Cryptography (Symmetric & Asymmetric)

Cryptography Basics

• Plaintext: The original, readable message (e.g., 'HELLO').
• Ciphertext: The scrambled, unreadable message (e.g., 'KHOOR').
• Encryption: The process of converting Plaintext to Ciphertext.
• Decryption: The process of converting Ciphertext back to Plaintext.
• Key: A secret password or mathematical value used in the algorithm to encrypt and decrypt the data.

1. Symmetric Key Encryption

• How it works: Both the sender and the receiver use the EXACT SAME KEY to encrypt and decrypt the data.
• Analogy: Locking a physical box with a key, and mailing the box to your friend. Your friend must have an identical copy of that same key to open the box.
• Pros: It is extremely fast and efficient. Ideal for encrypting large amounts of data (like a whole hard drive).
• Cons: The Key Distribution Problem. How do you safely send the secret key to your friend in the first place? If a hacker intercepts the key while you are sending it, they can read all your future messages.
• Examples: DES (Data Encryption Standard), AES (Advanced Encryption Standard).

2. Asymmetric Key Encryption

• How it works: It uses a mathematically related PAIR of keys for every user: a Public Key and a Private Key.
  1. The Public Key is shared with the whole world.
  2. The Private Key is kept strictly secret by the owner.
  3. Crucial Rule: Data encrypted with the Public Key can ONLY be decrypted by the matching Private Key.
• Example (Sending a message): If Alice wants to send a secret message to Bob, Alice encrypts the message using Bob's Public Key. Now, even Alice cannot decrypt it! Only Bob can decrypt it using his secret Private Key.
• Pros: Solves the key distribution problem. You don't need to securely share a secret key beforehand.
• Cons: Mathematically complex and very slow compared to symmetric encryption.
• Examples: RSA (Rivest-Shamir-Adleman), ECC (Elliptic Curve Cryptography).

3. Hashing (One-Way Encryption)

• Unlike symmetric and asymmetric encryption, hashing is a one-way function. You cannot decrypt a hash back into the original plaintext.
• How it works: It takes an input of any size (a word, a file, a whole movie) and produces a fixed-size string of characters.
• Usage: Used for verifying data integrity (checking if a file was corrupted or altered) and safely storing passwords in a database.
• Examples: MD5, SHA-1, SHA-256.