Cryptography Reference
In-Depth Information
3.13. Apply the expansion permutation described on page 119 to the input
(1010).
3.14. Given a symmetric key
k
= (11101000), employ the S-DES key generation
method described on pages 119 and 120 to
k
in order to produce the
subkeys
k
1
and
k
2
.
(
Hint: You should get
k
1
= (10000100)
and
k
2
= (01010110)
.
)
3.15. Apply the S-Boxes
S
0
and
S
1
to the input (1110).
(
Hint: You should get
S
0
(1110) = (11)
and
S
1
(1110) = (00)
.
)
3.16. Given
SK
= (01010110) and
t
= (11111011), compute
f
SK
(
t
), the S-DES
round function described on page 121.
(
Hint: The end result is
f
SK
(
t
) = (11011011)
.
)
3.17. Given input
m
= (01110111) and key
k
= (11101000), use the S-DES
encryption steps outlined on page 122 to find the ciphertext. Then use
the S-DES decryption steps outlined on pages 122 and 123 to verify that
your answer is correct.
(
Hint: You have already calculated
k
1
and
k
2
in Exercise 3.14.
)
3.18. Prove the DES complementation property highlighted on page 127.
(
Hint: Complementation does not affect out modulo
2
addition, namely,
c
(
x
)
⊕
c
(
y
)=
x
⊕
y
.
)
3.19. Verify that the deciphering method for CBC mode described on page 134
actually recovers the plaintext.
3.20. Show that for CFB mode described on page 135, the following decryption
method will recover the plaintext:
m
j
=
c
j
⊕
E
k
(
C
j
−
1
).
3.21. Verify that the CTR random access property, highlighted on page 137,
is indeed valid.
3.22. Show that Blowfish decryption, summarized on page 141, is indeed the
inverse of the algorithm's encryption technique, summarized on page 140.
3.23. Compare DES and AES from the perspective of their round functions.
Also, compare them with respect to the use of confusion and diffusion.
3.24. Given a key for a one-time pad:
k
= (1110001100100000111100001110001100100000011111000011100)
and ciphertext
c
= (0100011110101000000101100100110000101110000101001011000)
,
find the plaintext string.
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