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# 密码学代写｜7CCSONCR Cryptography

### 密码学代写｜7CCSONCR Cryptography

1. This question is about cryptography basics.

a. What properties of information security can be guaranteed by
implementing cryptography and which can not be guaranteed?
Give a minimum five examples; and for each example, say
whether it can be guaranteed by implementing cryptography and
explain why this is the case.
[5 marks]

b. Alice uses the same key k to encrypt two messages m1 and m2 to
get ciphertexts ci = E(k;mi) = k ⊕ mi. k, m1 and m2 are the same
m2 and to both ciphertexts c1 and c2. Is it possible for Charlie to
how to reconstruct message m1 with the available information.
[3 marks]

2. This question is about different ciphers.

a. Encrypt the plain text MEET ME AT TRAFALGAR SQUARE using the
Playfair cipher and the keyword MONARCHY. Identify the steps for
encryption. Discuss the strength of this cipher against frequency
attacks.
[6 marks]

b. Decrypt the ciphertext TTROAEEAAKATCTTYTHGD given that you
know that it was encrypted using transposition cipher with a key
31524 (spaces are removed). Identify the cipher type and steps of
analysis. Discuss the strength of the cipher against frequency
attacks.
[6 marks]

3. This question is about the Feistel cipher.

Consider the following scenario:

• a Feistel Cipher consists of 4 rounds (for both encryption and
decryption);
• the plaintext is LE0 and RE0, so LE5||RE5 is the ciphertext output at
the very end of the encryption;
• the key is K1, K2, K3, K4.

For this particular version of Feistel cipher:

i. Write the general equation that defines the input and output
of each encryption round i of the Feistel cipher.
[3 marks]

ii. Draw the diagram of encryption and decryption.
[4 marks]

iii. Identify the encryption round that corresponds to the
decryption round 2, and then prove the equivalence
mathematically. Write down the common equation that
connects each decryption round to the corresponding
encryption round.
[8 marks] 