You have to basically hash your passwords and store them in the database. You never decrypt them back but always compare the hashed versions. Some sample stuff is below.
private static string CreateSalt(int size)
{
//Generate a cryptographic random number.
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
byte[] buff = new byte[size];
rng.GetBytes(buff);
// Return a Base64 string representation of the random number.
return Convert.ToBase64String(buff);
}
private static string CreatePasswordHash(string pwd, string salt)
{
string saltAndPwd = String.Concat(pwd, salt);
string hashedPwd =
FormsAuthentication.HashPasswordForStoringInConfigFile(
saltAndPwd, "sha1");
return hashedPwd;
}
Store the PasswordHash and salt in the database in the user's account.
Then, when the user attempts to logon the next time, grab the salt from the database and hash it as usual with the password provided by the user during logon and compare that value to the PasswordHash in the database. If they are the same, the user provided the correct password (whatever that may be). If they are not the same, the password entered was incorrect.