Tham khảo tài liệu 'hacker professional ebook part 160', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Note that this definition is somewhat broader than the now common understanding of a huge and thus emulated Simple Substitution. But there are ciphers which require blocked plaintext and which do not emulate Simple Substitution and calling these something other than block ciphers negates the advantage of a taxonomy. 1. SUBSTITUTION CIPHER A codebook or simple substitution. Each code value becomes a distinguishable element. Thus substitution generally converts a collection of independent elements to a single related unit. Keying constitutes a permutation or re-arrangement of the fixed set of possible code values. Avalanche or data diffusion is a natural consequence of an arbitrary selection among all possible code values. The usual complete binary substitution distributes bitchanges between code values binomially and this effect can be sampled and examined statistically. Avalanche is two-way diffusion in the sense that later plaintext can change earlier ciphertext. A conventional block cipher is built from small components with a design intended to simulate a substitution table of a size vastly larger than anything which could be practically realized. sr Transposition Cipher Clearly it is necessary for all message elements which will be transposed to be collected before operations begin this is the block cipher signature. Any possible transposition is necessarily a subset of an arbitrary substitution thus transposition can be seen as a particular keying subset of substitution. Notice however that the usual avalanche signature of substitution is not present and of course the actual data values are not changed at all by transposition just moved about. Also notice that we are close to using the idea of permutation in two very different ways first as a particular n-bit to n-bit substitution and second as a particular re-arrangement of characters in the block. These have wildly different ciphering effects. STREAM CIPHER o A stream cipher does not need to accumulate some
