact format.


Stringa, Stringc, Stringd would all be known constants, and it would try to solve for stringb in the way i outlined above. Obviously i am paying for someones time and effort in creating an efficient script / addon to do just this.