[CTF] Base64题目

base64.txt:

bWFpbigpe2ludCBpLG5bXT17KCgoMSA8PDEpPDwgKDE8PDEpPDwoMTw8Cm==
ICAgICAgIDEpPDwoMTw8KDE+PjEpKSkrKCgxPDwxKTw8KDE8PDEpKSksKCgoMQp=
ICAgICAgIDw8ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAxKQq=
ICAgICAgIDw8ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAoMQp=
ICAgICAgIDw8MSk8PCgxPDwxKTw8ICAgICAgICAgKDE8PDEpKS0oKDE8PDEpPDwoCr==
ICAgIAkxPDwxKTw8KDE8PDEpKSsgICAgICAgICgoMTw8MSk8PCgxPDwoMT4+Ch==
ICAgICAgIDEpKSkrKDE8PCgxPj4xKSkpICAgICAgICAgICAgICAgICAgICAgICAgLAq=
ICAgICAgICgoKDE8PDEpPDwoMTw8MSkgICAgICAgICAgICAgICAgICAgICAgICA8PAo=
ICAgICAgICgxPDwxKTw8KDE8PDEpKS0oKDEgPDwxKTw8KDE8PDEpIDw8KDE8PCgxCl==
ICAgICAgID4+MSkpKS0oKDE8PDEpPDwoMTw8KDE+PjEpKSkpLCgoKDE8PDEpCp==
ICAgICAgIDw8KCAgICAxPDwgICAgICAgICAgICAgICAgICAgICAgICAgIDEpCt==
ICAgICAgIDw8KCAgICAxPDwgICAgICAgICAgICAgICAgICAgICAgICAgIDEpCu==
ICAgICAgIDw8KCAxPDwxKSktKCgxIDw8MSk8PCAoMSA8PDEpPDwoMSA8PAr=
ICAgICAgICgxPj4xKSkpLSgoMTw8MSk8PCgxPDwoMT4+MSkpKSksKCgoMTw8MSk8PAo=
ICAgICAgICgxPDwxKTw8KDE8PDEpPDwoMTw8MSkpLSgoMTw8MSk8PCgxPDwxKTw8KAr=
ICAgICAgIDE8PCgxPj4xKSkpLSgxPDwoMT4+ICAgICAgICAgIDEpKSksKCgoMTw8MSk8PAq=
ICAgICAgICgxPDwxKTw8KDE8PDEpKSsgICAgKCgxPDwxKSAgICA8PCgxPDwxKTw8Ch==
ICAgICAgICgxPDwoMT4+MSkpKS0oKCAgICAgMTw8MSk8PCggICAgICAxPDwoMT4+Co==
ICAgICAgIDEpKSkpLCgoMTw8MSk8PCAgICAgICgxPDwxKSAgICAgICAgPDwoMTw8MSkpCl==
ICAgICAgICwoKCgxPDwxKTw8KDE8PCAgICAgIDEpPDwoICAgICAgIDE8PDEpPDwoMTw8MSkpLQr=
ICAgICAgICgoMTw8MSk8PCgxPDwxKSAgICAgKS0oMTw8KDE+PjEpICAgICAgKSksKCgoCj==
ICAgICAgIDE8PDEpPDwoMTw8MSk8PCggICAgMTw8MSk8PCgxPDwxKSktICAgICgoMQp=
ICAgICAgIDw8ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgMQq=
ICAgICAgICkgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICA8PAp=
ICAgICAgICgxPDwxKTw8KDE8PCgxPj4xKSkpLSgxPDwoMT4+MSkpKSwgKCgoMTw8MQp=
ICAgICAgICk8PCgxPDwxKTw8KDE8PDEpPDwoMTw8MSkpLSgoMTw8MSk8PCAoMSAgCk==
ICAgICAgIDw8ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAxCp==
ICAgICAgICkgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIDw8Cn==
ICAgICAgICggICAgIDE8PCgxPj4xKSkpKyAgICAoMTw8MSkpLCgoKDEgICAgIDw8Cj==
ICAgICAgIDEpICAgIDw8KDE8PDEpPDwgICAgICAoMTw8MSk8PCgxICAgICAgIDw8Ck==
ICAgICAgIDEgICAgICkpLSgoMTw8MSk8PCAgICAoMTw8MSk8PCgxICAgICAgIDw8Cj==
ICAgICAgICggICAgIDE+PjEpKSktKCgxICAgICAgPDwxKTw8KDE8PCAgICAgICAoCj==
ICAgICAgIDEgICAgID4+MSkpKSksKCgoMSAgICAgPDwxKTw8KDEgICAgICAgIDw8Cg==
ICAgICAgIDEgICAgICk8PCgxPDwxKTw8KCAgICAgMTw8MSkpLSgoMSAgICAgIDw8Cm==
ICAgICAgIDEgICAgICk8PCgxPDwxKTw8ICAgICAoMTw8MSkpKygoICAgICAgICAxCv==
ICAgICAgIDw8MSk8PCgxPDwoMT4+MSkpKSksKCgoMTw8MSk8PCgxPDwxKSA8PCgxCm==
ICAgICAgIDw8MSkpKygxPDwoMT4+MSkpKSwoKCgxPDwxKTw8KDE8PDEpKSArKCgxCs==
ICAgICAgIDw8MSk8PCAoMTw8KDE+PjEpKSkgKyAoMTw8ICgxPj4xKSkpfSA7Zm9yCn==
ICAgICAgIChpPSgxPj4xKTtpPCgoKDE8PDEpPDwoMSA8PDEpKSsoKDEgPDwxKTw8KAr=
ICAgICAgIDE8PCgxPj4xKSkpKygxPDwxKSk7aSsrKSAgcHJpbnRmKCIlYyIsbltpXSk7fQp=

• get_base64_diff_value 方法，参数s1,s2。返回res：

    base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    res = 0
    for i in xrange(len(s1)):
        if s1[i] != s2[i]:
           return abs(base64chars.index(s1[i]) - base64chars.index(s2[i]))
  

• solve_stego 方法。返回res_str：

   with open('base64.txt', 'rb') as f:
       file_lines = f.readlines()
   bin_str = ''
   for line in file_lines:
       steg_line = line.replace('\n', '')
       norm_line = line.replace('\n', '').decode('base64').encode('base64').replace('\n', '')
       diff = get_base64_diff_value(steg_line, norm_line)
       #print diff
       pads_num = steg_line.count('=')
       if diff:
           bin_str += bin(diff)[2:].zfill(pads_num * 2)
           # print(bin(diff)[2:].zfill(pads_num * 2))
       else:
           bin_str += '0' * pads_num * 2
       res_str = ''
       #print bin_str
       for i in xrange(0, len(bin_str), 8):
           res_str += chr(int(bin_str[i:i+8], 2))
  

• 最后执行solve_stego()，结果：

  
  image.png