WP Crypto Polybius 棋盤密碼

title:polybius

description: 2020 | bjdctf | crypto | polybius

做題平台：ctfhub

密文：ouauuuoooeeaaiaeauieuooeeiea hint：vghligxlbmd0acbvzib0aglzihbsywlud**4ddogmtq= flag:解出明文後，**上bjd{}

首先題目描述為「polybius」，最直接想到的應該是「波利比奧斯矩陣」或「波利比奧斯棋盤」即棋盤密碼。再對應給出的密文是「a,e,i,o,u」這五個字母的重複，那就肯定了這個應該是棋盤密碼。

話說給出的hint為base64編碼，解碼後為「the length of this plaintext: 14」，而密文長度剛好為14*2=28，更加確定了這是棋盤加密。

key=[['a','b','c','d','e'],\

['f','g','h','i/j','k'],\
['l','m','n','o','p'],\
['q','r','s','t','u'],\
['v','w','x','y','z']]   #字母棋盤的生成
ordstr = ''
c = 'ouauuuoooeeaaiaeauieuooeeiea'
import itertools
num = ['a','e','i','o','u']
for num in itertools.permutations(num, 5):  #python內建庫itertools中指定元素全排列函式的使用
order = list(num)
for i in c :
a = str(order.index(i))
ordstr = ordstr + a   
ordstrlist = list(map(int,list(ordstr)))  #將密文中的aeiou轉化為對應數字並存入列表
ordstr = ''
i = 0
mstr2 = ''
while (i!=28):
mstr1 = key[ordstrlist[i]][ordstrlist[i+1]]  #兩兩一組獲取對應明文
mstr2 = mstr2 + mstr1
i = i+2
print(mstr2)  #輸出解密結果

在結果中查詢「flag」：

bjd

2.了解到了指定元素全排列函式itertools.permutations(list,int)的用法

3.不能定式思維誰說的「aeiou」順序必須按照字母表來？

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