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 復大软件学院 LiTLiJT 7 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