[WUSTCTF2020]B@se

碰到过类似的原题，但是还是记录一下，这样的思路很有趣。

这道题的原理就是将base64原本的替换顺序打乱，改成相应的字符顺序，然后使用新的字符顺序去进行base64的加解密。

class base64:
    def __init__(self,alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"):
        self.alphabet = alphabet
    def _EnInsideManage(self,strlist):
        strflag = ""
        temp = ord(strlist[0]) >> 2
        strflag += self.alphabet[temp]
        temp = ((ord(strlist[0])&3)<<4)|(ord(strlist[1])>>4)
        strflag += self.alphabet[temp]
        temp = ((ord(strlist[1])&15)<<2)|(ord(strlist[2])>>6)
        strflag += self.alphabet[temp]
        temp = (ord(strlist[2])&63)
        strflag += self.alphabet[temp]
        return strflag

    def enbase64(self,charString):
        encode = ""
        for i in range(len(charString)//3):
            encode += self._EnInsideManage(charString[i*3:i*3+3])
        if len(charString)%3!=0:
            if len(charString)%3 == 1:
                encode += self._EnInsideManage(charString[-1:]+chr(0)+chr(0))[:2]+"=="
            if len(charString)%3 == 2:
                encode += self._EnInsideManage(charString[-2:]+chr(0))[:3]+'='
        return encode

    def TenToBin(self,tenum):
        binstr = ""
        for i in range(5,-1,-1):
            if 1 == (tenum//(2**i)):
                binstr += '1'
                tenum = tenum%(2**i)
            else:
                binstr += '0'
        return binstr

    def BinToStr(self,strbin):
        "Turn the binary string to a ASCII string"
        strten = ""
        for i in range(len(strbin)//8):
            num = 0
            test = strbin[i*8:i*8+8]
            for j in range(8):
                num += int(test[j])*(2**(7-j))
            strten += chr(num)
        return strten

    def debase64(self,base64string):
        binstr = ""
        for i in base64string:
            binstr += self.TenToBin(self.alphabet.find(i))
        return self.BinToStr(binstr)



from itertools import combinations, permutations
for i in list(permutations(['3','4','j','u'], 4)):
	try:
		password  = 'JASGBWcQPRXEFLbCDIlmnHUVKTYZdMovwipatNOefghq56rs{}kxyz012789+/'.format(''.join(i))
		print password
		newobj = base64(alphabet=password)
		print(newobj.debase64("MyLkTaP3FaA7KOWjTmKkVjWjVzKjdeNvTnAjoH9iZOIvTeHbvD=="))
	except:
		pass