Cryptography

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Algorithms and Ciphers Chapter 4

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Module Agenda

Stream Ciphers

Block Ciphers

Modes of Symmetric Block Ciphers

Symmetric Algorithms

Asymmetric Algorithms

Hybrid Systems

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Categories of Cryptosystems

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Organization of Cryptosystems

Two-Way Cryptography

Symmetric

Asymmetric

Steganography

Message Integrity Controls

Stream

Block

Factoring the Product of Large Primes

Discrete Logarithms

Symmetric Key Algorithms

Same key used for both the encryption and decryption operations

Document

to Be Encrypted

Encryption Algorithm

(i.e., AES)

Encrypted

Document

Decrypted Document

Encryption Algorithm

Encrypted

Document

Key

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Stream Ciphers

A keystream (sequence of bits used as a key) is generated and combined with the plaintext using an Exclusive-OR (XOR) operation:

Statistically unpredictable and unbiased

Not linearly related to the key

Operates on individual bits or bytes

Functionally complex

Long periods with no repeats

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Stream Ciphers: Advantages and Disadvantages

Advantages:

Emulates a one-time pad

No size difference between plaintext and ciphertext

Very suitable for hardware implementation and serial communications

Disadvantages:

Can be difficult to implement correctly

Generally weaker than a block-mode cipher

Difficult to generate a truly random unbiased keystream

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Stream Cipher Uses

Wireless:

WEP, WPA

Audio/video streaming

GSM

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Examples of Stream-based Algorithms

RC-4: Ron Rivest of RSA Security (1987)

One of the most widely used stream ciphers today

TLS (Transport Layer Security), WEP (Wired Equivalent Privacy), WPA (Wi-Fi Protected Access), TKIP (Temporal Key Integrity Protocol), Microsoft XBox, Oracle SQL, Microsoft PPTP, Microsoft Office, and Adobe Acrobat.

SEAL: Software-optimized Encryption Algorithm

A5/1, A5/2, A5/3: These are stream ciphers used in GSM (Global Systems for Mobile Communications) systems.

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Block Cipher

Blocks of plaintext are encrypted into ciphertext blocks

Multiple modes of operation

Variable key size, block size, rounds

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Data Block

Cryptosystem

Cipher Block

Ciphertext

Key

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Block Cipher Uses

Data transport

AES (Advanced Encryption Standard): Created by NIST in 2001

Triple DES (Data Encryption Standard)

Data storage

AES in counter mode

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Advanced Encryption Standard (AES)

Based on Rijndael algorithm:

Developed by Daemen and Rijmen in 1998

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Rijndael

Block Options

AES

Block Options

128,192,256

Key Options

128, 192, 256

Rounds

10, 12, 14

128, 192, 256

Rounds

10, 12, 14

Key Options

128

Examples of Block Mode Algorithms

AES (Rijndael)

IDEA

RC2

RC5

RC6

Blowfish

Twofish

CAST

SAFER

Data Encryption Standard (DES)

3DES

Serpent

Mars

Skipjack

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