Computer Network Security

8Cryptography

This chapter will focus on the following topics:

• – basic concepts in cryptography;
• – different classifications in cryptography:
  • - traditional cryptography,
  • - modern cryptography,
  • - symmetric and asymmetric encryption;
• – key management:
  • - Diffie-Hellman key exchange;
• – hash functions;
• – HMAC codes;
• – asymmetric cryptography:
  • - numerical signatures,
  • - public key infrastructure.

8.1. Basic concepts in cryptography

8.1.1. Definition

Cryptography is a computer science discipline that makes it possible to protect messages that are judged to be confidential. If the message is intercepted by an unauthorized person, it should be incomprehensible and difficult to decipher.

The four objectives of cryptography are:

• – authentication: verifying the identity of the source and destination before beginning transmission;
• – integrity: ensuring that the data transmitted has not been modified during the transmission;
• – confidentiality: ensuring that only authorized persons can consult the data;
• – non-repudiation: ensuring that a message transmitted between two people cannot be questioned by one of the two parties.

8.1.2. Terminology

• – Plain text: denotes all data that we wish to transmit before modification. Plain text may include text, images, videos, audio etc.
• – Encryption: transforms plain text into incomprehensible data.
• – Encrypted text: refers to text obtained after applying the encryption algorithm on the plain-text, also cryptogram.
• – Decryption: refers to ...