• 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 in 1949 Claude Shannon introduced idea of substitu-
tion-permutation (S-P) networks called modern substitution-transposition product cipher
these form the basis of modern block ciphers
• S-P networks are based on the two primitive cryptographic operations we have seen before:
• substitution (S-box)
• permutation (P-box)
• provide confusion and diffusion of message
• diffusion – dissipates statistical structure of plaintext over bulk of ciphertext
• confusion – makes relationship between ciphertext and key as complex as possible
