All of the following are properties of RC5 except: Select one: A. Adaptability to processors of different word lengths B. Simplicity in the operation of the algorithm C. Relatively high memory requirements D. Variable-length of the key
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RC4 is a stream cipher that was widely used for more than thirty years (including TLS). Recently, it was proved that there is some bias in the output, and it is not recommended anymore. However, it has a very simple description and has good properties. It can be interpreted as a (vulnerable) pseudorandom function. We see it next. It is convenient to describe it in terms of integers. The algorithm has an initial state S = (S0, . . . , S255) ∈ {0, . . . , 255}256 that is secret, and evolves during the execution. The initial state is private (it can be seen as the input of the random function) and has the property that Si ≠ Sj for i ≠ j. That is, S represents a permutation of {0, . . . , 255}. There is an initial preprocessing of the state (omitted here) with the initialisation i = 0 and j = 0. The output is generated by iterating of the following algorithm, obtaining one bit b for each iteration: i ← (i + 1) mod 256, j ← (j + Si) mod 256, swap(Si, Sj), t ← (Si + Sj) mod 256, b ← St. Consider now the functions F that are variants of RC4 with S = (S0, . . . , Sn−1) ∈ {0, . . . , n − 1}n for some n defined as follows: (a) Let F be the function generated by the following algorithm: i ← (i + 1) mod n, b ← Si. Prove that F is not a pseudorandom. (b) Let F be the function generated by the following algorithm: i ← (i + 1) mod n, j ← (j + Si) mod n, swap(Si, Sj), t ← (Si · Sj) mod n, b ← St. Assuming that Si and Sj are uniformly distributed, prove that F is not pseudorandom. (Hint: Study the distribution of the output of Si · Sj mod n. Analyze the multiplication table of Z2l).
Akash M.
Which of the following answers are correct about data encryptions? 1. The cipher block chaining mode is a common method of applying data encryption to a long text message to take care of repeating patterns in the message. 2. 2DES/2 meets the security requirements for encrypting important messages. 3. AES is an implementation of Feistel's cipher scheme. 4. DES allows three different key lengths, namely, 128 bits, 192 bits, and 256 bits. 5. AES operations are finite-field operations on bytes, where each byte represents a polynomial of degree 7, to allow multiplication and inverse operations on bytes.
When using a 128-bit block cipher in CBC mode to encrypt disk sectors, one simple way to generate an initialization vector for each sector would be to take the MD5 hash of the sector number. Is this a good scheme? a. No - an attacker can use a watermarking attack to confirm that you are using this scheme. b. Yes - the MD5 algorithm introduces enough randomness into the IVs. c. No - an attacker can use a watermarking attack to recover the encrypted data. d. No - an attacker can use a birthday paradox attack to recover the IV for half the sectors.
Madhur L.
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