This week we talked about multiple anagramming (for breaking arbitrary transposition ciphers), the Jefferson wheel ciphers, and the Vernam cipher. Next week we will talk about rotors and start on the Enigma cipher.

This Thursday (5/26), your proposal for the final project is due.

We also talked in some detail about the recent lawsuit against dropbox. We discussed a blog post describing potential issues: there is a side-channel. However, the blog post's conclusion that "the company does in fact have access to the unencrypted data (if it didn't, it wouldn't be able to detect duplicate data across different accounts)" is erroneous. One can construct systems (and it has been done and it's probably what dropbox is doing) that combine de-duplication and secrecy.

1. (Transposition Ciphers: multiple ciphertexts with single method, 15pt)

You know that the four pieces of plaintext were encrypted using the exact same transposition method and the same key. The following are the resulting ciphertexts.

Solve this through multiple anagramming, i.e. move the columns around so that all four lines yield plaintext simultaneously; all four lines are parts of messages (telegraphic, secret agent style). There were several clues given in class, e.g. that q is always followed by u.

2. (Wheel Cipher, 15pt) You intercepted the message

    OFNSA QPAZQ FTIPV ZLGHY ZWBIQ KWZRU PDTWM GLARV FFMME IAYNM

which you suspect was encrypted using the M-94 wheel cipher (virtual M94 simulator). You don't know the keyword, but suspect the sender was lazy and used a keyword of three letters. That still leaves 26*26*26 possibilities to try. Too many to try by hand. However, the M-94 is very weak with short keys. Decrypt the message and explain how you did it. Hint: i) See what ordering of the disks you get when you try short keys, e.g. try buy and abc. From this create a small set of keys that are sufficient to try to break the message; ii) the ciphertext contains 50 letters; since there are only 25 disks, this means it was encrypted as two 25-letter blocks. The shift (up/down) used for the two blocks need not have been the same. The key, however, must have been the same.

3. (Vernam Cipher, 10pt) You want to encrypt the text "car" using the Vernam Cipher.

a) Using the Baudot code, translate "car" into binary (see Baudot table for the code).

b) You have two tapes, containing "101" and "11001". Use Morehouse's (cryptographically flawed) method to construct a long key, and use that key to encrypt your result from a).

4. (Extra Credit) You have intercepted the following ciphertext, for which you also know the corresponding plaintext:

apoetcansurviveeverything
OEZHCIHDKQDSFBRDTBXMJQGBQ

The encryption was performed with the cylinders listed in the M94 (virtual M94 simulator). Your goal is to determine the order of the disks that was used to get this result.