Lecture Overview

• Encryption - the encryption wikipedia page is about the level of detail I covered, including a brief overview of symmetric encryption and public-key cryptography.
  • encryption is used to allow two people to exchange messages without anyone who intercepts the message to be able to discover its contents.
  • encryption does NOT protect against forged messages or allow you to know who the sender was.
  • There are three pieces of data in an encryption scheme, the key, plaintext, and ciphertext. The key is a secret known to the sender and/or receiver and is not transmitted. The plaintext is the message to send, and the ciphertext is the result of using the key to encrypt the plaintext. The ciphertext is the only thing that is transmitted.
  • security lies in the fact that the key is not known to an attacker, only to the sender and receiver. It does NOT rely on security through obscurity.
• Substitution Ciphers
  • The substitution cipher substitutes one letter for another throughout the plaintext. The key consists of assignment of letters to their replacement.
  • Substitution is a symmetric cipher, meaning both the sender and receiver have the same key. This means that the security relies on another method of securely transmitting the key.

Here is python code for the substitution cipher.

# A key, which could be randomly generated using
# >>> x = list("abcdefghijklmnopqrstuvwxyz")
# >>> random.shuffle(x)
# >>> print x
key = "rbtvpjlwyaigusxzcdmnofhekq"

# The message to encrypt
plaintext = "Hide the gold in the tree stump."
# Replace spaces by x and switch to lower case
plaintext = plaintext.replace(" ", "x").lower()

alphabet = "abcdefghijklmnopqrstuvwxyz"
ciphertext = ""

for char in plaintext:
    # the variable char is one of the letters in the plaintext
    # First, check if char is an alphabet character
    if char in alphabet:
        # Lookup the index of char within alphabet.  This turns "a"
        # into 0, "b" into 1, "c" into 2, and so on, since the index
        # location of the letter in the alphabet string is its number.
        i = alphabet.index(char)

        # Now access the cooresponding index of key.  So if for
        # example char was "c", i becomes 2 so we use key[2] which
        # happens to be "t".
        ciphertext += key[i]
    else:
        # the character is not an alphabet character like the period, just add it directly.
        ciphertext += char

print(ciphertext)

Exercises

Implement code for decryption of the substitution cipher. You should be able to mostly copy the encryption code with a few modifications. Try it out to make sure it encrypts and decrypts the messages properly.