over 8 years ago
題目是一張 png，先拿工具來掃一下：
$ foremost final.png
Processing: final.png
|*|
$ cat output/audit.txt
Foremost version 1.5.7 by Jesse Kornblum, Kris Kendall, and Nick Mikus
Audit File
Foremost started at Mon Sep 29 16:14:32 2014
Invocation: foremost final.png
Output directory: /home/peter50216/ISG/output
Configuration file: /etc/foremost.conf
------------------------------------------------------------------
File: final.png
Start: Mon Sep 29 16:14:32 2014
Length: 3 MB (3845157 bytes)
Num Name (bs=512) Size File Offset Comment
0: 00000000.png 1 MB 0 (1440 x 900)
1: 00003754.png 1 MB 1922524 (1440 x 900)
Finish: Mon Sep 29 16:14:32 2014
2 FILES EXTRACTED
png:= 2
------------------------------------------------------------------
好呀其實有兩張圖接在一起。
把兩張圖拿來 diff 一下會發現，只有 184 個 pixel 的 R channel 值不一樣。
仔細一看會發現，第二張圖那幾個 pixel 的值只有 0 或 1，所以就寫個 code 吧~
require 'chunky_png'
require 'openssl'
i1 = ChunkyPNG::Image.from_file('./output/png/00000000.png')
i2 = ChunkyPNG::Image.from_file('./output/png/00003754.png')
val = []
(0...i1.height).each do |j|
(0...i1.width).each do |i|
r1 = ChunkyPNG::Color.r(i1[i, j])
r2 = ChunkyPNG::Color.r(i2[i, j])
val << r2 if r1 != r2
end
end
puts val.map{|x| x.to_s(2)}.join.to_i(2).to_bn.to_s(2)
Flag: ISG{E4sY_StEg4n0gR4pHy}
