unique key generator

Hi i want to generate a unique key.
I want it to be made up of letters and numbers.

I have been looking at RC2 using the following code:

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using System.Security.Cryptography;

static void Main(string[] args)
{
RC2 r = RC2.Create();
r.GenerateIV();
r.GenerateKey();

for (short i = 13; i < 200; i += 13)
{
byte[] data = BitConverter.GetBytes(i);
byte[] enc = r.CreateEncryptor().TransformFinalBlock(data, 0, 2);
Console.WriteLine("{0}\t= {1}", 13, Convert.ToBase64String(enc));
}
Console.ReadLine();
}

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The problem is that this generates a key that contains non alphanumeric characters:
i.e
/MQpLW6J+3o=  

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so is there anything out there that will generate a unique alphanumeric key (Not a guid) about 10-12 characters.  something that will be unique across time.

thanks very much.
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