2024年中国工业互联网安全大赛智能家电赛道选拔赛｜线上初赛Writeup

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key  = b'flag\x00{\x00this\x00isk\x00e\x00\x00\x00y\x00\x00\x00}f\x00\x00l\x00\x00\x00a\x00\x00g{xthisiskey}'
flag = b'kwam{dGawkqBxuiBlIHNjY3JniCBeZXNzYWjl}'

tmp = []
for x,y in zip(key, flag)
:
	if y not in range(97, 123):
		print(chr(y), end = '')
		tmp.append(y)
		continue
	print(chr(((y-x)%26) + 97), end = '')
	tmp.append(((y-x)%26) + 97)

tmp = bytes(tmp)
print()

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import base64

def check_key(input_str, l = 1):
    b_arr = [44, 66, 67, 77, 46, 43, 47, 71, 49, 68, 80, 50, 68, 83, 51, 49, 50, 68, 58, 56, 61, 53, 70, 57, 58, 79, 59, 78, 66, 67, 89, 68]
    # print(bytes(b_arr))
    b_arr2 = bytearray(32)
    
    for i in range(len(input_str)):
        b_arr2[i] = (input_str[i] + b_arr[i]) % 101
    
    for i in range(len(input_str)):
        b_arr2[i] = ((b_arr2[i] * 2) - i) % 256
    
    #print(bytes(b_arr2), bytes(b_arr2)[:l] == input_str[:l])
    return bytes(b_arr2)[:l] == input_str[:l]

tmp = []
for i in range(32):
	for x in range(32, 128):
		t = tmp + [x]
		#print(bytes(t))
		if check_key(bytes(t), i + 1):
			print(i, 'solved')
			tmp.append(x)
			break
	print('tmp->', tmp, bytes(tmp))

print(check_key(bytes(tmp), 32))

print(bytes(tmp))

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from Crypto.Cipher import ARC4

enc = b'\x1B\xB0\xAC\xAD\x4F\x23\x54\x2B\x59\x93\x78\x8F\x39\xF9\x78\xE5\x26\x13\xBD\x8C\xDE\x9D\x9A\xB7\x43\x99\x7E\x11\xEB\x46\xED\xE0'
key = b'rGF3ryrCpK4qN1rwvShmduTonEnIba6a'
cipher = ARC4.new(key)
m=cipher.decrypt(enc)

from hashlib import md5

print(md5(m).hexdigest())

1

var/www/html";echo ZmxhZ3szOTA4NEVFRjJEMjhFOTQxRjUzRTRBMUFBMUZBNjc2Nn0K|base64 -d > ./flag.txt;

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VxdTY2Ng==
cHkl67tL
c2NjZHBpZH
YfGyj56s

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===============================================================
======= Welcome to YAFU (Yet Another Factoring Utility) =======
=======             bbuhrow@gmail.com
===
   factor(71484438965393396388835335667806052411397994375702758854090697767967524655627)

fac: factoring 71484438965393396388835335667806052411397994375702758854090697767967524655627
fac: using pretesting plan: normal
fac: no tune info: using qs/gnfs crossover of 100 digits
fac: no tune info: using qs/snfs crossover of 75 digits
div: primes less than 10000
fmt: 1000000 iterations
rho: x^2 + 3, starting 1000 iterations on C77
rho: x^2 + 2, starting 1000 iterations on C77
rho: x^2 + 1, starting 1000 iterations on C77
nfs: searching for brent special forms...
nfs: searching for homogeneous cunningham special forms...
nfs: searching for XYYXF special forms...
nfs: searching for direct special forms...
nfs: snfs form detection took 0.188331 seconds
nfs: couldn't find special form
pm1: starting B1 = 150K, B2 = gmp-ecm default on C77
ecm: ECM executable does not exist at ../trosi_ecm_git/ecm
ecm: using internal single threaded ECM...
ecm: 30/30 curves on C77, B1=2k, B2=gmp-ecm default
ecm: ECM executable does not exist at ../trosi_ecm_git/ecm
ecm: using internal single threaded ECM...
ecm: 74/74 curves on C77, B1=11k, B2=gmp-ecm default
ecm: ECM executable does not exist at ../trosi_ecm_git/ecm
ecm: using internal single threaded ECM...
ecm: 149/149 curves on C77, B1=50k, B2=gmp-ecm default, ETA: 0 sec

starting SIQS on c77: 71484438965393396388835335667806052411397994375702758854090697767967524655627

==== sieving in progress (1 thread):   36240 relations needed ====
====           Press ctrl-c to abort and save state           ====
36449 rels found: 16437 full + 20012 from 233943 partial, (2406.79 rels/sec)

building matrix with 36449 columns
SIQS elapsed time = 109.2670 seconds.
Total factoring time = 135.2192 seconds


***factors found***

P41 = 79823191688166851259736970548355545692829
P36 = 895534711824738922785094048763390663

ans = 1

>>

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from Crypto.Util.number import *

n = 71484438965393396388835335667806052411397994375702758854090697767967524655627
e = 65537
c = 0x515b50d7407f4f321ddea14d0d99e4134c285ee6b7b92b77f3ed65f32212a529

# yafu
p = 79823191688166851259736970548355545692829
q = 895534711824738922785094048763390663

assert n == p * q

phi = (p - 1) * (q - 1)
d = inverse(e, phi)

m = pow(c, d, n)
m = long_to_bytes(m)


def subtract_one_from_ascii(data):
    result = bytearray()
    for byte in data:
        result.append(byte - 1)
    return bytes(result)

def xor_bytes(data, key):
    result = bytearray()
    for byte in data:
        result.append(byte ^ key)
    return bytes(result)

def decode_m(m):
    # step 1: 对 m XOR 12
    m_xor = xor_bytes(m, 12)
    # step 2: 对 res sub 1
    original_x = subtract_one_from_ascii(m_xor)
    return original_x

x = decode_m(m)
print("x:", x)

# Output
# b'\r_&S\x87\xef\x96g\xec\x08\x0bflag{954fbe80adb799}'