FIT2093 Lecture Notes - Lecture 6: Coprime Integers, Prime Number, Composite Number
30 May 2018
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Lecture 5 - Symmetric Encryption techniques
Symmetric Key Cryptography
-A symmetric key cipher is composed of 2 algorithms:
-Encryption Algorithm E
-Decryption Algorithm D
-The same key K is used for encryption & decryption
Feistel Cipher Parameters:
-Block size: larger block sizes mean greater security
-Key size: larger key size means greater security
-Number of rounds: multiple rounds off increasing security, typically 16 rounds
-Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis
-Round function: greater complexity means greater resistance to cryptanalysis
2 Types of Private Key Ciphers:
-Block Ciphers: encrypting block by block
-Stream Cipher or Vernam Cipher: encrypting byte or bit by bit
-Have a pseudo random key stream
-Combined (XOR) with plaintext bit by bit
-Must never reuse stream key
Symmetric Encryption Algorithms:
-Data Encryption Standard (DES) —> bloch cipher
-Most widely used encryption scheme
-Processed in 64-bit blocks
-58-bits key, 8 parity bits are stripped off form the full (8 characters)
-16 subkeys are generated for 16 rounds
Triple DES: apply DES algorithm three times
-use three keys and three executions of the DES algorithm
AES Encryption Algorithm:
-128- bit block cipher
-Supports three different key length: 128, 192 and 256 bits
-Supports 10 to 14 rounds
-Offers flexibility
-Efficient implementation in both software and hardware
Cryptanalysis:
-Ciphertext only attack: code breaker only has access to some samples of cipher text
-Known plaintext attack: code breaker knows some samples of cipher text with their
corresponding plaintext
-Chosen plaintext attack: can feed encryption algorithm with plaintext and obtain the matching
cipher texts
-Chosen cipher text attack (most severe): can feed decryption algorithm with ciphertext and
obtain the matching plaintexts
Encryption with Block Cipher
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
A symmetric key cipher is composed of 2 algorithms: The same key k is used for encryption & decryption. Block size: larger block sizes mean greater security. Key size: larger key size means greater security. Number of rounds: multiple rounds off increasing security, typically 16 rounds. Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis. Round function: greater complexity means greater resistance to cryptanalysis. Stream cipher or vernam cipher: encrypting byte or bit by bit. Combined (xor) with plaintext bit by bit. Data encryption standard (des) > bloch cipher. 58-bits key, 8 parity bits are stripped off form the full (8 characters) 16 subkeys are generated for 16 rounds. Use three keys and three executions of the des algorithm. Supports three different key length: 128, 192 and 256 bits. Efficient implementation in both software and hardware. Ciphertext only attack: code breaker only has access to some samples of cipher text.
