1. 題目信息
附件提供了背景介紹(Somebody leaked a still from the upcoming Happy Feet Three movie, which will be released in 4K, but Warner Bros. was smart enough to encrypt it. But those idiots used a black and white bmp format, and that wasn't their biggest mistake. Show 'em who's boss and get the flag.)，與一個bmp文件。
2. 分析
根據背景介紹，原圖片應該是bmp格式，經ecb工作模式加密得ecb.bmp；由於ecb工作模式不會掩蓋明文的統計規律，因此只要能夠修復ecb.bmp就可以見到明文，要修復ecb.bmp，則要求文件的前128字節為bmp文件格式特有的字節。
3. 解題
將ecb.bmp的前128字節替換為bmp文件格式特有的字節，則可以見到明文信息，實現的Python腳本如下：
from Crypto.Util.number import long_to_bytes
with open('ecb.bmp','rb') as f:
data=f.read()
pre=0x424d76483f00000000007600000028000000000f000070080000010004000000000000483f00000000000000000000000000000000000000000000008000008000000080800080000000800080008080000080808000c0c0c0000000ff0000ff000000ffff00ff000000ff00ff00ffff0000ffffff00ffffffffffffffffffffL
out=long_to_bytes(pre)+data[128:]
with open('out.bmp','wb') as g:
g.write(out)
out.bmp可被照片查看器打開，從而見到flag。