Let the encrypt function of standard DES be C = E(P, K) where C is the...

  

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Let the encrypt function of standard DES be C = E(P, K) where C is the ciphertext, P is the plaintext, and K is the key. Accordingly, let the decrypt function of DES be P = D(C, K). Suppose that Alice uses a customized cipher C" = D(E(P, K₁), K₂) where K₁ and K₂ are two keys, and Alice then sends C" to Bob. Suppose an attacker happens to know the plaintext P, but he does not know the two keys that was used in the cipher. (a) (2pt) What are the sizes of K₁ and K₂? (b) (6pt) Suppose an attacker knows the cipher text C'. Show that the attacker can figure out the two keys. Please elaborate your attack with diagrams. (c) (2pt) How could you modify the customized cipher and improve its security with- out adding another key? Why it becomes more secure? Please explain your answer. Let the encrypt function of standard DES be C = E(P, K) where C is the ciphertext, P is the plaintext, and K is the key. Accordingly, let the decrypt function of DES be P = D(C, K). Suppose that Alice uses a customized cipher C" = D(E(P, K₁), K₂) where K₁ and K₂ are two keys, and Alice then sends C" to Bob. Suppose an attacker happens to know the plaintext P, but he does not know the two keys that was used in the cipher. (a) (2pt) What are the sizes of K₁ and K₂? (b) (6pt) Suppose an attacker knows the cipher text C'. Show that the attacker can figure out the two keys. Please elaborate your attack with diagrams. (c) (2pt) How could you modify the customized cipher and improve its security with- out adding another key? Why it becomes more secure? Please explain your answer.

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a Sizes of K and K The sizes of K and K are both the same as the block size of the DES algorithm DES ... View the full answer