public async Task <string> GetTokenAsync(Option <TimeSpan> ttl)
{
DateTime startTime = DateTime.UtcNow;
string audience = SasTokenHelper.BuildAudience(this.iotHubHostName, this.deviceId, this.moduleId);
string expiresOn = SasTokenHelper.BuildExpiresOn(startTime, ttl.GetOrElse(this.defaultTtl));
string data = string.Join("\n", new List <string> {
audience, expiresOn
});
string signature = await this.signatureProvider.SignAsync(data).ConfigureAwait(false);
return(SasTokenHelper.BuildSasToken(audience, signature, expiresOn));
}