Easy

tsurai.py

#!/usr/bin/env python2

# too sleepy.....it's kimoi.
# N = p*q
# C = M*(M+B) mod N

import binascii
import itertools
import string

"""
793194894181: 868019 913799
792127391761: 875543 904727
531838213717: 597263 890459
"""

usestr = string.printable

data = [
        {'N': 793194894181, 'B': 0xffeee, 'C': 0x8d5051562b, 'p': 868019, 'q': 913799},
        {'N': 792127391761, 'B': 0xfffee, 'C': 0x5ffa0ac1a2, 'p': 875543, 'q': 904727},
        {'N': 531838213717, 'B': 0xfefef, 'C': 0x6008ddf867, 'p': 597263, 'q': 890459},
]

#M = int("0x"+"SECCO".encode('hex'), 16)

hint = "N{"
C = data[0]['C']
B = data[0]['B']
N = data[0]['N']

flag = "SECCO"

for x in itertools.product(usestr, repeat=3):
    token = "".join(x)
    token = hint+token
    token = token.encode('hex')
    M = int(token ,16)
    if M*(M+B)%N == C:
        print "OK"
        t = hex(M)[2:-1]
        t = t.decode('hex')
        flag += t
        break

C = data[1]['C']
B = data[1]['B']
N = data[1]['N']
hint = "}"
for x in itertools.product(usestr, repeat=4):
    token = "".join(x)
    token = token+hint
    token = token.encode('hex')
    M = int(token ,16)
    if M*(M+B)%N == C:
        print "OK"
        t = hex(M)[2:-1]
        t = t.decode('hex')
        flag += t
        break

with open('flag.txt', 'w') as f:
    f.write(flag)

waiwai.py

#!/usr/bin/env python2

import ctypes
import sys

libc = ctypes.cdll.LoadLibrary("/lib/libc.so.6")

def f(x):
    k = libc.rand()&0xff
    return chr((ord(x)^k)&0xff)

# for error
for i in range(-3, 3):
    t = 1416667590+i
    libc.srand(t)
    with open(sys.argv[1], 'rb') as fi:
        ebin = fi.read()
        with open(sys.argv[2]+"-"+str(t), 'wb') as fo:
            fo.write("".join(map(f, ebin)))