**Unformatted text preview: **or not, and prove your answer . In each case, say what the space C oF the ciphertexts is, and note the sizes oF the message space M and key space K . Do these sizes correlate somehow with whether or not the cipher is secure? Can you explain why? 1. Let M = S ℓ and K = { , 1 } 2 ℓ , i.e. both the message and the key are (2 ℓ )-long bit strings 1 However, not every (2 ℓ )-bit string can be a valid message. ±or example, For ℓ = 3, we could have m = [00 , 01 , 00] = 000100 but m = [11 , 00 , 11] = 110011 is not in M because 11 n∈ S . 2. Let M = { , 1 } 2 ℓ and K = S ℓ 3. Let M = K = S ℓ . [[Hint: This one is actually not perfectly secure. ..]] 1 We use notation A n to denote a set of n-long sequences [ A 1 , A 2 , ..., A n ] where each A i is an element of A . Using this notation, { , 1 } n denotes a set of all n-long binary strings. H1-1...

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- Spring '04
- Jarecki
- Cryptography, Substitution cipher, OTP cipher, OTP cipher variations