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}