18 Digit Unique ID - Code reliability

Question

I want a number that would be unique forever, I came up with the following code, it generates a number and adds a check digit to the end of it, I would like to know how reliable is this code?

public void GenerateUniqueNumber(out string ValidUniqueNumber) {
        string GeneratedUniqueNumber = "";

        // Default implementation of UNIX time of the current UTC time
        TimeSpan ts = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0);
        string FormatedDateTime = Convert.ToInt64(ts.TotalSeconds).ToString();
        string ssUniqueId = DateTime.UtcNow.ToString("fffffff");
        //Add Padding to UniqueId
        string FormatedUniqueId = ssUniqueId.PadLeft(7, '0'); 

        if (FormatedDateTime.Length == 10 && FormatedUniqueId.Length == 7)
        {
            // Calculate checksum number using Luhn's algorithm.
            int sum = 0;
            bool odd = true;
            string InputData = FormatedDateTime + FormatedUniqueId;
            int CheckSumNumber;

            for (int i = InputData.Length - 1; i >= 0; i--)
            {
                if (odd == true)
                {
                    int tSum = Convert.ToInt32(InputData[i].ToString()) * 2;
                    if (tSum >= 10)
                    {
                        string tData = tSum.ToString();
                        tSum = Convert.ToInt32(tData[0].ToString()) + Convert.ToInt32(tData[1].ToString());
                    }
                    sum += tSum;
                }
                else
                    sum += Convert.ToInt32(InputData[i].ToString());
                odd = !odd;
            }
            //CheckSumNumber = (((sum / 10) + 1) * 10) - sum;
            CheckSumNumber = (((sum + 9) / 10) * 10) - sum;

            // Compute Full length 18 digit UniqueNumber
            GeneratedUniqueNumber = FormatedDateTime + FormatedUniqueId + Convert.ToString(CheckSumNumber);
        }
        else
        {
            // Error
            GeneratedUniqueNumber = Convert.ToString(-1);
        }

        ValidUniqueNumber = GeneratedUniqueNumber;        
    }

EDIT: clarification GUID can not be used, the number will need to be entered into a IVR system via telephone keypad.

Answer 1

You cannot use GUIDs, but you can create your own format of unique number similar to a GUID, that is based on the machine's MAC address (space) and the current time and date (time). This is guaranteed to be unique if the machines all have synchronised clocks.

For more information, please see here

Answer 2

Why don't you just use a Guid?

Answer 3

There are a few problems with this method:

• You're basically just counting the number of milliseconds from January 1, 1970. You can get this from ts.TotalSeconds rounded to 0.0000001. All your conversion and millisecond calculation is unnecessary.

• 10 years is about 3×10¹¹ milliseconds. You are keeping 17 significant digits, so for the next 10 years the first 5 digits will never change and cannot be used to distinguish numbers. They are useless.

• Are you generating numbers for milliseconds between 1970 and now? If not, they also cannot be used to distinguish numbers and are useless.

• This is totally dependent on what machine is returning the date. Anyone who has access to this machine can generate whatever "unique" numbers they want. Is this is problem?

• Anyone who sees one of these numbers can tell when it was generated. Is this a problem?

• Anyone can predict what number will be generated when. Is this a problem?

• 10¹⁵ milliseconds is about 30000 years. After then, your algorithm will repeat numbers. Seems like a long time, but you specified "forever" and 30000 years is not "forever". Do you really mean "forever"?

Answer 4

If I understand your implementation correctly, it only uses the current date/time as a basis. That means that if you create two IDs simultaneously, they will not be unique.