Capture the Flag Exercise 1 - FAQ

  1. Q: How to set an environment on DeterLab for testing?
    A: Allocate three machines - one client, one server, one router connected to the client and to the server. Have client and server send any traffic to each other - e.g., run ping on one of them. On the router, use nfqueue to capture traffic.
  2. Q: Are we supposed to use the same key for every message if using homophonic cipher method? Since the letter frequency is different for each message.
    A: Homophonic cipher uses letter frequency in spoken language to set up set sizes, not frequency in actual messages. So there is just one key. The message letter frequency may or may not fit the one you anticipated but that's OK. Attackers looking to break the cipher must work off the frequency tables for spoken language too.
  3. Q: Can we encode same messages differently?
    A: If you end up using polygram cipher you can use CBC mode for it to make same messages appear differently. But in any case you have to use the same key for every message, and the same encryption method.
  4. Q:Should our encrypted messages be only in ASCII?
    A: No, they can be in binary too. Input messages are ASCII only, as specified in the exercise description.
  5. Q: For the monoalphabetic cipher, can we calculate a mapping scheme based on each individual message? Or does this count as a polyalphabetic cipher?
    A: No, you must use one mapping for every message.
  6. Q: Is the key only going to be used for stream ciphers?
    A: That depends on your implementation but yes, key is likely only going to be used in stream cipher.
  7. Q: When implementing stream cipher, do we have to do the bit XOR implementation or can I just generate a huge random number and add each corresponding digit to the ascii code for the letter of the message I am trying to encrypt?
    A: You can do either