2017-05-31_08-22-44.cs

GenerateKeys("0123456789ABCDEF", 2).Dump();

IEnumerable<string> GenerateKeys(IEnumerable<char> charset, int length)
{
    var charsetArray = charset.ToArray();
    var output = new int[length];
    
    if (length < 1) throw new ArgumentOutOfRangeException(nameof(length));
    
    while(true)
    {
        yield return string.Concat(output.Select(x => charsetArray[x]));
        
        //increment and "carry"
        for(int i = length - 1; i >= 0; i--)
        {
            output[i] = (output[i] + 1) % charsetArray.Length;
            
            if (output[i] != 0) //simple addition, didn't need to carry over to the next digit
                break;
            if (i == 0) //"carrying" at the last digit, we're done
                yield break;
        }
    }

}