BUUCTF:https://buuoj.cn/challenges
题目描述：
得到的 flag 请包上 flag{} 提交。
密文：
下载附件，解压得到.zip文件，再次解压得到两个文件。
解题思路：
1、打开ComeOn!.txt文件，发现一大串经过Base64加密的密文，如下图。
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Q2c9PQ0= SUNBZ0lDOHZjSEpwYm5SbUtDSWxaQzB0SldSY2JpSXNiR1Z1ZUN4c1pXNTVLVHNLDQ== Q2c9PQ0= SUNBZ0lITjBjbWx1WnlCaExDQmlMQ0JqTEdRN0NnPT0N SUNBZ0lHRWdQU0I0TG5OMVluTjBjaWd3TENCc1pXNTRMeklwT3dvPQ0= SUNBZ0lHSWdQU0I0TG5OMVluTjBjaWhzWlc1NEx6SXNJR3hsYm5ndk1pazdDZz09DQ== SUNBZ0lHTWdQU0I1TG5OMVluTjBjaWd3TENCc1pXNTVMeklwT3dvPQ0= SUNBZ0lHUWdQU0I1TG5OMVluTjBjaWhzWlc1NUx6SXNJR3hsYm5rdk1pazdDZz09DQ== SUNBZ0lIQnlhVzUwWmlnaUpXUXRMU1ZrTFMwbFpDMHRKV1F0TFMwdExTMHRMUzB0TFMwdExTMHRMUzB0TFMwdExTMHRMUzB0TFMwdExTMHRMUzB0TFMwdExTMHRMU1ZrTFNWa1hHNGlMR0V1YzJsNlpTZ3BMQ0JpTG5OcGVtVW9LU3dnWXk1emFYcGxLQ2tzSUdRdWMybDZaU2dwTENCNExuTnBlbVVvS1N3Z2VTNXphWHBsS0NrcE93bz0N SUNBZ0lDOHZZMjkxZER3OGVEdzhJam9pUER4aFBEd2lJQ0k4UEdJOFBDSWdJanc4WXp3OElpQWlQRHhrUER4bGJtUnNPd289DQ== SUNBZ0lITjBjbWx1WnlCaFl5QTlJRzExYkNoaExDQmpLVHNLDQ== SUNBZ0lITjBjbWx1WnlCaVpDQTlJRzExYkNoaUxDQmtLVHNLDQ== SUNBZ0lDOHZZMjkxZER3OGVEdzhJam9pUER4aFl6dzhJaUFpUER4aVpEdzhaVzVrYkRzSw0= Q2c9PQ0= SUNBZ0lITjBjbWx1WnlCaFlpQTlJRzFwYm5Vb1lTd2dZaWs3THk5aExXSUsN 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SUNBZ0lITjBjbWx1WnlCNExDQjVPd289DQ== SUNBZ0lIQnlhVzUwWmlnaTZLKzM2TDZUNVlXbDVMaWs1TGlxNUxxTTZMK2I1WWkyNXBXdzVhMlhPaUFpS1RzSw0= SUNBZ0lHTnBiajQrZUQ0K2VUc0sN SUNBZ0lITjBjbWx1WnlCaGJuTWdQU0J0ZFd3b2VDd2dlU2s3Q2c9PQ0= SUNBZ0lHTnZkWFE4UEdGdWN6c0sN ZlFvPQ0=
2、结合题目提示和密文特征，猜测为Base64隐写，使用如下Python脚本进行解密。
base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" binstr="" strings = open('./base64.txt').read() e = strings.splitlines() for i in e: if i.find("==") > 0: temp = bin((base64.find(i[-3]) & 15))[2:] # 取倒数第3个字符，在base64找到对应的索引数（就是编码数），取低4位，再转换为二进制字符 binstr = binstr + "0" * (4 - len(temp)) + temp # 二进制字符补高位0后，连接字符到binstr elif i.find("=") > 0: temp = bin((base64.find(i[-2]) & 3))[2:] # 取倒数第2个字符，在base64找到对应的索引数（就是编码数），取低2位，再转换为二进制字符 binstr = binstr + "0" * (2 - len(temp)) + temp # 二进制字符补高位0后，连接字符到binstr str = "" for i in range(0, len(binstr), 8): str = str + chr(int(binstr[i:i + 8], 2)) # 从左到右，每取8位转换为ascii字符，连接字符到字符串 print(str)
运行脚本，得到flag。
flag：
flag{6aseb4_f33!}
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