lecture 7.pdf

1 University of Science and Technology Faculty of Computer Science and Information Technology Department of Information and Communication Technology Lecture (7) Block Ciphers and the Data Encryption Standard Instructor: M...

1 University of Science and Technology Faculty of Computer Science and Information Technology Department of Information and Communication Technology Lecture (7) Block Ciphers and the Data Encryption Standard Instructor: Mashair Omer 2 Modern Block Ciphers We will now look at modern block ciphers one of the most widely used types of cryptographic algorithms provide secrecy and/or authentication services in particular will introduce DES (Data Encryption Standard) 3 Block vs Stream Ciphers block ciphers process messages in into 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 4 Block Cipher Principles  most symmetric block ciphers are based on a Feistel Cipher Structure.  using idea of a product cipher  Claude Shannon and Substitution-Permutation Ciphers  in 1949 Claude Shannon introduced idea of substitution- permutation (S-P) networks  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 5 Confusion and Diffusion cipher needs to completely obscure statistical properties of original message a one-time pad does this more practically Shannon suggested combining elements to obtain: diffusion – dissipates statistical structure of plaintext over bulk of cipher text confusion – makes relationship between cipher text and key as complex as possible  Feistel Cipher Structure 6 Horst Feistel devised the feistel cipher based on concept of invertible product cipher partitions input block into two halves process through multiple rounds perform a substitution on left data half based on round function of right half & subkey then have permutation swapping halves implements Shannon’s substitution- permutation network concept  Feistel Cipher Structure 7  Feistel Cipher Design Principles: 8  block size  increasing size improves security, but slows cipher  key size  increasing size improves security, makes exhaustive key searching harder, but may slow cipher  number of rounds  increasing number improves security, but slows cipher  subkey generation  greater complexity can make analysis harder, but slows cipher  round function  greater complexity can make analysis harder, but slows cipher  fast software en/decryption & ease of analysis  are more recent concerns for practical use and testing  Feistel Cipher Decryption 9 10 Data Encryption Standard  The most widely used encryption scheme is based on the Data Encryption Standard  1967: Feistel at IBM  Lucifer: block size 128; key size 128 bit  1972: NBS asks for an encryption standard  1975: IBM developed DES (modification of Lucifer) block size 64 bits; key size 56 bits  1975: NSA suggests modifications  1977: NBS adopts DES as encryption standard in (FIPS 46-1, 46- 2).  2001: NIST adopts Rijndael as replacement to DES. 11 DES Features  Block size = 64 bits  Key size = 56 bits (in reality, 64 bits, but 8 are used as parity-check bits for error control, see next slide)  Number of rounds = 16  16 intermediary keys, each 48 bits