CPSC 333 Homework #4
Daniel Jordan
daniel_jordan@csu.fullerton.edu
R-8.9
In the inverse of the S-box from figure 8.14, what is the substitution for e3, in hexadecimal?
The hex substitution for e3 is 4d
R-8.12
Bob is arguing that if you use Electronic Codebook (ECB) mode twice in a row to encrypt a long message, M,
using the same key each time, that it will be more secure. Explain why Bob is wrong in the case of using a
binary one-time pad encryption scheme.
By running a message through an ECB twice, using the same key, both cipher blocks will cancel each other out, and
return the original plain text message.
R-8.14
Compute the multiplicative inverse of 5 in Z21
x = 5-1 mod 21
x = 17
R-8.22
Demonstrate that the hash function H(x) = 5x + 11 mod 19 is not weakly collision resistant, for H(4), by
showing how easy it is to find such a collision.
H(4): 5x + 11 mod 19 = 12
Using a simple program, I was able to find collisions at x =23, 42, 61, 80, 99. Finding 5 collisions in x<100 shows
H(4) is not very collision resistant. It also reveals a pattern of finding a collision every 19 numbers
for(var i = 0; i< 100; i++){
var k = (5*i+11)%19;
if(k == 12){
console.log(i + " - " + k);
}
}
R-8.25
Explain the strengths and weaknesses of using symmetric encryption, like AES, versus a public-key
cryptosystem, like RSA
Some strengths of symmetric encryption is that it is much faster, and requires much less processing power than a
public-key system like RSA. A disadvantage of symmetric is that both parties must agree on a key beforehand, to
encrypt and decrypt the message. This is sometimes not feasible to do. Often times, a public-key system like RSA is
used to encrypt an AES key which is used to encrypt the contents of the transmission.
C-8.1
What is the plaintext for the following ciphertext, which was encrypted using a simple substitution cipher:
CJBT COZ NPON ZJV FTTK TWRTUYTFGT NJ DTN O XJL. Y COZ ZJV CPJVIK DTN O XJL
MYUCN
SOME SAY THAT YOU NEED EXPERIENCE TO GET A JOB. I SAY YOU SHOULD GET A JOB FIRST
B (1) - 1.61% - M
C (5) - 8.06%- S
D (2) - 3.23% - G
F (2) - 3.23% - N
G (1) - 1.61% - C
I (1) - 1.61% - L
J (7) - 11.29% - O
K (2) - 3.23% - D
L (2) - 3.23% - B
M (1) - 1.61% - F
N (6) - 9.68% - T
Frequency table generated using program from project 1
O (5) - 8.06% - A
P (2) - 3.23% - H
R (1) - 1.61% - P
T (9) - 14.52% - E
U (2) - 3.23% - R
V (3) - 4.84% - U
W (1) - 1.61% - X
X (2) - 3.23% - J
Y (3) - 4.84% - I
Z (4) - 6.45% - Y