For Monday, November 5

Read Denning, pp. 285-300.

Consider the following cryptosystem described in class. Suppose Alice wants to send 01000001 to Bob, she and Bob follow these steps:

• Alice uses her own secret key (that she shares with no one), let's say: 10111001 and encrypts the plaintext using XOR:

    01000001      
    10111001
    --------
    11111000
  

  and sends the result, 11111000 to Bob.

• Bob receives 11111000 and encrypts it again using his own secret key (that he shares with no one), let's say: 11001010:

    11111000
    11001010
    --------
    00110010
  

  He sends the result, 00110010 back to Alice.

• Alice receives 00110010 and decrypts it using her key (which means using XOR again):

    00110010
    10111001
    --------
    10001011
  

  and send the result, 10001011 back to Bob.

• Finally, Bob receives 10001011 and decrypts it using his key to recover the original plaintext:

    10001011
    11001010
    --------
    01000001
  

1. This system looks good - Alice and Bob never share their keys with anyone (thereby solving the key distribution problem) and they use one-time pads which offer perfect security. But there is a problem! Explain, in a clear and concise paragraph why this system is not secure.