1.1 Introduction to Cryptography

Introduction to Cryptography

• Cryptography is the art and science of protecting data, ensuring only intended recipients can read and process it, while blocking adversaries.
• Various aspects of information security, such as confidentiality, integrity, authentication, and non-repudiation, are central to modern cryptography.
• Modern cryptography combines mathematics, computer science, and electrical engineering
• Cryptography is used in ATMs, computer passwords, and electronic commerce.

Goals of Cryptography

• Confidentiality: Ensuring secret data remains secret from unauthorized access.
• Integrity: Verifying data hasn't been altered unauthorizedly.
• Non-repudiation: Proving data was created or modified by a specific individual.
• Authentication: Determining if someone or something is genuinely who or what it claims to be.

Two Kinds of Cryptography

• Symmetric Cryptography:
  • Alice and Bob agree on a cryptosystem and a key
  • Alice encrypts the plaintext message using the algorithm and key, creating ciphertext
  • Alice transmits ciphertext to Bob
  • Bob decrypts the message using the same algorithm and key.
• Asymmetric Cryptography:
  • Alice and Bob agree on a public-key cryptosystem.
  • Bob sends Alice his public key.
  • Alice encrypts her message with Bob's public key and sends it to Bob.
  • Bob decrypts the message with his private key.

Symmetric Cryptography Detail

• Uses the same key for encryption and decryption.
• Efficient for bulk data processing.
• Works well with smaller key sizes.
• Key distribution is a challenge.

Symmetric Key Algorithms

• Data Encryption Standard (DES)
• Triple DES (3DES)
• Blowfish & Twofish
• IDEA
• RC4, RC5, and RC6
• Advanced Encryption Standard (AES)

Asymmetric Cryptography Detail

• Uses two different but related keys.
• Encryption with one key, decryption with the other key.
• Solves key distribution challenges.
• Inefficient for large data blocks.
• Slower than symmetric cryptography.

Problems with Cryptography

• Symmetric: Keys need secure distribution. If a key is compromised, anyone can decrypt messages. Doesn't scale well for networks.
• Asymmetric: Significantly slower than symmetric. Vulnerable to attacks like chosen-plaintext attacks.

Public-Key Algorithms

• Not a substitute for symmetric algorithms.
• Used to encrypt keys (session keys) used by symmetric algorithms for message traffic.

Asymmetric Key Algorithms

• Rivest Shamir Adleman (RSA)
• Pretty Good Privacy (PGP)
• Open Standard PGP (RFC 4880) - implemented as Gnu Privacy Guard
• Elliptic Curve Cryptosystem (ECC)

Hybrid Cryptosystems

• Bob sends Alice his public key.
• Alice generates a session key, encrypts it with Bob's public key, and sends it to Bob.
• Bob decrypts the session key using his private key.
• Both use the same session key for further communication.

Hash Functions

• Computes a small hash value from large amounts of data.
• Different data results in different hash values.
• Protects data integrity - verifying no unauthorized alterations.
• Used in digital signatures.

Hash Algorithms

• Message Digest (MD) (e.g., MD2, MD4, MD5)
• Secure Hash Algorithm (SHA) (e.g., SHA-1, SHA-256, SHA-512)
• HAVAL

Digital Certificates

• Electronic documents containing information about the certificate (who issued it, whom it's issued to, valid until, public key, algorithms and key sizes, digital signature).
• Digital signatures verify data integrity.