fake_entropy.py

#!/usr/bin/env python

import os,fcntl,struct

"""
cat > test.c <<DEL
#include <stdio.h>
#include <linux/random.h>
main() { printf("%u\n",RNDADDENTROPY);}
DEL
gcc test.c -o a.out; ./a.out
"""
RNDADDENTROPY = 1074287107
entropy_dev = '/proc/sys/kernel/random/entropy_avail'

def write_rand(block_size,bc,entropy_ratio):
        fd = os.open("/dev/random", os.O_WRONLY)
        fakefd = os.open("/dev/urandom", os.O_RDONLY)

        fmt = 'ii%is' % (block_size)

        rnd = os.read(fakefd,block_size)
        os.close(fakefd)
        this = struct.pack(fmt, entropy_ratio * len(rnd), len(rnd), rnd)

        #refeeding the same block, nooooooo
        for _ in xrange(bc):
                fcntl.ioctl(fd, RNDADDENTROPY, this)

        os.close(fd)

def read(s):
        f = open(s,'r')
        rv = f.read()
        f.close()
        return rv

expect = 512

try:
        before = int(read(entropy_dev))
        while True:
                write_rand(512,10,1)
                after = int(read(entropy_dev))
                print after
                if after>=expect: break

        print 'Entropy was %s, now %s' % (before,after)
        print 'Your crypto karma just dropped below zero, Bruce Schneier hates you.'
except Exception, e:
        print e
        print 'You probably forgot to sudo'