Tài liệu thực hành môn Mật mã và An ninh mạng - Tutorial 2 - Nguyễn Hữu Hiếu
Faculty of Computer Science and Engineering
Ho Chi Minh City University of Technology
Cryptography and Network Security
Tutorial 2
Hieu Nguyen
August 28, 2016
Exercise 1 (2pt). Consider the substitution defined by row 1 of S-box S1 in Table 3.3.
Show a block diagram similar to Figure 3.2 that corresponds to this substitution
Exercise 2 (5pt).
This problem provides a numerical example of encryption using a one round version of
DES. We start with the same bit pattern for the key and the plaintext, namely:
In hexadecimal notation:
0 1 2 3 4 5 6 7 8 9 A B C D E F
In binary:
0000 0001 0010 0011 0100 0101 0110 0111
1000 1001 1010 1011 1100 1101 1110 1111
EXPRESS YOUR ANSWERS IN BINARY NOTATION IN 4-BIT GROUPS WITH
SPACE SEPERATORS (I.E., 0010 1100 1110, ETC.)!
Part a. Derive K1, the first round key.
Part b. Derive L0, R0
Part c. Expand R0 to get E[R0], where E is the expansion function of Table 3.2.
Part d. Calculate A = E[R0] ⊕ K1
Part e. Group the 48-bit result of part d into sets of 6 bits and evaluate the coresponding
S-box substitutions. Express your answers in decimal and binary.
1
Hint: Be sure you count 0, 1, 2, 3, etc for row and column position when doing the S-box
lookup.
Part f. Concatenate the results of part e to get a 32-bit result, B. Express the answer in
binary.
Part g. Apply the permutation to get P(B).
Part h. Calculate R1 = P(B) ⊕ L0
Part i. Write down the cipher text.
Exercise 3 (3pt). Using S-DES, decrypt the string (10100010) using the key (0111111101)
by hand.
Show intermediate results after each function (IP, FK, SW, FK, IP-1). Then decode the
first 4 bits of the plaintext string to a letter and the second 4 bits to another letter where
we encode A through P in base 2 (i.e., A = 0000, B = 0001, ..., P = 1111). Hint: As a
midway check, after the application of SW, the string should be (00010011).
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How DES Works.png
