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Cryptography hexionCTF 2020

“Really Smart Acronym”

Really Smart Acronym, of course, is RSA. Looking at the code, it uses PyCrypto to generate a RSA key to encrypt the flag. You also get one encryption and 1024 decrypts, but you only get the last bit of the decrypts. At first I thought it could be Franklin-Reiter related-message attack, but there is not […]

Categories
Cryptography hexionCTF 2020

“SSS”

We are given an sss.py. See https://pastebin.com/KQhdB3fa for source. I found that SSS stands for Shamir’s Secret Sharing by copy-pasting the loop from eval_at, which brought me to this Wikipedia page: https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing From there I learned that sss.py was basically giving out shares, with the flag as the secret. So we can just request enough […]

Categories
Cryptography hexionCTF 2020

“XOR”

We are given an encrypted flag and an encryption program. The encryption program generates a key of letters of length 8 to 15, inclusive. Then the flag is repeatedly xored by cycling the key for each index. What follows is a brute-force method to retrieve the flag. The script will probably print multiple flags (assuming […]