Description:
To complete this lab, follow all the instructions below very carefully and record your answers as you go.
When you feel comfortable with the material and confident about your answers please take the Lab 4: Security & Encryption Lab Quiz.
Cryptography Overview : Watch the cryptography video on Cryptography and the Ceasar Cipher at: https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/intro- to-cryptography . Other useful sites to investigate are: https://en.wikipedia.org/wiki/Encryption and https://en.wikipedia.org/wiki/Cipher which discuss encryption & ciphers in general.
Caesar Cipher:
The Caesar Cipher is a character shifting, symmetric cipher (encryption algorithm). Read about it on Wikipedia at http://en.wikipedia.org/wiki/Caesar_cipher. With this algorithm, the “key” is the number of characters to shift forward or backward.
For example, consider the following clear text message: COMPUTING
Using a key of shift forward by 3, the corresponding cipher text is: FRPSXWLQJ
To decrypt this example, shift each character backwards by 3 characters.
1. What is the corresponding cipher text for the clear text message: “PERIMETER”, when using a key of shift forward by 10? (Ignore the quotes)
2. What is the corresponding clear text for the cipher text message: “HWDUYTLWFUMD”, when using a key of shift forward by 5? (Ignore the quotes)
3. What is the corresponding cipher text for the clear text message: “GEORGIA”, when using a key of shift forward by 26? (Ignore the quotes)
Problems with the Caesar Cipher:
The Caesar Cipher is not very secure and is not used in modern times to secure very sensitive data. Since there are only 26 characters in our alphabet, it is feasible to do a manual brute-‐force attack to break this algorithm. See if you can “crack the code” and discover the clear text of a cipher text without knowing the key.
4. What is the corresponding clear text for the cipher text message: “LWNKWU” that was encrypted with the Caesar Cipher? (Ignore the quotes) . HINT: use the “Caesar Cipher Exploration” link at the Khan Academy site: https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/intro- to-cryptography .
Substitution Ciphers and Frequency Analysis
Read the introductory section and the “Simple substitution” section in Wikipedia at http://en.wikipedia.org/wiki/Substitution_cipher. The Caesar Cipher (or shift cipher) is a specific type of substitution cipher. We can create a substitution cipher algorithm stronger than the Caesar Cipher by substituting a somewhat random character for each character in the clear text instead of simply shifting. In this case, the “key” is the mapping from clear text character to cipher text character.
For example, consider the following key called “Key-‐1” in the following table:
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
G P R B V Y F X J T W S E H D Z U I L O Q N M A K C
The clear text characters are listed in the top row and the corresponding cipher text characters are listed in the bottom row.
Therefore, when decrypting with Key-‐1,
“A” becomes “G”
“B” becomes “P”
“C” becomes “R”
… and so on.
For example, the clear text message:
I HOPE YOU HAD A GREAT SPRING BREAK
Encrypts to:
J XDZV KDQ XGB G FIVGO LZIHF PIVGW
5. What would the following clear text message encrypt to:
“HAPPY FALL Y’ALL”
Frequency Analysis: Notice that, in most phrases, certain letters appear more than others. Consider the bonus round of the Wheel of Fortune game show in which a contestant needs to guess a phrase and is given a certain set of letters to start with. These starting letters are “R S T L N E” and these letters tend to appear more frequently than other letters. Keeping this in mind, determine the character frequency of the example clear text phrase above; in other words, determine how many times each character appears.
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
4 1 0 1 3 0 2 2 2 0 1 0 0 1 2 2 0 3 1 1 1 0 0 0 1 0
6. Does this frequency analysis align with your perceptions of how often each letter appears in common phrases and everyday English language usage?
Now complete the Lab Quiz: Security & Cryptography in iCollege before the deadline. You’ll be asked about the following:
• What is cryptography?
• What is a cipher?
• What is the relationship between the Caesar Cipher and the simple substitution cipher?
• Answers to the previous questions (# 1- 6 above).
