[CISCN2018]oldstreamgame

題目

key 000644 000765 000024 00000000144 13256665261 011660 0 ustar 00hehe staff 000000 000000 嬸?3?婂?=鶿z?V?]鬗|lE浳_肚gu哲悍鋳 <??`産硙桎x悃x~?s 暽釘B堧>.t}?U??蚷 stream.py 000644 000765 000024 00000001023 13256665314 013006 0 ustar 00hehe staff 000000 000000 flag = "flag{xxxxxxxxxxxxxxxx}" assert flag.startswith("flag{") assert flag.endswith("}") assert len(flag)==14def lfsr(R,mask):output = (R << 1) & 0xffffffffi=(R&mask)&0xfffffffflastbit=0while i!=0:lastbit^=(i&1)i=i>>1output^=lastbitreturn (output,lastbit)R=int(flag[5:-1],16) mask = 0b10100100000010000000100010010100f=open("key","w") for i in range(100):tmp=0for j in range(8):(R,out)=lfsr(R,mask)tmp=(tmp << 1)^outf.write(chr(tmp)) f.close()

解題
考點應該為LFSR

import os,sys os.chdir(sys.path[0]) from Crypto.Util.number import* f = open('key.txt','rb').read() key = bytes_to_long(f) bin_out = bin(key)[2:].zfill(100*8) # print(bin_out[:32]) #前32位就是key key = '00100000111111011110111011111000' mask = '10100100000010000000100010010100'R = '' tem = key for i in range(32):output = '?' + key[:31]ans = int(tem[-1-i]) ^ int(output[-3]) ^ int(output[-5]) ^ int(output[-8]) ^ int(output[-12]) ^ int(output[-20]) ^ int(output[-27]) ^ int(output[-30])R += str(ans)key = str(ans) + key[:31]# R = format(int(R[::-1],2),'z') R = str(hex(int(R[::-1],2))[2:]) print(R)

答案

flag{0x926201d7}

總結

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