
Death Note: L, Anonymity & Eluding Entropy - simonbrown
http://www.gwern.net/Death%20Note%20Anonymity
======
Eliezer
So now if criminals found only in widely available public sources begin dying
of heart disease and cancer at correctly distributed but far-above-average
rates, L will know that it's somebody who read this article - possibly Gwern
himself! Even if this article hadn't been published, such clever use of the
Death Note would certainly point to someone with high IQ and possibly involved
in the cryptographic community...

~~~
gwern
Well, all I can say is I don't remember writing this! Whomever did so must be
one clever fellow though, and I'd be happy to assist anyone in trying to catch
him...

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buu700
It's kind of funny that you posted this now, because at the moment I'm
actually hacking on the last level of Stripe's Capture the Flag with Low of
Solipsism, L's theme, Kira's theme, Near's theme, and Zetsubou Billy looping
in the background.

As a side note, has anyone else considered the possibility that it's entirely
possible that Light could have simply set up a server which would schedule his
victims' deaths for a certain randomised time, then at that time pull up an
image of them on X and print their name on one page of the death note which
was set up to be indefinitely recycled within the same printer?

At that point it'd have been trivial for him to set up a simple password-
protected REST interface which would accept POST requests of victims'
names/pictures, and he could have even done all his future recruiting through
Tor without taking the risk of using physical mail (and eventually
facilitating the recruitment of a local "Kira" in every country, possibly
vetting through social media comment histories in a similar fashion to the way
he evaluated Mikami).

This setup wouldn't necessarily have been possible, but the wording of the
Death Note looks like it might have allowed for it (contingent on how strictly
it defined "mind"). At the very least, printing through electronic media would
have been worth an experiment.

Assuming the server were reasonably secure, and he'd taken appropriate steps
to scale as time passed, the very fact of all that ink on one page would have
made it completely illegible, and thus inconclusive evidence. Then, he could
have just performed any Kira actions from a virtual machine with an encrypted
filesystem, eventually ceasing any direct killings as his army increased in
size. It would have scaled beautifully, and he'd have been almost perfectly
anonymous.

To put the icing on the cake, if he didn't want to put in so much grunt work,
he could have simply configured his server to automatically hook into every
well-maintained reputable public listing of convicted violent criminals (as
well as listings like the website of _America's Most Wanted_ ) and then
forgotten about the whole thing. And hell, if the whole thing is automated,
why not just put in a line of code that marks each death as being at the hands
of the most likely intelligence agency? It would have achieved the same
chilling effect ("OMG THE CIA IS KILLING EVERY CRIMINAL IN THE WORLD NOW!"),
and thus the same goal, without even realistically risking his own livelihood.

~~~
archangel_one
As I recall, part of the rules of the Death Note was that the user has to
picture the face of the victim as they write the name, so they have to have a
name and a face to kill. I assume a printer doesn't manage to do that,
although I do suspect at times that some printers are fairly malevolent.

~~~
buu700
".. would accept POST requests of victims' names/ _pictures_ ... at that time
_pull up an image of them on X_ and print their name on one page of the death
note ..." (emphasis added)

Like I said, this depends on a bit of a loose definition of "mind", so it
might not have worked at all.

Here is the exact wording of the rule in question: "This note will not take
effect unless the writer has the person’s face in their mind when writing
his/her name. Therefore, people sharing the same name will not be affected."

It specifically appeals to the (somewhat hazy) concept of mind, which would
imply the prerequisite of some sort of sentience. However, the rules are not
absolute, but rather based on Ryuk's imperfect interpretation (Ryuk didn't
know a priori how certain experiments would turn out, like writing causes of
death before names with the FBI agents); thus, the use of "mind" itself isn't
so important, as it merely implies that Ryuk's experience is limited to
sentient beings, which should be obvious.

In other words, there isn't any way to know whether a death note could be used
in this way without testing it on a "real" death note (or having the authors
write canonical material on it).

~~~
archangel_one
I agree that there's not any way to know for sure. I guess I saw the "mind"
thing a different way; that it has to be a sentient being thinking about it,
but obviously we can't know without one to test on :)

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ufo
I hate to be pedantic, but mistake #1 misses the point a bit. While the serial
killer doesn't want to be found, he certainly wants to be _known_.

~~~
Cushman
If you really want to scare people, you should make every cause of death
"Batman". Or maybe "space laser".

~~~
gwern
The death has to be possible; Light tests out the limits early and finds IIRC
that if you specify an imprisoned criminal dies outside the prison, the Death
Note just gives up and kills them normally or something like that. 'Batman'
and 'space laser' are right out, as is making criminals solve NP-hard or
Halting problems.

~~~
SamReidHughes
What are the odds that three victims would write out notes that combine to
encode a message about death gods eating apples? I wouldn't rule out their
writing solutions to NP-hard problems. Even halting problem problems. It's
just a matter of getting them to write a note that begins with a certain
letter, or that includes the word "yes" or "no".

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gburt
Can anyone explain what is going on here? Edit: to clarify, I honestly have no
idea what I'm reading: the title piqued my interest, but then it became... I'm
really not sure.

~~~
AznHisoka
It's about an anime named Death Note where the main character(named Light)
finds a notebook (dubbed death note), and has the ability to kill anyone if he
writes their name in the book. Noone knows he has it, but someone names "L" is
hired to find out, and eventually traces the deaths to Light.

~~~
Lockyy
Manga adapted into an anime and movie(s) to be pedantic.

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throwaway15213
How much can you trust each bit of information you gained though? For example,
it might be very possible that Light could be intentionally waking up at the
middle of the night to do his killings. In which case you would've ruled out
japan, which then ruins your subsequent analysis which is all somewhat based
on him being in japan. Verifying every bit of information gained or even
assigning a probability to how much you can trust it seems to be a hard
problem.

~~~
Natsu
The show itself used quite a few very interesting methods to do it. Basically,
L played a lot of hunches and won big several times.

The part about the time of day was found out long after L had narrowed things
down to a small region of Japan by provoking Light into killing someone who
appeared on a "worldwide" broadcast that was actually only broadcast in
Light's prefecture. The time of day part was actually used to hypothesize that
Light was a student, whereupon the pattern changed to show people killed every
hour of the day. And L saw through _that_ by assuming (correctly) that Light
had access to police data. It seemed like he was always watching Light's
reactions to his moves, rather than the moves themselves.

Probably one of the most clever traps of all was after they met in person,
when L was able to trap Light by getting evidence that Light knew something he
should have no way of knowing at all by means of false evidence. He played it
off as Light being too dumb to figure out the riddle, but the only reason he
couldn't solve it was because he thought the answer was impossible (and he
shouldn't have known that).

~~~
Lockyy
One problem with the article I realised earlier was that it described the
broadcast to Kanto as a gamble because it wasn't broadcast anywhere else,
which is wrong. It was broadcast in each region one after another to find out
which region it was in. It wasn't a gamble at all, it was a very clever way of
finding out where Kira was watching from.

~~~
Natsu
It might be possible for him to hear about the broadcast by other means had he
not been in the first region where it was broadcast.

But it never really turned out to matter, as you say. L was able to notice
that weird case where the evil guy who had taken school children hostage died
of a mysteriously timed heart attack and piece a lot of things together.

Actually, though, that broadcast was probably one of the most important puzzle
pieces. He was able to prove that Light could kill remotely with nothing but a
face & name, having cut a special deal with a criminal and kept the guy under
special observation the whole time. Light was very careless there.

~~~
Shaanie
It would be extremely hard (probably impossible) to hide the fact that you
need a criminals name and face to kill them. At most he could've gained a bit
of time by being more careful, and at that point he didn't believe the death
note would work anyway.

~~~
Natsu
Maybe if you assume that psychic killings are possible from the outset. While
people might be able to figure out that something is going on, proving that
someone is doing the killings remotely via a notebook is quite a leap and only
the crazy arrangement L tried made proving that possible.

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hardy263
As a curiosity, how many bits were gained (rather than lost) when the second
and third characters obtaining a death note were entered? (female and male
respectively)

Do the bits of entropy add on, or does it not matter at all?

~~~
gwern
Depends on the assumptions you make, I think. If we make the assumption that
#2 and #3 obtained Death Notes at a known time, then any observations before
then still pin down #1 - if there are kills at morning Japan-time, that serves
to pin him down to Japan, etc.

But once #2 and #3 become equally active, now any evidence like that serves to
narrow down the propositions 'any of #1, #2, #3 are in Japan', so if someone
asked you what's the odds that #2 lives in Japan, you would only have 1/3 the
evidence you did before - because any kill linked to Japan would only have 1/3
chance of having been #2. If a kill is made using information from a rural
Iowan newspaper with circulation of 100, well, all you know is that any of
1/2/3 had access to it (and maybe all 3!). And so on.

(In bit terms, if there were 4 Death Note users, then any observation has 1/4
the power it did, or 1/2^2; similarly, if there were 16 users, or 32 users...
Once you established someone was a Death Note user you would still need to do
that many more bits of work to figure out _which_ Death Note user you want.)

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ff0066mote
Just a note to the author:

I'm getting a lot of [Math Processing Errors] when I view your article:
<http://imgur.com/PSeMX> I haven't looked at the source to see what was going
on, so it might be that I'm blocking flash or something.

Otherwise, it was great! :) Thanks for a fun read!

~~~
gwern
No, it's all done via MathJax/JavaScript. That should be cross-browser without
any issues, I thought.

~~~
ff0066mote
Well, no worries. I was on a school computer with an outdated version of
Chrome.

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Tichy
I have recently thought about a possible future where everybody could kill
everybody else anonymously. No idea how that would play out - probably with
extinction?

I think technology could provide such a future, for example something like
those tiny poison drones in the movie Dune.

Or maybe it will be possible to genetically engineer a virus to just kill one
specific person - kind of like Stuxnet was targeted at one specific factory...

~~~
mikecane
See the Twilight Zone episode or Matheson's story:
[http://en.wikipedia.org/wiki/Button,_Button_%28The_Twilight_...](http://en.wikipedia.org/wiki/Button,_Button_%28The_Twilight_Zone%29)

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mikecane
I thought the article was brilliant. I stopped watching the series Numb3rs
early on, so I wonder if it ever really covered real math like that?

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Splines
Sort of reminds me of the board game "Scotland Yard". It'd be interesting to
see if an analysis with a mathematical bent would yield some tips for playing
the game.

