Question

I want to generate 1M random (appearing) unique alphanumeric keys and store them in a database. Each key will be 8 characters long and only the subset “abcdefghijk n pqrstuvxyz and 0-9” will be used.

The letters l,m,o and w are ditched. “m and w” are left out because of limited printing space, as each key will be printed on a product in a very small space. Dropping m and w enabled to increase the letter size with 2pt, improving readability. l and o were dropped because they are easily mixed up with 1, i and 0 at the current printing size. We did some testing characters 1,i, and 0 were always read correctly, l and o had to many mistakes. Capitals were left out for the same reason as ‘m and w”.

So why not a sequence? A few reasons: The keys can be registered afterwards and we do not want anyone guessing the next key in the sequence and register somebody else’s key. Appearance: we don’t need customers and competition to know we only shipped a few thousand keys.

Is there a practical way to generate the keys, ensure the uniqueness of each key and store them in a database? Thanks!

Answer 1

Edit: @CodeInChaos pointed out a problem: System.Random isn’t very secure, and the sequence could be reproduced without a great deal of difficulty. I’ve replaced Random with a secure generator here:

var possibilities = "abcdefghijknpqrstuvxyz0123456789".ToCharArray();
int goal = 1000000;
int codeLength = 8;
var codes = new HashSet<string>();
var random = new RNGCryptoServiceProvider();
while (codes.Count < goal)
{
    var newCode = new char[codeLength];
    for (int i = 0; i < codeLength; i++)
        newCode[i] = possibilities[random.Next(possibilities.Length)];
    codes.Add(new string(newCode));
}
// now write codes to database

static class Extensions
{
    public static byte Next(this RNGCryptoServiceProvider provider, byte maximum)
    {
        var b = new byte[1];
        while (true)
        {
            provider.GetBytes(b);
            if (b[0] < maximum)
                return b[0];
        }
    }
}

(the Next method isn’t very fast, but might be good enough for your purposes)