Lec3-CH02-CompSec4e.pptx - Chapter 2 Cryptographic Tools Basic Terminology Plaintext The original message Ciphertext The coded message Enciphering or

# Lec3-CH02-CompSec4e.pptx - Chapter 2 Cryptographic Tools...

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Chapter 2 Cryptographic Tools
Basic Terminology Plaintext The original message Ciphertext The coded message Enciphering or encryption Converting plaintext to ciphertext Deciphering or decryption Restoring the plaintext from the ciphertext 2
2 Types of Encryption Symmetric Asymmetric
Symmetric Encryption The universal technique for providing confidentiality for transmitted or stored data Also referred to as conventional encryption or single-key encryption Two requirements for secure use: Need a strong encryption algorithm Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure
Symmetric Encryption The only type of encryption in use prior to the development of public-key encryption in the 1970s Remains by far the most widely used of the two types of encryption 5
Plaintext input Y = E[ K , X ] X = D[ K , Y ] X K K Transmitted ciphertext Plaintext output Secret key shared by sender and recipient Secret key shared by sender and recipient Encryption algorithm (e.g., DES) Decryption algorithm (reverse of encryption algorithm) Figure 2.1 Simplified Model of Symmetric Encryption
Security through obscurity and Kerchoffs’ assumption S ecurity through obscurity The secrecy of the design or implementation is the main method of providing security. Kerchoffs’ assumption The adversary knows all details of the encryption function except the secret key 7
Cryptographic Protocols Design features: Two types of operations used for transforming plaintext to ciphertext Two types of operations used for transforming plaintext to ciphertext Substitutio n (confusion) Transpositi on (diffusion) Number of keys used Number of keys used Symmetric single-key Asymmetri c two-keys public encryption key private decryption key The way in which the plaintext is processed The way in which the plaintext is processed Block cipher Stream cipher 8
Security of Encryption Unconditionally secure o No matter how much time or computational power an opponent has, it is impossible to decrypt the ciphertext Computationally secure o The cost of breaking the cipher exceeds the value of the encrypted information o The time required to break the cipher exceeds the useful lifetime of the information 9
Attacking Symmetric Encryption Cryptanalytic Attacks Brute-Force Attacks Rely on: Nature of the algorithm Some knowledge of the general characteristics of the plaintext Some sample plaintext- ciphertext pairs Exploits the characteristics of the algorithm to attempt to deduce a specific plaintext or the key being used If successful all future and past messages encrypted with that key are compromised Try all possible keys on some ciphertext until an intelligible translation into plaintext is obtained On average half of all possible keys must be tried to achieve success Some degree of knowledge about the expected plaintext is needed, and some means of automatically distinguishing plaintext from garble

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