Encryption With Keys and Passwords (Vigenere Cipher)

Purpose: Students will use a Vigenère cipher to learn about the relationship between cryptographic keys/passwords and the Encryption & Decryption of a Message.

Vocabulary

• Vigenère Cipher
  (The Vigenère cipher is a method of encrypting alphabetic text where each letter of the plaintext is encoded with a different Caesar cipher, whose increment is determined by the corresponding letter of another text, the key.)
• Computationally Hard
  (Human can't...but computer can!)

Log on To: Code.org: Vigenere Cipher _ Click Here!
View the animated table (Puzzle 2) - As the message is being encrypted, you cross the line of each letter in order in the Secret Message with the line of the corresponding letter in the Key - where the two meet on the table is what will determine what the encrypted letter will be.

Try This: Read the explanation as to how the Vigenere Cipher tool works.

• Enter a test message.
• Start simple: enter a simple Text Message & enter a simple Key.
• Select Encrypt (Select a slow encryption speed.).
• Select play.
• When it finishes creating the Ciphertext…copy it and paste it in the “Text Message” field.
• Keep the same secret key.
• Select Decrypt.
• Select Play (but hover your mouse over the pause button.).
• The original message should now appear in the Cipher Text.

Go ahead and play with the Vigenere Cipher tool.

Journal: To Crack a Ciphertext, what would you need to know?

Activity: Test the Strength of your Password.
Your Password acts as a “Key”. Let’s test the strength of your password from being cracked. Go to “How Secure Is My Password.” Enter some fake passwords to see their strength. Add the following to check the strength:

• More characters (keep adding and testing).
• Capital
• Number
• Exclamation Point

Try This:

• Create a few passwords using 8 lowercase ASCII characters (a-z).  What’s the longest amount of time-to-crack you can generate?
• Using any characters on the keyboard, what’s the longest amount of time-to-crack you can generate with an 8-character password?
• As you try passwords, what seems to be the single most significant factor in making a password difficult to crack?  Why do you think this is?
• Opinion: Is an 8-character minimum a good password length for websites to require? Give your opinion, yes or no, and explain why you think that.

Video: Encryption & Public Keys
Journal:

• What is the difference between “cracking” a code and “decrypting” a message?
• Would you feel comfortable sending your password over the Internet using a substitution cipher? Explain.
• State one thing you can do to produce a better encrypted message using a Vigenere cipher?

If time remains…play the Encryption Terminology Matching Quiz…#6!