Let me pose an alternate question. What is your reason for using md5 for this? Even if it is not currently possible to reverse the hash (I believe the key space

Let me pose an alternate question. What is your reason for using md5 for this? Even if it is not currently possible to reverse the hash (I believe the key space is too large to feasibility break), it may become vulnerable in the future. If you want a fast one way hash, why not use something like an hmac with a secret key?