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'''
Author: @XOR_Hex
#flareon4
Challenge 3
' Three parts
' 0) Dump the ASM from IDA, Start @ 0x40107C and the length is 0x79 (see box @ loc_401029)
' bytes = idaapi.get_many_bytes(0x40107C, 0x79)
' with open('/home/firefly/buffer.asm', 'wb') as f:
' f.write(bytes)
' 1) Figure out what byte the code needs to be xor'ed with. Last word must match 0xfb5eL (loop at 401039, comparison check: 40105E)
' Code to be xor'ed and add'ed is located at 40107C and the length is 0x79 (see box @ loc_401029)
' To determine the input byte the modified (xord + add) bytes have to be passed through sub_4011E6 before the comparison check can be done
' 2) After the check passes the state memory from angr needs to be interpreted as code, so use capstone
' The op_str from capstone contains what looks like ASCII characters...
' 3) Take the capstone output and interpret the ASCII codes to get the flag
'''
import angr
import sys
p = angr.Project('greek_to_me.exe', load_options={'auto_load_libs': False})
f2 = None
# Interate through all of the possible byte values to find the correct "user" input to de-mask the flag
for buf in xrange(0x100):
print("Trying buf = {0}".format(buf))
# Variable to store the bits written to disk using IDA
asm = None
# Store the output from the first de-obfuscation routine
b2 = []
# Read in bytes written to file from IDA
with open('greek_to_me_buffer.asm', 'rb') as f:
asm = f.read()
# Re-implement loc_401039
dl = buf
for byte in asm:
bl = ord(byte)
bl = bl ^ dl
bl = bl & 0xff
bl = bl + 0x22
bl = bl & 0xff
b2.append(bl)
# Set up angr to "run" sub_4011E6
s = p.factory.blank_state(addr=0x4011E6)
s.mem[s.regs.esp+4:].dword = 1 # Angr memory location to hold the xor'ed and add'ed bytes
s.mem[s.regs.esp+8:].dword = 0x79 # Length of ASM
# Copy bytes output from loc_401039 into address 0x1 so Angr can run it
asm = ''.join(map(lambda x: chr(x), b2))
s.memory.store(1, s.se.BVV(int(asm.encode('hex'), 16), 0x79 * 8 ))
# Create a simulation manager...
#import pdb; pdb.set_trace()
simgr = p.factory.simulation_manager(s)
# Tell Angr where to go, though there is only one way through this function,
# we just need to stop after ax is set
simgr.explore(find=0x401268)
# Once ax is set, check to see if the value in ax matches the comparison value
for found in simgr.found:
#import pdb; pdb.set_trace()
print(' ax = %s' % hex(found.state.solver.eval(found.state.regs.ax)))
# Comparison check
if hex(found.state.solver.eval(found.state.regs.ax)) == '0xfb5eL':
# Upon success, dump the asm
code = ("%x" % found.state.solver.eval_upto(found.state.memory.load(1, 0x79), 1)[0]).decode('hex')
print('\n Winner is: {0}\n\n'.format(buf))
print(' %s' % code)
bl = None
dl = None
flag = []
# Using capstone, interpret the ASM
from capstone import *
md = Cs(CS_ARCH_X86, CS_MODE_32)
for i in md.disasm(code, 0x1000):
flag_char = None
# The if statements do the work of interpreting the ASCII codes to their value counterpart
if i.op_str.split(',')[0].startswith("byte ptr"):
flag_char = chr(long(i.op_str.split(',')[1], 16))
if i.op_str.split(',')[0].startswith('bl'):
bl = chr(long(i.op_str.split(',')[1], 16))
if i.op_str.split(',')[0].startswith('dl'):
dl = chr(long(i.op_str.split(',')[1], 16))
if i.op_str.split(',')[1].strip() == 'dl':
flag_char = dl
if i.op_str.split(',')[1].strip() == 'bl':
flag_char = bl
if (flag_char):
flag.append(flag_char.strip())
print(" 0x%x\t%s\t%s\t%s" %(i.address, i.mnemonic, i.op_str, flag_char))
print('\n\n')
print(''.join(flag))
sys.exit(0)
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