Challenge
Oh, you're here... Good, because I really need you at this moment. So, you know I'm ambitious and trying to be accepted at one of the best universities. Every year they are giving a challenge to students that want to be accepted at their university. An accepted student is called "Next Generation Child", you know all this Next-Gen hype... I believe this challenge is not so hard and I want to prove that they are Old-Gen. Ready?NOTE: When you get something intelligible enclose only the last part in timctf{}
Author: 0xcpu
Summary
The challenge is based in the following steps:- patch the binary and remove the anti-debug techniques;
- get 32 bytes from .data and perform a xor with the (n-odd character);
- perform a md5 to the result and check with the md5 hashes that they are in .rodata (You have the n-odd character);
- if the md5 is correct, you have to dissasemble the 32 bytes generated with the xor operation;
- the n-pair character is included in this assembly code;
- in one function, perform a call to this assembly code, disassemble it and now you know the character that you need;
- perform it 12 times (because 12 hashes and 12 binary strings); and
- get the flag.
Solution
File:ng_child: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=2d3481d0b7302267d6d01c8e4021ca735e5acf11, stripped
Before:
After:
Main code which perform the XOR, md5, and call
So, disassemble first function and look that it is a simple xor between the first 32 byte got from .data and the first character.
We can know all odd characters now.
Getting the odd characters:
#!/usr/bin/bash
import string
import md5
import binascii
hashes = ["95a196021fa4e9574cd821c9d0ba041f",
"5795b8d6e2435c5ecdac54375166b544",
"5cc3e196973fbb79c9aa4e18027f866b",
"fcc149646e5d3879134804646da90cea",
"484ec01fb9e6449c0d80a716077702d1",
"73c0dc9db84acd0c336522043b6fd4e3",
"b466c3a7d2618bf6f61c6077e3bd6aa0",
"e73273179f206a7db0d6960e96b84b82",
"01a3e2b4e7523506929b4e3f3ad6dbbb",
"bca5b1b2c5716cfd2c2e94cc07c26029",
"7339e454bbe37d732026780d70d87c9a",
"b5490761c3a6641e8f54de6a47a35eed"]
init = ["3c29373f97e120f8e829baef2000dc293429f8a8fc670f94abf89e492097e120",
"a1502936f1a140299ee12100d6213e3697e829a1f56ef1049ee629bae629a23e",
"203187f7388ef7381ec031e0b0593de03db0e47f0088f031b9f738b32831a2f9",
"23b941382186513800c229918738222b5138b0e47f158af0b68f38b32ab28e86",
"3400c8382838b97080f938b95834abf03c2038b9ed76e90bb68631b5bae9b041",
"20f839a8e120b89fa0972000d9f8a897e0a9fc67a9505cd89ca4abba492031a0",
"2039313720ba68299fe12097e92400d7209ee829a1f56e15f115a79720a2f13f",
"422ba3422b00d030f395422b9cea2b95f338a3f76c5298e33895422ba09cea2b",
"00dbfb3f2aab6a23645b97eb2aafe32a2afbafff642aabffa9ff64ab6a23a837",
"da283b3a28a958200792e928a9582100b9e1a95821abfd66b2e021aaf9322833",
"26fe3faee726be99a6912600dffeae91e6affa61af5631de9aa2adbc4f2637a6",
"2a00d6263626a76e9ee726a7462ab5ee223e26a7f368f73da8982faba4f7ae5f"]
index_init = 0
for h in hashes:
for i in string.printable:
m = md5.new()
d = ord(i)
out_x = ""
index = 0
while (index < len(init[index_init])):
f = init[index_init][index:index+2]
d_hex = int("0x"+f,16)^d
if f == "00":
out_x = out_x + "00"
elif len(hex(d_hex)) == 4:
out_x = out_x + hex(d_hex).replace("0x","")
elif len(hex(d_hex)) == 3:
out_x = out_x + hex(d_hex).replace("0x","0")
index = index + 2
cadena = ""
c1 = "".join(reversed([out_x[0:16][z:z+2] for z in range(0, 16, 2)])) #out_c[0:16]
c2 = "".join(reversed([out_x[16:32][z:z+2] for z in range(0, 16, 2)]))#out_c[0:32]
c3 = "".join(reversed([out_x[32:48][z:z+2] for z in range(0, 16, 2)]))#out_c[32:48]
c4 = "".join(reversed([out_x[48:64][z:z+2] for z in range(0, 16, 2)]))#out_c[48:64]
cadena = c1 + c2 + c3 + c4
m.update(binascii.unhexlify(cadena))
h_f = m.hexdigest()
if h_f == h:
print i, m.hexdigest()
index_init+=1
$ python hash.py
h 95a196021fa4e9574cd821c9d0ba041f
a 5795b8d6e2435c5ecdac54375166b544
p 5cc3e196973fbb79c9aa4e18027f866b
p fcc149646e5d3879134804646da90cea
y 484ec01fb9e6449c0d80a716077702d1
h 73c0dc9db84acd0c336522043b6fd4e3
a b466c3a7d2618bf6f61c6077e3bd6aa0
c e73273179f206a7db0d6960e96b84b82
k 01a3e2b4e7523506929b4e3f3ad6dbbb
n 7339e454bbe37d732026780d70d87c9a
g b5490761c3a6641e8f54de6a47a35eed
How to solve it? Disassembly, and understand the code, it is easy (obviously, each 32 bytes generated will have different code..)
So, we modified our script to disassembly this code automatically.
import string
import md5
import binascii
from pwn import *
hashes = ["95a196021fa4e9574cd821c9d0ba041f",
"5795b8d6e2435c5ecdac54375166b544",
"5cc3e196973fbb79c9aa4e18027f866b",
"fcc149646e5d3879134804646da90cea",
"484ec01fb9e6449c0d80a716077702d1",
"73c0dc9db84acd0c336522043b6fd4e3",
"b466c3a7d2618bf6f61c6077e3bd6aa0",
"e73273179f206a7db0d6960e96b84b82",
"01a3e2b4e7523506929b4e3f3ad6dbbb",
"bca5b1b2c5716cfd2c2e94cc07c26029",
"7339e454bbe37d732026780d70d87c9a",
"b5490761c3a6641e8f54de6a47a35eed"]
init = ["3c29373f97e120f8e829baef2000dc293429f8a8fc670f94abf89e492097e120",
"a1502936f1a140299ee12100d6213e3697e829a1f56ef1049ee629bae629a23e",
"203187f7388ef7381ec031e0b0593de03db0e47f0088f031b9f738b32831a2f9",
"23b941382186513800c229918738222b5138b0e47f158af0b68f38b32ab28e86",
"3400c8382838b97080f938b95834abf03c2038b9ed76e90bb68631b5bae9b041",
"20f839a8e120b89fa0972000d9f8a897e0a9fc67a9505cd89ca4abba492031a0",
"2039313720ba68299fe12097e92400d7209ee829a1f56e15f115a79720a2f13f",
"422ba3422b00d030f395422b9cea2b95f338a3f76c5298e33895422ba09cea2b",
"00dbfb3f2aab6a23645b97eb2aafe32a2afbafff642aabffa9ff64ab6a23a837",
"da283b3a28a958200792e928a9582100b9e1a95821abfd66b2e021aaf9322833",
"26fe3faee726be99a6912600dffeae91e6affa61af5631de9aa2adbc4f2637a6",
"2a00d6263626a76e9ee726a7462ab5ee223e26a7f368f73da8982faba4f7ae5f"]
assembly = []
index_init = 0
for h in hashes:
for i in string.printable:
m = md5.new()
d = ord(i)
out_x = ""
index = 0
while (index < len(init[index_init])):
f = init[index_init][index:index+2]
d_hex = int("0x"+f,16)^d
if f == "00":
out_x = out_x + "00"
elif len(hex(d_hex)) == 4:
out_x = out_x + hex(d_hex).replace("0x","")
elif len(hex(d_hex)) == 3:
out_x = out_x + hex(d_hex).replace("0x","0")
index = index + 2
# Divide en 4 partes e invierte
cadena = ""
c1 = "".join(reversed([out_x[0:16][z:z+2] for z in range(0, 16, 2)])) #out_c[0:16]
c2 = "".join(reversed([out_x[16:32][z:z+2] for z in range(0, 16, 2)]))#out_c[0:32]
c3 = "".join(reversed([out_x[32:48][z:z+2] for z in range(0, 16, 2)]))#out_c[32:48]
c4 = "".join(reversed([out_x[48:64][z:z+2] for z in range(0, 16, 2)]))#out_c[48:64]
cadena = c1 + c2 + c3 + c4
m.update(binascii.unhexlify(cadena))
h_f = m.hexdigest()
if h_f == h:
assembly.append(binascii.unhexlify(cadena))
print i, m.hexdigest()
#print out
index_init+=1
context.arch = 'amd64'
for a in assembly:
print "----------------------"
print disasm(a)
Result:
h 95a196021fa4e9574cd821c9d0ba041f
a 5795b8d6e2435c5ecdac54375166b544
p 5cc3e196973fbb79c9aa4e18027f866b
p fcc149646e5d3879134804646da90cea
y 484ec01fb9e6449c0d80a716077702d1
h 73c0dc9db84acd0c336522043b6fd4e3
a b466c3a7d2618bf6f61c6077e3bd6aa0
c e73273179f206a7db0d6960e96b84b82
k 01a3e2b4e7523506929b4e3f3ad6dbbb
n 7339e454bbe37d732026780d70d87c9a
g b5490761c3a6641e8f54de6a47a35eed
----------------------
0: 90 nop
1: 48 89 ff mov rdi,rdi
4: 57 push rdi
5: 5f pop rdi
6: 41 54 push r12
8: 41 b4 00 mov r12b,0x0
b: 48 87 d2 xchg rdx,rdx
e: 41 80 fc 67 cmp r12b,0x67
12: 0f 94 c0 sete al
15: 90 nop
16: 41 5c pop r12
18: 48 89 ff mov rdi,rdi
1b: 48 21 f6 and rsi,rsi
1e: 90 nop
1f: c3 ret
----------------------
0: 48 21 c0 and rax,rax
3: 90 nop
4: 57 push rdi
5: 48 31 c0 xor rax,rax
8: 57 push rdi
9: 5f pop rdi
a: 40 b7 00 mov dil,0x0
d: 40 80 ff 65 cmp dil,0x65
11: 90 nop
12: 0f 94 c0 sete al
15: 48 89 f6 mov rsi,rsi
18: 5f pop rdi
19: c3 ret
1a: 48 87 db xchg rbx,rbx
1d: 48 87 ff xchg rdi,rdi
----------------------
0: 48 87 fe xchg rsi,rdi
3: 48 87 f7 xchg rdi,rsi
6: 41 50 push r8
8: 90 nop
9: 4d 29 c0 sub r8,r8
c: 90 nop
d: 41 b0 6e mov r8b,0x6e
10: 41 80 f8 00 cmp r8b,0x0
14: 0f 94 c0 sete al
17: 4d 89 d2 mov r10,r10
1a: 41 58 pop r8
1c: c3 ret
1d: 48 87 c9 xchg rcx,rcx
----------------------
0: 48 21 f6 and rsi,rsi
3: 51 push rcx
4: 48 31 c9 xor rcx,rcx
7: 53 push rbx
8: 5b pop rbx
9: 52 push rdx
a: 48 f7 e1 mul rcx
d: 59 pop rcx
e: b2 00 mov dl,0x0
10: 80 fa 65 cmp dl,0x65
13: 0f 94 c0 sete al
16: 48 21 f6 and rsi,rsi
19: fe c2 inc dl
1b: 5a pop rdx
1c: c3 ret
1d: 48 ff c6 inc rsi
----------------------
0: 09 c0 or eax,eax
2: 41 51 push r9
4: 41 b1 00 mov r9b,0x0
7: 4d 89 d2 mov r10,r10
a: 4d 21 c0 and r8,r8
d: 41 80 f9 72 cmp r9b,0x72
11: 90 nop
12: 0f 94 c0 sete al
15: 41 59 pop r9
17: 45 38 c9 cmp r9b,r9b
1a: 90 nop
1b: c3 ret
1c: cc int3
1d: 48 ff cf dec rdi
----------------------
0: f7 d0 not eax
2: 48 89 c0 mov rax,rax
5: 51 push rcx
6: 90 nop
7: 48 ff c0 inc rax
a: 90 nop
b: b1 00 mov cl,0x0
d: 48 ff c8 dec rax
10: b0 34 mov al,0x34
12: 38 c1 cmp cl,al
14: 0f 94 c1 sete cl
17: 88 c8 mov al,cl
19: 59 pop rcx
1a: 48 21 d2 and rdx,rdx
1d: c3 ret
1e: cc int3
1f: f4 hlt
----------------------
0: 48 09 db or rbx,rbx
3: 41 56 push r14
5: 50 push rax
6: 58 pop rax
7: 41 b6 00 mov r14b,0x0
a: 45 88 f6 mov r14b,r14b
d: 41 80 fe 74 cmp r14b,0x74
11: 0f 94 c0 sete al
14: 48 89 ff mov rdi,rdi
17: 41 5e pop r14
19: 90 nop
1a: c3 ret
1b: 41 f6 c6 74 test r14b,0x74
1f: 90 nop
----------------------
0: 53 push rbx
1: b3 00 mov bl,0x0
3: 48 21 c0 and rax,rax
6: 48 21 f6 and rsi,rsi
9: 48 89 ff mov rdi,rdi
c: 48 21 f6 and rsi,rsi
f: 90 nop
10: 80 fb 31 cmp bl,0x31
13: 0f 94 c0 sete al
16: 5b pop rbx
17: 90 nop
18: 48 89 ff mov rdi,rdi
1b: c3 ret
1c: 48 21 f6 and rsi,rsi
1f: 5b pop rbx
----------------------
0: 48 01 c0 add rax,rax
3: 41 54 push r12
5: 90 nop
6: b0 00 mov al,0x0
8: 41 88 c4 mov r12b,al
b: 41 80 fc 30 cmp r12b,0x30
f: 0f 94 c0 sete al
12: 41 0f 94 c4 sete r12b
16: 90 nop
17: 41 5c pop r12
19: c3 ret
1a: 48 01 c0 add rax,rax
1d: 0f 94 c2 sete dl
----------------------
0: f7 d0 not eax
2: 48 89 c0 mov rax,rax
5: 51 push rcx
6: 90 nop
7: 48 ff c0 inc rax
a: 90 nop
b: b1 00 mov cl,0x0
d: 48 ff c8 dec rax
10: b0 5f mov al,0x5f
12: 38 c1 cmp cl,al
14: 0f 94 c1 sete cl
17: 88 c8 mov al,cl
19: 59 pop rcx
1a: 48 21 d2 and rdx,rdx
1d: c3 ret
1e: cc int3
1f: f4 hlt
----------------------
0: 09 c0 or eax,eax
2: 41 51 push r9
4: 41 b1 00 mov r9b,0x0
7: 4d 89 d2 mov r10,r10
a: 4d 21 c0 and r8,r8
d: 41 80 f9 5a cmp r9b,0x5a
11: 90 nop
12: 0f 94 c0 sete al
15: 41 59 pop r9
17: 45 38 c9 cmp r9b,r9b
1a: 90 nop
1b: c3 ret
1c: cc int3
1d: 48 ff cf dec rdi
Result:
# ./ng_child
New Generation Child Verifier
=============================
Are you a NG child or not!?
We are checking every child!
No one will pass by, no one!!!
(If you're not, come back next year ;-) )
hgaepnpeyrh4atc1k0inn_gZ
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Got it!
Good!
happyhacking <- odds
gener4t10n_Z <- pairs
Reading the challenge, we know that the flag is:
timctf{gener4t10n_Z}
Happy Hacking!!