When a stream cipher is referred to a synchronous, what does this mean? 2. How d

Question

When a stream cipher is referred to a synchronous, what does this mean? 2. How do the encryption and decryption operations differ in a binary additive synchronous stream cipher? What is the implementation advantage associated with this? For a Linear Feedback Shift Register (LFSR) of length L, a. What is maximum period of the LFSR output sequence? b. What is required to obtain a sequence of maximal length? c. What is the linear complexity of a maximal length LFSR output sequence? Describe two methods to produce a binary output sequence using LFSRs with increased linear complexity. Consider the LFSR shown in the diagram above: a. Using the initial state indicated, produce the first 40 terms of the LFSR output sequence. b. What is the period of the LFSR output sequence? c. Is this maximal?

Explanation / Answer

1)Stream cipher is referred to as synchronous only when if the keystream does not depend on the key alone.Because in synchronous stream cipher,the keys generated by the sender and receiver not only compatible but also be perfectly synchronized otherwise it will fail

2)Both the operations differ by different mode of text i.e plaintext for encrytpion and ciphertext for decryption.A stream of pseudo-random digits which are generated are combined with the plaintext for encrytption using the XOR operation. And, similarly with ciphertext for decryption.
Advantage :- Synchronous nature of data is maintained plus any data which is corrupted or damaged can be spotted out easily

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