PROGRAMME: ICE
COURSE CODE: EIE 524
ASSIGNMENT
REVIEW QUESTIONS
3.1 Most symmetric block encryption algorithms in current use are based on the Feistel block
cipher structure. Therefore, a study of the Feistel structure reveals the principles behind these
more recent ciphers.
3.2 A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. A
block cipher is one in which a block of plaintext is treated as a whole and used to produce a
ciphertext block of equal length.
3.3 If a small block size, such as n = 4, is used, then the system is equivalent to a classical
substitution cipher. For small n, such systems are vulnerable to a statistical analysis of the
plaintext. For a large block size, the size of the key, which is on the order of n ´ 2n, makes the
system impractical.
3.4 In a product cipher, two or more basic ciphers are performed in sequence in such a way that
the final result or product is cryptographically stronger than any of the component ciphers.
3.5 In diffusion, the statistical structure of the plaintext is dissipated into long-range statistics of
the ciphertext. This is achieved by having each plaintext digit affect the value of many ciphertext
digits, which is equivalent to saying that each ciphertext digit is affected by many plaintext
digits. Confusion seeks to make the relationship between the statistics of the ciphertext and the
value of the encryption key as complex as possible, again to thwart attempts to discover the key.
Thus, even if the attacker can get some handle on the statistics of the ciphertext, the way in
which the key was used to produce that ciphertext is so complex as to make it difficult to deduce
the key. This is achieved by the use of a complex substitution algorithm.
3.6 Block size: Larger block sizes mean greater security (all other things being equal) but
reduced encryption/decryption speed.
Key size: Larger key size means greater security but may decrease encryption/decryption speed.
Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate
security but that multiple rounds offer increasing security.
Subkey generation algorithm: Greater complexity in this algorithm should lead to greater
difficulty of cryptanalysis.
Round function: Again, greater complexity generally means greater resistance to cryptanalysis.
COURSE CODE: EIE 524
ASSIGNMENT
REVIEW QUESTIONS
3.1 Most symmetric block encryption algorithms in current use are based on the Feistel block
cipher structure. Therefore, a study of the Feistel structure reveals the principles behind these
more recent ciphers.
3.2 A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. A
block cipher is one in which a block of plaintext is treated as a whole and used to produce a
ciphertext block of equal length.
3.3 If a small block size, such as n = 4, is used, then the system is equivalent to a classical
substitution cipher. For small n, such systems are vulnerable to a statistical analysis of the
plaintext. For a large block size, the size of the key, which is on the order of n ´ 2n, makes the
system impractical.
3.4 In a product cipher, two or more basic ciphers are performed in sequence in such a way that
the final result or product is cryptographically stronger than any of the component ciphers.
3.5 In diffusion, the statistical structure of the plaintext is dissipated into long-range statistics of
the ciphertext. This is achieved by having each plaintext digit affect the value of many ciphertext
digits, which is equivalent to saying that each ciphertext digit is affected by many plaintext
digits. Confusion seeks to make the relationship between the statistics of the ciphertext and the
value of the encryption key as complex as possible, again to thwart attempts to discover the key.
Thus, even if the attacker can get some handle on the statistics of the ciphertext, the way in
which the key was used to produce that ciphertext is so complex as to make it difficult to deduce
the key. This is achieved by the use of a complex substitution algorithm.
3.6 Block size: Larger block sizes mean greater security (all other things being equal) but
reduced encryption/decryption speed.
Key size: Larger key size means greater security but may decrease encryption/decryption speed.
Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate
security but that multiple rounds offer increasing security.
Subkey generation algorithm: Greater complexity in this algorithm should lead to greater
difficulty of cryptanalysis.
Round function: Again, greater complexity generally means greater resistance to cryptanalysis.
