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2020 GKCTF writeup

这周做了一下由防灾科技学院 Ginkgo 战队主办的 GKCTF2020,题目难度不大,但是有些小地方卡住了(再加上周末睡懒觉起晚了

RE

Check in

用字符画画了一个笔记本电脑,提示说自己找开机密码.

没有仔细逆画字符画和移动的部分,直接搜字符串找到个类似base64的字符串,查找下引用发现是base58.

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import base58
print(base58.b58decode(b"2i9Q8AtFJTfL3ahU2XGuemEqZJ2ensozjg1EjPJwCHy4RY1Nyvn1ZE1bZe"))

EzMachine

VM题,一共22个handler,有寄存器及栈.指令不难分析,很容易写出parser

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code = [0x01, 0x03, 0x03, 0x05, 0x00, 0x00, 0x11, 0x00, 0x00, 0x01, 0x01, 0x11, 0x0C, 0x00, 0x01, 0x0D, 0x0A, 0x00, 0x01, 0x03, 0x01, 0x05, 0x00, 0x00, 0xFF, 0x00, 0x00, 0x01, 0x02, 0x00, 0x01, 0x00, 0x11, 0x0C, 0x00, 0x02, 0x0D, 0x2B, 0x00, 0x14, 0x00, 0x02, 0x01, 0x01, 0x61, 0x0C, 0x00, 0x01, 0x10, 0x1A, 0x00, 0x01, 0x01, 0x7A, 0x0C, 0x00, 0x01, 0x0F, 0x1A, 0x00, 0x01, 0x01, 0x47, 0x0A, 0x00, 0x01, 0x01, 0x01, 0x01, 0x06, 0x00, 0x01, 0x0B, 0x24, 0x00, 0x01, 0x01, 0x41, 0x0C, 0x00, 0x01, 0x10, 0x24, 0x00, 0x01, 0x01, 0x5A, 0x0C, 0x00, 0x01, 0x0F, 0x24, 0x00, 0x01, 0x01, 0x4B, 0x0A, 0x00, 0x01, 0x01, 0x01, 0x01, 0x07, 0x00, 0x01, 0x01, 0x01, 0x10, 0x09, 0x00, 0x01, 0x03, 0x01, 0x00, 0x03, 0x00, 0x00, 0x01, 0x01, 0x01, 0x06, 0x02, 0x01, 0x0B, 0x0B, 0x00, 0x02, 0x07, 0x00, 0x02, 0x0D, 0x00, 0x02, 0x00, 0x00, 0x02, 0x05, 0x00, 0x02, 0x01, 0x00, 0x02, 0x0C, 0x00, 0x02, 0x01, 0x00, 0x02, 0x00, 0x00, 0x02, 0x00, 0x00, 0x02, 0x0D, 0x00, 0x02, 0x05, 0x00, 0x02, 0x0F, 0x00, 0x02, 0x00, 0x00, 0x02, 0x09, 0x00, 0x02, 0x05, 0x00, 0x02, 0x0F, 0x00, 0x02, 0x03, 0x00, 0x02, 0x00, 0x00, 0x02, 0x02, 0x00, 0x02, 0x05, 0x00, 0x02, 0x03, 0x00, 0x02, 0x03, 0x00, 0x02, 0x01, 0x00, 0x02, 0x07, 0x00, 0x02, 0x07, 0x00, 0x02, 0x0B, 0x00, 0x02, 0x02, 0x00, 0x02, 0x01, 0x00, 0x02, 0x02, 0x00, 0x02, 0x07, 0x00, 0x02, 0x02, 0x00, 0x02, 0x0C, 0x00, 0x02, 0x02, 0x00, 0x02, 0x02, 0x00, 0x01, 0x02, 0x01, 0x13, 0x01, 0x02, 0x04, 0x00, 0x00, 0x0C, 0x00, 0x01, 0x0E, 0x5B, 0x00, 0x01, 0x01, 0x22, 0x0C, 0x02, 0x01, 0x0D, 0x59, 0x00, 0x01, 0x01, 0x01, 0x06, 0x02, 0x01, 0x0B, 0x4E, 0x00, 0x01, 0x03, 0x00, 0x05, 0x00, 0x00, 0xFF, 0x00, 0x00, 0x01, 0x03, 0x01, 0x05, 0x00, 0x00, 0xFF, 0x00, 0x00, 0x00]
ip = 0
sp = 0
bp = 0
stack = [0 for i in range(256)]
R = [0 for i in range(8)]

while ip < len(code):
op = code[ip]
if op == 0:
ip+=1
elif op == 1:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[oprnd1] = oprnd2
print("%3d: "%(ip-3),"mov R%d, %d"%(oprnd1, oprnd2))
elif op == 2:
oprnd1 = code[ip+1]
sp+=1
ip+=3
stack[sp] = oprnd1
print("%3d: "%(ip-3),"push %d"%oprnd1)
elif op == 3:
oprnd1 = code[ip+1]
sp+=1
ip+=3
stack[sp] = R[oprnd1]
print("%3d: "%(ip-3),"push R%d"%oprnd1)
elif op == 4:
oprnd1 = code[ip+1]

ip+=3
R[oprnd1] = stack[sp]
sp-=1
print("%3d: "%(ip-3),"pop R%d"%oprnd1)
elif op == 5:

ip+=3
print("%3d: "%(ip-3),"puts something")
elif op == 6:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[oprnd1] += R[oprnd2]
print("%3d: "%(ip-3),"add R%d, R%d"%(oprnd1, oprnd2))
elif op == 7:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[oprnd1] -= R[oprnd2]
print("%3d: "%(ip-3),"sum R%d, R%d"%(oprnd1, oprnd2))
elif op == 8:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[oprnd1] *= R[oprnd2]
print("%3d: "%(ip-3),"mul R%d, R%d"%(oprnd1, oprnd2))
elif op == 9:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[0] = R[oprnd1] // R[oprnd2]
R[1] = R[oprnd1] % R[oprnd2]
print("%3d: "%(ip-3),"div R%d, R%d"%(oprnd1, oprnd2))
elif op == 10:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[oprnd1] ^= R[oprnd2]
print("%3d: "%(ip-3),"xor R%d, R%d"%(oprnd1, oprnd2))
elif op == 11:
ip = 3*code[ip+1]-3
print("%3d: "%(ip-3),"jmp %d"%oprnd1)
elif op == 12:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
ip+=3
R[3] = R[oprnd1] - R[oprnd2]
print("%3d: "%(ip-3),"cmp R%d, R%d"%(oprnd1, oprnd2), R[oprnd1], R[oprnd2])
elif op == 13:
oprnd1 = code[ip+1]
print("%3d: "%ip,"jz %d"%(3*oprnd1-3))
if R[3]:
ip += 3
else:
ip = 3*code[ip+1]-3

elif op == 14:

oprnd1 = code[ip+1]
print("%3d: "%ip,"jnz %d"%(3*oprnd1-3))
if R[3]:
ip = 3*code[ip+1]-3
else:
ip += 3
elif op == 15:

oprnd1 = code[ip+1]
print("%3d: "%ip,"jg %d"%(3*oprnd1-3))
if R[3]<=0:
ip += 3
else:
ip = 3*code[ip+1]-3
elif op == 16:

oprnd1 = code[ip+1]
print("%3d: "%ip,"jb %d"%(3*oprnd1-3))
if R[3]>=0:
ip += 3
else:
ip = 3*code[ip+1]-3
elif op == 17:
# input
buf = [0 for i in range(17)]
buf = list(b"?lag{1234567890a}")
R[0] = len(buf)
ip+=3
print("%3d: "%(ip-3),"input")
elif op == 18:
length = code[ip+2] - code[ip+1]
buf[code[ip+1]:code[ip+2]] = [0 for i in range(length)]
ip=3
print("%3d: "%(ip-3),"memset")
elif op == 19:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
R[oprnd1] = stack[bp+R[oprnd2]]
ip+=3
print("%3d: "%(ip-3),"mov R%d, [bp+R%d]"%(oprnd1, oprnd2))
elif op == 20:
oprnd1 = code[ip+1]
oprnd2 = code[ip+2]
R[oprnd1] = buf[R[oprnd2]]
ip+=3
print("%3d: "%(ip-3),"mov R%d, buf[R%d]"%(oprnd1, oprnd2))
elif op == 0xff:
print("%3d: "%(ip-3),"death")
exit(0)

算法也不难,根据范围做简单的异或/加减运算,然后转hex.

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res = [7, 13, 0, 5, 1, 12, 1, 0, 0, 13, 5, 15, 0, 9, 5, 15, 3, 0, 2, 5, 3, 3, 1, 7, 7, 11, 2, 1, 2, 7, 2, 12, 2, 2][::-1]
cipher = []
for i in range(17):
cipher.append(res[2*i+1]*16+res[2*i])
plain = [[] for i in range(17)]
print(cipher)
for i in range(len(cipher)):
tmp = cipher[i]
# print((i-1)^71)
if 97<=((tmp-1)^71)<=122:
plain[i].append((tmp-1)^71)
print(i)
if 65<=((tmp+1)^75)<=90:
plain[i].append((tmp+1)^75)
print(i)
if tmp<65:
plain[i].append(tmp)
print(i)
if 90<tmp<97:
plain[i].append(tmp)
print(i)
if 122<tmp<127:
plain[i].append(tmp)
print(i)
for i in plain:
print(bytes(i))
print((plain))

BabyDriver

驱动逆向.Windows搞得不多,由于这题算法比较简单因此纯静态就能做了,不然还得想办法搭双机调试环境.

我也不太熟悉keyboard hook,做的时候也是瞎鸡儿做的.(以下内容分析的不一定正确,有空再研究…)总之就找到了一个关于按键的回调函数,很容易分析出是走迷宫.根据键盘扫描码应该是ikjl对应上下左右,(发现有个地方加6了,一开始以为是要加6对应wsad,因为看了一篇文章是hook KeyboardInterruptService的)交了下发现不对,也没找到其他的什么地方,又试了一下大写就对了.

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from hashlib import md5
print("flag{"+md5(b"LKKKLLKLKKKLLLKKKLLLLLL").hexdigest()+"}")

Chellys_identity

搜了下找到罪恶王冠,由于没看过就没继续看了

调试一下很容易分析出算法.

求小于n的斐波那契数质数(我也不知道我做题的时候怎么想到是斐波那契数的,可能是看到2 3 5了吧2333…,总之是dump下来的)之和再异或

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res = [0x000001B6, 0x00000498, 0x00000441, 0x00000179, 0x00000179, 0x00000640, 0x0000039C, 0x00000179, 0x0000064A, 0x0000039C, 0x0000027D, 0x0000027F, 0x00000178, 0x00000236, 0x00000344, 0x0000033E]
fib = [0x00000002, 0x00000003, 0x00000005, 0x00000007, 0x0000000B, 0x0000000D, 0x00000011, 0x00000013, 0x00000017, 0x0000001D, 0x0000001F, 0x00000025, 0x00000029, 0x0000002B, 0x0000002F, 0x00000035, 0x0000003B, 0x0000003D, 0x00000043, 0x00000047, 0x00000049, 0x0000004F, 0x00000053, 0x00000059, 0x00000061, 0x00000065, 0x00000067, 0x0000006B, 0x0000006D, 0x00000071, 0x0000007F]
dic = {}
def get_map(a):
s = 0
for i in fib:
if i<a:
s+=i
return s^a
# print(hex(get_map(ord('f'))))
for i in range(127):
dic[get_map(i)] = chr(i)
print(dic)
s = ""
for i in res:
s+=dic[i]
print(s)

WannaReverse

此题赛后解出

看这个名字应该是类似永恒之蓝的玩意,就没直接run了.

直接分析WannaReverse.exe静态编译的,但是熟悉函数结构的话不难分析出一些常见函数.main开头用srand(time(0))设定种子,然后用rand()%10生成32字节伪随机密钥.用exe中硬编码的公钥加密32字节密钥,然后用伪随机的32字节密钥AES加密flag.最后把WannaReverse字符串,RSA加密的AES密钥,密文写入文件.

一开始尝试分解N,无果

然后直接硬爆密钥了.想用flag筛选,筛选不出.

测试一下题目加密的结果,发现是用\x00padding的.然后用\x00 * n做结尾筛选,三个的时候筛选出了flag.一看原来是Unicode编码的,所以用flag字符串筛选不到

这里说一下,Win7的记事本在保存文件的时候可以指定Unicode编码,开头的\xFF\xFE表示是用Unicode,同时也可以指定UTF-8或Unicode大端序或ANSI.

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from Crypto.Cipher import AES
from base64 import *

def gen_key(seed):
k = b""
for i in range(32):
seed = seed * 214013 + 2531011
k+=bytes([0x30+(((seed>>16) & 0x7fff)%10)])
return k
# t = 1590310000
t = 1589530000

while True:
key = gen_key(t)
# print(key)
aes = AES.new(key, mode=AES.MODE_ECB)
cipher = b"\\\xbc\xea\x89\xba+\x18\'y?\x13\n\x8a\x97\xb4\x9b\xcdx\x9b\xd85\x92\x05EL\"\xa5i7\xebn+\x0e\xbd\x84\x0f\x91a8\xf6\xf1\xba\x99\x19Ar\x07\x91\xf0&h\x06a&\\ 5\xdd\xcf\xfcwWT\x81\xf2\xf2\xe4\xaf\xbf\xa2\x1d)\xael\x08;v\x1bf\xb8\xfer\xcb\xd6\x94\xc3\xd5j\xe7\x0cz(\xdc\xbc\xac\x80"
if aes.decrypt(cipher).endswith(b"\x00\x00\x00"):
print(t)
print(aes.decrypt(cipher))
print(aes.decrypt(cipher).decode("utf-16"))
exit()
t-=1
if t%10000 == 0:
print(t)

DbgIsFun

此题赛后解出

main函数利用SEH捕获int 3断点,改变控制流.

输入长度不为28会进入假逻辑(先减去28,然后检验EFLG为0x246,即是否相等)

真逻辑只是设置一个地方为1,然后sleep5秒,就没了

其实这里Sleep应该反应过来是有多线程在干什么事情,应该搜一下CreateThread之类的

然后这里就卡住了.比赛后重新看一遍,找到TLS回调函数,里面有SMC,这是正常解法(不知道为什么x86dbg没有在tls回调函数断下来,还是Windows做的少.一般情况下x86dbg在TLS断下来后我才会去看,很少主动找TLS)

解完SMC,这里是正常加密逻辑.先对sub_401540求和(防止patch与断点),然后RC4

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from Crypto.Cipher import ARC4
res = bytes([0x2D, 0xD4, 0x0F, 0xD0, 0x54, 0xEE, 0x75, 0xD0, 0xE0, 0x30, 0x96, 0xE1, 0x79, 0x8A, 0xE0, 0xFE, 0x18, 0x3A, 0x27, 0xE7, 0x2F, 0x86, 0xC9, 0xFE, 0x66, 0x43, 0xA7, 0x75])
key = b"GKCTF"
rc4 = ARC4.new(key)
print(k)
for k in range(256):
rc4 = ARC4.new(key)
print(bytes(list(map(lambda x: x^k, rc4.decrypt(res)))), k)
# flag{5tay4wayFr0m8reakp0int}