Info
Category: ppc Point: 500 Author: OAlienO @ BambooFox
Analysis
就是解 hamming(15,11) (15,11) 就是將 11 bits 的原始資料 加上 4 bits 的 parity 變成 15 bits 加了檢查碼的資料
題目要解的是 block data 也就是 bits 變成 bytes 上面的解釋就變成 (15,11) 就是將 11 bytes 的原始資料 加上 4 bytes 的 parity 變成 15 bytes 加了檢查碼的資料
看圖片比較清楚
Solutions
他給你一個檔案 就照著 Hamming Code 的定義 把他修補回來 就變回正常的照片檔案了
from __future__ import division
def bits(f):
bytes = (ord(b) for b in f.read())
array = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
now = 0
for b in bytes:
array[now] = b
if now == 14:
yield array
now = (now+1)%15
answer = ""
for b in bits(open('image_with_flag_defect.jpg.hamming', 'r')):
ans = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
multiply = 1
for i in xrange(8):
parity = [0,0,0,0]
for j in xrange(4):
for k in xrange(1,16):
if k&(1<<j):
parity[j] ^= b[k-1]%2
if parity[0] or parity[1] or parity[2] or parity[3]:
b[parity[0]*1+parity[1]*2+parity[2]*4+parity[3]*8-1] ^= 1
for j in xrange(1,16):
ans[j-1] += multiply*(b[j-1]%2)
b[j-1] //= 2
multiply *= 2
for i in xrange(1,16):
if i != 1 and i != 2 and i != 4 and i != 8:
answer += chr(ans[i-1])
with open("flag","w") as f:
f.write(answer)
