For this challenge, we were given two pcapng files, comm1.pcapng and comm2.pcapng, and a text file. The text file contains this: Bonjour Agent-CTF, Nous avons recemment interceptés un message, chiffré grace a un algorithme inconnu. Nous avons mis en relation ce message a un message anterieur, contenant une discussion suspecte entre deux membres de l’APT. Votre mission sera de dechiffrer les communications. Bonne chance Q.G. We can use tshark to view the raw data of the network captures:

For this challenge, we were given access to a service. The description told us that it was an encryption oracle, which used AES to encrypt what we sent it, concatenated with critical data. I’m writting this after the challenge ended, so I can’t include demos. The description should make us think about byte-at-a-time ECB decryption attacks, which are well explained here: Cryptopals - Byte-a-time ECB decryption (Simple) Why is Byte-at-a-time ECB decryption a vulnerability?

For this challenge, we were given access to a server which we can connect to: $ nc 144.217.73.235 27099 give me cmd|token example: ls|c9af5ac08978481063b711f031f38518a7c2d83d6db3eabacbd7726470e8a140 id|69a4061766769d0a19ab59e6f905f7ac5875691b62765cb6b3b5ee6ae08f776a ls|c9af5ac08978481063b711f031f38518a7c2d83d6db3eabacbd7726470e8a140 chall.py wrapper $ nc 144.217.73.235 27099 give me cmd|token example: ls|c9af5ac08978481063b711f031f38518a7c2d83d6db3eabacbd7726470e8a140 id|69a4061766769d0a19ab59e6f905f7ac5875691b62765cb6b3b5ee6ae08f776a whoami|c9af5ac08978481063b711f031f38518a7c2d83d6db3eabacbd7726470e8a140 Bad Token It executes the command we give it, given that we know the corresponding hash. The challenge description told us that the hash format is HASH(SECRET || CMD). This should instantly make us think of hash key length extension attacks.