I want my application to encrypt a user password, and at one time password will be decrypted to be sent to the server for authentication. A friend advise me to use HMAC. I wrote the following code in C#:

System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
byte[] key = encoding.GetBytes("secret");
HMACSHA256 myhmacsha256 = new HMACSHA256(key);
byte[] hashValue = myhmacsha256.ComputeHash(encoding.GetBytes("text"));
string resultSTR = Convert.ToBase64String(hashValue);
myhmacsha256.Clear();

How to decode the password (resultSTR, in this case)?

Thanks.