【Write-up】HITCON GIRLS 5th 新生招募 CTF Write-up - Crypto

本章節收錄

• Baby Easy RSA
• Weird flyer
• XOR

其他題目連結：

• HITCON GIRLS 5th 新生招募 CTF Write-up - Web
• HITCON GIRLS 5th 新生招募 CTF Write-up - Reverse
• HITCON GIRLS 5th 新生招募 CTF Write-up - Pwn
• HITCON GIRLS 5th 新生招募 CTF Write-up - Crypto
• HITCON GIRLS 5th 新生招募 CTF Write-up - Misc

Baby Easy RSA

Flag

hg_5th{4JXDL4BNG9}

write-up

題目描述

baby easy :D
What is RSA?

hint:use openssl and factordb

cipher和public key(pem檔)(兩個檔案連結)

1. 先用openssl去解pem檔可以拿到n和e
   

2. 網路隨便找個轉一下n的值(hex>dec)
   

3. 去factordb去解出質數
   

4. 寫code算出d然後推出原文

附個code

import libnum
import gmpy2

n=83092583783534841000145280642003842283533340442637642451258941907393275732996256523893438356692786223410880194199043046345864683398238392329295750150314289824255749149834103
e=65537

r=2262150367
q=3006300461
p=12218233223644524650141958853163065112163255395621655741865064529020634406575730714768264558014607893896434523845321502371618344594488810317052606914954669

d=gmpy2.invert(e,(q-1)*(p-1)*(r-1))

with open('cipher') as f:
    cipher=f.read()
    c=libnum.s2n(cipher)
    m=pow(c,d,n)

print libnum.n2s(m)

Weird flyer

Flag

hg_5th{RNDHMCBY3K}

write-up

tags: Write-up、HG_5th_CTF

用文字編輯器打開最底下（這邊用NotePad++）發現有一串Base64

解碼後發現是個提示 有youtube網址

把連結點開是一個介紹Elian Script的影片

google搜’Elian Script’ 就會發現有一個文件在介紹這個東西（如下）
https://www.ccelian.com/ElianScriptFull.html

其中有一張圖有對應的翻譯（？）

對著把元圖翻出來得到flag

XOR

Flag

hg_5th{33WN7W63GH}

write-up

題目描述

xor file is here.

xorcipher(檔案連結)

原始模樣

22061257011f125503181a1b571419135510120157031d1257131b16127d1f1a0057141a160f1e1912565654567d3d120510571e0657031d1257131b161257111a05570c1802554d337f1f102a42031d0c4446203942204146303f087d191c141255031855041210572e1a02574f277d3b120110055706160e55131e107d

使用xortool去解

1. xortool -o -x xorcipher

2. xortool -l 3 -c 20 -x xorcipher

cat xortool_out/1.out
or
grep -r hg xortool_out

key=‘wuw’