BUUCTF [GXYCTF2019]SXMgdGhpcyBiYXNlPw== 1

BUUCTF:https://buuoj.cn/challenges
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题目描述：
得到的 flag 请包上 flag{} 提交。

密文：
下载附件，解压得到flag.txt文件。
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解题思路：
1、打开flag.txt文件，内容如下。

Q2V0dGUgbnVpdCwK
SW50ZW5hYmxlIGluc29tbmllLAp=
TGEgZm9saWUgbWUgZ3VldHRlLAo=
SmUgc3VpcyBjZSBxdWUgamUgZnVpcwp=
SmUgc3ViaXMsCt==
Q2V0dGUgY2Fjb3Bob25pZSwK
UXVpIG1lIHNjaWUgbGEgdOmUmnRlLAp=
QXNzb21tYW50ZSBoYXJtb25pZSwK
RWxsZSBtZSBkaXQsCo==
VHUgcGFpZXJhcyB0ZXMgZGVsaXRzLAp=
UXVvaSBxdSdpbCBhZHZpZW5uZSwK
T24gdHJh5Y2vbmUgc2VzIGNoYeWNr25lcywK
U2VzIHBlaW5lcywK
SmUgdm91ZSBtZXMgbnVpdHMsCm==
QSBsJ2Fzc2FzeW1waG9uaWUsCl==
QXV4IHJlcXVpZW1zLAr=
VHVhbnQgcGFyIGRlcGl0LAq=
Q2UgcXVlIGplIHNlbWUsCt==
SmUgdm91ZSBtZXMgbnVpdHMsCp==
QSBsJ2Fzc2FzeW1waG9uaWUsCp==
RXQgYXV4IGJsYXNwaGVtZXMsCo==
Sidhdm91ZSBqZSBtYXVkaXMsCl==
VG91cyBjZXV4IHF1aSBzJ2FpbWVudCwK
TCdlbm5lbWksCu==
VGFwaSBkYW5zIG1vbiBlc3ByaXQsCp==
RumUmnRlIG1lcyBkZWZhaXRlcywK
U2FucyByZXBpdCBtZSBkZWZpZSwK
SmUgcmVuaWUsCq==
TGEgZmF0YWxlIGhlcmVzaWUsCh==
UXVpIHJvbmdlIG1vbiDplJp0cmUsCo==
SmUgdmV1eCByZW5h5Y2vdHJlLAp=
UmVuYeWNr3RyZSwK
SmUgdm91ZSBtZXMgbnVpdHMsCn==
QSBsJ2Fzc2FzeW1waG9uaWUsCq==
QXV4IHJlcXVpZW1zLAp=
VHVhbnQgcGFyIGRlcGl0LAq=
Q2UgcXVlIGplIHNlbWUsCo==
SmUgdm91ZSBtZXMgbnVpdHMsCm==
QSBsJ2Fzc2FzeW1waG9uaWUsCl==
RXQgYXV4IGJsYXNwaGVtZXMsCm==
Sidhdm91ZSBqZSBtYXVkaXMsCu==
VG91cyBjZXV4IHF1aSBzJ2FpbWVudCwK
UGxldXJlbnQgbGVzIHZpb2xvbnMgZGUgbWEgdmllLAp=
TGEgdmlvbGVuY2UgZGUgbWVzIGVudmllcywK
U2lwaG9ubmVlIHN5bXBob25pZSwK
RGVjb25jZXJ0YW50IGNvbmNlcnRvLAq=
SmUgam91ZSBzYW5zIHRvdWNoZXIgbGUgRG8sCo==
TW9uIHRhbGVudCBzb25uZSBmYXV4LAp=
SmUgbm9pZSBtb24gZW5udWksCo==
RGFucyBsYSBtZWxvbWFuaWUsCl==
SmUgdHVlIG1lcyBwaG9iaWVzLAq=
RGFucyBsYSBkZXNoYXJtb25pZSwK
SmUgdm91ZSBtZXMgbnVpdHMsCv==
QSBsJ2Fzc2FzeW1waG9uaWUsCn==
QXV4IHJlcXVpZW1zLAp=
VHVhbnQgcGFyIGRlcGl0LAo=
Q2UgcXVlIGplIHNlbWUsCm==
SmUgdm91ZSBtZXMgbnVpdHMsCp==
QSBsJ2Fzc2FzeW1waG9uaWUsCm==
RXQgYXV4IGJsYXNwaGVtZXMsCu==
Sidhdm91ZSBqZSBtYXVkaXMsCm==
VG91cyBjZXV4IHF1aSBzJ2FpbWVudCwK
SmUgdm91ZSBtZXMgbnVpdHMsCn==
QSBsJ2Fzc2FzeW1waG9uaWUgKGwnYXNzYXN5bXBob25pZSksCn==
Sidhdm91ZSBqZSBtYXVkaXMsCt==
VG91cyBjZXV4IHF1aSBzJ2FpbWVudA==

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2、经过base64编码的数据很多，感觉不是普通的base64解密，尝试base64隐写，得到flag。
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)