攻防世界-Decrypt-the-Message

一、题目

收到一首英文诗歌和一段密文，要求很简单，就是解密这个密文

二、解题

1、背景知识PoemCode

参考文章:https://blog.csdn.net/xiao__1bai/article/details/120250452

2、解密

了解加密原理即可，解密过程很复杂，可以直接用现成的脚本

• 脚本地址:https://github.com/abpolym/crypto-tools/tree/master/poemcode
• 脚本用法:`python poemcode.py poem.txt msg.txt
  
• 脚本用法参考:https://www.cnblogs.com/zhengna/p/14763913.html

3、答案

从输出中找到有实际意义的语段，即为明文

flag:ifyouthinkcryptographyistheanswertoyourproblemthenyoudonotknowwhatyourproblemisabcdefghijklmnopqrstu

3、脚本代码

import sys
import itertools
from os import listdir
from os.path import isfile, join

abc = 'abcdefghijklmnopqrstuvwxyz'


def loadlist(infile):
    tlist = []
    for line in open(infile, 'r'):
        for w in line.split(): tlist.append(w.lower())
    return tlist


def encrypt(code, poem, msg):
    # Load all words of the poem into a temporary list
    twords = loadlist(poem)

    # Select only those words specified in the code in a new list
    pwords = ''
    for c in code: pwords += twords[c].lower()
    plen = len(pwords)

    # We can only support encoding all alphabetical letters, a key length greater len(abc) is not reasonable here
    if plen > len(abc): sys.exit(3)

    # Assign an index for each letter in the key based on the alphabet
    pcode = [None] * plen
    count = 0
    while (count < plen):
        for al in abc:
            for pc, pl in enumerate(pwords):
                if al != pl: continue
                pcode[pc] = count
                count += 1

    # Load all words of the message into a string
    mwords = ''
    for line in open(msg, 'r'):
        for w in line.split(): mwords += w.lower()
    mlen = len(mwords)

    # Split message into chunks of size plen, append random (here alphabet) characters to fill the last chunk, if necessary
    cpairs = []
    curlen = plen
    while (curlen < mlen):
        cpairs.append(mwords[curlen - plen:curlen])
        curlen += plen
    rword = mwords[curlen - plen:curlen]
    rlen = len(rword)
    if rlen < plen: rword += abc[:plen - rlen]
    cpairs.append(rword)

    # Encrypt the message according to the key
    cip = ''
    for i in code: cip += abc[i]
    cip += ' '
    for i in pcode:
        for pair in cpairs:
            cip += pair[i]
        cip += ' '
    return cip


def decrypt(poem, cip):
    # Load all words of the poem into a temporary list
    twords = loadlist(poem)

    # Load all cipher chunks of the ciphertext into a list
    cwords = loadlist(cip)

    # Get the code rom the first chunk and remove it from the ciphertext list
    code = []
    for i in cwords.pop(0):
        code.append(abc.index(i))

    # Select only those words specified in the code in a new multi-arrayed list
    xwords = [[] for x in range(len(code))]
    for xcount, c in enumerate(code):
        tlen = c
        while (c < len(twords)):
            xwords[xcount].append(twords[c].lower())
            c += 26

    # Get all possible combinations
    for comb in itertools.product(*xwords):
        pwords = ''
        for c in comb: pwords += c
        plen = len(pwords)

        # Rearrange the chunks according to the key
        pcode = [None] * plen
        count = 0
        while (count < plen):
            for al in abc:
                for pc, pl in enumerate(pwords):
                    if al != pl: continue
                    pcode[count] = cwords[pc]
                    count += 1

        # Decrypt the ciphertext
        msg = ''
        wlen = len(pcode[0])
        for c in range(0, wlen):
            for word in pcode:
                msg += word[c]
        print(msg)


# first argument = poem
# second argument = ciphertxt or msg
if len(sys.argv) != 3:sys.exit(2)

# print encrypt([0, 5, 13, 16, 19], sys.argv[1], sys.argv[2])
decrypt(sys.argv[1], sys.argv[2])