CS 100 HW 5: Web Pages and Encryption

• Complete the problems in each section below which is labelled Problem
• Sections that are not labelled Problem are designed to help you solve the HW problems so make sure to read the information contained there.
• This is an individual assignment: you must do your own work and may not share or compare answers with other students. If you are struggling, ask questions on Piazza, seek help from the TA or professor, and review the class notes and tutorial links provided.
• Do your work in an acceptable electronic format which are limited to the following
  • A Microsoft Word Document (.doc or .docx extension)
  • A PDF (portable document format, .pdf)

  You may work in other programs (Apple Words, Google Docs, etc.) but make sure you can export/"save as" your work to an acceptable format before submitting it.

• Submit your work to our Blackboard site.
  • Log into Blackboard
  • Click on CS 100->HW Assignments->HW 5
  • Press the button marked "Attach File: Browse My Computer"
  • Select your file
  • Click submit on Blackboard
• If you want verbose feedback print a copy of your assignment and submit it in class on the due date. You must still submit an electronic version.
• Submit 2 files to our Blackboard site.
  • Log into Blackboard
  • Click on CS 100->HW Assignments->HW 5
  • Press the button marked "Attach File: Browse My Computer"
  • Select your HW Writeup file (.doc .docx .pdf .html .txt only)
  • Again: Press the button marked "Attach File: Browse My Computer"
  • This time select your hw5.html and attach it.
  • Click Submit
• Make sure the HW Writeup has your information in it:

  CS 100 HW 5 Writeup
  Turanga Leela tleela4 G07019321
  

In class we studied two very basic symmetric encryption techinques, the Caesar Cipher and the Vigenere Cipher. We covered these during Week 7-2 and 8-1. The next two problems will have you do a little encoding and code breaking.

Problems 3-5 use the following table of number to letter correspondences. Notice the underscore ( _ ) is the last character so there are 27 letters in this alphabet numbered 0 to 26. Underscore will be used in place of spaces in messages

For example the word HOMER_BOWLS would be converted to numbers as

7 14 12 4 17 26 1 14 22 11 18

Use the Caesar cipher to encrypt the following message using the given key.

• Message: EAT_MY_SHORTS
• Key: 14

Decrypt the following message using the given key.

• Message: XLBRP
• Key: 11

What to put in your HW Write-up

• Put your encrypted and decrypted messages in your write-up
• Make sure to show some work so you can garner some partial credit in case things go wrong and your final answer is a little off

The Python code below has functions to do both encryption and decryption of Caesar ciphered messages. It is demonstrated on the message AY_CARUMBA. To encrypt call the ceasear_encrypt(message,key) function

plain = "AY_CARUMBA"
key = 14
encrypted = caesar_encrypt(plain,key)

To decrypt, use the caesar_decrypt(message,key) function as in

decrypted = caesar_decrypt(encrypted,key)

Both encrypted and decrypted messages can then be printed as in

print("encrypted: "+encrypted)
print("decrypted: "+decrypted)

The full code to support the Caesar cipher is below. You can use this code the check your answers for Problem 3 and will need to use it for Problem 4.

# Caesar cipher python functions to encode and decode
# Valide letters are A-Z and _

# 27 letters: A-Z plus _
numbers_letters = [ (0,"A"), (1,"B"), (2,"C"), (3,"D"), (4,"E"), (5,"F"), (6,"G"), (7,"H"), 
                    (8,"I"), (9,"J"), (10,"K"), (11,"L"), (12,"M"), (13,"N"), (14,"O"), (15,"P"),
                    (16,"Q"), (17,"R"), (18,"S"), (19,"T"), (20,"U"), (21,"V"), (22,"W"), (23,"X"),
                    (24,"Y"), (25,"Z"), (26,"_"),]
alphabet_size = len(numbers_letters)

# Convert a number to a letter
def number2letter(num):
    for (n,l) in numbers_letters:
        if n==num:
            return l
    raise Exception("No letter for number "+str(num))

# Convert a letter to a number
def letter2number(let):
    for (n,l) in numbers_letters:
        if l==let:
            return n
    raise Exception("No number for letter "+str(let))

# Encrypt some text according to the caesar cipher using the give key
def caesar_encrypt(plain_text, key):
    encrypted_letters = []
    for plain_letter in plain_text:
        plain_number = letter2number(plain_letter)
        encrypted_number = (plain_number + key) % alphabet_size
        encrypted_letter = number2letter(encrypted_number)
        encrypted_letters.append(encrypted_letter)
    encrypted_text = "".join(encrypted_letters)
    return encrypted_text

# Decrypt some text according to the caesar cipher using the give key
def caesar_decrypt(encrypted_text, key):
    plain_letters = []
    for encrypted_letter in encrypted_text:
        encrypted_number = letter2number(encrypted_letter)
        plain_number = (encrypted_number + alphabet_size - key) % alphabet_size
        plain_letter = number2letter(plain_number)
        plain_letters.append(plain_letter)
    plain_text = "".join(plain_letters)
    return plain_text

# Demonstration of using the caesar_encrypt() and ceasar_decrypt() functions
plain = "AY_CARUMBA"
key = 14
encrypted = caesar_encrypt(plain,key)
decrypted = caesar_decrypt(encrypted,key)
print("plain:     "+plain)
print("encrypted: "+encrypted)
print("decrypted: "+decrypted)

# Try to decrypt this text. The secret key is between 0 and 26
secret_message = "L_XSKAEHKGFKSDANXSAFSKHJAFZYAXDW"

At the bottom of the python code above is a secret message.

# Try to decrypt this text. The secret key is between 0 and 26
secret_message = "L_XSKAEHKGFKSDANXSAFSKHJAFZYAXDW"

Copy all the python from the previous section into a file called caesar.py and then write a loop at the bottom to try every possible key in the Caesar cipher to decrypt the secret_message.. Use the function caesar_decrypt(encrypted_text,key) and print the strings it returns in the loop. Many of them will look like gibberish such as decoding using the keys 1 and 2 which should give the messages

key: 1
decrypted: KZWRJ_DGJFEJRC_MWR_ERJGI_EYX_WCV
key: 2
decrypted: JYVQIZCFIEDIQBZLVQZDQIFHZDXWZVBU

However, one of the keys should give a decrypted message that looks like actual English and it is the answer. Print out which key is used to try the decryption so you'll be able to determine the secret key for future use.

You can do this by hand if you like, but it's much faster to use Python in a loop to perform this brute force attack to try every possible key.

What to put in your HW Write-up

• Show the python code for the loop you used to do your brute force attack and try every possible key
• Show the key that you determined decrypted the mesage
• Show the decrypted message text (Hint: it's the setting for America's longest running primetime series)

Food Corp would like to determine the secret ingredient to the Flaming Moe, a popular beverage at a local food establishment. Unfortunately, the proprietor, Moe, is not willing to sell the recipe. Food Corp was able to intercept an encrypted messasge which they believe contains the secret ingredient. They know that Moe favors using the Vigenere cipher and know from his Facebook page that the secret key is likely to be the name of one of his two most stalwart customers.

Crack the secret message below by decrypting the Secret Message using one of the two keys below. Only one key will produce a meaningful message which will be the secret ingredient to the Flaming Moe.

• Secret Message: JBFKYGFJVKW
• Potential Key 1: BARNEY
• Potential Key 2: HOMER

What to put in your HW Write-up

• Show the key that you determined decrypted the mesage
• Show the decrypted message text (Hint: it's not love)
• Make sure to show some work so you can garner some partial credit in case things go wrong and your final answer is a little off

Every student at GMU web space on the mason server to set up a personal web site. This is very useful for advertising yourself to potential employers and can be generally fun. Problems 1 and 2 center around setting up the your personal site. You will need to perform several steps.

1. Create a personal web page on your local computer which is an HTML file along with any pictures you want to use (Problem 1)
2. Get a communications tool that allows your computer to talk to the mason server.
3. Set up your web directory on mason and transfer your personal web page and pictures from your personal computer to the mason server. (Problem 2)

The sections preceding Problem 1 walk you through the process of creating an HTML page on your personal computer.

The section preceding Problem 2 walks you through the steps to transfer the page to the mason web server.

Make sure to follow the instructions closely to finish the next two problems.

An web page is as simple as a text document with some code in a specific display language, the Hyper Text Markup Language, HTML for short. To create a web page on your computer, choose a sensible location (folder) such as Desktop/cs100/hw5/ and create a text file there. Unfortunately, Windows 7-10 and Mac OS X lack a good text editor for web pages so in this section gives instructions on how to download and install Sublime Text Editor which is available on all major computing platforms and does a good job of editing HTML pages.

Create a personal web page in a file called hw5.html.

For full credit your web page should have all of the following things in it

1. A proper title in the title bar using the <title> tags
2. Your full name and major
3. A picture of yourself, preferably one in which your face is visible, you are fully clothed, and that is an appropriate size. Make sure that your picture is in the same local folder as the hw5.html or you won't be able to see it.
4. A short paragraph on your interests ("When I'm not studying I like to…", "I really enjoy … and …").
5. A list of 3 things you have learned so far in CS 100. This list must be a numbered list coded in HTML using the <ol> tag.
6. A link to the CS 100 lectures slides. Visit the page and copy the link. Write the text CS 100 Lecture Slides for the clickable link text for this link.
7. A link to another student's HW 5 page. This means you'll have to find out the web address of someone else's hw5.html once they have it online so that your HTML page links to theirs. You may need to come back to this step after other students complete the HW. Make sure to include the name of the other student's page as the clickable link text. For example Chris Kauffman's personal web page.
8. A link to Prof. Kauffman's Home Page which is here: https://cs.gmu.edu/~kauffman/. The link should have the the following text associated with it

   Recent research has shown beard loss increases the computer power of professors.
   

   so that clicking that text directs one to Prof. Kauffman's page. This text will potentially influence internet search engine results so that a search for those words may show the linked web page as a top hit (a Google Bomb).

We will discuss most of the above concepts in HTML during lecture. The lecture notes and code for those discussions will be on the lecture slides page so review the slide forget to do something with HTML.

What to put in your HW Write-up

• Once you have your hw5.html finished, show it in the web browswer and take a screenshot of it. Put the screeshot of your HW Write-up.
• When submitting your write-up, also submit your hw5.html file to Blackboard.

In order to move files on your local computer to the mason server, we will use some simple programs that utilize the Secure File Transfer Protocol called SFTP for short. An older protocol was called just FTP as it did not use encryption to ensure the connection was secure. The protocol is a way for one computer to talk to another computer, similar to the Internet Protocol, however SFTP includes the ability to issue commands like "I have a file, put it on the server" or "The server has a file, get it from the server and put it on my computer" and "Server, make a directory (mkdir) for me to put files in."

The following instructions give an overview of how to use SFTP on Windows and Mac. After these individual sections, the next section outlines the commands you run in case you get into trouble and need to understand them more.

Windows does not come with a built in SFTP tool so you'll need to get one. Download and install the PuTTY communication tools. The download link is here: http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

Look for the link title A Windows installer for everything except PuTTYtel and click it

Once downloaded, run through the installer program to load all the putty programs onto your computer.

You should now be able to use the PSFTP program which you should be able to locate by doing a search.

When started, PSFTP looks a little barren like this:

You'll first want to open up a connection to the mason server. If it is your first time connecting, you'll likely be warned that you've never connected before to which you should just answer y for yes. Subsequent logins will not warn you.

Your username and password on mason are your GMU NetID (first part of your Mason email) and password. Type them in when prompted. Note that below the lines where you must type are highlighted in yellow but when you are working the lines will likely not be highlighted.

Note that as you type your password, no characters will appear on the screen so type carefully.

On successfully logging in you'll get a message about what the Remote Folder is.

Once you have logged into mason, you'll need to run a series of commands to move files from your local computer to the the remote mason server. The picture below shows what I typed which will be similar to what you'll need to type but not exactly the same as I have my hw5.html file in a folder on my desktop that might be different from where you put it and my user name is different than yours, both on my local machine and on mason. Refer to the sftp section for more details about what specifically to do. Note that the example below does not completely set up your web page so make sure the read the SFTP section to finish the process.

Mac OS X comes with a built in SFTP tool so you don't need to get any new programs. You will need to use the Terminal program to run sftp though.

You can access the Terminal by searching for it using the OS X search functionality. Every Mac has a terminal as OS X is a flavor of unix under the hood.

When started, the terminal looks similar to Python's interactive prompt.

Within, the terminal you'll use the sftp program to communicate with the server. On starting, the program requires you to tell it what server you are connecting to and what you username on the server is. By username is ckauffm2 and I am connecting to the machine mason.gmu.edu so I would start the sftp program as follows.

You will need to change your username to whatever your NetID is. If it is your first time connecting to mason, you'll get a warning to which you should just answer yes.

If all goes well you will be prompted for your password which is identical to your normal GMU password for Blackboard and email access. Type it in (but note that no characters will be shown on the screen).

When started, sftp looks a little barren like this:

Once you have logged into mason, you'll need to run a series of commands to move files from your local computer to the the remote mason server. The picture below shows what I typed which will be similar to what you'll need to type but not exactly the same as I have my hw5.html file in a folder on my desktop that might be different from where you put it and my user name is different than yours, both on my local machine and on mason. Refer to the sftp section for more details about what specifically to do. Note that the example below does not completely set up your web page so make sure the read the SFTP section to finish the process.

In order for the world to consume your new personal web page, follow the instructions in Mac SFPT or Windows PSFT and the instructions in SFTP commands to upload your personal web page and your pictures. Make sure to set the permissions right and to name your page hw5.html. Your personal web page should then be available to everyone on the web using an address similar to

http://mason.gmu.edu/~ckauffm2/hw5.html

When grading, we will check to see that the page exists at an address similar to the above.

What to put in your HW Write-up

• Include a clickable web link to your personal web page.