How to do MD5 with foreign characters?
like a user has stated before, you are using the wrong encoding if you will be using out of ascii chars (the case for korean, japanese, etc). You should be using Encoding.UTF8.GetBytes instead of Encoding.ASCII.GetBytes, take a look at https://msdn.microsoft.com/en-us/library/system.security.cryptography.md5%28v=vs.110%29.aspx for the example function GetMd5Hash. If you run ASCII md5, a different md5 will be generated.
The "salt" is the secret key you are using. $secretKey in PHP and secretKey in C#. You should read a little about security if you dont know what salt is, because if you dont you'll think you created a secure(r) system and you havent.
Hope it helps.