Part 1: Symmetric Encryption

Objective: Understand and apply symmetric encryption techniques.

Definition: Symmetric encryption is a type of encryption where the same key is used for both encrypting and decrypting the data. It is also known as secret key encryption.
Key Characteristics:
- Speed: Generally faster than asymmetric encryption.
- Key Distribution Challenge: The same key must be securely shared between parties.

Encryption and Decryption:
- Encryption: The process of converting plaintext (original data) into ciphertext (encrypted data) using a secret key.
- Decryption: The reverse process of converting ciphertext back to plaintext using the same secret key.
Block Cipher vs. Stream Cipher:
- Block Cipher: Encrypts data in fixed-size blocks (e.g., AES operates on 128-bit blocks).
- Stream Cipher: Encrypts data one bit or byte at a time (e.g., RC4).

AES (Advanced Encryption Standard):
- Most widely used symmetric encryption algorithm.
- Key Sizes: 128, 192, or 256 bits.
- Use Cases: Encrypting data at rest (e.g., file encryption), secure communication (e.g., VPNs).
DES (Data Encryption Standard) and 3DES (Triple DES):
- DES is an older encryption standard, now considered insecure due to its short key length (56 bits).
- 3DES applies DES encryption three times to each data block, increasing security.

Goal: Encrypt a plaintext message and then decrypt it using AES.
Tools: Online AES encryption tool or a simple program using a cryptographic library (e.g., OpenSSL, PyCrypto).

Steps:

Choose a plaintext message (e.g., “Hello, World!”).
Select an AES key (e.g., 128-bit key). Ensure it’s randomly generated for security.
Encrypt the plaintext using AES.
Decrypt the resulting ciphertext using the same AES key.
Verify that the decrypted text matches the original plaintext.

File Encryption: Use symmetric encryption to protect sensitive files.
Secure Communication: Employ symmetric encryption to secure data transmission in VPNs or other secure communication channels.