Base64 换表加密

[Nepuctf_2021] Real_base

这题涉及的是 base64 更换了码表，这种考法也挺多的，比如凯撒加密等中也会涉及到，更换码表

题目

# py2
import string
import random
from secret import flag, b_char


def encode(s):
    res = ''
    binstr = [bin(ord(s[i])).replace('0b', '').zfill(8) for i in range(len(s))]
    p1 = len(binstr) // 3
    p2 = len(binstr) % 3
    part1 = binstr[0:3 * p1]

    for i in range(p1):
        str_p1 = binstr[i * 3] + binstr[i * 3 + 1] + binstr[i * 3 + 2]
        tmp_str = [str_p1[x: x + 6] for x in [0, 6, 12, 18]]
        tmp_res = [b_char[int(x, 2)] for x in tmp_str]
        res += ''.join(tmp_res)

    if p2:
        part2 = binstr[3 * p1:]
        str_p2 = ''.join(part2) + (3 - p2) * '0' * 8
        tmp_str = [str_p2[x: x + 6] for x in [0, 6, 12, 18]][:p2 + 1]
        tmp_res = [b_char[int(x, 2)] for x in tmp_str]
        res += ''.join(tmp_res)
        res += '=' * (3 - p2)
    return res


m1 = random.sample(list(b_char), 50)
print ''.join(m1)
print encode(m1)
print encode(flag)

# rTcb1BR8YVW2EOUjweXpIiLt5QCNg7ZAsD9muq3ylMhvofnx/P
# 2Br9y9fcu97zvB2OruZv0D3Bwhbj0uNQnvfdtC2TwAfPrdBJ3xeP4wNn0hzLzCVUlRa=
# tCvM4R3TzvZ7nhjBxSiNyxmP28e7qCjVxQn91SRM3gBKzxQ=

根据最后三行可知，题目给了明文 m1 和加密后对应的密文，进行了 base64加密，但更换了码表

通过与标准的 base64 加密的对比，判断出此加密的码表

先找出使用的码表

import string
import base64

m1 = b'rTcb1BR8YVW2EOUjweXpIiLt5QCNg7ZAsD9muq3ylMhvofnx/P'
# 标准base64加密
a = base64.b64encode(m1)
# print(a)
a = b'clRjYjFCUjhZVlcyRU9VandlWHBJaUx0NVFDTmc3WkFzRDltdXEzeWxNaHZvZm54L1A='

b = b'2Br9y9fcu97zvB2OruZv0D3Bwhbj0uNQnvfdtC2TwAfPrdBJ3xeP4wNn0hzLzCVUlRa='
nvs = zip(a, b)
# print(nvs)
b64_char = string.ascii_uppercase + string.ascii_lowercase + string.digits + '+/'
new_char = ['*'] * 64

for i in nvs:
    s = i[0]
    ind = b64_char.find(chr(s))
    new_char[ind] = chr(i[1])
print(''.join(new_char))
# abcdef*h*j*l*n***r*tuvwxyz0*234**7*9ABCD*****J*L*NOPQR*TUV***Z*=

得到使用的加密码表应该为：

abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ+/

更换base64码表进行解密

import base64
import string

#自定义的码表，包含 64 个字符
custom_table = b'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ+/'
# 标准表
standard_table = b'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

ENCODE = bytes.maketrans(standard_table,custom_table)
DECODE = bytes.maketrans(custom_table,standard_table)

def encode(input):
    return base64.b64encode(input).translate(ENCODE)

def decode(input):
    return base64.b64decode(input.translate(DECODE))

enc = "tCvM4R3TzvZ7nhjBxSiNyxmP28e7qCjVxQn91SRM3gBKzxQ="
print(decode(enc))