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Symmetric Cryptography

Alice and Bob

Modern Cryptography Fundamentals

Alice and Bob

One of the two main branches of cryptography is symmetric cryptography. It includes encryption schemes as well as schemes concerned with authentication and integrity. Until the 1970s, all of cryptography would have consisted of symmetric encryption schemes.
The main discussion starts by looking at symmetric encryption schemes and making the crucial distinction between stream ciphers and block ciphers. We, then, turn to message authentication codes, which are schemes for ensuring message integrity and authenticity. Finally, we explore how symmetric encryption schemes and message authentication codes can be combined to ensure secure communication.
This chapter discusses various symmetric cryptographic schemes from practice in passing. The next chapter offers a detailed exposition of encryption with a stream cipher and a block cipher from practice, namely RC4 and AES respectively.
Before starting our discussion on symmetric cryptography, I want to briefly make some remarks on the Alice and Bob illustrations in this and subsequent chapters.
In illustrating the principles of cryptography, people often rely on examples involving Alice and Bob. I will do so as well.
Especially if you are new to cryptography, it is important to realize that these examples of Alice and Bob are only meant to serve as illustrations of cryptographic principles and constructions in a simplified environment. The principles and constructions, however, are applicable to a much wider range of real-life contexts.
Following are five key points to keep in mind about examples involving Alice and Bob in cryptography:
  1. They can easily be translated into examples with other types of actors such as companies or government organizations.
  2. They can easily be extended to include three or more actors.
  3. In the examples, Bob and Alice are typically active participants in creating each message and in the application of cryptographic schemes on that message. But in reality, electronic communications are largely automated. When you visit a website using transport layer security, for example, the cryptography is typically all handled by your computer and the web server.
  4. In the context of electronic communication, the “messages” that are sent across a communication channel are usually TCP/IP packets. These can belong to an e-mail, a Facebook message, a phone conversation, a file transfer, a website, a software upload, and so on. They are not messages in the traditional sense. Nevertheless, cryptographers will often simplify this reality by stating that the message is, for instance, an e-mail.
  5. The examples typically focus on electronic communication, but they can also be extended to traditional forms of communication such as letters.
Quiz
Quiz1/5
What type of symmetric encryption is primarily used for continuous communications, such as audio or video streams?