Cryptography and Network
Security
Third Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 3 – Block Ciphers and the
Data Encryption Standard
All the afternoon Mungo had been working on
Stern's code, principally with the aid of the latest
messages which he had copied down at the
Nevin Square drop. Stern was very confident.
He must be well aware London Central knew
about that drop. It was obvious that they didn't
care how often Mungo read their messages, so
confident were they in the impenetrability of the
code.
—Talking to Strange Men, Ruth Rendell
Modern Block Ciphers
•
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)
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
•
hence are focus of course
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 2
64
entries for a 64-bit block
•
instead create from smaller building blocks
•
using idea of a product cipher
