平成 16 年度

大学院博士 (前期) 課程入学者選抜学力試験

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I 次の記事は，コンピュータ同士をネットワーク上で直接接続して，互いに持つ情報をやり取りする方式である P2P(peer-to-peer) と呼ばれる通信形式について述べたものである．この記事を読み，以下の問いに答えよ．とくに指示がない場合は，問いには日本語で答えよ．（配点 100 点）

Napster is the pioneer of a technology known as peer-to-peer networking, or P2P forshort. The core idea of P2P is to allow individual computers to communicate directly
over the Internet. By bypassing central servers, the technology promises to transformthe way people use the Net. In the process, it could destroy the ability of anyone—
including corporations and governments—to control what happens in cyberspace.
Napster’s winning idea was to give P2P to the masses. It figured out that it didn’t
have to store everything itself. Instead, it acted like a dating agency, bringing musicfans—and their MP3 collections—together. Napster provided members with an index
of all the music stored on other members’ computers, and software that enabled themto hook into each other’s hard drives. Members could then swap files without the direct
involvement of Napster.
Napster was thus able to give its members access to massive amounts of music without
having to store a single note itself. It’s clear that most of the recordings were being
distributed in violation of copyright laws. If Napster had been storing pirated music on
its site, it would have been shut down in days.

The reason it lasted so long was thatit could quite credibly argue that it was an innocent intermediary. If users happened to
be trading pirated music it was no more Napster’s fault than it’s the fault of the postal
service if people mail home-taped cassettes to one another.
Napster hadn’t just found a way of dodging the copyright lawyers, it had solved
a problem plaguing many large networks, especially the Internet.

The client-server

models they are built on are hierarchies, and like all hierarchies they’re great as long as
you are near the top. But most small-time users are near the bottom, shackled to an
Internet service provider and its rules.
Napster’s Achilles’ heel was that it retained a trace of the client-server model. Because members were dependent on Napster for software and indexes, record companies
had a target to go after. And go after it they did. In December 1999, EMI, BMG,
Sony, Warner, Universal and the Recording Industry Association of America sued Napster for copyright infringement. Although the suit is not yet settled, Napster suffereda terminal blow last month when a US court of appeal ordered it to stop enabling— 1 —the exchange of copyrighted material. Napster has effectively thrown in the towel
and is now trying to find a way of charging for its services so it can pay royalties.
But there is a P2P network that looks capable of evading
the lawyers. Called Freenet, it’s a radical system created fromthe ground up to be anonymous and censorship注 1 -proof.
Freenet uses the Internet as a backbone to send and receive
information, and identifies each computer by its IP address.
But it covers its tracks whenever information is transferred.
Hooking your computer up to Freenet, first you download
the software from the Web. Then you contact other Freenet
computers, whereupon your computer becomes a Freenet”node”. Freenet is made up of thousands of these nodes, andeach one can make files available. When you ”insert” a file—
say an MP3—into Freenet it is encrypted and then copied toseveral other nodes. Each node knows which documents it
holds and also has information about documents stored on afew other nodes. Neighbouring nodes communicate routinely,
updating one another on additions to the network. But nosingle node knows about more than a fraction of the entire
network.
How do you get information out of a system like this? AsClarke注 2 explains it, the strategy is similar to the way people
navigated before maps. Starting out, a group of travellers
might have known only to go north. But the closer they got
to their goal, the more detailed was the information they got
from people they asked, until finally they found someone able
to tell them that yes, the minstrel they were looking for lived
right around the corner, second hovel on the right.
Before you start a Freenet search, you must know the titleof the document you’re looking for. Each document also has anumeric key that is cryptographically linked to the title, andit’s this you’re actually looking for during a Freenet search.
Let’s say you know the key is 123—though of course real keys
will be a lot more complex than that. Each node, including yours, knows what documents— 2 —it holds, and also has a list of documents held by a few other nodes. Your computer willlook to see if it has document 123. If not, it will look up to see if it knows a node that has
document 123. If it doesn’t, it contacts the node with the document that comes closest—
maybe document 135. That node might not know where 123 is either, but it knows which
node has document 119, so it sends the request there. The idea is that with each request
you get closer to the document you really want. When the document is found, it’s returnedalong the request chain (seeDiagram ). As the document is returned, each node along thechain makes a copy of it and stores it.
One consequence of this is that the more requests come in for a piece of information, themore copies there will be on the network, and the easier it will be to find. It also means
there’s no way of telling where the document originally came from. All you know is that you
asked a neighbouring node for it, and it fetched the document from somewhere. Conversely,
if you receive a request for a file, you have no idea who made it.
The result is a censorship-proof network. If the powers that be request a file from a node
they’ll get a copy. If they seize that node they’ll definitely find a copy. But it would beimpossible for them to prove that the file was there before they requested it, so the exercise
amounts to entrapment, Clarke says. And because documents are stored in encrypted form,
the node’s owners can argue truthfully that they had no idea any particular document is held
there. What’s more, as the act of requesting a document generates new copies, censorshipis self-defeating.
Not everyone accepts that Freenet is as censorship-proof as Clarke thinks. Creighton
reckons he can bring it down by getting the IP addresses of individual nodes, sending lettersto ISPs, and taking some users to court, just as he wants to do with Gnutella.
But if Clarke turns out to be correct, Freenet will usher in a different world. No one will
be able to stop you downloading free music files from the Internet. You’ll be able to criticise
the rich and powerful without fear of being silenced or punished. And you’ll be able to readwhichever spy memoir your government is trying to suppress at the moment.
By the same token, you’ll be powerless to stop people from plagiarising your copyrighted
work or telling lies about you. Napster set out to give us free music, but it seems to have put
us on the road to absolute freedom of speech. If so, the real challenge hasn’t even begun.
(New Scientist vol 169 issue 2281 - 10 March 2001, page 32-36 より一部修正の上引用)(注 1) censorship: 検閲 (注 2) Clarke: Freenet 作者の名— 3 —問 1 下線部，陵由を簡単に述べよ．

問 2 下線部△鯑

本語訳せよ．

問 3 下線部
問 4 下線部

示す図表の内容を，本文に即して説明せよ．に”The result is a censorship-proof network.” とあるように，ここに提案されているシステムはデータの検閲やデータの出所の特定が困難であると主張されている．その理由について本文に即して説明せよ．
問 5 この記事の内容を英文で要約せよ（100 words 程度）
問 6 Napster, Freenet などの P2P ファイル交換サービスは，原著論文などの個人の創作物・著作物の扱いについて影響力をもつと考えられている．このようなサービスの存在のもとで，個人の学術論文の価値を守り，かつ広く利用してもらうための方法について自由に論ぜよ．かならずしも記事の立場に立たなくても良い．

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