Lecture Notes on Basic Cryptography for CPSC 121 (Fall 2011), Study notes of Computer Science

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Today ...

• Quiz 9
• Basic cryptography (cont.)

Basic Cryptography Terms

A plaintext message is unencrypted (readable)

A ciphertext message is encrypted (unreadable)

Encryption converts plaintext to ciphertext

Decryption converts ciphertext to plaintext

def shift_encrypt(message, shift): cipher = "" for c in message: num = ord(c) if 32 <= num <= 126: if num + shift > 126: cipher += chr(31 + num + shift - 126) else: cipher += chr(num + shift) else: cipher += c return cipher

• Note we shift from 31 instead of 32
  • e.g., so that 126 ("~") + 1 gives 32 (" ")

Exercise

Q: Can we use shift encrypt to decrypt the message?

• No ... unless shift = 79

Write a shift decrypt(message, shift) function

• Takes a ciphertext message and a shift number, eturns plaintext message

def shift_decrypt(message, shift): text = "" for ch in message: num = ord(ch) if 32 <= num <= 126: if num - shift < 32: text += chr(127 - (32 - (num - shift))) else: text += chr(num - shift) else: text += ch return text

• You can also cheat ...

def shift_decrypt(message, shift): return shift_encrypt(message, 95 - shift)

• where shifting by 95 shifts to itself
• all assume shift ≤ 95 (get around this by shift = shift % 95)

Exercise

Write a railfence encrypt(message) function

• Hint: Use a range loop over the length of the message
• You’ll need to also use the subscript operator on message

def railfence_encrypt(message): cipher1 = "" cipher2 = "" for i in range(len(message)): if i % 2 == 0: cipher1 += message[i] else: cipher2 += message[i] return cipher1 + cipher

• Note that it is easy to use more rails ...