The World's Tiniest P2P Program

As you have probably sussed out by now, writing a peer-to-peer program is almost as simple as programming a network client and a network server into one application. But, did you ever wonder how small it could really get? A couple of years ago the US Congress was considering the prohibition of peer-to-peer applications in toto. To illustrate how fruitless such a prohibition would be, Ed Felten, a professor at Princeton University, and one of his (then) doctoral students, Alex Halderman, wrote a P2P application with Python in just 15 lines of code:

# tinyp2p.py 1.0 (documentation at http://freedom-to-tinker.com/tinyp2p.html)
import sys, os, SimpleXMLRPCServer, xmlrpclib, re, hmac # (C) 2004, E.W. Felten
ar,pw,res = (sys.argv,lambda u:hmac.new(sys.argv[1],u).hexdigest(),re.search)
pxy,xs = (xmlrpclib.ServerProxy,SimpleXMLRPCServer.SimpleXMLRPCServer)
def ls(p=""):return filter(lambda n:(p=="")or res(p,n),os.listdir(os.getcwd()))
if ar[2]!="client": # license: http://creativecommons.org/licenses/by-nc-sa/2.0
  myU,prs,srv = ("http://"+ar[3]+":"+ar[4], ar[5:],lambda x:x.serve_forever())
  def pr(x=[]): return ([(y in prs) or prs.append(y) for y in x] or 1) and prs
  def c(n): return ((lambda f: (f.read(), f.close()))(file(n)))[0]
  f=lambda p,n,a:(p==pw(myU))and(((n==0)and pr(a))or((n==1)and [ls(a)])or c(a))
  def aug(u): return ((u==myU) and pr()) or pr(pxy(u).f(pw(u),0,pr([myU])))
  pr() and [aug(s) for s in aug(pr()[0])]
  (lambda sv:sv.register_function(f,"f") or srv(sv))(xs((ar[3],int(ar[4]))))
for url in pxy(ar[3]).f(pw(ar[3]),0,[]):
  for fn in filter(lambda n:not n in ls(), (pxy(url).f(pw(url),1,ar[4]))[0]):
    (lambda fi:fi.write(pxy(url).f(pw(url),2,fn)) or fi.close())(file(fn,"wc"))

To run the server, simply type: python tinyp2p.py password server hostname portnum [otherurl]
The corresponding client is invoked: python tinyp2p.py password client serverurl pattern

The flexibility of the port number allows the user to run multiple servers at the same time. You can read more about this program at Felten's website: Freedom-to-Tinker.com.