CIS 4360 Introduction to Computer Security Fall WITH ANSWERS in bold. First Midterm

Transcription

1 CIS 4360 Introduction to Computer Security Fall 2010 WITH ANSWERS in bold Name: Number: First Midterm Instructions This is a closed-book examination. Maximum score 100 pts. There are 5 questions, each is weighted 20pts. You have 60 minutes. Q Q Q Q Q Total

2 1. This question concerns the basic protection mechanisms used in general purpose OS and attacks on passwords =20 pts (a) What is the Fundamental Dilemma of Computer Security. Functionality or Assurance: which one? (b) What are the five layers of an IT system? Application Services OS (Operating System) OS kernel Hardware (c) In a spoofing attack on passwords a user is fooled into believing that she/he is connected to a remote (imposter) site and reveals her/his user-name and password to the attacker. (d) List some advantages and disadvantages of using a cryptographically protected multiplepasswords file that can be accessed with a master-password. i. Advantages: Flexibility and security: ease of use & different passwords are used for different applications ii. Disadvantages: Single-point-of-failure: insecure if the master-password gets compromised (e) What is a brute force attack? A brute force attack or exhaustive key search is a strategy in which, systematically, all possible keys until the correct key is found. In the worst case, this would involve exhausting the entire search space. 2

3 2. This question concerns the protection mechanisms in general purpose Operating Systems =20 pts (a) Entity authentication is the process of verifying the identity of an entity. (b) Entity authentication is based on: i. Something the entity knows ii. Something the entity has iii. Something the entity does iv. Something the entity is v. the location of the entity (c) Short length passwords are insecure because they are subject to (d) with biometrics a stored pattern is compared to a taken measurement. i. What is a false positive? A pattern mismatch that is accepted. ii. What is a false negative? A pattern matching that is rejected. (e) List three defenses used for passwords. i. Use password checkers ii. Use password generators iii. Use password ageing dictionary attacks. 3

4 3. This question concerns the basic protection mechanisms used in general purpose OS and the basics of Cryptography =20 pts (a) Protecting the password file: To validate passwords the system compares the password entered against a value stored in the password file. There are two basic defenses. List them: i. cryptographic protection, ii. access control enforced by the OS. (b) One-way hash functions f are a cryptographic mechanism that may be used to protect the password file. How is this done? Answer: Instead of storing the password in the password list, the hash f(password) is stored. The password list is organized as a two column table of user IDs (usernames) and the corresponding hashed values f(password). When the user logs in and enters the password, this is hashed (locally) into f(password). This value is then compared with the stored value: if there is a match the client is given access to the username account. (c) Dictionary attacks can be prevented by using password salting. Describe how password salting is applied. Answer: Password salt consists of some random bits or a nonce (a number or bit string used only once). When salting a password additional information (the salt) is appended to it before it is hashed to get f(password salt). This implies that even if two users have the same password their salted hashes will be different. (d) What is Kerchoffs assumption? The adversary knows all details of the encryption algorithm except the secret key (e) What is symmetric key encryption? The encryption and decyption keys are the same (f) There are four basic modes of operation available for block ciphers. List two of them:: Electronic codebook mode: ECB Cipher block chaining mode: CBC Cipher feedback mode: CFB Output feedback mode: OFB 4

5 4. This question concerns Ceasar s cipher and perfect secrecy =20 pts (a) Ceasar s cipher is a substitution cipher in which each one of the 26 letters of the alphabet is substituted by a letter obtained by shifted it K places forward, where K is the key. For example, if the key is K = 3, then the word CEASAR is encrypted as: F HDV DU because if we shift the letters of the alphabet three places forward we get: C D E F E F G H A B C D S T U V A B C D and R S T U Letters are rolled over when reaching Z. So Y Z A B. What is the plaintext if the ciphertext is: (Hint: Shift all the letters forward one-at-a-time) Answer: O C D N O Z N O D N Z V N T P D E O P A O P E O A W O U Q E F P Q B P Q F P B X P V R F G Q R C Q R G Q C Y Q W S G H R S D R S H R D Z R X T H I S T E S T I S E A S Y (b) This question concerns the one-time-pad. Let the plaintext be x and the ciphertext y. i. TRUE OR FALSE: x is a random bit string. False ii. TRUE OR FALSE: y is a random bit string. True iii. Suppose x = and y = What is the key k. k = iv. TRUE OR FALSE: The key k is a random bit string. True v. What are: A. The strengths of the one-time-pad: Offers perfect secrecy: the adversary cannot do better than guessing. B. The weaknesses of the one-time pad: Very inefficient: a different random key is used for each encryption and the key length is equal to the length of the plaintext. 5

6 5. This question concerns the Data Encryption Standard, AES, and the one-time-pad =20 pts (a) DES is an iterated cipher with parameters: Block length: 64 bits Key length: 56 bits Number of rounds: 16. (b) Describe Triple DES: its mode of operation, and the size of the required (secret) key? Mode of operation: E K1 D K2 E K1. That is: first DES-encrypt with key K 1, then decrypt with key K 2 and finally encrypt with key K 1. Size of secret key: 112 = (c) There are several major attacks on DES. List two generic attacks. Linear Cryptanalysis Differential Cryptanalysis. (d) Is there an attack on DES that is a significant improvement on brute force? No. (e) AES is a acronym for a cipher system: what is the full name: Advanced Encryption Standard. AES is an iterated cipher, and comes in several versions. List the parameters of the basic version: Block length: 128 bits Key length: 128 bits. Number of rounds: 10. (f) Give an example of a one-time-pad encryption and decryption using the plaintext m = and key k = m = k = c = (encryption) m = (decryption) Mike Burmester 6