• Table of Contents

Principles of Public-Key Cryptosystems

If you recall asymmetric encryption, it employs two separate keys, distinct from symmetric encryption's single-key approach. This use of two keys holds significant implications for confidentiality, key distribution, and authentication, as we'll explore in the following sections.

Public-Key Cryptosystems

To discern between symmetric and asymmetric encryption, we refer to the key used in symmetric encryption as a secret key. The two keys used for asymmetric encryption are referred to as the public key and the private key.

1. Public Key: Available to anyone.
2. Private Key: Known only to the individual.

Applications for Public-Key Cryptosystems

In public-key cryptosystems, there are three main categories of usage, each involving different aspects of key management:

1. Encryption/Decryption:
   • The sender encrypts a message using the recipient's public key.
   • The recipient decrypts the message using their private key.
2. Digital Signature:
   • The sender signs a message with their private key.
   • Signing involves applying a cryptographic algorithm to the message or a small block of data derived from the message.
3. Key Exchange:
   • Two parties collaborate to exchange a session key.
   • A session key is a temporary secret key for symmetric encryption, valid for a specific transaction or session and short-lived.

Some algorithms are suitable for all three applications, whereas others can be used only for one or two of these applications. The table below indicates the applications supported by the algorithms discussed.

Table 2.1 Applications for public-key cryptosystems.

One in particular which will look further into is RSA.

Requirements for Public-Key Cryptography

The requirements for public-key cryptosystems can be simplified as follows:

1. Generating a key pair (public key $\text{K}{\text{pub}}$, private key $\text{K}{\text{priv}}$) should be computationally easy for the user.
2. It should be computationally easy for a sender to generate the ciphertext ($\text{C}$) using the recipient's public key.