## Homework 3 (due 4/26) CSC 233

We talked about Vigenere, Babbage (check him out, he was an interesting character, see his (reconstructed) difference engine in action), and started on the Kasiski text. We will continue talking (and performing) the Kasiski test and then go on to the Playfair cipher.

2. (Alberti Cipher disk, 10pt)

You have intercepted the ciphertext  MAOFKDIVXAPAXXFQOAJZAXXZX encrypted with our modernized Alberti disk (pictured above: download here if you lost it). You do not know the key letter that was used for this piece of text (but apparently it didn't change through this piece of text). Decrypt and determine the key letter used (the letter aligned with the index letter K).

Describe how you found the solution in detail.

3. (Autokeys, 10pt) Remember Vigenere's first autokey method: instead of repeating the keyword (as you do in the Vigenere cipher) you append the plaintext to the keyword and use the resulting word as they key to encrypt the plaintext. For example (and double-check that example using , encrypting "It was a dark and stormy night" using keyword Lytton gives:

GYRFCWMWZEUZFUVMDAIGEGEJUAKIPUVHLVWENJHYXZGLLUFBUUFSXSBNGGOOL

from a friend; you'd agreed on the keyword "charles" and the use of the Vigerene autokey idea. Decrypt the ciphertext, and include a short explanation of how you did it.

4. (Vigenere, 15pt) The following ciphertext was encrypted using a Vigenere cipher with a keyword of length 8.

Find the keyword and decrypt the text using the vigenere analysis program. Hint: keep in mind that it's possible that the automatic analysis doesn't get all the letters in the keyword right, so keep track of the top choices.

5. (Extra Credit, 20pt) This is again about the Vigenere cipher use with Vigenere's first auto-key idea. E.g. encoding "and did those feet" with "blake", we get

Plaintext:  anddidthosefeet
Key:        blakeanddidthos
Ciphertext: bydnmdgkrahylsl

Come up with (and describe) a general method to break such an autokey Vigenere cipher. Use it to break the following cryptotext:

VSITYZAEPE ACZOEIUGBF UYTBRWAGFY MIFBWDRJQD XVVBSUMIUL WSAFTSXYEF
XLTHZSCDPG KMVXTBPHRJ ZRAVLRTTCS VOKBVEWNBG GLOKNYSOWQ CHLVDITSQP
ZLVVMJFAEL XQSVPFLTVH YTSRVNUXAG OIEZHMLSBJ XIUARHFHWH QGXCKWYMLY
TKILRVYUSM UYOQDOLECZ RCHEMFIXNS NWFXWODNSE FLVNXRFZZL IXRVXLUYJX
UAGHCDRYAO OXPBIEBZWI GGVEEFLSJF TYG

Marcus Schaefer
Last updated: April 19th, 2011.