CTFshow 月饼杯crypto1_中秋月wp
下载附件得到内容
fsskryenvkm~jl{ejs}jwflzsnpgmifq{{j{|suhzrjppnx|qvixt~whu
自动钥匙⊕
明文全大写，得到后转小写，并以_连接单词。
题目hint：
某古典密码
经此古典密码加密后，密文还是大写
该古典密码的密钥形式：keyword+plaintext (+plaintext...+plaintext)
一开始我就找到了是Autokey密码，但是解不出来
看来了大佬的wp后发现这里有异或⊕
然后就自己尝试异或一下，脚本：
s='fsskryenvkm~jl{ejs}jwflzsnpgmifq{{j{|suhzrjppnx|qvixt~whu'
for i in range(255):
res=''
for j in range(0,len(s)):
temp = ord(s[j])^i
if 65<=temp<=90 or 97<=temp<=122: #由大小写字母构成
res += (chr(temp))
if len(res)==len(s):
print(res)
得到
ylltmfzqitrausdzulbuhyselqoxrvynddudcljwemuooqgcnivgkahwj
YLLTMFZQITRAUSDZULBUHYSELQOXRVYNDDUDCLJWEMUOOQGCNIVGKAHWJ
然后再次尝试使用break_autokey.py：
from ngram_score import ngram_score
from pycipher import Autokey
import re
from itertools import permutations
qgram = ngram_score('quadgrams.txt')
trigram = ngram_score('trigrams.txt')
ctext = 'YLLTMFZQITRAUSDZULBUHYSELQOXRVYNDDUDCLJWEMUOOQGCNIVGKAHWJ'
ctext = re.sub(r'[^A-Z]','',ctext.upper())
keep a list of the N best things we have seen, discard anything else
class nbest(object):
def __init__(self,N=1000):
self.store = []
self.N = N
def add(self,item):
self.store.append(item)
self.store.sort(reverse=True)
self.store = self.store[:self.N]
def __getitem__(self,k):
return self.store[k]
def __len__(self):
return len(self.store)
#init
N=100
for KLEN in range(8,20):
rec = nbest(N)
for i in permutations('ABCDEFGHIJKLMNOPQRSTUVWXYZ',3):
key = ''.join(i) + 'A'*(KLEN-len(i))
pt = Autokey(key).decipher(ctext)
score = 0
for j in range(0,len(ctext),KLEN):
score += trigram.score(pt[j:j+3])
rec.add((score,''.join(i),pt[:30]))
next_rec = nbest(N)
for i in range(0,KLEN-3):
for k in range(N):
for c in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
key = rec[k][1] + c
fullkey = key + 'A'*(KLEN-len(key))
pt = Autokey(fullkey).decipher(ctext)
score = 0
for j in range(0,len(ctext),KLEN):
score += qgram.score(pt[j:j+len(key)])
next_rec.add((score,key,pt[:30]))
rec = next_rec
next_rec = nbest(N)
bestkey = rec[0][1]
pt = Autokey(bestkey).decipher(ctext)
bestscore = qgram.score(pt)
for i in range(N):
pt = Autokey(rec[i][1]).decipher(ctext)
score = qgram.score(pt)
if score > bestscore:
bestkey = rec[i][1]
bestscore = score
print(bestscore,'autokey,klen',KLEN,':"'+bestkey+'",',Autokey(bestkey).decipher(ctext).lower())
得到
-306.4154316738783 autokey, klen 8 :"KYIFZYWC", ondonhdougomhlalafnianstllbprigusstolddcmubadndaboughnewi
-290.9667615558167 autokey, klen 9 :"NHSZZCZEI", letundamainhafazilthaynemidekvaazrmaybowendcosforeidimprv
-252.5124922641369 autokey, klen 10 :"KEYFORFLAG", ohnoyoufindtheflagtheflagforyouisdoyoulikeclassicalcipher
-272.6830354332479 autokey, klen 11 :"QJTTBDRTGFE", icsalcixconssadosdestlamlnaforgusdispleingawlioncenteallb
-284.1217857757366 autokey, klen 12 :"ELAJWLRDEVRL", ualkquineyapassperthdasplywinefgadcornnoripioneowvistssov
-281.0496306112152 autokey, klen 13 :"UIZCVRFYKENJJ", edmrrousyperloandunapadaufaxeseadouacrewhickonsingecotzuz
-261.79764432480465 autokey, klen 14 :"VXBIMDSUECYSHZ", doklachwertintalkabsaconsiberkoncluboyrodideaderthhitmeog
-267.71934867785404 autokey, klen 15 :"FLDGXNBQVQOAROL", tainpsyanddadesgudofpasrinoundstappoctsoryablyncytheritfl
-261.83884091332243 autokey, klen 16 :"HAXCFFMAOPRNRKLG", rlorhanqueandistdandayformokonguadhacneinageadainfogindow
-241.55027012885887 autokey, klen 17 :"UKXQTYVZFFMHNOQLF", ebodtherdofthenophageflaunasyouapondwheleshowssinththeasy
-246.7536189844891 autokey, klen 18 :"QYIMYUWMIBYTQFRZHI", indholdeasthenmandtheretimofyouardhajeffatmcaliotieddayse
-242.68397600907335 autokey, klen 19 :"GSZFZUBKEAYCYSSNUNJ", stmonlygettywalmayscomerasitycardsidethisidowineliedssesq
当klen为10时，ohnoyoufindtheflagtheflagforyouisdoyoulikeclassicalcipher。
结合提示flag为：flag{do_you_like_classical_cipher}。
