## Homework 3 (due 1/24) CSC 233

We have finished the parts of chapter 2 we are going to cover. If you get excited about long-open ciphers, check out the Voynich manuscript. There have been many attempts at deciphering it, but none of them convincingly successful so far.

Next week, we will see an known-plaintext attack on the Playfair cipher, talk about some transposition ciphers, and then continue with chapter 3.

The Lewis Carroll acrostic I mentioned is the epilogue to Through the Looking Glass.

1. (Reading Assignment) Finish reading chapter 2 of the book.

2. (Alberti Cipher disk)

The following text was encrypted using Alberti's cipher disk, using it as a progressive cipher (i.e. advancing the inner disk by one position in each step).

"JHCZ WWWBDP CYAFO"

Decrypt the message (include details of how you proceeded).

Since you don't know where the ciphertext started you can't be sure that the first letter was encoded with the A alphabet. Decode the message nevertheless. (Remember def and stu, why do these two not help you here directly?)

Describe how you found the solution in detail.

3. (Vigenere Cipher) The following text has been encrypted using a Vigenere cipher

BARPATFJHS XZJIIFAFQI MORXMTTUSM GNDSVXDFVO MORRMLOFYP WKFMXBZESX
MORXMHIAIG MAFXLXDFVO FPEHCHBZPM DLNSVDPKJE LJZREMLJQI BJRRWBARRH
EVFOEMPKJS KOFYVL

a) Find long (at least three letters) repeating sequences of letters  by hand (i.e. don't use any programs), and determine their distance. There are at least three different such sequences.

b) Based on your results from a), what are likely key lengths?

c) Decrypt the text using the vigenere.xls spreadsheet we saw in class (don't use any other program). Warning: the ciphertext contains spaces and line breaks. For the program to work correctly, you need to remove all spaces and linebreaks before analyzing it.

4. (Autokeys) Remember Vigenere's use of an autokey: 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 vigenere.xls), encrypting "It was a dark and stormy night" using keyword Lytton gives:

plaintext:  itwasadarkandstormynight
Key:        lyttonitwasadarkandstorm
ciphertext: TRPTGNLTNKSNGSKYRZBFBUYF

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

5. (Playfair Cipher) Read Appendix E on the Plaifair cipher, and encrypt the following text using the Playfair cipher with keyword "turing".

"Bletchley Park".

6. (Extra Credit - tricky) Suppose we extend the Vigenere cipher 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 a general method (partially based on what we have already done) 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

Hint: Try to reduce the problem to a regular Vigenere cipher; play around with some examples where the key is very short (a single letter, maybe).

Marcus Schaefer
Last updated: January 17th, 2006.