Home Work Assignment
CIS341 - Fall
2003
Due: Tuesday, October 14
Assignment
1. Hand in your decodings of the Vigenere ciphertexts found here.
NOTE: The programs have been amended to make your decryption work easier. And, although the ciphertexts are not new, redundancies have been eliminated. So be sure to download the new version of the assignment!!
2. Listen to the interview of Phil Zimmerman. The link to this interview is found at the Screensaver website. Be prepared to discuss its content.
Important Information
1. Since the following ciphertexts were found to be equivalent:
vig1=vig5; vig2=vig4; vig3=vig11
the redundancies were eliminated.
2. Of the 8 remaining files, 4 have key lengths of 7; 4 have key lengths of 11.
3. These files represent 4 separate plaintexts, each encoded with 7 & 11 shifts.
4. Of the key length 7 files, for two of them the shifts are the first choices in the frequency analysis given by the "xxx-sorted" file. For another one, 6 of the shifts are first choices. For the 4th, 3 are 1st choices, 3 are 2nd choices and the 7th is a 3rd choice.
5. Of the key length 11 files, one has 10 first choices; another has 9 first choices, one second choice and one 6th choice. The remaining two have only 5 and 2 first choices, respectively. Perhaps knowing the plaintexts from the key length 7 files will help in the decipherment process.
Additional Important Information
1. The reason why key length 11 files yielded such large trial-and-error files is that the number of possible combinations of the first 5 choices for shifts is 511, which is over 48 million. So the AutoDecrypt program was adjusted to hold down the size of the trial-and-error files. Do not use the old version of AutoDecrypt!!
2. In addition, as you use AutoDecrypt you will be asked if you want to use a short text. Unless you want a big file generated, you should always choose YES.
3. The new AutoDecrypt does not work with key lengths of 11. You will have to use the Decrypt.exe program and try the keys produced by V-analyze.