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Today, processors are fast, so software-based crypto is usually more than fast enough. Future of stream ciphers? Shamir declared “the death of stream ciphers” May be greatly exaggerated… Block Ciphers (Iterated) Block Cipher Plaintext and ciphertext consist of fixed-sized blocks Ciphertext obtained from plaintext by iterating a round function Input to round function consists of key and output of previous round Usually implemented in software Feistel Cipher: Encryption Feistel cipher is a type of block cipher Not a specific block cipher Split plaintext block into left and right halves: P = (L0, R0) For each round i = 1, 2,..., n, compute Li = Ri−1 Ri = Li−1 F(Ri−1, Ki) where F is round function and Ki is subkey Ciphertext: C = (Ln, Rn) Feistel Cipher: Decryption Start with ciphertext C = (Ln, Rn) For each round i = n, n−1, …, 1, compute Ri−1 = Li Li−1 = Ri F(Ri−1, Ki) where F is round function and Ki is subkey Plaintext: P = (L0, R0) Decryption works for any function F But only secure for certain functions F Data Encryption Standard DES developed in 1970’s Based on IBM’s Lucifer cipher DES was U.S.
Explain the steps involved in a known plaintext attack using a pre-computed table.
Pre-compute table of E(P,K1) for every possible key K1, then for each possible K2 compute D(C,K2) until a match in table is found.
What algorithm was ultimately selected as the replacement for DES in the AES competition?
Rijndael Algorithm
What are the key lengths available in the Advanced Encryption Standard (AES)?
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Explain the role of the ByteSub layer in AES.
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What operation is performed by the MixColumn layer in AES?
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How is the AddRoundKey layer implemented in AES?
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What is the main challenge in decrypting data encrypted with the Tiny Encryption Algorithm (TEA)?
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Explain the process involved in TEA encryption.
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How is decryption handled in a Feistel cipher?
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What was DES based on and when was it developed?
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