A proof of work challenge server that issues and checks POW challenges, keeping score

challenger_server.py

import socket, random, string, hashlib

'''
challenge request packet format
\x08[HASH] [START] [SEED]
sends a hash algorithm (SHA1, MD5, etc), what the hash must start with, and what the prehashed string must start with

challenge response packet format
\x09[FULL PRE-HASH]
The [FULL PRE-HASH] must start with [SEED] and when hashed with [HASH] algorithm, the result starts with [START]
'''

randomstring = lambda x:''.join([random.choice(string.printable[:62]) for i in range(x)])
sequential = lambda y: (lambda s: s(y, s))(lambda x, s: "" if not x else s(int((x-1) / 256), s) + chr((x % 256) - 1 if x % 256 != 0 else 255))

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('0.0.0.0', 9696))
sock.settimeout(1)

users = {}

while 1:
	try:
		data,addr = sock.recvfrom(64000)
		if data[0] == '\x05': # register me
			users[addr] = {'score':0}
			sock.sendto('\x05OK', addr)
			print 'registered',addr
		elif data[0] == '\x06': # unregister me
			del users[addr]
			sock.sendto('\x06OK', addr)
			print 'unregistered',addr
		elif data[0] == '\x07': # what is my score
			if addr in users.keys():
				sock.sendto('\x07' + str(users[addr]), addr)
			else:
				sock.sendto('\x07NOT REGISTERED', addr)
		elif data[0] == '\x08': # give me a challenge
			if addr in users.keys():
				hashseed = randomstring(5)
				sock.sendto('\x08SHA1 0000 ' + hashseed, addr)
				users[addr]['challenge'] = {'hash':'SHA1', 'start':'0000', 'seed':hashseed}
				print 'issued new challenge to',addr,'::',users[addr]['challenge']
			else:
				sock.sendto('\x08NOT REGISTERED', addr)
		elif data[0] == '\x09': # challenge answered
			if not addr in users.keys():
				sock.sendto('\x09NOT REGISTERED', addr)
				continue
			if not 'challenge' in users[addr].keys():
				sock.sendto('\x09NOT CHALLENGED', addr)
				continue
			
			prehash = data[1:]
			if prehash.startswith(users[addr]['challenge']['seed']) and hashlib.new(users[addr]['challenge']['hash'], prehash).hexdigest().startswith(users[addr]['challenge']['start']):
				sock.sendto('\x09OK', addr)
				users[addr]['score'] += 1
				del users[addr]['challenge']
				print addr,'solved their challenge'
			else:
				sock.sendto('\x09WRONG', addr)
				del users[addr]['challenge']
				print addr,'failed their challenge'
			
	except socket.timeout:
		pass