这一题比前面的要难不少!
学到了压缩包伪加密和base64隐写！

压缩包伪加密

下载附件后发现是一个压缩包，用WinRAR解压，结果发现有密码。但是能看到压缩包内是一个txt文件。

这一步我是改用7-Zip解压，提取出txt文件。
压缩包伪加密其实可以用winhex把指定位置处的09 00->00 00即可。
注：伪加密中压缩源文件目录区的全局方式位标记为09 00，无加密文件此位置应该是00 00
想了解更多点击此处

base64隐写

txt文件中的内容是：

U3RlZ2Fub2dyYXBoeSBpcyB0aGUgYXJ0IGFuZCBzY2llbmNlIG9m
IHdyaXRpbmcgaGlkZGVuIG1lc3NhZ2VzIGluIHN1Y2ggYSB3YXkgdGhhdCBubyBvbmV=
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Y3RzIHRoZSBleGlzdGVuY2Ugb2YgdGhlIG1lc3M=
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aGUgd29yZCBzdGVnYW5vZ3JhcGh5IGlzIG9mIEdyZWVrIG9yaWdpbiBhbmQgbWVhbnMgImNvbmNlYW==
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cml0ZSIuIFRoZSBmaXJzdCByZWNvcmRlZCB1c2Ugb2YgdGhlIHRlcm0gd2FzIGluIDE0OTkgYnkgSm9o
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aGVyIGNvdmVydGV4dCBhbmQsIGNsYXNzaWNhbGx5LCB0aGUgaGlkZGVuIG1lc3NhZ2UgbWF5IGJlIGluIGludmm=
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IGFkdmFudGFnZSBvZiBzdGVnYW5vZ3JhcGh5LCBvdmVyIGNy
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IHRvIHRoZW1zZWx2ZXMuIFBsYWlubHkgdmlzaWJsZSBlbmNyeXB0ZWQgbWVzc2FnZXOXbm8gbWF0dGVyIF==
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cHV0ZXIgZmlsZXMuIEluIGRpZ2l0YWwgc3RlZ2Fub2dyYXBoeSwgZWxlY3Ryb25pYyBjb21tdW5pY2F0aW9u
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ZSwgcHJvZ3JhbSBvciBwcm90b2NvbC4gTWVkaWEg
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L3dpa2kvU3RlZ2Fub2dyYXBoeQ0K

有点像base64加密后的字符串是不是？显然没这么简单。
首先，字符串太长，base64解密太麻烦。
这儿有个小技巧：
"flag"base64加密之后变成了"ZmxhZw=="，搜一下文本中没有出现。
这里就要提到base64加密隐写了：
简单来说，base64编码再解码这个过程中，会出现"="个数*2bit的字节数的无效字节，也就是说这些字节无论填充什么，都不会影响结果。（因为这些字节在解码的过程中被删掉了）
base64隐写其实就是在这些字节上做文章。
想了解更多base64隐写点击此处
最后贴一个网上找的base64隐写脚本：

import base64
b64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
with open('stego.txt', 'rb') as f:
    bin_str = ''
    for line in f.readlines():
        stegb64 = str(line, "utf-8").strip("\n")
        rowb64 =  str(base64.b64encode(base64.b64decode(stegb64)), "utf-8").strip("\n")#删除首尾换行

        offset = abs(b64chars.index(stegb64.replace('=','')[-1])-b64chars.index(rowb64.replace('=','')[-1]))
        #把末尾的'='去掉,offset = |给定字符串在b64表中的位置-解码字符串在b64表中的位置|
        equalnum = stegb64.count('=') #计算等号个数
        if equalnum:
            bin_str += bin(offset)[2:].zfill(equalnum * 2)#返回offset的二进制形式
        print(''.join([chr(int(bin_str[i:i + 8], 2)) for i in range(0, len(bin_str), 8)])) #8 位一组

flag{Base_sixty_four_point_five}