Symmetric & Asymmetric Encryption

Symmetric Encryption (Private-Key Cryptography)

• Also known as an conventional encryption and a single-key encryption.
• Is a form of cryptosystem in which encryption and decryption are performed using the same key or also as private-key encryption.
• Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure, hence this justifies the name private key encryption.
• With the use of symmetric encryption, the principal security problem is maintaining the secrecy of the key.
• Some Terms:
  • Plain-Text -> Original Message
  • Cipher-Text -> Coded message
  • The process of converting the plain-text to cipher-text -> Encryption/Enciphering.
  • Restoring the plain-text from the cipher-text -> Decrption/Deciphering.

How does Symmetric Encryption Work?

1. Key Generation
   • So the first step is to generate the the private-key, this is generated through two methods; that is the Password-based key Derivation(PBKD) and hardware random number generators
2. Encryption
   • The plain-text is transformed to cipher-text through encryption-algorithm.
3. Transfer of Cipher Text
   • Here the coded-message, is transfered through the internet, and even if it is intercepted, the message will be un-readable.
4. Decryption
   • In the final step, the recipient uses the same secret key and a reverse encryption algorithm to convert the cipher text back into the original message (plain text).

Types of Symmetric Encryption Algorithms

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Modes of Operation

• ECB(Electronic Codebook) -> Insecure, reveals patterns (identical plaintext -> identical ciphertext), identical plaintext blocks produce identical ciphertext blocks, making it easier for attackers to infer information..

  echo "katleho" | openssl enc -aes-128-ecb -nosalt -k passwd 2>/null/dev
  

  • "katleho" is our input
  • -k: stands for key; hence our key is passwd
  • 2>/null/dev silences the errors.
    

• CBC(Cipher Block Chaining) -> Uses an IV(Initialization Vector), vulnerable to padding oracle attacks.

  from Crypto.Cipher import AES
  from Crypto.Util.Padding import pad, unpad
  key = b'16_or_32_byte_key...'  # Must be 16, 24, or 32 bytes
  data = b"secret message"
  # Encrypt
  cipher = AES.new(key, AES.MODE_CBC)
  ct = cipher.encrypt(pad(data, AES.block_size))
  iv = cipher.iv  # Needed for decryption
  # Decrypt
  cipher = AES.new(key, AES.MODE_CBC, iv)
  pt = unpad(cipher.decrypt(ct), AES.block_size)
  print(pt)  # b'secret message'
  

• CTR(Counter Mode) -> Turns block cipher into stream cipher, no padding needed.
• GCM(Galois/Counter Mode) -> Secure, provides authentication.

Asymmetric Encryption (Public-Key Cryptography)

• Uses different keys, public key for encryption and private-key to decryption.

Types of Symmetric Encryption Algorithms

How RSA Works

1. Choose two large prime numbers p and q
2. Compute modulus(n) which is (p * q)
3. Compute phi(n) = (p-1)*(q-1)
4. Choose e (public exponent, often 65537)
5. Compute d(private exponent) = (e^-1) * mod(phi(n))
6. Encrypt: ciphertext = plaintext^e * mod(n)
7. Decrypt: plaintext = ciphertext^d * mod(n)

Key Differences