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25-26 IY2760/DC3760: Introduction to Information Security
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Pick all the correct statements. Marks may be deducted for wrong answers.
A secure stream cipher should have keystreams with large linear equivalence.
Suppose you intercepted 8 bits of ciphertext: .
Suppose you know that this ciphertext is obtained by encrypting some plaintext using a stream cipher, and suppose you know that the keystream has period 4. Suppose further that you managed to obtain some bits of the plaintext:
What can you say about the keystream?
This is a model of stream cipher. X is the input to a keystream generator, Y is the output. Y and W are inputs to an XOR operator, and Z is the output. What is X?
Consider the following cryptosystem:
Let the plaintext and ciphertext space be . The secret key is , where each for .
To encrypt a string of plaintext symbols using key , we perform the following operation to obtain ciphertext :
, if ;
, if ;
, if .
In other words, we add (mod 5) to the first position of the plaintext, to the second position of the plaintext, to the third position of the plaintext, and continue with adding to the fourth position of the plaintext, to the fifth position of the plaintext and so forth.
Suppose you intercepted the ciphertext 0 1 4 4 0 0 3 2 1, and you know that the plaintext in positions 4, 8 and 9 are 3, 4, 4, that is, the plaintext is 3 , 4, 4, where indicates that these symbols are unknown.
Determine the key.
The following substitution cipher is to be used to encrypt and decrypt some plaintext and ciphertext:
| Plaintext | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ciphertext | L | Y | N | B | R | G | M | T | Z | S | C | O | A | W | K | F | I | X | P | V | D | Q | U | H | J | E |
Decrypt the ciphertext RLPJ.
Enter your answer without spacing.
Match the types of attack to the amount of information the adversary Oscar has.
A block cipher uses binary plaintext blocks of length 64 and binary keys of length 128. What is the size of the key space? (Marks may be deducted for a wrong choice.)
An encryption algorithm outputs a ciphertext. What is/are the input(s)? Pick all the correct answers. (Marks may be deducted for wrong answers.)
Pick all the descriptions of active attacks. (Marks may be deducted for wrong choices.)