文档介绍：曹天杰
Tianjie Cao
******@cumt.
College puter Science and
Technology, China University of Mining and Technology, Xuzhou, China
中国矿业大学计算机科学与技术学院
Block ciphers Linear and Differential Cryptanalysis
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Block cipher
Definition An n-bit block cipher is a function E : VnKVn, such that for each key K K, E(P;K) is an invertible mapping (the encryption function for K) from Vn to Vn, written EK(P). The inverse mapping is the decryption function, denoted DK(C). P denotes that ciphertext results from encrypting plaintext P under K.
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Iterating Block ciphers
Definition A product bines two or more transformations in a manner intending that the resulting cipher is more secure than the ponents.
Definition An iterated block cipher is a block cipher involving the sequential repetition of an internal function called a round function. Parameters include the number of rounds Nr, the block bitsize n, and the bitsize k of the input key K from which Nr subkeys Ki (round keys) are derived. For invertibility (allowing unique decryption), for each value Ki the round function is a bijection on the round input.
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Iterating Block ciphers
1. Iterated block cipher
Random (binary) key K round keys: K1,..., KNr,
2. Round function g
w r = g(w r-1, K r),
where w r-1 is the previous state
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Iterated cipher …
Encryption operation:
w0 x
w1 = g(w0, K1),
w2 = g(w1, K2),
wNr = g(wNr-1, KNr),
y wNr
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Iterated cipher …
For decryption we must have:
g(.,K) must be invertible for all K
Then decryption is the reverse of encryption
(bottom-up)
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Diffusion and Confusion -- Shannon
Diffusion. The relationship between the statistics of the plaintext and the ciphertext is plex as possible: the value of each plaintext bit affects many ciphertext bits.
Confusion: the relationship between the statistics of the plaintext and the value of the key is plex as possible.
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
DEFGHIJKLMNOPQRSTUVWXYZABC
Substitution Table - Caesar‘s Cipher
Shannon‘s P