VolgaCTF 2017 Quals - VC - Crypto

Information#

Version#

By	Version	Comment
noraj	1.0	Creation

CTF#

• Name : VolgaCTF 2017 Quals
• Website : quals.2017.volgactf.ru
• Type : Online
• Format : Jeopardy
• CTF Time : link

Description#

There are files A.png and B.png. But where's the flag?

A.png

B.png

A.png:

B.png:

Solution#

If you know OTP you must know this famous attack on pad reused.

You can also see some black lines on image B. Now let's XOR image A and B (with ImageMagick):

$ convert A.png B.png -fx "(((255*u)&(255*(1-v)))|((255*(1-u))&(255*v)))/255" out.png

And now we get the result:

Flag is: VolgaCTF{Classic_secret_sharing_scheme}.

Note: VC is for Visual Cryptography