Symmetric Encryption

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Symmetric encryption is a two-way encryption scheme in which encryption and decryption are both performed by the same key.

• aka private key cryptography
• used for confidentiality
• used for encrypting large amounts of data
  • is very fast
• secret key must be kept to authorized persons only
• to use, both entities need to exchange the key using a secure channel
  • this key exchange is the main problem

Structure of Symmetric Key Cryptosystem

2 Challenges of Symmetric Cryptography

1. Key establishment/exchange

• agreement on a secret symmetric key requires to make use of secure channel or a specific key-agreement protocol.

2. Trust

• since both parties use the same key, there is an implicit requirement that they trust each other
• not practical to provide security when parties do not belong to the same circle

Advantages over asymmetric key cryptography:

• significantly faster
• use shorter keys for the same security strength
• based on simple bit operations
  • while asymmetric is based on mathematical notions

Key Exchange

Key exchange is the process of sharing the key between the sender and receiver.

• constitutes an entire section in cryptography
• # of keys required:
  • for a group of people
  • need keys
    • because each member shares a key with each of the other members
• Sharing a single key among all users of the system is a weakness

2 Types of Ciphers

• Block Cipher
• Stream Cipher

Symmetric Key Algorithms

• Common symmetric key algorithms:
  • DES
  • 3DES
  • AES
• Other well-known symmetric ciphers include:
  • Block
    • Twofish
    • Serpent
    • Blowfish
    • CAST5
    • RC6
    • IDEA
  • Stream
    • RC4
    • ORYX
    • SEAL

3 Differences Between AES and 3DES

1. 3DES is three rounds of DES, while AES uses a newer and completely different algorithm
2. AES uses longer and stronger keys than 3DES, as well as a longer block length, making AES harder to attack
3. 3DES is slower than AES