SHA2017 Teaser CTF - Crypto Engine

Hello my name is Flyingsky.

I’m pretty new and wanted to broaden my knowledge, so I took part in the SHA2017 teaser CTF. I thought let’s take on the crypto challange because you’ve never worked with cryptography. It was the only challenge I was able to solve though, because that day I only had 2 hours of time…

Anyways let’s get to the write up:

When you clicked on the challange you got greeted by this:

If you clicked on the “crypto engine” link you got presented a input box that allowed you to encrypt your own input.

If you clicked on the “flag” link you got this:

The first thing I tried was, what happens if I put in some random text so I put in 1 A and it told me that there is “no text to encrypt”. So there is a minimum input length. I tried AA with the same result. But with AAA I got this:

So the minimum is 3 characters. Next I put in AAAA, and this time it worked and gave me this result:

So everything above 3 characters works! Next I put in some random text and looked if there is anything useful to notice, and there was! I noticed that 3 characters make 1 block and if it’s less than 3 it displays a 2 or 4 digit number. Next I counted the blocks in the flag picture and calculated 12 * 3 + 2 that the flag was 38 characters long.

(38 * “A”)

From this picture I saw that the pattern repeats every 8 blocks (24 characters)!

Then I looked at the flag picture and tried to understand how to decode it. Soon came the idea of reading the hexvalues of each coloured block with the pipette. So I downloaded the flag picture and put it into krita and used the pipette to extract the hash from the picture.

545c50 502f38 353149 010d02 2c633e 364957 06032a 323f67 185359 012832 3c351c 540400 2e29

With this in my hands I finally was able to go about solving the challenge. I bruteforced the first 2 blocks to try to find a pattern… with no luck. But after the 3rd block I noticed that certain values seemed to be swapped around. If I put in 0x35 I got 0x65 and vice versa! From this I thought I could just put the whole hash into the encryption box and have it decrypt it for me, but that sadly wasn’t true.

I tried to understand what was going on and tried encrypting more things. At some point I noticed that the encrypted output changed depending on what preceded it. So I knew I had to decrypt it block for block! Which worked exactly as I thought it would!

I put in 0x353149 and got 0x656166, those are the printable ascii characters e, a and f! I put them after the 2 brute forced blocks and it really was true!

After I knew that decrypting the hash was only a matter of encrypting each block preceded by the right characters.

(the last 2 hex digits put into the encrypt field)

(the 0x327d put into the encrypt field, it yields the desired result)

With that I solved the Crypto challenge of the SHA2017 teaser round. The whole flag was:
flag{deaf983eb34e485ce9d2aff0ae44f852}