## Terminology

• Cryptosystem
• A system that describes how to encrypt or decrypt messages
• Plaintext
• Message in its original form
• Ciphertext
• Message in its encrypted form
• Cryptographer
• Invents encryption algorithms
• Cryptanalyst
• Breaks encryption algorithms or implementations

Security Benefits of Cryptography
• Confidentiality
• Integrity
• Authentication (as we will see)
• Non-repudiation

Cryptosystem

• Quintuple $(\mathcal{E}, \mathcal{D}, \mathcal{M}, \mathcal{K}, C)$
$-\mathcal{M}$ set of plaintexts
• $\mathcal{K}$ set of keys
• $C$ set of ciphertexts
$-\mathcal{E}$ set of encryption functions e: $\mathcal{M} \times \mathcal{K} \rightarrow C$
$-\mathcal{D}$ set of decryption functions $d: C \times \mathcal{K} \rightarrow \mathcal{M}$

Attacks

• Adversary is the person who wants to break the cryptosystem
• Assume adversary knowns the algorithm used, but not the key
• Is this a realistic assumption?
• ciphertext only
• known plaintext
• chosen plaintext

Basis for Attacks

• Mathematical attacks
• Finding flaws by analyzing the underlying mathematics of the cryptosystem
• Statistical attacks
• Make assumptions based on the underlying language
• Examine ciphertext, correlate properties with the assumptions
• Implementation attacks
• Implementation of cryptosystem introduces a flaw that is not in the mathematics of the cryptosystem

Classical Cryptography

• Sender and receiver share common key
• Keys may be the same, or trivial to derive from one another
• Called symmetric cryptography
• Two basic types
• Substitution ciphers
• Transposition ciphers
• Combinations are called product ciphers

Substitution Ciphers

• Change characters in plaintext to produce ciphertext
• Ceasar cipher
• HELLO WORLD
• Change each letter to the third letter following it $(X \rightarrow A, Y->B, Z->C, \ldots)$
• Key is 3 or written as a letter ‘D’
• KHOOR ZRUOG

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### CSE 365LR Introduction to Computer Security

View ScheduleCSE 365LR Introduction to Computer SecurityLecture

This is an undergraduate-level course intended for junior and senior-level students and will teach them introductory concepts of computer security. The main foci of this course will be network, web security, and application security. Part of the work will be dedicated to ethical aspects of security, and online privacy. The course will be heavily hands-on, as opposed to theoretical teaching.Credits: 4