Need a smaller alternative to GUID for DB ID but still unique and random for URL

Question

I have looked all of the place for this and I can't seem to get a complete answer for this. So if the answer does already exist on stackoverflow then I apologize in advance.

I want a unique and random ID so that users in my website can't guess the next number and just hop to someone else's information. I plan to stick to a incrementing ID for the primary key but to also store a random and unique ID (sort of a hash) for that row in the DB and put an index on it.

From my searching I realize that I would like to avoid collisions and I have read some mentions of SHA1.

My basic requirements are

• Something smaller than a GUID. (Looks horrible in URL)
• Must be unique
• Avoid collisions
• Not a long list of strange characters that are unreadable.

An example of what I am looking for would be www.somesite.com/page.aspx?id=AF78FEB

I am not sure whether I should be implementing this in the database (I am using SQL Server 2005) or in the code (I am using C# ASP.Net)

EDIT:

From all the reading I have done I realize that this is security through obscurity. I do intend having proper authorization and authentication for access to the pages. I will use .Net's Authentication and authorization framework. But once a legitimate user has logged in and is accessing a legimate (but dynamically created page) filled with links to items that belong to him. For example a link might be www.site.com/page.aspx?item_id=123. What is stopping him from clicking on that link, then altering the URL above to go www.site.com/page.aspx?item_id=456 which does NOT belong to him? I know some Java technologies like Struts (I stand to be corrected) store everything in the session and somehow work it out from that but I have no idea how this is done.

Answer 1

If you don't want other users to see people information why don't you secure the page which you are using the id?

If you do that then it won't matter if you use an incrementing Id.

Answer 2

You could randomly generate a number. Check that this number is not already in the DB and use it. If you want it to appear as a random string you could just convert it to hexadecimal, so you get A-F in there just like in your example.

Answer 3

Raymond Chen has a good article on why you shouldn't use "half a guid", and offers a suitable solution to generating your own "not quite guid but good enough" type value here:

GUIDs are globally unique, but substrings of GUIDs aren't

Answer 4

How long is too long? You could convert the GUID to Base 64, which ends up making it quite a bit shorter.

Answer 5

A GUID is 128 bit. If you take these bits and don’t use a character set with just 16 characters to represent them (16=2^4 and 128/4 = 32 chacters) but a character set with, let’s say, 64 characters (like Base 64), you would end up at only 22 characters (64=2^6 and 128/6 = 21.333, so 22 characters).

Answer 6

What you could do is something I do when I want exactly what you are wanting.

1. Create your GUID.

2. Get remove the dashes, and get a substring of how long you want your ID

3. Check the db for that ID, if it exists goto step 1.

4. Insert record.

This is the simplest way to insure it is obscured and unique.

Answer 7

[In response to the edit]
You should consider query strings as "evil input". You need to programmatically check that the authenticated user is allowed to view the requested item.

if( !item456.BelongsTo(user123) )
{
  // Either show them one of their items or a show an error message.
}

Answer 8

One interesting mechanism I've used in the past is to internally just use an incrementing integer/long, but to "map" that integer to a alphanumeric "code".

The following code shows a simple class that will change a long to a "code" (and back again!):

public static class ShortCodes
{
private static Random rand = new Random();

// You may change the "shortcode_Keyspace" variable to contain as many or as few characters as you
// please.  The more characters that aer included in the "shortcode_Keyspace" constant, the shorter
// the codes you can produce for a given long.
const string shortcode_Keyspace = "abcdefghijklmnopqrstuvwxyz0123456789";

// Arbitrary constant for the maximum length of ShortCodes generated by the application.
const int shortcode_maxLen = 12;


public static string LongToShortCode(long number)
{
    int ks_len = shortcode_Keyspace.Length;
    string sc_result = "";
    long num_to_encode = number;
    long i = 0;
    do
    {
        i++;
        sc_result = shortcode_Keyspace[(int)(num_to_encode % ks_len)] + sc_result;
        num_to_encode = ((num_to_encode - (num_to_encode % ks_len)) / ks_len);
    }
    while (num_to_encode != 0);
    return sc_result;
}


public static long ShortCodeToLong(string shortcode)
{
    int ks_len = shortcode_Keyspace.Length;
    long sc_result = 0;
    int sc_length = shortcode.Length;
    string code_to_decode = shortcode;
    for (int i = 0; i < code_to_decode.Length; i++)
    {
        sc_length--;
        char code_char = code_to_decode[i];
        sc_result += shortcode_Keyspace.IndexOf(code_char) * (long)(Math.Pow((double)ks_len, (double)sc_length));
    }
    return sc_result;
}

}

This is essentially your own baseX numbering system (where the X is the number of unique characters in the shortCode_Keyspace constant.

To make things unpredicable, start your internal incrementing numbering at something other than 1 or 0 (i.e start at 184723) and also change the order of the characters in the shortCode_Keyspace constant (i.e. use the letters A-Z and the numbers 0-9, but scamble their order within the constant string. This will help make each code somewhat unpredictable.

If you're using this to "protect" anything, this is still security by obscurity, and if a given user can observe enough of these generated codes, they can predict the relevant code for a given long. The "security" (if you can call it that) of this is that the shortCode_Keyspace constant is scrambled, and remains secret.

EDIT: If you just want to generate a GUID, and transform it to something that is still unique, but contains a few less characters, this little function will do the trick:

public static string GuidToShortGuid(Guid gooid)
{
    string encoded = Convert.ToBase64String(gooid.ToByteArray());
    encoded = encoded.Replace("/", "_").Replace("+", "-");
    return encoded.Substring(0, 22);
}

Answer 9

Take your auto-increment ID, and HMAC-SHA1 it with a secret known only to you. This will generate a random-looking 160-bits that hide the real incremental ID. Then, take a prefix of a length that makes collisions sufficiently unlikely for your application---say 64-bits, which you can encode in 8 characters. Use this as your string.

HMAC will guarantee that no one can map from the bits shown back to the underlying number. By hashing an auto-increment ID, you can be pretty sure that it will be unique. So your risk for collisions comes from the likelihood of a 64-bit partial collision in SHA1. With this method, you can predetermine if you will have any collisions by pre-generating all the random strings that this method which generate (e.g. up to the number of rows you expect) and checking.

Of course, if you are willing to specify a unique condition on your database column, then simply generating a totally random number will work just as well. You just have to be careful about the source of randomness.

Answer 10

I have just had an idea and I see Greg also pointed it out. I have the user stored in the session with a user ID. When I create my query I will join on the Users table with that User ID, if the result set is empty then we know he was hacking the URL and I can redirect to an error page.