/// <summary>
/// Generate the SAS token
/// </summary>
/// <param name="resourceUri">Contains [hubname].azure-device.net/devices/[deviceid]</param>
/// <param name="signingKey">Signing key from connection string</param>
/// <param name="expiresInMinutes">Number of minutes before the generated SAS token expires</param>
/// <returns></returns>
private string GeneratePassword(string resourceUri, string signingKey, int expiresInMinutes)
{
resourceUri = WebUtility.UrlEncode(resourceUri);
ExpiresAt = DateTime.UtcNow.AddMinutes(expiresInMinutes);
TimeSpan secondsFromBaseTime = ExpiresAt.Subtract(EPOCH);
string toSign = string.Format("{0}\n{1}", resourceUri, Math.Ceiling(secondsFromBaseTime.TotalSeconds));
string signed = "";
string result = "";
using (HMACSHA256 hmac = new HMACSHA256(Convert.FromBase64String(signingKey)))
{
signed = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(toSign)));
}
signed = WebUtility.UrlEncode(signed);
result = string.Format("sr={0}&sig={1}&se={2}", resourceUri, signed, Math.Ceiling(secondsFromBaseTime.TotalSeconds));
return(result);
}
/// <summary>
/// Returns true if the token is within 10 minutes of expiring.
/// </summary>
/// <returns></returns>
public bool IsCloseToExpiry()
{
return(ExpiresAt.Subtract(DateTime.UtcNow).TotalMinutes < 10);
}