# 1. Altering a text encrypted with an AES key under

## 1. Altering a text encrypted with an AES key under CBC.

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1. Altering a text encrypted with an AES key under CBC. [10%]

You are given an encryption of the message below, encrypted with an AES key (which you are not given) using CBC with PKCS5 padding.

Advance 5km East with all due speed: 7:00 on 2nd

a) Give a ciphertext (encrypted under the same AES key) in which 7:00 has been changed to 8:30 in the plaintext. [4%]

b)State what other parts of the plaintext will change as a result of the change in (a). [2%]

c) Explain why you cannot use the approach used to solve part(a) to change the word speed to the word haste within the plaintext. [4%]

2. RSA [10%] You are given an RSA public key (exponent and modulus), and a plaintext encrypted using the Javax implementation of RSA/ECB/NoPadding with that key.

There are two ways you can approach this problem:

(1) factorise the modulus of the public key to obtain p and q, then compute (p-1)(q-1) and work out the private key from that, to decrypt the ciphertext

(2) Use a dictionary attack (the coursework website provides a dictionary: english.txt)

a) Use one of these methods to obtain the plaintext, and explain why the approach you use is better than the other one. [5%]

b) Explain how you applied your method to this problem. [5%]

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