2.6. Assume we have a stream cipher whose period is quite short. We happen to kn
ID: 3886533 • Letter: 2
Question
2.6. Assume we have a stream cipher whose period is quite short. We happen to know that the period is 150–200 bit in length. We assume that we do not know anything else about the internals of the stream cipher. In particular, we should not assume that it is a simple LFSR. For simplicity, assume that English text in ASCII format is being encrypted. Describe in detail how such a cipher can be attacked. Specify exactly what Oscar has to know in terms of plaintext/ciphertext, and how he can decrypt all ciphertext.
Explanation / Answer
A plaintext or ciphertext attack can be done using a ciphertext-only Attack, which is a method of cryptanalysis, during which the attacker gets the access to a few ciphertexts present. The attacker does not necessarily have the plaintext present with him/her. the attacker has no idea about the data present in the plain text prior the plain text being encrypted. If the data in the plaintext is figured out by the attacker, using math or dedective reasoning, then the cipher onyl attackes can be used on the system. This is what oscar should know.
To decrypt all ciphertext, oscar should know about Encryption, decryption, encryption and decryption of the same function, modulo addition, nature of the ciphertext and streamed bits. The most important part is that oscar should understand the key stream, as the cippher completely depends on the key stream. with the help of the key stream, oscar does not have to guess the bit and decrypt the information but rather use the key stream to deduce and directly decrypt the data into a plaintext.
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