The Content Scrambling System is a protocol which was used for digital rights ma

Question

The Content Scrambling System is a protocol which was used for digital rights management for DVDs. Its design consists of a synchronous stream cipher built from two LFSRs, one 17 bits long and one 25 bits long The 40-bit key is loaded directly into the LFSRs, 2 bytes into the first LFSR and 3 bytes into the second LFSR with the remaining bits in each set to 1. The outputs of the LFSRs are then combined to produce the keystream (Note: I've left out some details which are not important for this discussion.) a. What is the maximum possible period of each LFSR? (1 mark) b. If we have two periodic functions f(t) and g(t) and consider the function h(t) (f(t)g(t)), then h will have period pq ged(p, q) where p and q are the period of f and g, respectively, and gcd is the greatest common divisor. Apply this principle to explain why CSS was chosen to have different lengths for the two LFSRs. (1 mark) c. What is the brute force attack complexity for this cipher? (1 mark) d. Imagine a he same method of loading the key and simply XORs the outputs similar cipher which uses t of each LFSR to produce the keystream. Explain how this fictitious cipher leaks easy information about the key into the keystream (1 mark) and suggest a known plaintext attack that retrieves the key with complexity around 221 or less (1 mark) e. CSS was an epic security fail. Do some reading and write one paragraph about the design decisions which led to its eventual break. (1 mark)

Explanation / Answer

Answer for a :

Since we have reached the initial state again, this LFSR produces a sequence with period 7. There are 2n possible states, but the all zero state cannot be achieved unless you start with it, so there are 2n - 1 possible states, so this is the maximum possible period.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.