BUUCTF Crypto [WUSTCTF2020]B@se wp

这道题主要是考一个base64，但是是变换了编码表的base64，首先，我们看到变化后的编码表缺少了4个字符，肯定不可能去数是什么的，先写个脚本康康缺少了啥吧

// An highlighted block
import string
s = "JASGBWcQPRXEFLbCDIlmnHUVKTYZdMovwipatNOefghq56rs****kxyz012789+/"
for i in string.ascii_letters + string.digits:
if(i not in s):
print i
#ju34

然后我们就可以上网找个脚本，再爆破这四位的组合就可以得到flag

from Crypto.Util.number import *
from gmpy2 import *
from functools import reduce
import sympy
import itertools
#JASGBWcQPRXEFLbCDIlmnHUVKTYZdMovwipatNOefghq56rs****kxyz012789+/
# coding:utf-8
def My_base64_encode(inputs,s):
bin_str = []
for i in inputs:
x = str(bin(ord(i))).replace('0b', '')
bin_str.append('{:0>8}'.format(x))
#print(bin_str)
outputs = ""
nums = 0
while bin_str:
temp_list = bin_str[:3]
if(len(temp_list) != 3):
nums = 3 - len(temp_list)
while len(temp_list) < 3:
temp_list += ['0' * 8]
temp_str = "".join(temp_list)
#print(temp_str)
temp_str_list = []
for i in range(0,4):
temp_str_list.append(int(temp_str[i*6:(i+1)*6],2))
#print(temp_str_list)
if nums:
temp_str_list = temp_str_list[0:4 - nums]

for i in temp_str_list:
outputs += s[i]
bin_str = bin_str[3:]
outputs += nums * '='
print("Encrypted String:\n%s "%outputs)

def My_base64_decode(inputs,s):
bin_str = []
for i in inputs:
if i != '=':
x = str(bin(s.index(i))).replace('0b', '')
bin_str.append('{:0>6}'.format(x))
#print(bin_str)
outputs = ""
nums = inputs.count('=')
while bin_str:
temp_list = bin_str[:4]
temp_str = "".join(temp_list)
#print(temp_str)
if(len(temp_str) % 8 != 0):
temp_str = temp_str[0:-1 * nums * 2]
for i in range(0,int(len(temp_str) / 8)):
outputs += chr(int(temp_str[i*8:(i+1)*8],2))
bin_str = bin_str[4:]
print("Decrypted String:\n%s "%outputs)
#s = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
h=['j','u','3','4']
h1=list(itertools.permutations(h,4))
for i in h1:
m="".join(i)
s = "JASGBWcQPRXEFLbCDIlmnHUVKTYZdMovwipatNOefghq56rs"+m+"kxyz012789+/"
input_str="MyLkTaP3FaA7KOWjTmKkVjWjVzKjdeNvTnAjoH9iZOIvTeHbvD=="
#My_base64_encode(input_str)
My_base64_decode(input_str,s)
#wctf2020{base64_1s_v3ry_e@sy_and_fuN}

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