[WARGAME NDH 2016] Write-Up – Crypto: One too many

06
Jul
2016
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Posted by: Yann C.  /   Category: Cryptanalyze / Cryptography / Cryptology / / / / /   /   No Comments

Write-up of the challenge “Crypto – One too many” of Nuit du Hack 2016 Wargame

The weekend of 02-03 july 2016 is the WARGAME of the Nuit du Hack 2016 as a Jeopardy CTF. Having had the opportunity and the time to participate with some colleagues and friends, here’s a write-up resolution of the challenges which we could participate.

  • Category: Crypto
  • Name: One too many
  • Description : We encrypted both parts of the flag with our revolutionary two-time-pad technique! Are you able to recover it?
  • URL : http://static.wargame.ndh/one_too_many_1.png and http://static.wargame.ndh/one_too_many_2.png
  • Points : 100

tl;dr : one_too_many_1.png XOR one_too_many_2.png (convert tool on kali)

Two PNG images are provided to us for this challenge:

one_too_many_1

one_too_many_1

one_too_many_2

one_too_many_2

The technique of “One-Time-Pad” was used to encrypt these images, and the key is identical between the two ciphers (two-time-pad).

The following relationship follows:

one_too_many_1.png = one_too_many_1_original.png XOR OTP
one_too_many_2.png = one_too_many_2_original.png XOR OTP
 
one_too_many_1.png XOR one_too_many_2.png = one_too_many_1_original.png XOR one_too_many_2_original.png

A simple Google search on a fast method to XOR two images, brings us to this topic, which details the use of the command “convert”. The methodology is applied:

convert one_too_many_1.png one_too_many_2.png -fx "(((255*u)&(255*(1-v)))|((255*(1-u))&(255*v)))/255" one_too_many_flag.png

And the flag is displayed : ndh2k16_88bcb95612bf8207da083e784f1855b8

one_too_many_flag

one_too_many_flag

Thank you to all the team of the NDH2K16 for this event and for the whole organization!

Greeting to nj8, St0rn, Emiya, Mido, downgrade, Ryuk@n and rikelm, 😉 // Gr3etZ

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Yann C.

About the Author : Yann C.

Consultant en sécurité informatique et s’exerçant dans ce domaine depuis le début des années 2000 en autodidacte par passion, plaisir et perspectives, il maintient le portail ASafety pour présenter des articles, des projets personnels, des recherches et développements, ainsi que des « advisory » de vulnérabilités décelées notamment au cours de pentest.