No need to decrypt the password. I got this to work yesterday.
1. Use a database query to obtain the hashed password and the salt key.
2. Take the password that the user enters and run it though the function below.
3. Compare the results of the function with the hashed password from the database. if both match, the logon is successful.
Public Overridable Function CreatePasswordHash(ByVal password As String, ByVal saltkey As String, Optional ByVal passwordFormat As String = "SHA1") As String
If [String].IsNullOrEmpty(passwordFormat) Then
passwordFormat = "SHA1"
End If
Dim saltAndPassword As String = [String].Concat(password, saltkey)
Dim hashedPassword As String = FormsAuthentication.HashPasswordForStoringInConfigFile(saltAndPassword, passwordFormat)
Return hashedPassword
End Function