Talk:Main Page/Archive 1

This page serves as an archive for clearly Handled topics, as Talk:Main Page is getting a little cluttered.

md5 Collisions?

What about collisions in the md5 algorithm?

What about them? In theory, two dates/stock market prices might end up with the same meetup point. Unlikely, and probably not a terribly big deal? Zigdon 07:39, 21 May 2008 (UTC)

There are situations where cryptographic collisions are a problem, but this isn't one of them. md5 is just being used as a pseudorandom number generator here, and it has a very small-entropy seed (someone call Debian!). So its cryptographic strenth isn't particularly relevant. --Xkcd 07:43, 21 May 2008 (UTC)xkcd

OMG geohashing predicts future dow prices!

http://irc.peeron.com/xkcd/map/data/2009/05/27 --Ryan the leach 12:33, 21 May 2008 (UTC)

heh yeh i saw that one too... this is deep magic SinJax

Yeah, I totally wasn't testing stuff, <_< Zigdon 14:54, 21 May 2008 (UTC)

Wow, now you only need to break MD5 to get rich! 81.167.17.12 12:40, 21 May 2008 (UTC)

could be future meetup? --Ryan the leach 12:33, 21 May 2008 (UTC)

Fairly inconsistent if so, what if the dow is different. WHAT THEN?!?

It won't ;) --DarkRat

well the tool wont say its different :P --Ryan the leach 12:33, 21 May 2008 (UTC)

geohash.org

There's already a thing called "geohash" which is a way to represent coordinates with arbitrary precision in a computer and query-friendly format. It's at http://geohash.org/ and it was designed and implemented by Gustavo Niemeyer.

I don't see how that's relevant. We are talking about geohashing. It's a different word.

To decimal

...now, i'm likely just too thick to get it, but how does one convert back from the half md5 hash to decimals?

It goes like this... each digit is below zero but not 10^-n but 16^-n....so the first hex digit is 16^-1, the next is 16^-2 and so on. And it goes from left to right so in the hex number "8d" you get:

f = (8 * 16^-1) + (13 * 16^-2) = 0.5507

i think anyway :) - SinJax

Or, interpret the bytes from md5 as two big-endian 64-bit integers, and divide both by 2^64.

Chicago area

In the northern Chicago suburbs, the vast majority of nearby GPS areas according to the implementation are dangerous pools of dihydrogen monoxide. Is there some workaround?

Chicago area residents could just use which ever of the four locations makes the most sense. (See the Chicago page.)

Believe Adelaide in South Australia Has a similair problem but nowhere quite as severe.

dihydrogen monoxide should be avoided at all costs, if you're careful, you could go to the nearest point at the shore of the dihidrogen monoxide pool, but be sure to not ingest any.

While i'm probably the only Milwaukeean who is going to be running around and playing, my region is about 80% Lake Michigan. No good.

My region is a similar amount English Channel and Solent. I guess this just makes the occasional on-land meetup more special - perversely, I think it might actually *keep* people's attention longer. 81.187.153.189 18:45, 21 May 2008 (UTC)

xkcd.com/geohashing Problems?

Hey - I can't get to [1], and none of the graticules' maps are working - is this just me, or the rest of the world, too?

Yes, irc.peeron.com is currently down (as of 20080523 11pm PDT), I know, I'm trying to get hold of someone who can do something about it. Zigdon 07:29, 23 May 2008 (UTC)

Why is the the xkcd.com/geohashing redirecting to irc.peeron.com anyway, shouldn't it be moved to xkcd? I think too many active graticulees are melting the server. --Opspin 08:07, 23 May 2008 (UTC)

it's working again!

Difference between the comic and the generator

Zigdon, xkcd:

The hash in the comic for 2005-05-26-10458.68 starts with db9318........

Here's debugging info for the map page's calculation of http://irc.peeron.com/xkcd/map/:

Graticule: (37, -123) - (38, -122)
Market open on 2005-05-26 = 10458.68
MD5(2005-05-26-10458.68 ): 357e5cac889681628fdd754c1a235919
Split: 357e5cac88968162, 8fdd754c1a235919
offset = 0.20895938122029104, 0.5619729338451526
37.20895938122029 -122.56197293384515

On my machine: $ md5 -s "2005-05-26-10458.68"
MD5 ("2005-05-26-10458.68") = db9318c2259923d08b672cb305440f97

Any idea what's going on here? Is that an extra space on the end there, Zig?
--FunkyTuba

   I've found a bunch of opening values at http://irc.peeron.com/xkcd/map/data/
   with carriage returns on the end, which I dont think were being stripped out before
   hashing. The website went down before I could fully confirm though.
   --ZorMonkey 11:49, 23 May 2008 (UTC)

That was indeed the case, fixed now. Zigdon 14:07, 23 May 2008 (UTC)

DOW source

What is our source for the DJIA figure? I only ask because http://irc.peeron.com/xkcd/map/dow.js has "data['2008-05-20']=13026.04", and http://irc.peeron.com//xkcd/map/data/2008/05/20 also says, "13026.04"

However, WSJ (http://online.wsj.com/mdc/public/npage/2_3051.html?symbol=DJIA), usually reliable for this kind of thing, has an opening of "12,958.06" for 5/20/2008.

I could be misinterpreting something. Mattflaschen 13:27, 21 May 2008 (UTC)

And http://finance.yahoo.com/q?d=t&s=%5EDJI does have, "13,026.04". Mattflaschen 13:28, 21 May 2008 (UTC)

We get the data from finance.google.com Zigdon 14:55, 21 May 2008 (UTC)

I think this may help explain the problem with opening values. Perhaps the closing value should just be used as that seems a bit more definitive -- someone may want to check the sources named in this section to see if they agree on closing value. If the closing values of all three of Dow, Nikkei and Euronext were summed, this would still allow for surprise most mornings of the week worldwide -- your surprise would just be based on the closing of a market partway around the world. See #Time_Zone_Discussion for discussion of combining multiple indices. —Christian Campbell 19:32, 21 May 2008 (UTC)

Well, I don't know. The article says, "the posted opening price on the Dow will be close to the previous day's closing price (which can be observed by looking at Dow price history) and will not accurately reflect the true opening prices of all its components." But that's a criticism that the posted opening is meaningless (from an economic point of view). It doesn't necessarily imply that different services post different openings.

The three (Google Finance, Yahoo Finance, and WSJ) do agree on today's closing price. However, I recommend we stick with opening and just state a definitive source. Mattflaschen 20:57, 21 May 2008 (UTC)

Okay. The statements above seemed to indicate that different sources post different openings, and I thought the article might explain that.

Regardless of opening vs closing, the definitive source solution seems helpful. —Christian Campbell 21:42, 21 May 2008 (UTC)

I realize that this is just for spontaneity, and that's great, but lest someone decide to use this for something serious, it ought to be noted that the entropy of the DOW isn't very high: the biggest change ever over the course of a single day is -684.81 (http://www.djindexes.com/mdsidx/index.cfm?event=showavgstats#no4), which is about 16 bits. Most daily differences are much smaller, around half that many bits. It's very amenable to a dictionary attack. --Mike Stay

True... adversaries could prepare to target the 32768 likeliest meetup sites tomorrow. I'll risk it.  ;-) —Christian Campbell 19:21, 22 May 2008 (UTC)

The entropy of the DJIA doesn't actually matter. A single digit change is enough to toss you completely to the other end of the graticule, since the MD5 would be *completely different*. Zigdon 23:49, 22 May 2008 (UTC)

Not just for games

This idea interests me but not necessarily just for games. My only problem is the use of the DOW. Say you maintained a group of people who you want to secretly convene at certain times and locations. The times are all set in advance. You don't want to transmit the location itself since it could be intercepted. Replace MD5 with your own convoluted scheme. Transmit the keys to that algorithm online, through numbers stations, whatever. The point is that you can select their location in advance, compute the key, transmit the key, and nobody would be able to figure out what it meant. If they did know it associated to a geolocation, then they would have to crack your algorithm.

You don't need to replace MD5. Just arrange on a password, and concatenate it with the date and number before doing the MD5. Then you can publish the number and even the method, and your location won't be determinable. -- lilac

Planning Ahead

I tried creating a geohash for june 26th, but it seems you are going off of market data, because it says "no market data for 6/26".

Um, yeah, that's rather the point. Did you actually read the cartoon? The reason for using something unknowable in advance is so that one does not know in advance (except at weekends) where each day's location will be.

Its about spontaneous fun adventures my deary! Just let go, find out where things are on the 25th and make it happen. see you there! SinJax

Map datum?

One thing left unspecified so far is "What map datum should the coordinates be interpreted in?" I suppose that everyone is using the same thing Google Maps uses, by default. Having searched for what it is, I see a lot of people speculating that it is WGS84, though the Google Maps API doesn't say so. --Recursive 18:31, 21 May 2008 (UTC)

It is. (as defined by the recent EPSG standard (that's actually based on MS VE, but Google use the same thing)) --Edgemaster 17:28, 22 May 2008 (UTC)