前言

懶狗一個，最近打比賽才知道還有base64隱寫，有很多大師傅已經記錄了原理，這里簡單寫一下。

base64

base64編碼就是用64個ascii字符作為基礎來編碼二進制內容的一種編碼方式。編碼后的數據是一個字符串, 包含的字符為: A-Za-z0-9+/共 64 個字符,= 是填充字符.

編碼與解碼

編碼

編碼時，將要編碼的內容轉換為二進制數據（一個字符對應8位二進制），每6位作為一組，從索引表中找到對應的字符

編碼的文本字節數是3的倍數時，剛好可以編碼成4個字符

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編碼的文本字節數不是3的倍數時，剩下1個字符時

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編碼的文本字節數不是3的倍數時，剩下2個字符時

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這就是base編碼方式

解碼

首先把填充字符=去掉

每個字符查表轉換為對應的6位索引，得到一串二進制字符串

接著從左到右，每8位一組，多余位丟掉，轉為對應的字符

base64隱寫原理

base64之所以可以隱藏信息，便是在于在解密過程中,在解碼的第3步中，會有部分數據被丟棄（即不會影響解碼結果），這些數據正是在編碼過程中補的0。也就是說，如果在編碼過程中不全用0填充，而是用其他的數據填充，仍然可以正常編碼解碼，因此這些位置可以用于隱寫。

解開隱寫的方法就是將這些不影響解碼結果的位提取出來組成二進制串（一行 base64 最多有 2 個等號, 也就是有 22 位的可隱寫位.），然后轉換成ASCII字符串。這就是為什么出base64隱寫時會有很多大段 base64 的原因*

這里借用下Tr0y師傅的圖

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例如第三幅圖，將最后兩位改成01、10、11其實都不會對結果產生任何影響

aGU= aGV= aGW= aGX= base64解碼后全為 he

需要注意的是等號的那部分 0 不能用于隱寫，因為修改那里的二進制值會導致等號數量變化, 解碼的第 1 步會受影響. 自然也就破壞了源字符串。

base64隱寫解密腳本

def get_base64_diff_value(s1, s2):base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'res = 0for i in xrange(len(s2)):if s1[i] != s2[i]:return abs(base64chars.index(s1[i]) - base64chars.index(s2[i]))return resdef solve_stego():with open('shy.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 diffpads_num = steg_line.count('=')if diff:bin_str += bin(diff)[2:].zfill(pads_num * 2)else:bin_str += '0' * pads_num * 2print 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_strif __name__ == '__main__':solve_stego()

題目實踐

湖湘杯中的顏文字之謎便考察了base64隱寫

直接跑腳本

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總結

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