Dumbledore's ancient magic (was: What is the purpose of the WW?)

[Tanya]

No: HPFGUIDX 48148

<snip>

Christopher Nuttall wrote:
Dumbledore uses an old magic, the exact nature of which is not disclosed, to
keep Harry safe.  I must question his skill with it, as Dobby, Fred and
George and Mr. Weasliy are all able to enter at one time or another.  Where
does that spell come from and why is it based on blood ties?

<snip>

Now me:
Maybe the magic that Dumbledore uses is similar to that used on the Mirror
of Erised in PS/SS. Perhaps only those seeking (seeking, but not desiring to
harm Harry) can find him? I've always wondered about that quote from
Dumbledore. "It was one of my most brilliant ideas, and between you and me,
that's saying something." (Don't have the book, sorry if that's not entirely
accurate).
-Tanya