2021-10-28 攻防世界-简单题-MISC-011(base64stego)

╰+攻爆jí腚メ 2022-08-28 02:50 187阅读 0赞

## base64stego ##

打开题目附件得到zip文件解压后得到文本如下  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5oSa5YWs5pCs5Luj56CB_size_20_color_FFFFFF_t_70_g_se_x_16]  
发现一堆base64，猜测位base64隐写，上脚本

#coding=utf-8
    def get_base64_diff_value(s1, s2):
        base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
        res = 0
        for i in xrange(len(s2)):
            if s1[i] != s2[i]:
                return abs(base64chars.index(s1[i]) - base64chars.index(s2[i]))
        return res
    
    def solve_stego():
        with open('1.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)
                else:
                    bin_str += '0' * pads_num * 2
                print goflag(bin_str)
    
    def goflag(bin_str):
        res_str = ''
        for i in xrange(0, len(bin_str), 8):
            res_str += chr(int(bin_str[i:i + 8], 2))
        return res_str
    
    if __name__ == '__main__':
        solve_stego()

运行后等到flag：`flag{Base_sixty_four_point_five}`

[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5oSa5YWs5pCs5Luj56CB_size_20_color_FFFFFF_t_70_g_se_x_16]: /images/20220828/1298a56529ac44f9a72eff02dfd0f564.png