Reverse MD5 of a 76 character string when the first 44 characters are known I have a situation where I would like to reverse a hash which is the MD5 of a concatenation of 76 characters. I know the first 44 characters but don't know the remaining 32 characters. A description of the problem is as follows:

Hash = MD5 (cat (known_string_of_length_44chars, unknown_string_of_length_32chars))


I can generate Hash values for multiple combinations of the known_string. I would like to determine the unknown_string. Can this be done? Is brute forcing a possibility?
You have two problems, the first is the computational complexity of preimage attack on MD5. It is currently around 2^123 Pre-image attack on MD5 hash
This is technically better than brute force but way outside what is feasible in the forseeabile future. 
But perhaps worse than that is the information theory limit. A preimage attack will find a pre-image, not the pre-image which was originally used. With 32 chars you get a space of 256 bi,  far more than 128 bits of MD5. This means there are expected to be many preimages of the required form.
This means if you want to find the original as opposed to just any value with the correct form and correct hash it is impossible even with infinite computing power.
The same thing also shows us that requiring a pre image of such a form can't improve the pre image attack complexity as we could have picked any prefix and still expect many solutions.
If you could have limited to a small preimage space you could brute force it. And if the space had some special form  the knowledge that a solution of a special form exist could theoretically make the attack even faster.