MIS 4850 Systems Security
Encryption Exercises
April 1, 2015
Student name: ______________________________________________
1) For their private communications, Bill and Joe use a 16-bit key for symmetric encryption. A cryptanalyst has
captured a large amount of ciphertext being sent to Joe.
2.1. What is the maximum number of keys that the cryptanalyst using the correct algorithm will have to
try in order to decrypt the ciphertext and crack the key?
Answer:
2.2. The cryptanalyst was able to decrypt the cipher and crack the key in 5000 tries. How can you explain
this?
2) Computers’ processing power is used to crack encryption keys. Today, the maximum processing speed of
microprocessors (based on clock rate and number of circuits) is about 3.5 GHz, and it is increasing by 30%
roughly every year. A symmetric session key needs to be, at least, 100 bits long to be considered strong
today. It will take processors that have 9 GHz speed to crack such a key.
a. How long, in terms of years, it will take until microcomputers are capable of cracking a 100 bit
symmetric key. Explain your answer.
b. A decision is made to increase the length of a 100-bit symmetric key by one bit. What microprocessor
speed will be required to break the key? Explain your answer.
c.
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3) A friend of yours who is running a small business wants to use symmetric encryption for confidentiality to
protect communications with his business partners. As a security expert you told him that, given the nature of
the transactions and the kind of risk involved, a 112-bit encryption key may be enough to protect the
communications. But because he is a high-risk aversion person, he wants to use a 256-bit key. Explain what
could be a downside of using a 256-bit key symmetric for encryption where a 112-bit is strong enough to
provide protection.
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