05crypto1 - Computer and Network Security c circlecopyrt...

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Unformatted text preview: Computer and Network Security c circlecopyrt Copyright 2000 R. E. Newman Computer & Information Sciences & Engineering University Of Florida Gainesville, Florida 32611-6120 [email protected] Basic Cryptography (Pfleeger Ch. 2, KPS Ch. 1-2, etc.) Basic Cryptography Why here? In computer security, crypto is but one of several ways of achieving isolation, and is not one of the more important ways. In network security, crypto is the main attraction - everything is done via message-passing (ultimately), so the only secure way to achieve confidentiality and the authentication needed for access control is through crypto. 1 Definitions and Models 1.1 Cryptography 1.2 Steganography 1.3 Cipher 1.4 Code 1.5 Plaintext 1.6 Ciphertext 2 Uses of steganography 2.1 Watermarks 2.2 Covert channels 3 Crypto types 3.1 Key Symmetry • Symmetric M = D ( E ( M,K ) ,K ) • Asymmetric M = D ( E ( M,K ) ,K − 1 ), where K negationslash = K − 1 in general 3.2 Block vs. Stream • Block Plaintext is broken into fixed-length blocks, which are fed to the cryptosystem for encryption one block at a time, and one block is output for each block input. • Stream Plaintext is fed to the cryptosystem for encryption one symbol at a time, and a symbol is output for each symbol input. 4 Basics 4.1 Basic Goals • Plaintext should not easily be obtained from ciphertext • Key should not easily be obtained from ciphertext • Keyspace should be large enough to resist brute-force attacks • Confusion - effect of small change in plaintext on ciphertext should not be predictable • Diffusion - small change in plaintext should affect large part of ciphertext (block ciphers, feed-forward) 4.2 Cryptanalysis Attacks classified by amount of knowledge available to the cryptanalyst about P ; it is assumed that the crypt- analyst knows C = E ( P,K ) but does not know K . The cryptanalyst’s goal may be to find just P or to find K also. • Ciphertext-only - only C is known; quantity counts • Recognizable plaintext - C is known, and there is a test to determine if D ( C,K ′ ) is valid plaintext for a candiate key K ′ • Guessed plaintext - C is known, and (part or all) of the corresponding P may be guessed • Partial plaintext - both C and some of P are known • Known plaintext - both C and all of P are known • Chosen plaintext - the attacker can submit any P for encryption and obtain C = E ( P,K ) 2 4.3 Systems Models insecure channel E(M,K) secure channel K Alice M M Bob K K E D Symmetric Cryptography System Model Figure 1: Symmetric cryptosystem model insecure channel E(M,K) secure channel K Alice M M Bob K E D Asymmetric Cryptography System Model K-1 Figure 2: Asymmetric cryptosystem model 3 4.4 Functions 4.4.1 Converting text to numbers • Number letters - A=0, B=1, ..., Z=25 (26 symbols) • Both cases, numbers, ‘.’ and ‘ ’ (64 symbols) • ASCII, EBCDIC, etc. (128 or 256 symbols) 4.4.2 Types of functions Function f : A → B is • total iff f is defined for every element...
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This note was uploaded on 01/18/2012 for the course CIS 4930 taught by Professor Staff during the Fall '08 term at University of Florida.

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05crypto1 - Computer and Network Security c circlecopyrt...

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