Reverse Engineering

Challenge	Link
Secure Bank (100 pts)	Here
Phish Market Order Management (100 pts)	Here
TriForce Recon (100 pts)	Here
Funny (449 pts)	Here

Funny (449 pts)

Description

What is this funny file? 🤪

Solution

Given .pyc file, at first i tried to decompile using pycdc but it failed. After that i tried to use pycdas to show the opcode of compiled python.

        136     LOAD_NAME                       34: input
        138     PUSH_NULL
        140     LOAD_CONST                      4: 'Key > '
        142     CALL                            1
        150     LOAD_ATTR                       71: encode
        170     CALL                            0
        178     STORE_NAME                      36: key
        180     LOAD_NAME                       34: input
        182     PUSH_NULL
        184     LOAD_CONST                      5: 'IV  > '
        186     CALL                            1
        194     LOAD_ATTR                       71: encode
        214     CALL                            0
        222     STORE_NAME                      37: iv
        224     LOAD_NAME                       38: len
        226     PUSH_NULL
        228     LOAD_NAME                       36: key
        230     CALL                            1
        238     LOAD_CONST                      6: 32
        240     COMPARE_OP                      88 (==)
        244     POP_JUMP_IF_FALSE               111 (to 468)
        248     LOAD_NAME                       38: len
        250     PUSH_NULL
        252     LOAD_NAME                       37: iv
        254     CALL                            1
        262     LOAD_CONST                      7: 16
        264     COMPARE_OP                      88 (==)
        268     POP_JUMP_IF_FALSE               99 (to 468)
        272     LOAD_NAME                       4: unpad
        274     PUSH_NULL
        276     LOAD_NAME                       1: AES
        278     LOAD_ATTR                       78: new
        298     PUSH_NULL
        300     LOAD_NAME                       36: key
        302     LOAD_NAME                       1: AES
        304     LOAD_ATTR                       80: MODE_CBC
        324     LOAD_NAME                       37: iv
        326     CALL                            3
        334     LOAD_ATTR                       83: decrypt
        354     LOAD_CONST                      8: b'\x1fQer\xec\xb6\x05\xee\xc0g/\x82\x0c\xdb\xecZ\x13\xf8\x08\x01\tB:\xe4\xbdkb/r\xba\xf6\xc0\xe9\xe6M\xf7\xa1\x02&\x17p\x7f)\xb4\xe0\xdf\xe8w'
        356     CALL                            1
        364     LOAD_NAME                       1: AES
        366     LOAD_ATTR                       84: block_size
        386     CALL                            2
        394     STORE_NAME                      43: decrypted
        396     LOAD_CONST                      9: b'INTIGRITI'
        398     LOAD_NAME                       43: decrypted
        400     CONTAINS_OP                     0 (in)
        404     POP_JUMP_IF_FALSE               30 (to 466)
        408     LOAD_NAME                       6: print
        410     PUSH_NULL
        412     LOAD_CONST                      10: 'gg'
        414     CALL                            1
        422     POP_TOP
        424     LOAD_NAME                       6: print
        426     PUSH_NULL
        428     LOAD_NAME                       43: decrypted
        430     LOAD_ATTR                       89: decode
        450     CALL                            0
        458     CALL                            1

Looking at above code we can see that there is 2 input processed (key and iv) and there is no value check. The check only for the length of IV and KEY, after that there will be check for the decrypted value which is the flag. So lets continue the analysis. Right after above code, there is a

        476     STORE_NAME                      45: i_wonder_what_this_decrypts_to
        478     LOAD_NAME                       4: unpad
        480     PUSH_NULL
        482     LOAD_NAME                       1: AES
        484     LOAD_ATTR                       78: new
        504     PUSH_NULL
        506     LOAD_CONST                      12: b'/\xa0\xaa<?\xb54q\xdb\x00\x8e\xfe w\xae\x89\xf3R3\xd7.\x83c\xe6\xb6E\xac|\xfd\x92\x95j'
        508     LOAD_NAME                       1: AES
        510     LOAD_ATTR                       80: MODE_CBC
        530     LOAD_CONST                      13: b'\x1fdu\xae\x8f\x9a\xa5\x8b,\x94\xa0T\xe5ib\xd0'
        532     CALL                            3
        540     LOAD_ATTR                       83: decrypt
        560     LOAD_NAME                       45: i_wonder_what_this_decrypts_to
        562     CALL                            1
        570     LOAD_NAME                       1: AES
        572     LOAD_ATTR                       84: block_size
        592     CALL                            2

There is key and iv also ciphertext (i_wonder_what_this_decrypts_to) in the code that can we use to do the decryption. It should be called but never printed, so lets construct by ourself.

from Crypto.Cipher import AES

ct = b'\xc0\xae\x13\x8a\xbb\x0b\xa1\x95\xf9\x8a\xe0\\\xb6\xc8\xd2\x1fHF*\x0bK\x11\xf2\x82\xfcIk\xd2|\xcdf\xd6@c\x17\xe9\xcc\xdd\x98O\xa5;=<\xea\\\x91\xf2\xc0I\x901)\x8bH\xab\xda}g\x95\x0f#<YN\x03\x94n\xd1\xed\xcf\x9a\xc4\xefU\x99\x92\xe7\xf0&^\xd9\xb9\xc7U\x7f\xce\x0eQ;9dU\xf2\x88\x17\xe2\xc0Q\x93&\xeb\x1f\xf9\'"$\x08?\x83~\xb1\xc8HK\x07\xce\xb2\x10\xd1\x9d\x99k\xba)X\xc5\x08\xec\xe7]\x81s\x9b\xdfsl\xad\xbc 8TM\xd9\xa1em;\xec\x03E\xa5EJ\xbaT:\'\xa3\xa9\xf4\xa9\x0b\xadL\xaer\xf1\x84\x02bI\xe3Ym-&\xb6\xae\\c\xa0\xbcJ@\xaf\xf9\x1d\x8bY\xa3\xa5\xe5b\xf3\xf1\x99b\xf2~\xbd\xf3\xd6\xf5m,\x82=u\x98G\x1b\x86s6\xcb\xf8x\xa9\x16\xfc\xe35\xd8\xca\x17K\xa4\xc6\x04d:\xbaN \xed\x15`\x8f\xd1\xe1\xfdHhr\xbc\xb08V\xb8\x8d\x83\x8a\xf2\x8e{\xe0M\xffs=\xfb\x8d;\xdf\xdd\xc1\x0fm\xeb{\xb5\x9bm\x83\xe7\xf9\x91F\xc0C\xdfP\rG\xd6\xfa\xd7Lb\x8eA\xe4\x008\xf2\xad\xd8\x0f\x9d\xb8\xbfM@\xf4\xca\xf4\xb2\xbbi\x0c\xff\x0fu\x9cT\x0b\xf8\x8e\x9c\x1e\xca\xbc\xcd\xaaG\xd9\x17xp\xd1B7\x0f\xe0\x914\xf1\xdb\xeczH\xbbc\xde\xfcyGA/\xce\x86\x91\xec\xe5:\xc1\x03\x8d]\x90J^\xdc\xa9+\x03Xh\xc3\xe9\x1cDxd\x1b\x0fF\x1f\x99\x18\x8e9DA{\x80\xea\xcf\xb0\xeb\xb0K\x8f\xe4\x07\x1f\x8e\x04n\xb8t\xe9\x13R\xf39\xf1\x17\x05\xfb\x00\xf8\xca\xfa}K\xa6\x8e+\xd5\xa9\x04\xb9I\xab+w\x93\xa8\xfek\xd3\x8bJt\xe3\xfea\xa1 \xb7\xb9\xae\x81\xe2:\xfc\x0b\x85\x15\xe4s\xef:xF\xd9\x83\xddi4\x9e6!\xc2\xbaP\\\xb4V6%\xc0"x\xd1\x15\xc1\x07\x10\xd0\xf9\x96\xee\xd6\xf1)\x9e\xe3\xb9\xe3T\xec\xdd\x8alF\x88\x101U\xed\xb8\xbcd\xfc\x02\xb3\xb4B\x04\xd4g\xc8\xc7\x83\x89^^\xc4\xfc\x02\xca\xcd\xb9\xa7\\|yU\xb71\xb4>\xa9\x1dxv4!\\\xe1\xc1\xe0?\xa3\xaf/(\x16\x0b\xcd\xd3\x99S\x0f\xe4\xfd\xa9\\\x17g\x9e\'N^\xca\x9f^\xec~G\xfa\xd9\xc5 R\xb1\x1f\x95\xaa\xe5:\xa7\x9f\xc2\x8bC\x88\xd2\xa1\xff\x83\x94a\x11J\xdb{\x1bb\x1aBs\x18\xef\xa7\xd7\x1e\xc3\xa4\xff\xa9v:M\xb2\xc59\x98\xe4f\x13!\xf5mz\xb1\x15\xdc\xf5x\xd3g\x0e%t\xda\x9aV\x0cQ\xe5Z{s5\xb1C\xe2!\xdf\xc4z\xaa9^/y\x01^\x99\xdb)4\x02,C\x16tQ\xa2Rj\x80f\xc5U\t\xbd\x1d\xaf/\xaf\x9b\x8cv\xd5\n\x0b\xf6$3\x97Ac\xd6\xff\xc0\x86\xe3\xe3\x1d\x87T\x1d\x8b\xab%S\x8a!\x9f\x1b\x9a$\xdb:\xf47\xa7,\x1cY\x9c*\xec\xcbt1\x9c\x9euGs\x18t\xf6\x81\xeb\xe0Dd\xedx\x02\xec"\xab\x80Bh\x03\xcb\x86\xe1*K\xa5O\xa3\xc2,q~\x87\x8a%\xcc%\x0c\xda-9F)\x00\x8b\xbe\x85\x17\xadl\xd2\xb0\x8c\x8e;U*\xfc4\xd6\r>\x04\xbe\x7f\x88\xc7T\xbd0\xa6\xe85\xa9.\x19\x8d\xfa%\x85\n\xe2\x84\xa8&\xcd\'\xb9\xd6\xebK\x8d\x10}\xc2\xd1\xde@9Uf\xa4_\xa2\xb9X\xd6;\xf9I\xcfc\xfc6\x94\xa5\x834\xbeBD\xfa\xfd\xb8.\x82\xfb)a\xf6L\x95V#\x05\xea}6\xe0C\xf7\xf4\xd3\xed\xba\x02v\xe5 \xcch,{\xad\xc7\xfc\xe1\xaf\xb3\x14R\xdb1\x07JK:g8\x8b\x9bw\x17\x1d\x13\x8ff\x8a\x0e\x98\xcd\xc4\xb1e^\x8c\xbai\x05\xa2\x9e\xc9\xd8{ D\x9c\x0f\xc1\x8a\x0b\x92\'\xed \x8f,\x00o\xa3\xb8%X\xa3\xde\xec\xfdu\x1a\xc9\xac\xfd\x1f\xf9<|\xd4G\x1a.\xac^\xfc\xc9I\x1fkG\xe9\xa1\x0e\xd2\x1b\x90\xb7\xfd\xd0.{\x00d6Y\x06\xcb,L\x9a\x82\xb5\xe6\x8f\xedp=\xbf\xbdFP\x88\xdf!\x0f}]\xecK\x0b3\x97f\xf6\'\x90\xe7\x9ex:xE\x9c\xe0\xbe\x14\xc8\x0f\xa3\xc8\xdag1\xe5{\xe1i|=\x14\x81wy\xb8W\xe7?j\x95;p\x1a\x95\xc7#6\xc3\x00\xec8\xae;\xc9\xf7\xcc\x0f\xb4K\x17w\x1f\xc9]\xa8D&\xb1\xacrs\xbeI\xc7\xab\xf29s\x85\xd7\xaf0\xb0|\x13\xa6\x96\x8f\xe2\x97\x84\x8ag\xf3\xb9ow!b\xa8\xd1\x83a[*$\x8a\xfdp\xc2\x7f\xb1f\xd4\x90z\xe3V\x16\xd8\xed\xa2@\n_\xb4\xd2)\xd4\x14\x85\xbay\xf65#3\xbf\x14\xbf\x1a\xb4\xcb\xd9\xc7\x0f\xe8\xa5!\xa0\xae\x00\x9a\xdb\xd8\xfbg\x8f\x99\xca&\x9f\xe7\x0c\xf3\x03\xd3rw\xe2P\x8c\x01\x0b\xa14-U\xb3\x8b?\x94\xfd\xed#\x99w4\xa2\xf7\xfaS\x10\xa5F\xcbY\x1a\xe7\x87I\xed\x02t,|\xb6c\x8f\xa4)\xa7\x8e\xe2\x18<\xaf\x05z g\xda\xa3\x19\xefq\xe4?\x92\xfdj+y}|\xc6\r}\xad\xd8\xecp\x96v\x8ah\x8b_\xec?\xfbpg\xc5\xfe\x9d=\x063\x17\x96\xe2>\xb1\xed-l\x95b\xb6\x81\xebF-\x1b\x12\xc2\x0b\xdeG\xbd\xdc\xe3\xf56u\xfd\x14\x17V\x8c\x87\x17_m?\xf7\xc3\xc2\xf0\x80;\x03\xea\xc9\xfd\x93~\xeff\x9b\xc5\r\x92\x9atb\xf9\x16\x8c&m4\xb5/F@\x12\')\xea\xff\x97\xf5C;\xd7\x08\xa0\xac`\xc5#\x8b\x80\t\xe4\xf1\x99\x81\xc9\xa7g\xe0\xb1\x02)CZVz\xa6\xcb\x84@!\x98\xf9n \xa0e.3\xa3I\xe8\xcd\x8a\x9d\xb6\xdf\x18G\xdd\xf8T6&\xaa\xde\xb6\x9d\xfb\xdb\x17\x00H\xe3Lm|\xc8\x80\xb0\x03\xd1\xd2\xaaw~\xa0\x00P\x1f\x1a_\x8b\t\x81\xc8p\xfa.\xfb\xd8;\x13\xbb\x80$\xe3\xa4\x05*\x91\x9eD$cc\xaf@\x9b\xaa\xb7&\xaf\x1a\xa1L/X\xc8\xafW\xa3\xb6\xd1\r\xa4\xe8\x91 ^4\xd4\xc7V7\xc6\xe4\x11\x12\xe0{x\x85\xbb\x92\x07\xc0\xd3\xec\xf2JcX\xdb\xcd\xda\xae\x80\xfa\x92\xae\xae&\x07\xaf\x86\xf8i\xabai\xed\x0c`'
key = b'/\xa0\xaa<?\xb54q\xdb\x00\x8e\xfe w\xae\x89\xf3R3\xd7.\x83c\xe6\xb6E\xac|\xfd\x92\x95j'
iv = b'\x1fdu\xae\x8f\x9a\xa5\x8b,\x94\xa0T\xe5ib\xd0'
cipher = AES.new(key, AES.MODE_CBC, iv)
print(cipher.decrypt(ct).decode())

Now come to the part that need us to guess what is the objective that the author want, by reading above text again by again i understand that we need to reconstruct the original code including the position (row and column). Before i got the objective i tried to do analysis on the variable initialized (a-z) and i found that all the values are 0 and the code only put the variable without doing nothing. Now, lets take a look on the opcode again

        32      LOAD_CONST                      0: 0
        34      COPY                            1
        36      STORE_NAME                      8: a
        38      COPY                            1
        40      STORE_NAME                      9: b
        42      COPY                            1
        44      STORE_NAME                      10: c
        46      COPY                            1
        48      STORE_NAME                      11: d
        50      COPY                            1
        52      STORE_NAME                      12: e
        54      COPY                            1
        56      STORE_NAME                      13: f
        58      COPY                            1
        60      STORE_NAME                      14: g
        62      COPY                            1
        64      STORE_NAME                      15: h
        66      COPY                            1
        68      STORE_NAME                      16: i
--------------------------------snip----------------------------------------
        600     POP_TOP
        602     LOAD_NAME                       9: b
        604     POP_TOP
        606     LOAD_NAME                       25: r
        608     POP_TOP
        610     LOAD_NAME                       32: y
        612     POP_TOP
        614     LOAD_NAME                       9: b
        616     POP_TOP
        618     LOAD_NAME                       12: e
        620     POP_TOP
        622     LOAD_NAME                       13: f
        624     POP_TOP
        626     LOAD_NAME                       12: e
        628     POP_TOP
        630     LOAD_NAME                       17: j
        632     POP_TOP
        634     LOAD_NAME                       21: n
        636     POP_TOP
        638     LOAD_NAME                       18: k
        640     POP_TOP
        642     LOAD_NAME                       16: i
        644     POP_TOP
        646     LOAD_NAME                       11: d
        648     POP_TOP
        650     LOAD_NAME                       22: o
        652     POP_TOP
        654     LOAD_NAME                       26: s
        656     POP_TOP
        658     LOAD_NAME                       9: b
        660     POP_TOP
        662     LOAD_NAME                       22: o
        664     POP_TOP
        666     LOAD_NAME                       14: g
        668     POP_TOP
        670     LOAD_NAME                       28: u
--------------------------------snip----------------------------------------

The code above that i said it does nothing. During the disassembling process of the pyc file using pycdas, i found there is option --pycode-extra and it will show Line Number Table like below

By trying to do the dis with our own code i found that Line Number table consist information about column and row, which is that all we need to solve this challenge. Below is my flow for solving the challenge.

• save dis output from line 24 until line 30

  • https://gist.github.com/kos0ng/36b5e319d0873b491f585570d446ed67 (output2.dis)
• reconstruct the original code by knowing each variable location (row and col)

• print the map to get the key

• get the value for each iv coordinate

import marshal
import types

f =  open('funny.pyc', 'rb')
f.read(16)
code_object = marshal.load(f)

list_col = []
for i in code_object.co_positions():
	if i[0] != None:
		if i[0] <= 30 and i[0] >= 24:
			list_col.append(i)
mapp = [[' ' for j in range(600)] for i in range(7)]

counter = -1
f2 = open("output2.dis", "r").read().split("\n")
for i in range(len(list_col)):
	assert list_col[i][3] - list_col[i][2] == 1
	if f2[i].split(" ")[2] != "":
		counter += 1
	if "LOAD_NAME" in f2[i]:
		mapp[counter][list_col[i][2]] = f2[i].split("(")[-1][0]
		if mapp[counter][list_col[i][2] + 1] != ' ':
			print("what")
			exit()
		else:
			mapp[counter][list_col[i][2] + 1] = ';'

for i in mapp:
	print(''.join(i))

For the IV just add the following code in the end

coord_iv = [(476, 6), (468, 5), (282, 6), (506, 6), (420, 3), (492, 0), (192, 6), (56, 6), (144, 3), (324, 0), (360, 1), (352, 6), (30, 1), (260, 0), (298, 1), (480, 3)]
iv = ""
for i in coord_iv:
	iv += mapp[i[1]][i[0]]
print(iv)

Last, just decrypt it.

from Crypto.Cipher import AES


key = b'P05IT1ON4L_INF0RM4TION_1S_GR34T_'
iv = b'xqamhngxojmobdnh'
ct = b'\x1fQer\xec\xb6\x05\xee\xc0g/\x82\x0c\xdb\xecZ\x13\xf8\x08\x01\tB:\xe4\xbdkb/r\xba\xf6\xc0\xe9\xe6M\xf7\xa1\x02&\x17p\x7f)\xb4\xe0\xdf\xe8w'

cipher = AES.new(key, AES.MODE_CBC, iv)
print(cipher.decrypt(ct))

Flag: INTIGRITI{y0u_7ruly_4r3_7h3_pyc_p4r51n6_m4573r}