🎃

2023/01/23に公開

# 結果

チーム参加。125位/848チーム

## 解けた問題

### Cryptography

• Xorathrust[345 solves]
• Factorie[505 solves]
• I Love Pi[221 solves]
• Jamal[142 solves]
• Encode Mania[186 solves]
• Toddler RSA[106 solves]

### Basics

• KnightCTF Discord Server[665 solves]

# Xorathrust [345 solves]

Read the script and solve the problem.

Demo Flag: KCTF{Fl4g_H3r3}

encrypt.py
def main():

flag_enc = ""

with open("flag.txt", "r") as infile:
flag = list(flag)

for each in flag:
each = chr(ord(each) ^ 0x66)
flag_enc += each

with open("flag.enc", "w") as outfile:
outfile.write(flag_enc)

if __name__ == "__main__":
main()

-%2 S9R9RS9V


pt = ""

with open("flag.enc.txt", "r") as f:
enc = list(enc)

for each in enc:
each = chr(ord(each) ^ 0x66)
pt += each

print(pt)


KCTF{ju5t_4_b45ic_x0r}

# Factorie [505 solves]

Have you ever heard of prime factors? The file challenge.txt contains a number that has two prime factors. Can you find them?

Demo Flag: KCTF{small-number_big-number}

n: 2174096211032823084932239036566496093206280423


SageMath に投げる

KCTF{39434538531451803895327_55131777675015246472249}

# I Love Pi [221 solves]

Isaac Newton left me this piece of code and a message. Can you help me decode this...

Demo Flag : KCTF{FL4G_H3R3}

i_love.py
import base64

lengths = [--REDACTED--]
flag = "KCTF{*******************************}"

# len(lengths) = 10
# len(flag) = 39

s = 0
encoded_flag = ""
for l in lengths:
seg = flag[s:s+l]
for _ in range(len(seg)):
seg = base64.b64encode(seg.encode('ascii')).decode('ascii')
s+=l
encoded_flag += seg

print(encoded_flag)

encoded_flag.txt
VXpCT1ZRPT0=Rg==V2xod1UxcHNWa0pRVkRBOQ==MQ==VmpCb2QxVXhjSE5UYTFaV1ZrUkJPUT09Vm0wd2QyVkhVWGhUV0doaFUwVndVRlp0TVZOV01XeFZVbTVrVlUxV2NIbFdNalZyVmpKS1NHVkliRmRpVkVaSVZtMTRTMk15VGtWUmJIQk9VakF4TkZkWGRHRmtNRFZ5VFZWV2FHVnFRVGs9U0RNPQ==Vm1wQ1UxRXlTbkpOVldSVFYwZFNjVlJVU1RSUFVUMDk=VjFSS2QxWXhjSEpPVldSYVpXcEJPUT09VGtac09RPT0=


length = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3]

ということがわかる。

VXpCT1ZRPT0=, 3, "KCT"
Rg==, 1, "F"
V2xod1UxcHNWa0pRVkRBOQ==, 4, "{4_P"
MQ==, 1, "1"
VmpCb2QxVXhjSE5UYTFaV1ZrUkJPUT09, 5, "_4_D4"
Vm0wd2QyVkhVWGhUV0doaFUwVndVRlp0TVZOV01XeFZVbTVrVlUxV2NIbFdNalZyVmpKS1NHVkliRmRpVkVaSVZtMTRTMk15VGtWUmJIQk9VakF4TkZkWGRHRmtNRFZ5VFZWV2FHVnFRVGs9, 9, "Y_K33P5_7"
U0RNPQ==, 2, "H3"
Vm1wQ1UxRXlTbkpOVldSVFYwZFNjVlJVU1RSUFVUMDk=, 6, "_H4CK3"
VjFSS2QxWXhjSEpPVldSYVpXcEJPUT09, 5, "r5_4W"
VGtac09RPT0=, 3, "4Y}"


KCTF{4_P1_4_D4Y_K33P5_7H3_H4CK3r5_4W4Y}

# Jamal [142 solves]

My friend Jamal has encrypted the flag and sent me the files. He now wants me to decrypt it and find out. Can you do it for me, please?

Demo Flag: KCTF{f4k3_fl4g}

jamal-encrypt.py
from random import randint, seed

def is_prime(n):
if n == 2 or n == 3:
return True
if n % 2 == 0 or n < 2:
return False
for i in range(3, int(n**0.5)+1, 2):
if n % i == 0:
return False
return True

def get_prime(n):
while True:
p = randint(2**(n-1), 2**n)
if is_prime(p):
return p

def encrypt(ch, g, p, y, k):
pt_int = int.from_bytes(ch.encode(), "big")
c1 = pow(g, k, p)
c2 = (pt_int * pow(y, k, p)) % p

return c1, c2

def main():

x = randint(2, 9999999)
p = get_prime(43)
g = get_prime(43)

while g == p:
g = get_prime(43)

if g > p:
g, p = p, g

y = pow(g, x, p)

with open("flag.enc", "w") as out:
for ch in list(flag):
#   encrypt
k = randint(2, 9999)
c1, c2 = encrypt(ch, g, p, y, k)
out.write(f"{c1},{c2}\n")

with open("key.pub", "w") as out:
out.write(f"p: {p}\n")

if __name__ == "__main__":
seed(0)
main()

jamal-flag.enc.txt
2542674357061,3229421908786
1047095110514,2703124715302
2241080680361,339282920484
2877082899121,213437588357
2962454333590,1146622182354
8667422468,2914668005810
3283797690288,5668747940970
92336276716,2578935770315
280618016374,4820189175032
1209866491592,146344916624
2253881281576,2448560213475
1161992879414,1674301627359
4818900557459,4874289912091
1794902833332,3885572444087
3082175285512,235300300457
1157352410430,3391910550439
1975682514838,4412581570619
940190944451,4668208507929
4892987552563,3969703431560
861276404486,4681646838677
5308919438905,740205209430
4882067017820,5684832423565
832490479211,3882413987731
2768698738377,2559643212287
4799150729489,2715004149410
2400910224808,2411154824523
871309115691,312836731476
32734373338,5274112237786
2427518930772,3618374377486
3075900055786,2047586052701
5395051057487,427396164824
663270611761,2916443609523

p: 5690411473163


シードが固定されているので、平文の各文字を brute-force すればよい。

seed(0)

x = randint(2, 9999999)

p = get_prime(43)
g = get_prime(43)

if g > p:
g, p = p, g

y = pow(g, x, p)

flg = []

for i in range (32):
k = randint(2, 9999)
c1 = cs[i][0]
c2 = cs[i][1]
for num in range (128):
g1, g2 = encrypt(chr(num), g, p, y, k)
if c1 == g1 and c2 == g2:
flg.append(chr(num))

print(''.join(c for c in flg))


KCTF{h4v3_y0u_h34rd_0f_3l64m4l?}

# Encode Mania [186 solves]

Encoding random stuffs is so cool! I just want to encode it over and over and over again ...

Demo Flag: KCTF{fl4g_h3r3}

encode_mania.py
import base64
from random import randint

flag = "kctf{************}"

def encrypt(s, option):
if option == 0:
ret = base64.b64encode(s)
elif option == 1:
ret = base64.b32encode(s)
elif option == 2:
ret = base64.b16encode(s)
else:
ret = base64.b85encode(s)

return ret

encrypted_flag = flag.encode('utf-8')

for _ in range(12):
option = randint(0, 3)
encrypted_flag = encrypt(encrypted_flag, option)

with open("encode_mania.txt", 'w') as f:
f.write(encrypted_flag.decode())


encode_mania.txt
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==


その方式でしか使えない文字があったら、その方式であると推測し、復号化した

import base64

enc = ""

with open('encode_mania.txt', 'r') as f:

chars16 = "0123456789ABCDEF"
chars32 = "GHIJKLMNOPQRSTUVWXYZ"
chars64 = "abcdefghijklmnopqrstuvwxyz+/"
chars85 = "!#\$%&()*-;<>?@^_{|}~"

while enc[0:4] != "KCTF":
mode = -1

if mode == -1:
for c85 in chars85:
if c85 in enc:
mode = 85
if mode == -1:
for c64 in chars64:
if c64 in enc:
mode = 64
break
if mode == -1:
for c32 in chars32:
if c32 in enc:
mode = 32
break
if mode == -1:
for c16 in chars16:
if c16 in enc:
mode = 16
break

if mode == -1:
print("ERR")
print(enc)
exit(-1)

if mode == 16:
enc = base64.b16decode(enc)
if mode == 32:
enc = base64.b32decode(enc)
if mode == 64:
enc = base64.b64decode(enc)
if mode == 85:
enc = base64.b85decode(enc)

enc = enc.decode()

print(enc)


KCTF{dfs_0r_b4u7e_f04c3}

# Toddler RSA [106 solves]

You may have heard of baby RSA. But did you know that toddler RSA is a thing as well?

Demo Flag : KCTF{fl4g_h3r3}

toddler_rsa.py
from Crypto.Util.number import getPrime, bytes_to_long
import random

flag1 = b"kctf{*****"
flag2 = b"******}"

p, q = getPrime(512), getPrime(512)
n = p * q
e = 0x10001
phi = (p - 1) * (q - 1)
m = bytes_to_long(flag1)
ct = pow(m, e, n)

primes = [p, q]

for _ in range(98):
primes.append(getPrime(512))

random.shuffle(primes)

primes2 = []

while True:
if len(primes2) == 16:
break
p = getPrime(128)
if p not in primes2:
primes2.append(p)

while True:
n2 = 1
for p in primes2:
r = random.randint(0, 1)
if r:
n2 *= p
if n2.bit_length() > 1024:
break

m2 = bytes_to_long(flag2)
ct2 = pow(m2, e, n2)

with open("infos.txt", 'w') as f:
f.write('{}\n'.format(ct))
f.write('{}\n'.format(ct2))

for p in primes:
f.write(str(p) + " ")
f.write("\n")

for p in primes2:
f.write(str(p) + " ")


infos.txt
66583536270887021807490397743997271455125755581160018003919916008658356150175910713440835182971996187562565226675351042460434353931585831584272555696179274126321107345396847166333122985792520485127336843400145192855625694720207592205790961842916053059876605678603849267317164063089140203608170094060063975862
2279780221943295570270982236035582893977993979407790219223966836692545701998684597925266035868375779180218333832926459880229000054158919717743575043736900857245622481911503420167128462730860113002412674566899699371968214333231947746623067855028026843801255651389667577056503904971101594424541057381520257614453843471818576395516786682190919979161
13222824171934158795160017601111983741654223888476625973950275883862159298970076592660290659087375028487839263506230550950954738263237932702764355101517599 8978218440360436772235286311777411873413406027349849093024924257750929134670580864086577595401547891869173780064243105867543502813494481721960549178734933 8798068787464414221197551296572936834120506461129636585457070924885719707767419223056437741312655425088750417646336045043935576206958190908227376649816311 8030010099167454614960222504466241738018348712555301693805378086778095226716475768328494310610463069062410668569126285398897237915737813051218561486949661 9055251760177547872940166101580078387797598200872674009939438706220395803028074529456868014854874943327211163891726007597467757106686723097692393955690209 6859436135120487087497059388407753889091359939418063353544340593288960333716530348130557020877661334085430098869425207812303618587906929317989878364445927 10131543868910545016763564342668644797467734878362450948600041668707417091596010549904617745787056134914271620187430566778590406046294200547007645850481303 13396508311817357141752215255099252623364698033324141891630501088864849832595658646432103953284005969465311565311391979907391999640050074419458123219155603 10181940734649931795215490119971369608085374553973301280505166110215912741573683069591890941461759051435534340056831272416988448284912031024385416313627321 10769230225612119942280183290912162659882855090406190026504946362498094977666994810977191766033026702436373553913732137087675964305415476940219266331973103 10908631376982577198348577535384506308458873896093144493231727775302012357896230599591384219644682356992105824827628783353708435819356317918959386749340197 7102379140576069248026402682339395072753597161688628753274478933837665833536300729759624302466228624341656091638243469719724214419688712644348878754757037 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from Crypto.Util.number import *

ct = 66583536270887021807490397743997271455125755581160018003919916008658356150175910713440835182971996187562565226675351042460434353931585831584272555696179274126321107345396847166333122985792520485127336843400145192855625694720207592205790961842916053059876605678603849267317164063089140203608170094060063975862

e = 65537

primes = []
with open("primes.txt", "r") as f:

for i in range(len(primes)):
for j in range(len(primes)):
if i >= j:
continue

p = primes[i]
q = primes[j]

n = p * q

phi = (p - 1) * (q - 1)

d = pow(e, -1, phi)

m = pow(ct, d, n)

m = long_to_bytes(m)

if m.count(b'KCTF'):
print(m)

from Crypto.Util.number import *

ct = 2279780221943295570270982236035582893977993979407790219223966836692545701998684597925266035868375779180218333832926459880229000054158919717743575043736900857245622481911503420167128462730860113002412674566899699371968214333231947746623067855028026843801255651389667577056503904971101594424541057381520257614453843471818576395516786682190919979161

e = 65537

primes = []
with open("primes2.txt", "r") as f:

for bit in range(2 ** 16):
if bit == 0:
continue

n = 1
phi = 1

for i in range(16):
if (bit & (1 << i)) == 0:
continue
n *= primes[i]
phi *= (primes[i] - 1)

if n.bit_length() > 1024:
continue

d = pow(e, -1, phi)

m = pow(ct, d, n)

m = long_to_bytes(m)

try:
print(m.decode('UTF-8'))
except:
pass


KCTF{rsa_and_only_rsa_ftw}`