[Day 3] 3. Encryption Schemes

Symmetric Algorithms

• Definition:
  In symmetric encryption, both the sender and receiver use the same key for both encryption and decryption.
• Key Management:
  The key must be kept confidential. If the key is compromised, the security of the system is lost, and the purpose of cryptography is undermined.
• Failure of PKI:
  If the symmetric key is compromised, the Public Key Infrastructure (PKI) system fails.

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Advantages of Symmetric Algorithms:

• Faster Performance:
  Symmetric encryption is faster than asymmetric encryption, making it suitable for encrypting large amounts of data.

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Disadvantages of Symmetric Algorithms:

1. Scalability and Key Management:

   • For communication between multiple people, the number of keys required grows quickly.
   • For example, communication between 10 people needs 45 keys, and as the number of people increases, managing these keys becomes complex.

2. Key Exchange Risk:

   • If the encryption keys are exchanged via any medium (e.g., email, physical transfer), there is a risk of the keys being tampered with during transit.

3. Authentication and Non-Repudiation:

   • Symmetric encryption does not provide a way to authenticate or verify who encrypted or decrypted the data.
   • Non-repudiation, which ensures a party cannot deny their actions, is also lacking.
   • So, symmetric algorithms cannot authenticate or provide non-repudiation.
   • This is often addressed with digital signatures in asymmetric encryption.

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Examples of Symmetric Algorithms:

• DES (Data Encryption Standard)
• Triple-DES (3DES)
• Blowfish
• Twofish
• IDEA
• RC4, RC5, RC6
• AES (Advanced Encryption Standard) - The most modern and widely used symmetric algorithm.

Note:
No logs are kept for who creates the key, which means there is no way to authenticate the creation of the key. This lack of logging is to avoid compromising security, as sensitive information must not be recorded.

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Types of Symmetric Algorithms:

1. Block Ciphers:
   These work on blocks of data (usually 128, 192, or 256 bits at a time).
   Examples: AES, DES

2. Stream Ciphers:
   These work on one bit at a time, processing data bit by bit.
   Examples: RC4

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