B FOR 204 Lecture Notes - Lecture 8: Key Size, Data Encryption Standard, Substitution Cipher
2 Jun 2018
School
Department
Course
Professor
Cryptography
Basic Terminology
• plaintext - the original message
• ciphertext - the coded message
• cipher - algorithm for transforming plaintext to ciphertext
• key - information used in cipher known only to sender/receiver
• encipher (encrypt) - converting plaintext to ciphertext
• decipher (decrypt) - recovering ciphertext from plaintext
• cryptography - study of encryption principles/methods
• cryptanalysis (codebreaking) - the study of principles/ methods of deciphering
ciphertext without knowing key
• cryptology - the field of both cryptography and cryptanalysis
Cryptographic Primitives
• Cryptographic primitives are concepts that are frequently used to build
cryptographic protocols and algorithms for computer security systems.
o Building blocks of cryptography
• Two major primitives
o Diffusion – dissipates statistical structure of plaintext over bulk of
ciphertext
o Confusion – makes relationship between ciphertext and key as complex
as possible
Implementing Confusion and Diffusion
Types of Ciphers
• Transposition ciphers - Rearranging bits or characters in data.
• Also called permutation ciphers
• Substitution ciphers - Replacing bits, characters, or blocks of characters with
substitutes.
• Block ciphers process messages into blocks, each of which is then encrypted or
decrypted.
• Electronic Code Book (ECB) mode
• Stream ciphers process messages a bit or byte at a time when
encrypting/decrypting.
Symmetric Cipher Model
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