56 - Cryptography.md

Introduction

• Cryptography is used to protect data when it's outside the operating system's control
• Converts data from one form to another in controlled ways
• Goal is to make data unintelligible to opponents who don't have the key

Basic Cryptography Concepts

Encryption and Decryption

• Plaintext (P) is converted to ciphertext (C) using encryption algorithm E() and key K
• C = E(P, K)
• Decryption reverses this: P = D(C, K)

Properties of Good Cryptography

• Deterministic - same inputs always produce same outputs
• Hard to determine P from C without knowing K
• Relies entirely on secrecy of the key

Symmetric vs Asymmetric Cryptography

• Symmetric: Same key used for encryption and decryption
• Asymmetric (public key): Different keys for encryption and decryption
  • Public key can be widely shared
  • Private key kept secret

Types of Cryptography

Symmetric Cryptography

• Uses single shared secret key
• Fast but key distribution is challenging

Public Key Cryptography

• Uses key pairs - public and private key
• Slower but solves key distribution problem
• Can be used for authentication and secure communication

Cryptographic Hashes

• One-way function, cannot be reversed
• Used for integrity checking, password storage, etc.

Uses of Cryptography in Operating Systems

At-Rest Data Encryption

• Encrypts data stored on disk/storage devices
• Protects against physical theft of storage media
• Full disk encryption vs selective encryption

Network Communication

• Protects data in transit between machines

Cryptographic Capabilities

• Unforgeable access tokens using encryption

Key Management

• Proper key selection and secrecy is critical
• Use truly random sources to generate keys
• Minimize storage and exposure of keys

Limitations of Cryptography

• Does not protect against compromised OS
• Vulnerable to software flaws in implementations
• Key management remains a major challenge