攻防世界- MISC新手區(qū)–第11題–base64隱寫

下載之后解壓，是stego.txt

打開stego.txt

顯然是base64編碼之后的結(jié)果，base64解碼，我還百度翻譯了一下，也沒什么發(fā)現(xiàn)。

于是想辦法，因為沒有MISC的基礎(chǔ)，就去看了WP，知道是base64隱寫。于是查閱資料了解了一下。

base64隱寫的原理

（1）起因：
俄羅斯有個叫 Olympic_ctf 的 CTF, 在 2014 年有道 misc 題是關(guān)于 Base64 的隱寫題.

大意就是給了你一段字符串, 讓你找 flag.
這里放上字符串, 便于讀者實驗

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（2）復(fù)習(xí)base64編碼

BASE64 是一種編碼方式, 是一種可逆的編碼方式.
編碼后的數(shù)據(jù)是一個字符串, 包含的字符為: A-Za-z0-9+/
共 64 個字符：26 + 26 + 10 + 1 + 1 = 64
其實是 65 個字符, = 是填充字符.

64 個字符需要 6 位二進(jìn)制來表示
2^5=32
2^6=64
表示成數(shù)值為 0~63.

比如, 字符串"Rascals"經(jīng)過 Base64 編碼后變?yōu)?#34;UmFzY2Fscw=="

上面的 圖，完整展示了為什么轉(zhuǎn)化后面會多出兩個=，原因是后面補(bǔ)了兩個000000

概括來說，出現(xiàn)等號是因為：
需在原數(shù)據(jù)二進(jìn)制值后面添加零, 使其字節(jié)數(shù)是 6 的倍數(shù).
然后, 在編碼后的字符串后面添加 1 個或 2 個等號"=", 表示所添加的零值字節(jié)數(shù).

按照上面的邏輯，同理：
長度為 3 個字節(jié)的數(shù)據(jù)經(jīng)過 Base64 編碼后就變?yōu)?4 個字節(jié) 38=64

（3）base64隱寫原理

從Base64編碼的原理可得Base64解碼的過程：

丟掉末尾的所有‘=’； 每個字符查表轉(zhuǎn)換為對應(yīng)的6位索引，得到一串二進(jìn)制字符串； 從頭開始，每次取8位轉(zhuǎn)換為對應(yīng)的ASCII字符，如果不足8位則丟棄。

以上面剛才用到的例子：

做解析：
UmFzY2Fscw== 首先丟掉末尾的== ，得到UmFzY2Fscw
然后 每個字符查表轉(zhuǎn)換為對應(yīng)的6位索引

后面的完整過程如下：

在解碼的過程中中，會有部分?jǐn)?shù)據(jù)被丟棄（即不會影響解碼結(jié)果），這些數(shù)據(jù)正是我們在編碼過程中補(bǔ)的0
也就是說，如果我們在編碼過程中不全用0填充，而是用其他的數(shù)據(jù)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(jié)果的位提取出來組成二進(jìn)制串，然后轉(zhuǎn)換成ASCII字符串。

（4）base64隱寫腳本（解密）

python腳本：

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cyBwcmVzZW50ZWQgdG8gdGhlIFVTLCBjcmV3IG1lbWJlcnMgZ2F2ZSAidGhlIGZpbmdlciIgdG8g dGhlIHVuc3VzcGVjdGluZyBOb3J0aCBLb3JlYW5zLCBpbiBhbiBhdHRlbXB0IHRvIE== ZGlzY3JlZGl0IHBob3RvcyB0aGF0IHNob3dlZCB0aGVtIHNtaQ== bGluZyBhbmQgY29tZm9ydGFibGUuDQoNCi0tDQpodHRwOi8vZW4ud2lraXBlZGlhLm9yZw== L3dpa2kvU3RlZ2Fub2dyYXBoeQ0K''' e=d.splitlines() binstr="" base64="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" for i in e :if i.find("==")>0:temp=bin((base64.find(i[-3])&15))[2:]#取倒數(shù)第3個字符，在base64找到對應(yīng)的索引數(shù)（就是編碼數(shù)），取低4位，再轉(zhuǎn)換為二進(jìn)制字符binstr=binstr + "0"*(4-len(temp))+temp #二進(jìn)制字符補(bǔ)高位0后，連接字符到binstrelif i.find("=")>0:temp=bin((base64.find(i[-2])&3))[2:] #取倒數(shù)第2個字符，在base64找到對應(yīng)的索引數(shù)（就是編碼數(shù)），取低2位，再轉(zhuǎn)換為二進(jìn)制字符binstr=binstr + "0"*(2-len(temp))+temp #二進(jìn)制字符補(bǔ)高位0后，連接字符到binstr str="" for i in range(0,len(binstr),8):str=str+chr(int(binstr[i:i+8],2)) #從左到右，每取8位轉(zhuǎn)換為ascii字符，連接字符到字符串 print(str) #結(jié)果是 Base_sixty_four_point_five轉(zhuǎn)換為

輸出：

得到flag就是flag{Base_sixty_four_point_five}

總結(jié)

base64隱寫解密，以后碰到直接用腳本，先成為腳本小子（小白沒辦法）

總結(jié)

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