题目描述：

三份文件

#!/usr/bin/python2.7
# -*- coding: utf-8 -*-

from Crypto.Cipher import AES
from Crypto.Random import random
from Crypto.Util.number import long_to_bytes

def MyOwnCBC(key, plain):
	if len(key)!=32:
		return "error!"
	cipher_txt = b""
	cipher_arr = []
	cipher = AES.new(key, AES.MODE_ECB, "")
	plain = [plain[i:i+32] for i in range(0, len(plain), 32)]
	print plain
	cipher_arr.append(cipher.encrypt(plain[0]))
	cipher_txt += cipher_arr[0]
	for i in range(1, len(plain)):
		cipher = AES.new(cipher_arr[i-1], AES.MODE_ECB, "")
		cipher_arr.append(cipher.encrypt(plain[i]))
		cipher_txt += cipher_arr[i]
	return cipher_txt
	
key = random.getrandbits(256)
key = long_to_bytes(key)

s = ""
with open("flag.txt","r") as f:
	s = f.read()
	f.close()

with open("flag_cipher","wb") as f:
	f.write(MyOwnCBC(key, s))
	f.close()

题目分析：

• 可以看出是通过将ecb模式下的加密进行改写得到不那么正宗的cbc加密模式
  为什么说不那么正宗呢，嗯，缺少了异或操作

• 加密过程：

  • 先取32字节的初始密钥，再将明文按32字节进行分组
  • 用初始密钥对第一组明文进行ecb模式下的加密，得到第一组密文
  • 然后用第一组密文作为密钥，对第二组明文进行ecb模式下的加密
  • 依次进行（即用上一组密文做这一组的密钥对这一组明文进行加密，得到这一组的密文）
  • 将各组密文拼接起来得到最终完整密文

• 解密过程即为加密的逆过程，直接看代码理解吧（加密代码都能理解，解密代码就更不在话下了）

import os,sys
os.chdir(sys.path[0])
from Crypto.Cipher import AES
cipher = open('flag_cipher','rb').read()
key = cipher[0:32]
# print(key)
def MyOwnCBC(key,cipher):
    m_txt = b''
    c = [cipher[i:i+32] for i in range(0,len(cipher),32)]
    tempkey = key
    for i in range(1,len(c)):
        dic_cipher = AES.new(tempkey,AES.MODE_ECB)
        m_txt += dic_cipher.decrypt(c[i])
        tempkey = c[i]
    return m_txt

print(MyOwnCBC(key,cipher))
# afctf{Don't_be_fooled_by_yourself}

（注意：其中涉及的变量都是以字节形式存在！！！）

• 解法二：

（只能说转的弯不是一点点！）

from Crypto.Cipher import AES
import libnum

with open("flag_cipher","rb") as f:
    c=f.read()
    c=libnum.s2n(c)
# print(c)
c = 21292656373488011978364418024107789700887686551530454270445522828464720337206248499124844312171155259641500274278874017314457372773151331307172407714869749176143440687749839770567442331846034966132632446746437743579678545367396741376061652325905716827679715476783532048746086027349867619979913477315160624714430557596381579504025605249954196240156509940191971341082933976267130950808869988591597008953378645992765167331660427602201651289002183061261253340235784861206432341842664224641825531290318599065681883152692316579041346092358484268916741013748596889639508853744660658343891543516724158172382242657896353529023140453270810386977724714267621910713968037150164139128609665536958336097235701367197979335798416236251207007729252523052830385454973965146392046407155155180272651727459783130365548712217044673706002207419100213480033557361008211299219461128812963986646428886085942368169280785236274441447545888762161205585353130687151876162503128053548528182768921141882448760029624045029806765051107885242962127420601194487411325732712519675955560543192779813151302722721338552649859145727955350575192701503422131510568120070728345711761932972505439044647126890223095102720945738677370950849864657467995850857925200505619267270311413264029241173761160789157852659613357304828148364937932250964647387762693941262230355257464245554320440192562478599947973106196183763919883358341668877020119637477763648287503361958502502893410408445479094343127543928237244283218218476337913735881996297103758071297338546709574020805624508427467792568708722397326982249486419815864837966709325287892953951540599625469925089738327786403689628541008420194024823843525393418212344154913334264894965628656130429574500479743436414538636333787030506711510106428148761057631139622050661188746540182565579488253957773470822102760043738678246762502499612888782232054596589007483350152753750569061916109598309458617340741355292650821798820214122488781096100414742756004208744078698162863984819630892780667270791666811885161949926089770963998688543707862983719866929241402237771330607815260605330329173274642894787672412332242774166602507323483948496539313151023644571247043176255279863758836264858421976416173227907442339415245105298343064893438803603612461986125704634296301009994966582752217246159527366850195241306878313150178758573581996729576930975802940869868416271773909052695649237941425747946600717100491378083818202448245490587697888326643535453668425455398848045263489798238106558806599430386848437146067101032116948347867604304936676326896380704663582301937444582539538193684596607309426055261338225821401977930137075632797406023364085546408323097370664918046312088623960825316302416835352498190495277093383182325163990419438929079710426991251445921649332188614107717689033701160555792276504520444051407345616793959373440323290054877445375288913862263689404640230922070060907993291831309033279086376357240572001939273547215645094557628726844335244989439429517424112316118941357594308890394130052372156987033266085760402268414175209456903502544829608600534487631205295191189909711081930563473180896335589147129495537694277321861460768544386763591959557989939286846981077310662114247589536261454359436815263020670634725488926563438773167948562715097859414315538623160751073867890465153478699163393781362433932104820449278800177775834189159954853181387514425004293801318345091108784782726615923996894455593089821429787276069341449612273919400148171170416377733437875864384655589256209926784271181271526166931851845258074095439363817296858238387036176754704529581496698666729002152982970396676356910182791107500953069313005774632591975634341340380709111925574116651544495145299463147718630661482967606012895581859296393549313767483396402302558579626292822192231713835803570651242457779742275512749524355870423112183936833763572266104269114605480071591476091026668845547360426465805633548122957435282625167193959977436453557128333628187898160955120807872244828122004450770947879506746375528475282047679955191297732287428640196362451637128618935694068987143793672503871659501725099541713552917194626851742
m = b''
# c = bin(c)
c_bin = '1110010111011111100101000111001101001010110000101100110100000100111010111011011111001111000001010010100010111110100110000101110011101001110000110101111000011111001000011111101111101010001101101101101001100011000111111111111010010000001100011011101100010011010110110010101110110101011111001011001101011111000001100000100010100111001100010100101001001100111001001111001000000101100010000001110111100001011000111101100100111001111101101011100100100111001111100110001110111011011010011000010010000000011011000001100001110011011101001101111100001001010110011001000111110101011010110110001011110101001101011110111101100100010110110110100001000011100001000011010000001001011110001001010111011100000101001011101101000000101101010111010100010010111111000000101001001000010000110010100111001100011101101110100010000110111000011100001011010111110101101011010111000011111101000010100001100111110101101001100110001110100101010010101001001101101011110101111011011110000001110011110100010110110001010100011111100111111110101101010011111100011011100011100011010110001101101110001000011110101001111011000111110101011110111101101110010000001111111001011101100000011110100000001001010011001001110010111111111101010010001101011011000010010011011100011111110110110110110000110001110101001111010111110011100111100001010001000000011000101010010110101110101001000001010010110101010010100011101000101101010011110110000000011111000111111100110100111000101100011000011100110110011000010000010100010010010001000011100000100011000010101101100011000010111110001000000111111001000000110100110010111101010011111010001000100000110111001110000010011000000000001100010100001110001100000001010100001000001011101011011001101101011100000110011000010101111100000000100101100000001110100111111011010000110011001110111010101101110111100011001011001001100001110111001000100010011110101011000001011111111000001100101011101110011111101100000111101010111100000100111010101101011011010110001110000110001010111110011110110111011000100111101111010000111010000010001101001100111010000011000100100010011010110110100110110000001100111110111011100111010000000000011110010001101101000001101111100101100101101011101111110010111110000111010001100010001001001010110101100100000000000010011111100000010111101101101110100000111000110001001011011001110111101110100000011010011010100100110001001110111111101100100001101011110111011000110000001100000010110100100111110110110001110001111001111111100100000000011010011000001100100001110111111110101111000010101011110001100010110110011010011010110010000001101100100001110101110001001111001100111001011010110010100101101011111101110011001011101011110001010000011010010001100001100111111010011111011101000101011101001011101010101110001100110001101001111001001111000100010010101101000111111001000100100001010101000011001111100100010101111110100010110110111111000111110010001100110110001101111010010110110011100101000010101101011011000111111000011000101110011000011001101001110110111100101011101101011011001111110111000001101010101101110100011101000000001111100101001010001001000100100100101100000110101110110001100010111111100100110010111101000111000111110010100100011100110100001011011000101000111101110101010100111110001111110010001010010010000000011110000000111010010100011011111010111011110001111010111010001001001110001110000101010101011001000111101001011011110010010111111100100110010101010010010010100100011101010111101111101001001110100101011001101010001001111111010110101100111000010101000010100111010111011101101011111010000100100010001000011110110011111010100011110001000100001100001101000110000001000100010110111000100110010101011011101110011010010011111111000011101100010110010001010111110001011011011110001010101000011111111111011111101001100111111010000111011001101000001110100001100111010010001000101110101111101110010111001010001010110111110011001010111110100001111011110010101011101001100101100101011001011101111010010001101010010111100001000001010101010111110001111011000111011111111011011001010111101001001100011010100100000111110110010010100010111011101111110011100110101001110001111011010011110010010001001001011111000111011100110111100010100110001001111001100111101100000000101110000011011011111010101000000111110101110110101001001010000011101001011000100111111011010000000001101010010010010111110101011011011001000101001001110000001110011000001001101000110001101111001001100111110011100110001100011110100101010000001111000110111010110010111100001101001110000000001011001011001100000100101100011100100011101111011100111110010000000001100110100000001100111001111000111011010000001001111111010100111110111000111010110100110011000110100001010100111110010011010001000100011100110010101010111011010110000011001011110001001110101101101111110111111000111000010101111001101001111100111101101101100010011011111101011111010110011001111001000111011011111010001001100011100000001001110000110000010011010100100101011100010110101010100010100100001000111011001101101011010110111001100110011001011010000010110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int_list = [int(c_bin[i:i+8], 2) for i in range(0, len(c_bin), 8)] # 将二进制字符串转换为整数列表
print(int_list)
c_bytes = bytes(int_list) # 将整数列表转换为字节串对象 amazing!
cipher = [c_bytes[i:i+32] for i in range(0,len(c_bytes),32)]
for i in range(1,len(cipher)):
    key = cipher[i-1]
    mm = AES.new(key,AES.MODE_ECB,)
    m += mm.decrypt(cipher[i])
print(m)

收获：

知道了如何将2进制转化为字节形式：

bin_str = '0110100001100101011011000110110001101111' # 二进制字符串
int_list = [int(bin_str[i:i+8], 2) for i in range(0, len(bin_str), 8)] # 将二进制字符串转换为整数列表
byte_data = bytes(int_list) # 将整数列表转换为字节串对象

print(byte_data)  # b'hello'