Cryptography and Network Security Chapter 9

Private-Key Cryptography

• Traditional cryptography utilizes a single key shared between sender and receiver, known as private or secret key cryptography.
• If this shared key is disclosed, the security of the communication is compromised.
• This method is symmetric, meaning both parties hold equal status, but does not protect the sender from forgery claims by the receiver.

Public-Key Cryptography

• Considered a monumental advancement in the 3000-year evolution of cryptography.
• Utilizes two distinct keys: a public key available to anyone and a private key known only to the recipient.
• This approach is asymmetric; parties do not hold equal power in the process.
• It employs number theoretic concepts, complementing rather than replacing symmetric key cryptography.

Reasons for Public-Key Cryptography

• Developed to resolve issues related to key distribution, avoiding reliance on a Key Distribution Center (KDC) for secure communication.
• Facilitates digital signatures, which authenticate that a message originates from the claimed sender.
• Public invention credited to Whitfield Diffie and Martin Hellman in 1976, although earlier known in classified circles.

Key Components of Public-Key Cryptography

• The public key can encrypt messages and verify digital signatures, while the private key is used to decrypt messages and create signatures.
• It is computationally infeasible to derive the private key from the public key, ensuring security.
• Asymmetric nature means those who encrypt or verify cannot simultaneously decrypt or sign.

Public-Key Applications

• Public-Key systems can be categorized into three primary applications:
  • Encryption/Decryption for ensuring message secrecy.
  • Digital Signatures for guaranteeing message authenticity.
  • Key Exchange for secure session keys.
• Some algorithms serve multiple uses, while others are specialized for a single function.

Public-Key Requirements

• Effective Public-Key algorithms depend on two critical requirements:
  • It must be computationally infeasible to discover the decryption key with only knowledge of the algorithm and encryption key.
  • It should be computationally straightforward to encrypt or decrypt messages when the corresponding key is available.
• In some algorithms, either key may serve for encryption, the other must decrypt.
• Necessitates a trapdoor one-way function to ensure secure operations.