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@Rurik

Rurik/asm_find_math.py

Last active Oct 13, 2020
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Detect subroutines that may have encryption/encoding routines by finding XOR and shift routines.
# Automatically find XOR/SHL/SHR routines from an executable
# Uses IDAW (text IDA)
# @bbaskin - brian @ thebaskins.com
# While other, more powerful scripts like FindCrypt find known
# algorithms this is used to find custom encoding or modified
# encryption routines
"""
Script results:
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
sub_402E51 shr eax, 1
sub_402E51 xor eax, 0EDB88320h
sub_402E51 shr eax, 1
sub_402E51 shr eax, 8
sub_402E51 xor eax, dword_4161C0[ebx*4]
sub_402EBB xor edx, edi
sub_402EBB shr eax, 8
sub_402EBB xor eax, dword_4161C0[edx*4]
sub_402EBB shr edi, 3
sub_402EBB xor eax, [ecx]
sub_402EBB shr ebx, 10h
sub_402EBB shr edx, 18h
sub_402EBB xor edx, dword_4175C0[ebx*4]
sub_402EBB shr ebx, 8
sub_402EBB xor edx, dword_4179C0[ebx*4]
sub_402EBB xor edx, dword_4165C0[ebx*4]
sub_402EBB xor edx, dword_4169C0[ebx*4]
sub_402EBB xor edx, dword_416DC0[ebx*4]
sub_402EBB xor edx, dword_4161C0[ebx*4]
sub_402EBB xor edx, dword_417DC0[eax*4]
sub_402EBB xor edx, edi
sub_402EBB shr eax, 8
sub_402EBB xor eax, dword_4161C0[edx*4]
"""
from argparse import ArgumentParser
import os
import subprocess
import sys
#IDA_EXE = 'C:\Program Files (x86)\IDA 6.95\idaw.exe'
#IDA_EXE = 'C:\Program Files\IDA Pro 7.5\idat.exe'
IDA_EXE = 'C:\Program Files\IDA Pro 7.5\idat64.exe'
IDA_ARGS = ' -A -B '
IDA_CMD = '"%s" %s' % (IDA_EXE, IDA_ARGS)
#sep = '\t' # Use for IDA 6
sep = ' ' # Use for IDA 7
def file_exists(fname):
"""
Determine if a file exists
Arguments:
fname: path to a file
Results:
boolean value if file exists
"""
return os.path.exists(fname) and os.access(fname, os.X_OK)
def CreateASM(exefile, exefile_asm):
"""
Use IDA Pro text (idaw) to create an ASM dump of the PE file
Arguments:
exefile: path to PE file
exefile_asm: output ASM file
"""
print('[*] Creating %s' % (exefile_asm))
cmdline = IDA_CMD + exefile
result = subprocess.Popen(cmdline)
print('[*] Executing: %s' % (cmdline))
result.wait()
if not file_exists(exefile_asm):
print('\n\n' + '-=' * 20 + '-')
print('[!] Could not find ASM file: {}'.format(exefile_asm))
print('[!] If the following is shown above, adjust DOS screen buffer height to < 300:')
print('>>> Fatal error: the window is too high (max 300 rows)!')
quit()
def ScanASM(data, find_shifts):
"""
Process ASM data to find XOR/SHL/SHR
Arguments:
data: contents of ASM file
"""
print('Script results:')
print('-=' * 20 + '-')
# Unknown_Sub is for when IDA could not identify initial subroutines
currentsub = 'Unknown_Sub'
for line in data:
if line.startswith('sub_'):
currentsub = line.split()[0]
if 'xor' + sep in line:
items = line.split()
if items[1].strip(",") != items[2]:
print(currentsub + '\t' + line.strip())
if find_shifts:
if 'shl' + sep in line or 'shr' + sep in line:
print(currentsub + '\t' + line.strip())
def main():
if not file_exists(IDA_EXE):
print('Cannot find IDA: {}'.format(IDA_EXE))
quit()
parser = ArgumentParser()
parser.add_argument('exefile')
parser.add_argument('-s', help='Display shift operations', required=False, action='store_true')
args = parser.parse_args()
if not args.exefile:
print('[!] Specify executable to scan on the command line')
quit()
else:
exefile = args.exefile
exefile_asm = exefile + '.asm'
if not file_exists(exefile_asm):
CreateASM(exefile, exefile_asm)
else:
print('[*] ASM file already found: {}'.format(exefile_asm))
data = open(exefile_asm, 'r').readlines()
find_shifts = True if args.s else False
ScanASM(data, find_shifts)
if __name__ == '__main__':
main()
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