第3单元 分组密码和数据加密标准.pptVIP

第3章 分组密码与数据加密标准 Modern Block Ciphers modern block ciphers one of the most widely used types of cryptographic algorithms provide secrecy /authentication services focus on DES (Data Encryption Standard) to illustrate block cipher design principles Block vs Stream Ciphers block ciphers process messages in blocks, each of which is then en/decrypted like a substitution on very big characters 64-bits or more stream ciphers process messages a bit or byte at a time when en/decrypting many current ciphers are block ciphers broader range of applications Block Cipher Principles most symmetric block ciphers are based on a Feistel Cipher Structure needed since must be able to decrypt ciphertext to recover messages efficiently block ciphers look like an extremely large substitution would need table of 264 entries for a 64-bit block instead create from smaller building blocks using idea of a product cipher Ideal Block Cipher Claude Shannon and Substitution-Permutation Ciphers Claude Shannon introduced idea of substitution-permutation (S-P) networks in 1949 paper form basis of modern block ciphers S-P nets are based on the two primitive cryptographic operations seen before: substitution (S-box) permutation (P-box) provide confusion diffusion of message key Confusion and Diffusion cipher needs to completely obscure statistical properties of original message a one-time pad does this more practically Shannon suggested combining S P elements to obtain: diffusion – dissipates statistical structure of plaintext over bulk of ciphertext confusion – makes relationship between ciphertext and key as complex as possible Feistel Cipher Structure Feistel devised Feistel cipher based on concept of invertible product cipher partitions input block into two halves process through multiple rounds which perform a substitution on left data half based on round function of right half subkey then have permutation swapping halves implements Shannon’s S-P net concept Feistel Cipher Structure F