CH02-CompSec3e.pptx - Chapter 2 Cryptographic Tools Symmetric Encryption \u2022 \u2022 \u2022 The universal technique for providing confidentiality for

# CH02-CompSec3e.pptx - Chapter 2 Cryptographic Tools...

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Chapter 2 Cryptographic Tools
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
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
Attacking Symmetric Encryption Cryptanalytic Attacks Brute-Force Attack 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
Table 2.1 Comparison of Three Popular Symmetric Encryption Algorithms DES Triple DES AES Plaintext block size (bits) 64 64 128 Ciphertext block size(bits) 64 64 128 Key size(bits) 56 112 or 168 128, 192, or 256 DES = Data Encryption Standard AES = Advanced Encryption Standard
Data Encryption Standard (DES) The most widely used encryption scheme FIPS PUB 46 Referred to as the Data Encryption Algorithm (DEA) Uses 64 bit plaintext block and 56 bit key to produce a 64 bit ciphertext block Strength concerns: Concerns about algorithm DES is the most studied encryption algorithm in existence Use of 56-bit key Electronic Frontier Foundation (EFF) announced in July 1998 that it had broken a DES encryption
Table 2.2 Average Time Required for Exhaustive Key Search Key size (bits) Cipher Number of Alternative Keys Time Required at 10 9 decryptions/s Time Required at 10 13 decryptions/s 56 DES 2 56 ≈ 7.2 ´ 10 16 2 55 ns = 1.125 years 1 hour 128 AES 2 128 ≈ 3.4 ´ 10 38 2 127 ns = 5.3 ´ 10 21 years 5.3

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