How to figure out when your ACS Auth Token Expires

If you’re using the Windows Azure AppFabric Access Control Service (ACS) you’ve probably encountered a Simple Web Token (SWT) accessed via the Web Resource Authorization Protocol (WRAP). Doing this recently, I wanted to figure out when my token expired (so that I could cache it until a couple of minutes before expiry, improving my app's performance) – you might find this snippet useful if you want to figure out when a SWT token expires:

    /// <summary>
/// Gets the UTC time that the specified token expires.
/// </summary>
/// <param name="token">An Access Token from the Access Control Service .</param>
/// <returns>The time that the token expires as a UTC DateTime.</returns>
public static DateTime GetExpiryTime(string token)
{
        var swt = token.Substring("wrap_access_token=\"".Length, token.Length - ("wrap_access_token=\"".Length + 1));
var tokenValue = Uri.UnescapeDataString(swt);
var properties = (from prop in tokenValue.Split('&')
let pair = prop.Split(new[] { '=' }, 2)
select new { Name = pair[0], Value = pair[1] })
.ToDictionary(p => p.Name, p => p.Value);

var expiresOnUnixTicks = int.Parse(properties["ExpiresOn"]);
var epochStart = new DateTime(1970, 01, 01, 0, 0, 0, 0, DateTimeKind.Utc);

return epochStart.AddSeconds(expiresOnUnixTicks);

}

The code takes an access token from ACS (of the format wrap_access_token=”[Insert-SWT-Here]” ) and extracts the ‘ExpiresOn’ value from the SWT. According to the OAuth WRAP Spec, this is a Unix time (i.e., the number of seconds past January 1, 1970 at 12am) expressed as a UTC time. So, don’t forget that if you want to figure out when the token expires in your local time zone, you’ll need to convert it using the TimeZone type:

    Console.WriteLine("Your auth token expires at {0}.", TimeZone.CurrentTimeZone.ToLocalTime(this.AuthTokenExpiry));

I hope this helps,

Will