Problem 20
In the SSL record, there is a field for SSL sequence numbers. True or False?
Problem 21
Suppose Alice and Bob are communicating over an SSL session. Suppose an attacker, who does not have any of the shared keys, inserts a bogus TCP segment into a packet stream with correct TCP checksum and sequence numbers (and correct IP addresses and port numbers). Will SSL at the receiving side accept the bogus packet and pass the payload to the receiving application? Why or why not?
Problem 21
What is the purpose of the random nonces in the SSL handshake?
Problem 22
Suppose an SSL session employs a block cipher with CBC. True or False: The server sends to the client the IV in the clear?
Problem 23
Suppose Bob initiates a TCP connection to Trudy who is pretending to be Alice. During the handshake, Trudy sends Bob Alice’s certificate. In what step of the SSL handshake algorithm will Bob discover that he is not communicating with Alice?
Problem 24
Consider the following pseudo-WEP protocol. The key is 4 bits and the IV is 2 bits. The IV is appended to the end of the key when generating the keystream. Suppose that the shared secret key is 1010. The keystreams for the four possible inputs are as follows: 101000: 0010101101010101001011010100100 . . . 101001: 1010011011001010110100100101101 . . . 101010: 0001101000111100010100101001111 . . . 101011: 1111101010000000101010100010111 . . . Suppose all messages are 8-bits long. Suppose the ICV (integrity check) is 4-bits long, and is calculated by XOR-ing the first 4 bits of data with the last 4 bits of data. Suppose the pseudo-WEP packet consists of three fields: first the IV field, then the message field, and last the ICV field, with some of these fields encrypted. a. We want to send the message m = 10100000 using the IV = 11 and using WEP. What will be the values in the three WEP fields? b. Show that when the receiver decrypts the WEP packet, it recovers the message and the ICV. c. Suppose Trudy intercepts a WEP packet (not necessarily with the IV = 11) and wants to modify it before forwarding it to the receiver. Suppose Trudy flips the first ICV bit. Assuming that Trudy does not know the keystreams for any of the IVs, what other bit(s) must Trudy also flip so that the received packet passes the ICV check? d. Justify your answer by modifying the bits in the WEP packet in part (a), decrypting the resulting packet, and verifying the integrity check.
Problem 25
Provide a filter table and a connection table for a stateful firewall that is as restrictive as possible but accomplishes the following: a. Allows all internal users to establish Telnet sessions with external hosts. b. Allows external users to surf the company Web site at 222.22.0.12. c. But otherwise blocks all inbound and outbound traffic. The internal network is 222.22/16. In your solution, suppose that the connection table is currently caching three connections, all from inside to outside. You’ll need to invent appropriate IP addresses and port numbers.
Problem 26
An IKE SA and an IPsec SA are the same thing. True or False?
Problem 26
Suppose Alice wants to visit the Web site activist.com using a TOR-like service. This service uses two non-colluding proxy servers, Proxy1 and Proxy2. Alice first obtains the certificates (each containing a public key) for Proxy1 and Proxy2 from some central server. Denote K1 +( ), K2 +( ), K1 –( ), and K2 –( ) for the encryption/decryption with public and private RSA keys. a. Using a timing diagram, provide a protocol (as simple as possible) that enables Alice to establish a shared session key S1 with Proxy1. Denote S1(m) for encryption/decryption of data m with the shared key S1. b. Using a timing diagram, provide a protocol (as simple as possible) that allows Alice to establish a shared session key S2 with Proxy2 without revealing her IP address to Proxy2. c. Assume now that shared keys S1 and S2 are now established. Using a timing diagram, provide a protocol (as simple as possible and not using public-key cryptography) that allows Alice to request an html page from activist.com without revealing her IP address to Proxy2 and without revealing to Proxy1 which site she is visiting. Your diagram should end with an HTTP request arriving at activist.com.
Problem 28
In WEP, an IV is sent in the clear in every frame. True or False?
