Classified along three independent dimensions: The type of operations used for transforming plaintext to ciphertext The number of keys used symmetric (single key) asymmetric (two-keys, or public-key encryption) The way in which the plaintext is processed | Chapter 2 Conventional Encryption Message Confidentiality Henric Johnson Blekinge Institute of Technology, Sweden Henric Johnson Outline Conventional Encryption Principles Conventional Encryption Algorithms Cipher Block Modes of Operation Location of Encryption Devices Key Distribution Henric Johnson Conventional Encryption Principles An encryption scheme has five ingredients: Plaintext Encryption algorithm Secret Key Ciphertext Decryption algorithm Security depends on the secrecy of the key, not the secrecy of the algorithm Henric Johnson Conventional Encryption Principles Henric Johnson Cryptography Classified along three independent dimensions: The type of operations used for transforming plaintext to ciphertext The number of keys used symmetric (single key) asymmetric (two-keys, or public-key encryption) The way in which the plaintext is processed Henric Johnson Average time required for exhaustive key . | Chapter 2 Conventional Encryption Message Confidentiality Henric Johnson Blekinge Institute of Technology, Sweden Henric Johnson Outline Conventional Encryption Principles Conventional Encryption Algorithms Cipher Block Modes of Operation Location of Encryption Devices Key Distribution Henric Johnson Conventional Encryption Principles An encryption scheme has five ingredients: Plaintext Encryption algorithm Secret Key Ciphertext Decryption algorithm Security depends on the secrecy of the key, not the secrecy of the algorithm Henric Johnson Conventional Encryption Principles Henric Johnson Cryptography Classified along three independent dimensions: The type of operations used for transforming plaintext to ciphertext The number of keys used symmetric (single key) asymmetric (two-keys, or public-key encryption) The way in which the plaintext is processed Henric Johnson Average time required for exhaustive key search Key Size (bits) Number of Alternative Keys Time required at 106 Decryption/µs 32 232 = x 109 milliseconds 56 256 = x 1016 10 hours 128 2128 = x 1038 x 1018 years 168 2168 = x 1050 x 1030 years Henric Johnson Feistel Cipher Structure Virtually all conventional block encryption algorithms, including DES have a structure first described by Horst Feistel of IBM in 1973 The realisation of a Fesitel Network depends on the choice of the following parameters and design features (see next slide): Henric Johnson Feistel Cipher Structure Block size: larger block sizes mean greater security Key Size: larger key size means greater security Number of rounds: multiple rounds offer increasing security Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis. Fast software encryption/decryption: the speed of execution of the algorithm becomes a concern Henric Johnson Henric Johnson Conventional Encryption .
