Web Forms :: Forms Authentication With (Encryption) Encrypted Password?

I was able to encrypt the password and save it in the database but I need to encrypt the password now when the users try to login.  For example, user comes and types his/her username and password but the password is already encrypted in the database, how can I decrypt and authenticate the user? 

protected void ValidateUser(object sender, EventArgs e)
{
int userId = 0;
string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
using (SqlCommand cmd = new SqlCommand("Validate_User"))

[code].....

here is the decrypt function

private string Decrypt(string cipherText)
{
string EncryptionKey = "MAKV2SPBNI99212";
byte[] cipherBytes = Convert.FromBase64String(cipherText);
using (Aes encryptor = Aes.Create())

[Code]......