SI110 Homework Alpha: ________ Name: __________________
Page 1 of 2
Collaboration Policy: Default
MIDN Last, F.
choose one: □ None □ XS110 □ EI with:
(or more)
□ MGSP
□ Discussed with: ______________________
Homework: /SI110/Cyber Security Tools/Symmetric Encryption
1. [ 10 / 8 / 5 / 0 ] Decrypt the cipher text: milob zsvvkbc; which was encrypted using the
Caesar Shift Cipher with shift value 10. You may use the course resources to help, but otherwise
do this by hand!
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
2. [ 10 / 8 / 5 / 0 ] You are in class, with everyone sitting at their desks, unable to get up for any
reason. You have no cell phones or computers. The only way you can communicate is by passing
notes. There's a new student sitting across the room from you, whom you've never met before.
Explain why you can't carry on a Caesar Shift encrypted note passing correspondence with the
new person with any real hope of confidentiality ignoring a brute force attack, and no computers
means frequency analysis is impractical. Use complete sentences; spelling and grammar count.
3. Use the Caesar Cipher Frequency Analysis page from the SI110 web site Resources page (link
below) to fill in the table below. Note: Refreshing the page clears all fields.
http://rona.academy.usna.edu/~si110/resources/caesar-shift/index.html
a. [ 10 / 8 / 5 / 0 ] Use the above tool to crack the cipher text fragment: dat .
Suppose you recover the larger cipher text fragment dat eo qoaz wo odknpdwjz .
b. [ 10 / 8 / 5 / 0 ] What do you now believe is the plaintext for: dat .
c. [ 10 / 8 / 5 / 0 ] What does this example show us about the effectiveness of frequency
analysis?
SI110 Homework
Alpha: ________
Name: __________________
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4. [ 5 / 2 / 0 ] Circle the correct answer. When using the Vigenere Cipher, with a longer key …
a. communication is less secure.
b. communication is more secure.
c. the security of the communication is unchanged.
d. communication is too complex to be useful.
5. [ 10 / 8 / 5 / 0 ] In what situation does the Vigenere Cipher provide provably perfect security –
describe the situation thoroughly? What alternative name is used for this class of Vigenere
Cipher?
Name:
Description:
6. [ 10 / 8 / 5 / 0 ] Continuing, explain one of the key challenges with this type of perfect security;
i.e. what is impractical about the technique? Use complete sentences; spelling and grammar
count.
7. [ 10 / 8 / 5 / 0 ] Encrypt the plain text BEACHFRONT with the key TAKE using the Vigenere
Cipher (link below). Show your work using the below table, given a Vigenere table you should
be able to complete this by hand.
http://rona.academy.usna.edu/~si110/resources/vigenere/vctable.html
Key
Plain Text
Cipher Text
8. [ 10 / 8 / 5 / 0 ] Using a chosen-plain text attack, you’ve tricked your enemy into sending a
message with plaintext STENOGRAPHIC. You intercept the ciphertext XBZRTOMEUPDG, and you
suspect that your enemy is using the Vigenere Cipher. Figure out your enemy’s secret key! Show
your work using the below table, given a Vigenere table you should be able to complete this by
hand.
Key
Plain Text
Cipher Text
[ 5 / 2 / 0 ] Secret Key: