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Which of the following is not an attribute of a good stream cipher?

There are two main types of symmetric ciphers: stream and block. Which of the following is not an attribute of a good stream cipher?

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A.
Statistically unbiased keystream

B.
Statistically predictable

C.
Long periods of no repeating patterns

D.
Keystream not linearly related to key

Explanation:
B: The two main types of symmetric algorithms are block ciphers and stream ciphers. A block cipher performs mathematical functions on blocks of bits at a time. A stream cipher, on
the other hand, does not divide a message ilnto blocks. Instead, a stream cipher treats the message as a stream of bits and performs mathematical functions on each bit individually.
Good stream ciphers offer the following: unpredictable statistical results, long periods of no repeating patterns, a statistically unbiased keystream, and a keystream that is not linearly
related to the key. If a stream cipher is statistically predictable, then it will be possible for an attacker to uncover the key and break the cipher.
A is incorrect because a statistically unbiased keystream is an attribute of a good stream cipher. A statistically unbiased keystream means that there are as many zeros as there are
ones. There should be no dominance in the number of zeros or ones in the keystream.
C is incorrect because long periods of no repeating patterns within keystream values is a characteristic of a good stream cipher. The ultimate goal of any encryption is to provide a
high level of randomness so that an attacker cannot reverse engineer and uncover the key that was used during the encryption process.
D is incorrect because a keystream not linearly related to a key is an attribute of a good stream cipher. This means that if someone figures out the keystream values, that does not
mean he now knows the key value. This is important because the key provides the randomness of the encryption process. Most encryption algorithms are public, so people know how
they work. The secret to the secret sauce is the key. The key provides randomness, so that the stream of bits that are XORed to the plaintext are as random as possible

One Comment on “Which of the following is not an attribute of a good stream cipher?

  1. Joe says:

    Good stream ciphers offer the following: unpredictable statistical results, long periods of no repeating patterns, a statistically unbiased keystream, and a keystream that is not linearly related to the key.

    If a stream cipher is statistically predictable, then it will be possible for an attacker to uncover the key and break the cipher.




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