Network Security - symmetric key encryption

1. Symmetric Key Encryption

Definition:
Symmetric key encryption is a type of encryption where the same key is used for both encryption and decryption.

• Both sender and receiver must share the secret key securely.

• It’s generally faster than asymmetric encryption.

• Main challenge: secure key distribution.

2. DES (Data Encryption Standard)

• Developed: 1977 by IBM; standardized by NIST.

• Key size: 56 bits (effective)

• Block size: 64 bits

• Algorithm type: Block cipher

• Structure: Uses Feistel network with 16 rounds.

• Process:

  1. Plaintext is divided into 64-bit blocks.

  2. Each block goes through 16 rounds of substitution and permutation using the key.

  3. Resulting ciphertext is 64-bit blocks.

Pros:

• Was widely adopted and standardized.

• Simple structure.

Cons:

• Key length is too short by today’s standards → vulnerable to brute-force attacks.

• Mostly replaced by AES.

3. AES (Advanced Encryption Standard)

• Developed: 2001 by NIST (Rijndael algorithm)

• Key sizes: 128, 192, 256 bits

• Block size: 128 bits

• Algorithm type: Block cipher

• Structure: Substitution-Permutation Network (SPN)

• Process:

  1. Plaintext is divided into 128-bit blocks.

  2. Each block undergoes multiple rounds (10, 12, or 14 depending on key size) of:

     • SubBytes: Byte substitution using a fixed S-box

     • ShiftRows: Row-wise permutation

     • MixColumns: Column mixing for diffusion

     • AddRoundKey: XOR with round key

  3. Produces ciphertext of 128-bit blocks.

Pros:

• Strong security, resistant to known attacks

• Efficient in both hardware and software

• Standard for government and commercial applications

Cons:

• Slightly more complex than DES, but manageable with modern systems.