Encrypt application name in c# and decrypt in SQL Server
I don't believe this is possible natively in SQL Server since MD5 is a one-way (apparently loss-full) hash. Here's a good StackOverflow answer that goes over MD5 in general.
SQL Server has a built in function to generate hashes called HASHBYTES() which maybe you could leverage (and one of the other hashing algorithms that are loss-less) to potentially generate a hash string that is reversible. But I have feeling even that might not be possible to do.