ECE 646 – Fall 2010 Multiple-choice test
1. (0.5 pt) Arrange the following ciphers in the order of the increasing measure of roughness for the ciphertext obtained by encrypting 1000-letter message with a given cipher (start from the cipher giving the smallest value of the measure of roughness; group together, using an “=” sign, ciphers for which the measure of roughness is the same or very close to each other, e.g. A=B=C, D, E=F, G)
A. running key cipher B. transposition cipher C. general monoalphabetic cipher D. shift cipher E. Enigma F. Vigenère cipher with the period d=5 G. general polyalphabetic cipher with the period d=3 H. letter-based Vernam cipher with the encryption described by the equation ci = mi + ki mod 26, where mi is a code of the plaintext letter, ci is a code of the ciphertext letter, and ki is a code of the keystream letter; the keystream is generated at random and can be used only once.
2. (0.5 pt) In the ciphertext obtained using the Vigenère cipher, a four-character sequence RFW appears at positions 31, 136, and 283. Additionally, the index of coincidence for this ciphertext is equal to 0.044. Based on this information, the most likely period of the Vigenère cipher is: A. 2 B. 3 C. 4 D. 6 E. 7 F. 9 G. 21
3. (0.5 pt) Select an attack type which is the most efficient against a Playfair cipher A. Kasiski’s method B. linear cryptanalysis C. frequency analysis of single ciphertext characters D. method of index of coincidence E. frequency analysis of ciphertext digrams F. exhaustive key search
4. (0.5 pt) The major weaknesses of the inverse CBC mode of DES, for which encryption transformation is given below are that (more than one answer may be correct):
A. decryption is not possible B. IV must be kept secret C. encryption is more time consuming than decryption D. encryption cannot be parallelized E. the same plaintext block is always encrypted to the same ciphertext block F. the mode is not suitable for a fast random read access to the block mj G. exhaustive key search is easier to mount than in the ECB mode H. analysis of the ciphertext may reveal patterns (e.g., repeating blocks) in the plaintext I. decryption circuit takes more area than the encryption circuit
5. (0.5 pt) Match the following ciphers with the number of different round keys they use during encryption A. DESX B. Triple DES with two keys K1 and K2 C. IDEA D. RC5 32/12/16 E. RC5 64/26/32
a. 52 b. 26 c. 16 d. 54 e. 32
6. (0.5 pt) Divide the following transformations performed when exchanging a signed only message using PGP, into those
A. performed only on the sender’s side B. performed only on the receiver’s side C. performed on both sides D. not performed at either side
Transformations: a. generating a random session key b. accessing a private key ring c. decompression using ZIP algorithm d. generating hash value of a message e. accessing a public key ring f. public key transformation determined by a private key of the receiver g. public key transformation determined by a private key of the sender h. verifying certificate of the sender
Short problems
1. (3 pts) Break the affine cipher (i.e., find the key (k1, k2)) based on the knowledge that the cipher encrypts “R” to “I”, and “H” to “C”. Using the obtained key, decrypt the ciphertext: “RFP”.
2. (3 pts) Encrypt the message "BIRDS OF A FEATHER FLOCK TOGETHER" using the Vigenère cipher with the key "ORNITHOLOGIST". Compute the index of coincidence of the obtained ciphertext (you do not have to reduce it to a single number, but you need to replace variables in the I.C. equation with actual numbers). Can the period of this cipher be determined using the Kasiski’s method based on the obtained ciphertext? If so, how?
3. (3 pts) What is the output from the first subround of IDEA (half of the first round) for the following values of inputs and internal keys. X1 = 0010, K1 = 1234 X2 = ABCD, K2 = 5433 X3 = 1234, K3 = 5678 X4 = FFFE, K4 = 0000 Hint: Note that all values can be computed without using a calculator by using the following formula:
ab mod (216+1) = ab mod 216 – ab div 216 (mod 216+1)
4. (3 pts) Suppose the DES Mangler function F mapped every 32-bit input R, regardless of the value of the 48-bit input K, to a) 32-bit string of ones, b) bitwise complement of R. What would be the expression for R16L16 as a function of L0R0 during encryption? What would be the expression for L0R0 as a function of R16L16 during decryption? Hint: You can use the following properties of the xor operation: (A xor B) xor C = A xor (B xor C) A xor A = 0 A xor 0 = A A xor 1 = bitwise complement of A where A, B, C are n-bit strings of bits 0 is an n-bit string of zeros 1 is an n-bit string of ones
5. (3 pt) What is a result of decryption using RC5-16/1/8 (ONE round, word size w=16) for the ciphertext C=AB (where A, B are 16-bit words) assuming the following contents of the internal key array (all in hexadecimal notation): S[0]=0012, S[1]=0123, S[2]=1234, S[3]=2345, A=9ABC, B=ABCD.