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@arushiro

SECCON 2017 Online CTF write-up

Last updated at Posted at 2017-12-11

概要

2017/12/09 ～ 10に行われたSECCON 2017 Online CTFのwrite-upです。
10日中にwrite-up書けないダメな遅筆人間ですね。

チームnanimodekinaiで参加し、1400Ptsで136位でした。

解いた問題

スコア

Vigenere3d (100Pts / Crypto)

普通のヴィジュネルを1つずつずらしながら3次元方向に重ねた感じのヴィジュネル。
それってヴィジュネルの結果をヴィジュネルすれば出るんじゃね？的な発想で逆変換の式、

c += s[(((s.find(a) - s.find(k1[i1])) % len(s)) - s.find(k2[i2])) % len(s)]

(コードは元コードのc +=の行をそのまま置換したもの)
を書いてVigenere3d2.pyとかして保存しておきました。

そんでもって、ヴィジュネル暗号はKeyがAの時変化しなかったような覚えがあるので、
KeyをA埋めの文字列から始めて、元のVigenere3d.pyで暗号化した結果の暗号文の先頭から7文字が
合うようにKeyを見つけ、

$ python Vigenere3d.py SECCON{AAAAAAAAAAAAAAAAAAAAAAAAAA} _KP2Za_AAAAAAA
POR4dny_aZ2PK__KP2Za__aZ2PK__KP2Z9

先程作った復号コードで戻すと、

$python Vigenere3d2.py POR4dnyTLHBfwbxAAZhe}}ocZR3Cxcftw9 _KP2Za_AAAAAAA
SECCON{Welc0me_to_SECCON_CTF_2017}

フラグが出ました。細かい条件とかはちゃんと調べてないので出たのはラッキーでした。やったね。

SECCON{Welc0me_to_SECCON_CTF_2017}

Run me! (100Pts / Programming)

フィボナッチ数列の第11011項の先頭32桁を求める問題。
Wolfram Alphaで一瞬で出るよとチムメンが解いてくれてました。

SECCON{65076140832331717667772761541872}

putchar music (100Pts / Programming)

putcharで音楽作る系らしい。
これ多分IOCCCとかでみた/dev/dspに流すと出てくるやつやろって適当に実行結果を流し込もうとしたけど/dev/dspがない。
じゃあaplayで再生するか～って

$ gcc file.c -lm
$ ./a.out > sound.wav
(出力が長いので途中でCtrl-cで打ち切り)
$ aplay sound.wav

したら有名な曲が流れ始めました。
レトロな感じが良かったです。大好き。

SECCON{STAR_WARS}

Qubic Rube (300Pts / Programming)

今年のQR枠。昨年は挫折したので今年はがんばった。

Qubic Rube

こんな感じのルービックキューブを解いて、面のどれかに次の問題(最後はフラグ)があるので読み込んでく問題。

PNG画像を問題URLから各画像のURLに変換してDLして、いい感じに反転処理とかさせて(底面だけXY反転必要だった気がする)、3×3で切り出して、色検出して各色抜き出して9パズルみたいにする処理を書いて、んでもって配置が間違ってると微妙にずれるので、人力で解けば割と出来るんじゃねと、

9パズル

こんな感じのソフトを作って、クリックすると回転や入れ替えが出来るようにして9パズルにしてチムメンでひたすら解いてました。
達成感がすごかったです。

SECCON{Thanks to Denso Wave for inventing the QR code}

SHA-1 is dead (100Pts / Crypto)

有名なSHA-1衝突PDFのやつ。
2017kB2017KBと2018kB2018KBの間なファイルで同様にSHA-1が衝突して、
SHA-256で衝突しないものを上げればOK。
どうやらSHA-1が衝突している2つのファイルの両方の末尾に同じ大きさの同じデータをくっつけたら、
同様にSHA-1が衝突するらしい。
https://shattered.io/ から衝突PDFを両方取ってきて、双方に同じ大きさのデータをつけて指定されたサイズにすればOK。

$ dd if=/dev/zero of=test.dmp bs=1 count=1643073

でちょうどいい大きさの0埋めデータ作って、

$ cat test.dmp >> shattered-1.pdf

みたいに後にくっつければ出来る。

×1000なのか×1024なのか両方やる必要あったのが微妙にめんどかったです。(大した面倒じゃないけど)

SECCON{SHA-1_1995-2017?}

Powerful_Shell (300Pts / Binary)

難読化されたPower Shellコードを読んでいく問題。
まず

$ECCON="";
$ECCON+=[char](3783/291);
$ECCON+=[char](6690/669);
$ECCON+=[char](776-740);
$ECCON+=[char](381-312);
$ECCON+=[char](403-289);
$ECCON+=[char](-301+415);
$ECCON+=[char](143-32);
$ECCON+=[char](93594/821);
$ECCON+=[char](626-561);
$ECCON+=[char](86427/873);
$ECCON+=[char](112752/972);
$ECCON+=[char](43680/416);
$ECCON+=[char](95127/857);
$ECCON+=[char](-682+792);
$ECCON+=[char](-230+310);
$ECCON+=[char](-732+846);
...

って感じのやばいコードがあるので、いい感じに$ECCONをデコードします。
そうすると、


$ErrorActionPreference = "ContinueSilently"
[console]::BackgroundColor = "black";[console]::ForegroundColor = "white";cls;Set-Alias -Name x -Value Write-Host;$host.UI.RawUI.BufferSize = New-Object System.Management.Automation.Host.Size 95,25;$host.UI.RawUI.WindowSize = New-Object System.Management.Automation.Host.Size 95,25;$host.UI.RawUI.BufferSize = New-Object System.Management.Automation.Host.Size 95,25;$host.UI.RawUI.WindowSize = New-Object System.Management.Automation.Host.Size 95,25;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 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'  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x;x '  ' -b 15 -n;x '  ' -b 12 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 12 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x '  ' -b 0 -n;x '  ' -b 15 -n;x;x '  ' -b 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' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x '  ' -b 15 -n;x;x;

<# Host Check #>
Write-Host -b 00 -f 15 Checking Host... Please wait... -n
Try{
	If ((Get-EventLog -LogName Security | Where EventID -Eq 4624).Length -Lt 1000) {
		Write-Host "This host is too fresh!"
		Exit
	}
}Catch{
	Write-Host "Failed: No admin rights!"
	Exit
}
Write-Host "Check passed"

$keytone=@{'a'=261.63}
$pk='a'
ForEach($k in ('w','s','e','d','f','t','g','y','h','u','j','k')){
	$keytone+=@{$k=$keytone[$pk]*[math]::pow(2,1/12)};$pk=$k	
}
Write-Host -b 00 -f 15 "Play the secret melody."

Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n '   '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '   '
Write-Host -b 00 -f 15 -n ' w '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' e '
Write-Host -b 15 -f 00 -n '   |   '
Write-Host -b 00 -f 15 -n ' t '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' y '
Write-Host -b 15 -f 00 -n '  '
Write-Host -b 00 -f 15 -n ' u '
Write-Host -b 15 -f 00 '   |    ' 
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00  '    '
Write-Host -b 15 -f 00 -n '  a |'
Write-Host -b 15 -f 00 -n '  s |'
Write-Host -b 15 -f 00 -n '  d |'
Write-Host -b 15 -f 00 -n '  f |'
Write-Host -b 15 -f 00 -n '  g |'
Write-Host -b 15 -f 00 -n '  h |'
Write-Host -b 15 -f 00 -n '  j |'
Write-Host -b 15 -f 00  '  k '
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00 -n '    |'
Write-Host -b 15 -f 00  '    '
Write-Host
$stage1=@();$f="";
While($stage1.length -lt 14){
	$key=(Get-Host).ui.RawUI.ReadKey("NoEcho,IncludeKeyDown")
	$k=[String]$key.Character
	$f+=$k;
	If($keytone.Contains($k)){
		$stage1+=[math]::floor($keytone[$k])
		[console]::beep($keytone[$k],500)
	}
}
$secret=@(440,440,493,440,440,493,440,493,523,493,440,493,440,349)
If($secret.length -eq $stage1.length){
	For ($i=1; $i -le $secret.length; $i++) {
		If($secret[$i] -ne $stage1[$i]){
			Exit
		}
	}
	x "Correct. Move to the next stage."
}
$text=@"
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TBEzFVdDQU8RUxdTbEwTNxVVQUNOEFEVUUwdQBVXQ0NOE1EWUUwRQRtVQ0FME1EVUU8RThdVTUNM
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"@

$plain=@()
$byteString = [System.Convert]::FromBase64String($text)
$xordData = $(for ($i = 0; $i -lt $byteString.length; ) {
	for ($j = 0; $j -lt $f.length; $j++) {
		$plain+=$byteString[$i] -bxor $f[$j]
		$i++
		if ($i -ge $byteString.Length) {
			$j = $f.length
		}
	}
})
iex([System.Text.Encoding]::ASCII.GetString($plain))

こんな感じのコードが出てくるので、

<# Host Check #>
Write-Host -b 00 -f 15 Checking Host... Please wait... -n
Try{
	If ((Get-EventLog -LogName Security | Where EventID -Eq 4624).Length -Lt 1000) {
		Write-Host "This host is too fresh!"
		Exit
	}
}Catch{
	Write-Host "Failed: No admin rights!"
	Exit
}

あたりを削って実行します。

さくらさくら

あら綺麗。
コード見ると周波数的なのが書かれているので、
さくらさくらのメロディー、「ララシ ララシ ラシドシ ラシラファ」(ドは高い方)
を奏でると次に進めました。
処理的にはいかにもBase64っぽい文字列をBase64デコードしたやつを、
さっきのメロディーのキー、hhjhhjhjkjhjhfと
XORしたものが次のコードになっているみたいです。

${;}=+$();${=}=${;};${+}=++${;};${@}=++${;};${.}=++${;};${[}=++${;};
${]}=++${;};${(}=++${;};${)}=++${;};${&}=++${;};${|}=++${;};
${"}="["+"$(@{})"[${)}]+"$(@{})"["${+}${|}"]+"$(@{})"["${@}${=}"]+"$?"[${+}]+"]";
${;}="".("$(@{})"["${+}${[}"]+"$(@{})"["${+}${(}"]+"$(@{})"[${=}]+"$(@{})"[${[}]+"$?"[${+}]+"$(@{})"[${.}]);
${;}="$(@{})"["${+}${[}"]+"$(@{})"[${[}]+"${;}"["${@}${)}"];"${"}${.}${(}+${"}${(}${|}+${"}${(}${)}+${"}${(}${)}+${"}${)}${|}+${"}${)}${&}+${"}${(}${+}+${"}${&}${@}+${"}${+}${=}${+}+${"}${|}${)}+${"}${+}${=}${=}+${"}${[}${]}+${"}${)}${@}+${"}${+}${+}${+}+${"}${+}${+}${]}+${"}${+}${+}${(}+${"}${.}${@}+${"}${[}${]}+${"}${&}${=}+${"}${+}${+}${[}+${"}${+}${+}${+}+${"}${+}${=}${|}+${"}${+}${+}${@}+${"}${+}${+}${(}+${"}${.}${@}+${"}${.}${|}+${"}${(}${|}+${"}${+}${+}${=}+${"}${+}${+}${(}+${"}${+}${=}${+}+${"}${+}${+}${[}+${"}${.}${@}+${"}${+}${+}${(}+${"}${+}${=}${[}+${"}${+}${=}${+}+${"}${.}${@}+${"}${+}${+}${@}+${"}${|}${)}+${"}${+}${+}${]}+${"}${+}${+}${]}+${"}${+}${+}${|}+${"}${+}${+}${+}+${"}${+}${+}${[}+${"}${+}${=}${=}+${"}${.}${|}+${"}${+}${.}+${"}${+}${=}+${"}${)}${.}+${"}${+}${=}${@}+${"}${[}${=}+${"}${.}${(}+${"}${(}${|}+${"}${(}${)}+${"}${(}${)}+${"}${)}${|}+${"}${)}${&}+${"}${.}${@}+${"}${[}${]}+${"}${+}${=}${+}+${"}${+}${+}${.}+${"}${.}${@}+${"}${.}${|}+${"}${&}${=}+${"}${[}${&}+${"}${+}${+}${|}+${"}${(}${|}+${"}${+}${+}${[}+${"}${.}${(}+${"}${)}${@}+${"}${]}${+}+${"}${[}${|}+${"}${[}${|}+${"}${.}${|}+${"}${[}${+}+${"}${+}${@}${.}+${"}${+}${.}+${"}${+}${=}+${"}${|}+${"}${&}${)}+${"}${+}${+}${[}+${"}${+}${=}${]}+${"}${+}${+}${(}+${"}${+}${=}${+}+${"}${[}${]}+${"}${)}${@}+${"}${+}${+}${+}+${"}${+}${+}${]}+${"}${+}${+}${(}+${"}${.}${@}+${"}${.}${|}+${"}${)}${+}+${"}${+}${+}${+}+${"}${+}${+}${+}+${"}${+}${=}${=}+${"}${.}${@}+${"}${)}${[}+${"}${+}${+}${+}+${"}${|}${&}+${"}${.}${.}+${"}${.}${|}+${"}${]}${|}+${"}${+}${.}+${"}${+}${=}+${"}${|}+${"}${&}${)}+${"}${+}${+}${[}+${"}${+}${=}${]}+${"}${+}${+}${(}+${"}${+}${=}${+}+${"}${[}${]}+${"}${)}${@}+${"}${+}${+}${+}+${"}${+}${+}${]}+${"}${+}${+}${(}+${"}${.}${@}+${"}${.}${[}+${"}${&}${.}+${"}${(}${|}+${"}${(}${)}+${"}${(}${)}+${"}${)}${|}+${"}${)}${&}+${"}${+}${@}${.}+${"}${.}${(}+${"}${(}${|}+${"}${(}${)}+${"}${(}${)}+${"}${)}${|}+${"}${)}${&}+${"}${+}${@}${]}+${"}${.}${[}+${"}${+}${.}+${"}${+}${=}+${"}${+}${@}${]}|${;}"|&${;}

これ知ってる……jjencode的なやつだ……
後回しにしてたらチムメンがそれを直してくれて、

${;}="$(@{})"["14"]+"$(@{})"[4]+"${;}"["27"];"${"}36+${"}69+${"}67+${"}67+${"}79+${"}78+${"}61+${"}82+${"}101+${"}97+${"}100+${"}45+${"}72+${"}111+${"}115+${"}116+${"}32+${"}45+${"}80+${"}114+${"}111+${"}109+${"}112+${"}116+${"}32+${"}39+${"}69+${"}110+${"}116+${"}101+${"}114+${"}32+${"}116+${"}104+${"}101+${"}32+${"}112+${"}97+${"}115+${"}115+${"}119+${"}111+${"}114+${"}100+${"}39+${"}13+${"}10+${"}73+${"}102+${"}40+${"}36+${"}69+${"}67+${"}67+${"}79+${"}78+${"}32+${"}45+${"}101+${"}113+${"}32+${"}39+${"}80+${"}48+${"}119+${"}69+${"}114+${"}36+${"}72+${"}51+${"}49+${"}49+${"}39+${"}41+${"}123+${"}13+${"}10+${"}9+${"}87+${"}114+${"}105+${"}116+${"}101+${"}45+${"}72+${"}111+${"}115+${"}116+${"}32+${"}39+${"}71+${"}111+${"}111+${"}100+${"}32+${"}74+${"}111+${"}98+${"}33+${"}39+${"}59+${"}13+${"}10+${"}9+${"}87+${"}114+${"}105+${"}116+${"}101+${"}45+${"}72+${"}111+${"}115+${"}116+${"}32+${"}34+${"}83+${"}69+${"}67+${"}67+${"}79+${"}78+${"}123+${"}36+${"}69+${"}67+${"}67+${"}79+${"}78+${"}125+${"}34+${"}13+${"}10+${"}125|${;}"|&${;}

と出たらしいから、数字をASCIIコードに直したら、

$ECCON=Read-Host -Prompt 'Enter the password'

If($ECCON -eq 'P0wEr$H311'){

	Write-Host 'Good Job!';

	Write-Host "SECCON{$ECCON}"

}

となって無事フラグ($ECCONは変数なのに注意)が得られました。マジ感謝……

SECCON{P0wEr$H311}

Ps and Qs (200Pts / Crypto)

チムメンが解いてくれたのでよくわかってません……
Slackの記述みて解いてみて解けたら追記します……

JPEG file (100Pts / Binary)

どっかの1bitが反転してるJPEGファイルを読めるようにする問題。
ファイルサイズがおよそ80000bitなんで先頭から順に1bitずつ反転させてって、
それぞれJPEGとして読めるなら復号したものを動画っぽくして眺めれば答え出るだろ～
って適当にやったら先頭から4985bit付近に出ました(処理の都合で8,7,6,5,4,3,2,1,16,15...のような順番に読んでるので付近です)

アルゴリズム的にはこんな感じです(C#)

Image i = Image.FromFile(@"tktk.jpg");
byte[] orig = (byte[])imgconv.ConvertTo(i, typeof(byte[]));
int count = 0;

while(count < 8 * orig.Length) {
   byte[] dst = (byte [])(orig.Clone());
   dst[count / 8] = (byte)(dst[count / 8] ^ (1 << count % 8));
   Image img = (Image)imgconv.ConvertFrom(dst);
   pictureBox1.Image = img;
   pictureBox1.Refresh();
   count++;
}

元はFormアプリケーションなので、アルゴリズムだけを抜粋しました。
これだけだと動かないと思うので動かす時はいい感じに整形して下さい。

JPEGの何のお勉強にもなってないので皮肉っぽいフラグになってしまいました。

SECCON{jp3g_study}

Thank you for playing! (100Pts / Thank you!)

楽しかったです！！！ありがとうございました！！！

SECCON{We have done all the challenges. Enjoy last 12 hours. Thank you!}

感想

Qubic Rubeの達成感が高かったです。時間は損してた気がします。
次やるCTFではもっと解けるようになりたいです……

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