Fidelius (long, contains spoilers for PoA and OtP)

[Wildean]

No: HPFGUIDX 95006

'Fudge dropped his voice and proceeded in a sort of low rumble. "Not many
people are aware that the Potters knew You-Know-Who was after them.
Dumbledore, who was of course working tirelessly against You-Know-Who, had a
number of useful spies. One of them tipped him off, and he alerted James and
Lily at once. He advised them to go into hiding. Well, of course,
You-Know-Who wasn't an easy person to hide from. Dumbledore told them that
their best chance was the Fidelius Charm."

"How does that work?" said Madam Rosmerta, breathless with interest.
Professor Flitwick cleared his throat.

"An immensely complex spell," he said squeakily, "involving the magical
concealment of a secret inside a single, living soul. The information is
hidden inside the chosen person, or Secret-Keeper, and is henceforth
impossible to find -- unless, of course, the Secret-Keeper chooses to
divulge it. As long as the Secret-Keeper refused to speak, You-Know-Who
could search the village where Lily and James were staying for years and
never find them, not even if he had his nose pressed against their sitting
room window!"'
(from "Harry Potter and the Prisoner of Azkaban")

>From this discussion, we know that the Potters were concealed by the
Fidelius Charm. We've also discovered that the Secret-Keeper for the Potter
family was changed at the last minute from Sirius Black to Peter Pettigrew,
with tragic results.

While they were hidden under the Fidelius charm, I assue that the Potters
(Lily, James and Harry) were confined to their hiding place. It would be of
no use using such an "immensely complex spell" if you nipped out for a bag
of chips on Saturday night, allowing anyone you met - whether Muggles,
Wizards, Death Eaters or You-Know-Whom (good grammar is everything!)  to
watch you adding the salt and vinegar.
The Secret that was being hidden inside Pettigrew's soul was that the
Potters were hiding in [whatever address] in Godric's Hollow. Presumably,
other than the Potters and Peter Pettigrew, not one single person would be
aware of where they were. When the spell was cast, I assume that anyone who
had known where they were would instantly forget.

Naturally, it would be of prime importance that You-Know-Whom didn't find
out where the Potters were hiding, otherwise he would kill them. At first,
Sirius was selected to be the Secret-Keeper, but at his instigation, they
changed their minds and chose Peter, because Sirius was the obvious choice,
and You-Know-Whom would be sure to go after him first. Lupin wasn't told of
the change, in case he was spying for You-Know-Whom. It makes sense for as
few people as possible to know who the Secret-Keeper was. Even Dumbledore
didn't know that Peter had been chosen.

Put yourself in their place for a moment. It's 1981. You want to go into
hiding. You need a Secret-Keeper. Whom do you choose?
Dumbledore - too much on his plate already.
Sirius - too obvious, plus Lupin thinks he may be a spy. (at the end of
"PoA" they apologised to each other for suspecting this.)
Lupin - Sirius thinks he may be a spy.
Others - (I'm speculating here that maybe Lily and James don't want to [put
at] risk people they arent sure of, or aren't as close to)

Which leaves Peter. If Lupin and Sirius suspect each other, then isn't there
the slightest doubt in your mind that maybe Peter is a spy?

I know who I'd choose.

What's wrong with James or Lily? If you need to keep a secret in the most
secure way possible, then why not keep the identity of the secret keeper as
part of the secret to be kept (or if that's getting too self-referential,
make James the primary Secret-Keeper concealing thelocation of the Potters)
and perform another charm to let Lily keep the secret of the identity of the
first secret-keeper.
That way, if Peer goes to You-Know-Whom, he can't betray them. Nor can
anyone else. Unless James or Lily divulge the fact that they are hiding in
Godric's Hollow, they quite simply are completely and utterly safe.
Of course then You-Know-Whom wouldn't be (almost) killed, Harry wouldn't be
the boy who lived, and we'd all be talking about a completely different set
of books.
As my mother would put it, "If my auntie had balls, she'd be my uncle."

Actually, now I come to think of it, there must be something we're not being
told about the Fidelius charm.

As we know, "The information is hidden inside the chosen person, or
Secret-Keeper, and is henceforth impossible to find -- unless, of course,
the Secret-Keeper chooses to divulge it."
Dumbledore has used the Fidelius charm to conceal 12 Grimmauld Place,
particularly in its aspect as the HQ-Quarters of the Order of the Phoenix.
Presumably, members of the Order of the Phoenix know where they're based, as
part of the secret that's being Kept.
But if anyone outside the Order finds out, then the secret is divulged and
the place is no longer protected by the Fidelius charm.
But there are several non-members of the Order of the Phoenix who know all
about 12 Grimauld Place, and the fact that it's the Order's HQ. Namely Harry
Potter, Ron, Fred, George and Ginny Weasley, Hermione Granger, Kreacher, Mrs
Black's portrait.
I'd say pretty much that constitutes divulgence. In that case, what's
protecting 12 Grimmauld Place now?

Wildean