Cryptography Fundamentals

Cryptography is the science of securing communication and data through mathematical algorithms. It is the foundation of confidentiality, integrity, and authentication in cybersecurity.

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Core Concepts

Term	Definition
Plaintext	The original readable data
Ciphertext	The encrypted (unreadable) data
Encryption	Converting plaintext to ciphertext
Decryption	Converting ciphertext back to plaintext
Key	A secret value used by the algorithm to encrypt/decrypt
Algorithm (cipher)	The mathematical function used for encryption

Plaintext  ──[Encryption + Key]──▶  Ciphertext
Ciphertext ──[Decryption + Key]──▶  Plaintext

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Symmetric Encryption

Symmetric encryption uses the same key for encryption and decryption:

Algorithm	Key Size	Notes
AES	128, 192, 256 bits	Industry standard — used everywhere
3DES (Triple DES)	168 bits	Legacy — being phased out
ChaCha20	256 bits	Modern, fast on mobile devices
Blowfish / Twofish	Up to 448 bits	Older alternatives

AES (Advanced Encryption Standard)

AES is the most widely used symmetric cipher:

• Adopted by NIST in 2001
• Used by governments, banks, and nearly every HTTPS connection
• AES-256 is approved for top-secret classified data

AES Modes of Operation

Mode	Full Name	Use Case
CBC	Cipher Block Chaining	General-purpose, requires IV
GCM	Galois/Counter Mode	Authenticated encryption (encryption + integrity)
CTR	Counter Mode	Stream-like encryption

Tip: Always use AES-GCM when you need both encryption and integrity verification. Never use ECB mode — it leaks patterns.

Pros and Cons

• Fast — suitable for encrypting large amounts of data
• Key distribution problem — both parties must share the secret key securely

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Asymmetric Encryption

Asymmetric encryption uses a key pair — a public key and a private key:

Algorithm	Use Case	Notes
RSA	Encryption, digital signatures	Widely used, key sizes 2048–4096 bits
ECC (Elliptic Curve)	TLS, mobile, IoT	Smaller keys, same security (256-bit ECC ~ 3072-bit RSA)
Diffie-Hellman	Key exchange	Establishes shared secret over insecure channel
Ed25519	Digital signatures	Modern, fast, used in SSH and TLS

How It Works

Alice (Sender)                           Bob (Receiver)
     │                                        │
     │  Encrypts with Bob's PUBLIC key         │
     │──────────── Ciphertext ────────────────▶│
     │                                         │
     │           Decrypts with Bob's PRIVATE key│

• Public key — shared with everyone (used to encrypt or verify signatures)
• Private key — kept secret (used to decrypt or create signatures)

Pros and Cons

• Solves key distribution — no need to share a secret key
• Slower — not practical for large data; typically used to encrypt a symmetric key

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Hashing