Mật mã, tổng quan về mật mã

[...]... the other letters in ordinary English text Hence the letter occurring most frequently in a sequence of ciphertext blocks is most likely to correspond to the letter in the plaintext By observing a modest quantity of ciphertext blocks, a cryptanalyst can determine the key S eỷ ệ ặ ẫ ĐS Cí S ầ Homophonic substitution ciphers 1.28 Denition To each symbol , associate a set of strings of symbols, with the . Information security and cryptography The concept of information will be taken to be an understood quantity. To introduce cryp- tography, an understanding of issues related to information security. evaluated with respect to various criteria such as: 1. level of security. This is usually difficult to quantify. Often it is given in terms of the number of operationsrequired (using thebest methods. ciphertext blocks is most likely to correspond to the letter in the plaintext. By observing a modest quantity of ci- phertext blocks, a cryptanalyst can determine the key. Homophonic substitution ciphers 1.28