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Basic Cryptography

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Cryptography is a fundamental aspect of cybersecurity, involving techniques to secure information by transforming it into an unreadable format for unauthorized users. This section will cover the basic concepts, types of cryptographic algorithms, and practical examples to help you understand how cryptography works.

Key Concepts in Cryptography

  1. Encryption and Decryption:

    • Encryption: The process of converting plaintext (readable data) into ciphertext (unreadable data) using an algorithm and a key.
    • Decryption: The process of converting ciphertext back into plaintext using an algorithm and a key.

  2. Keys:

    • Symmetric Key: The same key is used for both encryption and decryption.
    • Asymmetric Key: Different keys are used for encryption and decryption (public key and private key).

  3. Cryptographic Algorithms:

    • Symmetric Algorithms: Examples include AES (Advanced Encryption Standard) and DES (Data Encryption Standard).
    • Asymmetric Algorithms: Examples include RSA (Rivest-Shamir-Adleman) and ECC (Elliptic Curve Cryptography).

  4. Hash Functions:

    • A function that converts an input (or 'message') into a fixed-size string of bytes. Hash functions are used for ensuring data integrity.

Symmetric vs. Asymmetric Cryptography

Feature Symmetric Cryptography Asymmetric Cryptography
Key Usage Same key for encryption and decryption Different keys for encryption and decryption
Speed Faster Slower
Key Distribution Challenging (securely sharing the key) Easier (public key can be shared openly)
Example Algorithms AES, DES RSA, ECC

Practical Examples

Symmetric Encryption Example (AES)

from Crypto.Cipher import AES
from Crypto.Random import get_random_bytes

# Generate a random key
key = get_random_bytes(16)  # AES-128

# Create a cipher object
cipher = AES.new(key, AES.MODE_EAX)

# Encrypt some data
data = b"Secret Message"
ciphertext, tag = cipher.encrypt_and_digest(data)

print("Ciphertext:", ciphertext)

Explanation:

  • get_random_bytes(16): Generates a random 16-byte key for AES-128 encryption.
  • AES.new(key, AES.MODE_EAX): Creates a new AES cipher object in EAX mode.
  • cipher.encrypt_and_digest(data): Encrypts the data and generates a tag for integrity checking.

Asymmetric Encryption Example (RSA)

from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP

# Generate RSA key pair
key = RSA.generate(2048)
public_key = key.publickey()

# Encrypt with the public key
cipher = PKCS1_OAEP.new(public_key)
ciphertext = cipher.encrypt(b"Secret Message")

print("Ciphertext:", ciphertext)

# Decrypt with the private key
cipher = PKCS1_OAEP.new(key)
plaintext = cipher.decrypt(ciphertext)

print("Plaintext:", plaintext)

Explanation:

  • RSA.generate(2048): Generates a 2048-bit RSA key pair.
  • PKCS1_OAEP.new(public_key): Creates a new cipher object for encryption using the public key.
  • cipher.encrypt(b"Secret Message"): Encrypts the message using the public key.
  • PKCS1_OAEP.new(key): Creates a new cipher object for decryption using the private key.
  • cipher.decrypt(ciphertext): Decrypts the ciphertext back to plaintext.

Exercises

Exercise 1: Symmetric Encryption with AES

Task: Write a Python script to encrypt and decrypt a message using AES in CBC mode.

Solution:

from Crypto.Cipher import AES
from Crypto.Random import get_random_bytes
from Crypto.Util.Padding import pad, unpad

# Generate a random key and IV
key = get_random_bytes(16)  # AES-128
iv = get_random_bytes(16)   # Initialization Vector

# Create a cipher object
cipher = AES.new(key, AES.MODE_CBC, iv)

# Encrypt some data
data = b"Secret Message"
ciphertext = cipher.encrypt(pad(data, AES.block_size))

print("Ciphertext:", ciphertext)

# Decrypt the data
cipher = AES.new(key, AES.MODE_CBC, iv)
plaintext = unpad(cipher.decrypt(ciphertext), AES.block_size)

print("Plaintext:", plaintext)

Exercise 2: Asymmetric Encryption with RSA

Task: Write a Python script to generate an RSA key pair, encrypt a message with the public key, and decrypt it with the private key.

Solution:

from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP

# Generate RSA key pair
key = RSA.generate(2048)
public_key = key.publickey()

# Encrypt with the public key
cipher = PKCS1_OAEP.new(public_key)
ciphertext = cipher.encrypt(b"Secret Message")

print("Ciphertext:", ciphertext)

# Decrypt with the private key
cipher = PKCS1_OAEP.new(key)
plaintext = cipher.decrypt(ciphertext)

print("Plaintext:", plaintext)

Common Mistakes and Tips

  1. Key Management: Ensure that keys are stored securely and not hard-coded in the source code.
  2. Initialization Vector (IV): Always use a unique IV for each encryption operation to ensure security.
  3. Padding: When using block ciphers, ensure that the data is properly padded to match the block size.

Conclusion

In this section, we covered the basics of cryptography, including key concepts, symmetric and asymmetric encryption, and practical examples. Understanding these fundamentals is crucial for implementing secure systems and protecting sensitive information. In the next module, we will delve into network security, exploring how to protect data in transit.

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