private string GetUserAgentAuthorization(SessionInfo sessionInfo)
{
string a1 = Crypto.GetMd5(sessionInfo.UserAgent + ":" + sessionInfo.UserAgentAuth);
return "Digest " + Crypto.GetMd5(a1 + ":" + (sessionInfo.RetsRequestId ?? string.Empty) + ":" + (sessionInfo.RetsSessionId ?? string.Empty) + ":" + sessionInfo.RetsVersion);
}