Cryptography

Cryptography is the science of securing information by transforming it so that only authorized parties can understand or use it.

Core Goals of Cryptography

1. Confidentiality – Keep data secret from unauthorized users.
2. Integrity – Ensure data has not been altered.
3. Authentication – Verify the identity of users or systems.
4. Non-repudiation – Prevent someone from denying they performed an action.

Main Types of Cryptography

1. Symmetric-Key Cryptography

• Uses the same key for encryption and decryption.
• Fast and efficient.
• Example: AES

Plaintext + Secret Key → Ciphertext
Ciphertext + Same Secret Key → Plaintext

2. Asymmetric-Key Cryptography

• Uses a public key and a private key.
• Public key encrypts; private key decrypts.
• Example: RSA

Public Key → Encrypt
Private Key → Decrypt

3. Hash Functions

• Convert data into a fixed-size value (hash).
• One-way operation; cannot be reversed.
• Used for passwords and integrity checks.
• Examples: SHA-256, SHA-3

Common Applications

• HTTPS secure web browsing
• Online banking
• Digital signatures
• Password storage
• Secure messaging apps
• Cryptocurrencies such as Bitcoin

Example

Suppose Alice wants to send a secret message to Bob:

1. Alice encrypts the message using a cryptographic algorithm and key.
2. The encrypted message (ciphertext) is sent over the internet.
3. Bob uses the correct key to decrypt it.
4. Anyone intercepting the ciphertext cannot understand it without the key.

Example Cipher (Caesar Cipher)

Shift each letter by 3 positions:

HELLO → KHOOR

This is a historical cipher and is not secure by modern standards.

If you'd like, I can also explain:

• Classical cryptography (Caesar, Vigenère)
• Modern cryptography (AES, RSA, ECC)
• Public-key infrastructure (PKI)
• Digital signatures
• Cryptanalysis and attacks
• Cryptography for beginners or advanced learners.