use this code to authenticate with SharePoint online office 360.
string userName = Your user Name;
string password = Pasword;
string baseurl = "Share point URL";
Uri uri = new Uri(baseurl);
var securePassword = new SecureString();
foreach (var c in password) { securePassword.AppendChar(c); }
var credentials = new SharePointOnlineCredentials(userName, securePassword);
var authCookie = credentials.GetAuthenticationCookie(uri);
var fedAuthString = authCookie.TrimStart("SPOIDCRL=".ToCharArray());
if (fedAuthString != null)
{
result.UserToken = fedAuthString;
result.Message = "User has been logged in successfully";
}
else
{
result.Message = "Invalid User!!";
}