# 6 (a) Explain how you would quickly determine if a piece of cipherte

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6 (a) Explain how you would quickly determine if a piece of
ciphertext was likely to be the result of applying
transposition.(2 marks)
(b) Given the speed of a current ordinary computer, how
would you estimate the amount of time necessary to crack
a DES encryption by brute force? (4 marks)
(c) Why is it recommended to change the encryption key
from time to time? How often should a key be changed? (4
marks)
Solution
(a)
One plaintext symbol can convert to several ciphertext
symbols, or vice versa. For example, Morse code is a form
of substitution of alphabetic letters to dots and dashes.
Two plaintext characters could map the same ciphertext
character as long as the recipient could distinguish
between the two. Each cipher contributes its own strength,
so ideally the strength of the product is at least the product
of the strengths of the input ciphers. A substitution cipher
contributes confusion, whereas a transposition performs
diffusion. The DES and AES algorithms both use a
combination of relatively simple functions. Obviously,

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however, just composing two ciphers is not guaranteed to
result in a stronger combination
To test a piece of ciphertext to determine quickly if it was
likely the result of a transposition by Letter frequency
count.
(b)Let us assume that speed of a computer that should be
estimated as,the household computer has a 2GHZ
processor. Also we assumethat a machine takes a hundred
cycles per brute force against a single 56-bit DES keyor
128 bit AES key.
To crack a DES encryption, we need:
(2^56 key)*100
cycles/60sec/60min/24hour/365days/2000000000hz =
114.246566
years
To crack a AES encryption, we need:
(2^128 key)*100
cycles/60sec/60min/24hour/365days/2000000000hz =
5.39514154 ×
1023 years
(c) why to change the encryption key from time to time
Sensitive data (such as credit card numbers) that must be
stored to the database are protected with Advanced
Encryption Standard (AES) cryptography. AES is a keyed
encryption we need a secret password to encrypt and

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6 (a) Explain how you would quickly determine if a piece of ciphertext was likely to be the result of applying transposition.(2 marks) (b) Given the speed of a current ordinary computer, how would you estimate the amount of time necessary to crack a DES encryption by brute force? (4 marks) (c) Why is it recommended to change the encryption key from time to time? How often should a key be changed? (4 marks) Solution (a) One plaintext symbol can convert to several ciphertext symbols, or vice versa. For example, Morse code is a form of substitution of alphabetic letters to dots and dashes. Two plaintext characters could map the same ciphertext character as long as the recipient could distinguish between the two. Each cipher contributes its own streng th, so ideally the strength of the product is at least the product of the strengths of the input ciphers. A substitution cipher contributes confusion, whereas a transposition performs diffusion. The DES and AES algorithms both use a combination of relatively simple functions. Obviously, however, just composing two ciphers is not guaranteed to result in a stronger combination To test a piece of ciphertext to determine quickly if it was ...
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