 # Block Ciphers: Workhorses of Cryptography COMP 1721 A Winter 2004.

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Block Ciphers: Workhorses of Cryptography COMP 1721 A Winter 2004

Basic Scenario Basic Scenario Two communicating parties: Alice Bob insecure channel Attacker

Information Security Services Information Security Services u secrecy - attacker listening cannot gain any useful information from the communication Communicating parties may require: u authentication - communicating parties are ensured to be who/what they claim to be u integrity - data cannot be modified by attacker without detection u non-repudiation - Alice and Bob cannot deny transmission/receipt of certain data

Terminology Terminology Cryptography is the study of mathematical techniques used to provide information security Cryptanalysis is the study of techniques used to thwart (break) cryptographic techniques Cryptology is the field which encompasses both cryptography and cryptanalysis

Cryptographic Primitives Cryptographic Primitives The basic cryptographic “building blocks” used to provide these information security services are called primitives EXAMPLES: è symmetric-key ciphers (block ciphers) è public-key ciphers è digital signatures è cryptographic hash functions

Using Ciphers for Secrecy Using Ciphers for Secrecy A cipher is an encryption/decryption algorithm, parameterized by keys decryption algorithm encryption algorithm kdkd keke plaintext (p) ciphertext (c) Attacker

Categories of Ciphers Categories of Ciphers u Symmetric-key Ciphers u Public-key (Asymmetric) Ciphers u k e = k d (or they are easily derived from each other) u the key is known only to Alice and Bob u the recipient (Bob) has a key pair (k e, k d ) u k e (the public key) is widely distributed - on Bob’s Web page, in a directory, etc u k d (the private key) is known only to Bob

Advantages/Disadvantages Advantages/Disadvantages u Symmetric-key ciphers u Public-key ciphers u Alice and Bob must establish a shared key (key distribution problem) u solves key distribution problem u very fast (used for bulk encryption) u slow (approximately 1/1000 speed of symmetric- key ciphers)

Hybrid Approach Hybrid Approach Suppose Alice has a large file she wants to encrypt and send to Bob 1. Alice randomly generates a key (k) to be used in a symmetric-key cipher 2. Alice encrypts k with Bob’s public key and sends it to Bob 3. Bob decrypts k using his private key 4. Alice and Bob communicate using k and an agreed-upon symmetric-key cipher

Block Ciphers Block Ciphers u Block ciphers are an important class of symmetric-key ciphers u A block cipher maps N bits to N bits (plaintext to ciphertext) and is parameterized by a key u typical block sizes: 64 bits, 128 bits u name comes from fixed-length nature of input (as compared to stream ciphers, which encrypt their input as a continuous stream) u N is called the block size

Block Cipher Architectures Block Cipher Architectures Two fundamental block cipher architectures: u Substitution-Permutation Networks (SPNs) u Feistel Networks è Both encrypt a plaintext through a series of weaker steps called rounds è Feistel networks modify only half the input to each round (simplifies decryption process), whereas SPNs modify the entire input

u An n  n s-box is a mapping from n bits to n bits that can be inverted (so two different inputs must map to two different outputs) S-Boxes and Subkeys S-Boxes and Subkeys u The subkeys (round keys) are typically derived from k via a key-scheduling algorithm u k is sometimes called the master key

u In its simplest form, a linear transformation is simply a permutation (shuffling) of bits (hence the name substitution-permutation network) The Linear Transformation The Linear Transformation u In general, a applying a linear transformation means multiplying the input (an N-bit vector) by a binary N  N matrix

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