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# Lecture04 - Block Cipher Principles Two types stream block...

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Block Cipher Principles Two types: stream, block A stream cipher encrypts one bit or byte at a time, often times adapting the encrypting key based upon the previous bit or byte encrypted. A block cipher breaks the plaintext into blocks of equal size and uses the same algorithm to encrypt each block. The total number of reversible mappings between n bit blocks is 2 n ! This is a very, very large number! Notice that if n is small, what we have is something similar to a substitution cipher. However, for large n, (56 for DES), frequency analysis is clearly impossible. Feistel wanted to approximate a large substitution cipher through easier means. (The key for an arbitrary substitution cipher of 56 bits is ridiculously large. This is the block size for DES.) Claude Shannon proposed that a good cipher system should use both diffusion and confusion. Feistel incorporated these ideas. Ideally, the statistics of the ciphertext should be independent of the plaintext language and the key. Diffusion: Have one plaintext character affect many ciphertext characters. Can be achieved by repeated permutations and functions Confusion: reduce the relationship between the ciphertext and the key Basic structure: Plaintext is size 2w. Split into L0, R0. Pass R0 with K1 through a function and then xor it with L0 to create R1. L1 is just R0, and repeat a bunch. Key terms: block size, key size, number of rounds, subkey generation algorithm, round function You want: fast encryption/decryption, ease of analysis Decryption for feistel: Use subkeys in reverse order.

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